WHAT is Dusting?
Formation of loose powder resulting from disintegration of surface of hardened concrete is called dusting or chalking. The characteristics of such surfaces are:
a. They powder under any kind of traffic
b. They can be e
asily scratched with a nail or even by sweeping.
WHY Do Concrete Floors Dust?
A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by:
a. Any finishing operation performed while bleed water is on the surface or before the concrete has finished bleeding. Working this bleed water back into the top ¼-inch [6 mm] of the slab produces a very high water-cement ratio and, therefore, a low strength surface layer.
b. Poor finishing practices such as broadcasting dry cement to speed up finishing or sprinkling water to the surface while finishing
c. Floating and/or troweling operations following the condensation of moisture from warm humid air on cold concrete. In cold weather concrete sets slowly, in particular, cold concrete in basement floors. If the humidity is relatively high, water will condense on the freshly placed concrete, which, if troweled into the surface, will cause dusting.
d. Inadequate ventilation in enclosed spaces. Carbon dioxide from open salamanders, gasoline engines or generators, power buggies or mixer engines may cause a chemical reaction known as carbonation, which greatly reduces the strength and hardness of the concrete surface.
e. Insufficient curing. This omission often results in a soft surface skin, which will easily dust under foot traffic.
f. Inadequate protection of freshly placed concrete from rain, snow or drying winds. Allowing the concrete surface to freeze will weaken the surface and result in dusting.
HOW to Prevent Dusting:
a. Concrete with the lowest water content with an adequate slump for placing and finishing will result in a strong, durable, and wear-resistant surface. In general, use concrete with a moderate slump not exceeding 5 inches [125 mm]. Concrete with a higher slump may be used provided the mixture is designed to produce the required strength without excessive bleeding and/or segregation. Water-reducing admixtures are typically used to increase slump while maintaining a low water content in the mixture. This is particularly important in cold weather when delayed set results in prolonged bleeding.
b. NEVER sprinkle or trowel dry cement into the surface of plastic concrete to absorb bleed water. Remove bleed water by dragging a garden hose across the surface. Excessive bleeding of concrete can be reduced by using air-entrained concrete, by modifying mix proportions, or by accelerating the setting time.
c. DO NOT perform any finishing operations with water present on the surface or while the concrete continues to bleed. Initial screeding must be promptly followed by bull floating. Delaying bull floating operations can cause bleed water to be worked into surface layer. Do not use a jitterbug, as it tends to bring excess mortar to the surface. DO NOT add water to the surface to facilitate finishing operations.
d. Do not place concrete directly on polyethylene vapor retarders or non-absorptive subgrades as this can contribute to problems such as dusting, scaling and cracking. Place 3 to 4 inches [75 to 100 mm] of a trimable, compactible fill, such as a crusher-run material, over vapor retarders or non-absorptive subgrade prior to concrete placement. When high evaporation rates exist, lightly dampen absorptive subgrades just prior to concrete placement, ensuring that water does not pond or collect on
the subgrade surface. However, it may essential to place concrete directly on polyethylene vapor retarders for interior slabs that can receive floor coverings at any point in its service life (CIP 29). For such cases take special care to ensure that finishing operations are performed after all bleed water has dissipated from the surface.
e. Provide proper curing by using liquid membrane curing compound or by covering the surface with water, wet burlap, or other curing materials as soon as possible after finishing to retain moisture in the slab. It is important to protect concrete from the environment at early ages.
HOW to Repair Dusting:
a. Sandblast, shot blast or use a high-pressure washer to remove the weak surface layer.
b. To minimize or eliminate dusting, apply a commercially available chemical floor hardener, such as sodium silicate (water glass) or metallic zinc or magnesium fluosilicate, in compliance with manufacturer’s directions on thoroughly dried concrete. If dusting persists, use a coating, such as latex formulations, epoxy sealers or cement paint.
c. In severe cases, a serviceable floor can be obtained by wet-grinding the surface to durable substrate concrete. This may be followed by properly bonded placement of a topping course. If this is not practical, installation of a floor covering, such as carpeting or vinyl tile covering, is the least expensive solution to severe dusting. This
option will require some prior preparation since adhesives for floor covering materials will not bond to floors with a dusting problem and dusting can permeate through carpeting.
Follow These Rules to Prevent Dusting:
1. Use moderate slump concrete not exceeding 5 inches [125 mm].
2. Do not start finishing operations while the concrete is bleeding.
3. Do not broadcast cement or sprinkle water on concrete prior to or during finishing operations.
4. Ensure that there is adequate venting of exhaust gases from gas-fired heaters in enclosed spaces.
5. Use adequate curing measures to retain moisture in concrete for the first 3 to 7 days and protect it from the environment, especially freezing conditions.
1. Guide for Concrete Floor and Slab Construction, ACI 302.1R. American Concrete Institute, Farmington Hills, MI.
2. Slabs on Grade, Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, MI.
3. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL
4. The Effect of Various Surface Treatments, Using Zinc and Magnesium Fluosilicate Crystals on Abrasion Resistance of Concrete Surfaces, Concrete Laboratory Report No. C-819, U.S. Bureau of Reclamation.
5. Residential Concrete, National Association of Home Builders, Washington, DC.
6. Trouble Shooting Guide for Concrete Dusting, Concrete Construction, April 1996.
WHAT is Scaling?
Scaling is local flaking or peeling of a finished surface of hardened concrete as a result of exposure to cycles of freezing and thawing. Generally, it starts as localized small patches which later may merge and extend to expose large areas. Light scaling does not expose the coarse aggregate. Moderate scaling exposes the aggregate and may involve loss of up to ⅛ to ⅜ inch [3 to 10 mm] of the surface mortar. In severe scaling more surface has been lost and the aggregate is clearly exposed and stands out.
Note—Occasionally concrete peels or scales in the absence of freezing and thawing. This type of scaling is not covered in this CIP. Often this is due to the early use of a steel trowel, over-finishing or finishing while bleed water is on the surface.
WHY Do Concrete Surfaces Scale?
Concrete slabs and surfaces of other members that are saturated with water and exposed to cycles of freezing and thawing are susceptible to scaling. When concrete is saturated with water and temperature approaches freezing, water expands as it forms ice and this causes stresses within concrete. As the number of cycles of freezing and thawing increases, the potential for scaling increases. Deicing chemicals exacerbate this by increasing the saturation of concrete at the surface and the number of freezing and thawing cycles. Air entrained concrete contains millions of small air bubbles that accommodate the expanding water and ice and prevent the stress buildup.
Most scaling is caused by:
a. The use of non-air-entrained concrete or too little entrained air, especially at the surface.
b. Using concrete that has a low strength that allows permeation to water.
c. Using the improper concrete mixture or mixture proportions for the application.
d. Application of excessive amounts of deicing chemicals, especially on newly installed concrete that tends to be saturated and of lower strength.
e. Improper finishing procedures of concrete slabs.
f. Insufficient curing resulting in a weak concrete surface.
HOW to Prevent Scaling:
The potential for scaling in concrete slabs can be reduced by using good quality dense concrete with entrained air, following good practice for installing and curing, and by minimizing the use of deicing chemicals.
For concrete that will be continuously moist, exposed to freezing temperatures and will be subject to the use of deicing chemicals, the following recommendations should be followed:
a. For exterior slabs, order concrete with specified strength of 4000 psi [28 MPa], consistent with the requirements of ACI 332, Code for Residential Concrete. For concrete that will not be continuously moist or where deicing chemicals will not be applied, the specified strength should be 3500 psi [24 MPa].
b. Concrete should be air-entrained. The recommended total air content for concrete containing ¾-inch [19 mm] or 1-inch 25 mm] coarse aggregate is 6 percent.
c. The quantity of supplementary cementitious materials (SCM) should not exceed one of the following: 25% fly ash, 50% slag cement or 10% silica fume, expressed as percent by weight of the cementitious materials. SCMs are beneficial to concrete, however, at higher quantities change the rate of setting, bleeding, and strength gain. These impact the process of finishing. With appropriate modifications of the finishing procedures, it is possible to use higher quantities of SCMs, but these need to be evaluated.
d. For most slab construction, place concrete at a slump in the range of 3 to 5 inches [75 to 125 mm]. Do not add excessive water at the jobsite. High slump obtained by adding water increased the potential for segregation and excessive bleeding and can result in weak mortar layer at the surface. Water reducing admixtures can provide improved workability and retain good concrete quality.
e. Placing and finishing procedures can reduce the entrained air content in concrete, making it more susceptible to scaling.
f. Do not use a jitterbug or vibrating screed with high-slump concrete as it increases segregation and result in a weak mortar layer at the surface.
g. Do not perform finishing operations with bleed water present on the surface. Bull floating must promptly follow initial screeding. Delay subsequent finishing until bleed water has risen and dissipated from the surface. This is critical when placing air-entrained concrete in dry and windy conditions where the surface may appear to be dry while concrete is continuing to bleed. The use of fog sprays or evaporation retardants are recommended in these conditions. See CIP 14 for finishing concrete.
h. Do not overwork the surface of concrete. Excessive finishing reduces entrained air in the surface layer. For most exterior surfaces a broom finish is adequate.
i. Provide proper curing by using pigmented liquid membrane curing compound or by covering the surface of newly placed slab with wet burlap and plastic sheets. Proper curing involves maintaining concrete at adequate temperature and moisture for optimum performance.
j. Protect concrete from the harsh winter environment. Apply a commercially available silane or siloxane- based breathable concrete sealer or water repellent specifically designed for use on concrete slabs. Follow the manufacturer’s recommendations. The concrete should be reasonably dry prior to the application of a sealer. Late summer with a few dry days preceding application is an ideal time.
k. Be cautious about placing exterior concrete in late fall, winter or early spring when conditions are such that it will be exposed to freezing temperatures shortly after placement while concrete is still saturated.
l. Avoid using deicing chemicals on newly placed concrete, if possible. Use clean sand for traction. When used, deicing chemicals should be applied in moderate amounts. Excessive applications increases potential for scaling. When conditions permit, hose off accumulation of salt deposited by cars on driveways and garage slabs. Deicing chemicals composed of calcium chloride and sodium chloride (rock salt) are considered acceptable for concrete. Never use ammonium sulfate or ammonium nitrate or magnesium-based salts as a deicer; these are chemically aggressive and destroy concrete surfaces. Magnesium-based salts are used for pre-snow deicing of roads and can be tracked by cars and accumulate on concrete surfaces. Poor drainage causing salt solutions to accumulate on concrete surfaces increases the severity of the exposure and may cause scaling.
HOW to Repair Scaled Surfaces:
Minor scaling is a cosmetic issue and may not need to be repaired. On the other hand, repairing concrete slabs with excessive and progressing scaling may not be feasible.
It is possible to repair light to moderately scaled surfaces. The repaired surface will only be as strong as the base surface to which it is bonded. The surface should be prepared to remove the unsound surface and should be free of dirt, oil or paint. The surface receiving the repair must be sound. To accomplish this, use a hammer and chisel, sandblasting, high-pressure washer, or jack hammer. The clean, rough, textured surface can be repaired with thin bonded resurfacing such as:
a. Portland cement concrete resurfacing
b. Latex modified concrete resurfacing
c. Polymer-modified cementitious-based repair mortar
Repair material will not match the color and characteristics of the original concrete.
1. Guide to Durable Concrete, ACI 201.2R, American Concrete Institute, Farmington Hills, MI.
2. Scale-Resistant Concrete Pavements, IS117.02P, Portland Cement Association, Skokie, IL.
3. Protective Coatings to Prevent Deterioration of Concrete by Deicing Chemicals, National Cooperative Highway Research Program Report No. 16.
4. Code Requirements for Residential Concrete, ACI 332, American Concrete Institute, Farmington Hills, MI.
5. Residential Concrete, National Association of Home Builders, Washington, DC.
6. Slabs on Grade, Concrete Craftsman Series CCS-1, American Concrete Institute, Farmington Hills, MI.
7. Eugene Goeb, Deicer Scaling: An Unnecessary Problem, Concrete Products, February 1994.
8. Concrete in Practice Series, CIP 5, 11, 14, NRMCA, Silver Spring, MD.
WHAT is Crazing?
Crazing is the development of a network of fine random cracks or fissures on the surface of concrete or mortar caused by shrinkage of the surface layer. These cracks are rarely more than ⅛ inch [3 mm] deep and are more noticeable on steel-troweled surfaces. The irregular hexagonal areas enclosed by the cracks are typically no more than 1½ inch [40 mm] across and may be as small as ½ or ⅜ inch [12 or 20 mm] in unusual instances. Generally, craze cracks develop at an early age and are apparent the day after placement or at least by the end of the first week. Often they are not readily visible until the surface has been wetted and it is beginning to dry out. Crazing cracks are sometimes referred to as shallow map or pattern cracking. They do not affect the structural integrity of concrete and rarely do they affect durability or wear resistance. However, crazed surfaces can be unsightly. They are particularly conspicuous and unsightly when concrete contains calcium chloride, a commonly used accelerating admixture.
WHY Do Concrete Surfaces Craze?
Hard steel-troweled slab surfaces often have craze cracks due to shrinkage of the concentrated dense paste layer at the surface. Concrete surface crazing can also occur because one or more of the rules of “good concrete practices” were not followed. The most frequent factors when crazing occurs are:
a. Poor or inadequate curing. Environmental conditions conducive to high evaporation rates, such as low humidity, extremes in ambient temperature, direct sunlight, and drying winds on a concrete surface when the concrete is just beginning to gain strength, cause rapid surface drying resulting in craze cracking. Avoid the delayed application of curing or even intermittent wet curing and drying after the concrete has been finished.
b. Too wet a mix, excessive floating, the use of a jitterbug or procedures that will depress the coarse aggregate and produce an excessive concentration of cement paste and fines at the surface.
c. Finishing operations performed while bleed water remains on the surface or the use of a steel trowel in a manner that the smooth surface of the trowel brings up excessive water and cement fines. Use of a bull float or darby with water on the surface or while the concrete continues to bleed will produce a high water-cement ratio at surface resulting in a weak surface layer that will be susceptible to crazing, dusting, scaling and other surface defects.
d. Sprinkling cement on the surface to dry up the bleed water is a frequent cause of crazing. This concentrates fines on the surface. Spraying water on the concrete surface during finishing operations will result in a weak surface susceptible to crazing or dusting.
HOW to Prevent Crazing:
a. To prevent crazing, start curing the concrete as soon as possible. Curing retains moisture required for proper reaction of cement with water, called hydration. Keep the surface wet by either flooding with water or by covering it with damp burlap and keeping it continuously moist for a minimum of 3 days. An alternative is to spray the surface with a liquid-membrane curing compound. Avoid alternate wetting and drying of concrete surfaces at an early age. Curing retains the moisture required for proper reaction of cement with water, called hydration. Keep the surface wet by either flooding with water or by covering it with damp burlap and keep it continuously moist for a minimum of 3 days. An alternative is to spray the surface with a liquid-membrane curing compound. Avoid alternate wetting and drying of concrete surfaces at an early age.
b. When Placing, use moderate slump (3 to 5 inches [75 to 125 mm]) concrete. Higher slump (up to 6 or 7 inches [150 to 175 mm]) can be used provided the mixture is designed to produce the required strength without excessive bleeding and/or segregation. This is generally accomplished by using water-reducing admixtures.
c. NEVER sprinkle or trowel dry cement or a mixture of cement and fine sand on the surface of the plastic concrete to absorb bleed water. DO NOT sprinkle water on the slab to facilitate finishing. If necessary, remove bleed water by dragging a garden hose across the surface. DO NOT perform any finishing operation while bleed water is present on the surface or before the bleeding process is completed. DO NOT overwork or over-finish the surface.
d. When high evaporation rates are anticipated, lightly dampen the subgrade prior to concrete placement to prevent it absorbing too much water from the concrete. If a vapor is required on the subgrade, cover it with 3 to 4 inches of a compactible, granular fill, such as a crusher-run material except when the slab will receive a vapor-sensitive floor covering or will be in a humidity controlled environment. See CIP 29 that discusses the location of vapor retarders.
Follow These Rules to Prevent Crazing:
1. Use moderate slump (3-5 inches) concrete with reduced bleeding characteristics.
2. Follow recommended practices and timing, based on concrete setting characteristics, for placing and finishing operations:
a. Avoid excessive manipulation of the surface, which can depress the coarse aggregate, increase the cement paste at the surface, or increase the water-cement ratio at the surface.
b. DO NOT finish concrete before the concrete has completed bleeding. DO NOT dust any cement onto the surface to absorb bleed water. DO NOT sprinkle water on the surface while finishing concrete.
c. When steel troweling is required, delay it until the water sheen has disappeared from the surface.
3. Cure properly as soon as finishing has been completed.
1. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
2. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS 177T, Portland Cement
Association, Skokie, IL.
3. Ward Malisch, Avoiding Common Outdoor Flatwork Problems, Concrete Construction, July 1990.
4. Ralph Spannenberg, Use the Right Tool at the Right Time, Concrete Construction, May 1996.
Concrete, like other construction materials, contracts and expands with changes in moisture and temperature, and deflects depending on load and support conditions. Cracks can occur when provisions to accommodate these movements are not made in design and construction. Some forms of common cracks are:
Fig. A: Plastic shrinkage cracks (CIP 5).
Fig. B: Cracks due to improper jointing (CIP 6).
Fig. C: Cracks due to continuous external restraint. Example: Cast-in-place wall restrained along bottom edge of footing.
Fig. D: Cracks due to lack of isolation joints (CIP 6).
Fig. E: D-Cracks from freezing and thawing.
Fig. F: Craze Cracks (CIP 3).
Fig. G: Settlement cracks.
Most random cracks that appear at an early age, although unsightly, rarely affect the structural integrity or the service life of concrete. Two exceptions are:
a. D-cracks, which occur due to freeze-thaw deterioration of some types of porous aggregate in concrete. These cracks initiate at joints at the bottom of exterior slabs and typically appear at later ages.
b. Cracking due to alkali aggregate reactions will lead to long term structural damage (CIP 43).
WHY do Concrete Surfaces Crack?
The majority of concrete cracks occur due to improper design and construction practices, such as:
a. Omission of isolation and contraction joints and improper jointing practices.
b. Improper subgrade preparation.
c. The use of high slump concrete or excessive addition of water on the job.
d. Improper finishing.
e. Rapid loss of moisture from newly placed concrete in dry conditions.
f. Inadequate or no curing.
HOW to Prevent or Minimize Cracking:
All concrete has a tendency to crack and it is not possible to produce completely crack-free concrete. However, cracking can be reduced and controlled if the following basic concreting practices are followed:
a. Subgrade and Formwork. All topsoil and soft spots should be removed. The soil beneath the slab should be compacted soil or granular fill, well compacted by rolling, vibrating or tamping. The slab, and therefore, the subgrade, should be sloped for proper drainage. In winter, remove snow and ice prior to placing concrete and do not place concrete on a frozen subgrade. Smooth, level and uniformly compacted subgrades help prevent cracking. All formwork must be constructed and braced so that it can withstand the pressure of the concrete without movement. Vapor retarders directly under a concrete slab increase bleeding and greatly increase the potential for cracking, especially with high-slump concrete. When it is required to place concrete directly on polyethylene vapor retarders (CIP 29) take special care to ensure that finishing operations are performed after all bleed water has dissipated from the surface. In dry conditions, lightly dampen subgrade, formwork and reinforcement immediately prior to concrete placement.
b. Concrete. In general, use concrete with a moderate slump (not to exceed 5 inches [125 mm]). Higher slump can be used provided the mixture is designed to produce the required strength without excessive bleeding and/or segregation. This is generally accomplished by using water-reducing admixtures. Use air-entrained concrete for outdoor slabs exposed to freezing weather (CIP 2). Concrete mixtures can be designed for reduced shrinkage to minimize cracking.
c. Finishing. Initial screeding must be promptly followed by bull floating. DO NOT perform subsequent finishing operations with water present on the surface or before the concrete has completed bleeding. Do not overwork or over-finish the surface. For better traction on exterior surfaces, use a broom finish. When ambient conditions are conducive to a high evaporation rate, use means to avoid rapid drying and associated plastic shrinkage cracking by using wind breaks, fog sprays, and covering the concrete with wet burlap or polyethylene sheets between finishing operations.
d. Curing. Curing is an important step to ensure durable crack-resistant concrete. Start curing as soon as possible. Spray the surface with liquid membrane curing compound or cover it with damp burlap and keep it moist for at least 3 days. A second application of curing compound the next day is a good quality assurance step.
e. Joints. Anticipated volume changes due to temperature and/or moisture should be accommodated by contraction joints saw cut or tooled at the proper time with a depth of about ¼ to ⅓ the thickness of the slab, and with a spacing between 24 to 36 times the slab thickness. A maximum 15 feet spacing for contraction joints is often recommended. Panels between joints should be square and the length should not exceed about 1.5 times the width. Isolation joints to the full thickness of the slab should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated—such as where floors meet walls, columns, or footings. See CIP 6 for information on joints.
f. Reinforcement. Wire mesh and reinforcement in slabs cannot prevent cracking. When placed at the proper location, reinforcement can reduce crack width. Providing sufficient concrete cover (at least 2 inches [50 mm]) to keep salt and moisture from contacting the steel should prevent cracks in reinforced concrete caused by expansion of rust on reinforcing steel.
Follow These Rules to Minimize Cracking:
1. Design the members to handle all anticipated loads.
2. Provide proper contraction and isolation joints.
3. In slab on grade work, prepare a stable uniformly compacted subgrade.
4. Place and finish according to recommended and established practices.
5. Protect and cure the concrete properly.
1. Control of Cracking in Concrete Structures, ACI 224R, American Concrete Institute, Farmington Hills, MI.
2. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
3. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL.
4. Grant T. Halvorson, Troubleshooting Concrete Cracking During Construction, Concrete Construction, October 1993.
5. Cracks in Concrete: Causes, Prevention, Repair, A collection of articles from Concrete Construction Magazine, June 1973.
WHAT is Plastic Shrinkage Cracking?
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. These cracks appear mostly on horizontal surfaces. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab. Plastic shrinkage cracking is more likely to occur when high evaporation rates cause the concrete surface to dry out before it has set.
Plastic shrinkage cracks are unsightly but rarely impair the strength of concrete floors and pavements. These cracks, however, can permit the ingress of other aggressive chemicals that impact durability and as weak points for the initiation of later age cracking due to other reasons. The development of these cracks can be minimized if appropriate measures are taken prior to and during placing and finishing concrete.
Note: Plastic shrinkage cracks should be distinguished from other early or pre-hardening cracks caused by settlement of the concrete around reinforcing bars, formwork movement, early age thermal cracking, or differential settlement at a change from a thin to a deep section of concrete.
WHY do Plastic Shrinkage Cracks Occur?
Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci between the fine particles of cement and aggregate causing a tensile force to develop in the surface layers. If the concrete surface has started to set and has developed sufficient tensile strength to resist the tensile forces, cracks do not form. If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic shrinkage cracks will surely form as soon as the concrete starts to stiffen. Synthetic fiber reinforcement incorporated in the concrete mixture can help resist the tension when concrete is very weak.
Conditions that cause high evaporation rates from the concrete surface, and thereby increase the possibility of plastic shrinkage cracking, include:
• Wind velocity in excess of 5 mph
• Low relative humidity
• High ambient and/or concrete temperatures
Small changes in any one of these factors can change the rate of evaporation of water from the concrete surface. ACI 305R provides a chart to estimate the rate of evaporation and indicates when special precautions might be required. However, the chart isn’t infallible because many factors other than rate of evaporation are involved.
Concrete mixtures with an inherent reduced rate of bleeding or quantity of bleed water are susceptible to plastic shrinkage cracking even when evaporation rates are low. Factors that reduce the rate or quantity of bleeding include high cementitious materials content, high fines content, reduced water content, entrained air, high concrete temperature, and thinner sections. Concrete containing silica fume requires particular attention to avoid surface drying during placement due to its very low rate of bleeding.
Any factor which delays setting increases the possibility of plastic shrinkage cracking. Delayed setting can result from a combination of one or more of the following: cool weather, cool subgrades, high water contents, lower cement contents, retarders, some water reducers, and supplementary cementitious materials.
HOW to Minimize Plastic Shrinkage Cracking:
Attempts to eliminate plastic shrinkage cracking by modifying the concrete mixture composition to affect bleeding characteristics have not been found to be consistently effective. To reduce the potential for plastic shrinkage cracking, it is important to recognize ahead of time, before placement, when weather conditions conducive to plastic shrinkage cracking will exist. Precautions can then be taken to minimize its occurrence.
a. When adverse conditions exist, erect temporary windbreaks to reduce the wind velocity over the surface of the concrete and, if possible, provide sunshades to control the surface temperature of the slab. If conditions are critical, schedule placement to begin in the later afternoon or early evening. However, in very hot conditions, early morning placement can afford better control on concrete temperatures.
b. In the very hot and dry periods, use fog sprays to discharge a fine mist upwind and into the air above the concrete. Fog sprays reduce the rate of evaporation from the concrete surface and should be continued until suitable curing materials can be applied.
c. If concrete is to be placed on a dry absorptive subgrade in hot and dry weather, dampen the subgrade but not to a point that there is freestanding water prior to placement. The formwork and reinforcement should also be dampened.
d. The use of vapor retarders under a slab on grade can increase the risk of plastic shrinkage cracking. However, it may be necessary for interior slabs that will have a floor covering at any point during its service life (CIP 29).
e. Have proper manpower, equipment and supplies on hand so that the concrete can be placed and finished promptly. If delays occur, cover the concrete with moisture-retaining coverings, such as wet burlap, polyethylene sheeting or building paper, between finishing operations. Some contractors find that plastic shrinkage cracks can be prevented in hot dry climates by spraying an evaporation retardant on the surface behind the screeding operation and following floating or troweling, as needed, until curing is started.
f. Start curing the concrete as soon as possible. Spray the surface with liquid membrane curing compound or cover the surface with wet burlap and keep it continuously moist for a minimum of 3 days.
g. Consider using synthetic fibers (ASTM C1116) to minimize plastic shrinkage cracking.
h. Accelerate the setting time of concrete and avoid large temperature differences between concrete and ambient air temperatures.
If plastic shrinkage cracks should appear during final finishing, the finisher may be able to close them by refinishing. However, when this occurs, precautions, as discussed above, should be taken to avoid further cracking.
Follow These Rules to Minimize Plastic Shrinkage Cracking:
1. Dampen the subgrade and forms when conditions for high evaporation rates exist.
2. Prevent excessive surface moisture evaporation by providing fog sprays and erecting windbreaks.
3. Cover concrete with wet burlap or polyethylene sheets between finishing operations.
4. Use cooler concrete in hot weather and avoid high concrete temperatures in cold weather.
5. Cure properly as soon as finishing has been completed.
1. Hot Weather Concreting, ACI 305R, American Concrete Institute, Farmington Hills, MI., www.concrete.org.
2. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
3. Standard Practice for Curing Concrete, ACI 308, American Concrete Institute, Farmington Hills, MI.
4. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL, www.cement.org.
5. Bruce A. Suprenant, Curing During the Pour, Concrete Construction, June 1997.
6. Eugene Goeb, Common Field Problems, Concrete Construction, October 1985.
7. Vapor Retarders Under Slabs on Grade, CIP 29, NRMCA, Silver Spring, MD, www.nrmca.org.
WHAT are Joints?
Concrete expands and shrinks with changes in moisture and temperature. The overall tendency is to shrink and this can cause cracking at an early age. Irregular cracks are unsightly and difficult to maintain but generally do not affect the integrity of concrete. Joints are simply pre-planned cracks. Joints in concrete slabs can be created by forming, tooling, sawing and placement of joint formers.
Types of joints include:
a. Contraction joints are intended to create weakened planes in the concrete and control the location where cracks, resulting from dimensional changes, will occur.
b. Isolation or expansion joints separate or isolate slabs from other parts of the structure, such as walls, footings or columns; and driveways and patios from sidewalks, garage slabs, stairs, light poles and other points of restraint. They permit independent vertical and horizontal movement between adjoining parts of the structure and help minimize cracking when such movements are restrained.
c. Construction joints are surfaces where two successive placements of concrete meet. They are typically placed at the end of a days’ work but may be required when concrete placement is stopped for a period such that the previously placed concrete has set and hardened. In slabs they may be designed to permit movement and/or to transfer load. The location of construction joints should be planned. It may be desirable to achieve bond and continue reinforcement through a construction joint for load transfer.
WHY are Joints Constructed?
Cracks in concrete cannot be prevented entirely, but they can be controlled and minimized by properly designed joints. Concrete cracks because:
a. Concrete is weak in tension and, therefore, if its natural tendency to shrink is restrained, tensile stresses that exceed its tensile strength can develop, resulting in cracking.
b. At early ages, before the concrete dries out, most cracking is caused by temperature changes or by the slight contraction that takes place as the concrete sets and hardens. Later, as the concrete dries, it will shrink further and either additional cracks may form or preexisting cracks may become wider.
Joints provide relief from the tensile stresses, are easy to maintain and are less objectionable than uncontrolled or irregular cracks.
HOW to Construct Joints:
Joints must be carefully designed and properly constructed if uncontrolled cracking of concrete flatwork is to be avoided. The following recommended practices should be observed:
a. The maximum joint spacing should be 24 to 36 times the thickness of the slab. For example, the joint spacing for a 4-inch [100 mm] thick slab should be about 10 feet [3 m]. It is further recommended that joint spacing be limited to a maximum of 15 feet [4.5 m].
b. All panels should be square or nearly so. The length should not exceed 1.5 times the width. Avoid L‑shaped panels.
c. For contraction joints, the joint groove should have a minimum depth of ¼ the thickness of the slab, but not less than 1 inch [25 mm]. Timing of jointing operations depends on the method used:
• Preformed plastic or hard board joint strips are inserted into the concrete surface to the required depth before finishing.
• Tooled joints must be run early in the finishing process and rerun later to ensure groove bond has not occurred.
• Early-entry dry-cut joints are generally run 1 to 4 hours after completion of finishing, depending on the concrete’s setting characteristics. These joints are typically not as deep as those obtained by the conventional saw-cut process, but should be a minimum of 1 inch [25 mm] in depth.
• Conventional saw-cut joints should be run within 4 to 12 hours after the concrete has been finished.
d. Raveling during saw cutting is affected by the strength of the concrete and aggregate characteristics. If the joint edges ravel during sawing, it must be delayed. However, if delayed too long, sawing can become difficult and uncontrolled cracking may occur.
e. Use pre-molded joint filler such as asphalt-impregnated fiber sheeting, compressible foam strips, or similar materials for isolation joints to separate slabs from building walls or footings. At least 2 inches [50 mm] of sand over the top of a footing will also prevent bond to the footing.
f. To isolate columns from slabs, form circular or square openings, which will not be filled until after the floor has hardened. Slab contraction joints should intersect at the openings for columns. If square openings are used around columns, the square should be turned at 45 degrees so the contraction joints intersect at the diagonals of the square.
g. If the slab contains wire mesh, cut out alternate wires, or preferably discontinue the mesh, across contraction joints. Note that wire mesh will not prevent cracking. Mesh tends to keep the cracks and joints from opening up.
h. Construction joints key the two edges of the slab together either to provide transfer of loads or to help prevent curling or warping of the two adjacent edges. Galvanized metal keys are sometimes used for interior slabs, however, a beveled 1 by 2 inch [25 by 50 mm] strip, nailed to bulkheads or form boards, can be used in slabs that are at least 5 inches [125 mm] thick to form a key which will resist vertical loads and movements. Keyed joints are not recommended for industrial floors. Metal dowels should be used in slabs or pavements that will carry heavy loads. Dowels must be carefully lined up and parallel or they may induce restraint and cause random cracking at the end of the dowel.
i. Joints in industrial floors subject to heavy traffic require special attention to avoid spalling of joint edges. Such joints should be filled with a material capable of supporting joint edges. Manufacturer’s recommendations and performance records should be checked before use.
Follow These Rules for Proper Jointing:
1. Plan exact location of all joints, including timing of contraction joint sawing before construction.
2. Provide isolation joints between slabs and columns, walls and footings, and at junctions of driveways with walks, curbs or other obstructions.
3. Provide contraction joints and joint filling materials as outlined in specifications.
1. Joints in Concrete Construction, ACI 224.3R, American Concrete Institute, Farmington Hills, MI.
2. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
3. Slabs on Grade, ACI Concrete Craftsman Series CCS- 1, American Concrete Institute, Farmington Hills, Ml.
4. Joint Planning Primer, Concrete Construction, August 1997.
5. Bruce A. Suprenant, Sawcutting Joints in Concrete, Concrete Construction, January 1995.
WHAT Types of Cracks May Occur?
Cast-in-place concrete basements provide durable, high quality living space. Cracking of concrete is a natural occurrence and at times can be undesirable. Most common causes of cracking include:
a. Temperature and drying shrinkage cracks. Generally, newly placed concrete is at its largest volume. As concrete hardens it dries and starts to shrink. Temperature variations cause concrete to expand and contract. When these volume changes are restrained, cracking results.
b. Re-entrant corner cracking occurs diagonally from the corners of windows, doors or openings in the concrete walls. These cracks result from shrinkage.
c. Pour lines are visible demarcations between placement of two concrete loads, typically due to a delay in placing between the loads and if proper consolidation was not performed to homogenize the two portions across the separation. Pour lines are often perceived as cracks. In extreme cases they may perform as cracks if the first placement has partially hardened before the second placement. This is often referred to as a cold joint.
d. Vertical form lines occur between form panels and can sometimes cause weak zones due to the use of form ties that support two layers of formwork during concrete placement. Cracks may initiate at form lines.
e. Restraint cracks may form in some portions of walls where contact with footings restrains the shrinkage of the concrete wall.
f. Crazing and surface cracking may occur due to a lack of adequate curing and protection if construction is during extreme cold or hot weather.
g. Settlement cracks occur from non-uniform support of footings or occasionally from expansive soils.
h. Structural cracks may occur during backfilling if concrete strength is not adequate or the walls are not adequately supported as the design intends. This is most likely to occur when heavy equipment gets too close to the walls during the backfill process or when pressure due to backfill material exceed that anticipated in the design, for example with liquefied soils.
WHY do Basement Cracks Occur?
Some cracking is normal in concrete basement walls. Volume changes and other movements at an early age result in different types of cracks, as discussed earlier. These cracks can grow if the walls are not properly designed, due to the continued horizontal pressures applied by soils, water and temperature. Cracking can be minimized and problems prevented if the design and construction practices that follow are implemented.
Most builders or third party providers offer limited warranties for basements. A typical warranty will require repair only when cracks leak, have measurable vertical displacement, or if the crack width exceeds ⅛-in. (3 mm). The National Association of Home Builders (NAHB) requires repair or corrective action when cracks in basement walls cause leaks into the basement.
HOW to Design & Construct Quality Basements:
Cast-in-place concrete basement walls are the strongest and most effective foundation for a residence. However, climate conditions, unusual or unforeseen loads, material quality and workmanship may impact the quality of the finished basement. Proper design and construction is important. The following steps should be followed:
a. Site conditions and excavation. Soil type and conditions should be properly assessed for appropriate design and construction of foundations specific to the building site. The excavation should be at least to the level of the bottom of the basement slab and can be to the bottom of the footing. Soil or granular fill beneath the entire area of the basement should be well compacted by rolling, vibrating or tamping. Footings must bear on undisturbed soil or well compacted fill. Uniform soil bearing capacity should be ensured or the design should accommodate any variation.
b. Formwork and reinforcement. Formwork must be installed and braced to withstand the pressure of the fresh and flowing concrete. Reinforcement is used to control crack width. Wall thickness and reinforcement should be provided in accordance with International Residential Code (IRC), ACI 332, or locally adopted Code.
c. Joints. Some cracks in basement walls can be controlled to occur in properly located formed joints.
d. Concrete. Use concrete with adequate strength in accordance with the Code and project specifications. Excess water should not be added to concrete in the truck mixer. Water-reducing admixtures can be used to increase flow. Air-entrained concrete should be used for walls that may be exposed to moisture and freezing temperatures.
e. Placement and curing. Place concrete in a continuous operation to avoid cold joints and segregation. Adding excess water to concrete to facilitate placement will increase segregation, cause honeycombing or excessive cracking, and reduce strength. Consider placement points no greater 20 or 30 feet around the perimeter of the wall to minimize segregation. Properly designed higher slump concretes with admixtures will flow horizontally for long distances and placement points can be spread out. Curing should begin after placement. Forms should be left in place 5 to 7 days or at least until concrete attains adequate strength to support itself. Forms removed too early can result in premature drying and may cause cracking. In cold weather, forms should be insulated with blankets or other materials to retain heat. During hot dry weather, forms should be covered with wet burlap to retain moisture. Liquid membrane-forming curing compounds can be sprayed at the required coverage after forms are removed to prevent excessive drying.
f. Waterproofing and drainage. Waterproof membranes are best applied to the exterior of foundation walls. These are spray-applied, painted or mechanically fastened sheet systems. Positive side waterproofing (exterior) is generally better than negative side (inside) to keep water from leaking through cracks. Drainage systems should be designed to remove excessive soil moisture along the basement wall. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing and properly connect them to a removal system or drain to daylight. Surface and roof drainage should direct water away from the residence. Water should be drained to lower elevations suitable to receive storm water runoff.
g. Backfilling and final grading. Backfilling should be done carefully to avoid damaging the walls. Brace the walls, if possible, or backfill after first floor or other structural systems are in place. Finish grade to slope ½ to 1-in. per foot (40 to 80-mm per m) for at least 8 to 10 feet (2.5 to 3 m) to drain water away from the foundation. Considering settlement, maintain this final grade to prevent water from standing along the foundation and exceeding the designed wall pressure.
h. Crack repair. Cracking is not necessarily a sign of poor materials or workmanship or a structural problem with the concrete wall. If repair is necessary, epoxy injection, dry-packing, or routing and sealing techniques can be used to repair and stabilize cracks. Before repairing leaking cracks, the drainage around the structure should be checked and corrected if necessary. Details of these and other repair methods are provided in Ref. 1. Seek professional advice to evaluate and repair active cracks that are widening with time.
1. Causes, Evaluation and Repair of Cracks, ACI 224.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
2. Code Requirements for Residential Concrete, ACI 332-14, American Concrete Institute, Farmington Hills, MI.
3. International Residential Code, International Code Council, Washington, DC, www.iccsafe.org
4. Residential Concrete, National Association of Home Builders, Washington, DC, www.nahb.org
5. Residential Construction Performance Guidelines, National Association of Home Builders, Washington, DC.
6. Casting Residential Foundation Walls in Cold Weather, Concrete Foundations Association, Mount Vernon, IA, www.cfawalls.org.
7. Backfilling Foundation Walls, Concrete Foundations Association, Mount Vernon, IA.
8. Cracking in Foundation Walls, Concrete Foundations Association, Mount Vernon, IA.
WHAT is Concrete Yield?
Concrete yield is defined as the volume of freshly mixed concrete from a known quantity of ingredients. Ready mixed concrete is sold on the basis of the volume of fresh, unhardened concrete-in cubic yards (yd3) or cubic meters (m3) as discharged from a truck mixer.
The basis for calculating the volume is described in the ASTM C94 – Specification for Ready Mixed Concrete. The volume of freshly mixed and unhardened concrete in a given batch is determined by dividing the total weight of the materials by the average unit weight or density of the concrete determined in accordance with ASTM C138. Three unit weight tests must be made, each from a different truck.
ASTM C94 notes: It should be understood that the volume of hardened concrete may be, or appears to be, less than expected due to waste and spillage, over-excavation, spreading forms, some loss of entrained air, or settlement of wet mixtures, none of which are the responsibility of the producer.
Further, the volume of hardened concrete in place may be about 2 percent less than its volume in a freshly mixed state due to reduction in air content, settlement and bleeding, decrease in volume of cement and water, and drying shrinkage.
WHY do Yield Problems Occur?
Most yield complaints concern a perceived or real deficiency of concrete volume. Concerns about yield should be evaluated using unit weight measurements to calculate the yield. Apparent under-yield occurs when insufficient concrete is ordered to fill the forms and to account for contingencies discussed below. If unit weight and yield calculations indicate an actual under-yield, it should be corrected.
Apparent concrete shortages are sometimes caused for the following reasons:
a. Miscalculation of form volume or slab thickness when the actual dimensions exceed the assumed dimensions by a fraction of an inch. For example, a ⅛-inch (3-mm) error in a 4-inch (100-mm) slab would mean a shortage of 3 percent or 1 yd3 in a 32-yd3 (1 m3 in a 32-m3) order.
b. Deflection or distortion of the forms resulting from pressure exerted by the concrete.
c. Irregular subgrade, placement over granular fill, and settlement of subgrade prior to placement.
d. Over the course of a large job, the small amounts of concrete returned each day or used in mud sills or incidental footings.
An over-yield can be an indication of a problem if the excess concrete is caused by too much air or aggregate, or if the forms have not been properly filled.
Differences in batched weights of ingredients and air content in concrete, within the permitted tolerances, can result in discrepancies in yield.
HOW to Prevent Yield Discrepancies:
To prevent or minimize concrete yield problems:
a. Check concrete yield by measuring concrete unit weight in accordance with ASTM C138 early in the job. Repeat these tests if a problem arises. Be sure that the scale is accurate, that the unit weight bucket is properly calibrated, that a flat plate is used for strike off and that the bucket is cleaned prior to weighing. Concrete yield in cubic feet (m3) is total batch weight in pounds (kg) divided by unit weight in pounds per cubic foot (kg/m3). The total batch weight is the sum of the weights of all ingredients from the batch ticket. As a rough check, the mixer truck can be weighed empty and full. The difference is the total batch weight.
b. Measure formwork accurately. Near the end of large pours, carefully measure the remaining volume so that the order for the last 2 or 3 trucks can be adjusted to provide the required quantity of concrete. This can prevent waiting for an extra ½ yd3 after the plant has closed or the concrete trucks have been scheduled for other jobs. Order sufficient quantity of concrete to complete the job and reevaluate the amount required towards the end of the pour. Disposal of returned concrete has environmental and economic consequences to the concrete producer.
c. Estimate extra concrete needed for waste and increased placement dimensions over nominal dimensions. Include an allowance of 4 to 10 percent over plan dimensions for waste, over-excavation and other causes. Repetitive operations and slip form operations permit more accurate estimates of the amount of concrete that will be needed. On the other hand, sporadic operations involving a combination of concrete uses such as slabs, footings, walls, and as incidental fill around pipes, etc., will require a bigger allowance for contingencies.
d. Construct and brace forms to minimize deflection or distortion.
e. For slabs on grade accurately finish and compact the subgrade to the proper elevation.
Follow These Rules to Avoid Under-Yield:
1. Measure volume needed accurately. Reevaluate required volume towards the end of the pour and inform the concrete producer.
2. Estimate waste and potential increased thickness – order more than required by at least 4 to 10 percent.
3. To check yield use the ASTM C138 unit weight test method on three samples from three different loads – yield is the total batch weight divided by the average unit weight or density.
1. ASTM C94, Standard Specification for Ready Mixed Concrete, American Society for Testing and Materials, West Conshohocken, PA.
2. ASTM C138, Standard Test Method for Unit Weight, Yield and Air Content (Gravimetric) of Concrete, American Society for Testing and Materials.
3. Ready Mixed Concrete, Gaynor, R.D. NRMCA Publication 186, NRMCA, Silver Spring, Maryland.
4. No Minus Tolerance on Yield, Malisch, W. R. and Suprenant, B. A., Concrete Producer, May 1998
5. Causes for Variation in Concrete Yield, Suprenant, B. A., The Concrete Journal, March 1994
6. An Analysis of Factors Influencing Concrete Pavement Cost, by Harold J. Halm, Portland Cement Association Skokie, Illinois.
WHAT Constitutes Low Cylinder Strength?
Strength test results of concrete cylinders are used as the basis of acceptance of ready mixed concrete when a strength requirement is specified. Cylinders are molded from a sample of fresh concrete, cured in standard conditions and tested at an age indicated in the specification, usually at 28 days. For strength test results to be reliable, procedures must be in accordance with ASTM standards. The average strength of a set of two 6×12 in. (150×300 mm) or three 4×8 in. (100×200 mm) cylinders made from the same concrete sample constitutes one test result. In some cases cylinders are tested at 7 days to get an early indication of the potential strength, but these test results are not to be used to determine the acceptability of the concrete. Cylinders used for acceptance of concrete should not be confused with field-cured cylinders. Tests of field-cured cylinders are used to evaluate whether the in-place concrete has been properly cured and protected, to estimate the early-age strength in the structure to strip forms or for post-tensioning, and to continue construction activity.
The ACI Building Code, ACI 318, and the Specification for Structural Concrete, ACI 301, recognize that when mixtures are proportioned to meet the requirements of the standards, strength test results will fail to comply with acceptance criteria about one time in 100 tests due to normal variability.
The strength acceptance criteria used are:
a. The average of three consecutive tests equals or exceeds the specified strength, and
b. No single test is lower than the specified strength by more than 500 psi (3.5 MPa) when the specified strength is less than or equal to 5000 psi (35 MPa), or
No single test is lower than (0.9 × specified strength) when the specified strength is greater than 5000 psi (35 MPa).
An example of these strength acceptance criteria is provided in the table. If the strength test results fail either condition (a) or (b), steps must be taken to increase the strength of the concrete. If the results fail condition (b), an investigation should be made to ensure structural adequacy of that portion of the structure.
WHY are Compressive Tests Low?
The major reasons for low compressive strength tests are:
1. Improper cylinder handling, curing and testing – this is typically the reason in most cases;
2. The addition of excessive water to the concrete mixture at the jobsite due to delays in placement or requests for a higher slump to facilitate placement and finishing;
3. High air content in the concrete (and test specimens); and
4. Errors in production and unanticipated factors during delivery.
When low compressive strength test results are reported:
1. Collect all test reports and analyze the results before taking action. Labs should not provide reports of only failing tests.
2. Look at the pattern and numbers of reported strength results.
• Considering the sequence of results—is there a violation on compliance with the strength acceptance criteria discussed above?
• The strength range of two or three cylinders prepared from the same sample should rarely exceed 8.0% or 9.5% of the strength average, respectively.
• Do the results indicate that the cylinders are being loaded to complete failure?
3. Do the test reports provide any causal reasons?
• Review the dates and times of batching, sampling, pick up from jobsite and delivery to the lab.
• Review concrete and ambient temperatures, number of days cylinders were left in the field, procedures used for initial curing in the field, duration of transportation, and subsequent curing in the lab.
• Review the slump, air content, and density, if measured.
• Review for any reported cylinder defects.
It is important that procedures are conducted in accordance with ASTM standards. Almost all deficiencies in handling and testing cylinders will result in a lower measured strength. All violations add up to cause significant reductions in measured strength. The more significant factors are: improperly finished surfaces; initial curing over 80°F (27°C); frozen cylinders; extra days in the field; damage during transportation; delay in stripping molds and curing at the lab; improper capping; and insufficient care in breaking cylinders.
The laboratory should be held responsible for deficiencies in its procedures. Field-testing technicians and laboratory personnel should be certified; construction workers untrained in concrete testing must not make and handle cylinders. ACI 318 requires laboratories to conform to ASTM C1077 for their quality system and personnel qualifications. Laboratories should be inspected by the Cement and Concrete Reference Laboratory (CCRL) laboratory inspection or an equivalent program. Field testing personnel must have a current ACI Grade I Field Testing Technician certification or equivalent. Laboratory personnel should have the ACI Grade I and II Laboratory Testing Technician and/or the ACI Strength Testing Certification, or equivalent.
If the deficiency justifies investigation, first verify testing accuracy and then compare the structural requirements with the measured strength. If testing is deficient or if strength is greater than actually needed in that portion of the structure, there is little point in investigating the in-place strength. However, if procedures conform to the standards and the strength as specified is required for the structural capacity of the member in question, further investigation of the in-place concrete may be required (CIP 10).
HOW to Reduce Low Strength Tests:
1. Ensure that sample of concrete at the jobsite is obtained in accordance with ASTM C172
2. Ensure that the cylinders are made and cured in accordance with the standard curing requirements in ASTM C31.
3. Ensure that cylinders are handled with care at the jobsite and during transportation.
4. Ensure the cylinders are tested in the laboratory in accordance with ASTM C39.
1. ASTM Standards C31, C39, C172, C1077, ASTM Book of Standards, Volume 04.02, American Society for Testing and Materials, West Conshohocken, PA. www.astm.org
2. Specification for Structural Concrete, ACI 301, American Concrete Institute, Farmington Hills, MI, www.concrete.org
3. Building Code Requirements for Reinforced Concrete, ACI 318, American Concrete Institute, Farmington Hills, MI.
4. In-Place Concrete Strength Evaluation-A Recommended Practice. NRMCA Publication 133, NRMCA, Silver Spring, MD., www.nrmca.org
5. Effect of Curing Condition on Compressive Strength of Concrete Test Specimens, NRMCA Publication 53, NRMCA Silver Spring, MD.
6. Review of Variables that Influence Measured Concrete Compressive Strength, David N. Richardson, NRMCA Publication 179, NRMCA, Silver Spring, MD.
7. Low Strength Tests? Maybe Not!, E.O. Goeb, Concrete Products, December 1992.
8. Why Low Cylinder Tests in Hot Weather? E.O. Goeb, Concrete Construction, Jan. 1986.
9. CIP 10 – Strength of In-Place Concrete; CIP 34—Making Concrete Cylinders in the Field, CIP Series, NRMCA, Silver Spring, MD.
WHAT is the Strength of In-Place Concrete?
Concrete structures are designed to carry dead and live loads during construction and in service. Samples of concrete are obtained during construction and standard ASTM procedures are used to measure the potential strength of the concrete as delivered. Cylinders are molded and cured at 60 to 80°F (17 to 27°C) for one day and then moist cured in the laboratory until broken in compression, normally at an age of 7 and 28 days or at an alternative specified age.
The in-place strength of concrete will not be equivalent to that measured on standard cylinders and will generally be lower. Job practices for handling, placing, consolidation, and curing concrete in structures are relied upon to provide an adequate percentage of that potential strength, measured on cylinders, in the structure. Structural design principles recognize this and the ACI Building Code, ACI 318, has a process of assuring the structural safety during construction. In cold weather, a slower rate of in-place strength gain can be expected.
Means of measuring, estimating, or comparing the strength of in-place concrete include: rebound hammer, penetration probe, pullouts, maturity, cast-in- place cylinders, tests of drilled cores, and load tests of the structural member or system.
Cores drilled from the structure is one of the methods of evaluating whether the structural capacity of a concrete member is adequate. Drilled cores generally test lower than standard-cured cylinders. The ACI Building Code (ACI 318) recognizes that concrete construction can be considered structurally adequate if the average of three cores from a region represented by non-compliant concrete strength tests is equal to or exceeds 85 percent of specified strength, ƒ´c with no single core less than 75 percent of ƒ´c. Measured core strengths are not corrected for age. TIP 11 discusses core testing for acceptance of concrete. ACI 214.4R provides detailed guidance on core testing, evaluating existing structure capacity using in-place strengths, and determining an equivalent ƒ´c value for evaluating the structural capacity of an existing structure. The latter process should not be used to determine the acceptability of concrete furnished to a project.
WHY Measure In-Place Strength?
Tests of in-place concrete may be needed when standard cylinder strengths are low and do not comply with strength acceptance criteria outlined in ACI 318. However, do not investigate in-place without first checking to be sure that: the concrete strengths actually failed to meet the specification provisions, low strengths are not attributable to faulty testing practices, or the specified strength is really needed. (See CIP 9 and TIP 11) In many cases, the concrete can be accepted for the intended use without in-place strength testing.
There are many other situations that may require the investigation of in-place strength. These include: shore and form removal, post-tensioning, or early load application; investigation of damage due to freezing, fire, or adverse curing exposure; evaluation of older structures; and when a lower strength concrete is placed in a member by mistake. When cores or other in-place tests fail to assure structural adequacy, additional curing of the structure may provide the necessary strength. This is particularly possible with concrete containing fly ash, slag cement or some blended cements.
HOW to Investigate In-Place Strength:
If only one set of cylinders is low, often the question can be settled by comparing rebound hammer or penetration probe results on concrete in areas represented by acceptable cylinder results. Where the possibility of low strength is such that large portions need to be investigated, a well-organized study will be needed. Establish a grid and obtain systematic readings including good and questionable areas. Tabulate the hammer or probe readings. If areas appear to be low, drill cores from both low and high areas. If the cores confirm the hammer or probe results, the need for extensive core tests is greatly reduced.
Core Strength, ASTM C42 – If core drilling is necessary observe the following:
a. Test a minimum of 3 cores for each location in the structure represented by low strength tests;
b. Obtain cores with a minimum diameter of 3.7 in. (85 mm) or at least twice the nominal maximum aggregate size. Smaller diameter cores are permitted when it is not feasible to obtain the required size;
c. The length to diameter ratio (L/D) should be around 2, but try to obtain cores with L/D of at least 1½;
d. Avoid drilling cores from the top layers of columns, slabs, walls, or footings, which will be 10 to 20 percent weaker than cores from the mid or lower portions; and
e. Store the cores in sealed watertight bags or containers and transport to the laboratory. Test the cores in accordance with ASTM C42. Saw or grind core ends within 2 days after drilling. Keep cores in a sealed condition for at least 5 days after last wetted. Review the requirements for conditioning cores in current versions of ACI 318 and ASTM C42.
Probe Penetration Resistance, ASTM C803 – Probes or pins driven into concrete can be used to study relative strength of in-place concrete:
a. Different size probes or pins, or a change in driving force may be necessary for large differences in strength or concrete density;
b. Accurate measurement of the exposed length of the probe is required;
c. Probes should be spaced at least 7 in. apart and not be close to the edge of the concrete;
d. Probes not firmly embedded in the concrete should be rejected;
e. Develop a strength calibration curve for the materials and conditions under investigation; and
f. Surface conditions, moisture conditions and aggregate characteristics can affect the results.
Rebound Hammer, ASTM C805 – This method is also used to evaluate the relative in-place strength:
a. Wet all surfaces for several hours or overnight because drying affects rebound number;
b. Don’t compare readings on concrete cast against different form materials, concrete of varying moisture content, readings from different impact directions, on members of different mass, or results using different hammers;
c. Don’t grind off the surface unless it is soft, finished or textured;
d. Test structural slabs from the bottom;
e. Do not test frozen concrete; and
f. Surface conditions, moisture conditions, and aggregate characteristics can affect the results.
Maturity, ASTM C1074 – If concrete maturity is used to estimate the in-place strength please refer to CIP 39.
Advance Planning – When it is known in advance that in-place testing is required, such as for shore and form removal, other methods can be considered such as: cast-in-place, push-out cylinders and pullout strength measuring techniques covered by ASTM C873 and C900.
1. ACI 318, Building Code Requirements for Structural Concrete, ACI, Farmington Hills, MI, www.concrete.org.
2. ACI 228.1R, In-Place Methods to Estimate Concrete Strength, ACI, Farmington Hills, MI.
3. ACI 214.4R, Guide for Obtaining Cores and Interpreting Compressive Strength Results, ACI, Farmington Hills, MI.
4. ASTM C31, C39, C42, C805, C803, C873, C900, ASTM Book of Standards, Vol. 04.02, American Society for Testing and Materials, West Conshohocken, PA
5. Guide to Nondestructive Testing of Concrete, G.I. Crawford, Report FHWA-SA-97-105, Sept. 1997, Federal Highway Administration, Washington, DC.
6. CIP 9, 39, Concrete in Practice Series, NRMCA, Silver Spring, MD, www.nrmca.org.
7. TIP 11, Testing Concrete Cores, Technology in Practice Series, NRMCA, Silver Spring, MD,
8. In-Place Strength Evaluation – A Recommended Practice, NRMCA Publication 133, NRMCA, Silver Spring, MD.
9. Understanding Concrete Core Testing, Bruce A. Suprenant, NRMCA Publication 185, NRMCA, Silver Spring, MD.
WHAT is Curing?
Curing is the maintaining of an adequate moisture content and temperature in concrete at early ages so that it can develop properties the mixture was designed to achieve. Curing begins immediately after placement and finishing so that the concrete may develop the desired strength and durability.
Without an adequate supply of moisture, the cementitious materials in concrete cannot react to form a quality product. Drying may remove the water needed for this chemical reaction called hydration and the concrete will not achieve its potential properties.
Temperature is an important factor in proper curing, since the rate of hydration, and therefore, strength development, is faster at higher temperatures. Generally, concrete temperature should be maintained above 50°F (10°C) for an adequate rate of strength development. Further, a uniform temperature should be maintained through the concrete section while it is gaining strength to avoid thermal cracking.
For exposed concrete, relative humidity and wind conditions are also important; they contribute to the rate of moisture loss from the concrete and could result in cracking, poor surface quality and durability. Protective measures to control evaporation of moisture from concrete surfaces before it sets are essential to prevent plastic shrinkage cracking (See CIP 5).
Several important reasons are:
a. Predictable strength gain. Laboratory tests show that concrete in a dry environment can lose as much as 50 percent of its potential strength compared to similar concrete that is moist cured. Concrete placed under high temperature conditions will gain early strength quickly but later strengths may be reduced. Concrete placed in cold weather will take longer to gain strength, delaying form removal and subsequent construction.
b. Improved durability. Well-cured concrete has better surface hardness and will better withstand surface wear and abrasion. Curing also makes concrete more watertight, which prevents moisture and water-borne chemicals from entering into the concrete, thereby increasing durability and service life.
c. Better serviceability and appearance. A concrete slab that has been allowed to dry out too early will have a soft surface with poor resistance to wear and abrasion. Proper curing reduces crazing, dusting and scaling.
HOW to Cure:
Moisture Requirements for Curing:
Concrete should be protected from losing moisture until final finishing using suitable methods like wind breaks, fogger sprays or misters to avoid plastic shrinkage cracking. After final finishing, the concrete surface must be kept continuously wet or sealed to prevent evaporation for a period of at least several days after finishing. See the table for examples.
Systems to keep concrete wet include:
a. Burlap or cotton mats and rugs used with a soaker hose or sprinkler. Care must be taken not to let the coverings dry out and absorb water from the concrete. The edges should be lapped and the materials weighted down so they are not blown away.
b. Straw that is sprinkled with water regularly. Straw can easily blow away and, if it dries, can catch fire. The layer of straw should be 6 inches thick and should be covered with a tarp.
c. Damp earth, sand, or sawdust can be used to cure flatwork, especially floors. There should be no organic or iron- staining contaminants in the materials used.
d. Sprinkling on a continuous basis is suitable provided the air temperature is well above freezing. The concrete should not be allowed to dry out between soakings, since alternate wetting and drying is not an acceptable curing practice.
e. Ponding of water on a slab is an excellent method of curing. The water should not be more than 20°F (11°C) cooler than the concrete and the dike around the pond must be secure against leaks.
Moisture retaining materials include:
a. Liquid membrane-forming curing compounds must conform to ASTM C309. Apply to the concrete surface about one hour after finishing. Do not apply to concrete that is still bleeding or has a visible water sheen on the surface. While a clear liquid may be used, a white pigment will provide reflective properties and allow for a visual inspection of coverage. A single coat may be adequate, but where possible a second coat, applied at right angles to the first, is desirable for even coverage. If the concrete will be painted, or covered with vinyl or ceramic tile, then a liquid compound that is non-reactive with the paint or adhesives must be used, or use a compound that is easily brushed or washed off. On floors, the surface should be protected from the other trades with scuff-proof paper after the application of the curing compound.
b. Plastic sheets – either clear, white (reflective) or pigmented. Plastic should conform to ASTM C171, be at least 4 mils thick, and preferably reinforced with glass fibers. Dark colored sheets are recommended when ambient temperatures are below 60°F (15°C) and reflective sheets should be used when temperatures exceed 85°F (30°C). The plastic should be laid in direct contact with the concrete surface as soon as possible without marring the surface. The edges of the sheets should overlap and be fastened with waterproof tape and then weighted down to prevent the wind from getting under the plastic. Plastic can make dark streaks wherever a wrinkle touches the concrete, so plastic should not be used on concretes where appearance is important. Plastic is sometimes used over wet burlap to retain moisture.
c. Waterproof paper – used like plastic sheeting, but does not mar the surface. This paper generally consists of two layers of kraft paper cemented together and reinforced with fiber. The paper should conform to ASTM C171.
Note that products sold as evaporation retardants are used to reduce the rate of evaporation from fresh concrete surfaces before it sets to prevent plastic shrinkage cracking. These materials should not be used for final curing.
Control of temperature:
In cold weather do not allow concrete to cool faster than a rate of 5°F (3°C) per hour for the first 24 hours. Concrete should be protected from freezing until it reaches a compressive strength of at least 500 psi (3.5 MPa) using insulating materials. Curing methods that retain moisture, rather than wet curing, should be used when freezing temperatures are anticipated. Guard against rapid temperature changes after removing protective measures. Guidelines are provided in Reference 7.
In hot weather, higher initial curing temperature will result in rapid strength gain and lower ultimate strengths. Water curing and sprinkling can be used to achieve lower curing temperatures in summer. Day and night temperature extremes that allow cooling faster than 5°F (3°C) per hour during the first 24 hours should be protected against.
1. Effect of Curing Condition on Compressive Strength of Concrete Test Specimens, NRMCA Publication No. 53, National Ready Mixed Concrete Association, Silver Spring, MD.
2. How to Eliminate Scaling, Concrete International, February 1980. American Concrete Institute, Farmington Hills, MI.
3. ASTM C309, Specification for Liquid Membrane Forming Compounds for Curing Concrete, American Society for Testing Materials, West Conshohocken, PA.
4. ASTM C171, Specification for Sheet Materials for Curing Concrete, American Society for Testing Materials, West Conshohocken, PA.
5. Standard Practice for Curing Concrete, ACI 308, American Concrete Institute, Farmington Hills, MI.
6. Standard Specification for Curing Concrete, ACI 308.1, American Concrete Institute, Farmington Hills, MI.
7. Cold Weather Concreting, ACI 306R, American Concrete Institute, Farmington Hills, MI
WHAT is Hot Weather?
Hot weather, as defined by ACI 305R, is any combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise causing detrimental results:
• High ambient temperature
• High concrete temperature
• Low relative humidity
• High wind speed, and
• Solar radiation
Hot weather problems are most frequently encountered in the summer, but the associated climatic factors of high winds, low relative humidity and solar radiation can occur at any time, especially in arid or tropical climates. Hot weather conditions can produce a rapid rate of evaporation of moisture from the surface of the newly placed concrete and accelerate setting time, among other problems. Generally, high relative humidity tends to reduce the effects of high temperature.
WHY Consider Hot Weather?
Hot weather should be taken into consideration when planning concrete projects because of the potential effects on fresh and newly placed concrete. High concrete temperature causes increased water demand, which, in turn, will increase the water-cementitious materials ratio and result in lower strength and reduced durability. Higher temperatures tend to accelerate the rate of slump loss and can cause loss of entrained air. Temperature also has a major effect on the setting time of concrete: At higher temperatures, concrete will set quicker and finishing operations will need to occur at a faster rate. Concrete that is cured at high temperatures at an early age will not be as strong at later ages as the same concrete cured at temperatures in the range of 70°F (20°C).
High temperatures, high wind velocity, and low relative humidity can affect fresh concrete in two important ways: the high rate of evaporation can result in plastic shrinkage before concrete sets or early-age drying shrinkage cracking. The evaporation rate removes surface water necessary for hydration unless proper curing methods are employed. Thermal cracking may result from rapid changes in temperature, such as when concrete slabs or walls are placed on a hot day followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for thermal cracking in thicker concrete sections.
HOW to Concrete in Hot Weather:
The keys to successful hot weather concreting are:
1. Recognizing the factors that affect concrete; and
2. Planning to minimize their effects.
Use proven local recommendations for adjusting concrete mixture composition and proportions, such as the use of water-reducing and set-retarding admixtures. Extended-set control admixtures may also be used for long haul deliveries or in extremely high temperatures. Modifying concrete mixtures to reduce the heat generated by cement hydration, such as the use of an ASTM Type II moderate heat cement, blended cements with a low heat option, and the use of fly ash and slag cement can reduce potential problems with high concrete temperature. Advance planning to schedule concrete delivery to avoid interruptions and delays of placing and finishing is essential. Trucks should be able to discharge immediately and adequate personnel should be available to place and handle the concrete. When possible, avoid the hottest part of the day to place and finish concrete. Do not sprinkle water on the surface of slabs to facilitate finishing. Limits on maximum concrete temperature may be waived by the purchaser if the concrete consistency (slump) is adequate for the placement and excessive water addition is not required.
In the case of extreme temperature conditions or with thicker (mass) concrete sections, the concrete temperature can be lowered by using chilled water or ice as part of the mixing water. Chilled water can reduce concrete temperature by up to 10°F (6°C); ice can reduce temperature by up to 20°F (12°C). The ready mixed concrete producer uses other measures, such as sprinkling and shading the aggregate, to help lower the temperature of the concrete. For greater reductions in concrete temperature, liquid nitrogen can be injected into concrete mixers. This needs additional setup costs and appropriate precautions to prevent damage to blades and mixer drum.
If low humidity and high winds are predicted, windbreaks, sunscreens, mist fogging, or evaporation retardants may be needed to minimize the potential plastic shrinkage cracking in slabs.
Follow These Rules for Hot Weather Concrete:
1. Make appropriate modifications to concrete mixtures to manage rate of slump loss, setting time and other characteristics. Retarders, water reducers, mid and high-range water reducers, extended set-control admixtures, moderate heat of hydration cement, pozzolanic materials, slag cement, or other proven local solutions may be used. Reduced cement content, while ensuring that concrete strength will be attained, may be appropriate. Synthetic fibers may be used to minimize plastic shrinkage cracking (CIP 24).
2. Have adequate manpower to place, finish and cure the concrete. Schedule the rate of concrete delivery that can be managed by available crew and placement equipment.
3. Limit the addition of water at the jobsite—do not exceed the quantity of mixing water established for the concrete mixture. Adding water to concrete that is more than 1½ hours old should be avoided.
4. Slabs on grade placed directly on vapor retarders (CIP 29) will need special precautions when finishing and curing to avoid cracking.
5. On dry and/or hot days, when conditions are conducive for plastic shrinkage cracking, dampen the subgrade, forms and reinforcement prior to placing concrete. Do not allow excessive water to pond.
6. Begin final finishing operations as soon as the water sheen has left the surface; start curing as soon as finishing is completed. Continue curing for at least 3 days; cover the concrete with wet burlap and plastic sheeting to prevent evaporation or use a liquid membrane curing compound, or cure slabs with water (CIP 11). Using white pigmented membrane curing compounds will help with proper coverage and reflect heat from the concrete surface.
7. Protect test cylinders at the jobsite to maintain temperature and moisture for initial curing. Field curing boxes with ice or refrigeration may be necessary to ensure maintaining the required 60 to 80°F (17 to 27°C) for initial curing of cylinders (CIP 9 and 34).
8. Accelerators may be used in hot weather to expedite finishing operations and to avoid plastic shrinkage cracking.
1. Hot Weather Concreting, ACI 305R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
2. Hot-Weather Concreting, Chapter in Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, IL., www.cement.org
3. CIP 9, 11, 24, 29, 34, Concrete in Practice Series, NRMCA, Silver Spring, MD, www.nrmca.org
4. Cooling Ready Mixed Concrete, NRMCA Publication No. 106, NRMCA, Silver Spring, MD.
5. Effect of Temperature and Delivery Time on Concrete Proportions, R.D. Gaynor, R.C. Meininger, T.S. Khan, NRMCA Publication 171, NRMCA, Silver Spring, MD.
6. Keeping Concrete Cool in the Heat of Summer, K.C. Hover, Concrete Construction, June 1993.
WHAT are Blisters?
Blisters are hollow, low-profile bumps on the concrete surface, typically from the size of a dime up to 1 inch (25 mm), but occasionally even 2 or 3 inches (50 – 75 mm) in diameter. A dense troweled skin of mortar about ⅛ in. (3 mm) thick covers an underlying void that moves around under the surface during troweling. Blisters may occur shortly after the completion of finishing operation. In poorly lighted areas, small blisters may be difficult to see during finishing and may not be detected until they break under traffic.
WHY do Blisters Form?
Blisters may form on the surface of fresh concrete when either bubbles of entrapped air or bleed water migrate through the concrete and become trapped under the surface, which has been sealed prematurely during the finishing operations. These defects are not easily repaired after concrete hardens.
Blisters are more likely to form if:
1. Insufficient or excessive vibration is employed. Insufficient vibration prevents the entrapped air from being released and excessive use of vibrating screeds works up a thick mortar layer on the surface.
2. An improper tool is used for floating the surface or it is used improperly. The surface should be tested to determine which tool, whether it be wood or magnesium bull float, does not seal the surface. The floating tool should be kept as flat as possible.
3. Excessive evaporation of bleed water occurs and the concrete appears ready for final finishing operations (premature finishing), when, in fact, the underlying concrete is still releasing bleed water and entrapped air. High rate of bleed water evaporation is especially a problem during periods of high ambient temperatures, high winds and/or low humidity.
4. Entrained air is used or is higher than normal. Rate of bleeding and quantity of bleed water is greatly reduced in air-entrained concrete giving the appearance that the concrete is ready to float and further finish causing premature finishing.
5. The subgrade is cooler than concrete. The top surface sets faster than the concrete in the bot- tom and the surface appears ready to be floated and further finished.
6. The slab is thick and it takes a longer time for the entrapped air and bleed water to rise to the surface.
7. The concrete is cohesive or sticky from higher content of cementitious materials or excessive fines in the sand. These mixtures also bleed less and at a slower rate. Concrete mixtures with lower con- tents of cementitious materials bleed rapidly for a shorter period, have higher total bleeding and tend to delay finishing.
8. A dry shake is prematurely applied, particularly over air-entrained concrete.
9. The slab is placed directly on top of a vapor retarder or an impervious base, preventing bleed water from being absorbed by the subgrade.
HOW to Prevent Blisters:
The finisher should be wary of a concrete surface that appears to be ready for final finishing before it would normally be expected. Emphasis in finishing operations should be on placing, striking off and bull floating the concrete as rapidly as possible and without working up a layer of mortar on the surface. After these operations are completed, further finishing should be delayed as long as possible and the surface covered with polyethylene or otherwise protected from evaporation. If conditions for high evaporation rates exist, place a cover on a small portion of the slab to judge if the concrete is still bleeding. In initial floating, the float blades should be flat to avoid densifying the surface too early. Use of an accelerating admixture or heated concrete often prevents blisters in cool weather. It is recommended that non-air entrained concrete be used in interior slabs and that air entrained concrete not be steel troweled.
If blisters are forming, try to either flatten the trowel blades or tear the surface with a wood float and delay finishing as long as possible. Under conditions causing rapid evaporation, slow evaporation by using wind breaks, water misting of the surface, evaporation retarders, or a cover (polyethylene film or wet burlap) between finishing operations. Further recommendations are given in ACI 302.1R and ACI 305.
Follow These Rules to Avoid Blisters:
1. Do not seal surface before air or bleed water from below have had a chance to escape.
2. Avoid dry shakes on air-entrained concrete.
3. Use heated or accelerated concrete to promote even setting throughout the depth of the slab in cooler weather.
4. Do not place slabs directly on vapor retarders. If vapor retarders are essential (CIP 28), take steps to avoid premature finishing.
5. Protect surface from premature drying and evaporation.
6. Do not use a jitterbug or excessive vibration such as a vibratory screed on slumps over 5 inches (125 mm).
7. Air entrained concrete should not be steel troweled. If required by specifications, extreme caution should be exercised when timing the finishing operation.
1. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org
2. Slabs on Grade, ACI Concrete Craftsman Series, CCS 1, American Concrete Institute, Farmington Hills, MI.
3. Hot Weather Concreting, ACI 305R, American Concrete Institute, Farmington Hills, MI.
4. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS 177, Portland Cement Association, Skokie, IL. www.cement.org
5. Concrete Surface Blistering—Causes and Cures, Carl O. Peterson, Concrete Construction, September 1970. www.concreteconstruction.net
6. CIP 14 – Finishing Concrete Flatwork; CIP 20 – Delamination of Troweled Concrete Surfaces, NRMCA CIP Series. www.nrmca.org.
7. Finishing, Concrete Construction, August 1976, p. 369.
8. Finishing Problems and Surface Defects in Flatwork, Concrete Construction, April 1979.
WHAT is Finishing?
Finishing is the operation of creating a concrete surface of a desired texture, smoothness and durability. The finish can be strictly functional or decorative.
WHY Finish Concrete?
Finishing makes concrete attractive and serviceable. The final texture, hardness, and joint pattern on slabs, floors, sidewalks, patios and driveways depend on the concrete’s end use. Warehouse or industrial floors usually have greater durability requirements and need to be flat and level, while other interior floors that are covered with floor coverings do not have to be as smooth and durable. Exterior slabs must be sloped to carry away water and must provide a texture that will not be slippery when wet.
HOW to Place Concrete:
Prior to the finishing operation, concrete is placed, consolidated and leveled. These operations should be carefully planned. Skill, knowledge and experience are required to deal with a variety of concrete mixtures and field conditions. Having the proper manpower and equipment available, and timing the operations properly for existing conditions, is critical. A slope is necessary to avoid low spots and to drain water away from buildings.
Complete all subgrade excavation and compaction, formwork, and placement of mesh, rebars or other embedments as required prior to concrete delivery. Delays after the concrete arrives create problems and can reduce the final quality of flatwork.
General guidelines for placing and consolidating concrete are:
a. A successful job depends on selecting the correct concrete mixture for the job. Consult your Ready Mixed Concrete Producer. Deposit concrete as near as possible to its final location, either directly in place from the truck chute or use wheelbarrows, buggies or pumps. Avoid adding excessive water to increase the concrete’s slump. Start at the far end placing concrete into previously placed concrete and work towards the near end. On a slope, use concrete with a stiffer consistency (lower slump) and work up the slope.
b. Spread the concrete using a short-handled, square-ended shovel, or a come-along. Never use a garden rake to move concrete horizontally. This type of rake causes segregation.
c. All concrete should be well consolidated. For small flatwork jobs, pay particular attention to the edges of the forms by tamping the concrete with a spade or piece of wood. For large flatwork jobs, consolidation is usually accomplished by using a vibrating screed or internal vibrator.
d. When manually striking off and leveling the concrete, use a lumber or metal straightedge (called a screed). Rest the screed on edge on the top of the forms, tilt it forward and draw it across the concrete with a slight sawing motion. Keep a little concrete in front of the screed to fill in any low spots. Do not use a jitterbug or vibrating screed with concrete slump exceeding 3 inches (75mm). Vibrating screeds should be moved rapidly to ensure consolidation but avoid working up an excessive layer of mortar on the surface.
HOW to Finish Concrete:
1. LEVEL the concrete further using a bull float, darby, or highway straightedge as soon as it has been struck-off. This operation should be completed before bleed water appears on the surface. The bull float or darby embeds large aggregate, smoothes the surface, and takes out high and low spots. Keep the bull float as flat as possible to avoid premature sealing of the surface.
2. WAIT for the concrete to stop “bleeding.” All other finishing operations must wait until the concrete has stopped bleeding and the water sheen has left the surface. Any finishing operations done while the concrete is still bleeding will result in later problems, such as dusting, scaling, crazing, delamination and blisters. The waiting period depends on the setting and bleeding characteristics of the concrete and the ambient conditions. During the waiting period, protect against evaporation from the concrete surface if conditions are hot, dry and windy. Cover a small test portion of the slab to evaluate if the concrete is still bleeding. General guidance regarding whether the concrete has sufficiently set for final finishing operations is when a footprint indentation of a person standing on the slab is between ⅛ to ¼ inch (3 to 6 mm).
3. EDGE the concrete when required. Spade the concrete to break any bond with the form with a small mason’s trowel. Use the edging tool to obtain durable rounded edges.
4. JOINT the concrete when required. The jointing tool should have a blade one-fourth the depth of the slab. Use a straight piece of lumber as a guide. A shallow-bit groover should only be used for decorative grooves. When saw-cutting is required, it should be done as soon as the concrete is hard enough not to be torn by the blade. Early entry saw cutting can be done before the concrete has completely hardened. See CIP 6 for jointing practices and spacing.
5. FLOAT the concrete by hand or machine in order to embed the larger aggregates. Floating also levels and pre- pares the surface for further finishing. Never float the concrete while there is still bleed water on the surface.
6. TROWEL the concrete when required for its end use. For sidewalks, patios, driveways and other exterior applications, troweling is not usually required. Air entrained concrete should not be troweled. If trowel finishing of air-entrained concrete is required by specifications, extreme caution should be exercised when timing the finishing operation. For a smooth floor, make successive passes with a smaller steel trowel and increased pressure. Repeated passes with a steel trowel will produce a smooth floor that will be slippery when wet. Excessive troweling may create dark “trowel burns.” Improperly tilting the trowel will cause an undesirable “chatter” texture.
7. TEXTURE the concrete surface as required after floating or troweling. For exterior concrete flatwork (sidewalks, patios or driveways) texture the concrete surface after the floating operation with a coarse or fine push-broom to give a non-slip surface. For interior flatwork texture the concrete surface after final troweling. Concrete can be finished with several decorative treatments, such as exposed aggregate, dry shake color, integral color, and stamped or patterned concrete. Decorative finishes need much more care and experience.
8. NEVER sprinkle water or cement on concrete while finishing it. This may cause dusting or scaling.
9. CURE the concrete as soon as all finishing is completed to provide proper conditions for cement hydration, which provides the required strength and durability to the concrete surface. In severe conditions, slab protection may be needed even before finishing is complete. See CIP 11 for more information on curing concrete.
10. AVOID concrete burns to skin by following proper safety practices.
Follow These Rules to Finish Concrete:
1. Place and move concrete to its final location using procedures that avoid segregation.
2. Strike off and obtain an initial level surface without sealing the surface.
3. Wait until the bleed water disappears from the surface before starting finishing operations.
4. Use the appropriate surface texture as required for the application.
5. Avoid steel troweling air-entrained concrete.
6. Cure the concrete to ensure it achieves the desired strength and durability.
1. Concrete in Practice (CIP) Series, National Ready Mixed Concrete Association, Silver Spring, Maryland. www.nrmca.org
2. Guide for Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI. www.concrete.org.
3. Slabs on Grade, ACI Concrete Craftsman Series, CCS-1, American Concrete Institute, Farmington Hills, MI.
4. Cement Mason’s Guide, PA122, Portland Cement Association, Skokie, IL. www.cement.org.
5. Residential Concrete, National Association of Home Builders, Washington, D.C.
6. Sealing Effects of Finishing Tools, Bruce Suprenant, Concrete Construction, September 1999. www.concreteconstruction.net.
7. Finishing Tool Primer, Kim Basham, Concrete Construction, July 2000.
WHAT are Admixtures?
Admixtures are natural or manufactured chemicals added to the concrete before or during mixing. The most often used chemical admixtures are air-entraining agents, water reducers, water-reducing retarders and accelerators.
WHY Use Admixtures?
Admixtures are used to give special properties to fresh or hardened concrete. Admixtures may enhance the workability of fresh concrete and the durability strength of hardened concrete. Admixtures are used to overcome difficult construction situations, such as hot or cold weather placements, pumping requirements, early-age strength requirements, or specifications that require low water-cementitious materials ratio. Admixtures can be used to optimize the cementitious composition of concrete mixtures for performance and sustainability.
HOW to Use Admixtures:
Consult your ready mixed concrete supplier about admixture(s) appropriate for your application. Admixtures are evaluated for compatibility with cementitious materials, construction practices, job specifications and economic benefits before being used. Purchasers of ready mixed concrete should avoid requiring the use of specific brands or using products of their own accord.
Follow This Guide to Use Admixtures:
1. AIR-ENTRAINING ADMIXTURES are liquid chemicals added when batching concrete to produce microscopic air bubbles, called entrained air, produced by the mixing action. These air bubbles improve the concrete’s resistance to damage caused by exposure to cycles of freezing and thawing and deicing salt application. In fresh concrete, entrained air improves workability and reduces bleeding and segregation. For exterior flatwork (parking lots, driveways, sidewalks, pool decks, patios) subject to freezing and thawing cycles, or in areas where deicer salts are used, an air content of 4% to 7% of the concrete volume is used depending on the size of coarse aggregate (see Table). Air entrainment is not necessary for interior structural concrete since it is not subject to freezing and thawing. Entrained air should be avoided for concrete flatwork that will have a smooth troweled finish. In concretes with higher cementitious materials content, entrained air will reduce strength by about 5% for each 1% of air added; but in low cement content concretes, adding air has less effect and can reduce segregation and result in a modest increased strength due to the reduced water needed for required slump. Air entraining admixtures for use in concrete should meet the requirements of ASTM C260 – Specification for Air-Entraining Admixtures for Concrete.
2. WATER REDUCERS are used for two different purposes: (1) to lower the water content in fresh concrete and to increase its strength; and (2) to obtain higher slump without adding additional water. Water-reducers reduce the required water content of a concrete mixture for a target slump. These admixtures disperse the cement particles in concrete and make more efficient use of cement. This increases strength or allows the use of less cement to achieve a similar strength. Water-reducers are useful for pumping concrete and in hot weather, to offset the increased water demand. Some water-reducers may cause an increased rate of slump loss with time. Water-reducers should meet the requirements for Type A in ASTM C494 – Specification for Chemical Admixtures for Concrete.
Mid-range water reducers are now commonly used and are used for a greater water reduction than typical water reducers. These admixtures are popular as they improve the finishability of concrete flatwork. Mid-range water reducers must at least meet the requirements for Type A in ASTM C494. There is separate classification for these products in ASTM C494.
3. HIGH RANGE WATER REDUCERS (HRWR) is a special class of water reducer. Often referred to as superplasticizers, HRWRs reduce the water content of a given concrete mixture between 12% and 40% to maintain the same slump. HRWRs are therefore used to increase strength and reduce permeability of concrete by reducing the water content in the mixture. They greatly increase the slump to produce “flowing” concrete or self-consolidating concrete (CIP 37) by using less water. These admixtures are essential for producing high strength and high performance concretes that contain higher contents of cementitious materials and mixtures containing silica fume. Some HRWRs may cause a higher rate of slump loss with time. In some cases, HRWRs may be added at the jobsite in a controlled manner to provide the required slump for placement. HRWRs are covered by ASTM Specification C494. Types F and G, and Types 1 and 2 in ASTM C1017 – Specification for Chemical Admixtures for Use in Producing Flowing Concrete.
4. RETARDERS are chemicals that delay the initial setting of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. For large jobs, or in hot weather, concrete with retarder allows more time for placing and finishing. Retarders are typically a component of water reducers. Retarders should meet the requirements for Type B or D in ASTM C494.
5. ACCELERATORS reduce the initial setting time of concrete and produces higher strength at early ages. Accelerators do not prevent concrete from freezing; rather, they speed up the setting to permit finishing concrete earlier; and increase the rate of strength gain, thereby making the concrete stronger to resist damage from freezing in cold weather. Accelerators are also used in fast track construction requiring early form removal, opening to traffic, or load application on structures. Liquid accelerators should conform to ASTM C494 Types C and E. There are two kinds of accelerating admixtures: chloride based and non-chloride based. Calcium chloride is a commonly used effective and economical accelerator, which is available in liquid or flake form. Calcium chloride must meet the requirements of ASTM D98. For non-reinforced concrete, calcium chloride can be used to a limit of 2% by the weight of the cement. Because of concerns with corrosion of reinforcing steel induced by chloride, lower limits on chlorides apply to reinforced concrete. Prestressed concrete and concrete with embedded aluminum or galvanized metal should not contain any chloride-based materials because of the increased potential for corrosion of the embedded metal. Non-chloride based accelerators are used where there is concern of corrosion of embedded metals or reinforcement in concrete.
Besides these standard types of admixtures, there are products available for enhancing concrete properties for a wide variety of applications. Some of these products include: corrosion inhibitors, shrinkage reducing admixtures, anti-washout admixtures, hydration stabilizing or extended set retarding admixtures, admixtures to reduce potential for alkali aggregate reactivity, pumping aids, permeability reducing admixtures, workability retaining admixtures, rheology and viscosity modifying admixtures and a variety of colors and products that enhance the aesthetics of concrete. Consult with your local ready mixed concrete producer on admixture products that add value to your project.
1. ASTM C260, C494, C1017, D98, ASTM International, West Conshohocken, PA, www.astm.org.
2. Chemical and Air-Entraining Admixtures for Concrete, ACI Educational Bulletin, E4, American Concrete Institute, Farmington Hills, MI, www.concrete.org.
3. Chemical Admixtures for Concrete, ACI 212.3R, American Concrete Institute, Farmington Hills, MI.
4. Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, Farmington Hills, MI.
5. Understanding Chloride Percentages, NRMCA Publication No. 173, NRMCA, Silver Spring, MD, www.nrmca.org.
6. Self-Consolidating Concrete, CIP 37, NRMCA Concrete in Practice Series, Silver Spring, MD, www.nrmca.org.
WHAT is Flexural Strength?
Flexural strength is one measure of the tensile strength of concrete. It is a measure of an unreinforced concrete beam or slab to resist failure in bending. It is measured by loading 6 x 6-inch (150 x 150-mm) concrete beams with a span length at least three times the depth. The flexural strength is expressed as Modulus of Rupture (MR) in psi (MPa) and is determined by standard test methods ASTM C78 (third-point loading) or ASTM C293 (center-point loading).
Flexural MR is about 10 to 20 percent of compressive strength depending on the type, size and volume of coarse aggregate used. However, the best correlation for specific materials is obtained by laboratory tests for given materials and mix design. The MR determined by third-point loading is lower than the MR determined by center-point loading, sometimes by as much as 15%.
WHY Test Flexural Strength?
Designers of pavements use a theory based on flexural strength. Therefore, laboratory mix design based on flexural strength tests may be required, or a cementitious material content may be selected from past experience to obtain the needed design MR. Some also use MR for field control and acceptance of pavements. Very few use flexural testing for structural concrete. Agencies not using flexural strength for field control generally find the use of compressive strength convenient and reliable to judge the quality of the concrete as delivered.
HOW to Use Flexural Strength:
Beam specimens must be properly made in the field. Pavement concrete mixtures are stiff (½ to 2½-inch slump). Consolidate by vibration in accordance with ASTM C31 and tap sides to release air pockets. For higher slump, after rodding, tap the molds to release air pockets and spade along the sides to consolidate.
Never allow the beam surfaces to dry at any time. Immerse in saturated limewater for at least 20 hours before testing.
Specifications and investigation of apparent low strengths should take into account the higher variability of flexural strength results. Standard deviation for concrete flexural strengths up to 800 psi (5.5 MPa) for projects with good control range from about 40 to 80 psi (0.3 to 0.6 MPa). Standard deviation values over 100 psi (0.7 MPa) may indicate testing problems. There is a high likelihood that testing problems, or moisture differences within a beam caused from premature drying, will cause low strength.
Where a correlation between flexural and compressive strength has been established in the laboratory, core strengths by ASTM C42 can be used for compressive strength to check against the desired value using the ACI 318 criteria of 85 percent of specified strength for the average of three cores. It is impractical to saw beams from a slab for flexural testing. Sawing beams will greatly reduce measured flexural strength and should not be done. In some instances, splitting tensile strength of cores by ASTM C496 is used, but experience is limited on how to apply the data.
Another procedure for in-place strength investigation uses compressive strength of cores calibrated by comparison with acceptable placements in proximity to the concrete in question:
WHAT are the Problems with Flexure?
Flexural tests are extremely sensitive to specimen preparation, handling and curing procedure. Beams are very heavy and can be damaged when handled and transported from the jobsite to the lab. Allowing a beam to dry will yield lower strengths. Beams must be cured in a standard manner and tested while wet. Meeting all these requirements on a jobsite is extremely difficult, often resulting in unreliable and generally low MR values. A short period of drying can produce a sharp drop in flexural strength.
Many state highway agencies have used flexural strength but are now changing to compressive strength or maturity concepts for job control and quality assurance of concrete paving. Cylinder compressive strengths are also used for concrete structures.
The data point to a need for a review of current testing procedures. They suggest also that, while the flexural strength test is a useful tool in research and in laboratory evaluation of concrete ingredients and proportions, it is too sensitive to testing variations to be usable as a basis for the acceptance or rejection of concrete in the field. (Reference 3)
NRMCA and the American Concrete Pavement Association (ACPA) have a policy that compressive strength testing is the preferred method of concrete acceptance and that certified technicians should conduct the testing. ACI Committees 325 and 330 on concrete pavement construction and design and the Portland Cement Association (PCA) point to the use of compressive strength tests as more convenient and reliable.
The concrete industry and inspection and testing agencies are much more familiar with traditional cylinder compression tests for control and acceptance of concrete. Flexure can be used for design purposes, but the corresponding compressive strength should be used to order and accept the concrete. Any time trial batches are made, both flexural and compressive tests should be made so that a correlation can be developed for field control.
1. How Should Strength be Measured for Concrete Paving? Richard C. Meininger, NRMCA TIL 420, and Data Summary, NRMCA TIL 451, NRMCA, Silver Spring, MD.
2. Concrete Strength Testing, Peggy Carrasquillo, Chapter 14, ASTM STP 169C, Significance of Tests and Properties of Concrete and Concrete-Making Materials, American Society for Testing and Materials, West Conshohocken, PA.
3. “Studies of Flexural Strength of Concrete, Part 3, Effects of Variations in Testing Procedures,” Stanton Walker and D. L. Bloem, NRMCA Publication No. 75, NRMCA, Silver Spring, MD.
4. Variation of Laboratory Concrete Flexural Strength Tests, W. Charles Greer, Jr., ASTM Cement, Concrete and Aggregates, Winter, 1983, American Society for Testing and Materials, West Conshohocken, PA.
5. “Concrete Mixture Evaluation and Acceptance for Air Field Pavements” Richard C. Meininger and Norm Nelson, NRMCA Publication 178, September 1991, NRMCA, Silver Spring, MD.
6. Compression vs. Flexural Strength for Quality Control of Pavements, Steve Kosmatka, CTT PL 854, 1985, Portland Cement Association, Skokie, IL.
7. Time to Rein in the Flexure Test, Orrin Riley, ACI Concrete International, August, 1994, American Concrete Institute, Farmington Hills, MI.
WHAT is Flowable Fill?
Flowable fill is a self-compacting low strength material with a flowable consistency that is used as an economical fill or backfill material as an alternative to compacted granular fill. Flowable fill is not concrete nor is it used to replace concrete. Terminology used by ACI Committee 229 is Controlled Low Strength Material (CLSM). Other terms used for this material are unshrinkable fill, controlled density fill, flowable mortar or lean-mix backfill.
In terms of its flowability, the slump, as measured for concrete, is generally greater than 8 inches (200 mm). It is a self-leveling material and can be placed with minimal effort and does not require vibration or tamping. It hardens into a strong material with minimal subsidence.
While the broader definition includes materials with compressive strength less than 1200 psi (8.3 MPa), most applications use mixtures with strength less than 300 psi (2.1 MPa). The late-age strength of removable CLSM materials should be in the range of 30 to 200 psi (0.2 to 1.4 MPa) as measured by compressive strength of cylinders. It is important that the expectation of future excavation of flowable fill material be stated when specifying or ordering the material.
WHY is Flowable Fill Used?
Flowable fill is an economical alternative to compacted granular fill considering the savings in labor costs, equipment and time. Since it does not need manual compaction, trench width or the size of excavation is significantly reduced. Placing flowable fill does not require people to enter an excavation, a significant safety concern. CLSM is also an excellent solution for filling inaccessible areas, such as underground tanks, where compacted fill cannot be placed.
Uses of Flowable Fill include:
1. BACKFILL – sewer trenches, utility trenches, bridge abutments, conduit encasement, pile excavations, retaining walls, and road cuts.
2. STRUCTURAL FILL – foundation sub-base, subfooting, floor slab base, pavement bases, and conduit bedding.
3. OTHER USES – abandoned mines, underground storage tanks, wells, abandoned tunnel shafts and sewers, basements and underground structures, voids under pavement, erosion control, and thermal insulation with high air content flowable fill.
How is Flowable Fill Ordered?
Ask for it by intended use and indicate whether excavatability in the future is required. Ready mixed concrete producers generally have developed mixture proportions for flowable fill products that make best use of economical aggregates, fly ash and other materials. Frequently site-excavated materials and materials that do not meet standards for use in concrete can be incorporated in flowable fill mixtures.
Strength – For later excavatability the ultimate strength of the flowable fill must be kept below 200 psi (1.4 MPa) to allow excavation by mechanical equipment, like back- hoes. For manual excavation the ultimate strength should be less than 50 psi (0.3 MPa). Mixtures containing large amounts of coarse aggregate are more difficult to excavate. Mixtures with entrained air in excess of 20% by volume are used to keep the strength low.
Higher strength structural fills can be designed for a specific required strength. Compressive strength of 50 to 100 psi (0.3 to 0.7 MPa) provides an allowable bearing pacity similar to well-compacted soil.
Setting and Early Strength may be important where equipment, traffic, or construction loads must be carried or subsequent construction needs to be scheduled. Judge the setting characteristics by scraping off loose accumulations of water and fines on top and see how much force is necessary to cause an indentation in the material. ASTM C403 or ASTM D6024 may be used to estimate the load carrying ability of the flowable fill. Penetration values by C403 between 500 and 1500 psi are adequate for loading flowable fill.
Density in place is usually in the 115 to 145 lb./cu. ft. range for non-air entrained or conventionally air-entrained mixtures. These densities are typically higher than most compacted fills. If lightweight fills are needed to reduce the weight or to provide greater thermal insulation, high entrained air (greater than 20%) mixtures, preformed foam or lightweight aggregates may be used.
Flowability of flowable fill is important, so the mixture will flow into place and consolidate due to its fluidity without vibration or puddling action. The flowability can be varied to suit the placement requirements of most applications. Hydrostatic pressure and floatation of pipes should be considered by appropriate anchorage or by placing in lifts.
Subsidence of some flowable fill mixtures with high water content is on the order of ¼ inch per foot (20 mm percameter) of depth as the solid materials settle. Mixtures with high air content use less water and have little or no subsidence.
Permeability of flowable mixtures can be varied significantly to suit the application. Most mixtures have permeability similar to or lower than compacted soil.
Durability – Flowable fill materials are not designed to resist freezing and thawing, abrasive or most erosive actions, or aggressive chemicals. If these properties are required, use a high quality concrete. Fill materials are usually buried in the ground or otherwise confined. If flowable fill deteriorates in place it will continue to act as a granular fill.
How is Flowable Fill Delivered and Placed?
Flowable fill is delivered by ready mixed concrete truck mixers and placed easily by chute in a flowable condition directly into the cavity to be filled. To avoid segregation, the drum should be kept agitating. Flowable fill can be conveyed by pump, chutes or buckets to its final location. For efficient pumping, some granular material is needed in the mixture. Due to its fluid consistency it can flow long distances from the point of placement.
Flowable fill does not need to be cured like concrete but should be protected from freezing until it has hardened.
Testing Flowable Fill Mixtures
Quality assurance testing is not necessary for pre-tested standard mixtures of flowable fill. Visual checks of mixture consistency and performance have proven adequate. Test methods and acceptance criteria for concrete are generally not applicable. Testing may be appropriate with new mixtures or if non-standard materials are used.
• Obtain samples for testing flowable fill mixtures in accordance with ASTM D5971.
• Flow consistency is measured in accordance with ASTM D6103. A uniform spread diameter of at least 8 in. without segregation is necessary for good flowability. Another method of measuring flowability is with a flow cone, ASTM C939. The mixture tested should not contain coarse aggregate retained on the No. 4 (4.75-mm) sieve. An efflux time of 10 to 26 seconds is generally recommended.
• Unit weight, yield and air content of flowable fill are measured by ASTM D6023.
• Preparing and testing cylinders for compressive strength is described in ASTM D4832. Use 3 x 6 in. (75 x 150 mm) plastic cylinder molds, fill to overflowing and then tap sides lightly. Other sizes and types of molds may be used as long as the length to diameter ratio is 2 to 1. Cure cylinders in the molds (covered) until time of testing (or at least 14 days). Strip carefully using a knife to cut plastic mold off. Capping with sulfur compounds can damage these low strength specimens. Neoprene caps have been used but high strength gypsum plasters seem to work best.
• Penetration resistance tests such as ASTM C403 may be useful in judging the setting and strength development. Penetration resistance numbers of 500 to 1500 indicate adequate hardening. A penetration value of 4000, which is roughly 100 psi (0.7 MPa) compressive cylinder strength, is greater than the bearing capacity of most compacted soil. Another method of testing for adequate hardening after placement is the ball drop test, ASTM D6024. A diameter of indentation of less than 3 in. (75 mm) is considered adequate for most load applications. A relationship between the strength gain of the flowable fill and the penetration resistance can be developed for specific mixtures.
1. Flowable fill while fluid is a heavy material and during placement will exert a high fluid pressure against any forms, embankment, or walls used to contain the fill.
2. Placement of flowable fill around and under tanks, pipes, or large containers, such as swimming pools, can cause the container to float or shift.
3. In-place fluid flowable fill should be covered or cordoned off for safety reasons.
1. Controlled Low Strength Materials, ACI 229R, American Concrete Institute, Farmington Hills, MI.
2. Recommended Guide Specification for CLSM (Flowable Fill), NRMCA Publication 2PFFGS, National Ready Mixed Concrete Association, Silver Spring, MD.
3. ASTM Book of Standards, Volumes 04.09 and 04.02, American Society for Testing and Materials, West Conshohocken, PA.
4. Controlled Low Strength Materials, ACI SP-150, ed. W.S. Adaska, American Concrete Institute, Farmington Hills, MI.
5. The Design and Application of Controlled Low-Strength Materials (Flowable Fill), ASTM STP 1331, ed. A.K. Howard and J.L. Hitch, American Society for Testing and Materials, West Conshohocken, PA.
6. Controlled Low-Strength Materials, W.S. Adaska, Concrete International, April 1997, pp. 41-43, American Concrete Institute, Farmington Hills, MI.
WHAT is Radon?
Radon is a colorless, odorless, radioactive gas which occurs naturally in soils in amounts dependent upon the geology of the location. The rate of movement of radon through the soil is dependent primarily upon soil permeability and degree of saturation, and differences in air pressure within the soil. Soil gas enters buildings through cracks or openings in the foundation, slab, or basement walls when the air pressure in the building is less than that of the soil.
Radon gas decays to other radioactive elements in the uranium series. Called “radon progeny,” they exist as solid particles rather than as a gas.
WHY be Concerned about Radon Levels in Buildings?
The concern is due to an association with the development of lung cancer. Radon progeny can become attached to dust particles in the air. If inhaled, they can lodge in the lung. Energy emitted during radioactive decay while in the lung can cause tissue damage, which has been linked to lung cancer.
The level of health risk associated with radon is related to the concentration of radon in the air and the time a person is exposed to that air. The U.S. Environmental Protection Agency (EPA) has developed a risk profile for radon exposure at various concentrations, and established an action level concentration above which efforts should be made to reduce radon levels.1 It is prudent to take measures during construction which will reduce the amount of radon entering a building.
HOW to Construct Radon Resistant Concrete Buildings:
Solid concrete is an excellent material for use in constructing radon resistant buildings. It is an effective barrier to soil gas penetration if cracks and openings are sealed.
Solid concrete slabs and basement walls are commonly used in residential buildings. Buildings resistant to radon may be easily constructed with concrete. In concrete construction, the critical factor is to eliminate all entry routes through which gases can flow from the soil into the building.
The construction of radon resistant buildings requires adhering to accepted construction practices with attention to a few additional details. In instances where high radon levels are expected, installation of a sub-slab ventilation system incorporating an open-graded aggregate base beneath the slab may be warranted during construction. These systems provide a positive means of evacuating soil gas from beneath the slab, diverting it directly to the outside.2,3
Follow these Guidelines to Reduce Radon Entry
1. Design to minimize utility openings. Sump openings should be sealed and vented outdoors.²
2. Minimize random cracking by using control and isolation joints in walls and floors. Planned joints can then be easily sealed.⁵ If done properly, any cracks will occur at the joints and can be easily sealed.
3. Monolithic slab foundations are an effective way to minimize radon entry.²,⁴,⁶ For slab on grade homes in warm climates, pour foundation and slab as a single monolithic unit.
4. Use materials which will minimize concrete shrinkage and cracking (larger aggregate sizes and proper water-cementitious ratio).
5. When using polyethylene film beneath the slab, place a layer of sand over the polyethylene. See CIP 5 and 7.
6. Remove grade stakes after striking off the slab. (If left, they can provide entryways through the slab.)²
7. Construct the joints to facilitate caulking.⁵
8. Cure the concrete adequately. See CIP 11.
9. Caulk and seal all joints and openings in the walls or floor. (If cracks occur, they should be widened, and then caulked and sealed.)²,³
1. “A Citizen’s Guide to Radon—What It Is and What To Do About It,” U.S. Environmental Protection Agency, OPA-86004, 1986, 13 pp. Available from state radiation protection offices or EPA regional offices.
2. “Radon Reduction in New Construction—An Interim Guide,” U.S. Environmental Protection Agency, OPA-87- 009, 1987, 7 pp. Available from EPA, (513) 569-7771.
3. “Radon Reduction Techniques for Detached Houses — Technical Guide,” U.S. Environmental Protection Agency, EPA/625/5-86/019,19N, 50 pp. Available from EPA Center for Environmental Research Information, (513) 569-7562.
4. “Production of Radon-Resistant Slab on Grade Foundations,” Florida Institute of Phosphate Research, Bartown, Florida, 1987, 9 pp.
5. “Guide to Residential Cast-In-Place Concrete Construction,” ACI 332R, American Concrete Institute, Farmington Hills, MI.
6. “Production of Radon-Resistant Foundations,” A. G. Scott and W. O. Findlay, American ATCON, Inc., Wilmington, Delaware, 1987, 54 pp. Available from NTIS, Alexandria, Virginia, PB89-116149/WBT, (703) 487-4650.
7. Technical information on radon-resistant construction is available from the National Association of Home Builders, National Research Center, Radon Research Program, (301) 249-4000.
WHAT is Curling?
Curling is the distortion of a slab into a curved shape by upward or downward bending of the edges. The occurrence is primarily due to differences in moisture and/or temperature between the top and bottom surfaces of a concrete slab. The distortion can lift the edges or the middle of the slab from the base, leaving an unsupported portion. The slab section can crack when loads exceeding its capacity are applied. Slab edges might chip off or spall due to traffic when the slab section curls upwards at its edges. In most cases, curling is evident at an early age. Slabs may, however, curl over an extended period.
WHY do Concrete Slabs Curl?
Changes in slab dimensions that lead to curling are most often related to moisture and temperature gradients in the slab. When one surface of the slab changes size relative to the other, the slab will warp at its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners. One primary characteristic of concrete that affects curling is drying shrinkage. Anything that increases drying shrinkage of concrete will tend to increase curling.
The most common occurrence of curling is when the top surface of the slab dries and shrinks with respect to the bottom. This causes an upward curling of the edges of a slab (Figure 1A). Curling of a slab soon after placement is most likely related to poor curing and rapid surface drying. In slabs, excessive bleeding due to high water content in the concrete or water sprayed on the surface, or a lack of surface moisture due to poor or inadequate curing, can create increased surface drying shrinkage relative to the bottom of the slab. Bleeding is accentuated in slabs placed directly on a vapor retarder (polyethylene sheeting) or when topping mixtures are placed on concrete slabs. Shrinkage differences from top to bottom in these cases are larger than for slabs on an absorptive subgrade.
Thin slabs and long joint spacing tend to increase curling. For this reason, thin unbonded toppings need to have a fairly close joint spacing.
In industrial floors, close joint spacing may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints.
The other factor that can cause curling is temperature differences between the top and bottom of the slab. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing a downward curling of the edges (Figure 1B). Alternately, during a cold night when the top surface cools and contracts relative to the bottom surface in contact with a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials.
HOW to Minimize Slab Curling:
The primary factors controlling dimensional changes of concrete that lead to curling are drying shrinkage, construction practices, moist or wet subgrades, and day-night temperature cycles. The following practices will help to minimize the potential for curling:
1. Use the lowest practical water content in the concrete.
2. Use the largest practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage.
3. Take precautions to avoid excessive bleeding. In dry conditions place concrete on a damp, but absorptive, subgrade so that all the bleed water is not forced to the top of the slab. This may not be appropriate for interior slabs on which a moisture sensitive floor covering would be placed.
4. Avoid using polyethylene vapor retarders unless covered with at least four inches (100 mm) of a trimable, compactible granular fill (not sand). If a moisture-sensitive floor covering will be placed on interior slabs, the concrete will generally be placed directly on a vapor retarder (see CIP 29) and other procedures may be necessary.
5. Avoid a higher than necessary cement content. Use of pozzolan or slag is preferable to very high cement content.
6. Cure the concrete thoroughly, including joints and edges. If membrane-curing compounds are used, apply at twice the recommended rate in two applications at right angles to each other.
7. When minimizing curling is critical, use a joint spacing not exceeding 24 times the thickness of the slab.
8. For thin toppings, clean the base slab to ensure bond and consider use of studs and wire around the edges and particularly in the slab corners.
9. Use a thicker slab, or increase the thickness of the slab at edges.
10. The use of properly designed and placed slab reinforcement may help reduce or eliminate curling. Load transfer devices that minimize vertical movement should be used across construction joints.
11. Certain types of breathable sealers or coatings on slabs can work to minimize moisture differentials and reduce curling.
When curling in a concrete slab application cannot be tolerated alternate options include the use of shrinkage reducing admixtures, shrinkage-compensating concrete, post tensioned slab construction or vacuum dewatering. These options should be decided before the construction and could increase the initial cost of the project.
Some methods of remedying slab curling include ponding the slab to reduce curl followed by sawing additional contraction joints, grinding slab joints where curling has occurred to restore serviceability and injecting a grout to fill voids under the slab to restore support and prevent break-off of uplifted edges.
1. Guide for Concrete Floor and Slab Construction, ACI 302.1R American Concrete Institute, Farmington Hills, MI www.concrete.org
2. Slabs on Grade, ACI Concrete Craftsman Series, CCS- 1 American Concrete Institute, Farmington Hills, MI.
3. Shrinkage and Curling of Slabs on Grade, Series in three parts, R. F. Ytterberg, ACI Concrete International, April, May and June 1987, American Concrete Institute.
4. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL, www.cement.org
5. Pavement Design, Transportation Research Record 1207, National Research Council, Washington, D.C., 1988, p. 44.
6. Controlling Curling and Cracking in Floors to Receive Coverings, J. Holland and W. Walker, Concrete Construction, July 1998, www.worldofconcrete.com.
7. Where to Place the Vapor Retarder, B. Suprenant and W. Malisch, Concrete Construction, May 1998.
8. Repairing Curled Slabs, B. Suprenant, Concrete Construction, May 1999.
9. Residential Concrete, National Association of Home Builders, Washington, DC, www.nahb.com.
WHAT are Delaminations?
In most delaminated concrete slab surfaces, the top ⅛ to ¼ inch (3 to 6 mm) is densified, primarily due to premature and improper finishing, and separated from the base slab by a thin layer of air or water. The delaminations on the surface of a slab may range in size from several square inches to many square feet. The concrete slab surface may exhibit cracking and color differences because of rapid drying of the thin surface during curing. Traffic or freezing may break away the surface in large sheets. Delaminations are similar to blisters, but much larger (see CIP 13).
Delaminations form during final troweling. They are more frequent in early spring and late fall when concrete is placed on a cool subgrade with rising daytime temperatures, but they can occur at any time depending on the concrete characteristics and the finishing practices used.
Corrosion of reinforcing steel near the concrete surface or poor bond between two-course placements may also cause delaminations (or spalling). The resulting delaminations are generally thicker than those caused by improper finishing.
Delaminations are difficult to detect during finishing but become evident after the concrete surface has set and dried. Delaminations can be detected by a hollow sound when tapped with a hammer or with a heavy chain drag. A procedure is described in ASTM D4580 – Standard Practice for Measuring Delaminations in Concrete Bridge Decks by Sounding. More sophisticated techniques include acoustic impact echo and ground-penetrating radar.
WHY does Delamination Occur?
Bleeding is the upward flow of mixing water in plastic concrete as a result of the settlement of the solids. Delamination occurs when the fresh concrete surface is sealed or densified by troweling while the underlying concrete is still plastic and continues to bleed and/or to release air. Delaminations form fairly late in the finishing process after floating and after the first troweling pass. They can, however, form during the floating operation if the surface is overworked and densified. The chances for delaminations are greatly increased when conditions promote rapid drying of the surface (wind, sun, or low humidity). Drying and higher temperature at the slab surface makes it appear ready to trowel while the underlying concrete is plastic and can still bleed or release air. Vapor retarders placed directly under slabs force bleed water to rise and compound the problem.
Factors that delay initial set of the concrete and reduce the rate of bleeding will increase the chances for delaminations. Entrained air in concrete reduces the rate of bleeding and promotes early finishing that will produce a dense impermeable surface layer. A cool subgrade delays set in the bottom relative to the top layer.
Delamination is more likely to form if:
1. The underlying concrete sets slowly because of a cool subgrade.
2. The setting of the concrete is retarded due to concrete temperature or mixture ingredients.
3. The concrete has entrained air or the air content is higher than desirable for the application.
4. The concrete mixture is sticky from higher cementitious material or sand-fines content.
5. Environmental conditions during placement are conducive to rapid drying causing the surface to “crust” and appear ready to finish.
6. Concrete is excessively consolidated, such as the use of a jitterbug or vibrating screed that brings too much mortar to the surface.
7. A dry shake is used, particularly with air-entrained concrete.
8. The slab is thick.
9. The slab is placed directly on a vapor retarder.
Corrosion-related delaminations are formed when the upper layer of reinforcing steel rusts thereby breaking the bond between the steel and the surrounding concrete. Corrosion of steel occurs with reduced concrete cover and when the concrete is relatively more permeable causing chlorides to penetrate to the layer of the steel (See CIP 25).
HOW to Prevent Delamination:
Accelerators or heated concrete often prevent delamination in cool weather.
Be wary of a concrete surface that appears to be ready to trowel before it would normally be expected. Emphasis in finishing should be on screeding, straight-edging, and floating the concrete as rapidly as possible— without working up an excessive layer of mortar and without sealing the surface layer. In initial floating, the float blades should be flat to avoid densifying the surface too early.
Final finishing operations to produce a smooth surface should be delayed as long as possible, and the surface covered with polyethylene or otherwise protected from evaporation.
Delamination may be difficult to detect during finishing operations. If delamination is observed, tear the surface with a wood float and delay finishing as long as possible. Any steps that can be taken to slow evaporation should help.
If a vapor retarder is required, place at least four inches (100 mm) of a trimable, compactible granular fill (not sand). Do not place concrete directly on a vapor retarder. If a moisture-sensitive floor covering will be placed on interior slabs, concrete will generally be placed directly on a vapor retarder (see CIP 29), and other procedures may be necessary.
Do not use air-entrained concrete for interior floor slabs that have a hard troweled surface and that will not be subject to freeze-thaw cycles or deicing salt application. If entrained air is necessary to protect interior slabs from freezing and thawing cycles during construction avoid using air contents over 3%.
Delaminated surfaces can be repaired by patching after the surface layer is removed and the underlying concrete is properly cleaned. Extensive delamination may need to be repaired by grinding and overlaying a new surface. Delaminated surfaces due to steel corrosion will additionally require sandblasting to remove rust from the steel.
Follow These Rules to Avoid Delamination
1. Do not seal surface early—before air or bleed water from below have escaped.
2. Avoid dry shakes on air-entrained concrete.
3. Use heated or accelerated concrete to promote even setting throughout slab depth.
4. Avoid placing concrete directly on vapor retarders, if the application allows.
5. Do not use air-entrained concrete for interior slabs that will receive a trowel finish.
6. Avoid placing concrete on substrate with a temperature of less than 40°F (4°C).
1. Guide for Concrete Floor and Slab Construction, ACI 302.1R American Concrete Institute, Farmington Hills, MI www.concrete.org
2. Slabs on Grade, ACI Concrete Craftsman Series, American Concrete Institute, Farmington Hills, MI.
3. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177, Portland Cement Association, Skokie, IL, www.cement.org
4. Diagnosing Slab Delaminations – Series in three parts, B. Suprenant, Concrete Construction, January, February and March 1998, www.worldofconcrete.com.
5. Using the Right Finishing Tool at the Right Time, R.H. Spannenberg, Concrete Construction, May 1996.
6. Concrete in Practice Series, NRMCA, Silver Spring, Mary- land, www.nrmca.org.
7. Residential Concrete, National Association of Home Builders, Washington, DC, www.nahb.com.
8. ASTM D 4580, Annual Book of ASTM Standards, Vol 04.03, ASTM International, West Conshohocken, PA, www.astm.org.
WHAT is Air Loss in Pumping?
Increasingly, specifiers are requiring concrete to be tested for air content at the discharge end of concrete pumps at the point of placement in the concrete structure. In some cases it is observed that air contents are much lower than that in samples tested at discharge from the truck chute. It is normal to find a loss of about 0.5 to 1.0 percent air as concrete is conveyed through a pump. However, with long boom pumps that have the boom in an orientation with a long, near vertical downward section of pipe, the air content at discharge may be less than half of that of the concrete going into the pump hopper. When the boom is upward or horizontal, or if there is a 12-ft (3.6-m) section of rubber hose placed horizontally at the discharge end, there generally is no significant loss of air. Certainly, air loss through a pump doesn’t occur every time. However, it does occur often enough to be considered seriously until better solutions are developed.
WHY is Air Lost?
There are several mechanisms involved, but air loss will occur if the weight of concrete in a vertical downward section of pipe is sufficient to overcome frictional resistance to allow a slug of concrete to slide down the pipe. As the slug of concrete slides down the pipe, it develops a vacuum on the upper end that greatly expands the size of the air bubbles; and when the concrete hits an elbow in the boom or a horizontal surface, the bubbles collapse. The effect of this impact can be demonstrated by dropping concrete 15 or 20 ft. (4.5 to 6 m). The loss of air can be further exacerbated due to the transition from a high pressure in the pump to a near vacuum condition in the pump line.
Most field experience suggests that air loss is greatest with high cement content, flowable concrete mixtures which slide down easier; however, air loss has also been experienced in mixtures with a moderate cement factor at about 500 lb./yd3 (300 kg/m3) and moderate slump. Loss of air content in pumped concrete will not reduce freeze thaw durability of concrete as long as the air void system is not compromised.
The air loss due to pumping can be determined by measuring the air content of samples discharged from the ready mixed concrete truck and at discharge from the pump. Testing concrete as discharged from the pump alongside the pump will require the most critical boom configuration that will cause the highest loss of air content. If concrete at a higher air content, to compensate for this loss, is placed at a less critical, more horizontal boom configuration, the concrete placed in the structure will be at a high air content and lower strength.
HOW to Prevent Air Loss:
To minimize the loss of air in concrete through a pump, procedures should attempt to keep concrete from sliding down the line under its own weight. Ensure that there is a continuous stream of concrete within the pump and inside the pump line. Where possible, avoid vertical or steep downward boom sections. Be cautious with high slump, and particularly with high cementitious content mixtures. Steady, moderately rapid pumping may help somewhat to minimize air loss, but will not solve most problems.
a) Try inserting a loop in the pipeline just before the rubber hose. (Do not do this unless pipe clamps are designed to comply with all safety requirements). This method helps, but won’t be a perfect solution. In some cases it may cause an increase in the air content.
b) Use a slide gate at the end of the rubber hose to restrict discharge and provide resistance.
c) Use of a 6-ft. (2-m) diameter loop in the rubber hose with an extra section of rubber hose is reported to be a better solution than (a) or (b).
d) Lay 10 or 20 ft. (3 to 6 m) of hose horizontally on deck pours. This doesn’t work in columns or walls and requires additional labor to manage the extra hose.
e) Reduce the rubber hose size from 5 to 4 in. (125 to 100 mm). A transition pipe of length 4 feet (1.2 m) or longer should be used to avoid blockages.
Conduct a pre-pour conference in accordance with the agenda outlined in CIP 32 with the contractor, pump operator, and ready mixed concrete supplier present. Discuss the necessity for care in pumping air-entrained concrete, and list the precautions to take when pumping air-entrained concrete. Maintain communication between all parties during the placement process.
a) Before the pour, plan alternative pump locations and decide what will be done if air loss occurs. Be prepared to test for air content frequently.
b) Sampling from the end of a pump line can be very difficult and potentially hazardous. Wear proper personal protective equipment. Never sample the initial concrete through the pump line. It is recommended that sampling be done from the concrete placed in the structure as opposed to the end of a pump line.
c) Sample the first load on the job after pumping 3 or 4 yd3 (2 to 3 m3). Temper it to the maximum permissible slump. Swing the boom over near the pump to get the maximum length of vertical downward pipe and drop the sample in a wheel barrow. If air is lost, take precautions and sample to measure air content at the point of placement.
d) If air loss occurs, do not try to solve the problem by increasing the air content delivered to the pump beyond the upper specification limit. High air content concrete with low strength could, or almost surely will, be placed in the structure if boom angles are reduced or somewhat lower slump concrete is pumped.
e) Research has indicated that when the loss of air content is not too high (less than about 3%), the air void system in the concrete may still be adequate for freeze-thaw resistance of concrete. This is because most of the air lost is the larger air bubbles that do not significantly affect the durability of concrete.
1. Yingling, James; G.M. Mullings; and R.D. Gaynor, “Loss of Air Content in Pumped Concrete,” Concrete International, Volume 14, Number 10, October 1992, pp. 57-61.
2. ACI 304.2R, “Placing Concrete by Pumping Methods,” American Concrete Institute, Farmington Hills, MI.
3. Hoppe, Julian J., “Air Loss in Free-Falling Concrete,” Queries on Concrete, Concrete International, June 1992, p. 79.
4. Gorsha, Russel P., “Air Loss in Free-Falling Concrete”, Queries on Concrete, Concrete International, August 1992, p. 71.
5. “Effects of Pumping Air Entrained Concrete,” Washington Aggregates and Concrete Association, March 20, 1991, 12 pp.
6. Dyer, R.M., “An Investigation of Concrete Pumping Pressure and the Effects on the Air Void System of Concrete,” Master’s Thesis, Department of Civil Engineering, University of Washington, Seattle, Washington, 1991.
7. Hover, K.C., Phares, R.J., “Impact of Concrete Placing Method on Air Content, Air-Void System Parameters, and Freeze-Thaw Durability”, Transportation Research Record 1532, Transportation Research Board, Washington, DC.
WHAT is Grout?
ACI defines grout as “a mixture of cementitious material and water, with or without aggregate, proportioned to produce a pourable consistency without segregation of the constituents.” Grout may also contain fly ash, slag, and liquid admixtures.
The terms grout and mortar are frequently used interchangeably but there are clear distinctions. Grout need not contain aggregate whereas mortar contains fine aggregate. Grout is supplied in a pourable consistency whereas mortar is not. Grout fills space whereas mortar bonds elements together, as in masonry construction.
Grout is often identified by its application. Some examples are: bonded pre-stressed tendon grout, auger cast pile grout, masonry grout, and pre-placed aggregate grout. Controlled low strength material (flowable fill) is a type of grout.
WHY is Grout Used?
Grout is used to fill space or cavities and provide continuity between building elements. In some applications, grout will act in a structural capacity, such as in reinforced masonry construction. In building construction, grout can improve fire ratings, acoustic performance, blast resistance and the thermal mass properties of the building elements.
In projects where small quantities of grout are required, it is proportioned and mixed on site. The ready mixed concrete producer is generally called upon when large quantities are needed.
HOW to Specify Grout:
For masonry grout, ASTM C476 provides prescribed proportions by loose volumes that are convenient for small quantities of grout mixed on site. Alternatively ASTM C476 has provisions for establishing grout proportions on the basis of specified compressive strength. The specified compressive strength must be at least 2000 psi. Grout mixtures meeting the proportion table of ASTM C476 have high cement contents and tend to produce much higher strengths than specified compressive strength requirements of ASTM C476, ACI 530 or Model Codes. Two types of masonry grouts are defined in ASTM C476: fine grout with aggregates smaller than ⅜ inch (9.9 mm) and coarse grout that allows aggregate sizes up to ½ inch (12.5 mm). Choice of grout type depends primarily on the clear dimensions of the space being filled by the grout. Grouting of masonry construction should comply with the governing building code provisions. Information on grouts for masonry construction is available from the National Concrete Masonry Association (NCMA).
When grout is ordered from a ready mixed concrete producer, the specifications should be based on consistency and compressive strength. Converting loose volume proportions into batch weights per cubic yard is subject to errors and can lead to controversies on the job.
Specifications should address the addition of any required admixtures for grout. Conditions of delivery, such as temperature, time limits, and policies on job site addition of water, should be specified. The contractor will need to ensure that the grout consistency is sufficiently flowable. Testing frequency and methods of acceptance must be covered in specifications.
HOW to Test Grout:
The consistency of grout affects its strength and other properties. It is critical that grout consistency permit the complete filling of void space without segregation of ingredients.
Consistency of masonry grout may be measured with a slump cone (ASTM C143), and slumps of 8-11 in. are generally required for both fine and coarse grout. Self-consolidating grout is a highly fluid and stable grout mix that does not require consolidation. These grouts are tested using the slump flow test, ASTM C1611, which measures the spread of the grout using the slump cone.
For other types of grouts without aggregate, or only fine aggregate passing a No. 8 sieve, consistency is best determined with a flow cone (ASTM C939). For flow values exceeding 35 seconds, use the flow table in ASTM C109, modified to use 5 drops in 3 seconds.
Masonry grout (“blockfill”) for strength tests specimens should be cast in molds formed by masonry units having the same absorption characteristics and moisture content as the units used in construction (ASTM C1019). Never use nonabsorbent cube or cylinder molds for this purpose.
Strength of other types of grout is determined using 2 in. cubes per ASTM C942. Method C942 allows for field preparation, recognizes fluid consistency, and also affords a means for determining compressive strength of grouts that contain expansive agents or grout fluidifiers. This is extremely important since “expansive” grouts can lose substantial compressive strengths if cubes are not confined. However, cylindrical specimens (6 x 12 in. or 4 x 8 in.), may give more reliable results for grouts containing coarse aggregate.
Special application grouts often require modification of standard test procedures. All such modifications should be noted in the specifications and discussed prior to the start of the job.
1. “Cement and Concrete Terminology,” ACI Committee 116R, ACI Manual of Concrete Practice, Part 1.
2. Cementitious Grouts and Grouting, S. H. Kosmatka, Portland Cement Association, 1990.
3. ASTM C476, “Standard Specification for Grout for Masonry,” and ASTM C 1019, “Standard Method of Sampling and Testing Grout”, Annual Book of ASTM Standards, Vol. 04.05, ASTM International, West Conshohocken, PA, www.astm.org
4. ASTM C143, C 939, C 942, C 1611, Annual Book of ASTM Standards, Vol 04.02.
5. ASTM C109, Annual Book of ASTM Standards, Vol. 04.01.
6. Hedstrom, E. G., and Hogan, M. B., “The Properties of Masonry Grout in Concrete Masonry,” Masonry: Components to Assemblages, ASTM STP 1063, ed. John H. Matthys, 1990, pp. 47-62.
7. “Building Code Requirements for Masonry Structures (ACI 530.1-05/ASCE 6-05/TMS 602-05) and Specifications for Masonry Structures (ACI 530.1-05/ASCE 6-05/TMS 602- 05),” ACI-ASCE Standards, American Concrete Institute/American Society of Civil Engineers/The Masonry Society, 2005.
8. NCMA TEK 9-4A, Grout for Concrete Masonry, TEK 3-2A, Grouting Concrete Masonry Walls and TEX 18-8B, Grout Quality Assurance, National Concrete Masonry Association, Herndon, VA, www.ncma.org.
WHAT is Discoloration?
Surface discoloration is the non-uniformity of color or hue on the surface of a single concrete placement. It may take the form of dark blotches or mottled discoloration on flatwork surface, gross color changes in large areas of concrete caused by a change in the concrete mix, or light patches of discoloration caused by efflorescence. In this context, it is not intended to include stains caused by foreign material that comes in contact with the concrete surface after placement and curing, such as storm water runoff, irrigation, corrosion products, oil leaking from automobiles, etc.
WHY does Discoloration Occur?
Discoloration due to changes in cementitious materials or fine aggregate sources in subsequent batches in a placement sequence could occur, but is generally rare and insignificant. Concrete with a higher water to cementitious materials ratio (w/cm) will generally be lighter in color. Inconsistent use of admixtures, insufficient mixing time, and improper timing of finishing operations can also cause discoloration. A yellowish to greenish hue may appear on concrete containing ground slag as a cementitious material. This will disappear with time, generally within a one year period. Concrete containing slag cement does, however, have a generally lighter color. The discoloration of concrete cast in forms or in slabs on grade is usually the result of a change in either the concrete composition or a concrete construction practice. In most studies, no single factor seemed to cause discoloration.
Factors found to influence discoloration are: the use of calcium chloride, variation in cement alkali content, delayed hydration of the cement paste, admixtures, hard-troweled surfaces, inadequate or inappropriate curing, concreting practices and finishing procedures that cause surface variation of the water-cementitious materials ratio, and changes in the concrete mixture proportions or constituents.
HOW to Avoid Discoloration:
1. Calcium chloride in concrete can cause concrete discoloration. Flake or pelletized calcium chloride, when not mixed uniformly, discolors more than liquid calcium chloride.
2. The type, kind, and condition of formwork can influence surface color. Forms with different rates of absorption will cause surfaces with different shades of color. A change in the type or brand of a form release agent can also change concrete color.
3. Eliminate trowel burning (hard troweling of surface after it has become too stiff to trowel properly) of the concrete. Concrete which has been hard-troweled may have dark discoloration as a result of densifying the surface, which reduces the w/cm. The resulting low w/cm affects the hydration of the cement ferrites which contributes to a darker color. Concrete surfaces that are troweled too early will increase the w/cm at the surface and lighten the color.
4. Concrete which is not properly or uniformly cured may develop discoloration. Uneven curing will affect the degree of hydration of the cement. Curing with polyethylene may also cause discoloration. When portions of the plastic sheeting are in direct contact with the concrete while other portions are not, it will cause variations in color. Using an even application of a quality spray or curing compound may be the better alternative.
5. The discoloration of a slab may be minimized or prevented by moistening absorptive subgrades, following proper curing procedures, and adding proper protection of the concrete from drying by the wind and sun.
HOW to Remove Discoloration:
Certain treatments have been found to be successful in removing or decreasing the surface discoloration of concrete flatwork. Discoloration caused by calcium chloride admixtures and some finishing and curing methods can be reduced by repeated washing with hot water and a scrub brush. The slab should be alternately flushed and brushed, and then dried overnight until the discoloration disappears.
If a discoloration persists, a dilute solution (1% concentration) of hydrochloric (muriatic) acid or dilute solutions (3% concentration) of weaker acids like acetic or phosphoric acid may be tried. Prior to using acids, dampen the surface to prevent it from penetrating into the concrete and flush with clean water within 15 minutes of application.
The use of a 20% to 30% water solution of diammonium citrate (2 lbs. in 1 gallon of water) has been found to be a very effective treatment for most discoloration. Apply the solution to a dried surface for 15 minutes. A whitish gel that forms should be diluted with water and agitated by brushing. Subsequently, the gel should be completely washed off with water. More than one treatment may be required.
Some types of discoloration, such as trowel burning, may not respond to any treatment. It may be necessary to paint or use another type of coating to eliminate the discoloration. Some types of discoloration may, however, fade with wear and age.
Chemical methods to remove discoloration may significantly alter the color of concrete surfaces. Inappropriate or improper use of chemicals to remove discoloration may aggravate the situation. A trial treatment on an inconspicuous area is recommended. Acids should be thoroughly flushed from a concrete surface.
The user of chemicals should refer to a Material Safety Data Sheet (MSDS) or manufacturer guidelines to be aware of the toxicity, flammability, and/or health hazards associated with the use of the material. The appropriate safety procedures, such as the use of chemical resistant gloves, goggles, respirators, and chemical resistant clothing may be required in the MSDS.
1. Greening, N. R. and Landgren, R., Surface Discoloration of Flatwork, Portland Cement Association RD 203, 1966.
2. Steve Kosmatka, Discoloration of Concrete, Causes and Remedies, Concrete Products, April 1987.
3. Neal, R. E., Discoloration of Concrete Flatwork, Lehigh Portland Cement Company, 1977.
4. “Removing Stains and Cleaning Concrete Surfaces,” IS214.02T, PCA, 1988, www.cement.org.
5. “Discoloration: Myths, Causes and Cures,” Rech, D. P., Owl Rock Products, 1989.
6. C.F. Ferrington, R.L. Stowe, W.G. Miller, Removing Stains from Mortar and Concrete, Corps of Engineers, Vicksburg, Mississippi, Miscellaneous Paper C-68-8.
7. Eugene, O. Goeb, Discolored Concrete Surfaces, Concrete Products, Vol. 96, No. 2, February, 1993.
8. Concrete Slab Surface Defects: Causes, Prevention, Repair, IS177.04T, PCA, 2001, www.cement.org.
WHAT are Synthetic Fibers?
Synthetic fibers engineered for use in concrete can withstand the long-term alkaline environment of concrete. These fibers are manufactured polymer-based materials such as polypropylene, nylon, or polyethylene. Synthetic fibers are added to concrete before or during the mixing operation. The use of synthetic fibers at typical addition rates of 1 to 2 lbs./yd3 does not require any modification to concrete mixtures. At higher addition rates, workability may be reduced and water-reducing admixtures may be required to retain slump.
WHY Use Synthetic Fibers?
Synthetic fibers benefit the concrete in both the plastic and hardened state. Benefits include:
• reduced plastic settlement cracks
• reduced plastic shrinkage cracks
• increased toughness and impact resistance
• provides energy absorption
Macro-synthetic fibers, typically at a higher dosage rate, can be used for crack control in hardened concrete or temperature/shrinkage reinforcement in some applications. Documentation on the use of fibers for these applications should be available.
HOW do Synthetic Fibers Work in Early-Age Concrete?
Early-age volume changes in concrete cause weakened planes and cracks to form due to stresses that exceed the strength of the concrete at a specific time. This is beneficial to minimize plastic shrinkage cracking. The growth of these micro shrinkage cracks is inhibited by mechanical blocking action of the synthetic fibers. The internal support system of the synthetic fibers inhibits the formation of plastic settlement cracks. The uniform distribution of fibers throughout the concrete discourages the development of large capillary channels caused by bleed water migration to the surface. These bleed water capillaries can provide locations for later age cracking.
HOW do Synthetic Fibers Work in Hardened Concrete?
Benefits seen to early-age performance of concrete continue to contribute to the performance of hardened concrete. Prevention of early age cracking in the freshly mixed stage reduces the potential for increased cracking in the hardened state. Hardened concrete attributes provided by synthetic fibers are improved toughness for energy absorption and resistance to impact forces.
The ability to resist tensile forces can be enhanced with the use of synthetic fibers to the concrete. When plain concrete develops tensile stresses that exceeds its tensile strength, due to bending or changes in temperature and shrinkage, cracking occurs. Synthetic fibers can prevent the effect of excessive tensile stresses by bridging and dispersing cracks and holds concrete tightly together. These benefits are enhanced with the use of a higher dosage than typically used for control of plastic shrinkage cracking.
Macro-synthetic fibers reduce the amount of plastic (early age) and post-hardening crack formation. Macro-synthetic fibers are thicker fibers and are used at a higher dosage rate of around 5 lbs./yd3. In these uses and with the higher modulus of these fibers improves toughness, resistance to cracking and crack tightness.
Synthetic fibers help concrete develop its optimum long-term integrity by the reduction of plastic and drying shrinkage crack formation, increased energy absorption and resistance to impact forces. Synthetic fibers are compatible with chemical admixtures, pozzolans, slag cement, silica fume, metakaolin, and cement chemistries.
HOW are Synthetic Fibers Used as Secondary Reinforcement?
Synthetic fibers which meet certain hardened concrete criteria can be used as non-structural temperature/shrinkage or post-crack control reinforcement. These fibers should have documentation, including ASTM C1609 test results of residual flexural strength confirming their ability to hold concrete together after cracking. The uniform distribution of synthetic fibers throughout the concrete ensures the critical positioning of its use as secondary reinforcement.
Fibers used to control plastic shrinkage cracks, reducing shrinkage and temperature cracking, and in composite steel deck construction should meet the criteria of ICC Evaluation Service AC 32 (ref. 5).
Use Synthetic Fibers For:
• Reduction of concrete cracking as a result of plastic shrinkage.
• An alternate system of nonstructural shrinkage/temperature reinforcement (with documentation).
• Greater toughness and resistance to impact.
• Internal support and cohesiveness; concrete for steep inclines, shotcrete, and slip-formed placements.
• Reduction of concrete cracking as a result of plastic settlement.
• Applications where nonmetallic materials are required.
Do Not Use Synthetic Fibers For:
• Control of cracking as a result of external forces.
• Higher structural compressive or flexural strength development.
• Replacement of any movement-resisting or structural steel reinforcement.
• Decreasing the thickness of slabs on grade.
• The elimination or reduction of curling and/or creep.
• Increasing control joint spacing.
• Reduction in the size of the support columns.
• Reducing the thickness of bonded or unbonded overlay sections.
1. ASTM C1116, Specification Fiber Reinforced Concrete, ASTM International, West Conshohocken, PA, www.astm.org.
2. ASTM C1609, Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third- Point Loading), ASTM International, West Conshohocken, PA, www.astm.org.
3. ACI 544.1R, Report on Fiber Reinforced Concrete, American Concrete Institute, Farmington Hills, MI, www.concrete.org
4. Non-structural Cracks in Concrete, Concrete Society Technical Report No. 22.
5. ICC Evaluation Service, Inc., AC 32, Acceptance Criteria for Concrete with Synthetic Fibers, December 2010.
WHAT is Corrosion of Steel?
ASTM terminology defines corrosion as the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties. For corrosion of steel, oxygen and moisture are required for the electrochemical reaction to occur. Corrosion results in the formation of rust that has two to four times the volume of the original steel and none of its good mechanical properties. Corrosion also produces pits or holes in the surface of reinforcing steel, reducing strength capacity as a result of the reduced cross-sectional area.
WHY is Corrosion of Steel a Concern?
Structural concrete uses reinforcing steel where tensile stresses are anticipated. This provides structural capacity to members subjected to tensile and flexural loads due to traffic, winds, dead loads, and thermal cycling. However, when reinforcement corrodes, the larger volume of rust formed leads to internal stresses and subsequent delamination and spalling of the concrete cover. Reduction in the cross-sectional area of steel reduces the structural capacity of the member. If left unchecked, the integrity of the structure can be affected. Corrosion is especially detrimental to the performance of tensioned strands in pre-stressed concrete as failure can be catastrophic.
WHY Does Steel in Concrete Corrode?
Steel embedded in concrete is in a non-corroding, passive condition because of the high alkalinity (pH>13) within concrete. However, when water-soluble chlorides are present, the passive layer protecting steel is disrupted and corrosion begins. Chlorides can be from external sources for concrete exposed to severe environments, like sea water or when deicing salts are applied, or from internal sources, primarily from materials used to make concrete.
Carbonation of concrete is another cause of steel corrosion. Atmospheric carbon dioxide reacts with lime in the concrete to form calcium carbonate. This reaction reduces the alkalinity of the concrete that protects the steel. When the pH at the level of the reinforcing steel falls below 9, corrosion begins. Chloride-induced corrosion is more common than that resulting from carbonation.
Corrosion is aggravated by factors including moisture, high temperatures, cracking, stray currents and galvanic effects.
HOW to Prevent Corrosion:
Corrosion prevention strategies should ensure that reinforcing steel is embedded in good quality concrete with the minimized potential for chloride exposure and carbonation.
ACI 318 Building Code for Structural Concrete establishes exposure classes related to corrosion of reinforcing steel:
• C0—Concrete that will be dry in service
• C1—Concrete that will be exposed to moisture in service
• C2—Concrete that will exposed to moisture and an external source of chlorides in service
For exposure class C2, ACI 318 establishes a maximum water to cementitious materials ratio (w/cm) of 0.40 and minimum specified strength of 5000 psi. No w/cm limit is set for exposure classes C0 and C1 because penetration of external chlorides is not a concern. Good quality concrete, however, reduces the rate of carbonation.
Chloride limits are established for internal sources of water-soluble chlorides based on percent by weight of cement. For reinforced concrete, the limits are 1.0% for C0; 0.3% for C1; and 0.15% for C2. For pre-stressed concrete the limit is 0.06% for all exposure classes. Water soluble chlorides are measured in accordance with ASTM C1218 on powder specimens extracted from concrete cylinders at an age between 28 and 42 days.
Adequate cover over reinforcing steel is necessary. Increasing cover reduces the rate of chloride penetration and carbonation exponentially and delays the onset of corrosion. Minimum cover requirements in ACI 318 should be increased for concrete exposed to corrosive environments. Concrete containing larger aggregates require more cover. Adequate reinforcement should be provided to keep cracks tight. ACI 224 provides guidance to minimize the formation of cracks. Allowable crack widths for concrete exposed to chlorides are about 0.006-in. Adequate drainage of water away from concrete members should be ensured.
Chloride ingress can be reduced by using membranes and sealers. Onset of corrosion can be minimized or delayed by using corrosion resistant reinforcement, such as stainless steel, galvanized steel and epoxy-coated steel.
Life-365 is available software that models the expected service life and costs of different corrosion protection strategies. It can be used to demonstrate lower life cycle cost with higher initial cost of some options.
Quality concrete with a low permeability slows down the penetration of chloride salts and the development of carbonation. Low permeability can be obtained with a lower w/cm ratio in the range of 0.40 to 0.50. A w/cm much less than 0.40 may result in problems with placement and increase the potential for thermal and drying shrinkage cracking. Another factor that reduces the permeability of concrete is the use of supplementary cementitious materials (SCM). Typical dosage in percent by weight of cementitious materials is 5% silica fume, 25% fly ash and 50% slag cement and combinations thereof. Low permeability of concrete mixtures can be demonstrated by indicator tests. Excessive cementitious materials increases the volume of paste and the potential for cracking. Concrete materials should not contribute chlorides to the mixture that exceed the chloride limits. Concrete exposed to freezing should be air-entrained.
Corrosion inhibiting admixtures delay the onset of corrosion. Water repellent materials may reduce the ingress of moisture and chlorides to a limited extent in low permeability concrete.
Delamination, cracking and scaling accelerate corrosion of reinforcing steel. Placement and finishing should be properly scheduled with adequate crew and resources. Concrete must be adequately consolidated and cured. Curing should be performed preferably for at least 7 days. Concrete temperature should be maintained above 50°F. Early-age curing is especially important for concrete mixtures containing SCM. Numerous studies show that concrete porosity is reduced significantly with increased curing times and, correspondingly, corrosion resistance is improved.
HOW to Minimize Corrosion:
1. Evaluate the anticipated exposure of concrete members and establish appropriate requirements.
2. Use good quality concrete with SCM and a w/cm of about 0.40, when concrete will be exposed to chlorides.
3. Provide adequate cover to reinforcing steel.
4. Ensure that the concrete is adequately cured.
5. For critical structural members requiring long service life, consider advanced corrosion protection strategies.
1. Building Code Requirements for Reinforced Concrete, ACI 318, American Concrete Institute, Farmington Hills, MI. www.concrete.org
2. ACI 222R, Corrosion of Metals in Concrete, American Concrete Institute, Farmington Hills, MI.
3. ACI 224R, Control of Cracking in Concrete Structures, American Concrete Institute, Farmington Hills, MI.
4. ASTM Standards C 1218, ASTM Book of Standards, Volume 04.02, American Society for Testing and Materials, West Conshohocken, PA. www.astm.org
5. Berke, N.S., Corrosion of Reinforcing Steel, ASTM STP 169D, 2006, pp. 164-173. www.astm.org
6. Berke, N.S., “Corrosion Inhibitors in Concrete,” Concrete International, Vol. 13, No. 7, 1991, pp. 24-27.
7. Life-365 Software, www.life-365.org
WHAT is Jobsite Addition of Water?
This is the addition of water to ready mixed concrete in a truck mixer after arrival at the location of the concrete placement. Such tempering of concrete may be done with a portion of the design mixing water which was held back during the initial mixing (referred to as trim water), or with water in excess of the design mixing water, at the request of the purchaser. The design mixing water is the quantity of water set by the mixture proportions for required performance of the concrete.
WHY is Water Added at the Jobsite?
Water is added to concrete at the jobsite to increase its slump. When concrete arrives at the jobsite at a slump that is lower than that allowed by design or specification and/or is of such consistency so as to adversely affect the placeability of the concrete, water can be added to the concrete to bring the slump up to an acceptable or specified level. This can be done when the truck arrives on the jobsite provided the specified slump and/or water-cementitious materials ratio (w/cm) is not exceeded. Such an addition of water is in accordance with ASTM C94 – Specification for Ready Mixed Concrete.
The ready mixed concrete supplier establishes the proportions of materials for concrete mixtures according to industry standards to provide the intended performance. Addition of water in excess of the design mixing water will affect concrete properties, such as reducing strength (Figure 1), and increasing its susceptibility to cracking. If the purchaser requests additional water, in excess of the design mixing water, the purchaser assumes responsibility for the resulting concrete quality. The alternative of using a water reducing admixture or superplacticizer to increase concrete slump should be considered. Increasing the slump of concrete using admixtures usually will not alter concrete properties provided the mixture does not segregate. Consistent use of admixtures at the jobsite can reduce batch to batch variability. This option should be decided at a pre-pour conference as qualified personnel may need to be available at the jobsite.
HOW to Add Water at the Jobsite:
a. The maximum allowable slump of the concrete must be specified or determined from the specified nominal slump plus tolerance.
b. Prior to discharging concrete on the job, the actual slump of the concrete must be estimated or measured. If slump is measured, it should be on a preliminary sample obtained after discharging the first ¼ yd3 [0.2 m3]. The measured slump on this sample should be used as an indicator of concrete consistency and not an acceptance test. Tests for acceptance of concrete should be on samples obtained in accordance with ASTM C172.
c. At the jobsite, water should be added before any significant quantity of concrete has been discharged from the batch so that the volume of concrete being retempered is known. Water addition can be in several increments accompanied by mixing to evaluate change in slump.
A rule of thumb that works reasonably well is—1 gallon, or roughly 10 lb., of water per cubic yard for 1 inch increase in slump [5 liters, or 5 kg, of water per cubic meter for 25 mm increase in slump].
d. All water added to concrete on the jobsite must be measured and recorded on the delivery ticket. A designated representative of the purchaser should sign or initial the delivery ticket to acknowledge the water addition and the quantity added.
e. ASTM C94 requires an additional 30 revolutions of the mixer drum at mixing speed after the addition of water. In some cases, 10 revolutions will be sufficient if the truck is able to mix at 20 revolutions per minute (rpm) or faster.
f. The amount of water added should be controlled so that the maximum slump and/or w/cm, as indicated in the specification, is not exceeded. After more than a small portion of the concrete is discharged, no water addition is permitted.
g. Upon obtaining the desired slump and/or maximum w/cm, no further addition of water on the jobsite is permitted.
h. A pre-placement conference should be held to establish proper procedures to be followed, to determine who is authorized to request a water addition, and to define the method to be used for documentation of water added at the jobsite.
To ensure that the design mixing water or specified w/cm is not exceeded, it is good practice for the concrete supplier to indicate on the delivery ticket the amount of trim water held back when concrete was batched. This sets the limit of the jobsite water addition.
When project specifications prohibit the jobsite addition of water, the concrete supplier should be notified so that the design mixing water can be added at the plant and provisions made to adjust the slump of concrete at the jobsite, if necessary, with the use of admixtures.
Some truck mixers are equipped with automated devices that monitor slump of concrete and add water to maintain a target slump. This occurs while the concrete is being transported to the jobsite. The device should be able to record the amount of water added and to terminate the addition based on set limits. ASTM C94 recognizes the use of these systems.
ASTM C94 Jobsite Water Addition
1. Establish the maximum allowable slump and water content permitted by the specification.
2. Estimate or determine the concrete slump from the first portion of concrete discharged from the truck.
3. Add an amount of water such that the maximum slump or w/cm, according to the specification or designed mixture proportions, is not exceeded.
4. Measure and record the amount of water added. Water in excess of that permitted should be authorized by a designated representative of the purchaser. Purchaser should initial the ticket.
5. Mix the concrete for 30 revolutions of the mixer drum at mixing speed.
6. Do not add water if:
a. the maximum w/cm is reached,
b. the maximum slump is obtained, or
c. more than about ¼ yd3 (0.2 m3) has been discharged from the mixer.
1. ASTM C94, Specification for Ready Mixed Concrete, ASTM International, West Conshohocken, PA.
2. NRMCA Publication 186, Ready Mixed Concrete, Richard D. Gaynor and Colin Lobo, NRMCA, Silver Spring, Maryland.
3. Checklist for Pre-Construction Conference, Joint publication of ASCC and NRMCA, NRMCA, Silver Spring, MD.
4. NRMCA Publication 188, Truck Mixer Driver’s Manual, NRMCA, Silver Spring, MD.
5. Adding Water to the Mix: It’s Not all Bad, Eugene O. Goeb, Concrete Products, January 1994.
6. Adjusting Slump in the Field, Bruce A. Suprenant, Concrete Construction, January 1994.
7. Slump Retention of Fly Ash Concrete With and Without Chemical Admixtures, Dan Ravina, ACI Concrete International, April 1995.
WHAT is Cold Weather?
Cold weather is defined as a period when for more than 3 consecutive days the average daily temperature is less than 40°F [5°C] and the air temperature is not more than 50°F [10°C] for more than one-half of any 24-hr period. These conditions warrant special precautions when placing, finishing, curing and protecting concrete against the effects of cold weather. Since weather conditions can change rapidly in the winter months, good concrete practices and proper planning are critical.
WHY Consider Cold Weather?
Successful cold-weather concreting requires an understanding of the various factors that affect concrete properties.
In its fresh state concrete freezes if its temperature falls below about 25°F [-4°C]. The potential strength of frozen concrete can be reduced by more than 50% and it will not be durable. Concrete should be protected from freezing until it attains a compressive strength of 500 psi [3.5 MPa] – about two days after placement.
Concrete at a low temperature has a slower setting and rate of strength gain. A rule of thumb is that a drop in concrete temperature by 20°F [10°C] will approximately double the setting time. These factors should be accounted for when scheduling construction operations, such as form removal.
Concrete that will be in contact with water and exposed to cycles of freezing and thawing should be air-entrained. Newly placed concrete is saturated with water and should be protected from cycles of freezing and thawing until it has attained a compressive strength of at least 3500 psi [24.0 MPa].
The reaction between cement and water, called hydration, generates heat. Insulating concrete retains heat and maintains favorable curing temperatures. Temperature differences between the surface and the interior of concrete should be controlled. Thermal cracking may occur when the difference exceeds about 35°F [20°C]. Insulation or protective measures should be gradually removed to avoid thermal shock.
HOW to Place Concrete in Cold Weather:
Recommended concrete temperatures at the time of placement are shown below. The ready mixed concrete producer can control concrete temperature and furnish concrete to comply.
Concrete temperature should not exceed these temperatures by more than 20°F [10°C]. Concrete at a higher temperature requires more mixing water, has a higher rate of slump loss, and is more susceptible to cracking. Concreting in cold weather provides the opportunity for better quality, as cooler initial concrete temperature will typically result in higher ultimate strength and improved durability.
In cold weather, slower setting time and rate of strength gain of concrete can delay finishing operations and form removal. Chemical admixtures and other materials can be used to offset these effects. Accelerating admixtures, conforming to ASTM C494 – Types C (accelerating) and E (water-reducing and accelerating) – are commonly used.
Calcium chloride is an effective accelerating admixture, but should not exceed a dosage of 2% by weight of cement. Non-chloride, non-corrosive accelerators should be used for pre-stressed concrete or when corrosion of steel reinforcement or metal in contact with concrete is a concern. Accelerating admixtures do not prevent concrete from freezing and their use does not preclude the requirements for appropriate curing and protection from freezing.
Rate of setting and strength gain increases by increasing Portland cement content or by using a Type III cement (high early strength). The quantity of fly ash or slag cement in concrete may be reduced in cold weather for a similar effect. This may not be possible if a minimum quantity of SCM is required for durability. The selected solution should be economical and not compromise on the required concrete performance.
Concrete should be placed at the lowest practical slump. Adding water to achieve slump can delay setting time and prolong the duration of bleeding, thereby impacting finishing operations.
Adequate preparations should be made prior to concrete placement. Snow and ice should be removed and the temperature of surfaces and metallic embedments in contact with concrete should be above freezing. This might require insulating or heating subgrades and contact surfaces prior to placement.
Materials and equipment should be in place to protect concrete from freezing temperature and for adequate curing, both during and after placement. Insulated blankets and tarps, as well as straw covered with plastic sheets, are commonly used measures. Enclosures and insulated forms may be needed for additional protection depending on ambient conditions. Corners and edges are most susceptible to heat loss. Fossil-fueled heaters in enclosed spaces should be vented for safety reasons and to prevent carbonation of newly placed concrete surfaces, which causes dusting.
The concrete surface should not be allowed to dry before it sets as this can cause plastic shrinkage cracks. Subsequently, concrete should be adequately cured. Water curing is not recommended when freezing temperatures are imminent. Use membrane-forming curing compounds or impervious paper and plastic sheets for concrete slabs.
Forming materials, except for metals, maintain and evenly distribute heat and provide adequate protection in moderately cold weather. In extremely cold temperatures, insulating blankets or forms should be used, especially for thin sections. Forms should not be stripped for 1 to 7 days depending on rate of strength gain, ambient conditions, and anticipated loading on the structure. Field-cured cylinders or nondestructive methods should be used to estimate in-place concrete strength prior to stripping forms or applying loads. Removal of protective measures and formwork should not cause thermal shock to the concrete.
Concrete test specimens used for acceptance of concrete should be carefully managed. In accordance with ASTM C31, cylinders should be stored in insulated containers, which may need temperature controls, to insure that they are cured at 60°F to 80°F [16°C to 27°C] for the first 24 to 48 hours. A minimum/maximum thermometer should be placed in the curing box to maintain a temperature record.
Cold Weather Concreting Guidelines:
1. Use air-entrained concrete when exposure to moisture and freezing and thawing conditions are expected.
2. Keep surfaces in contact with concrete free of ice and snow and at a temperature above freezing prior to placement.
3. Place and maintain concrete at the recommended temperature.
4. Place concrete at the lowest practical slump.
5. Protect fresh concrete from freezing or drying.
6. Protect concrete from early-age freezing and thawing cycles until it has attained adequate strength.
7. Limit rapid temperature changes when protective measures are removed.
1. Cold Weather Concreting, ACI 306R, American Concrete Institute, Farmington Hills, MI.
2. Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, IL.
3. ASTM C94 Standard Specification for Ready Mixed Concrete, ASTM, West Conshohocken, PA.
4. ASTM C31 Making and Curing Concrete Test Specimens in the Field, ASTM, West Conshohocken, PA.
5. Cold-Weather Finishing, Concrete Construction, November 1993.
WHAT is the Problem?
Concrete slab moisture can cause problems with the adhesion of floor-covering material, such as tile, sheet flooring, or carpet and bond-related failures of non- breathable floor coatings. Many adhesives used for installation of floor coverings are more water-sensitive than in the past, due to restrictions on the use of volatile organic compounds (VOCs). To warranty their products, manufacturers require that the moisture emission from the hardened concrete slab be less than some threshold value prior to installing floor coverings or coatings. Fast-track construction schedules exacerbate the problem when floor-surfacing material is installed before the concrete slab has dried to an acceptable level.
WHAT are the Sources of Concrete Slab Moisture?
a. Ground water sources and when the floor slab is in contact with saturated ground, or if drainage is poor. Moisture moves to the slab surface by capillary action or wicking. Factors affecting this include depth of the water table and fineness of soil below the slab. Fine grained soil promotes moisture movements from considerable depths compared to coarser subgrade material.
b. Water vapor from damp soil will diffuse and condense on a concrete slab surface that is cooler and at a lower relative humidity due to a vapor pressure gradient.
c. Wetting of the fill course/blotter layer, if any, between the vapor retarder and the slab prior to placing the slab will trap moisture with the only possible escape route being through the slab. A blotter layer is not recommended for interior slabs on grade (CIP 29).
d. Residual moisture in the slab from the original concrete mixing water will move towards the surface. It may take anywhere from six weeks to one year or longer for a concrete slab to dry to an acceptable level under normal conditions, as illustrated in Figure 1. Factors that affect the drying rate include the original water content of the concrete, type of curing, and the relative humidity and temperature of the ambient air during the drying period. This is the only source of moisture in elevated slabs. Any wetting of the slab after final curing will elevate moisture levels within the slab and lengthen the drying period.
HOW do You Avoid Problems?
Avoiding problems associated with high moisture content in concrete can be accomplished by the following means:
• Protect against ingress of water under hydrostatic pressure by ensuring that proper drainage away from the slab is part of the design.
• Use a 6 to 8 inch [150 to 200 mm] layer of coarse gravel or crushed stone as a capillary break in locations with fine-grained soil subgrades.
• Use a vapor retarder membrane under the slab to prevent water from entering the slab. Ensure that the vapor retarder is installed correctly and not damaged during construction. Current recommendation of ACI Committee 302 is to place the concrete directly on a vapor retarder for interior slabs on grade (CIP 29).
• Use a concrete mixture with a moderately low water-cementitious material (w/cm) ratio (about 0.50). This reduces the amount of residual moisture in the slab, will require a shorter drying period, and result in a lower permeability to vapor transmission. Water reducing admixtures can be used to obtain adequate workability and maintain a low water content. The water tightness of concrete can be improved by using fly ash or slag in the concrete mixture.
• Curing is an important step in achieving excellent hardened concrete properties. However, moist curing will increase drying time. As a compromise, curing the concrete under plastic sheeting for 3 days is recommended and moist curing times greater than 7 days must be avoided. Avoid using curing compounds on floors where coverings or coatings will be installed.
• Allow sufficient time for the moisture in the slab to dry naturally while the floor is under a roof and protected from the elements. Avoid maintenance and cleaning operations that will wet the concrete floor. Use heat and dehumidifiers to accelerate drying. Since moisture transmission is affected by temperature and humidity, maintain the actual service conditions for a long enough period prior to installing the floor covering.
• Test the slab moisture condition prior to installing the floor covering.
When concrete slab moisture cannot be controlled, consider using decorative concrete, less moisture-sensitive floor coverings, breathable floor coatings, or install moisture vapor suppression systems (topical coatings).
HOW is Concrete Slab Moisture Measured?
Various qualitative and quantitative methods of measuring concrete slab moisture are described in ASTM E1907. Test the moisture condition of the slab in the same temperature and humidity conditions as it will be in service. In general, test at three random sample locations for areas up to 1000 ft2 [100 m2] and perform one additional test for each additional 1,000 ft2 Ensure that the surface is dry and clean. Record the relative humidity and temperature at the time of testing. Some of the common tests are:
Polyethylene Sheet Test (ASTM D4263) – is a simple qualitative test, where an 18 by 18 inch [450 by 450 mm] square plastic sheet is taped tightly to the concrete and left in place for at least 16 hours. The presence of moisture under the plastic sheet is a positive indication that excess moisture is likely present in the slab. However, a negative indication is not an assurance that the slab is acceptably dry below the surface.
Mat Test – where the adhesive intended for use is applied to a 24 by 24 inch [600 by 600 mm] area and a sheet vinyl flooring product is placed face down on the adhesive and sealed at the edges. A visual inspection of the condition of the adhesive is made after a 72-hour period. This test is no longer favored since it can produce false negative results.
Test Strip – in which a test strip of the proposed primer or adhesive is evaluated for 24 hours to predict its behavior on the floor. This procedure is not very reliable.
Moisture Meters – Measure electrical resistance or impedance to indicate slab moisture. Electronic meters can be useful survey tools that provide comparative readings across a floor but should not be used to accept or reject a floor because they do not provide an absolute measure of moisture conditions within the slab.
Gravimetric – This is a direct and accurate method of determining moisture content by weight in the concrete slab. Pieces of concrete are removed by chiseling or stitch-drilling and dried in an oven to constant weight. The moisture content is then calculated as a percentage of the dry sample weight. This is rarely recommended by floor covering manufacturers.
Nuclear Density and Radio Frequency – This nondestructive test instrument is relatively expensive and can take a long time to properly correlate correction factors for each individual project. The instrument has a radioactive source and therefore requires licensed operators.
Anhydrous Calcium Chloride Test (ASTM F1869) – is specified by most floor covering manufacturers for pre-installation testing. A measured amount of anhydrous calcium chloride is placed in a cup sealed under a plastic dome on the slab surface and the amount of moisture absorbed by the salt in 60 to 72 hours is measured to calculate the moisture vapor emission rate (MVER). Maximum limits of vapor transmission generally specified are 3 to 5 pounds of moisture per 1000 square feet per 24 hours. This test is relatively inexpensive, and yields a quantitative result. However, it has some major short- comings: it determines only a portion of the free moisture at a shallow depth of concrete near the surface of the slab. The test is sensitive to the temperature and humidity in the building. It provides only a “snapshot in time” of current moisture conditions and does not predict if the sub-slab conditions will cause a moisture problem later in the life of the floor.
Relative Humidity Probe (ASTM F2170) – This procedure involves measuring the relative humidity of concrete at a specific depth from the slab surface inside a drilled or cast hole in a concrete slab. The relative humidity is measured after allowing 72 hours to achieve moisture equilibrium within the hole. Typically a relative humidity of 75% to 80% is targeted for installation of floor coverings. Relative humidity probes can determine the moisture profile from top to bottom in a slab, conditions below the slab, and can monitor the drying of a slab over time, leading to predictions of future moisture conditions. These instruments have been used for many years in Europe and are becoming more popular in the United States.
1. Guide to Concrete Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
2. ASTM Standards E1907, F1869, D4263, F2170, ASTM International, West Conshohocken, PA, www.astm.org.
3. Bruce Suprenant, Moisture Movement Through Concrete Slabs, Concrete Construction, November 1997.
4. Bruce Suprenant, Design of Slabs that Receive Moisture-Sensitive Floor Coverings, Concrete International, Vol. 25, No. 3, April 2003, www.concrete.org.
5. Thomas K. Butt, Avoiding and Repairing Moisture Problems in Slabs on Grade, The Construction Specifier, December, 1992.
6. Malcolm Rode and Doug Wendler, Methods for Measuring Moisture Content in Concrete, Concrete Repair Bulletin, March-April, 1996.
7. Steven H. Kosmatka, Floor-Covering Materials and Moisture in Concrete, Portland Cement Association, Skokie, IL, www.cement.org.
WHAT are Vapor Retarders?
Vapor retarders are materials that will minimize the transmission of water vapor from the sub-slab support system into a concrete slab. Vapor retarders are typically specified according to ASTM E1745 and have a permeance of less than 0.3 US perms (0.2 metric perms), when tested by ASTM E96. Low-density polyethylene film is commonly used and a minimum thickness of 10 mils (0.25 mm) is recommended for reduced vapor transmission and durability during and after its installation. Membrane material specifically designed for use as true vapor barriers with permeance ratings of 0.0 perms per square foot per hour, as measured by ASTM E96, are also available.
WHY are Vapor Retarders Used?
Vapor retarders are frequently specified for interior concrete slabs on grade where moisture protection is desired. Protection from moisture is required when floors will be covered with carpet, tile, wood, resilient, and seamless polymeric flooring, or when moisture-sensitive equipment or products will be placed on the floor. Permeation of water vapor through concrete slabs can cause failure of moisture-sensitive adhesives or coatings, resulting in delamination, distortion or discoloration of flooring products, trip-and-fall hazards, and possibly fungal growth and odors.
Low-permeability membranes below floor slabs on grade, in conjunction with sealed joints, also provide a barrier to radon penetration into enclosed spaces when such conditions exist.
WHAT Conditions Require Vapor Retarders?
A floor is part of the building envelope and should be constructed to eliminate moisture infiltration into the slab and into the occupied building space. For many years, vapor retarders were specified only for floor slabs intended to receive floor coverings. However, even floors intended for “bare” use in service, such as warehouses, mechanical rooms, and unfinished expansion areas, often are converted to other uses and then moisture-sensitive flooring is installed. Such “adaptive re-use” cannot be predicted during design and construction of a new building. Therefore, it is sensible planning to include a vapor retarder under every interior floor slab in every building. Vapor retarders are generally not necessary when placing exterior slabs on grade.
Vapor retarders do not prevent migration of residual moisture from within the concrete slab to the surface. It is important to use a concrete mixture with the lowest water content that will afford adequate workability. Chemical and mineral admixtures are generally used to minimize the water content in a concrete mixture and provide adequate workability for placement. After proper curing, the concrete slab should be allowed to dry out and tested to ensure that moisture is not being transmitted through the slab prior to installing flooring materials (CIP 28).
HOW to Place Concrete on Vapor Retarders:
Current recommendation of ACI Committee 302 is to place a concrete slab directly on top of a vapor retarder when the concrete slab surface will receive a vapor sensitive floor covering. If environmental conditions exist for increased possibility of plastic shrinkage cracking, placing concrete directly on the vapor retarder can help alleviate the plastic shrinkage cracking somewhat by enhancing bleed water.
Placing concrete directly on the vapor retarder can also create potential problems. If environmental conditions do not permit rapid drying of bleed water from the slab surface then the excess bleeding can delay finishing operations. Bleed water trapped below a finished surface can cause delaminations (CIP 20) or blisters (CIP 13) if finishing operations are not performed at the correct time after bleed water has disappeared from the surface. Concrete may stiffen slower, which means that trowel finishing operations must be delayed; thus increasing the susceptibility of plastic shrinkage cracking. Curling (CIP 19) can occur due to differential drying and related shrinkage at different levels in the slab. Most of these problems can be alleviated by using a concrete with a low water content, moderate cement factor, and well-graded aggregate with the largest possible size. With the increased occurrence of moisture-related floor covering failures, minor cracking of floors placed on a vapor retarder and other problems discussed here are considered a more acceptable risk than failure of floor coverings.
The sub-grade and base should be adequately compacted.
The base should be well draining and stable to support construction traffic. A clean fine-graded, preferably crushed, material with about 10 to 30 percent passing the No. 100 [150 mm] sieve and free of clay or organic material is generally recommended. Concrete sand should not be used as it is easily displaced during construction.
If recommended in the geotechnical evaluation of the jobsite, install a 6 to 8 inch [150 to 200 mm] layer of coarse gravel or crushed stone as a capillary break. Note that a coarse stone capillary break will not reduce moisture vapor transmission from the subgrade. A vapor retarder is still required above a capillary break.
If a capillary break layer of coarse stone is used, choke the top surface with 2-in. of graded, fine-grained compactable fill to prevent damage to the vapor retarder from sharp corners of the coarse stone. Place the vapor retarder on top of the smooth, compacted fill.
Vapor retarder sheets should be overlapped by 6 inches [150 mm] at the seams and taped and sealed around utility or column openings, grade beams, footings, and foundation walls.
If an interior concrete slab will not have a vapor-sensitive floor covering but will be located in a humidity controlled area it may be placed over the granular fill/blotter layer provided the slab and base material is placed with waterproof roof membrane in place. Further, the granular material should not be subject to future moisture infiltration.
When the choice is made to place the concrete over a granular blotter layer, a minimum 4 inch [100 mm] layer of compactable, easy-to-trim, granular fill material should be used. A “crusher-run” material graded from 1½ in. [37.5 mm] to dust size works well. If this is not practical, cover the vapor retarder with at least 3 inches [75 mm] of crushed stone sand. Do not use concrete sand. To reduce slab friction, top off the crusher-run layer with a layer of fine-graded material. The granular layer should ideally be placed under cover and should be dry prior to concrete placement to function as a blotter and remove water from the fresh concrete.
Follow These Rules When Using Vapor Retarders:
1. Provide a vapor retarder directly under all interior floor slabs.
2. Place the vapor retarder on a smooth base and ensure it is vapor tight to moisture sources below the slab and at its edges and at penetrations.
3. Order a concrete mixture designed for minimum shrinkage and follow good concrete practices for finishing and curing to reduce potential water vapor emission. If the concrete slab will receive a vapor-sensitive floor covering, cure the concrete under plastic sheeting for 3 days and in no case moist cure the concrete for more than 7 days.
1. ASTM Standards E96-00, Standard Test Methods for Water Transmission of Materials, ASTM International, West Conshohocken, PA, www.astm.org.
2. ASTM E1745-97, Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs, ASTM International, West Conshohocken, PA, www.astm.org.
3. Guide to Floor and Slab Construction, ACI 302.1R, American Concrete Institute, Farmington Hills, MI.
4. ASTM E1643, Standard Practice for Installation of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs, ASTM, West Conshohocken, PA.
5. Slabs on Grade, Concrete Craftsman Series – CCS-1, 2nd edition, American Concrete Institute, Farmington Hills, MI.
6. R. H. Campbell, Job Conditions Affect Cracking and Strength of Concrete In-Place, et al., ACI Journal, Jan 1976, pp.10 – 13.
7. C. Bimel, No Sand, Please, The Construction Specifier, June 1995, pp. 26.
8. Robert W. Gaul, Moisture-Caused Coating Failures: Facts and Fiction, Concrete Repair Digest, February – March, 1997
WHAT are SCMs?
In its most basic form, concrete is a mixture of Portland cement, sand, coarse aggregate and water. The principal cementitious material in concrete is Portland cement. Today, most concrete mixtures contain supplementary cementitious materials (SCMs) that make up a portion of the cementitious component in concrete. These materials are generally byproducts from other processes or natural materials. They may or may not be further processed for use in concrete. Some of these materials are called pozzolans, which by themselves do not have any cementitious properties, but when used with Portland cement, react to form cementitious compounds. Other materials, such as slag cement and ASTM C618 Class C fly ash, do exhibit cementitious properties.
For use in concrete, SCMs need to meet requirements of established standards. They may be used individually or in combination in concrete. They may be added to the concrete mixture as a blended cement or as a separately batched ingredient at the ready mixed concrete plant.
Some examples of these materials are discussed below.
Fly Ash is a byproduct of coal-fired furnaces at power generation facilities and is the non-combustible particulates removed from the flue gases. Fly ash used in concrete should conform to specification ASTM C618. The amount of fly ash in concrete can vary from 15% to 65% by mass of the cementitious materials, depending on the source and composition of the fly ash and the performance requirements of the concrete. Characteristics of fly ash can vary significantly depending on the source of the coal. Class F fly ash is normally produced when burning anthracite or bituminous coal and generally has a low calcium content. Class F fly ash is pozzolanic. Class C fly ash is produced when subbituminous coal is burned and it typically has cementitious and pozzolanic properties. As defined in ASTM C618, the sum of silicon, aluminum, and iron oxides should be greater than 50% for Class C fly ashes and should be greater than 70% for Class F fly ashes.
Slag Cement is a non-metallic manufactured byproduct from a blast furnace when iron ore is reduced to pig iron. The liquid slag is rapidly cooled to form granules, which are then ground to a fineness similar to Portland cement. Slag cement used as a cementitious material should conform to the specification ASTM C989. Three grades – 80, 100, and 120 – are defined in C989, with the higher grade contributing more to strength potential. Slag cement has cementitious properties but these are enhanced when it is used with Portland cement. Slag is used at 20% to 70% by mass of the cementitious materials.
Silica Fume is a byproduct from the manufacture of silicon or ferro-silicon metal and is a highly reactive pozzolanic material. It is collected from the flue gases from electric arc furnaces. Silica fume is an extremely fine powder, with particles about 100 times smaller than an average cement particle. Silica fume is available as a densified powder. Silica fume for use in concrete should conform to specification ASTM C1240. It is generally used at 3 to 10% by mass of cementitious materials. Applications include concrete structures that need high strength or significantly reduced permeability to water and chemicals. Special procedures are warranted when handling, placing and curing silica fume concrete.
Natural Pozzolans. Various naturally occurring materials possess, or can be processed to possess, pozzolanic properties. These materials are also covered under the specification ASTM C618. Natural pozzolans are generally derived from volcanic origins. In the US, commercially available natural pozzolans include metakaolin and calcined shale or clay. These materials are manufactured by controlled calcining (firing) of naturally occurring materials. Metakaolin is produced from relatively pure kaolinite clay and it is used at 5% to 15% by mass of the cementitious materials. Calcined shale or clay is used at higher percentages by mass. Other natural pozzolans include volcanic glass, zeolitic trass or tuffs, rice husk ash and diatomaceous earth.
WHY are SCMs Used?
SCMs can be used for improved concrete performance in its fresh and hardened state. They are primarily used to enhance the workability, durability and strength of concrete. These materials allow the concrete producer to design and modify the concrete mixture to meet the performance requirements of the concrete application. Concrete mixtures with high Portland cement contents are susceptible to cracking and increased heat generation. These effects can be controlled to a certain degree by using SCMs.
SCMs such as fly ash, slag cement and silica fume enable the concrete industry to use hundreds of millions of tons of byproduct materials that would otherwise be landfilled as waste. Furthermore, their use reduces the consumption of Portland cement per unit volume of concrete. Portland cement has high energy consumption and emissions associated with its manufacture, which is conserved or reduced when the amount used in concrete is reduced.
HOW do SCMs Affect Concrete Properties?
Fresh Concrete – In general, SCMs improve the consistency and workability of fresh concrete because an additional volume of fines is incorporated in the mixture. Concrete with silica fume is typically used at low water contents with high range water reducing admixtures and these mixtures tend to be cohesive and stickier than plain concrete. Fly ash and slag cement generally reduce the water demand for required concrete slump. Concrete setting time may be slower with some SCMs used at higher percentages. This can be beneficial in hot weather. The slower setting time is offset in winter by reducing the percentage of supplementary cementitious material in the concrete and be using accelerating admixtures. Because of the additional fines, the amount and rate of bleeding of these concretes is often reduced. This is especially significant when silica fume is used. Reduced bleeding, in conjunction with slower setting characteristics, can cause plastic shrinkage cracking and may warrant special precautions during placing and finishing (See CIP 5).
Strength – Concrete mixtures can be proportioned to produce the required strength and rate of strength gain as required for the application. With SCMs other than silica fume, the rate of strength gain might be lower initially, but strength gain continues for a longer period compared to mixtures with only Portland cement, frequently resulting in higher ultimate strengths. Silica fume is often used to produce concrete compressive strengths in excess of 10,000 psi [70 MPa]. Concrete containing supplementary cementitious material generally needs additional consideration for curing of both the test specimens and the structure to ensure that the potential properties are attained.
Durability – SCMs can be used to reduce the heat generation associated with cement hydration and reduce the potential for thermal cracking in massive structural elements. These materials modify the microstructure of concrete and reduce its permeability thereby reducing the penetration of water and water-borne salts into concrete. Watertight concrete will reduce various forms of concrete deterioration, such as corrosion of reinforcing steel and chemical attack. Most SCMs can reduce internal expansion of concrete due to chemical reactions such as alkali aggregate reaction and sulfate attack. Resistance to freezing and thawing cycles requires the use of air-entrained concrete. Concrete with a proper air void system and strength will perform well in these conditions.
The optimum combination of materials will vary for different performance requirements and the type of SCMs. The ready mixed concrete producer, with knowledge of the locally available materials, can establish the mixture proportions for the required performance. Prescriptive restrictions on mixture proportions can inhibit optimization and economy. While several enhancements to concrete properties are discussed above, these are not mutually exclusive and the mixture should be proportioned for the most critical performance requirements for the job with the available materials.
1. ASTM Standards C618, C989, C1240, Volume 04.02, ASTM International, West Conshohocken, PA, www.astm.org.
2. Use of Natural Pozzolans in Concrete, ACI 232.1R, American Concrete Institute, Farmington Hills, MI, www.concrete.org
3. Use of Fly Ash in Concrete, ACI 232.2R, American Concrete Institute, Farmington Hills, MI.
4. Guide to Use of Slag Cement in Concrete and Mortar, ACI 233R, American Concrete Institute, Farmington Hills, MI.
5. Guide for the Use of Silica Fume in Concrete, ACI 234R, American Concrete Institute, Farmington Hills, MI.
6. Pozzolanic and Cementitious Materials, V.M. Malhotra and P. Kumar Mehta, Gordon and Breach Publishers.
7. CIP 5, Plastic Shrinkage Cracking, NRMCA Concrete in Practice Series, Silver Spring, MD, www.nrmca.org.
WHAT is Ready Mixed Concrete?
Concrete is a mixture of cementitious materials, water, and aggregate, usually sand and gravel or crushed stone. There is a common misunderstanding that cement and concrete are one and the same. Cement is a powdered ingredient that provides the glue that binds the aggregates together in a mass called concrete.
Ready mixed concrete is that which is delivered to the customer in a freshly mixed and unhardened state. Due to the ability to customize its properties for different applications and its strength and durability to withstand a wide variety of environmental conditions, ready mixed concrete is one of the most versatile and popular building materials.
Concrete mixtures are proportioned to obtain the required properties for the application. It should have the correct consistency, or slump, to facilitate handling and placing, and adequate strength and durability to withstand applied loads in the anticipated environment and service conditions. The design quantities of concrete ingredients are accurately weighed and mixed, either in a mixer at the concrete plant or in a concrete truck mixer. It is delivered in a truck mixer or agitation unit, which keeps the concrete uniformly mixed until it is discharged at the placement location. Concrete remains plastic for several hours depending on the type of mixture and conditions during placement so that there is sufficient time for it to be placed and finished. Concrete normally sets or hardens within two to twelve hours after mixing and continues to gain strength for months or even years if it is properly cured during the first few days.
WHY use Ready Mixed Concrete?
Concrete, in its freshly mixed state, is a plastic workable mixture that can be cast into virtually any desired shape. The properties of concrete can be customized for almost any application to serve in a wide variety of extreme environments. Concrete is a very economical building material that can serve its function for several years with minimum maintenance, provided the proper mixture relative to the application and established construction practices are used. A wide variety of options with color, texture and architectural detail are available to enhance the aesthetic quality of the concrete application.
HOW to Order Ready Mixed Concrete:
The key to placing an order for ready mixed concrete is to provide all the basic detailed information and to keep the requirements as simple as possible and relevant to the application. The ready mixed concrete producer has several mixture formulations for a wide variety of applications and can help with deciding the required mixture characteristics.
Some of the basic requirements to keep in mind when placing a concrete order are as follows:
Size of Coarse Aggregate – the important information is the nominal maximum size required, which should be smaller than the narrowest dimension through which concrete should flow, such as the thickness of the section and spacing of the reinforcing steel, if any. For most applications, nominal maximum size of coarse aggregate is ¾ or 1 inch (19.0 or 25.0 mm).
Slump – Concrete slump, a measure of its consistency, should be indicated. A stiffer mixture will have a low slump value. Typical slump range for most applications is 3 to 5 inches (75 to 100 mm). For slip-form construction a maximum slump of 2 inches (50 mm) is required, while higher slump to a maximum of 7 inches (175 mm) is typical for basement walls. The tolerance on the slump as delivered is ±1 to 1½ inches (25 to 38 mm). Addition of water at the jobsite to increase slump is permitted, provided it is not excessive enough to cause segregation and reduce strength and durability.
Entrained Air – Air-entrained concrete should be used if concrete will be exposed to freezing temperatures in service, or even during construction. In many locations air-entrained concrete is the default option. When non air-entrained concrete is required, this should be clearly stated at the time of ordering. Target air content depends on the size of the coarse aggregate and the typical range is 4 to 6% of the concrete volume. The tolerance on air content as delivered is ±1.5%. The concrete supplier is permitted to make an adjustment for air content at the jobsite if, when tested, it is lower than the required amount.
Quality Level Required – The purchaser specifies the concrete quality, in terms of its properties or composition.
The preferred method for ordering concrete is by specifying performance requirements, which is generally the concrete strength. Other performance characteristics, such as permeability, shrinkage or various durability requirements, may be specified when required. The producer should be made aware of anticipated exposure and service conditions of the structure. The concrete producer is best equipped to proportion, mix and furnish concrete for the desired performance. The strength level is generally dictated by the design of the structure to withstand anticipated loads during construction and in service. A minimum strength of 3500 to 4000 psi (25 to 28 MPa) may ensure durable concrete, such as resistance to wear, abrasion, and freezing and thawing cycles.
Another option is to order concrete by specifying prescriptive requirements. The purchaser specifies limits on the quantities and types of ingredients in the mixture. In this case the purchaser should generally accept responsibility for concrete strength and performance. The prescriptive limits may indicate minimum cement content, maximum water-cement ratio, and limits on the quantities of pozzolans, slag or admixtures. Frequently, this approach is used when a particular prescriptive mixture formulation has worked well in the past. This approach does not allow the producer much flexibility on the economy of the mixture or to accommodate changes in material sources or characteristics that may affect concrete’s performance.
Specifying performance and prescriptive requirements is discouraged as the performance requirements may conflict with the prescriptive limits.
Quantity of Concrete – Concrete is sold by volume, in cubic yards (cubic meters), in a freshly mixed unhardened state as discharged from the truck mixer. The delivered volume, or yield, is calculated from the measured concrete density or unit weight. One cubic yard of concrete weighs about 4000 lb. (2 short tons). One cubic meter (approximately 1.3 cubic yards) weighs about 2400 kg. The typical capacity of a truck mixer is 8 to 12 cubic yards (5 to 9 cubic meters).
Order about 4% to 10% more concrete than is estimated from a volumetric calculation of the plan dimensions. This will account for waste or spillage, over-excavation, spreading of forms, loss of entrained air during placement, settlement of a wet mixture, truck mixer holdback and change in volume – hardened concrete volume is 1% to 2% less than that of the fresh concrete. Reevaluate the needs during placement and communicate any changes to the concrete supplier.
Disposal of returned concrete has environmental and economic implications to the ready mixed concrete producer. Make a good estimate of concrete required for the job before placing an order. Avoid ordering small clean-up loads, less than 4 cubic yards (2.5 cubic meters).
Additional Items – A variety of value-added options are available from the ready mixed concrete producer. Chemical admixtures can accelerate or retard the setting characteristics of concrete to aid in placing and finishing during hot or cold weather. Water reducing admixtures are used to increase concrete slump without adding water to the concrete. Synthetic fibers can reduce the potential for plastic shrinkage cracking. Using color or special aggregates can enhance aesthetic characteristics. The concrete contractor can also be a resource for innovative finishes and textures to concrete.
Scheduling Delivery – Schedule the delivery of concrete to accommodate the construction schedule. Inform the producer of the correct address, location and nature of the pour, and an estimated delivery time. Call the ready mixed concrete producer well in advance of the required delivery date. Concrete is a perishable product and the construction crew should be ready for concrete placement when the truck arrives. Notify the producer of any schedule changes or work stoppage immediately.
Ensure that the truck mixer has proper access to the placement location. The concrete truck weighs in excess of 60,000 lbs. (27,000 kg) and may not be able to maneuver on loose dirt and residential curbs and pathways.
WHAT are the Responsibilities?
The responsibilities of the various parties involved in the construction process should be addressed at a pre-construction meeting, especially on a large job. These responsibilities should be documented and distributed to all concerned for reference during the construction. Some items are discussed below.
• The concrete producer is responsible for the concrete slump as specified for a period of 30 minutes after the requested time or the time the truck arrives at the placement site, whichever is later.
• The concrete producer is required to deliver concrete at the requested slump and air content, within the accepted tolerances addressed above, as measured at the point of discharge from the transportation unit.
• When placing procedures can potentially alter the characteristics of the fresh concrete, it is the responsibility of the purchaser to inform the producer of changes to the mixture requirements to accommodate these effects. An example is pumping concrete in place.
• When a job uses more than one type of concrete mixture, it is the purchaser’s responsibility to verify the mixture delivered and direct it to the correct placement location.
• The purchaser should check and sign the delivery ticket and document any special occurrences on the ticket.
• The concrete producer cannot be responsible for the quality of concrete when any modification or additions are made to the mixture at the jobsite. These include addition of excessive water, admixtures, fibers or special products, or if the truck has to wait for an extended period before discharging the concrete.
• When strength tests are used for acceptance of concrete, the samples should be obtained at the point of discharge from the transportation unit. The purchaser or his representative should ensure that proper facilities are available for curing the test specimens at the jobsite and that standard practices are followed for subsequent curing and testing. Certified personnel should conduct the tests. Test reports should be forwarded to the producer in a timely manner to ensure that deficiencies are rectified.
Fresh concrete can cause severe chemical burns to skin and eyes. Keep fresh concrete off your skin. When working with concrete use rubber work-boots, gloves, protective eyeglasses, clothing and knee-boards. Do not let concrete or other cement products soak into clothing or rub against your skin. Wash your skin promptly after contact with fresh concrete with clean water. If fresh concrete gets into your eyes, flush immediately and repeatedly with water and consult a doctor immediately. Keep children away from dry cement powder and all freshly mixed concrete.
1. ASTM C 94, Standard Specification for Ready Mixed Concrete, Vol. 04.02, American Society for Testing and Materials, West Conshohocken, PA.
2. Ready Mixed Concrete, Richard D. Gaynor, NRMCA Publication 186, NRMCA, Silver Spring, MD.
3. Guide for Measuring, Mixing, Transporting and Placing Concrete, ACI 304R, American Concrete Institute, Farmington Hills, MI
WHAT is a Pre-Construction Conference?
Prior to the start of a job, especially for a major project, a concrete pre-construction conference (sometimes called a pre-pour meeting) should be held to define and allocate responsibilities of the entire construction team. It is imperative that all members of the team meet to establish the responsibilities of the ready mixed concrete supplier, owner, architect, structural engineer, general contractor, subcontractors, testing agencies, and inspectors. This meeting should be held well in advance of the project to ensure there is sufficient time for all parties to be absolutely clear on what their responsibilities would entail.
WHY have a Pre-Construction Conference?
Every construction project brings together different companies, personnel and procedures, who may or may not have worked together before. Two jobs are never the same, even when working with the same companies, as personnel changes can realign the perception of individual responsibilities. Pre-construction conferences are needed to sort out the details of how a job will be executed, identify the authorized contacts for various aspects, and what should be done if some things do not go as planned. In far too many cases, projects are started without a clear understanding of assigned responsibilities resulting in extra work, lost time and major expenses. In some cases, a simple pre-construction conference could have prevented some, if not all, of these problems from occurring. Having this meeting serves to document the chain of responsibilities, which can be referenced when needed.
HOW to Conduct Pre-Construction Conferences?
The pre-construction conference agenda should contain the following to ensure that all details are addressed prior concrete placement.
Purpose: To define and allocate individual responsibilities of the concrete construction team
Subject: Pre-construction agenda, concrete mix designs, placement, inspection and testing
Project Name and Location: Establish the project name and address.
Personnel to Attend: Contractor’s project manager, owner’s representative, concrete subcontractor, architect, engineer, testing lab supervisor, pumping contractor, concrete producer’s quality control director, inspector and construction manager, if applicable, and anyone else with the need to know.
Minutes of the Meeting: Assign someone to take minutes. Establish a meeting distribution list.
Concrete Mix Design and Specifications: Have the mix designs been approved and what is the approval process? Are there any special concrete performance requirements or conditions? Are value-added admixtures approved for use and who can authorize them?
Ordering Concrete and Scheduling Deliveries: Ensure that concrete delivery schedules are in place. Establish the lead-time needed to place the order, especially for large placements or special concrete, and establish links of communication for last minute cancellations. Establish who has the authority to place and cancel concrete orders. Establish truck staging areas and location to wash out trucks and disposing of excess concrete.
Plant Inspections: Are plant inspections required? If so, who will do the inspections and what will it entail? Will an NRMCA certification be accepted?
Job Inspections: Who is responsible for inspection and approval of forms and rebar prior to concrete placement? Who is responsible for approving adequacy of subgrade preparation for concrete slabs on grade? Who is responsible for placing and consolidation of concrete? Who will ensure that proper methods of finishing and curing are employed? What method will be used and for how long will concrete be cured? What is the minimum concrete strength required for stripping form? Will there be a formal report form for stripping forms? Will there be any in-place strength testing? Who is responsible to authorize form removal? Where will field-cured cylinders be stored and for what purpose will they be tested?
Sampling and Testing: What procedure will be followed for acceptance samples? What is the frequency for sampling and testing concrete? Will concrete be sampled as it is discharged from the truck mixer or at another location? What tests will be performed? Who will conduct the testing and who will verify that the technicians are certified? How many test cylinders will be made, how will they be cured, and at what ages will they be tested? What procedure is followed for non-conformance to specification?
Acceptance and Rejection Responsibilities for Fresh Concrete: Who has the authority to add water to the concrete on site? Who has the authority to reject concrete delivery? For what reasons can concrete be rejected? What are the tolerances for slump, air content, unit weight, and temperature? Establish re-test procedures for concrete prior to rejection.
Specimen Handling: How will cylinders be stored at the jobsite? Who is required to provide the initial curing environment for the test cylinders and how will controlled temperature and moisture be maintained? How will test cylinders be transported on weekends or non-workdays and who will arrange for access on to the site? What curing procedure is used at the testing facility? Verify that cylinders will be handled, transported and cured in accordance with ASTM C31 or other applicable standards.
Report Distribution and Acceptance Criteria: Define the time frame for the report distribution and who will get copies of test reports. What will be on the reports and what will be the strength acceptance criteria: ACI 318, ASTM C94 or other?
Testing of In-Place Concrete: The meeting should address what situations will require additional testing. How will the test results be evaluated, and by whom? Who incurs the expense for additional evaluations?
The items listed above are examples of some of the issues that should be discussed at a pre-construction conference. It also provides the opportunity for all involved parties to thoroughly review the specification and contract documents and, if necessary, make changes and improvements to them. It will also provide an understanding of responsibilities, which should be documented, for future reference.
SUGGESTED PRE-CONSTRUCTION CONFERENCE AGENDA ITEMS
• Project information and schedule
• Project participants
• Construction sequence and processes
• Base/subgrade construction and acceptance
• Site access
• Power, lighting, water
• Formwork and removal
• Placing concrete – equipment and procedures
• Vapor retarders/barriers
• Requirements for surface finishes
• Curing and sealing
• Protection of concrete
• Hot and cold weather precautions
• Concrete materials and mixtures
• Specification requirements for concrete
• Jobsite adjustments
• Special materials
• Ordering and scheduling concrete delivery
• Quality control /Quality assurance
• Report distribution
• Corrective actions
• Test specimen storage, transportation and testing
• Acceptance/rejection of fresh and hardened concrete
• In-place concrete strength evaluation
• Dispute resolution and cost assignment
• Jobsite environmental management
• Jobsite safety
1. Ready Mixed Concrete Quality Control Checklist, Quality Control Manual – Section 1, NRMCA, Silver Spring, MD.
2. Concrete Pre-Construction Checklist, Georgia Concrete & Products Association, 1st Edition.
3. NRMCA-ASCC Checklist for the Concrete Pre-Construction Conference, NRMCA, Silver Spring, MD.
WHAT is High Strength Concrete?
It is a type of high performance concrete generally with a specified compressive strength of 6,000 psi (40 MPa) or greater. The compressive strength is measured on 6 ´ 12 inch (150 ´ 300 mm) or 4 ´ 8 inch (100 ´ 200 mm) test cylinders generally at 56 or 90-days or some other specified age depending upon the application. The production of high strength concrete requires more research and more attention to quality control than conventional concrete.
WHY do we Need High Strength Concrete?
A. To put the concrete into service at a much earlier age; for example, opening the pavement at 3 days.
B. To build high-rise buildings by reducing column sizes and increasing available space.
C. To build the superstructures of long-span bridges and to enhance the durability of bridge decks.
D. To satisfy the specific needs of special applications such as durability, modulus of elasticity, and flexural strength. Some of these applications include dams, grandstand roofs, marine foundations, parking garages, and heavy duty industrial floors. (Note that high strength concrete does not guarantee durable concrete.)
HOW to Design High-Strength Concrete Mixture:
Optimum concrete mixture design results from selecting locally available materials that make the fresh concrete placeable and finishable and that ensure the strength development and other desired properties of hardened concrete as specified by the designer. Some of the basic concepts that need to be understood for high strength concrete are:
1. Aggregates should be strong and durable. They need not necessarily be hard and of high strength but need to be compatible, in terms of stiffness and strength, with the cement paste. Generally smaller maximum size coarse aggregate is used for higher strength concretes. The sand may have to be coarser than that permitted by ASTM C33 (fineness modulus greater that 3.2) because of the high fines content from the cementitious materials.
2. High strength concrete mixtures will have a high cementitious materials content that increases the heat of hydration and possibly higher shrinkage leading to the potential for cracking. Most mixtures contain one or more supplementary cementitious materials such as fly ash (Class C or F), ground granulated blast furnace slag, silica fume, metakaolin or natural pozzolanic materials.
3. High strength concrete mixtures generally need to have a low water-cementitious materials ratio (w/cm). W/cm ratios can be in the range of 0.23 to 0.35. These low w/cm ratios are only attainable with quite large doses of high range water-reducing admixtures (or superplasticizers) conforming to Type F or G by ASTM C494. A Type A water-reducer may be used in combination.
4. The total cementitious material content will be typically around 700 lbs./yd3 (415 kg/m3) but not more than about 1100 lbs./yd3 (650 kg/m3).
5. The use of air entrainment in high strength concrete will greatly reduce the strength potential.
More attention and evaluation will be necessary if the job specification sets limits for other concrete properties such as creep, shrinkage, and modulus of elasticity. The engineer may set limits on these properties for the design of the structure. Current research may not provide the required guidance for empirical relationships of these properties from traditional tests and some of these tests are quite specialized and expensive to conduct for mixture evaluation. From theoretical considerations, lower creep and shrinkage, and high modulus of elasticity can be achieved with higher aggregate and lower paste volumes in the concrete. Using the largest size aggregate possible and medium to coarsely graded fine aggregate can attain this. Smaller maximum size aggregate such as 3/8 inch (9.5mm) can be used to produce very high compressive strength but required properties like creep, shrinkage, and modulus of elasticity may be sacrificed. If difficulty is encountered in achieving high strength, simply adding more cementitious material may not increase strength. Factors such as deleterious materials in aggregates, aggregate coatings, coarse aggregate fracture faces, shape and texture, and testing limitations may prevent higher strength from being achieved. Final concrete mixture proportions are determined by trial batches either in the laboratory or by small size field production batches. The production, transportation, placement and finishing of high strength concrete can differ significantly from procedures used for conventional concrete. For critical projects, it is highly recommended that a trial pour and evaluation be conducted and included as a pay item in the contract. Pre-bid and pre-construction meetings are very important to ensure the success of projects using high strength concrete. During construction, extra measures should be taken to protect against plastic shrinkage and thermal cracking in thicker sections. High strength concrete may need longer time before formwork is removed.
High strength concrete test cylinders should be carefully molded, cured, capped, and tested. Extra care and attention to handling of test cylinder specimens at very early age is necessary. Slower setting time of high strength concretes may be experienced. The ASTM Standards are continuously being revised to account for additional special precautions needed when testing high strength concrete. Particular attention should be paid to the type of mold, curing, type of cylinder capping material, and characteristics and load capacity of the testing machine.
1. State-of-the-Art Report on High Strength Concrete, ACI 363R, ACI International, Farmington Hills, MI, www.aci-int.org.
2. Guide to Quality Control and Testing of High Strength Concrete, ACI 363.2R, ACI International Farmington Hills, MI.
3. Creating a Balanced Mix Design for High Strength Concrete, Bryce Simons, The Concrete Producer, October 1995, www.worldofconcrete.com.
4. Getting Started with High-Strength Concrete, Ron Burg, The Concrete Producer, November 1993.
5. Effects of Testing Variables on the Measured Compressive Strength of High Strength (90 MPa) Concrete, Nick J. Carino, et al., NISTIR 5405, October 1994, National Institute of Standards and Technology, Gaithersburg, MD, www.nist.gov.
6. 10,000 psi Concrete, James E. Cook, ACI Concrete International, October 1989, ACI International, Farmington Hills, MI.
WHAT are Concrete Test Cylinders?
Most commonly, the compressive strength of concrete is measured to ensure that concrete delivered to a project meets the requirements of the specification and for quality control. For testing the compressive strength of concrete, cylindrical test specimens of size 4 x 8-in. (100 x 200-mm) or 6 x 12-in. (150 x 300-mm) are cast and stored in the field until the concrete hardens in accordance with the requirements of ASTM C31 – Standard Practice for Making and Curing Concrete Test Specimens in the Field.
Technicians making cylinders in the field should be certified by the ACI Field Testing Certification Grade I, or an equivalent program. When making cylinders for acceptance of concrete, the field technician must test other properties of the fresh concrete to include temperature, slump, density (unit weight) and air content. This information should be included on the strength test report for a particular pour or pour location. A strength test result is always the average of at least two specimens tested at the same age. A set of 2 to 6 cylinders may be made from the same sample of concrete at a minimum for every 150 yd3 (115 m3) of concrete placed.
WHY Make Concrete Test Cylinders?
According to ASTM C31, strength results of standard-cured cylinders are used for:
• Acceptance testing for specified strength;
• Verifying mixture proportions for strength; and
• Quality control.
It is important that the specimens are made and cured following standard procedures. Any deviation from standard procedures will result in a lower measured strength and cause undue concern, cost, and delay to the project.
Field-cured cylinders are used for:
• Determining when structure can be put into service;
• Comparing with results of standard-cured specimens or in-place tests;
• Evaluating the adequacy of curing and protecting concrete in the structure; and
• Scheduling removal of forms or shoring.
Because of the different purposes for strength test results, procedures for standard-curing differ from field-curing and the two should not be confused. Refer to ASTM C31 for details.
HOW to Make Concrete Test Cylinders:
Equipment needed at the job site:
• Molds for casting specimens
• Standard tamping rod or vibrator
• Standard rubber or rawhide mallet
• Shovel, scoop, handheld float or trowel
• Wheelbarrow or other sample container
• Water tank or curing box capable of maintaining curing environment during initial curing period
• Safety equipment to handle fresh concrete
It is critical that the sample of concrete obtained from the delivery vehicle is representative of the load. Sampling should be conducted in accordance with ASTM C172. Concrete should be sampled from the middle of the load. The first or last 10% of the discharge will not be representative and should not be used for the sample. From a truck mixer, the entire discharge stream should be diverted into a wheelbarrow. At least two portions are necessary to obtain a composite sample. The time between the first and final portion of the composite sample must not exceed 15 minutes. Minimum sample size required is 1 ft3 (28 L).
Prior to Casting Cylinders:
Cover the sample to protect it from evaporation, sunlight and contamination. Move the sample to the location where the fresh concrete tests will be conducted and where the cylinders will be stored for the initial curing period. Remix the concrete in the wheelbarrow. Begin slump, density (unit weight) and air content tests within 5 minutes and start molding cylinders within 15 minutes after the sample was obtained.
Casting Test Cylinders:
• Cylinder identification labels should be placed on the outside of the mold and not on the lids or tops
• Place the cylinder molds on a level surface
• Consolidation—Use vibration for concrete slump less than 1-in. (25-mm); rodding or vibration is permitted when slump 1-in. (25-mm) or higher
• Layers—For samples that are vibrated, fill mold in two equal layers; for rodded samples, place concrete in 3 equal layers for 6 x 12-in. cylinders and in 2 equal layers for 4 x 8-in. cylinders
• Distribute concrete inside the mold with the scoop. Rod each layer 25 times evenly distributed. For vibration, insert it long enough until large air bubbles are released. Two insertions of the vibrator are required for 6 x 12-in. and one insertion for 4 x 8-in. cylinders. Avoid over-vibration. Consolidate bottom layer throughout its depth; for upper layers penetrate 1-in. (25-mm) into underlying layer.
• Tap sides of the mold 10-15 times with the mallet to close any insertion holes formed during consolidation.
• Strike off the top with a wood float or trowel to produce a flat and even surface level with the edge of mold. Cover with a plastic lid or a plastic bag.
Storing and transporting test cylinders:
• Move cylinder molds with fresh concrete very carefully by supporting the bottom
• Place the cylinders on a flat surface and in a controlled environment. Maintain temperature during initial curing in the range of 60-80⁰F (16-27⁰C). When concrete specified strength is greater than 6000 psi (40 MPa), the maintain temperature in the range of 68-78⁰F (20- 26⁰C). Immersing cylinders, completely covered in water, is an acceptable and preferred procedure that ensures more reliable strength results. Temperature in storage containers should be controlled using heating and cooling devices as necessary. The maximum and minimum temperature should be recorded and reported.
• Protect cylinders from direct sunlight or radiant heat and from freezing temperatures in winter.
• Cylinders must be transported back to the laboratory within 48 hours of casting. Cylinders should not be moved until at least 8 hours after final set.
When transporting cylinders, they should be protected to prevent damage, maintain temperature and prevent loss of moisture. Transportation duration from the jobsite to the laboratory should not exceed 4 hours.
Responsibilities and Reports
ACI 301 states that it is the contractor’s responsibility to provide space and source of electrical power on the project site for initial curing of concrete test specimens. In some locations, it is customary for the contractor to provide equipment and storage for initial curing of test cylinders. If not provided, it is incumbent on the testing agency to have such equipment available. The strength test report should include information required by ASTM C39—storage and curing of specimens before testing; location in the work represented by each strength test, date and time of sampling and batch ticket number. Distribution of test reports to all stakeholders, including concrete producers, should be done within 7 days according to ACI 301. Distribution of all strength test reports is also required by ACI 318.
Fresh concrete can cause severe chemical burns to skin and eyes. Keep fresh concrete off your skin. When working with concrete use rubber work boots, gloves, protective eyeglasses and clothing. Do not let concrete or other cement-based products soak into clothing or rub against your skin. Wash your skin promptly after contact with fresh concrete with clean water. If fresh concrete gets into your eyes, flush immediately and repeatedly with water. Consult a doctor immediately. Keep children away from all freshly mixed plastic concrete.
Follow These Procedures to Make and Cure Standard-Cured Strength Test Specimens:
1. Obtain a representative concrete sample.
2. Place the concrete in layers in the molds and consolidate using standard equipment and procedures.
3. Finish the surface smooth and cover the cylinder with a cap or plastic bag.
4. For initial curing, store cylinders in the required temperature range. Protect from direct sunlight or extreme weather.
5. Transport the cylinders to the laboratory, properly protected, within 48 hours after they are made.
1. ASTM Standards C31, C39, C172—Annual Book of ASTM Standards, Volume 04.02 , ASTM, West Conshohocken, PA, www.astm.org
2. How Producers can Correct Improper Test-Cylinder Curing, Ward R. Malisch, The Concrete Producer, Nov 1997, pp. 782 – 805, www.theconcreteproducer.com
3. NRMCA/ASCC Checklist for Concrete Pre-Construction Conference, NRMCA, Silver Spring, MD, www.nrmca.org
4. ACI 301 and ACI 318, American Concrete Institute, Farmington Hills, MI, www.concrete.org
WHAT is the Compressive Strength of Concrete?
Concrete mixtures can be designed to provide a wide range of mechanical and durability properties to meet the design requirements of a structure. The compressive strength of concrete is the most common performance attribute used by the engineer when designing structures. Compressive strength is measured by breaking cylindrical concrete specimens in a compression-testing machine. Compressive strength is calculated from the failure load divided by the cross-sectional area resisting the load and reported in units of pound-force per square inch (psi) or megapascals (MPa). Concrete compressive strength can vary from 2500 psi (17 MPa) for residential concrete to 4000 psi (28 MPa) and higher in commercial structures. Some applications use higher strengths, greater than 10,000 psi (70 MPa).
WHY is Compressive Strength Determined?
Compressive strength results are used to ensure that the concrete mixture as delivered meets the requirements of the specified strength, ƒ´c, in the job specification.
Strength test results from cast cylinders may be used for quality control, acceptance of concrete, for estimating the strength in a structure, or for evaluating the adequacy of curing and protection afforded to the structure. Standard-cured cylinders are tested for acceptance and quality control. Field-cured cylinders are tested for estimating the in-place concrete strength. Procedures for standard-curing and field-curing are described in ASTM C31. Cylindrical specimens are tested in accordance with ASTM C39. Standard sizes of test specimens are 4×8 in. (100×200 mm) or 6×12 in. (150×300 mm) concrete cylinders. The smaller specimens tend to be easier to make and handle in the field and the laboratory.
A strength test result is the average of at least two strength specimens made from the same concrete sample and tested at the same age. In most cases strength requirements for concrete are at an age of 28 days.
Design engineers use the specified strength ƒ´c to design concrete members. The specified strength is stated in project specifications. The concrete mixture is designed to produce an average strength, ƒ´cr, higher than the specified strength so that the possibility of strength tests failing the acceptance criteria is very low. Historical strength test records from a similar concrete are used to establish the target average strength of concrete mixtures. TIP 2 discusses the process of establishing the required average strength for concrete mixtures.
To comply with the strength requirements of a specification, both the following acceptance criteria apply:
• The average of 3 consecutive tests should equal or exceed the specified strength, ƒ´c.
• No single strength test should fall below ƒ´c by more than 500 psi (3.5 MPa); or by more than 0.10 ƒ´c when ƒ´c is more than 5000 psi (35 MPa).
The same strength acceptance criteria are applicable for either cylinder size. It is important to understand that an individual test falling below ƒ´c does not constitute non-compliance with the strength acceptance criteria. The probability of not complying with these acceptance criteria is about 1% and that for an individual strength tests to be less than the specified strength is about 10%. This is based on the assumption that the average of strength tests are around the required average strength, ƒ´c, at the same level of variability of the assumed standard deviation
When strength test results indicate that the concrete delivered fails to comply with the acceptance criteria, it is possible that the failure may be in the testing, and not the concrete. This is especially true if the fabrication, handling, curing and testing of the cylinders are not conducted in accordance with standard procedures. See CIP 9 – Low Concrete Cylinder Strength.
HOW to Test the Strength of Concrete:
• Requirements for strength-testing machines are stated in ASTM C39.
• The diameter of the cylinder used should be at least 3 times the nominal maximum size of the coarse aggregate used in the concrete.
• Recording the weight of the specimens before testing provides useful information in case of disputes.
• To provide for a uniform load distribution when testing, cylinders are capped with sulfur mortar (ASTM C617) or neoprene pad caps (ASTM C1231). Sulfur caps should be applied at least two hours and preferably one day before testing. Neoprene pad caps can be used to measure concrete strengths between 1500 and 12,000 psi (10 to 80 MPa). Durometer hardness requirements for neoprene pads vary from 50 to 70 depending on the strength level tested. Pads should be replaced after 100 uses; 50 when testing strength between 7000 and 12,000 psi (50 and 80 MPa). TIP 5 discusses capping concrete specimens.
• Cylinders should not be allowed to dry prior to testing.
• The cylinder diameter should be measured in two locations at right angles to each other at mid-height of the specimen and averaged to calculate the cross-sectional area. If the two measured diameters differ by more than 2%, the cylinder should not be tested.
• The ends of the specimens should not depart from perpendicularity with the cylinder axis by more than 0.5⁰ and the ends should be plane to within 0.002 inches (0.05 mm).
• Cylinders should be centered in the compression-testing machine and loaded to complete failure. The loading rate on a hydraulic machine should be maintained in a range of 28 to 42 psi/s (0.20 to 0.30 MPa/s) during the latter half of the loading phase. The type of break should be recorded. A common break pattern is a conical fracture as seen in the figure.
• The concrete strength is calculated by dividing the maximum load at failure by the average cross-sectional area. C39 has correction factors if the length-to-diameter ratio of the cylinder is between 1.75 and 1.00, which is rare. At least two cylinders are tested at the same age and the average strength is reported as the test result to the nearest 10 psi (0.1 MPa).
• Information to be included in the report include the specimen identification, average measured diameter, cross-sectional area, test age, maximum load applied, compressive strength, type of fracture, any defects in the cylinders or caps and, when determined, the density to the nearest 1 lb./ft3 (10 kg/m3). Information required by C31 should be available—location of concrete in the structure represented by test specimens; date, time and individual molding cylinders; slump, air content, temperature and density measured on the concrete sample used to make test specimens; maximum and minimum temperatures during initial curing for standard cured specimens, or details of field curing.
• Most deviations from standard procedures for making, curing and testing concrete test specimens will result in a lower measured strength.
• The coefficient of variation between companion cylinders tested at the same age should be about 2 to 3%. If the difference between companion cylinders exceeds 8% for two or 9.5% for three more than 1 time in 20, the testing procedures should be evaluated and rectified.
• Strength test results made by different labs on the same concrete sample should not differ by more than about 14% of the average.
• If one or both of a set of cylinders break at strength below ƒ´c, evaluate the cylinders for obvious problems and hold the tested cylinders for later examination, possibly for petrographic examination. This provides an opportunity to correct a problem. In some cases, additional reserve cylinders are made and can be tested if one cylinder of a set breaks at a lower strength.
• A 3 or 7-day test may help detect potential problems with concrete quality or testing procedures at the lab but is not a basis for rejecting concrete, with a requirement for 28-day or other age strength.
• ACI 318, ACI 301 and ASTM C1077 requires that laboratory technicians involved in testing concrete must be certified.
• Reports of compressive strength tests provide valuable information to the project team for the current and future projects. The reports of all strength tests should be forwarded to the concrete producer, contractor and the owner’s representative as expeditiously as possible.
1. ASTM C31, C39, C617, C1077, C1231, Annual Book of ASTM Standards, Volume 04.02, ASTM, West Conshohocken, PA, www.astm.org
2. CIP 9, Low Concrete Cylinder Strength, Concrete in Practice Series, NRMCA, Silver Spring, MD, www.nrmca.org
3. TIP 2 and 5, Technology in Practice Series, NRMCA, Silver Spring, MD.
4. In-Place Strength Evaluation – A Recommended Practice, NRMCA Publication 133, NRMCA RES Committee, NRMCA, Silver Spring, MD
5. NRMCA/ASCC Checklist for Concrete Pre-Construction Conference, NRMCA, Silver Spring, MD
6. Review of Variables That Influence Measured Concrete Compressive Strength, David N. Richardson, NRMCA Publication 179, NRMCA, Silver Spring, MD
7. Tips on Control Tests for Quality Concrete, PA015, Portland Cement Association, Skokie, IL, www.cement.org
8. ACI 214R, 301 and 318, American Concrete Institute, Farmington Hills, MI, www.concrete.org
WHAT is Structural Lightweight Concrete?
Structural lightweight concrete has an in-place density (unit weight) on the order of 90 to 115 lb./ ft3 (1440 to 1840 kg/m3) compared to normal-weight concrete with a density in the range of 140 to 150 lb./ ft3 (2240 to 2400 kg/m3). For structural applications, the concrete strength should be greater than 2500 psi (17.0 MPa). The concrete mixture is made with a lightweight coarse aggregate. In some cases a portion or the entire fine aggregate may be a lightweight product. Lightweight aggregates used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure. Other products such as air-cooled blast furnace slag are also used. There are other classes of non-structural lightweight concretes with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete. These are typically used for their insulation properties. This CIP focuses on structural lightweight concrete.
WHY use Structural Lightweight Concrete?
The primary use of structural lightweight concrete is to reduce the dead load of a concrete structure, which then allows the structural designer to reduce the size of columns, footings and other load bearing elements. Structural lightweight concrete mixtures can be designed to achieve similar strengths as normal-weight concrete. The same is true for other mechanical and durability performance requirements. Structural lightweight concrete provides a more efficient strength-to-weight ratio in structural elements. In most cases, the marginally higher cost of the lightweight concrete is offset by size reduction of structural elements, less reinforcing steel and reduced volume of concrete, resulting in lower overall cost.
In buildings, structural lightweight concrete provides a higher fire-rated concrete structure. Structural lightweight concrete also benefits from energy conservation considerations as it provides higher R-values of wall elements for improved insulation properties. The porosity of lightweight aggregate provides a source of water for internal curing of the concrete that provides continued enhancement of concrete strength and durability. This does not preclude the need for external curing.
Structural lightweight concrete has been used for bridge decks, piers and beams, slabs and wall elements in steel and concrete frame buildings, parking structures, tilt-up walls, topping slabs and composite slabs on metal deck.
HOW is Structural Lightweight Concrete Used?
Lightweight concrete can be manufactured with a combination of fine and coarse lightweight aggregate or coarse lightweight aggregate and normal weight fine aggregate. Complete replacement of normal-weight fine aggregate with a lightweight aggregate will decrease the concrete density by approximately 10 lb./ft3 (160 kg/m3).
Designers recognize that structural lightweight concrete will not typically serve in an oven-dry environment. Therefore, structural design generally relies on an equilibrium density (sometimes referred to as air-dry density); the condition in which some moisture is retained within the lightweight concrete. Equilibrium density is a standardized value intended to represent the approximate density of the in-place concrete when it is in service. Project specifications should indicate the required equilibrium density of the lightweight concrete. Equilibrium density is defined in ASTM C567, and can be calculated from the concrete mixture proportions. Field acceptance is based on measured density of fresh concrete in accordance with ASTM C138. Equilibrium density will be approximately 3 to 8 lb./ ft3 (50 to 130 kg/m3) less than the fresh density and a correlation should be agreed upon prior to delivery of concrete. The tolerance for acceptance on fresh density is typically ±3 lb./ ft3 (±50 kg/m3) from the target value.
Lightweight aggregates must comply with the requirements of ASTM C330. Due to the cellular nature of lightweight aggregate particles, absorption typically is in the range of 5% to 20% by weight of dry aggregate. Lightweight aggregates generally require wetting prior to use to achieve a high degree of saturation. Some concrete producers may not have the capability of pre-wetting lightweight aggregates in cold weather if temperature-controlled storage is not available. Some lightweight aggregate suppliers furnish vacuum-saturated aggregate.
With the exception of bridges and marine structures, specifications for structural lightweight concrete do not typically have a requirement for maximum water-to-cementitious materials (w/cm) ratio. The w/cm ratio of structural lightweight concrete cannot be precisely determined because of the difficulty in determining the absorption of lightweight aggregate.
Air content of structural lightweight concrete must be closely monitored and controlled to ensure that the density requirements are being achieved. Testing for air content must be according to the volumetric method, ASTM C173 or calculated using the gravimetric method described in ASTM C138. Virtually all lightweight concrete is air-entrained.
Finishing lightweight concrete requires proper attention to detail. Excessive amounts of water or excessive slump will cause the lightweight aggregate to segregate from the mortar. Bull floating will generally provide an adequate finish. If the surface for an interior floor is to receive a hard troweled finish, use precautions to minimize the formation of blisters or delamination. See CIPs 13 and 20 for discussions on blisters and delamination, respectively.
Due to the inherent higher total moisture content of lightweight concrete, it typically takes a longer time than normal-weight concrete to dry to levels that might be considered adequate for application of floor covering materials.
The splitting tensile strength of lightweight concrete is used in structural design criteria. The design engineer may request the information for a particular source of lightweight aggregate prior to the design. The splitting tensile strength corresponding to the specified compressive strength is determined in laboratory evaluations. Splitting tensile strength testing is not used as a basis for field acceptance of concrete.
Ensure that the requirements of the designer relative to fire resistance or insulation properties of lightweight concrete building elements are in conformance with applicable industry standards. For a successful project, information is available from the supplier of lightweight aggregate and the ready mixed concrete producer. With proper planning, structural lightweight concrete can provide an economical solution to many engineering applications.
Guidelines for Pumping
Lightweight concrete placements frequently employ pumps and this can be done successfully when a few precautions are considered prior to the actual placement.
1. Aggregate should be adequately pre-soaked as pressure during pumping will drive water into the aggregate pores and cause slump loss that may result in plugging of the pump line and difficulties in placement and finishing.
2. Pump lines should be as large as possible, preferably 5-inch (125-mm) diameter, with a minimum number of elbows, reducers or rubber hose sections.
3. The lowest practical pressure should be used.
4. Pump location should be such that vertical fall of the concrete is minimized.
5. Adjustments to mixture characteristics, such as slump, aggregate content and air content may be necessary to ensure adequate pumpability for the job conditions.
6. Decide on where concrete samples for acceptance tests will be taken and what implications this would have on the concrete mixture proportions and properties as delivered to the jobsite.
1. Guide for Structural Lightweight Aggregate Concrete, ACI 213R, American Concrete Institute, Farmington Hills, MI, www.concrete.org.
2. Guide for Determining the Fire Endurance of Concrete Elements, ACI 216R, American Concrete Institute, Farmington Hills, MI.
3. ASTM C94, C138, C173, C330 and C567, Annual Book of ASTM Standards, Volume 04.02, ASTM International, West Conshohocken, PA, www.astm.org.
4. Lightweight Concrete and Aggregates, Tom Holm, ASTM 169C, Chapter 48, ASTM International, West Conshohocken, PA.
5. Pumping Structural Lightweight Concrete, Info Sheet #4770.1, Expanded Shale Clay and Slate Institute, Salt Lake City, Utah, www.escsi.org.
WHAT is Self-Consolidating Concrete?
Self-consolidating concrete (SCC), also known as self-compacting concrete, is a highly flowable, non-segregating concrete that can spread into place, fill the formwork and encapsulate the reinforcement without any mechanical consolidation. The flowability of SCC is measured in terms of spread when using a modified version of the slump test (ASTM C143). The spread (slump flow) of SCC typically ranges from 18 to 32 inches (455 to 810 mm) depending on the requirements for the project. The viscosity, as visually observed by the rate at which concrete spreads, is an important characteristic of plastic SCC and can be controlled when designing the mix to suit the type of application being constructed.
WHY is SCC Used?
Some of the advantages of using SCC are:
1. Can be placed at a faster rate with no mechanical vibration and less screeding, resulting in savings in placement costs.
2. Improved and more uniform architectural surface finish with little to no remedial surface work.
3. Ease of filling restricted sections and hard-to-reach areas. Opportunities to create structural and architectural shapes and surface finishes not achievable with conventional concrete.
4. Improved consolidation around reinforcement and bond with reinforcement
5. Improved pumpability.
6. Improved uniformity of in-place concrete by eliminating variable operator-related effort of consolidation.
7. Labor savings.
8. Shorter construction periods and resulting cost savings.
9. Quicker concrete truck turn-around times enabling the producer to service the project more efficiently.
10. Reduction or elimination of vibrator noise potentially increasing construction hours in urban areas.
11. Minimizes movement of ready mixed trucks and pumps during placement.
12. Increased jobsite safety by eliminating the need for consolidation.
HOW is SCC Achieved?
Two important properties specific to SCC in its plastic state are its flowability and stability. The high flowability of SCC is generally attained by using high-range-water-reducing (HRWR) admixtures and not by adding extra mixing water. The stability or resistance to segregation of the plastic concrete mixture is attained by increasing the total quantity of fines in the concrete and/or by using admixtures that modify the viscosity of the mixture. Increased fines contents can be achieved by increasing the content of cementitious materials or by incorporating mineral fines. Admixtures that affect the viscosity of the mixture are especially helpful when grading of available aggregate sources cannot be optimized for cohesive mixtures or with large source variations. A well distributed aggregate grading helps achieve SCC at reduced cementitious materials content and/or reduced admixture dosage. While SCC mixtures have been successfully produced with 1½ inch (38 mm) aggregate, it is easier to design and control with smaller size aggregate. Control of aggregate moisture content is also critical to producing a good mixture. SCC mixtures typically have a higher paste volume, less coarse aggregate and higher sand-coarse aggregate ratio than typical concrete mixtures.
Retention of flowability of SCC at the point of discharge at the jobsite is an important issue. Hot weather, long haul distances and delays on the jobsite can result in the reduction of flowability whereby the benefits of using SCC are reduced. Job site water addition to SCC may not always yield the expected increase in flowability and could cause stability problems.
Full capacity mixer truck loads may not be feasible with SCCs of very high flowability due to potential spillage. In such situations it is prudent to transport SCC at a lower flowability and adjust the mixture with HRWR admixtures at the job site. Care should be taken to maintain the stability of the mixture and minimize blocking during pumping and placement of SCC through restricted spaces. Formwork may have to be designed to withstand fluid concrete pressure and conservatively should be designed for full head pressure. SCC may have to be placed in lifts in taller elements. Once the concrete is in place it should not display segregation or bleeding/settlement.
SCC mixtures can be designed to provide the required hardened concrete properties for an application, similar to regular concrete. If the SCC mixture is designed to have a higher paste content or fines compared to conventional concrete, an increase in shrinkage may occur.
HOW to Test SCC:
Several test procedures have been successfully employed to measure the plastic properties of SCC. The slump flow test (see Figure 1), using the traditional slump cone, is the most common field test and is in the process of being standardized by ASTM. The slump cone is completely filled without consolidation, the cone lifted and the spread of the concrete is measured. The spread can range from 18 to 32 inches (455 to 810 mm). The resistance to segregation is observed through a visual stability index (VSI). The VSI is established based on whether bleed water is observed at the leading edge of the spreading concrete or if aggregates pile at the center. VSI values range from 0 for “highly stable” to 3 for unacceptable stability.
During the slump flow test, the viscosity of the SCC mixture can be estimated by measuring the time taken for the concrete to reach a spread diameter of 20 inches (500 mm) from the moment the slump cone is lifted up. This is called the T20 (T50) measurement and typically varies between 2 and 10 seconds for SCC. A higher T20 (T50) value indicates a more viscous mix which is more appropriate for concrete in applications with congested reinforcement or in deep sections. A lower T20 (T50) value may be appropriate for concrete that has to travel long horizontal distances without much obstruction.
The U-Box and L-Box tests are used for product development or prequalification and involve filling concrete on one side of the box and then opening a gate to allow the concrete to flow through the opening containing rebar. The J-ring test is a variation to the slump flow, where a simulated rebar cage is placed around the slump cone and the ability of the SCC mix to spread past the cage without segregation is evaluated. The U-box, L-box and J-ring tests measure the passing ability of concrete in congested reinforcement. Another test being standardized is a column test which measures the coarse aggregate content of concrete at different heights in a placed columnar specimen as an indication of stability or resistance to segregation.
HOW to Order or Specify SCC:
When ordering and/or specifying SCC, consideration must be given to the end use of the concrete. Ready mixed concrete producers will generally have developed mixture proportions based on performance and applications. The required spread (slump flow) is based on the type of construction, selected placement method, complexity of the formwork shape and the configuration of the reinforcement. ACI Committee 237 is completing a guidance document that will provide guidelines to select the appropriate slump flow for various conditions. The lowest slump flow required for the job conditions must be specified. This will ensure SCC can be attained easily with required stability and at the lowest possible cost. The hardened concrete properties should be specified by the design professional based on structural and service requirements of the structure. For the most part, hardened concrete properties of SCC are similar to conventional concrete mixtures. Based on the requirements of each project, SCC concrete designs can be submitted by the producer only after specification provisions regarding the performance of the concrete in its plastic and hardened state are clearly defined.
1. Emerging Technology Series on Self-Consolidating Concrete (under development), ACI 237, ACI International, Farmington Hills, MI, http://www.concrete.org.
2. Proceedings of the International Workshop on Self-Compacting Concrete, Kochi, Japan, August 1998.
3. Specification and Guidelines for Self-Compacting Concrete, EFNARC (European Federation of National Trade Associations), Surrey, UK, February 2002, http://www.efnarc.org/.
4. Proceedings of the First North American Conference on the Design and Use of Self-Consolidating Concrete, Chicago, USA, November 2002.
WHAT is Pervious Concrete?
Pervious concrete is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other sources to pass through it, thereby reducing the runoff from a site and recharging ground water levels. The high porosity is attained by a highly interconnected void content. Typically pervious concrete has little to no fine aggregate and has just enough cementitious paste to coat the coarse aggregate particles while preserving the interconnectivity of the voids. Pervious concrete is traditionally used in parking areas, areas with light traffic, pedestrian walkways, and greenhouses. It is an important application for sustainable construction.
WHY Use Pervious Concrete?
The proper utilization of pervious concrete is a recognized Best Management Practice by the U.S. Environmental Protection Agency (EPA) for providing first-flush pollution control and storm water management. As regulations further limit storm water runoff, it is becoming more expensive for property owners to develop real estate, due to the size and expense of the necessary drainage systems. Pervious concrete reduces the runoff from paved areas, which reduces the need for separate storm water retention ponds and allows the use of smaller capacity storm sewers. This allows property owners to develop a larger area of available property at a lower cost. Pervious concrete also naturally filters storm water and can reduce pollutant loads entering into streams, ponds and rivers. Pervious concrete functions like a storm water retention basin and allows the storm water to infiltrate the soil over a large area, thus facilitating recharge of precious groundwater supplies locally. All of these benefits lead to more effective land use.
Pervious concrete can also reduce the impact of development on trees. A pervious concrete pavement allows the transfer of both water and air to root systems allowing trees to flourish even in highly developed areas.
HOW to Install Pervious Concrete Pavement?
An experienced installer is vital to the success of pervious concrete pavements. As with any concrete pavement, proper subgrade preparation is important. The subgrade should be properly compacted to provide a uniform and stable surface. When pervious pavement is placed directly on sandy or gravelly soils it is recommended to compact the subgrade to 92% to 96% of the maximum density (ASTM D1557). With silty or clayey soils, the level of compaction will depend on the specifics of the pavement design and a layer of open graded stone may have to be placed over the soil. Engineering fabrics are often used to separate fine grained soils from the stone layer. Care must be taken not to over-compact soil with swelling potential. Moisten the subgrade prior to concrete placement, and wheel ruts from the construction traffic should be raked and re-compacted. Moistening the subgrade prevents pervious concrete from setting and drying too quickly.
Typically pervious concrete has a water-to-cementitious materials ratio (w/cm) of 0.35 to 0.45 with a void content of 15% to 25%. The mixture is composed of cementitious materials, coarse aggregate and water with little to no fine aggregates. Addition of a small amount of fine aggregate will generally reduce the void content and increase the strength, which may be desirable in certain situations. This material is sensitive to changes in water content, so field adjustment of the fresh mixture is usually necessary. The correct quantity of water in the concrete is critical. Too much water will cause segregation, and too little water will lead to balling in the mixer and very slow mixer unloading. Too low a water content can also hinder adequate curing of the concrete and lead to a premature raveling surface failure. A properly proportioned mixture gives the mixture a wet-metallic appearance or sheen.
A pervious concrete pavement may be placed with either fixed forms or slip-form paver. The most common approach to placing pervious concrete is in forms on grade that have a riser strip on the top of each form such that the strike off device is actually ⅜ – ½ in. (9 to 12 mm) above final pavement elevation. Strike off may be by vibratory or manual screeds, although vibratory screens are preferable. After striking off the concrete, the riser strips are removed and the concrete compacted by a manually operated roller that bridges the forms. Rolling consolidates the fresh concrete to provide strong bond between the paste and aggregate, and creates a smoother riding surface. Excessive pressure when rolling should be avoided as it may cause the voids to collapse. Rolling should be performed immediately after strike off.
Jointing pervious concrete pavement follows the same rules as for concrete slabs on grade, with a few exceptions. With significantly less water in the fresh concrete, shrinkage of the hardened material is reduced significantly, thus, joint spacings may be wider. The rules of jointing geometry, however, remain the same (See CIP 6). Joints in pervious concrete are tooled with a rolling jointing tool. This allows joints to be cut in a short time, and allows curing to continue uninterrupted. Proper curing is essential to the structural integrity of a pervious concrete pavement. Curing ensures sufficient hydration of the cement paste to provide the necessary strength in the pavement section to prevent raveling. Curing should begin within 20 minutes of concrete placement and continue through 7 days. Plastic sheeting is typically used to cure pervious concrete pavements.
HOW to Test and Inspect Pervious Concrete Pavement:
Pervious concrete can be designed to attain a compressive strength ranging from 400 psi to 4000 psi (2.8 to 28 MPa) although strengths of 600 psi to 1500 psi (2.8 to 10 MPa) are more common. Pervious concrete, however, is not specified or accepted based on strength. More important to the success of a pervious pavement is the void content. Acceptance is typically based on the density (unit weight) of the in-place pavement. An acceptable tolerance is plus or minus 5 lb./ft3 (80 kg/m3) of the design density. This should be verified through field testing. The fresh density (unit weight) of pervious concrete is measured using the jigging method described in ASTM C29. Slump and air content tests are not applicable to pervious concrete. If the pervious concrete pavement is an element of the storm water management plan, the designer should ensure that it is functioning properly through visual observation of its drainage characteristics prior to opening of the facility. Questions have been raised about the freeze-thaw durability of pervious concrete. Even though most of the experience with pervious concrete has been in warmer climates, recently there have been several pervious concrete projects in colder climates. Pervious concrete in freeze-thaw environment must not become fully saturated. Saturation of installed pervious concrete pavement can be prevented by placing the pervious concrete on a thick layer of 8 to 24 inches (200 to 600 mm) of open graded stone base. Limited laboratory testing has shown that entrained air may improve the freeze-thaw durability even when the pervious concrete is in a fully saturated condition. However, the entrained air content cannot be verified by any standard ASTM test procedure.
EPA recommends that pervious concrete pavement be cleaned regularly to prevent clogging. Cleaning can be accomplished through vacuum sweeping or high pressure washing. Even though pervious concrete and the underlying soil provide excellent filtration capabilities, all the contaminants may not be removed. In critical situations to preserve the quality of ground water, storm water testing is recommended.
1. Pervious Pavement Manual, Florida Concrete and Products Association Inc., Orlando, FL. http://www.fcpa.org.
2. Richard C. Meininger, “No-Fines Pervious Concrete for Paving,” Concrete International, Vol. 10, No. 8, August 1988, pp. 20-27.
3. Storm Water Technology Fact Sheet Porous Pavement, United States Environmental Protection Agency, EPA 832-F-99-023, September 1999. www.epa.gov/npdes.
4. Recommended Specifications for Portland Cement Pervious Pavement, Georgia Concrete and Products Association, Inc. Tucker, GA, www.gcpa.org.
5. Pervious Concrete Pavement – A Win-Win System, Concrete Technology Today, CT 032, August 2003, Portland Cement Association, Skokie, IL, http://www.cement.org.
6. ASTM D1557-00, “Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort,” Annual Book of ASTM Standards, Vol. 04.08, ASTM International, West Conshohocken, PA, www.astm.org.
7. Pervious Concrete, ACI 522R Report, (under review), ACI International, Farmington Hills, MI, http://www.concrete.org.
WHAT is Concrete Maturity?
The maturity concept uses the principle that concrete strength (and other properties) is directly related to both age and its temperature history. Maturity methods provide a relatively simple approach for reliably estimating the in-place early-age compressive (and flexural) strength of concrete (14 days or less) during construction. The maturity concept assumes that samples of a concrete mixture of the same maturity will have similar strengths, regardless of the combination of time and temperature yielding the maturity. The measured maturity index of in-place concrete, a function of temperature history and age, is used to estimate its strength development based on a pre-determined calibration of the time-temperature-strength relationship developed from laboratory tests for that mixture.
WHY use Maturity Methods?
Maturity methods are used as a more reliable indicator of the in-place strength of concrete during construction in lieu of testing field-cured cylinders. The traditional approach of measuring the strength of field-cured cylinders, cured in the same conditions as the structure, are used to schedule construction activities such as removal of forms or reshoring, backfilling walls, schedule pre-stressing and post-tensioning operations, determining the time for opening the pavements or bridges to traffic, sawing joints, and to determine when protection measures can be terminated in cold weather.
Maturity methods use the fundamental concept that concrete properties develop with time as the cement hydrates and releases heat. The rate of strength development at early ages is related to the rate of hydration of cement. Heat generated from the hydration reaction will be recorded as a temperature rise in the concrete. The main advantage of the maturity method is that it uses the actual temperature profile of the concrete in the structure to estimate its in-place strength. The traditional approach of using field-cured cylinders does not replicate the same temperature profile of the in-place concrete and likely does not estimate its in-place strength as accurately. With maturity methods strength information is provided in real-time since maturity measurements are made on-site at any time. As a result, construction workflow is optimized, and construction activity timing can be based on more accurate in-place strength information.
HOW are Maturity Methods Used?
The procedure for estimating concrete strength using maturity concepts is described in ASTM C1074 – Standard Practice for Estimating Concrete Strength by the Maturity Method. The temperature-time-strength relationship of a concrete mixture is developed in laboratory tests. This establishes one of two maturity functions (explained below) for that mixture. During construction, a maturity index is determined from measured temperature and age. The maturity index is used to estimate the in-place strength from the pre-established maturity-strength relationship. This is illustrated in Figure 1.
The maturity concept is governed by the underlying assumption that concrete samples of a given mixture will have the same strength when they have the same maturity index. For example, concrete cured at a temperature of 50⁰F (10⁰C) for 7 days may have the same maturity index as concrete cured at 80⁰F (27⁰C) for 3 days and therefore would have similar strengths.
ASTM C1074 provides two types of maturity functions: The Nurse-Saul function assumes that the rate of strength development is a linear function of temperature. The maturity index is expressed as a temperature-time factor (TTF) from the product of temperature and time in ⁰C-hours or ⁰C-days. The method requires a value for a datum temperature below which it is assumed that no cement hydration occurs. ASTM C1074 provides a procedure to determine this value for the specific concrete mixture or suggests assuming a value of 0⁰C. The accuracy of the Nurse-Saul prediction breaks down when there are wide ranges of curing temperatures, but its accuracy is considered adequate for most applications.
The Arrhenius function assumes that the rate of strength development follows an exponential relationship with temperature. The maturity index is expressed in terms of an equivalent age at a reference temperature. Actual age is typically normalized to an equivalent age at 20⁰C or 23⁰C. A value of the activation energy is needed for this maturity function. ASTM C1074 provides a procedure to determine the activation energy or alternatively suggests that a value of 40,000 to 45,000 J/mol is a reasonable assumption for concrete with a Type I cement. Using the established maturity function, the actual age and measured temperature is converted to an equivalent age to predict the concrete strength.
The Arrhenius function is considered to be more scientifically accurate. However, the Nurse-Saul function is more commonly used by the various state highway agencies in the United States mainly due to its simplicity.
The maturity method involves the following steps:
• Determine a strength-maturity relationship for the concrete mixture to be used in the structure using materials and mixture proportions proposed for the project. Monitor the temperature history of the test specimens using temperature probes embedded in one or more of the cylinders. Measure the compressive strength of standard-cured test cylinders at various ages. These data are used to establish the maturity function (Nurse-Saul or Arrhenius).
• Measure the temperature history of the concrete in the structure by embedding sensors at locations in the structure that are critical in terms of exposure conditions and structural requirements.
• Calculate the maturity index from the recorded temperature and age.
• Estimate the in-place strength of the field concrete from the calculated maturity index and the predetermined strength-maturity relationship (Figure 1).
Some of the limitations of maturity methods that can lead to erroneous estimation of in-place strength are:
a. Concrete used in the structure is not representative of that used for the laboratory calibration tests, due to changes in materials, batching accuracy, air content, etc.
b. High early-age temperatures will result in incorrect prediction of long-term strength;
c. Concrete should be properly placed, consolidated and cured; conditions should permit continued cement hydration;
d. Use of datum temperature or activation energy values that are not representative of the concrete mixture.
Points (a) and (b) above are inherent limitations of maturity methods. ASTM C1074 suggests that supplementary tests be conducted prior to performing safety-critical operations such as formwork removal or post-tensioning. While these additional tests are not always required, it is a good idea to periodically verify that the established maturity-strength relationship for the specific concrete is still valid. Suggested methods include:
(1) In-place non-destructive tests ASTM C803 (penetration resistance), ASTM C873 (cast-in-place cylinders), or ASTM C900 (pullout strength).
(2) Test method C918 that projects later-age strength from early-age tests.
(3) Using accelerated curing of test specimens to estimate later-age strength according to ASTM C684.
(4) Early-age tests of field molded cylinders instrumented with maturity instruments.
Strength-maturity relationships, datum temperature and activation energy are concrete mixture specific. Therefore, any significant modifications to the mixture design or material source should be accompanied by a re-calibration of these values.
Several maturity devices are commercially available that continuously measure concrete temperature, calculate maturity and display the maturity index digitally at any time. An unlimited number of locations can be monitored simultaneously. It is important to select a system that is rugged, provides uninterruptible and unalterable data, supports the maturity function being used for the project, and allows adjustment of the appropriate maturity constants.
It is important to realize that maturity is not intended to replace standard-cured cylinder testing. Maturity used in conjunction with other non-destructive testing can replace field-cured cylinder testing and facilitate decision making for construction operations. It can be a good tool for quality control while reducing the amount of strength tests performed. Because of maturity testing, projects are proceeding more quickly, safely, and economically as a result of having the right information at the right place and at the right time.
1. ASTM C1074-04, “Standard Practice for Estimating Concrete Strength by the Maturity Method,” ASTM International, www.astm.org.
2. Guide to Non Destructive Testing of Concrete, FHWA, Publication No. FHWA-SA-97-105, Sep. 1997, www.fhwa.dot.gov/pavement/.
3. Significance of Tests and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C, ed. Klieger, P., and Lamond, J.F.,1994, www.astm.org.
4. Carino, N.J., “The Maturity Method,” Chapter 5 in Handbook on Nondestructive Testing of Concrete, 2nd Edition, Malhotra, V.M., and Carino, N.J., Eds., CRC Press Inc, Boca Raton, FL, and ASTM International, 2004.
WHAT is Popout?
A “popout” is a small, generally cone-shaped cavity in a horizontal concrete surface left after a near-surface aggregate particle has expanded and fractured. Generally, part of the fractured aggregate particle will be found at the bottom of the cavity with the other part of the aggregate still adhering to the point of the popout cone. The cavity can range from ¼ in. (6 mm) to few inches in diameter.
WHY do Concrete Popouts Occur?
The aggregate particle expands and fractures as a result of a physical action or a chemical reaction:
The origin of a physical popout usually is a near-surface aggregate particle having a high absorption and relatively low relative density (specific gravity). As that particle absorbs moisture or if freezing occurs un- der moist conditions, its swelling creates internal pressures sufficient to rupture the particle and the overlying concrete surface. The top portion of the fractured aggregate particle separates from the concrete surface taking a portion of the surface mortar with it. In some cases the aggregate forces water into the surrounding mortar as it freezes thus causing the surface mortar to pop off, exposing an intact aggregate particle. Clay balls, coal, wood or other contaminants can uptake water and swell even without freezing, but the resulting pressure rarely is great enough to cause popouts. Also, there are reported cases of grain (soybeans, corn) contamination of aggregate shipments that have resulted in surface popouts. Such occurrences are not within the scope of this document.
Popouts as a result of physical action are typically only a problem with exterior flatwork in climates subject to freezing and thawing under moist conditions and resulting expansion. Even aggregates which meet the requirements of ASTM C33 Class 5S, for architectural concrete in severe exposure, allow several types of particles which may cause popouts when exposed to freezing and thawing in the saturated condition. The most common type of particles resulting in popouts are low density chert in natural aggregate deposits.
Crushed aggregates are less likely to contain light-weight, absorptive particles which are more susceptible to popouts.
The cause of a popout due to a chemical reaction is often related to alkali-silica reaction (ASR). Alkalis from cement or other source cause an environment of high pH (high concentrations of OH ions) causing the breakdown of silica and formation of an ASR gel, which absorbs water and expands, removing a small portion of the surface mortar with it. These are called ASR popouts. They are typically small and are often accompanied by a small spot that is discolored and/or appears to be damp. The aggregate particle does not often fracture and split as is the case of popouts from physical action. However, the ASR phenomenon can result in micro-fractures within the aggregate particles. Some ASR popouts can occur within a few days after the concrete is placed.
HOW to Avoid Concrete Popouts:
Most popouts are aesthetic defects that do not impact the structural performance of the concrete members. A large number of popouts however make it easier for water and other harmful chemicals to enter the concrete, which can ultimately lead to other forms of deterioration, such as corrosion of steel reinforcement. The following steps can be taken to avoid concrete popouts.
1. Avoid using aggregates which contain particles which may cause popouts or that have a history of popouts. However, in some parts of the United States, the available natural gravels contain some particles that are likely to result in surface popouts. Due to the unavailability of economical alternate aggregates, the occurrence of popouts on sidewalks and pavements is an accepted, albeit undesirable, likelihood in those locations.
2. If popouts are unacceptable, an alternate source of aggregates must be located. If appropriate, two-course construction can be used, whereby the popout susceptible aggregate is used for the lower course and the pop-out free aggregate that is likely to be more expensive is used for the surface course.
3. Aggregates can be beneficiated to remove light-weight materials, but the added cost of beneficiation can be prohibitive for most uses.
4. Reduce the water-to-cementitious materials ratio of the concrete, as this will reduce the likelihood of saturation and will increase the resistance to swelling forces. Provide proper curing for exterior flatwork, as this results in improved strength of the cementitious materials, especially on the surface. This will reduce permeability thereby lowering the amount of water migrating to coarse aggregate particles. These steps can reduce the frequency, but will not necessarily, eliminate popouts.
5. Reduce the maximum aggregate size, as smaller aggregates will develop lower stresses due to freezing, and fewer popouts will occur. Those that do will be smaller and less objectionable.
1. Use a low-alkali cement or a non-reactive aggregate. This is often not a practical option in many regions.
2. Flush the surfaces with water after the concrete has hardened and before applying the final curing. This will remove the alkalis that may have accumulated at the surface as a result of evaporation of bleed water.
3. Permit the use of Class F fly ash or slag cement as a partial cement substitute to reduce the permeability of the paste and mitigate deleterious reactions due to ASR.
HOW to Repair Concrete Popouts:
Prior to undertaking a repair program, it is advisable to confirm the cause of the popouts by obtaining core samples containing one or more typical popouts and having them studied by a qualified petrographer.
Popouts can be repaired by chipping out the remaining portion of the aggregate particle in the surface cavity, cleaning the resulting void, and by filling the void with a proprietary repair material such as a dry pack mortar, epoxy mortar, or other appropriate material following procedures recommended by the manufacturer. It will be difficult to match the color of the existing concrete. If the popouts in a surface are too numerous to patch individually, a thin bonded concrete overlay may be used to restore a uniform surface appearance. Specific recommendations for such overlays are beyond the scope of this publication.
1. Popouts: Causes, Prevention, Repair, Concrete Technology Today, PL852, Portland Cement Association, www.cement.org.
2. Guide to Residential Cast-in-Place Concrete Construction, ACI 332R, American Concrete Institute, Farmington Hills, MI, www.concrete.org.
3. Closing in on ASR Popouts, Concrete Technology Today, CT022, Portland Cement Association, www.cement.org.
4. R. Landgren, and D. W. Hadley, Surface Popouts caused by Alkali-Aggregate Reaction, Portland Cement Association, RD121, www.cement.org.
5. N. E. Henning, K. L. Johnson, and L. J. Smith, Popouts, Construction Bulletin, March 4, 1971, Upper Midwest News Weekly.
6. Richard H. Campbell, Wendell Harding, Edward Misenhimer, and Leo P. Nichelson, Surface Popouts: How are they Affected by Job Conditions?, ACI Journal, American Concrete Institute, June 1974, pp. 284-288.
WHAT is Acceptance of Concrete?
Acceptance testing is the process of testing representative samples of concrete furnished to a project. Acceptance testing includes tests on plastic concrete for slump, air content, density (unit weight), temperature, and tests on hardened concrete for strength and other durability properties as required in contract documents or project specifications.
Acceptance testing on hardened concrete is conducted in accordance with standardized procedures to determine whether the concrete as delivered has the potential of developing the desired properties intended by the design professional. These test results are not intended to imply the actual properties of concrete in the structure. There are several variables during construction that will impact in-place concrete properties that are beyond the control of the concrete supplier.
WHY Conduct Acceptance Testing?
Acceptance testing is conducted to quantitatively verify that concrete conforms to the requirements of the purchaser. The requirements of the purchaser, relative to the tests and acceptance criteria, are generally stated in writing in project specifications or are invoked by reference to industry standards, such as ACI 301, ACI 318 and ASTM C94.
Contractors are legally bound to facilitate or to conduct acceptance testing by local jurisdictions which adopt model codes such as such as the International Building Code. These model codes in turn refer to the ACI 318 Building Code.
It is important for those involved in testing to realize that the results of acceptance testing have significant implications on the project schedule, cost to project participants, and may impact the safety of the structure and its inhabitants.
HOW Should Acceptance Testing be Conducted?
Acceptance testing must be conducted by certified technicians who have demonstrated a written and practical knowledge of performing tests in accordance with the pertinent standards. Certification programs are offered by the American Concrete Institute (ACI) and other organizations for test conducted in the field and laboratory. Laboratories performing acceptance tests should conform to the requirements of ASTM C1077. Laboratories should be proficient in testing concrete, should have been through quality system audit by an independent evaluation organization and participate in reference sample testing programs to evaluate their testing proficiency and correct processes, if necessary. Laboratory inspections and reference sample programs of the Cement and Concrete Reference Laboratory (CCRL), or equivalent, are established standards.
All acceptance testing of concrete must be conducted in accordance with established standards referenced in contract documents. Any deviation from standard procedures is adequate reason for invalidating test results so obtained.
It is important that the process of conducting acceptance testing and the responsibilities of all involved parties for proper sampling, specimen storage, handling, transportation to the laboratory, jobsite sample disposition and subsequent laboratory testing are clearly defined prior to the start of a project. In medium to large projects, a pre-construction conference is strongly recommended to establish processes, contingencies and responsibilities (CIP 32).
Sampling: Samples of concrete from concrete delivery vehicles for acceptance tests should be obtained in accordance with ASTM C172. The sample should be obtained at the end of the truck chute. Two or more portions of concrete as discharged from the middle portion of the load are composited to obtain a sample that is representative of the load. When the specification requires additional tests to be conducted at the point of placement in the structure after concrete has been moved through some conveying means (such as a pump, bucket or conveyor), sampling procedures should be conducted such that the means of conveyance is not temporarily shut off or relocated to ease sampling as this can temporarily change the properties measured. ASTM C94 permits a preliminary sample to be obtained after 0.25 yd3 (0.20 m3) has been discharged to measure slump and air content and make appropriate adjustments to the load at the jobsite. The preliminary sample should not be used to make specimens for acceptance tests of hardened concrete.
Slump and Air Content: When the slump and air content measured on the preliminary sample are lower than specified, jobsite adjustments with water or admixtures followed by adequate mixing are permitted. If slump and air contents are higher, then a retest is made immediately, and if the retest fails, then the concrete is considered to have failed the requirements of the specification.
Slump of concrete is measured in accordance with ASTM C143. The tolerance on slump varies by slump level as ordered or specified. The slump tolerances of ASTM C94 are summarized in the table below. There is no established tolerance for the slump flow of self-consolidating concrete, which is measured in accordance with ASTM C1611.
The air content of concrete is measured in accordance with the pressure method, ASTM C231 or by the volumetric method, ASTM C173 for lightweight concrete or for aggregates with high absorptions. For air-entrained concrete, the tolerance on air content as ordered or specified is ±1.5%.
Density and Yield: When samples are obtained for strength tests, ASTM C94 requires measuring the density (unit weight) of the concrete in accordance with ASTM C138. This can be done by determining the weight of the air meter container after the sample has been prepared. The minimum container size based on the nominal maximum size of the aggregate in the concrete mixture should be followed. Density measurements can also be correlated with air content measurements and can be an indicator of the water content in the mixture. When determining yield, ASTM C94 requires that the density should be determined on separate samples from three different loads of concrete and compliance with volume of concrete ordered be done on that basis (CIP 8).
Temperature: The temperature of concrete is measured in accordance with ASTM C1064. Temperature is measured to determine conformance to temperature limits in a specification and is a required test when strength test specimens are prepared. It is permitted to measure the temperature of concrete in place when it is not measured in conjunction with strength tests.
Hardened Concrete Tests: ASTM C31 describes the procedures for preparing cylinders and beams for compressive and flexural strength tests, respectively. It describes the procedures for storing specimens at the jobsite and transporting specimens to the laboratory. ASTM C31 requires the test specimens to be maintained in a moist condition in a temperature range of 60 to 80°F (16 – 27°C) in the field. For high strength concrete with a specified strength greater than 5000 psi (35 MPa), the storage temperature limits are tighter at 68 to 78°F (20 – 26°C). A record of the temperature conditions during field storage of the specimens should be maintained. A curing box with max/min temperature recording device is generally required to verify conformance to these requirements. The same procedures should be adhered to for test specimens prepared for other tests. Test specimens should not be stored at the jobsite for longer than 48 hours. Specimens should be protected with adequate cushioning when transported to the laboratory. Transportation time should not exceed 4 hours. Specimens delivered to the laboratory should be stripped of molds, logged and placed in moist curing as defined in ASTM C31 as soon as possible and no later than about 6 hours. More details can be found in CIPs 9 and 34.
While most specifications delegate the contractor with the responsibility for providing adequate facilities for storage of specimens at the jobsite, it is also incumbent on the testing technicians and the individual certifying test results to ensure that standard procedures are followed. Concrete is very sensitive to temperature and moisture at early ages and any deviation from standard procedures is a basis for rejecting results of these acceptance tests as it increases the likelihood of failing test results of acceptable concrete. This has implications to the project cost and schedule. A significant number of low strength results can be attributed to cylinders being subjected to non-standardized initial curing at the job site (CIP 9).
Test reports with data on all tests conducted, as well as other reporting requirements addressed in the standards, should be distributed to the owner or his representative, contractor and concrete producer in a timely manner. This is very important to the ongoing project quality and serves as documentation for the ability of the concrete producer to furnish quality concrete for future projects.
1. International Building Code 2006, International Code Council, Inc. Falls Church, Virginia, www.iccsafe.org.
2. ACI 301 and 318, American Concrete Institute, Farmington Hills, Michigan, www.concrete.org.
3. ASTM C31, C94, C138, C143, C172, C173, C231, C1064, C1077, C1611, Annual Book of ASTM Standards, Volume 4.02, ASTM International, West Conshohocken, Pennsylvania.
4. CIP 8, 9, 32, 34, Concrete in Practice Series, NRMCA, Silver Spring, Maryland, www.nrmca.org.
5. Technical Bulletins #1, #2, #3, Virginia Ready-Mix Concrete Association, Charlottesville, Virginia.
WHAT is Thermal Cracking?
Thermal cracking occurs due to excessive temperature differences within a concrete structure or its surroundings. The temperature difference causes the cooler portion to contract more than the warmer portion, which restrains the contraction. Thermal cracks appear when the restraint results in tensile stresses that exceed the in-place concrete tensile strength. Cracking due to temperature can occur in concrete members that are not considered mass concrete.
WHY Does Thermal Cracking Occur?
Hydration of cementitious materials generates heat for several days after placement in all concrete members. This heat dissipates quickly in thin sections and causes no problems. In thicker sections, the internal temperature rises and drops slowly, while the surface cools rapidly to ambient temperature. Surface contraction due to cooling is restrained by the hotter interior concrete that doesn’t contract as rapidly as the surface. This restraint creates tensile stresses that can crack the surface concrete as a result of this uncontrolled temperature difference across the cross section. In most cases, thermal cracking occurs at early ages. In rarer instances thermal cracking can occur when concrete surfaces are exposed to extreme temperature rapidly.
Concrete members will expand and contract when exposed to hot and cold ambient temperatures, respectively. Cracking will occur if this bulk volume change resulting from temperature variations is restrained. This is sometimes called temperature cracking and is a later-age and longer term issue.
The main factor that defines a mass concrete member is its minimum dimension. ACI 301 suggests that a concrete member with a minimum dimension of 4 feet (1.3 m) should be considered as mass concrete. Some specifications use a volume-to-surface ratio. Other factors where precautions for mass concrete should be taken even for thinner sections are with higher heat generating concrete mixtures – higher cementitious materials content or faster hydrating mixtures.
The main concern with mass concrete is a high thermal surface gradient and resulting restraint as discussed above. These conditions can result during the initial stages due to heat of hydration and during the later stages due to ambient temperature changes. Another factor is a temperature differential between a mass concrete member and adjoining elements. As the mass member cools from its peak temperature, the contraction is restrained by the element it is attached to, resulting in cracking. Examples are thick walls or dams restrained by the foundation.
Temperature cracking can occur in structures that are not mass structures. The upper surface of pavements and slabs are exposed to wide ranges of temperature while the bottom surface is relatively protected. A significant temperature differential between the surface and the protected surface can result in cracking. Concrete has a thermal coefficient of expansion in the range of 3 to 8 millionths/°F (5.5 to 14.5 millionths/°C). A concrete pavement cast at 95°F (35°C) during the summer in Arizona may reach a maximum temperature of 160°F (70°C) and a minimum temperature in winter of 20°F (- 7°C), resulting in an annual temperature cycle of 140°F (75°C). Expansion joints and spacing between joints have to be designed to withstand such temperature induced expansion and contraction to prevent cracking.
HOW to Recognize Thermal Cracking:
Thermal cracks caused by excessive temperature differentials in mass concrete appear as random pattern cracking on the surface of the member. Checkerboard or patchwork cracking due to thermal effects will usually appear within a few days after stripping the formwork. Temperature-related cracks in pavements and slabs look very similar to drying shrinkage cracks. They usually occur perpendicular to the longest axis of the concrete. They may become apparent any time after the concrete is placed, but usually occur within the first year or summer-winter cycle.
HOW to Minimize Thermal Cracking:
The key to reducing thermal or temperature-related cracking is to recognize when it might occur and to take steps to minimize it. A thermal control plan that is tailored to the specific requirements of the project specification is recommended. See Ref. 2 for guidance.
Typical specifications for mass concrete include a maximum temperature and a maximum temperature differential. The maximum temperature addresses the time it takes for the concrete member to reach a stable temperature and will govern the period needed for protective measures. Excessively high internal concrete temperatures also have durability implications. A temperature differential limit attempts to minimize excessive cracking due to differential volume change. A limit of 35°F (20°C) is often used. However, concrete can crack at lower or higher temperature differentials. Temperature differential is measured using electronic sensors embedded in the interior and surface of the concrete.
The peak temperature of a concrete mixture can be estimated assuming perfectly insulated conditions. See Ref. 1 and 2. Thermal modeling can also be used to predict temperature and potential for cracking based on thermal controls planned. Two models are HIPERPAV (www.hiperpav.com) for pavements and ConcreteWorks (www.texasconcreteworks.com) for pavements and other mass concrete members. Consultants can also assist with these analyses.
A large part of the responsibility to minimize thermal cracking lies with the designer and contractor. Steps include establishing the concrete mixture, specification limits for temperature of concrete as delivered and in the structure, insulating the structure and termination of protective measures, and in critical conditions, post-cooling of structural members.
Some steps to minimize thermal cracking are:
• Concrete mixture – Reduce heat of hydration by optimizing the cementitious materials using supplementary cementitious materials like fly ash or slag; or using a Portland cement that generates a lower heat of hydration. Avoid specifying an excessively low water-to-cementitious materials ratio (w/cm). Retarding chemical admixtures may delay but not reduce peak concrete temperatures. A cooler initial concrete temperature will reduce the peak temperature in the structure but needs to be balanced with practical feasibility and project costs.
• Mass concrete – Ensure that thermal control measures are agreed upon in a pre-construction meeting. Some things to consider include placement method and details, establishing temperature requirements for concrete as delivered and temperature monitoring of in-place concrete, curing methods and duration that do not increase temperature differentials, use of insulation – including when and how the insulation will be removed, and use of cooling pipes, if necessary. Placing concrete in lifts along with timing of successive lifts can minimize the overall peak temperature and time of thermal control, but this needs to be balanced against construction joint preparation and the design requirements. Water curing will cool concrete surfaces, and water retention curing methods may be more appropriate. Wood forms provide insulation while metal forms do not. Covering forms with insulating blankets may be necessary. The removal of insulation or formwork should be scheduled based on monitored in-place temperature and thermal shock to the surface should be avoided. Reinforcing steel protruding from a massive beam can act as a heat sink to draw heat out of the interior of the beam. When needed, cooling pipes, typically plastic, can be embedded in the concrete about 3 feet (1m) apart to reduce peak internal temperatures.
• Pavements and slabs – Reduce heat gain from solar radiation by misting slabs and pavements or providing shade for the work. Placing concrete in the early morning may result in a more critical situation if the peak temperature from hydration coincides with peak ambient temperature. Wind breaks may increase heat gain if they inhibit evaporative cooling of the concrete. Curing blankets can reduce heat loss from slabs and pavements during cold weather conditions.
The key to reducing thermal cracking is good communication between the designer, contractor, and concrete producer.
HOW to Repair Thermal Cracking:
Repairs to concrete structures must be undertaken with the advice and consent of the designer. Inappropriate repair techniques can result in greater damage later. Pavements and slabs can be repaired using acceptable and compatible repair materials or by cutting out the cracked areas and replacing them with infill strips. Repair of mass concrete members will depend on the crack width and the service conditions of the structure. Fine hairline cracks are aesthetically unpleasing and may not require any repair. However, these cracks may prove to be a future durability problem depending on the service conditions. Wider cracks may need to be sealed by epoxy injection followed by a seal coating. Recommendations for crack repair are provided in ACI 224.1R and by the International Concrete Repair Institute (www.icri.org).
1. ACI 207.2R, Report on Thermal and Volume Change Effects on Cracking of Mass Concrete, American Concrete Institute, www.concrete.org.
2. Mass Concrete for Buildings and Bridges, John Gajda, EB547, Portland Cement Association, www.cement.org.
3. ACI 224.1R, Causes, Evaluation, and Repair of Cracks in Concrete Structures, American Concrete Institute
4. Contractor’s Guide to Mass Concrete, Bruce A. Suprenant and Ward R. Malisch, Concrete International, ACI, Jan 2008, pp. 37-40.
5. Controlling Temperatures in Mass Concrete, John Gajda and Martha VanGeem, Concrete International, Jan 2002, pp. 59-62.
Alkali Aggregate Reaction (AAR) results in deleterious expansive cracking of concrete occurring at later ages after construction. While mostly inert, some concrete aggregates can react in the highly alkaline environment in concrete resulting in internal expansion that causes deleterious cracking. Alkalis include sodium and potassium that are minor constituents in Portland cement but can be from other concrete ingredients or from external sources. Expansion due to AAR is a slow process and results in visible deterioration 10 to 15 years after the concrete structure has been built. In rare cases deterioration at earlier ages may be observed.
Two forms of AARs are recognized: Alkali carbonate reactions (ACR) occur with dolomitic limestone aggregate of a specific mineralogy and microstructure. Sources of these aggregates is relatively rare. ACR is typically a more aggressive reaction and occurs earlier in the life of the structure.
Alkali silica reactions (ASR) occur with certain forms of silica (SiO2) minerals in aggregates that react in a high alkaline (pH) medium in concrete creating an expansive gel. The gel expands by absorbing moisture that causes the expansion of concrete and subsequent damage. Three conditions are required for deleterious ASR to occur:
1. reactive forms of silica in aggregate;
2. high alkali pore solution (pH) in concrete; and
3. presence of moisture.
WHY is AAR a Concern?
Deterioration to concrete structures due to AAR does not generally result in catastrophic failures. Where dimensional stability is important, such as in dams, the expansions can impact the functioning of the structure. In most cases, synergy with other deterioration processes like cycles of freezing and thawing and corrosion of reinforcement exacerbates the rate of deterioration of concrete structures. ASR in concrete pavements and transportation infrastructure can result in spalling of cracked sections. Moisture, additional alkalis from deicing salts, and traffic loading accelerate the process.
HOW is the Potential for AAR Determined?
Aggregates with a distinct mineralogy of dolomite crystals embedded in a clay matrix cause ACR. A qualified petrographer can identify this. Quarries in North America where these aggregates occur are known and their use in hydraulic cement concrete is avoided. Test methods for determining potential for ACR include a rock cylinder expansion test, ASTM C586 and an expansion test of concrete prisms, ASTM C1105.
Cases of ASR have been noted in most areas in North America. Existing signs of ASR in concrete structures in a region is the most definitive way of establishing that the problem exists. A petrographic evaluation of an aggregate source, ASTM C295, can identify potentially reactive silica minerals in aggregates but will not definitively establish whether an ASR problem will occur when the aggregate is used in concrete.
The more reliable test method that has been correlated to actual deterioration in field structures or field-exposed test specimens is an expansion test using concrete prisms, ASTM C1293. This test requires a one-year period and may not be conducive to project schedules if not conducted ahead of time. Aggregates are considered to be potentially reactive when the expansion exceeds 0.04% at 12 months.
A more common test is an accelerated mortar bar expansion test, ASTM C1260. This test provides a result in about 2 weeks. Aggregates are considered potentially reactive when the expansion exceeds 0.20% (ASTM C33). Many agencies, however, use an expansion criterion of 0.10% at 14 days. ASTM C1260 is an aggressive test and aggregates that do not cause deleterious ASR reactions in the field are often characterized as reactive by the test. It is recommended that ASTM C1260 results should be supplemented with other information in determining the potential reactivity of an aggregate source.
Other test methods, like the quick chemical test, ASTM C289 and a longer term mortar bar expansion test, ASTM C227 are not considered to be reliable.
The Appendix of ASTM C33, Specification for Concrete Aggregates, provides guidance on AAR test methods, criteria and mitigation methods.
HOW is AAR Avoided?
There are no recommended methods of preventing deleterious expansion when the available aggregate source has been verified to be ACR reactive. The only recourse is to use an alternative source of aggregate.
For deleterious ASR expansion to occur, the three factors discussed above are required: alkalis, reactive silica and exposure to moisture. Concrete that remains dry inside buildings and not in contact with soil will typically not need preventive measures. In other situations various strategies can be used to avoid damage due to ASR.
One option is to avoid the use of aggregate sources that are determined to be reactive. This may not be feasible because alternative non-reactive aggregates may not be economically available or data may not exist as to their potential performance.
Another option is the use of a low alkali cement, typically characterized as one with Na2Oeq, less than 0.60%. Low alkali cement, however, is not available in many regions. Alternatively, limiting the total alkali content in concrete is often considered a better option. Only the alkali from Portland cement is considered. The total alkali content in concrete is determined by multiplying the cement content by the alkali content. Concrete alkali content is typically limited to a 5.0 lb./yd3 (3.0 kg/m3) or lower for more critical structures, like in concrete dams. With this option — low alkali cement or low concrete alkali content — it should be recognized that alkali concentrations can build up in concrete during service conditions from exposure to external sources like deicing chemicals and sea water, or from migration of alkalis within concrete due to drying.
The more accepted option to mitigate deleterious ASR is to incorporate supplementary cementitious materials (SCMs) in concrete. SCMs include fly ash, natural pozzolan, slag cement or silica fume. SCMs bind alkalis in the hydration products and prevents the deleterious expansion from occurring. One exception is fly ashes that have calcium oxide contents greater than 20%, typically characterized as Class C fly ash. Class C fly ashes typically need higher dosage levels to mitigate the reaction.
The quantity of SCMs required will depend on the reactivity of the aggregate, the alkali loading in the concrete, the type of SCM and the exposure of the concrete to external sources of alkalis. In many regions, historically established SCM contents required to mitigate ASR are used and work well. Alternatively, the effectiveness of an SCM can be evaluated by testing. The SCM contents evaluated should cover a range typical of those proposed for construction. The more common test methods are ASTM C441, ASTM C1293, and ASTM C1567. These test methods accelerate the reaction either by using highly reactive artificial aggregates, elevating the alkali loading in the test mixture, exposure to highly alkaline solutions, use of elevated temperature, or some combination thereof. The concrete prism test, ASTM C1293 is performed for a 2-year period at which point the expansion should be less than 0.04%. This tends to be too long for typical project submittals. More commonly SCMs are evaluated using ASTM C1567 with a 14-day expansion criterion of 0.10%. Research supports that these methods provide a conservative estimate of the quantity of SCM needed to mitigate ASR in concrete. Regardless of the process used to establish the minimum SCM content required, the impact on other project requirements for concrete must be considered. These include, but are not limited to, setting time, bleeding characteristics, workability, and early and later age strength development.
Chemical admixtures, primarily lithium nitrate, have been shown to be effective to mitigate deleterious ASR. Manufacturer recommendations should be sought to establish effective dosage levels for specific concrete mixtures. The Corps of Engineers method CRD-C 662 is referenced to evaluate the effectiveness of the lithium admixture dosage. In some cases, combinations of these options, such as the use of SCM and lithium admixtures, have proven successful.
Because test methods accelerate the reaction, none evaluate potential for deleterious ASR of the actual composition of concrete mixtures proposed for projects. No test evaluates the effectiveness of the alkali content of Portland cement. Test methods evaluate single aggregate sources. When the fine and coarse aggregates are determined to be reactive, the dosage of SCM that mitigates the more reactive aggregate should be used.
AASHTO PP65 provides a step-by-step method for evaluating aggregates and a prescriptive and performance-based methodology to minimize the potential for damage in field concrete. The methodology requires consideration of the risk level for the occurrence of ASR in structure to establish preventative measures.
1. ACI 221.1R, 2008, Report on Alkali-Aggregate Reactivity, ACI Manual of Concrete Practice, ACI, Farmington Hills, MI, www.concrete.org.
2. Guide and Guide Specification for Concrete subject to Alkali Silica Reaction, Portland Cement Association, Skokie, IL, IS413.02 and IS417.07www.cement.org.
3. ASTM Standards referenced, Annual Book of ASTM Standards, Volume 4.02, ASTM International, West Conshohocken, Pennsylvania, www.astm.org.
4. AASHTO PP 65-10 Practice for Determining the Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in New Concrete Construction, AASHTO, Washington DC, www.aashto.org.
5. Resources on ASR from Federal Highway Administration -www.fhwa.dot.gov/pavement/concrete/asr/resources.