CIP-26
 
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.

 
 

References
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.