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.
Groundwater sources and floor slabs in contact with saturated soil, or where drainage is poor, can contribute to moisture problems. Moisture moves to the slab surface through capillary action (wicking). The extent of this movement depends on factors such as the water table depth and the soil type beneath the slab. Fine-grained soils allow moisture to travel from greater depths than coarse-grained subgrade materials.
Water vapor from damp soil can diffuse upward and condense on a concrete slab surface when the slab is cooler and has a lower relative humidity. This occurs due to a vapor pressure gradient between the soil and the slab surface.
Wetting of the fill course or blotter layer located between the vapor retarder and the slab before concrete placement can trap moisture beneath the slab. Since the moisture can only escape through the concrete, drying is significantly delayed. For this reason, a blotter layer is generally not recommended for interior slabs-on-grade (CIP 29).
Moisture remaining from the original concrete mixing water gradually migrates toward the slab surface as the concrete dries. Under normal conditions, drying to an acceptable moisture level may take anywhere from six weeks to more than one year. The drying rate is influenced by factors such as the initial water content, curing methods, ambient temperature, and relative humidity. In elevated slabs, this is the primary source of moisture. Additional wetting after curing can increase internal moisture levels and further extend the drying period.
Avoiding problems associated with high moisture content in concrete can be accomplished by the following means:
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).
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.
