High moisture content and relative humidity (RH) in concrete slabs can seriously damage all types of flooring systems. While there are some general rules of thumb and testing methods used in the flooring industry to estimate moisture levels, relying on these loose guidelines can result in costly flooring failures and liabilities.
Age and appearance of a concrete slab are poor indicators of conditions beneath the surface. Concrete must always be properly tested according to ASTM standards and manufacturer specifications prior to installation to prevent problems like discoloration, cupping, buckling, blistering, peeling, adhesive failure, concrete deterioration, and mold growth.
There are various reasons why concrete may contain too much moisture. The water-to-cement ratio can affect the way concrete cures and dries. Water will continue to migrate upward through capillaries in the concrete and evaporate for about a month after a new pour. Moisture can also result from poor drainage or improper vapor barrier below the slab-on-grade, plumbing problems, rainfall, and high humidity in the air.
There are several concrete slab moisture testing methods, the gold standard of which is ASTM F2170 in-situ relative humidity (RH) testing. RH testing measures relative humidity to 40% depth of a concrete slab, or 20% if measuring from two sides, using in-situ probes inserted into holes drilled into the concrete. At least three sensors are placed in the first 1,000 sq. ft. of the concrete being tested, and one sensor is needed per additional 1,000 sq. ft. The ASTM F2170 standard states that the concrete slab must be below 75% relative humidity or meet the flooring manufacturer’s minimum requirements, which can sometimes range upwards of 80%.
Calcium chloride testing determines moisture levels at or near the surface, but results can be inconsistent due to external factors such as humid or rainy weather. Hand-held moisture meters can be used for general evaluation purposes and to help locate high-moisture areas where more precise testing may be required. Meters can also be used to verify moisture conditions remain acceptable at the time of installation. The plastic sheeting method is considered unreliable.
For more information regarding concrete moisture and RH testing, flooring installation, and concrete moisture mitigation, please contact SRI & Co. at (973) 832-7627.
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