Concrete Moisture Tests : PennCoat, Inc.

Testing Concrete Moisture

Putting down epoxy floor coatings, or for that matter, flooring, on a moist concrete slab is not going to have a happy ending. Concrete cures, which means it takes time to develop full strength and give up its moisture. Floor coating will not adhere properly to moist concrete and will be prone to premature failure. Flooring laid on top of moist concrete can create bumps and gaps. In addition, the trapped moisture encourages nasty creatures — mold, mildew and bacteria. It is much cheaper to practice patience and allow a concrete floor to finish curing.

Taking the Cure

As you have no doubt surmised from our previous blogs, concrete is a mixture of cement, sand, water and aggregates (rocks). Two types of cements are common:

  • Portland cement is hydraulic, meaning that it will harden in any moisture conditions, even underwater.
  • Gypsum plaster is an anhydrous cement that must dry properly or else lose its strength.

Concrete receives additives, such as lime and fly ash, to modify its properties, including its strength, color and curing time. Generally, a four-inch slab of concrete takes about a month to cure after pouring. However, this doesn’t mean the slab is dry enough to accept a coating — it might retain 2/3 of its original moisture after a month of curing. Ultimately, we have to test the slab to ensure its dry enough for further work, because environmental conditions can make a big difference in drying times.

Calcium Chloride Testing

One of the popular tests for concrete surface moisture, known as ASTM F1869, uses calcium chloride to determine the moisture vapor emission rate (MVER). The test results are expressed as the pounds of water that evaporate from a 1,000 square foot concrete surface in a 24-hour period — although the test might take up to three days. It’s not a difficult test to perform. A small dish of calcium chloride is placed on the concrete surface and sealed in a plastic dome. The weight gained by the sample after the test period translates into an MVER. The test has received some criticism in the last decade:

  • The test kits are not calibrated, meaning the kits from different companies can give different results
  • The test only measures moisture within 1/2 to 3/4 of an inch below the surface, but yields no information about the rest of the slab.
  • Moist environments could skew the test
  • The test is sensitive to the concrete mix, additives, floor thickness, aggregate type and other factors
  • Testing delays further work on the floor
  • The test is not appropriate for lightweight concrete, that is, concrete with gypsum cement or with lightweight aggregate

In Situ Testing

The second popular test, ASTM F2170, is more accurate because it measures moisture in the full thickness of the slab. The test requires us to drill a series of small holes in the slab and drop in relative humidity probes. The ideal depth of the hole is 40 percent of the slab depth, or about 1.6 inches for a 4-inch slab. A test meter connects to the probe and reads out the slab’s relative humidity. This testing method resolves some of the problems with the calcium chloride test:

  • The test meters can be calibrated according to national standards
  • The relative higher cost of the meter is offset by savings in labor and testing time
  • This test isn’t skewed by factors that affect the calcium concrete test
  • The test is predictive of potential failures with floor coverings and coating applied too soon
  • The test is quick

The test isn’t perfect, because the measured relative humidity is not exactly correlated to MVER. In addition, many U.S. manufacturers of coatings, adhesives and coverings don’t endorse the test, and the test gear costs more.

The best data is provided by running both tests. With the costs of concrete repair and replacement, not to mention coverings, so steep, it’s prudent to go the extra mile and conduct both tests.