Before you coat concrete, especially horizontal surfaces, you should test the concrete for moisture. These tests are primarily designed for new concrete slabs, but can be used on existing slabs, too, especially when applicators are unsure if a moisture/vapor barrier exists. Coating concrete floors that are high in moisture vapor transmission (MVT) rates, or the alkalinity associated with high MVT, can result in a variety of issues.

Below are three of the best ways to prevent these kinds of coating failures. It is important to note that results from all three tests are limited to a particular time period and may not reflect year-long environmental conditions. Additionally, it is recommended to conduct concrete moisture tests after the building’s HVAC system has operated for at least 48 hours.

The Plastic Sheet Method

The most basic test for measuring concrete readiness is ASTM D4263-05, Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method. Called the Plastic Sheet Method, it involves taping a 457 x 457-mm (18 x 18 in.) square plastic sheet to the concrete floor and waiting at least 24 hours before removing it. (A heat source, such as an incandescent lamp, is sometimes placed close to the plastic sheet to help promote moisture migration). Once the plastic sheet is removed, the concrete is inspected for darkening or other signs of moisture. Although this test has existed for decades and is still used, most flooring manufacturers do not favor its use due to several flaws. First, it does not quantify the amount of moisture; it simply indicates whether or not moisture is present. Second, it only detects moisture in the upper portion of the slab, not in the middle or lower areas where moisture is more apt to reside.

The Calcium Chloride Test

The Calcium Chloride Test, per ASTM F1869-11, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, is a better test because it provides a quantifiable result. For this test, a calcium chloride disk is placed under a sealed plastic sheet and left to collect moisture vapor. After 24 hours, the disk is retrieved and weighed, and compared to the disk’s pretest weight. This weight difference indicates how much moisture vapor has emerged from the slab in 24 hours.

Relative Humidity Test

A more recent test providing the most complete data is ASTM F2170-11, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. This method requires holes to be drilled into the concrete and a humidity probe inserted at 40 percent of its total depth. The quantitative result and the measurement within the middle of the slab make this test a more accurate and reliable way to measure MVT.

Other Options for High Moisture Areas

Floor coatings will have varying tolerance to moisture vapor levels, and this should be reflected on the product data sheet. As a general rule, moisture vapor transmission rates should not exceed 1.3 kg (3 lb)/24 hrs or 75 to 80 percent relative humidity (RH) for most conventional floor primers. However, there are specialty primers and floor toppings designed specifically for higher MVT rates, up to 9 kg (20 lb)/24 hrs, and up to 99 percent RH.

If you need help evaluating your concrete slab before applying a coating system, contact your local coatings representative. They can help you determine which coating would be ideal for your project's given concrete condition.