Installments: Progress in concrete moisture testing

Home Columns Installments: Progress in concrete moisture testing

January 5/12, 2015; Volume 28/Number 14

By Jason Spangler

Human nature often leads us to stick with the familiar. It seems safer that way. If that’s how people have always done it, it must be right, right? Well, as it pertains to measuring the moisture condition in concrete, advances in science are rapidly leading the industry away from its most common test method.

The most widely used test in America is the calcium chloride test (CaCl test). The earliest known reference to it is found in a flooring manufacturer’s installation guide from 1941.

In the test, desiccant crystals were placed at the slab’s surface and covered with a glass dome. Installers would let that sit for a night and then check in the morning to see if the crystals were wet. They called it the “dampness test,” but it was really a subjective “eyeball test.” If the crystals looked wet, the rationale went, the concrete must not be sufficiently dry.

By the 1960s, the qualitative dampness test had evolved into a quantitative test with numerical results. This newer version of the CaCl test purported to measure the volume (by weight) of moisture released across 1000 square feet over a 24-hour period. The test results were presented in pounds.

The ASTM later standardized the quantitative version of the CaCl test with language known as the F1869 standard. The calculated Moisture Vapor Emission Rate (MVER) of the test holds that an MVER below three pounds generally means the slab should be ready for installing flooring.

Reviewing the results

For many years, while other countries explored different ways to test the moisture conditions of concrete, use of the CaCl test and ASTM F1869 prevailed in the United States. Today, nearly half a million of these tests are conducted each year. However, along the way, floor installers noticed that moisture-related flooring failures remained a common and expensive problem. This was a condition begging for scientific inquiry.

In the 1990s, CTLGroup’s sophisticated testing laboratory conducted a series of scientific tests spanning a decade to assess the accuracy of the CaCl test. The group found that not only did the test not accurately present vapor emission rates, but also that MVER isn’t a reliable measure of the overall moisture condition of concrete.

The results clearly demonstrated that the CaCl test is—at best—only an indicator of surface moisture and fails to account for the moisture condition deeper within the slab. Every slab has a significant moisture gradient that moves toward equilibrium after floor installation; this means the CaCl test doesn’t really get at the critical question needed to determine when flooring can be installed: What will be the long-term moisture condition of the slab after floor installation?

Enter the relative humidity test

In contrast, the now widely recognized in-situ relative humidity (RH) test measures deeper into the concrete. A probe is inserted into 40% of the depth of the slab in order to measure the RH as a percentage. It has been shown that the moisture at a depth of 40% (when the slab is drying from one side) reflects the moisture condition of the slab that the floor covering will “see” over the long run.

Innovations in RH testing in the United States occurred rapidly once the F2170 standard came into being in 2002. RH testing has become fast and affordable. For more information about RH test kits, visit


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