Acceptable Concrete Moisture - July 2013

In April, Concrete Construction magazine published the article “Why Three Pounds?” that questions why the flooring industry ever abided by what it calls a “one size fits all” standard for evaluating concrete slab moisture. According to the article, “The literature from many flooring installers and manufacturers states that installation of glued-on flooring cannot begin, and warranties on flooring materials will not be honored, if MVER [moisture vapor emission rates] results are greater than 3.0 pounds/1000 square feet/24 hours (abbreviated in this article as simply 3 pounds); some manufacturers today have increased that to 5 pounds. It is not clear if this value refers to the results of any single test, of all tests, or the average of all tests required by ASTM F 1869 to be performed on the slab simultaneously.” So is the three-pound requirement as arbitrary as the authors of the article suggest?

The concrete moisture issue is one that causes continual frustration to nearly every segment of the flooring industry: manufacturers, contractors, installers and specifiers. This problem has become more prevalent in recent years due to the accelerated timelines for construction and the types of adhesives used today.

A three-pound moisture vapor emission rate (MVER) was for years recognized as the limit of moisture that could be safely tolerated by flooring materials and adhesives. An MVER above three pounds was believed to put many types of flooring materials and installations at risk.

The issue the authors raise isn’t only why an MVER limit was originally set to three pounds, but also how one can reliably determine that the moisture vapor emission rate is actually at or below three pounds—as well as whether reaching a three pound limit is really necessary. The authors argue that there is no direct correlation between a three-pound emission rate and the occurrence of flooring failures.

Since the calcium chloride method of measuring the MVER has been around for many years, much has been learned about what the test method does and does not indicate. Some of the limitations with the test hinge on the fact that the moisture content of a slab is mutable and that MVER results are in great part affected by the ambient conditions that the slab has been exposed to before and during the test period. Even if a concrete slab is considered acceptably dry, this does not mean that the moisture level within the concrete will remain at a static level if the underside of the slab is not adequately protected from moisture below. In addition it has been shown that the MVER test only reflects the amount of moisture present in the top 1/2” to 3/4” of the concrete slab.

According to concrete floor specialist Peter Craig, the most important element in the design and construction of a concrete subfloor where a moisture-sensitive finish is to be applied is an effective, low-permeance vapor barrier placed directly beneath the slab. If the barrier below the slab is absent, the results of any type of moisture test cannot be relied upon, as the moisture content of the slab will increase over time once the slab is covered.

Keep in mind that the relative humidity in the ground below the slab will be close to 100%, regardless of the depth of the water table, and that the humidity at the top of a concrete slab cannot be lower than the humidity in the air above it. In a humid environment, like the Southeast, the top portion of a concrete slab can only dry to the level of the air, so an MVER test will measure a higher level than, for instance, a slab in a dry desert environment. And, of course, this level will change with the air, so a slab will have a higher moisture content reading on a more humid day, when the surface of the slab absorbs more moisture. An MVER test only reveals what is happening in the very top portion of the slab, which is impacted by the air; it does not indicate the humidity at the slab’s core or the transmission of moisture through the slab.

According to Craig, with an effective barrier in place, the best means of measuring a slab’s moisture condition is to perform both MVER and in-situ RH testing (ASTM F2170-11). Measuring the RH level of a properly protected slab-on-ground at 40% of the slab’s thickness is a predictor of what the uniform RH level will be in the slab after it is covered.

As adhesives are developed that can withstand greater exposure to moisture and elevated pH conditions, tolerable moisture levels will likely rise, and with them, warranties will change. In the meantime, experts agree that multiple tests in multiple areas is the best way to determine the moisture condition of a slab, a crucial determination to ensure the successful installation of a floorcovering.

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