The simple question seems to be: is the concrete dry enough, or isn’t it?
Concrete moisture testing is typically conducted to determine whether a concrete slab is dry enough to have flooring materials installed on it. At first glance, this type of testing might seem simple -- either it’s dry enough, or it isn’t. However, there’s much more to concrete moisture testing than is readily apparent, and a single test is often not enough to provide the reliable data necessary to create decisions about flooring installations, especially when costs can easily run in the tens of thousands of dollars.
What makes concrete moisture testing complex?
First of all, there isn’t just one type of concrete moisture test. Test methods used to measure moisture in concrete are either qualitative or quantitative. While qualitative tests provide a general indication of the moisture content in concrete, quantitative tests produce a numerical result. Both types of tests can provide useful information; however qualitative tests should not be relied on to make a final determination of whether the concrete moisture level is acceptable for the application of flooring.
Why not? Because these tests aren’t always reliable.
The reason is that a qualitative test is only reliable when it produces a negative test result -- one that indicates excessive moisture in the concrete. In that case, it’s clear that the concrete isn’t ready to receive adhesive and floor covering. However a qualitative test result that’s positive -- that does not indicate excessive moisture -- can lead to the assumption that the concrete is dry enough to receive flooring. But this test must be followed by a quantitative test to be certain that the floor is in fact acceptably dry. In other words, qualitative tests usually don’t give false positive results, but can possibly give misleading negative results.
The plastic sheet test is an example of a potentially misleading qualitative test. This type of test involves taping a sheet of plastic over a specified area of the concrete for a minimum of 16 hours, then assessing the plastic, and the concrete floor underneath it. If the plastic is wet, and the floor below the plastic sheet looks damp, it can be concluded that excessive moisture is present. But what’s missing from this test is evidence of whether moisture might be entering the slab from below. This can occur when, for example, no vapor retarder is present under the slab.
Using this test, it’s also possible to get a negative result – showing no apparent moisture – if the temperature of the slab surface is warm enough that the moisture in the slab won’t appear on it.
The unreliability of these tests as a single measure suggests that one type of testing is not sufficient.
What about quantitative tests?
Concrete moisture testing that produces quantitative results are more accurate than qualitative ones, whether positive or negative. Still though, only one quantitative test isn’t always sufficient, depending on the answers you need.
Recommended by the Resilient Floor Covering Institute (1995) and the Carpet and Rug Institute (1996), the Moisture Vapor Emission Rate (MVER) test (ASTM F 1869-04 Test Method for Measuring Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride) is the most commonly used test in the United States. Known as a calcium chloride test, more than 300,000 of these tests are performed annually.
To conduct this test, a pre-measured amount of calcium chloride is placed on the concrete slab, which pulls moisture out of the top few centimeters of the slab. After 60 to 72 hours, the net weight gain of the calcium chloride is measured, indicating the amount of water absorbed.
However, because the MVER test determines moisture emitted only from the upper portion (less than an inch) of a concrete slab, it’s not a good indicator of the moisture content deep in the slab. Additionally, as with the qualitative tests, a high MVER result indicates a floor is not ready to receive flooring, but a low MVER result only indicates an acceptable moisture level in the upper portion of the concrete. Finally, the MVER test indicates only a moment-in-time of moisture emission. Especially if no vapor retarder is present below the slab, the moisture content of concrete can vary.
Uncovered vs covered concrete slab
A floor covering placed on top of a slab restricts the evaporation of moisture from the top surface of the slab. As a result, moisture within the slab distributes itself to achieve an equilibrium. Therefore, over time, adhesive and flooring are exposed to the equilibrium moisture level at the top of the slab. In order to determine the moisture throughout the slab, it’s necessary to perform a relative humidity test.
Relative Humidity (RH) testing: ASTM F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.
By drilling a hole into the concrete, RH probes can measure the relative humidity before or after the floor is covered. These probes must be placed at a depth equal to 40% of the thickness of the concrete when the slab is exposed on one side; 20% if the slab is exposed on two sides.
A recent update to the ASTM standard has reduced the minimum equilibrium time to 24 hours, making this test more viable in time-sensitive situations.
So, when you ask yourself whether one type of concrete moisture test is the “end all, be all” of testing, the answer in short is no. There are too many different variables, including slab conditions, epoxies, adhesives and floor types. Concrete moisture testing is complex, not necessarily in the tests themselves, but in interpreting the risks involved. Incomplete testing methods could lead to an inaccurate solution. That’s why it’s best to rely on a certified concrete moisture testing professional to ensure that the right tests are conducted correctly.