Relative humidity testing
It has been about 13 years since ASTM approved a “new method” of concrete moisture testing: the relative humidity probe method, also known as the RH test or the “in situ” test. ASTM F 2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes was published in 2002. This “new method” had actually been used in Europe for decades, but came to North America via ASTM because many concrete and floor covering experts grew concerned about the accuracy of such other test methods as the calcium chloride test (ASTM F1869) and hand-held moisture meters.
Today, the RH test is more widely accepted all the time, and is easier to do than ever because of better equipment that is easier to use and more widely available. However, there are still many questions as more people in our industry are being asked to test concrete floors. Here is a brief overview of the method and some answers to common questions.
The methodology involves a minimum of three tests for the first 100 square meters (1000 square feet) and one test per 100 square meters thereafter. Start by drilling a hole in the slab to a depth of 20 percent or 40 percent of the slab thickness, depending on the type of slab. The 40-percent depth is used for a slab that dries one side only, such as a slab in contact with the ground with a vapor retarder beneath it, or an elevated slab placed over a metal deck. The 20 percent depth is for slabs that dry from both sides, such as an elevated slab not placed on a metal deck. After vacuuming the hole to remove all debris, a special plastic sleeve with openings at specific locations is placed into the hole and allowed to sit for a prescribed length of time. Then, a relative humidity probe is placed in the hole, and a reading is taken of the relative humidity, expressed as a percentage. Generally, 75 – 85 percent is the acceptable upper limit. By comparison, the Moisture Vapor Emission rate (MVER) per the calcium chloride test, is expressed in pounds of moisture vapor per 1000 square feet per 24 hours, or just “pounds.” Generally, three to five pounds is an acceptable limit.
The industry is still generally more familiar with the calcium chloride test, so a common question is, “Why is this one better?” Scientific testing showed that the calcium chloride test only measures the MVER from the top 25mm or so of the concrete slab. Since most concrete slabs dry from the top down, the top surface could measure as “dry” while there is more moisture down below. This moisture could potentially move to the surface, since moisture vapor moves upward in the natural process of evaporation. In addition, once the slab is sealed by the application of a floor covering, moisture will “equalize” throughout the slab so eventually the moisture level will be the same top to bottom. That means the dry surface could become quite damp, affecting the floor covering adhesive. This explains how sometimes a slab with an acceptable MVER winds up “wet” after the flooring is installed, because the dampness in the lower part of the slab moves up to the surface.
Relative humidity (RH) probes can be placed at a specific depth in the concrete to indicate the actual moisture condition deeper in the concrete. Calcium chloride tests give a “snapshot” of the moisture condition whereas RH tests can monitor trends and indicate the rate of drying, providing a more accurate prediction of when a floor will be dry enough for installation.
Another very important factor involves the testing of lightweight, concrete slabs, often used for high-rise construction. Testing these slabs becomes more important than ever because these lightweight slabs take much longer to dry than a standard mix. The calcium chloride test method is NOT RECOMMENDED for lightweight concrete, so moisture must measured by RH probes.
Another advantage of the RH test is repeatability. The calcium chloride test takes three or four days to run, every time you run it. The method involves using a grinder to clean a 50 cm square of the slab before testing. If the test fails and you have to come back later to test, you have to grind a new section and do the procedure again. Not so with the RH test, which involves just drilling a hole. Although the initial F 2170 test involves a three-day waiting period, in the same scenario of a failed test, it’s easy to re-test. Once the sleeve is placed and capped, you can come back at any time and get a very quick reading. There is no three-day waiting period once the hole is drilled and the sleeve is placed.
I am often asked which test I prefer, and, although I lean to the RH test, it’s the manufacturer of the floor covering, adhesive and/or patching compound that specifies which method. If they don’t have a method in their published guidelines, I recommend contacting them and getting something in writing to state which method shall be used and what result is acceptable.
There is no correlation between the F 1869 and F 2170 because they measure different properties of the concrete. That means you need to have a manufacturer’s recommendation of the specific test and the limits. In other words, MVER of three pounds does not equal a RH of 75 percent, so you can’t substitute one method for the other. Test per the manufacturer’s guidelines, always. It’s sometimes a good idea to do both tests, depending on the circumstances. Why? If there is a low MVER and a high relative humidity, that can show that the concrete is drying from the top down, which is fairly normal. If it’s the other way around (high MVER and low RH), this may indicate that the slab was recently exposed to moisture on the surface such as a water leak. If the floor is adequately dry deeper down, you might simply need to let the surface dry some more before installation.
Concrete moisture testing is a requirement for all types of floor covering, although I’ll bet many of our readers are not doing these tests regularly. There is now a certification program for testing, offered by The International Concrete Repair Institute (ICRI). According to their website, “The purpose of this program is to help improve the performance of concrete slab moisture testing to result in more consistent, accurate, and reliable results that will help flooring manufacturers, architects, engineers, and contractors to make better decisions as to when a concrete floor is ready for a floor covering installation.” Visit www.icri.org for more information and get a copy of the printed test methods from ASTM at www.astm.org. And, I can’t stress enough to check with the floor covering manufacturer to learn what method they require and what test results are for the floor covering you plan to install.
Christopher Capobianco has been in the floor covering industry since the 1970s in various roles including retail and commercial sales, technical support, consulting, journalism, education and volunteer work. He currently is part of the sales team for Spartan Surfaces in New York City. You can reach him via [email protected].