Q:I’ve got a concrete slab that’s sup­posed to get a moisture-sensitive coat­ing. I’ve also found a ton of confusing information about various tests I should run first. Are they necessary? How do I make sense out of all this information?

Yes, testing is necessary.

In my research as Chair of the International Concrete Restoration Institute’s Technical Activities Committee, and a member of ICRI’s Coatings and Water­ proofing Committee 710, I’ve seen some interesting numbers about concrete floor­ing failures. I’ve heard it suggested that flooring failures, most of which are mois­ture related, cost hundreds of millions of dollars annually. I’ve even seen $1 billion mentioned as the annual flooring failure price tag. I once saw a 2005 study that reported more than 80 percent of new and re­model/adaptive reuse floors never get tested for moisture. When I connect those dots, it’s clear to me that moisture-related flooring failure and lack of testing for moisture are linked.

What’s not so clear and you are definitely not alone in wondering about this – is how to make sense of all the testing ad­vice and information out there, and how to make it count. Right away I can think of eight separate tests for moisture in con­crete, from cheap and quick to pricey and invasive.
They all return good data, but each one tells you something different. Without crete Floor Slabs Using in situ Probes, for instance, requires drilling holes in the floor to 40 percent of the slab thickness. The method specifies three test locations for up to 1,000 square feet (93 square me­ters), and one test per each additional 1,000 square feet. If you’ve got a big slab, say 100,000 square feet (9,290 square me­ters), that’s a lot of tests, and a lot of holes, and the kits aren’t cheap. On the other hand, electronic moisture meters and ASTM D4263 Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method are easy and cheap. Most moisture meters work on the principles of electrical impedance, radio frequency, or electrical conductivity/resistance. You generally check relative mois­ture content by placing two contacts on the concrete, pushing a button, then converting the reading signal to a relative moisture measurement. The plastic sheet method involves duct­ taping an 18 square-inch (6.45 square-em) sheet of clear plastic to the floor and leav­ing it for 16 to 24 hours. If there’s condensation inside at test’s end, you’ve got moisture. Should you use these cheap, easy tests instead of the pricier, more specific ones like F2170? Many contractors do. They’re the ones that often go out business after their first or second floor. These tests don’t give you precise num­bers on moisture content, just a relative in­dication that there is moisture. Folks may see a reading below 50 on the moisture meter scale of 0 to 100 and think that’s good. But they still don’t know what that number means in terms of absolute mois­ture in the concrete.
Proceeding on that basis isn’t much bet­ter than not testing at all.

Map the Slab
That said, the moisture meter check has its place and only takes a few seconds. encourage people to begin the testing process by walking the entire floor with the meter and getting a snapshot of the relative readings throughout the floor. Check doorways, check the middle of the slab, check where pipes may be buried, check areas where there’s airflow and sunlight on the floor, and where there isn’t. Check every few feet. With a moisture meter, you map the problems, so you know in general where the slab appears wet, dry and in-between. Now you proceed to more specific tests like ASTM F1869 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhy­drous Calcium Chloride to find out exactly how much moisture vapor is coming up through the floor in the areas you’ve mapped. Commonly called the “calcium chloride test,” you place a dish of calcium chloride under an airtight dome on the concrete. After 72 hours, weigh the dish and report the results as pounds of water emitted from 1,000 square feet of surface in 24 hours. These results will show how likely your coating is to fail as a result of water vapor coming out of the slab. Many flooring manufacturers recommend a maximum rate of 3 pounds ( 1.3 kilos) per 1,000 square feet in 24 hours before application
of their materials.

These test kits are expensive, but since you’ve mapped the areas of moisture in the floor, you don’t have to use so many. Just test representative areas of wet, dry and in-between. The test only measures moisture con­tent in the top surface which is not always representative of the moisture deeper in the slab. Because of this, experienced testers also use a modified version of ASTM F1869 to see how moisture will redistribute after the flooring installation. This involves sim­ulating the floor covering or coating by taping down plastic sheeting or other vapor-impermeable material, and leaving it for few days.Then you cut a hole in the material and run ASTM F1869 alongside the standard F1869 test where the floor was not covered. That can spotlight other possible prob­lems besides delamination, such as os­motic blistering, which is a whole other column. It gives you information similar to what you get from the next stage of testing, ASTM F2170 Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes. Do you need it, if you’ve already run F1869 and its modified cousin? To answer that question you have to ask yourself three more questions.
How moisture-sensitive is the flooring? How critical is it that the floor doesn’t fail? And, as Clint Eastwood might ask, do you feel lucky? Drilling holes and insert­ing probes per F2170 in areas identified as prob­lematic by F1869 tells you how much and how deep the moisture is in the floor. You can have dry concrete for the first inch or so, with more moisture deeper down. Installing the floor­ing traps that moisture at the surface as it gradually rises from the depths, until that once-dry top-inch or so is all wet, causing prob­lems.

In ASTM F2170 , probes determine the relative hu­midity of the concrete slab at a specific depth. A hole drilled into the slab to a predetermined depth gets a sleeve and probe inserted to take a reading. This method is more predictable than F1869 because it requires a depth of 40 percent of the slab thickness for single­ sided drying on grade, when taking meas­urements. This indicates the potential moisture reservoir within the concrete that can develop into an issue once a floor­ing system changes the moisture distribu­tion within the concrete. The test can also tell you there’s not much of a reservoir, and that the high vapor emission rate defined by F1869 means the concrete will soon be dry. You can leave the sleeves and probes in the concrete and monitor them over time. Note that concrete reinforced with metal fibers can produce false readings. Many flooring manufacturers recommend 75 per­cent or less internal relative humidity be­fore you apply their flooring systems. As you can see, there’s a hierarchy to testing, from simple and general to techni­cal and specific.

Avoid Problems
Your tests may reveal that moisture is not a problem, and that you can install the flooring with a high certainty of success. They may also alert you that moisture is a problem. In that case, as noted , you might try dehumidification, a mitigation system, or less moisture-sensitive flooring.
The only way you can know for cer­tain, however, is to follow a testing hierar­chy from initial visual survey to F2170. Each test has its value and its place in the hierarchy, and the tests referenced here are not the only ones. To get a true picture of the slab’s moisture condition, test systematically, mapping out general problem areas with simple easy qualitative tests then definitively testing those problem areas with more quantitative methods.

Only then will you have the actionable information you need to keep your floor – and your reputation – from being all wet.

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About the Author

Concrete O+A columnist Fred Goodwin is a chemist with more than 30 years of experi­ence in the con­struction chemicals in­dustry, includ­ing cement
manufacture , research , development and technical support of cementitious mortars. He is a fellow scientist of BASF Construc­tion Systems, a fellow of ICRI and ACI, and a Journal of Protective Coatings & Linings Top Thinker. He serves on ACI Technical Activities Committee (TAG) and chairs ICRI TAG and ACI 515 Protective Systems.
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