Concrete moisture testing is being done more and more frequently, but a lot of people seem to be keeping their head in the sand and not even bothering. Even some architects and general contractors have an attitude of “Well, I’ve never had a problem,” and don’t specify that slabs be tested. Many in the industry, myself included, have been trying hard to make this change because we have seen what happens when floors fail due to moisture.

If you’ve been reading this column since I started with FCIin 2003, you’ve seen some photos of some pretty ugly failures due to moisture. However, the worst part of a moisture-related failure may be the “blame game” that happens when fingers get pointed over whose fault it is.

A lot of times, attorneys are the one who make a lot of money on these jobs, not the flooring trade. Unfortunately, when blame is fixed, most of the time it comes down to the flooring installer and/or the dealer who sold the job. You can’t see, feel or smell a concrete moisture problem, so be aware of the industry standards for concrete and make sure to work testing into your regular processes for concrete substrate preparation.

Why do testing?The simplest reason why is to follow the industry standard: ASTM F710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring. The language in the standard is very simple, and uses legal terms like “all,” “shall” and “regardless” that don’t allow for any wiggle room. “All concrete slabs shall be tested for moisture, regardless of age or grade level.”

Most manufacturers of floor coverings, adhesives and patching compounds say the same thing. If you don’t test, the finger WILL get pointed at you, believe me. If your customer doesn’t want to bear the cost or take the time to have testing done, make sure that you document this fact.

As far as the methodology for moisture testing, remember moisture is invisible, so touching, smelling or looking at the slab cannot determine if there is moisture present. The old “tape a plastic sheet to the floor” does not work, no matter what you’ve heard before. Handheld electronic moisture meters are often used as a “spot” test, but they are not recognized as a “go or no go” test for moisture.

ASTM F1869. The two “quantitative” methods (meaning they produce a number) are the calcium chloride test (ASTM F1869) and the Relative Humidity or RH test (ASTM F2170). The F1869 Calcium Chloride test has been around for over 50 years, but it does not measure past about 3/4” down. This is a serious limitation because the test will miss a condition where the concrete could be dry on top and very damp inside, and since this moisture tends to move up to the surface, that could be trouble later on.

Many find F1869 more difficult because you have to use a grinder to prepare the concrete, and most often have to wait 24 hours after grinding to place the tests, meaning it takes three trips to test the slab. Plus, if the test results are too high, you need to retest at a later date, and it takes three days. Honestly, there is a better way.

ASTM F2170. The Relative Humidity Moisture (RH) test for concrete floor slabs is “officially” known as ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using In Situ Probes.RH testing measures moisture inside the slab, so this is a better way to “predict the future,” so to speak.

This method has been used in Europe for decades, and became ASTM F2170 in 2002. Today, it’s continuing to grow in popularity as more people understand the method, and the equipment becomes easier and easier to use. We still get questions about the test all the time, so here is a review of how and why the test is done.

Once you have the equipment, it’s as easy as drilling a hole. There are two options. Holes are drilled to 40% of the slab thickness in cases where the slab is drying from only one side, or only 20% in the case of a slab drying from BOTH sides. Drying from one side means any slab on or below grade, or an above-grade slab on a metal deck.  Drying from both sides would be a structural concrete slab above grade.

It may be tough to tell the difference, but if you can get a look at the slab from below then you can figure it out. If you see a corrugated metal deck, it’s drying from one side. If you see concrete, then it’s drying from both sides. So, for 40% thickness if the slab is 4” thick, drill the hole to 1 1/2”; or if it’s 20% of the slab depth, that same 4” slab would get a 3/4” hole.

If you don’t know how thick the slab is, it may take a little research to ask for the plans. I’ve not had much trouble getting this information, but I have heard of technicians that have and even drill all the way through the slab to determine how thick it is, or “assume the slab is 5” thick,” and drill a 2” hole. I am not fond of either of these methods; they’re not very scientific.

Once the hole is drilled, it’s important to clean it out. Wire brushes and special vacuum cleaner attachments are made for just that purpose. Once clean, a plastic sleeve is inserted into the hole that effectively seals off the hole except at the very bottom, which is where the measurement needs to be taken.

After a recommended waiting time, a relative humidity probe is used to determine the equilibrium relative humidity in the cavity at the bottom of the hole, which is expressed as a percentage of relative humidity. 75% - 100% are numbers you’d typically see. “Acceptable” numbers vary based on product and adhesive, so check with the manufacturer. Usually 75-80% is the “sweet spot” for a slab that’s ready for a floor.

The first test for F2170 (Relative Humidity) takes the same amount of time as the F1869 (Calcium Chloride) test; the test method calls for a three-day waiting period after you first drill the hole. However, if the test result is too high, you can leave the sleeve in place indefinitely and come back to insert a probe into the hole for a result in less than an hour. You can even leave the probe in place, and check on the RH readings on a daily basis. There is also technology to wire the probes into a “central station” of sorts that ties into the Internet so you can monitor the results from your desk.

Which test? With Calcium Chloride and RH tests both being readily available, this is a common question. I also get asked if there is a correlation between the two tests.  That question usually goes something like, “The adhesive says I need five pounds on the calcium chloride test, but I did an RH test and got 75%. Is that the same thing?” The answer is no, it’s not that easy.

When the F2170 method was first published, I did a column for National Floor Trends(now Floor Trends) to introduce the industry to the method. I interviewed Howard Kanare of Construction Technology Laboratories (CTL) of Skokie, Ill., whom I met through my work on ASTM Committee F.06 on Resilient Flooring. He is recognized as one of the foremost authorities on the issue of concrete floor slab moisture problems.

His responses to my questions in 2004 are still solid today. I asked him if there is a correlation between the two methods, and he said no, explaining, “They measure different properties of the concrete. F1869 determines a portion of ‘free moisture’ near the surface that can be ‘pulled out of the concrete’ over a short time period. F2170 measures the existing relative humidity within the slab at a specific depth.”

He added: “Calcium chloride tests only tell us about the moisture situation in the upper region of the concrete. Relative humidity (RH) probes can be placed at a specific depth in the concrete to indicate the actual moisture condition deeper in the concrete, even below the floor slab where moisture originates. F1869 tests give a snapshot of the moisture condition, whereas F2170 RH tests can monitor trends and indicate the rate of drying, allowing us to predict when a floor will be dry enough for installation.”

Since moisture moves up and concrete usually dries from the top down, the F2170 method was developed to measure moisture down in the slab, assuming that moisture will be coming to the top. I typically recommend doing bothtests in order to have a clearer picture of what’s happening with the slab. For example, a high reading on the F1869 test and a passing F2170 result may indicate the slab got flooded and needs to be dried out at the surface. If the results are the other way around, the slab is drying from the top down and hasn’t fully dried yet.

Some suppliers are still referencing the F1869 Calcium Chloride test only. I am not sure if they are behind the times or they haven’t updated their literature. If you are working “from the book,” be sure you don’t have an old installation guide – go online and make sure you have the latest document. Of course, you should do the test that is recommended, but if only the F1869 test is listed, call the supplier and ask them to clarify which method they would prefer. Get their recommendations in writing, and keep that document with the file for the job. I’d recommend always doing the F 2170 test. The new equipment out there makes it easier than ever to do the test.