3 Tips for Concrete Moisture Testing in the Real World

Everyone knows: There’s the manual – and then there’s the real world. People who do no more than put together basic furniture from instructions printed by a fine Scandinavian furniture company understand this immutable fact of life.

For the last year or so, those of us who measure moisture in concrete floor slabs have been excited that the ASTM F2170 standard now allows official documentation of in situ relative humidity (RH) test results after just 24 hours, instead of 72 hours. Who doesn’t want to cut two full days out of their project timeline?

This also sparked a thought that while flooring inspectors, installers, and general contractors (GCs) are charged with following the standard when doing RH testing, real life isn’t always entirely cut and dry (no pun intended). We thought now would be a good time to hear from people on the ground. What are your experiences measuring concrete moisture?

To find out, we posted this question on LinkedIn:

When doing concrete moisture testing in the real world, what difficulties have you had or what issues have you seen that impacted getting accurate results? What lessons have you gleaned from these experiences?

We went through all the responses (thanks so much to everyone who joined the conversation) and found a few themes. Here are several tips from your peers about how to put theory into practice.

#1. You Can’t Just Follow the Instructions

Concrete Moisture Testing with Rapid RH L6

Make sure the people who conduct RH testing understand how to follow the ASTM F2170 standard so that you can be assured of accurate, reliable results.

This one surprised us a bit. In situ RH testing is not rocket science. And yet, people cited poor training or execution as a key issue. One aspect of this is making sure that testing is done consistently and correctly every time. For example, always making sure the right number of RH probes are placed in the concrete for the square footage being tested.

However, the issue really went deeper than this. Some people described their challenge in finding people who have the right training to perform RH testing so they have the assurance of getting accurate, reliable results.

David P. said this: “Getting the right personnel to perform the testing might be perhaps the biggest challenge I see. Very few individuals are properly trained (certified notwithstanding) to know what they are doing. Certification does not underscore competency.”

Again, this is not to say RH testing is difficult. It is not. The methodology is easy enough that anybody can do it. But at the same time, there are important things to know to do it right. This is why Wagner Meters, for instance, offers a free one-hour moisture testing webinar and live Q&A twice monthly for all its customers.

As you’ll see from other comments below, understanding the science behind how concrete dries and how it interacts with its environment is critical to getting the testing done right and having confidence in the results.

#2. Look at the World Around You, and Be Sure to Check Under the Hood

Pay attention to what’s under the concrete slab because the risk of a moisture problem is high when no vapor retarder has been installed.

People responded about how important it is to pay attention to the ambient conditions where the testing is happening. ASTM F2170 requires conducting the test at service conditions. This refers to the ambient conditions that people will experience inside the building once it is placed into service. Testing at anything other than service conditions will mess with your results. The job site must be given time to acclimate per ASTM F2170.

Another key issue people raised was the “cleanliness” of the test hole itself – in other words, making sure the hole is not dusty and there’s no silica in the air to skew readings.

It’s not just ambient conditions that matter, but also what’s underneath the slab. A number of people mentioned that even if the slab is on grade, they don’t always know the substrate conditions or if a vapor retarder was ever installed under the slab.

Making matters more challenging, it can sometimes be difficult to find out. Making matters even worse, there’s not a whole lot you can do about it if a vapor retarder was not installed. Except perhaps to note to the GC or project manager that most flooring, coating, and adhesive manufacturers require a proper vapor retarder for their warranty to apply.

One poster named JD summed it this way: “[T]esting any concrete that has questionable substrate conditions is of limited use. I am called far too often to confirm the concrete is suitable for a moisture-sensitive coating only to find that nobody has ever even looked for a proper under-slab moisture retarder. Sure I can tell you what the slab RH is at now, but it is probably open to absorbing unlimited amounts of moisture from the soil it sits on.”

#3. Know the Limits of Your Test

Know the limits of the RH test and how you may want to handle test results that are close to the manufacturer’s specification for a given flooring product.

ASTM F2170 requires the use of RH sensors that are calibrated within +/- 2% between 50 to 90% RH, which begs the question: How accurate is accurate?

This two percent factor should not be glossed over. One poster named JD gave an example where the test result showed the slab at 94% RH and within the specification for an adhesive that’s rated to sustain up to 95% RH. Does this guarantee the flooring installation won’t fail because it meets the specification? Will this result protect you in court if there is a flooring failure?

He went on to suggest, “Products manufactured to withstand a certain level of moisture or alkalinity must have a fudge factor built into their specification to encompass variables in testing. If they do not, the manufacturer [may well be] in a position of legal liability for failures.”

Another poster, Richard K., offered these words of wisdom for getting results that are as accurate as possible: “[T]esting … exactly as required [per the standard] increases the certainty of our ability to attain accurate readings and reduce random errors. Any change to the conditions specified in the standard has a direct bearing on the … percent accuracy.”

Yet another poster noted that complying with ASTM standards is about all that can be asked of anyone—that’s why they’re the standards. The fingers will start pointing in all directions if a flooring failure occurs, but what more can anyone do, but be diligent in following the standard and properly executing the RH test.

So in spite of the above considerations encountered in the real world, we can minimize the risk of failure by taking the right action. And the RH test has a scientific pedigree that no other industry-accepted method for testing concrete moisture can offer. Its proven reliability and ease of use are why the RH test has rapidly grown to become the #1 moisture test for assessing the readiness of concrete slabs for flooring installations.

Thanks again to everyone who chimed in with observations and suggestions about overcoming the inherent challenges when testing moisture in the real world.

If you enjoyed the article read our Top 10 Concrete Moisture Testing Myths article to learn more about concrete moisture testing.

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