The Decision Made Easy: CaCl or RH Testing

It’s a well-proven fact. Installing finished floor products on a wet concrete slab can be disastrous. But because you want the slab dry enough to avoid costly moisture-related problems, you measure the slab’s moisture.

And now, thanks to a recent update to the ASTM F2170 standard for in situ relative humidity (RH) testing, you can get accurate, reliable information about moisture conditions in just one day. That’s not the case with other test methods.

In the United States, two test methods are commonly used for measuring moisture in concrete slabs: (1) calcium chloride (CaCl or MVER) and (2) RH testing. They’re both widely used, so which one do you select?

To help you decide, let’s compare them side by side.

How the Tests Originated

The first reference to the CaCl test method appeared in an Armstrong installation book on linoleum published in 1941. It was called a “dampness test,” and involved drilling a small 1-inch deep hole into the concrete and placing over it a small glass dish containing calcium chloride crystals.

If the crystals looked wet the next day, then the floor was considered wet. This was strictly a qualitative assessment. Then in the 1960s, without any documented scientific basis, the industry turned it into a quantitative test. A pounds-per-1000-square-feet expression was determined by weighing the desiccant before and after testing. This raised an important question: Is this really a practical test method for determining a slab’s true moisture condition?

According to Howard Kanare, a leading expert in issues related to concrete and moisture, what’s striking about the CaCl test is that “there’s no scientific documentation of why the original test conditions were established. There are no reasons or published data for the dimension of the dish and dome, the amount of CaCl used, or even why CaCl was used in the first place.”

In other words, the CaCl test has no scientific pedigree.

The RH test, on the other hand, does. In the 1950s and 1960s, researchers at the Portland Cement Association developed tools to assess moisture in concrete floor slabs using hygrometers and RH sensors. These methods preceded our modern standard test methods.

Meanwhile, several countries outside the U.S. developed scientific standards in the 1980s for measuring RH. This gives the RH test method several advantages over other concrete moisture measurement techniques.

Unlike the CaCl test, RH testing has been specifically designed and tested to provide a quantitative measurement of the moisture conditions present within a concrete slab. This measurement, taken from a specified depth, is scientifically proven to be an excellent indicator of the amount of moisture that the finished floor will “see” once the slab is sealed with a floor covering or sealer. With RH testing, flooring installers can be confident that they are minimizing the risk of moisture-related flooring problems.

It’s interesting to note that no other country outside the U.S. uses the CaCl test method.
Kanare notes that even though the test has been around for a long time. “We’ve learned that it’s actually not fundamentally accurate or useful in the way we had thought it had been for many years.”

In fact, over the last twenty years, both laboratory and field tests have revealed many things wrong with the CaCl test.

What Each Test Actually Measures

The CaCl test tells you what’s going on only at the surface of the concrete – about the top ¾” of the slab. That makes it sensitive to ambient relative humidity and temperature. Research also shows that CaCl testing is vulnerable to false readings, such that you can get both false positive and false negative results.

Moisture vapor emission levels tend to change. Using the CaCl test, you might measure a concrete slab today, think that it is dry enough, and then put flooring down. When a failure occurs, you could come back and repeat the test and get a higher value later on. That could be due to changed ambient conditions, or because moisture deeper down has moved to the surface, or both.

Kanare stresses that any test that gives different results depending on the weather, the temperature, and the particular day you do the test is not providing useful or reliable information.

Another thing to keep in mind is that while a slab is drying, concrete moisture typically exists in a gradient, with less moisture at the surface and higher levels of moisture deeper down. As the slab dries, moisture migrates to the surface and evaporates.

The CaCl test only measures moisture in the top ¾” of the slab. Once the slab is sealed with a direct finish or with an installed floor system, the slab’s moisture gradient will largely disappear. This means more moisture in the top ¾” of the slab. For a slab that has not dried sufficiently, disaster could follow if enough moisture rises to the surface and exceeds the moisture tolerance of the sealant, adhesive, or flooring system.

RH testing does not have the same limitations. RH sensors are placed within the slab at a specific depth scientifically proven to be an indicator of the final moisture condition of the slab once the surface is sealed – 40% of the slab depth for a slab drying from one side, or 20% for a slab drying from two sides. In other words, RH test results give an accurate picture of the true moisture condition of a concrete slab and are a far more reliable predictor of whether or not a floor covering or coating on a concrete slab will succeed or fail.

Labor Comparison

Before testing with CaCl, you must clean, scrape and ground the floor to ensure a clean surface. After opening the kits, you then must weigh the desiccant and record the data, after which you place a dish of the pre-weighed CaCl on the floor and cover it with a plastic “dome” for the 72-hour test period. Once the test is complete, you must weigh the desiccant again, record the data, clean the test sites, and dispose of the test materials.

The RH test, on the other hand, simply requires drilling and cleaning the test hole and installing a sensor. Multiple readings can be taken from each sensor. Recording data is also easy and convenient. Simply add the Rapid RH^® DataMaster^™ app in tandem with the Bluetooth^® Smart Reader for wireless reading, recording, and reporting of RH data.

Cost Comparison

For a contractor on a tight budget, the CaCl test appears at first glance to win this comparison hands down. But while the CaCl test kit initially costs less than the RH test materials, you end up paying a premium in the time and labor needed to prepare and conduct the test. That premium is significant, about 45 times as much as the RH test. Remember, time is money. More hours at the job site means more money spent, and that can make a big difference when it comes to a contractor’s budget.

There’s also a good possibility the CaCl test may have to be redone – jacking up the time and cost even more. For instance, if the seal is jarred at all, or the domes do not seal down, the test becomes invalid and must be repeated. Repeat readings require you to purchase new test kits each time. You’ll also have to weigh and calculate the desiccant again for each test spot at your job site.

RH testing does not have the same limitations. Installed inside the slab, the RH sensors are easily protected. Initial readings, capable of informing business-critical decisions, can be obtained quickly, and readings can be taken multiple times. Once the sensors are in place, they are operational for the duration.

The 24-Hour Advantage

CaCl testing (ASTM F1869) requires waiting a full 72 hours before accessing the test results. Not with RH testing. You can have scientifically valid results in only 24 hours.

That’s because the ASTM F2170 Committee has updated the F2170 standard for in situ RH testing. The update changes the length of time required for moisture equilibrium to occur in the RH test hole from 72 hours to 24 hours, thus giving you two additional days to move your project forward more quickly.

If you’re a builder or concrete/flooring professional, you now have the green light to get official, documentable RH readings in just one day. But with the CaCl test, you’re required to wait 72 hours to assess if the slab is ready for a flooring installation.

Remember what we said: Time is money. The RH test now saves you even more time and money than before, while also allowing you to make important project decisions much sooner. This is one more major advantage over CaCl testing.

Decision Time: CaCl or RH?

The facts are clear. They show RH testing is the better and only method necessary because it reveals the true moisture condition within concrete, which is what the floor covering will see after it’s installed.

The CaCl test simply can’t do that. Aside from giving you the amount of moisture escaping the slab’s surface at a specific point in time, it is incapable of determining the amount of moisture that remains deep within the slab. Often its test results are misleading and unreliable – and that may well lead to a flooring disaster.

The CaCl test or the RH test. You’ve got the facts. Now, which one will you choose?

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Last updated on May 20th, 2021