Moisture in concrete is the cause of many flooring failures, so it is important for installers to have a thorough understanding of how to test for this common problem. Richard Tanski of Dependable Floor Products details the testing methods and equipment used to test moisture in concrete.

In the previous article we discussed identifying possible sources of moisture and the need for moisture testing. This article will identify currently available moisture tests for concrete and the standards that are in place.

First, keep in mind that moisture testing identifies present moisture. Depending upon the source of the moisture, the level can vary.

Over the years there have been a wide variety of methods used to check the concrete surface for moisture prior to installing floor coverings or coatings. The plastic sheet test, mat test, and surface meters have all been used. These and others are identified in ASTM E1907-04. The flooring industry, however, requires more specific testing that provides objective and quantitative results. The most common test at this time is the calcium chloride test, the procedures for which are outlined in ASTM F 1869.

Chart 1

Calcium Chloride Test - ASTM F1869

The most common test at this time is the calcium chloride test, the procedures for which are outlined in ASTM F 1869. To summarize, the calcium chloride test involves:
  • Preparing the environment (to its serviceable condition for at least 48 hours prior to setting the test) and the slab. The slab must be completely bare. If it is ground to prepare for the test, it must acclimate for 24 hours prior to setting the test.
  • Weighing the CaCl Petri dish, placing it on the floor, covering it with an air tight dome, and protecting it.
  • Setting 3 tests for the first 1,000 SF and one for every 1,000 SF after that.
  • The test runs for 60 - 72 hours.
  • The dish is retrieved, weighed and a calculation is run.

The result is given in pounds of water over 1,000 square feet in 24 hours. Most floor covering manufacturers, along with ASTM F710, recommend a maximum rate of 3 lbs. There has been mounting concern for the accuracy of the CaCl test. Common problems in conducting the test include not acclimating the area (HVAC in full operation), insufficient surface preparation, and insufficient number of tests. When the test is properly conducted, scientific studies have shown that it is only testing the top 1 inch of the concrete. Concrete dries from the top down. Because the CaCl test identifies moisture in the top portion of the concrete, it misses the moisture that is deeper in the slab and is therefore less predictive.

Relative Humidity Probe Test - ASTM F2170

In response to the concern of the CaCl test other methods are being adopted. The relative humidity or probe test, outlined by ASTM F2170, reports the relative humidity of the concrete slab at a certain depth, through drilling a hole in the slab and inserting a sleeve and probe to a predetermined depth. Having been used in Europe for years, the method has a very good track record. Because the probe is testing within the concrete (ASTM F2170 calls for a depth of 40 percent slab thickness when there is a vapor barrier in place) the results are more predictive. The moisture indicated is available to transmit to the surface over time.

Although the method is less reliant on atmospheric conditions, the standard does require the area to be at serviceable conditions for 48 hours prior to testing. If the area is not acclimated (building completely enclosed and HVAC running) however, the test may be run and a reading obtained. With the maximum reading of 75 percent established by ASTM F710, the obtained reading may at least be compared, with the understanding that a change of condition will influence the Relative Humidity Test. The sleeve or probe may then be left in the concrete where it may be easily monitored. As the job schedule progresses these up to the minute results allow the owner to make a more informed decision on the plan of action. The RH Test is being used in the field and flooring manufacturers are starting to include it into their specifications.

Chart 2

The Insulated Hood Test - ASTM F2420

The other method being introduced to the U.S. is the hood method (ASTM F2420). One motive for this test was to provide a similar measure for emission to the CaCl test. The hood test consists of securing an air tight box to a bare concrete surface and the relative humidity within the box is obtained through a probe which is inserted into the box. The test calls for the area to conditioned at its serviceable level for 48 hours prior to starting the test. This method has also been used in Europe and is very good for those situations where drilling may not be advisable.

Chart 1 compares the three methods for moisture testing. Although the equipment to run ASTM F2170 can be costly, the ability to obtain meaningful data at any time during the life of the concrete, regardless of the environment, can not be understated, whereas with the other two methods an unacclimated area will produce inaccurate results that are meaningless, the ability to obtain instant results after the first test (vs. the 60 - 72 hour time for the CaCl test) allows quick periodic checks on the slab to track its drying progress. Currently, the biggest draw back to this test is the lack of agencies that have the equipment to perform the test.

Comparing CACL AND RH

One question that is asked frequently is, "How can the information provided by the CaCl test and the RH test be compared?" There really is no comparison. The two methods are testing entirely different things. The CaCl method is testing the amount of moisture coming out of the concrete where as the RH method is identifying the humidity level at a certain depth. We have conducted these tests at the same time on the same slab. Results have varied due to the mix design, curing method and porosity. Just as outlined in this article series, there are many factors on each project that should be considered before a decision is made on the floor covering installation, and testing is just one.

Figure 1

Moisture Meters

There are a handful of companies that manufacture meters that can be used to run ASTM F2170. Chart 2 compares the five most popular brands. Important characteristics include:
  • Speed / timing - although ASTM F2170 calls for the hole to acclimate for 3 days prior to obtaining a reading, some manufacturers are claiming accurate readings within hours
  • Multi-function - some of the meters can also be used to obtain readings at the surface. Although surface meters can not provide the test information called for by the standards or floor covering manufacturers, they may be used to identify wet locations to conduct further testing.
  • Information storage & down load - large commercial projects can become very complex, having traceable information which is quick and easy to download and process is a major benefit.


Along with moisture testing, the slab should also be tested for pH. Often, when experiencing a moisture problem, the concrete will also exhibit a high pH reading. As the moisture works its way to the concrete surface, it can bring with it free salts from within the concrete. It is often the combination of moisture and alkalinity that causes problems for the floor covering adhesive. Most floor covering and adhesive manufacturers recommend that the pH not exceed 9. Figure #1 shows that on the pH scale 7 is neutral. Newly placed concrete will exhibit a pH of 13 or so. As the concrete is exposed to the air, carbonation occurs, reducing the pH at the surface. pH may be tested with pH paper, pH pencils and pH meters. Identifying the pH of the surface is important before moving on with floor covering installation or the use of a mitigation system.

Before moving forward, consider the moisture test results along with everything else you know about the concrete and the building. Remember, the moisture test is simply identifying the amount of moisture present (MVER through CaCl testing or humidity level through RH testing) at that given point in time.