40%: Why Testing at the Correct Concrete Depth Is Crucial

All concrete slabs contain moisture and they always will. This is because concrete is composed of three main ingredients: water, cement, and aggregate. The three function together to form a concrete slab.

Moisture problems occur when flooring installers and contractors do not permit sufficient time for the process of drying concrete. During this process, some (not all) of the water migrates to the slab’s surface and evaporates. It is the only way moisture can leave a concrete slab. Once the slab is sealed with a finished floor product, the process of drying stops.

The appropriate level of “dryness” of a concrete slab will depend on the desired finish treatment. The target that installers should keep in mind prior to applying a particular flooring adhesive or finish is the manufacturer’s relative humidity (RH) specification for the moisture that’s within the slab.

Achieving this target will help assure success when the flooring product is applied to the slab. Many flooring failures occur when installers apply floor coverings over slabs with high RH, thus trapping excess moisture.

Historical Industry Practices

Historically, over a period of decades, the flooring industry has wrestled with various methodologies for concrete moisture measurement. In the mid-20th century, people began utilizing the anhydrous calcium chloride test to determine concrete slab readiness. This test measured the moisture vapor emission rate (MVER) at the slab surface.

Results were tabulated by the number of pounds of water that evaporated for every 1000 square feet of the concrete slab over a 24-hour period. If the MVER was within a specified range, contractors believed it was safe to proceed with the installation of the floor covering.

However, in recent decades, industry experts have questioned the integrity of the anhydrous calcium chloride test. They have discovered that its test results can mislead, many times providing a “false negative” result when moisture emissions are low and a “false positive” result when emission levels are higher.

Experts have also questioned the test’s utility because it fails to indicate moisture levels deeper within the slab. The test only evaluates the slab’s moisture condition to a depth of about 20mm or ¾ inch.

We are now in an entirely new age of concrete moisture assessment. RH testing is rapidly becoming preferred over the calcium chloride test. RH testing takes into account the process of moisture migration and the moisture gradient that exists within the slab during drying. And unlike the calcium chloride test, it is not highly sensitive to ambient conditions that affect the rate at which moisture is absorbed and released from concrete slabs.

RH testing is generally conducted using an in situ method of moisture measurement. This means that RH sensors are placed below the surface of the concrete (40% below, to be precise, when slabs are drying from one side, or 20% below when drying from two sides).

Based on extensive testing in the 1990s, scientists have demonstrated that RH readings taken at these depths will indicate the true moisture condition of the slab. In essence, the industry has been shifting to a much more scientific approach for determining slab readiness.

drying concrete slab“Dry” Concrete is Not about Mathematics

Concrete “dries” as water rises from deep within the slab to its surface in order to evaporate. It is a process that takes time, and the amount of time can be influenced by the ambient conditions around the slab as it dries. This means the drying process is not one of mere mathematical calculation (slab depth x # of days = dry slab) but a process that will require accurate monitoring of the slab’s RH over time.

Free Download – Which Rapid RH Sensor is Right For You?

Two Crucial RH Specifications

RH testing of concrete actually utilizes either of two important depths to assess moisture conditions and avoid moisture-related problems.

When constructing structural or cast-in-place slabs, installers can determine concrete readiness by conducting RH tests to 20% of slab depth. However, this depth is appropriate only if a slab is drying on two sides. If the moisture content is interacting with air on two sides and evaporation is occurring from two sides, RH testing at 20% gives the most reliable results.

For slabs at or below grade (installed with vapor barriers over ground) or elevated slabs poured in pan decking, contractors must assess moisture content at 40% depth. Installers drill holes in the slab and insert RH probes for assessing the moisture condition at that depth.

Learn more about how to conduct moisture tests for concrete floors in our more in-depth article.

RH test kits, such as the Wagner Meters Rapid RH® L6 or 5.0, are designed and engineered for RH sensors to be deployed at the appropriate depths in the concrete for fast and accurate testing.

By using these kits and obtaining accurate, scientifically-based test results, the floor installer or contractor is armed with reliable information to make the right decision about when to install the finished floor.

For concrete slabs to pass moisture content muster, always test for moisture deep within the slab. That’s best done with in-situ RH testing.

Last updated on February 25th, 2022


  1. larry ward says:

    Can a slab absorb moisture from a broken water line that soaks a 40 oz, woven wool/nylon carpet, glued to a 1/4″ thick, vulcanized closed-cell rubber cushion glued to the slab? The water sat on the carpet for about an hour, then extracted and commercial drying operations set up formtwo weeks.

    • Eric says:

      The simple answer to this is yes. A slab is like a sponge. As far as how much, that is really going to depend on how permeable the closed cell cushion and carpet are.

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