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

Testing at the correct concrete depth is crucial to ensuring the success of flooring installations.

Concrete slabs naturally contain moisture, and if not properly tested and monitored, this moisture can lead to a range of issues, such as buckling, cracking, and adhesive failures.

In this article, we will explore why testing at the correct concrete depth is so important and how it can help prevent these moisture-related problems.

We will also discuss the historical industry practices and the shift towards more scientific approaches, such as relative humidity (RH) testing.

RH testing a concrete floor

By understanding the concrete drying process and measuring moisture deep within the slab, you can make informed decisions and avoid potential flooring failures.

So, let’s dive in and discover why testing at the correct concrete depth is crucial for successful flooring installations.

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.

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 retarders 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.

Why Measure Moisture Deep Inside a Concrete Slab?

All concrete floor slabs contain moisture. Consider this true even if the slab was poured many years earlier. It’s just the nature of concrete. Concrete’s three main ingredients (water, cement, and aggregate) function together to give concrete its characteristic hardness, strength, and durability.

Without water, concrete would simply not be concrete at all.

The deception of a dry surface

When preparing to install a finished floor product over a concrete slab, you want to be sure that you won’t run into problems stemming from the concrete’s moisture condition. But it’s not a question of whether the concrete is dry or wet.

No matter how dry the slab may look at the surface, rest assured, it’s not completely dry. Again—by design—all slabs contain water.

Instead, you want to know if the slab has sufficiently dried so its moisture condition won’t lead to any serious moisture-related flooring issues later. Those issues range from one extreme to the next—buckling, crowning, warping, cracking, splitting, adhesive failures, mold, or mildew.

No one wants those kinds of headaches.

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

The drying process of a concrete slab

It’s important to consider the process of drying. After the initial pour, water begins evaporating from the slab’s surface. As the surface moisture leaves the concrete, a moisture gradient will begin to form, meaning the moisture levels will be higher deep in the slab than at the surface.

The moisture inside the slab will now move toward the surface to evaporate as well. But once the surface of the slab is sealed with an impermeable floor product, the drying process comes to a halt.

If there’s excess moisture in the concrete, that moisture now has nowhere to escape. Instead, the slab’s moisture gradient will gradually disappear and as it does, moisture will migrate to the surface where it will interact with the finished floor. The result: too much moisture and you are at risk of a flooring failure.

Once you understand this drying process, it’s clear that surface moisture testing should never be the basis for making decisions about the concrete’s readiness for your finished floor product. What you really need to know is the moisture condition deep down in the concrete.

And that’s because—in the future—your finished floor will certainly interact with a significant portion of the moisture lurking there.

moisture-related flooring failure

Understanding the concrete drying process and measuring moisture deep within the slab will help you avoid a moisture-related flooring failure.

Scientific studies validate measuring deep

In the 1990s, studies at Lund University in Sweden examined questions related to concrete moisture and the most appropriate depth for taking measurements. What did they learn?

If you measure the relative humidity at 40 percent of the depth of the concrete for a slab drying from one side (or 20 percent of the depth for a slab drying from two sides), this gives you a highly reliable, accurate indication of the amount of moisture the finished floor will “see” after it is installed. Subsequent studies validated what the Swedish researchers found.

These scientific findings became the driver for the development of the in-situ relative humidity (RH) test procedure that’s now standardized in the guidelines found in ASTM F2170.

Today, RH testing is increasingly the “go-to” test method used by contractors, installers, and inspectors, while surface-based methods of evaluating concrete moisture, such as the anhydrous calcium chloride test, are used less and less.

In-situ RH: the fast, easy, reliable test method

An in-situ RH test not only gives you the concrete moisture information you need for ensuring a successful floor installation, it’s also fast and easy to perform. Basically, it involves drilling a small hole in the concrete, waiting 24 hours for the air in the test hole to equilibrate, employing an RH sensor at the specified depth, and obtaining an RH measurement.

These measurements can then be compared to the manufacturer’s specification to decide if the concrete is dry enough for the finished floor product that you wish to install. Since various floor products exhibit different moisture tolerances, this specification will vary from product to product.

In recognition of RH testing’s significant value for anyone in the flooring industry who works with concrete, several different manufacturers have developed easy-to-use RH test kits. The most popular is the Rapid RH L6, which includes a highly accurate RH sensor along with a completely digitized system for sending all your test data wirelessly to your mobile device.

Given the convenience and simplicity of today’s technologies for RH testing, there’s really no reason to consider using surface-based test methods.

rapid rh l6 datamaster app for accurate testing

Today’s advanced tools for RH testing make accurate testing of the moisture deep within the concrete slab both fast and easy.

Make the right decision for your floor

As a flooring contractor, you’ll want to be armed with reliable information for making the right decision about when to install the finished floor. You should never assume that what’s going on at the surface is indicative of what’s going on inside the concrete.

The science clearly dictates that you should assess the moisture deep within the slab. And that’s best done with an in-situ RH test.

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 April 19th, 2024


  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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