The ASTM D4263 Concrete Moisture Test

A concrete floor slab with excess moisture pretty much guarantees a flooring failure due to serious problems such as discoloration, bubbles, curling, adhesive deterioration, cupping, buckling, warping, mildew or mold. That’s why it’s important for flooring installers and general contractors to evaluate the moisture content of the concrete before moving forward with a floor installation.

“Eyeballing” for Moisture

Some methods of concrete moisture testing use a simple, qualitative approach. This means no numbers or data, just simple observation or “eyeballing” of the test location and testing materials for signs of excessive moisture.

One such test is the Plastic Sheet Test, ASTM D4263. It’s been an old standby for some people, and can sometimes provide useful information. However, it is NOT recommended.

How to Perform the ASTM D4263 Concrete Moisture Test

astm d4163 Concrete Moisture TestThe test method involves taping an 18-inch (460mm) square of polyethylene film onto a concrete slab and waiting at least 16 hours. Afterward, the underside of the sheet is examined for signs of moisture. Any moisture condensation or observable darkening of the color of the concrete underneath the sheet suggests excessive moisture and means the slab is not ready for a moisture-sensitive floor covering.

It sounds simple enough, right? Why invest in specialized testing equipment when ASTM D4263 gives you the answer you need?

Accurate Concrete Moisture Measurement?

The problem is the misleading nature of the ASTM D4263 test results. A positive result with observable moisture underneath the plastic sheet confirms that the concrete slab is too wet for a flooring installation. However, the reverse is not true. A negative test result a dry plastic sheet with no observable moisture underneath does not necessarily confirm that the slab is sufficiently dry.

Several factors can contribute to a misleading test result, in which one concludes that the slab is dry when it’s really not. Observable moisture on the underside of the plastic sheet depends on the dew point, which in turn depends on the surface temperature of the concrete.

And, since it’s a surface test only, ASTM D4263 concrete moisture test does not show what is happening within the slab. Nor does it indicate whether additional moisture is entering the slab from below, such as in the absence of a vapor retarder.

By design, surface tests such as the Plastic Sheet Test only inform about moisture conditions at the slab’s surface. They’re not designed to inform about conditions any deeper than the first inch into the slab, and that’s a real problem.

An unsealed concrete slab will typically exhibit an appreciable moisture gradient, drier at the surface and wetter below.

Does This Moisture Gradient Really Matter?

Yes, it matters a great deal.

When the slab’s surface gets sealed by a finished floor, moisture is trapped. It can no longer evaporate through the surface. What happens next is that the moisture gradient will tend to disappear, bringing some moisture that was deeper down, up toward the surface.

Ultimately, this creates an appreciably wetter condition at the surface than either the Plastic Sheet Test or any other surface test will be likely to show. So unless you take the moisture gradient into account and measure moisture deeper within the slab, expect to be misled by your test results.

And more importantly, expect flooring failures.

How Does One Account for the Moisture Gradient?

The key is to use a moisture test that measures moisture within the slab, not just at the slab’s surface.

The most reliable and accurate method is ASTM F2170, otherwise known as the in situ relative humidity (RH) test. Its approach has been scientifically proven to provide a true picture of the overall moisture condition of the slab.

Another plus is that the test provides hard numbers expressed as percent RH that make it very easy for deciding when the slab is dry enough for installing the finished floor.

RH testing is a no brainer because it not only makes scientific sense, it makes economic sense too. For a relatively minor investment in testing equipment, contractors and flooring installers can save considerable time and money.

No more surprises. No more costly, moisture-related callbacks. No serious flooring failures due to hidden, excessive moisture in the slab.

Don’t be misled by the Plastic Sheet Test. Instead, always rely on in situ RH moisture testing. For more information about RH testing, click here.