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. 

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. 

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

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.