How Concrete Relative Humidity Readings Can Save Your Business Money

Most major floor covering and adhesive manufacturers in North America and Europe provide concrete relative humidity specifications in their installation guidelines. Meeting those specifications by accurately measuring the relative humidity of your floor slab is key to avoiding costly warranty or flooring failure expenses.

But surface testing and “moisture” meters are simply inadequate for reflecting an accurate picture of the drying condition of a concrete slab. “Moisture” meters should never be used to make the final determination as to whether or not a concrete slab is dry enough for a flooring installation. Why? “Moisture” meters only measure the top “drier” portion of a concrete slab.

By their very design, hand held meters that generate their readings through an electromagnetic field only measure approximately a one-inch depth at the surface of a concrete slab. If the field of electromagnetic signal went deeper, any steel in the slab, such as reinforcements or wire mesh, would “amplify” the readings of the meter as they interacted with the signal. As Howard Kanare of CTLGroup points out*, “It [the steel] will transmit that electromagnetic signal and the handheld meter will then indicate the floor was soaking wet … even if it isn’t.” For this reason, these surface moisture meters were designed to not penetrate deeply and they cannot see what is going on at the critical depth in the concrete. Additionally, the various mixes, aggregates, etc. in the concrete can affect the accuracy of these moisture meter readings.

Other tests such as the calcium chloride test method (moisture vapor emission), and the UK-based ‘hood’ method (RH) also have similar problems, only indicating conditions on the surface of the concrete slab. In reality, the surface of the concrete will more closely reflect the RH in the room or building but cannot determine the conditions down in the slab. And that is why problems occur.

Once a floor covering is placed on a concrete slab, the relative humidity (RH) within the slab will equilibrate throughout the thickness of the slab. This means that a slab that may have been “dry” (low RH) at the surface without a floor covering, will see a higher RH (migrated from down in the concrete) at the surface when the floor covering has been installed. Relative humidity conditions do not completely equilibrate until a floor covering is installed. (The only time this is not true is if the slab has been down for a long time with a vapor retarder directly underneath the slab.)

A relative humidity reading from within the slab (a drilled hole with a sensor installed at 40% depth for slabs drying from one side) is a much more accurate and efficient means to accurately test a slab’s relative humidity. It also means making informed choices about the best applications for the adhesives, flooring or coating that will be applied over the slab. Correct readings from within the slab are crucial to following the manufacturer’s installation guidelines, meeting industry standards and preventing costly flooring failures.

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