Concrete Moisture Meter- enough or not?

Successful concrete slab installation rests on two key pieces of knowledge regarding moisture: an understanding of concrete moisture content (MC), and the limitations of a concrete moisture meter. Builders and consumers must know that concrete floor moisture test methods produce MC readings for different purposes. The bottom line is that a concrete moisture meter does not indicate overall concrete slab readiness.

Concrete Moisture Meter

A concrete moisture meter is a type of moisture meter used by installers of flooring to measure the moisture levels of concrete. These meters have been used for decades to measure the moisture content in different materials and substances. Concrete meters have evolved from the successful wood moisture meter as flooring contractors tried to use their wood meters to measure the moisture in concrete. They are designed to be used as a relative test. The meters are used to “‘Spot check’ the top surface at one particular location on the slab.” The results can determine the best place to put a concrete relative humidity test.

Limitations

There is no ASTM standard for using a concrete moisture meter to determine a final moisture content reading.

Concrete moisture meters, either non-pin or pin meters are affected by what it sees in the concrete. This can be anything from the density of the concrete and aggregate size to the chemical properties of the slab.

Concrete is formed by mixing cement, water and aggregate; modern mixes produce more subtle blends of these materials which may include more or less water. That, in turn, can influence the concrete slab’s drying time. Other components, such as rebar (metal bar reinforcements) can produce false moisture content readings on non-pin meters. The varying chemical and physical characteristics can skew “moisture” readings by creating electrical resistance variances within the slab. No metered concrete floor moisture test can detect that dynamic from a surface reading.

So, installers may best use a concrete moisture meter as a qualitativeassessment; that is, an indication of where to best place RH sensors which produce quantitative moisture content results.

The Composite: Meters and Probes

How best to measure moisture content in concrete slabs? The answer: a combination of qualitative (surface) assessment and quantitative (internal) assessment.

Uncovered concrete dries from the top down. Concrete moisture meters measure only the top inch at most and this area is drier than the concrete further down. Once a floor covering has been installed the moisture in the slab equilibrates. In order to ensure the equilibrated moisture will be a safe level for a floor covering, a relative humidity sensor must be drilled and placed at 40% of the depth of the slab, This depth has been proven to be the relative humidity percentage that the slab will equilibrate once the top has been covered by a floor covering.

Every concrete moisture meter provides qualitative information by which installers can then accurately conduct quantitative RH testing inside the slab. Based on the combined results, contractors can determine concrete slab readiness.

Increasingly, building materials manufacturers include relative humidity specifications in the installation guidelines for their products. This assists builders greatly to ensure that concrete floor moisture test results conform with the optimum conditions needed for floor covering success.