Concrete Meters: A Limited Indicator of Concrete Moisture
The concrete moisture meter has evolved, but the need to clearly assess concrete moisture content (MC) remains as it always has: crucial.
Concrete Moisture Constituents
Water is a core component of any concrete mixture. In fact, concrete MC has become an exacting science of fitting core constituents – cement, aggregate, admixtures and water – with their intended application.
However, professionals need concrete moisture testing in order to assess the slab’s moisture gradient, its level of MC inside the slab. There is approximately twice as much water in a concrete slab than is necessary for the chemical process of hydration to occur. This additional water is there strictly as “water of convenience” for placement and finishing. Once poured, concrete slabs need time to dry in order for this excess moisture to dissipate. Time is a key ingredient for a successful concrete drying process.
Too often, builders use surface-based concrete moisture test results (either MVER or handheld meters) to declare that a slab is “dry” when, in fact, there may still be substantial MC beneath the surface. In this context, we see that a concrete moisture meter is useful, but not entirely adequate.
Water vapor migration can change the drying timeline due to relative humidity (RH) within the slab but is also affected by ambient RH and temperature that influence conditions at the slab surface. When ambient RH is high, water vapor cannot evaporate as quickly from the slab surface; when ambient RH is low, water vapor dissipates into the environment much more quickly. Either scenario will affect the rate at which moisture can move up through the slab gradient. So only a concrete moisture test which addresses MC at optimal slab depth can truly indicate the status of concrete dryness.
Concrete flooring failure is more likely to occur when contractors presume slab dryness and mistakenly seal in MC with a floor covering. Put simply, the water has nowhere to go and cannot evaporate. This can lead to substandard floor performance, adhesive breakdown and (at worst) total floor failure.
Timing Rules of Thumb
Nonetheless, contractors do need a general sense of how much time to allow for successful concrete drying. Several pounds of MC must evaporate from every square foot of a slab in order to safely install floor coverings. Generally, the industry needs to allow one month of concrete drying time for every inch of slab thickness. But a realistic schedule does not move forward until RH testing shows the slab is ready for the specified application.
And a concrete moisture test cannot adequately inform contractors of overall slab dryness unless the test accurately assesses MC levels inside the slab. Any surface concrete moisture meter provides only a superficial picture of slab dryness. In other words, builders need quantitative MC results.
The Bottom Line
Concrete moisture test technology makes concrete moisture assessment increasingly accurate.
Wagner Meters’ Rapid RH® deploys sensors which are inserted into holes in concrete for the purpose of measuring concrete MC at 40 percent deep into the slab. The Rapid RH® cost-effectively provides MC (RH%) measurement at varying locations throughout the slab as determined by industry standard ASTM F2170. Rapid RH® probes equilibrate much faster than any other brand and provide MC reading to within three percent of the final MC (RH%) reading within minutes instead of days. Complete information means complete confidence when moving forward with the building schedule.
Take the time to test concrete dryness correctly, and your drying time achieves a measure of unmitigated success.
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