As a concrete slab dries, there is a gradual and diffuse process that must take place before it is ready for a flooring or coating application. When concrete is freshly poured, liquid water must evaporate from the surface. As that liquid evaporates, more liquid moves up through the slab to continue evaporating from the surface until only water vapor remains to move through the slab in the drying process. But by its very nature, the process means that a slab’s surface will be “drier” than deeper into the concrete. And it is the amount of water vapor, or relative humidity that must be monitored in order to meet ASTM standards and to ensure a proper flooring application.
Keep in mind that an uncovered concrete slab will indeed have a relative humidity gradient (typically drier at the surface; much wetter at depth) throughout its thickness until a floor covering is put on top. Under normal conditions, the RH at roughly 50% slab depth will be significantly higher than the surface unless the slab has been down for a long time, and a vapor retarder is directly underneath the slab. This normal relative humidity gradient is why problems occur when relying on the use of surface tests.
Moisture meters, no matter what type, fail to provide consistently accurate ‘moisture’ readings across the different mixes and densities of concrete. Additionally, other components (metal reinforcing bar, aggregate size and amount, etc.) can cause false indications of ‘moisture’ especially with non-pin meters. Pin-type ‘moisture’ meters are also not practical for moisture measurement because variable chemical and physical characteristics in concrete can cause false readings due to changes in electrical resistance that have nothing to do with moisture.
The calcium chloride test method (moisture vapor emission), and the UK-based ‘hood’ method (RH) both have the same problem. They are surface tests and in reality, the surface of the concrete will more closely reflect the RH in the room or building than the conditions down inside the slab.
The only way you can get the true “moisture” picture, then, is by putting a probe down into the concrete and actually measuring the relative humidity within the body of the concrete. The industry standard (developed at Lund University in Sweden and adopted by the ASTM) demonstrates that a probe set at approximately 40% of the concrete’s total depth provides the best indicator of the final equilibration once a floor covering is applied, and an accurate indicator of the conditions that will be in contact with the final flooring or coating.
As surface moisture tests, hand held meters (qualitative), or calcium chloride or hood method tests (quantitative) are fine. But if you want the true, critical moisture picture of what’s happening deeper in the concrete, you have to get below the surface with an in situ relative humidity probe. An accurate relative humidity reading can inform your business decisions and prevent costly problems in the long run.