When is Concrete Really Dry?

March 1, 2012

The real answer? Never.

As with most materials, concrete retains a certain amount of water when in equilibrium with the built environment. However, concrete is a particularly wet material. Using ASTM #2170, the equilibrium relative humidity (ERH) test, most flooring manufacturer guidelines require the concrete maximum moisture to be between 70 and 75% ERH before placement of the floor will be warranted. Furthermore, 70 to 75% ERH is usually about 0.70 to 0.75 water activity. That makes concrete one of the wettest of our “dry” building materials.

As with any drying project, make sure you have identified the source of the water that has made the concrete too wet to apply flooring. Except for prolonged flooding, most water losses only cover the concrete with for as little as a few hours to as long as a day. This is seldom enough time for the water to penetrate the surface and saturate the concrete. Therefore, in most water losses, the moisture on the surface of the slab will usually evaporate in the time it takes to dry the structure – especially if the structure is timber framed.

If the slab shows that it is still too wet to place flooring after the rest of the building is brought back to its normal equilibrium, there is usually an underlying reason for the excess moisture. Reasons may include an insufficient moisture barrier under the slab, a water pipe or hydronic heating leak, garden sprinklers spraying on the building, etc. You must determine the original source of the water in order to prevent the problem from reoccurring.

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