Drying Time of Concrete Slabs

Subfloors & Underlayments

After the concrete is cured, the flooring contractor needs to ensure that it dries properly. The elimination of free water is essential for the formation of a bond between the adhesives, the flooring materials and the concrete. In the presence of free water, water-based adhesives will not set up, and solvent-type adhesives will not adhere. In the case of asphaltic adhesives already bonded to dry concrete, the asphalt will eventually be displaced by water if the availability of water is sustained.

Alkaline salts in solution with moisture, which exude from drying cement or which work their way up from the earth in concrete on grade and below grade, have a tendency to prevent or destroy satisfactory bonding of adhesives by sheer physical displacement. They can leave unsightly salt deposits at the seams of sheet materials and joints of tiles. They can also have a deteriorating effect on the overall installation.

Too much ground moisture can create problems for on-grade and below- grade areas of commercial and residential buildings over and beyond those relating to the installation and use of resilient flooring. These problems vary from merely slight but unpleasant dampness to actual structural damage. Moisture in a concrete slab varies as the weather changes. The amount of moisture in the slab usually approximates the dampness of the subsoil. What may be a relatively dry slab at one time can be quite different at another.

a. Suspended Concrete

Usually, suspended concrete is protected from all sources of moisture except the moisture initially in the mix and water vapor in the atmosphere. However, it should be kept damp for some time to permit chemical reaction to cure the concrete.

Suspended concrete poured on a metal deck is often produced with lightweight aggregate which can retain excess water longer than standard weight aggregate. Because drying is only possible from the top surface, such construction usually takes additional drying time. The use of curing compounds and sealers prolong the drying times.

When resilient floors are to be installed, allow suspended concrete to dry thoroughly for several months with good ventilation, lower humidity and dry heat. When suspended concrete is known by tests to be sufficiently dry, then a resilient floor may be installed.

b. On-Grade and Below-Grade Concrete

Although the concrete used for slabs at all grade levels may be identical, certain differences are important as they influence the drying. Slabs in contact with the ground, without a vapor retarder, are exposed to ground moisture. When covered with resilient flooring, these slabs will be approximately as moist as the soil on which they are placed. A concrete slab on grade or below grade is always damp in the absence of a perfect moisture barrier.

Updated on October 26, 2022