To many, concrete appears to be dry, solid, and impermeable. But in truth, concrete more closely resembles a very hard sponge. The concrete matrix includes thousands of tiny pores that are filled with water when the concrete is placed. As the hydration reaction consumes some of the liquid water and as concrete near the surface evaporates, the pores dry out. The liquid water may be gone but there is always some level of water vapor in the pores—just like in the air around us.
Interior concrete floors that are going to have a flooring material installed must be as dry as possible prior to installing the flooring. The moisture coming from a concrete floor will cause adhesives to liquefy, will delaminate flooring materials or coatings, and can cause mildew in carpeting. This problem can be eliminated by allowing the floor enough time to dry out and by making sure there is an impermeable vapor barrier directly below the concrete slab. The vapor barrier prevents moisture in the soil below the slab from moving into the floor.