After pouring, concrete can cure and gain strength only if it remains nearly saturated with water for a minimum of 28 days. The curing process stops if the relative humidity of the concrete drops below 80 percent Improperly cured concrete will develop structural weaknesses and cracks that can become pathways for unwanted moisture and soil gases to creep into the building. Rushing the drying process may also weaken the surface so that concrete dust continually sloughs from the surface into the home. Pouring slabs in cold weather is also risky since cold temperatures impede the curing process. The use of propane heaters may cause the concrete surface to become weak and crack as combustion gases enter the material and interfere with its chemical composition.
Improperly cured concrete will have high alkalinity levels that can cause certain substrates and finishing materials to breakdown or rot. Testing pH can help determine whether a concrete surface has been properly cured and whether problems will develop with certain finish applications. This simple test is described in the materials testing section of Division 13.
