Moisture Can Cause Problems with Concrete Flooring
Concrete interacts with the environment around it. Conditions such as temperature and humidity can affect the amount of moisture in a concrete slab. Moisture causes problems with concrete floors that require hundreds of millions of dollars to correct every year across North America. It is much easier and less expensive to deal with moisture issues before a concrete floor is installed than to repair damage later.
Moisture in concrete can come from free water, which is water in the concrete that is in excess of what is needed to hydrate the cement and create a workable consistency, or from moisture that rises from beneath the slab. Moisture that rises from under the slab usually occurs because a vapor barrier was not installed under the slab.
Natural systems have a tendency to move toward a state of equilibrium. Water vapor will move from an area with a higher to a lower concentration in a process called diffusion.
If the relative humidity of a concrete slab is different than the air above it, moisture will move in one direction or the other. If the concrete slab is more humid, it will dry out. If a vapor barrier has not been installed, the relative humidity in the concrete slab can be as high as 100 percent. The relative humidity in the air is usually less, which means that humidity will move from the slab into the air. When the surface dries, moisture will be drawn up from the bottom of the slab.
If water vapor escapes from a concrete slab into the environment, it can make the room humid. This can be problematic with modern tight home construction methods. If the concrete on the ground underneath the slab is very wet, the floor can be damp. This can cause condensation under objects placed on the slab and can lead to the growth of mold.
If carpet, vinyl, or wood flooring is placed over a damp concrete slab, the moisture and high alkalinity can cause warping of the flooring and emulsification of adhesives. This can be prevented by using concrete with a low water-cement ratio of 0.5 or less and installing a vapor barrier under the slab. A vapor barrier can prevent damage caused by moisture from the ground, but it has no effect on the moisture within the concrete. That moisture can still damage the flooring above.
Moisture being released from decorative concrete can cause several problems. If a stained slab loses a lot of moisture, the floor can become blackened or discolored, especially if the stain contains copper salts. When water vapor moves through the concrete slab, it can condense and form liquid water. The water vapor can also leach calcium hydroxide that can cause efflorescence on the surface of the slab and cause high pH levels.
The most significant problem with moisture moving through a decorative concrete slab is that it exerts pressure on the surface. If there is an excessive amount of moisture in the concrete and an impermeable layer is placed on top of it, the movement of moisture can delaminate the sealer or overlay.
Before a concrete floor is installed, the substrate surface should be tested for moisture emission. This can be done with a Relative Humidity Test, which involves drilling holes in the concrete slab, or with a Calcium Chloride Test, which involves leaving a small dish of calcium chloride on a concrete floor under a plastic dome to see how much moisture is absorbed.
If there is a lot of moisture underneath a concrete slab, its effects can be mitigated in several ways. Reactive penetrants can be used to reduce moisture and the transfer of soluble alkali from the slab. Moisture retarding coatings can slow down the emission of moisture from the slab and prevent soluble alkalis from raising the pH. Epoxy-based or epoxy-modified coating overlays can be used to form a layer to separate the concrete and the top layer of flooring. This can prevent the alkali salts in the concrete floor from reaching the adhesive. Dispersive membranes can use a fiber mat membrane to diffuse moisture beneath the flooring materials.