BY JENNIFER CRISMAN
Coating voids and pores in a concrete floor. The Euclid Chemical Company
HOW TO PREVENT COATING DEFECTS CAUSED BY CONCRETE OUTGASSING
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oncrete seeks equilibrium with its surroundings by releasing air and water vapor that expands when temperatures increase and absorbs air and moisture when temperatures decrease. The phenomenon of air expanding and releasing itself through the concrete pores is called “outgassing.” As air moves in and out of concrete with changes in temperature and barometric pressure, the expanding air travels through the pores and capillaries to the surface. However, because concrete is sponge-like and contains moisture, the air often brings moisture with it—often described as moisture vapor transmission (MVT). A primary function of surface coatings is to provide a barrier to the intrusion of water and other chemicals into the concrete, but when coatings are applied to a concrete surface that is outgassing, the air becomes trapped under the coating and the pressure of this trapped air causes a defect or pinhole in the liquid coating.
IDENTIFY & PREVENT Outgassing defects are rarely found across an entire surface. It is more common to find outgassing defects scattered seemingly at random around a coated concrete surface. Defects can be clustered in areas directly exposed to sunlight, where
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mechanical surface preparation was more aggressive, or even where there’s increased air circulation over the slab. • Monitor Environmental Conditions The primary method to reduce defects caused by concrete outgassing is to monitor the environmental conditions before and during coating application. A continuous flow of air over a porous concrete slab could entice entrapped air to flow outward, leading to outgassing and coating defects. Ensure that heating/cooling systems and open doors/windows are not blowing air across the concrete surface during the coating application and curing process. • Track Temperature Changes An increasing surface temperature is a strong indication that outgassing will occur. When weather fronts are on the horizon, it is best to wait before coating. Hot concrete and/or rising ambient temperatures cause air to move within the slab at a more extreme rate. Barometric pressure changes can also push air into concrete and vacuum it out repeatedly. When barometric pressure fluctuates, outgassing may occur even when all other conditions are ideal. Application of the coating in a consistently HVAC-controlled environment, or in the evening when the air temperature is decreasing, can reduce the possibility of outgassing.
• Seal with a Primer To reduce outgassing, especially on porous or rough surfaces, seal the concrete after surface preparation with a low-viscosity, deeply penetrating primer. Because of its thin consistency, primers are less likely to exhibit pinholes or other defects from outgassing because the air is able to make its way through. Primers soak into the concrete to fill the voids and capillaries that can hold air, thereby reducing the chances of outgassing. Most primers are moisture tolerant, and some are formulated to act as MVT reducers or oil blockers as well.
TIME TO REPAIR Repairing outgassing issues requires recoating with the same material. Since most outgassing defects occur scattered around a project, reapplying only the affected areas will lead to an undesirable, patchy appearance. To correct outgassing defects both aesthetically and functionally, it may require an entire floor or wall to be recoated. This can be time consuming and costly. Being aware of the conditions that lead to outgassing, thoughtful planning, and following precautions is the ultimate method to prevent and significantly reduce the possibility of outgassing. Jennifer Crisman is director of marketing services at Euclid Chemical.
www.ForConstructionPros.com/Concrete
11/30/21 12:58 PM
