What is concrete outgassing?
Outgassing is the escape of air, moisture, and other gases trapped in concrete pores during the curing process, typically resulting in pinholes or bubbles in coated finishes.
When concrete cures, water and gas trapped within its pore structure begin to escape. This natural migration of moisture and air toward the surface is called outgassing, and it occurs most visibly in newly placed slabs that have not had sufficient time to dry. The escaping gases carry moisture upward through the concrete matrix, reaching the interface between the slab and any coating applied to it.
If an epoxy, polyurethane, or other coating is installed before the concrete has dried adequately, the rising vapor and gas pressure cannot escape freely. Instead, it becomes trapped beneath the coating, creating small defects in the finish. These appear as pinholes, tiny craters, or larger bubble formations across the floor surface. The damage typically reveals itself within days or weeks after application, often after the coating has begun to cure.
New concrete slabs are the most vulnerable because they contain the highest moisture content. Indoor slabs poured in spring or summer, or those exposed to humidity, are particularly prone to extended outgassing periods. Concrete aged outdoors or in dry conditions may outgas more slowly or completely before coating. Understanding outgassing is critical for epoxy floor installers because improper moisture testing or premature coating application directly causes job failure and costly rework. Proper surface preparation, moisture testing with calcium chloride or relative humidity meters, and adequate concrete cure time are essential steps to prevent defects.