Outgassing is when air or moisture vapor trapped in porous concrete escapes up through a wet coating, leaving pinholes, bubbles, or craters in the finish.
Understanding Outgassing
Concrete is a porous material full of tiny air pockets. As temperature rises during or after application, usually because the sun heats the slab or the concrete is still warm from a recent pour, the air trapped inside those pores expands and pushes up through the surface. If a wet coating is sitting on top when that happens, the escaping air pushes through the film and leaves behind pinholes, bubbles, or small craters.
What Triggers Outgassing
Outgassing is most common on new concrete that has not fully cured, on slabs coated during a rising temperature window such as mid morning, and on highly porous or poorly prepared surfaces where more air can move through the pores at once. Direct sunlight hitting a slab shortly after coating is a frequent trigger, since it heats the concrete faster than the air above it.
Preventing and Fixing Outgassing
Mechanically opening the surface with grinding or shot blasting before coating gives trapped air a more direct path out before the coating goes down. Applying a primer coat, sometimes two on especially porous concrete, lets air escape through a thinner film rather than the final finish coat. If bubbles do appear, a light sanding followed by an additional coat once the slab has stabilized usually resolves the issue.
Why It Catches Contractors Off Guard
Because outgassing depends on temperature and timing rather than anything visibly wrong with the concrete, it can affect a slab that passed every moisture test. Experienced contractors watch the weather and schedule coating work for stable or falling temperatures, and keep extra material on hand in case a second pass is needed.
Also Called
Off-Gassing, Gassing
