Air bubbles from the concrete slab.

by Victor
(Spokane, Washington)

Q. My garage slab is about five years old. I was washing it and air bubbles appeared where I was washing. What is causing this? Should I worry?

A. Don't worry, your concrete is just fine. Typically concrete is very porous, even though it looks very smooth on the surface, the concrete has thousands of tiny air-voids in it's matrix. These air-voids, called air-entrainment are put there by the concrete company when they batch the concrete. Air-entrained concrete helps the concrete survive the many freeze-thaw cycles it will go through in it's lifetime.

The bubbles you see are caused by the water you used to wash the concrete with, settling into these air-voids and pushing the air out of the voids, making bubbles in the surface water. A concrete's matrix is very much like a sponge, once the water has soaked into all the voids in the concrete the bubbles will disappear. This is normal and nothing for you to worry about.