Question: We've got some holes that look like popouts in our concrete sidewalk. But you can normally see an aggregate particle at the bottom of most popouts, and there's no particle in most of these. Can materials other than aggregates cause popouts?

Clay lumps near the surface can cause popouts. So can lumps of undissolved calcium chloride, sometimes found in concrete placed during cold weather. These materials might wash out of the hole after the piece of concrete popped out. However, there's another possibility. Are the holes all the same size?

And is there any pattern?

Answer: Our contributing editor, Bruce Suprenant, found an interesting case of surface damage to sidewalks in Florida. There were a number of holes in the sidewalk but they were spaced in a regular pattern and were all near the edge. Bruce thinks it's most likely that the damage was caused by a soil aerator being pulled over a part of the sidewalk.

Holes in a concrete sidewalk appeared at first to be popouts. However, they were regularly spaced and it's more likely that they were caused by a soil aerator.