Question: We supplied concrete for a large floor, which the concrete contractor placed using a laser-guided self-propelled screed and riding power trowels. He initially floated the surface with pans and then used combo blades for later floating and troweling. During the late troweling stages, the blades were tilted at a pretty good angle and started dislodging 1/2- to 1-inch-diameter aggregate particles. Some smaller particles actually rolled across the floor.

When we later examined the floor, we also found small pockmarks where coarse aggregate particles were exposed, and there were annular voids about 1/8-inch wide surrounding the particle. It looked as if the top of a hard-boiled egg had been cut off and you could see the yolk (aggregate particle) and the white (void). One of these pockmarks appeared about every 100 square feet.

Is this problem caused by the concrete properties or the finishing method?

Answer: We have never heard of this problem, nor have several finishers we talked with. If any of you, our readers, have encountered this situation, did you find a solution?