Q.: I built a patio in my backyard last July and did the concrete work myself. I used a 6-bag mix with 5 percent entrained air and the concrete wasn't too soupy--not more than a 5-inch slump. It was a very hot day but my neighbor and I gave the surface a steel trowel finish before it had set up too much. I got some burlap bags from the local feed store and covered the surface immediately after finishing, then wet it down well and covered the whole thing with polyethylene sheets. I kept the burlap wet for a week. When I took it off there were several places on the surface where it looked as though the concrete still hadn't set. Some were as much as 6 inches across and others were only a couple of inches in diameter. The spots weren't more than 1/8-inch deep. The coarse aggregate was showing in these spots and yet most of the patio looked fine. What could have caused this?
A.: One possible cause for this problem is that some of the burlap bags you used were contaminated with an organic substance such as sugar that retards concrete setting. Because you gave the patio a steel trowel finish and may have finished it too early, it's also possible that blistering was the problem. Blisters occur when entrapped air or bleed water gets trapped in the concrete. However, since you said that it looked as though the concrete hadn't set in the affected areas, our first guess is probably the more likely answer.