Q. We're having a problem with popouts in the sides of a pool in southeastern South Dakota. The concrete was poured in August, when the temperature was in the mid-80s, using a 4000 psi mix with a .49 w/c ratio and 6% air entrainment. It was also stamped and sealed. A few popouts seem to be from the natural sand, which is quartzite. However, we haven't had problems with popouts on anything else. What could cause just the color stamped concrete next to a pool to pop? I have color stamped concrete and sealed other concrete at about the same time that doesn't show any popouts.
A. Based on your information, it's hard to determine the exact cause of the popouts. But the fact that most of them occur on the deck area located closest to the pool suggests that water is involved, because that portion of the deck would be wetted more frequently than deck areas farther away from the pool. You also mention that quartzite aggregates were incorporated in the mix and were responsible for some popouts, so the problem is probably due to alkali silica reaction (ASR).
Three conditions must be present in order for ASR to occur: a wet environment, reactive aggregates, and high alkalinity. When these conditions occur, the aggregates expand and break the concrete matrix around them.
You also may find a portion of the aggregate in the bottom of the hole. If the popouts are fairly small and conical in shape, it's likely that the quartzite sand is the culprit.
Another possibility is that there isn't enough air entrainment in the mix. You mention 6% but it's not clear whether this was the design percentage or the percentage determined by testing at the jobsite. If it's not based on test results, you may have less entrainment in the slab than what is needed to protect it during freeze-thaw cycles. But if that were the problem, you would typically observe scaling rather than popouts on the concrete surface. The slab would have to have gone through a winter for scaling to occur.
There is one other possibility. Your concrete probably contains a small percentage of chert aggregates, a porous stone that absorbs water. When that water freezes, it breaks up the aggregate. You will usually see part of the stone imbedded in the concrete at the bottom of the popout.
In any case, the popouts are likely coincidental with, and not directly related to, the coloring and stamping.