Hairline cracks in retaining wall

Q: Last July, I poured an 8-in.-thick concrete retaining wall in northern Utah. It’s built on thick footings placed at the frost line and has plenty of rebar. We poured the concrete for the wall in one lift and used a vibrator to settle it in. But the temperature was about 100°F on the day of the pour. Now there are vertical hairline cracks approximately every 10 ft. in the wall. What could have caused them?

A: Rick Arnold , author of Working With Concrete, replies: Unless you altered the mix to accommodate the temperature—using type II cement or having a retarder added—the concrete would have cured at an accelerated rate, which can cause cracking.

Also, if the wall wasn’t covered and kept moist for the first week, then the water inside the concrete would have evaporated prematurely, causing the concrete to dry and crack rather than cure. If the pattern of the cracks matches that of the steel, then hot steel could be the cause. Steel at the ambient temperature of 100°F is hot enough to drive the water from the concrete immediately surrounding it, causing a differential cure rate and cracks.

I wonder if, on such a hot day, you added water to the concrete partway through the pour. Doing so would change the dynamics of one section of the wall, resulting in unequal curing and possible cracking. One more cause of the cracking could be overzealous use of a concrete vibrator. Any more than about a five-second burst at any one spot can separate the aggregate from the mix, causing weak spots that are susceptible to cracking.

Fortunately, hairline cracks are more cosmetic than structural. Fill the cracks with an expandable masonry caulk to keep out the water, preventing damage caused by freeze/thaw conditions, and keep an eye on the wall to see if the cracks get any bigger.