Hairline Cracks In Retaining Wall
George B asks, Last year I poured an 8 inch thick concrete retaining wall on thick footings at the frost line with plenty of rebar. We poured the wall in a single pour & used a vibrator to settle it, the temperature that day was around 100 degrees F. Now there are vertical hairline cracks at about every 10 feet in the wall. What would have caused this?
There are several things that can cause cracking, adding water to the mix partway through the pour will result in unequal curing & possible cracking. Over using a concrete vibrator at any one spot can separate aggregate from the mix, causing weak spots that are susceptible to cracking, & unless you altered the mix to allow for the temperature, using type II cement or adding a retarder, the concrete would have cured at an accelerated rate, which can cause cracking.
Also, if the wall was not covered & kept moist for the first week, then the water would have evaporated prematurely, causing the concrete to dry & crack rather than cure. If the pattern of the cracks matches that of the rebar, then hot rebar could be the cause. That rebar at a temperature of 100 degrees is hot enough to drive the water from the concrete that is surrounding it, causing a differential cure rate & cracks.
Fortunately, hairline cracks are more cosmetic than structural. Seal the cracks with a masonry caulk that will prevent water intrusion & damage than can be caused by freeze/thaw cycles & monitor to make sure the cracks do not get bigger.
