Q.: On a hot day in August, we poured 40 lineal feet of concrete foundation wall for a house addition. The next day when we stripped the 2-foot-wide plywood-faced form panels we noticed about 10 to 15 cracks at the top of the wall. As shown in the photos, the cracks went from face to face but didn't penetrate to the full wall depth. They terminated at a depth of about 3 to 4 inches. What causes these cracks?
A.: In one of your photos, the crack coincides with the edge of a 2-foot-wide form panel. Cracks at these locations may be caused by plastic settlement of the concrete over form ties. However, your other photo shows a crack near an anchor bolt but not at the edge of a form panel. If the temperature dropped over night, it's possible that this crack was caused by thermal contraction. Plastic shrinkage cracking is another possibility.