Q. We are experiencing intermittent shrinkage cracking while placing extruded concrete curb. Generally the cracking occurs along the face of the curb and the presence of tight radii does not seem to make much difference. What can we do to stop this?
A. The concrete is probably cracking due to the low water content in the mix resulting in evaporation rates that are too fast relative to strength gain. Cover the exposed concrete curb with plastic sheeting and the problem will most likely go away. Another tip is to make sure that the subgrade the curb will be placed on is uniformly damp. Remember, subgrades must be wet down to a depth of 4 inches to prevent concrete that is placed above it from cracking. Another suggestion is to use synthetic fibers to help reduce plastic shrinkage cracking.