Q. Why does good concrete crack at random locations even though the sidewalk has been jointed?

A. This picture (right) shows two random cracks. Before taking the picture I wetted the concrete on one side of one of the cracks so it would show up better. I also wetted the joint area so you can clearly see that a joint is there.

The joint appears to be so shallow that it doesn't do any good. The modern rule is that joints that are made for the purpose of controlling cracking should penetrate to at least 1/4 the depth of the concrete. The joint in the curb may have helped initiate the crack in the concrete. But nature's forces were unable to identify this shallowly tooled joint as part of a weakened plane that they could follow. Consequently they found their own weakest location to continue their cracking.The crack at the right side of the picture may have been caused by a change in the cross section. The concrete may be too thin in this area.