Q.: What caused a crack to form early in our recent parking lot construction? We used air-entrained concrete estimated to attain 5500 psi compressive strength and placed it at a slump of 5 inches and finished it with a vibrating screed. The air temperature was 80 degrees F or slightly higher. When the slab, which was 5 inches thick, was between 21 and 23 hours old we sawed it into 12-foot-square panels by cutting 1 1/2 inches deep. By then the slab had already cracked in one place on a line about 1 to 2 feet from the intended location of one of the joints. This crack extended across a 24-foot width of pavement. As a result we decided to omit the joint that we had intended to cut at that location.

A.: The general rule is that slabs should be sawed as soon as sawing can be accomplished without causing the joints to ravel. The ACI 302 Guide for Concrete Floor and Slab Construction says that at normal temperatures this can usually be done by sawing within 4 to 12 hours after placing. Since you placed the concrete in warm weather, it seems obvious that you were sawing at a dangerously late time.

Assuming you do not plan to repair the crack (for example by epoxy injection), it was a reasonable decision not to saw a joint adjacent to the crack. That would have produced additional irregularly shaped small narrow panels. The existing panels adjacent to the crack should function much like the other panels since they are of comparable sizes. A disadvantage is that a crack is more likely than a joint to undergo progressive deterioration. Another obvious disadvantage is that the crack does not look as neat as the joints and would be more difficult to seal.