Q: In December 1996 we placed a floor slab for a manufacturing plant, and a year later we were called back by the owner because one section had cracked badly. Most of the floor is 8 inches thick, but the part that cracked was a 6-inch-thick aisle adjacent to an 8-inch-thick slab for the plant's production area. Specifications required a 5000-psi concrete containing 658 pounds of cement per cubic yard. We placed the concrete on a 6-inch-thick stone base with no vapor barrier and used 6x6 W2.9xW2.9 welded wire fabric, as required by the specification.

This slab was the last pour, and we left the job after completing it. We heard that there was construction traffic on the slab within seven days after it was placed because the doorway leading to the 8-inch-thick slab was blocked. The sketch shows the cracking pattern that developed in the first year after the building was turned over to the owner.

What causes this kind of cracking pattern?

A.: The cracking pattern in this 6-inch-thick slab indicates that the cracking probably is due to corner curling at contraction-joint intersections.

Much of the cracking appears to be related to corner curling at contraction-joint intersections. Curling is usually caused by unequal drying shrinkage: It's greater at the top of the slab than at the bottom. This causes the corners to curl upward, leaving them unsupported. Traffic loads then cause corner cracks like those that have occurred at nearly every contraction-joint intersection (see drawing, above). The multiple cracking in some of the panels may be caused by restrained drying shrinkage or thermal contraction.

Because the floor was placed in the winter, some of the curling may have been caused by the top of the slab cooling faster than the bottom. In an unheated building, the top of the slab would contract more than the bottom, creating corner curling similar to that caused by uneven drying shrinkage. If the slab was loaded after only seven days, the combination of unsupported corners and low strength at this age would increase the likelihood of corner cracking.