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If random cracking occurs before saw cutting the contraction joints, they typically extend across the entire width of the slab and likely are oriented perpendicular to the long direction of the slab. If cracks reverse direction and have unusual orientations with clusters, most likely they initiated on the bottom of the slab because of significant restraints caused by the subbase. In either case, sawing was too late to control the early-age cracking.

Joint depth and spacing

Contraction joints are commonly installed using conventional wet-cut or early-entry dry-cut saws fitted with a diamond blade. Joints sawed with conventional wet-cut saws are made within 4 hours during hot weather and 12 hours in cold weather. The waiting period for joints installed with early-entry dry-cut saws is much shorter — in hot weather, saw cuts are typically installed within one hour after finishing and 4 hours for cold weather. Using an early-entry dry-cut saw reduces the risk of cracking by allowing saw cuts to be installed sooner than conventional wet-cut saws.

Saw cut depths for a conventional wet-cut saw should be ¼ of the slab thickness or a minimum of 1 inch. For early-entry dry-cut saws, the depth of the saw cut should be 1 inch minimum for slabs up to 9 inches. Typically, most specifications require either an absolute minimum saw cut depth of 1 inch or specify a 1¼ with a tolerance of ± ¼ inch when using early-entry dry-cut saws.

Historically, designers and contractors used a rule-of-thumb that joint spacing should fall between two and three times the slab thickness. For a 6-inch-thick slab, this approach yields a joint spacing between 12 and 18 feet. Because of column spacings or economics, many times the actual joint spacing approaches or exceeds the maximum recommended joint spacing of 18 feet for a 6-inch-thick slab. This approach does not account for the shrinkage potential of the concrete, subbase friction, slab restraints, environmental factors and sensitivity of the owner to random cracking. Designers and contractors should take these factors into consideration when choosing joint spacings. The risk of random cracking increases as the joint spacing increases from two to three times the slab thickness.

In 2006, ACI modified the joint spacing criteria to directly account for the shrinkage potential of the concrete. Now, joint spacings are based on three categories of concrete shrinkage as shown below.