How to Control Early-age Cracking in Concrete

by Kim Basham, PhD PE FACI
Created: October 16, 2008

Learn why early-age cracking occurs and how to prevent it.

If slabs-on-ground are free to shrink as shown, hypothetically in (a) cracking does not occur. However, in reality slabs are not free from shrinkage because of subbase friction and other restraints...
If slabs-on-ground are free to shrink as shown, hypothetically in (a) cracking does not occur. However, in reality slabs are not free from shrinkage because of subbase friction and other restraints. Shrinkage and restraints cause cracking.
Figure 2. A sawcut slot creates a weakened section in the slab to control the location of cracking.
Figure 2. A sawcut slot creates a weakened section in the slab to control the location of cracking.
Figure 4. Sawing too early causes excessive joint raveling, but sawing too late causes random cracking.
Figure 4. Sawing too early causes excessive joint raveling, but sawing too late causes random cracking.

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 1 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 one-fourth 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 as shown in Figure 5.

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.

On your next job, saw cut contraction joints before shrinkage stresses exceed the tensile strength of the concrete, use an early-entry saw to saw cut joints to a minimum depth of 1 inch and select a joint spacing that reflects the dry shrinkage potential of the concrete. Also, select smaller joint spacings if slabs are highly restrained or the owner is sensitive to random cracking.

References:
ACI 302.1R-04 Guide for Concrete Floor and Slab Construction, American Concrete Institute, 38800 Country Club Dr., Farmington Hills, MI 48331.

ACI 360R-06 Design of Slabs-on-Ground, American Concrete Institute, 38800 Country Club Dr., Farmington Hills, MI 48331.