Spacing for Concrete Control Joints

Control Joint

• Cracks that form unpredictably at different sites in the concrete could impair the integrity of the structure and make the concrete surface look less aesthetically appealing. Consequently, builders ensure that shrinkage cracking will take place in a predictable location by incorporating control joints, although these control joints will only have the desired effect if they are spaced properly. If they are too far apart, then shrinkage cracking could take place between them.

Spacing

• Assuming the aggregate in the concrete mix has a maximum size of three-quarters of an inch, the distance between adjacent control joints as expressed in feet should be no more than 2 to 3 times the thickness of the slab as expressed in inches, and ideally the distance as expressed in feet should be about 2 times the slab thickness as expressed in inches. If the slab is 4 inches thick, for example, control joints should be spaced 8 feet apart. For slabs more than 8 inches thick with heavy loads, however, the spacing between control joints should be limited to 15 feet.

Large Aggregate

• In cases where the maximum aggregate size exceeds 3/4 inches, you can increase the distance between control joints as expressed in feet to 2 and 1/2 times the thickness as expressed in inches. If the aggregate size is greater than three-quarters of an inch and the slab is 5 inches thick, for example, you could space the control joints 13 feet apart, although the same 15-foot maximum still applies. Each control joint should be cut to one-quarter of the depth of the slab.

Considerations

• You can cut the control joints into the fresh concrete with a grooving tool or into the hardened concrete with a saw. Ideally you want to conceal the control joints by situating them in places where they will not be seen. You could, for example, place control joints beneath carpeted walkways. This precaution is not necessary for the structural integrity of the concrete, but is desirable for aesthetic reasons.