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Controlling Early-age Cracking

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Figure 1. A sawcut slot creates a weakened section in the slab to control the location of cracking.

Figure 3. Sawing too early causes excessive joint raveling, but sawing too late causes random cracking.

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By Kim Basham, PhD PE

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.

Kim Basham is president of KB Engineering LLC, which provides engineering and scientific services to the concrete industry. Basham also teaches seminars and workshops dealing with all aspects of concrete technology, construction and troubleshooting.

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