Avoid Unjointed Elongated Slabs
to Control Cracking

Hotline Problem

A contractor headed for a preconstruction conference wanted to know the maximum recommended length-to-width ratio for sections between joints of slabs on grade. No wire mesh was to be used in the slabs, but some of the sections were twice as long as they were wide.

Troubleshooting Response

The rule of thumb is that the ration of panel length to width shouldnt exceed 1.5. This is recommended in "Guide for Concrete Floor and Slab Construction," (ACI 302.1R-80), in Section 2.3.2 on contraction joints.

Panels with excessive length-to-width ratio are likely to crack at the midpanel point or at some other location between joints.

The maximum recommended joint spacing in feet is usually two to three times the slab thickness in inches. Thus, for a 6-inch thick slab the recommended joint spacing would be 12 to 18 feet.

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