Q.: What's the maximum recommended length-to-width ratio for sections between joints in a slab on grade? The drawings for a job I'm building show some bays that are twice as long as they are wide. No wire mesh is called for in the 6-inch-thick slab.

A.: The rule of thumb is that the ratio of panel length to width shouldn't exceed 1.5. This value 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 ratios are likely to crack at the mid-panel point or at some other location between joints.

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

Reader response:

A consulting engineer called to warn about the dangers of designing joint spacings at the extremes of these limits. He contends that mid-panel cracking is likely in a 6-inch-thick slab with joints at 18 feet or in a panel with a 1.5 length-to-width ratio. In hot, dry weather or when poor construction practices are used, even 12-foot joint spacings for a 6-inch-thick slab may be excessive.

If panels do crack, the design may be criticized as unconservative in a claims action. The engineer had such an experience involving slab cracking. The contractor claimed the engineer was responsible for slab cracking because his design joint spacing "pushed the limits" of accepted practice. Our caller advised that having used excessive joint spacings to reduce joint sawing costs is a weak defense when the slab cracks. He said to be especially careful when designing thick pavements. The Federal Aviation Administration (FAA) recommends 25-foot maximum joint spacing regardless of slab thickness.