Q.: Is there a published thickness tolerance for residential driveways? I'm a concrete consultant, and one of my clients is a homeowner who had a concrete driveway built. The contract specified a 5-inch thickness. After the concrete had cured, the owner cored it and found that the thickness was only 3 1/2 inches. When the contractor was told this, his only response was that the driveway would perform well even though it wasn't 5 inches thick.

A.: Surprisingly, we couldn't find a thickness tolerance for residential driveways or even for pavements in Standard Specifications for Tolerances for Concrete Construction and Materials (ACI 117-90). The thickness tolerance for concrete slabs in section 4.4.1 of that document is +3/8 and -1/4 inch, but it applies only to cast-in-place concrete buildings. Section 12, "Pavements and Sidewalks," gives only tolerances for lateral alignment of dowels and for level alignment.

We also found no reference to thickness tolerances in Guide for Design and Construction of Concrete Parking Lots (ACI 330R-92). But section 5.4 cautions that proper elevation control is critical because insufficient thickness due to poor grade control can be a serious deficiency.

State and local highway agencies generally publish thickness tolerances in their standard specifications. It's highly unlikely that any of these specs would permit a 1 1/2-inch tolerance in a 5-inch-thick pavement (see Problem Clinic, Concrete Construction, January 1997, pp. 66-67).