Q.: What is a common variance in thickness of floors on grade? Or what is an acceptable tolerance in such floor thickness? We had cores taken from a new slab on grade and found that thicknesses varied from 2 3/4 inches to 5 inches for a floor that was supposed to be 4 inches thick. The contractor claimed to have prepared the subgrade carefully, using a vibrating compactor to level the sand.

A.: ACI 117-81, "Standard Tolerances for Concrete Construction and Materials," in Section 2.1.5, gives thickness tolerances of +3/8 to -1/4 inch for slabs of 12 inches thickness or less, and +1/2 to -3/8 inch for slabs more than 12 inches thick. We don't know how widely existing floors conform to these requirements or whether data have been published that would answer your first question.