Q.: What tolerances have been established for the thickness of parking lots? We have a 5-inch pavement that averages about 0.35 inch less than specified thickness (measured by ASTM C 174). In one place the deficiency in thickness is more than 1.5 inch.

A.: We don't know of any standard tolerances for parking lots. Various state highway departments have thickness tolerances for highway pavement slabs. Some highway specifications indicate a range of deficiencies in thickness that are considered acceptable (within tolerance limits); in another higher range deficiencies affect the amount of payment; in a third range, higher deficiencies may be sufficient to cause rejection of the pavement. Most states and Canadian provinces have established their own standards. Some of the more stringent specifications allow only 0.01 foot deficiency in thickness for an acceptable pavement, but 0.2 inch is more typical and some values are as high as 0.5 inch. Some states impose a penalty if deficiencies are above these values. The amount of penalty is sometimes written into the job specifications. There are other higher values (0.5 inch in some states but up to 1 inch in others), above which no payment will be made. In such cases the pavement may have to be torn out and replaced. These and many other details on pavement specifications may be found in "Charted Summary of Concrete Highway Pavement Practices in the United States," IS201P, available for $1.05 from Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.