Q: Questions such as the one in the March 1997 Problem Clinic (p. 313), where a reader asks if there is a published thickness tolerance for residential driveways, bring out the importance of placing important construction parameters in easily accessible formats. There are recommended tolerances for thickness in Guide for Design and Construction of Concrete Parking Lots (ACI 330R-92). But evidently one has to know where to look, and that doesn't serve the occasional user very well.

A.: The last paragraph in section 4.2, "Subgrade Preparation," says: "Suggested tolerances for fine grading are no more than 1_4 inch above or 1_2 inch below the design grade. Deviations greater than these tolerances can jeopardize pavement performance because small variations in thickness of thin pavements significantly affect load-carrying capacity." As the initial chairman of ACI Committee 330, I'm embarrassed that we did such a good job of hiding that important recommendation.

ACI Committee 330 is currently revising ACI 330R-92, and I shall recommend that a section on recommended tolerances be added to collect and tabulate the recommendations scattered throughout the report.

Richard O. Albright, Arsee Engineers Inc. Noblesville, Ind.