Q.: I have a project erecting a metal building on a good-quality 10-year-old 4-inch slab. I foresee no problem with the wear resistance of the slab, but my client wants to use the facility for servicing his tractors and semitrailers, sometimes fully loaded. Do you see any problems or remedies?

The 70x90-foot slab does have a very few cracks 1/8 to 1/4 inch wide, not in straight lines.

A.: This is a design problem which we are not equipped to answer ourselves. The ACI 302.1R-80 "Guide for Concrete Floor and Slab Construction" contains a chart (Figure for designing slab thickness on the basis of anticipated axle loads, tire contact area, wheel spacing, subgrade support and flexural strength of the concrete. Safety factors are recommended that depend on the kind of service the floor will have to provide. Calculations should be made by a design engineer.

Is seems clear that the slab will have to be thicker. The engineer can advise whether or not a completely new slab will be needed. Possibly the present slab can be strengthened by adding a sufficiently thick bonded topping. If a topping will be adequate, care should be taken to make sure the topping is securely bonded. This involves thorough cleaning of the base slab, then scarifying or acid etching it, followed by scrubbing a portland cement grout into the surface and placing the topping immediately behind the grouting operation. An epoxy bonding agent should not be substituted for the grout because it would produce an intermediate layer of low permeability that could lead to subsequent delamination after migrating ground moisture builds up under the epoxy. Further details on preparing the slab and applying the topping are given in "Resurfacing Concrete Floors," Concrete Information IS144.03T, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.

You should probably also get the engineer to advise whether repairing the cracks in the base slab would assure that the slab and its topping would function monolithically and not crack again at the same locations. Cracks 1/8 to 1/4 inch wide are likely to contain dirt that would keep any repair material from bonding. It might be necessary to saw the cracks wider, clean the laitance off the newly sawed surfaces, and fill with concrete. The concrete should be bonded to the adjacent walls with a freshly scrubbed-in grout like the one used to bond the floor topping.

It might be that the simplest answer to your whole problem would be to regard the existing slab simply as a subbase and cast a new slab of adequate thickness over it, without bonding.