Q.: We contracted to remove and replace a cracked and broken topping in a 6 x 40-foot area of the second floor of a two-story concrete garage. The structural steel of this building, built about 50 years ago, is buried in the concrete. Although the owner thought that the old topping was a few inches thick, it turned out to vary from only 1/2 to 1 1/2 inches thick, and part of this variation occurred because the surface itself was far from flat. Because the topping was so thin I had to give up the idea of using mesh or even coarse aggregate, and we ended up using a 4000-psi air-entrained mix of cement, sand and water. We kept the base concrete damp but not moist, kept the slump low, placed concrete about 3 P.M., gave it a steel-troweled finish and cured it with a membrane curing compound that we use on state work.

A week later the owner noticed several cracks. By age 6 weeks random cracking is extensive, cracks are wider, and pieces of mortar can be broken out without much effort. The only thing I know that we may have done wrong was to allow cars to be parked on the repaired area on the third day after we placed the concrete. What should we have done or what should we do?

A.: We suspect that the trouble lies in the relatively high drying shrinkage that most 4000-psi mortars would be expected to have. Normally you could solve that problem by simply applying a topping of concrete, instead of mortar, so that there would be coarse aggregate present to reduce the drying shrinkage. But that would mean either (a) using a thicker topping, which would add to the elevation and to the dead load, or (b) cutting into the structural steel below the topping. Probably neither of these would be allowed.

The following suggestions for repair are based on the assumption that you would completely remove the existing topping and then roughen and clean the surface adequately to accept and bond to a new topping. No matter what topping you use, it might be wise to ensure a good bond by scrubbing fresh 1:1 cement-sand grout into the substrate just ahead of where you're placing the topping.

You might consider using a mix that incorporates polypropylene fibers to minimize crack propagation. Such fibers are made by two companies that we know of: Fibermesh Company, 4019 Industry Drive, Chattanooga, Tennessee 37416; and Forta Corporation, 100 Forta Drive, Grove City, Pennsylvania 16127. We also know of two companies that make packaged repair materials that contain fibers in the mix: Gemite Unique Products Inc., 328 Carlingview Drive, Rexdale, Ontario M9W 5G5, Canada; and MAC-USA, Suite 441, 1900 Glades Road, Boca Raton, Florida 33431.

Perhaps you should also investigate acrylic concretes. These are made by some of the companies listed under "Concrete Polymer Materials" in the Buyers' Guide (yellow pages) of our December 1984 issue and are discussed in the article "Acrylic Concrete for Fast Repair," page 881 of our October 1984 issue. Rohm and Haas Company, 727 Norristown Road, Spring House, Pennsylvania 19477, manufacturer of a high molecular weight methacrylate (HMWM) used by various formulators of repair materials, can provide information about HMWM products.