Q.: We recently installed a concrete walk that was damp cured. We have noticed thin but deep cracks in a few spots. The contractor called them "heat cracks." Though they are not wider than one's fingernail I am concerned that they will worsen as the freeze-thaw season approaches. Is there available an epoxy or something that I can use to seal these cracks? There is no cure-and-seal membrane to interfere with anything we apply.

A.:We gather that these are short cracks that begin and end somewhere in the body of the concrete--not cracks that begin at the edge and run for some distance or all the way across. If so, they are perhaps what are usually known as plastic shrinkage cracks.

Plastic shrinkage cracks occur when the rate of evaporation is high during construction. Conditions that contribute to rapid evaporation are high temperature, low humidity, wind, a slow rate of bleeding or some combination of these. On hot, dry windy days, or at other times when plastic cracking might be threatening, a variety of measures can be taken to minimize rapid evaporation. These include using sunshades and windbreaks, dampening the subgrade and formwork, and initiating the curing process promptly.

Your problem, however, is what to do about the cracks. We have recently looked at some plastic shrinkage cracks in a street in Wauwatosa, Wisconsin (a Milwaukee suburb) that developed when the street was built about 36 years ago. These cracks were considerably wider than yours and there was public concern at the time over what was perceived as a deficiency in the new concrete. However, the city was persuaded that the cracks would do no harm as far as service was concerned, so they made no repairs at the time and the pavement is still in otherwise good condition even though the street has been routinely salted in winter. The main difference is that the cracks have become filled with dirt and so they're harder to find. The appearance has also changed slightly because of some surface wear. This street has been discussed in the article "Common Field Problems," October 1985, page 855.

It is possible to seal the cracks by some such means as epoxy injection. But the usual result is to accentuate their presence. If the cracks are in fact plastic shrinkage cracks, the chances are that they won't get any worse and that the best thing you can do for their appearance is to leave them as they are. If the concrete is properly air entrained it should survive cyclic freezing and thawing.

If, however, the cracks are long and caused by drying shrinkage, perhaps they should be repaired or the sidewalk replaced. From your description, it doesn't seem likely that they are of this type.