Q.: On a spring day we were placing slabs that cracked open before we could finish them. The ambient high temperature was 40 degrees F. We then asked the ready mixed concrete company to send concrete containing retarders and they tried two different retarders in different proportions. The concrete then began hardening on the surface even faster.

Now the only admixture we ask for is an air entraining agent, but we still get cracking. One day this spring, a slab that took us four hours to place from start to finish cracked all over. There was some sun and wind, and the temperature was 60 degrees F. The concrete was a 5-½-bag mix with a 5-½-inch slump placed on a rock fill. We had to use the 5-½-inch slump because of the high absorption of the lava rock and poor sand used for the fill.

We have less trouble with 6-inch floors than 4-inch, but 6-inch floors crack also. The cement is not excessively hot. Will too much water reducer cause trouble?

A.: The problem seems to be what is known as "plastic cracking" or "plastic shrinkage cracking." This can occur under any conditions where evaporation is rapid whether hot, dry weather or any kind of windy weather (hot or cold). What often happens is the surface dries rapidly and shrinks enough to cause many cracks, usually parallel to one another, before the concrete has been finished.

Because plastic shrinkage cracking is related to evaporation rather than rate of chemical hardening, the use of retarders to control it is ineffective. The air content should not affect the problem. The effect of water reducers may be minor.The solution is to do everything possible to protect against rapid evaporation. Some measures that can be taken are described in two articles from Concrete Construction: "How Cracking is Controlled." November 1973, page 517 and "Finishing Problems and Surface Defects in Flatwork," April 1979, page 247.

A revised version of the ACI 302 Standard published in Concrete International, June 1980, pages 51 to 96, includes a discussion of this subject.