To give the straight simple definition of shrinkage would be to say that it is a reduction of volume caused by loss of water. This loss occurs both physically and chemically. Loss of water due to obvious physical agencies can easily be visualized. Sedimentation (bleeding) is one such agency; absorption by forms or porous aggregate another. Loss of water by the chemical reaction of the hardening process is a little more complex. During hydration the water present in the mix becomes a component of the solid products of the process without any loss of weight occurring. However, the reaction is accompanied by a change in volume; the water loses about 235 percent of its volume in combining with the compounds present in the cement. In wet mixes, chemical shrinkage is generally far less important than physical shrinkage, whereas in dry mixes chemical shrinkage becomes increasingly important, even to the point where it may be necessary not only to prevent evaporation of water but also actually to replace the water used up chemically. In practice, shrinkage means that all designs must allow for it. If concrete is restrained sufficiently against movement, something must give when it shrinks. Since creep (see "Some Facts, Theories and Guesses Concerning Creep," Concrete Construction, February 1960, page 36) also enters into the problem, the hypothesis is not quite as easy as it sounds. We can isolate shrinkage and say that it must be allowed for, but just how much of a tendency a particular mix has to crack is more difficult to establish. Much research work still remains to be done; whether it will eventually result in a practical formula for shrinkage control is still unpredictable. There are a few things we can do to avoid the effects of excessive shrinkage. Good design with well placed joints is obviously essential. In the field, three factors having a major influence on shrinkage are with in the direct control of the contractor: choice of aggregate, control of water content, and proper curing. The aggregates used should be as rigid and dense as possible and the proportion of coarse material should be as high as possible. Avoidance of the use of excess water is always good practice; if lowering the water/cement ratio means a reduction in workability, this can always be offset by the use of air-entraining agents, since these have a negligible effect on ultimate shrinkage. The last factor (proper curing) is probable the most important. The curing techniques which control evaporation are generally common practice for any experienced contractor. You should be sure to dampen the subgrade and forms before placing the concrete. In cold weather, care should be taken not to overheat the concrete and in hot wether to lower the concrete temperature. Finally, hold the period between start of placing and start of curing to the minimum.