Most people, and engineers especially, are inclined to think that whenever a pavement or any other structure fails it must be because it is too weak and that the way to correct the situation is to make the next design stronger. However, making concrete pavements thicker and stronger and reinforcing with steel has not solved all the problems. Anyone designing a concrete slab to rest directly on the ground must allow for the fact that there is a great deal of difference between the behavior of a relatively rigid monolithic slab and the underlying soil when both are subject to loads. When a concrete pavement slab is subjected to a heavy wheel load it may bend or flex a small amount, but if this bending goes very far the slab will break. On the other hand, the natural earth, if not too wet, may sustain the same load although it will compress or deflect farther in the process. Therefore, it is easy to see that when a concrete pavement is subjected to load the slab may break before the underlying soil support has been compressed or "mobilized" sufficiently to carry the load. It is, therefore, essential that there be some transition zone between the pavement and the native soil. This zone may consist of one or more layers which are commonly called the base and subbase, respectively, and which should be more rigid and incompressible than the underlying soils but somewhat less so than the concrete slab above. Another characteristic of concrete pavement slabs which is not helpful or conducive to good performance is the susceptibility to volume change. This can be minimized by following these suggestions: (1) use an aggregate gradation which will make workable concrete with the least amount of water; (2) make sure that both the sand and the coarse aggregates are clean and that there is no clay present; (3) use portland cement that is low in tricalcium aluminate and low in alkalies; (4) the portland cement should have the optimum amount of gypsum which varies for different cements; (5) make sure that the concrete is thoroughly compacted; and (6) protect the concrete while curing.
