Cracks rarely affect the structural action or the durability of concrete significantly, but they look bad and they let water in more easily and thus may accelerate weathering or rusting in some cases. There are many kinds of cracks. With regard to depth, there are surface, shallow, deep, and through cracks. With regard to direction at the surface, there are two main kinds: map cracks or pattern cracks are rather uniformly distributed short cracks running in all directions, roughly in hexagonal patterns; they indicate restraint of the surface layer by the inner concrete or backing. The other main kind: single continuous cracks run in definite directions, often in parallel at rather definite intervals; they indicate restraint in the direction perpendicular to them.
It is evident that no one property can be singled out as the sole or even the principal cause of cracking. There has been a tendency, which seems unduly emphasized, to take drying shrinkage of hardened concrete as a criterion of susceptibility to cracking; but actually a concrete with high shrinkage tendency may have low cracking tendency because of other characteristics that are favorable. It would be better to evaluate crack resistance directly, both by observation of existing structures and by direct test.
What causes cracking? There are many reasons. Some of the factors, discussed in this article, are: amount of water per bag of cement or per cubic yard of concrete; the amount of cement; shrinkage-compensating cement; mineral composition, shape, surface texture and grading of aggregate; admixtures; bleeding; placing; curing; temperature; exposure; and restraint.