Some of the many variables that affect the cracking of concrete are: materials and design; placing and finishing techniques; temperature and humidity; curing practices; ground conditions; and structural loads. There is no standard condition for evaluating cracking tendencies. Many engineers equate the cracking potential of concrete with its drying shrinkage characteristics. The drying shrinkage is usually measured according to ASTM C 157, "Standard Test Method for Length Change of Hardened Cement Mortar and Concrete."
One problem with the ASTM method is a lack of agreement between laboratory and field testing. Unfortunately, the shrinkage observed in ASTM test is much higher than the shrinkage results obtained from a structure containing the same concrete. The author believes that this difference is due in part to the size of the test specimen--4x4x11 inches. Based on considerable field experience, the author recommends the 4x4-foot slab 8 inches thick as a new standard test specimen. This size of specimen will give shrinkage test values similar to structure shrinkage measurements.