Here's a little article I found on why concrete cracks.....
The cracking phenomenon is complex and depends upon a number of things; rate and amount of drying, drying shrinkage, tensile strength, tensile strain, creep, elasticity, degree of restaint, and other factors.
While most types of cracking do not affect structural stability or durability, all cracks are unsightly and in extreme cases cracking can reduce the use and servicability of the structure. For this reason, cracking should be kept to a minimum.
In the laboratory, drying shrinkage tests are the most easily and most frequently performed tests in relation to shrinkage/cracking problems. However, there is sometimes too much emphasis on the drying shrinkage of hardened concrete as the criterion of susceptibility to cracking.
Drying shrinkage alone is influenced by many factors - water/cement ratio; amount, fineness and composition of cement; mineral composition, stiffness, shape, surface texture, and grading of the aggregate; characteristics and amount of any admixture; size and shape of the concrete mass; and conditions of humidity and temperature.
Preventative measures at the time of placing and curing remain the best means of minimizing cracking.
Some types and causes
In most cases where cracks appear in concrete the crack can be identified and the cause of cracking established. An extensive survey carried out revealed that concrete cracking can be attributed to the following:
•Construction and supervision problems - approx. 36%.
•Design defects - approx. 27%.
•Ambient conditions (temperature, humidity, etc.) - approx. 21%.
•Quality of materials - approx 17%.
In addition, all cracks can be grouped into two broad categories:
•Cracks occurring before and during hardening.
•Cracks occurring after hardening of concrete.
