Perhaps the most common complaint concrete suppliers hear from customers is that the concrete cracked. There are many causes for concrete cracking and many ways to minimize or eliminate cracking. A basic knowledge of concrete shrinkage is fundamental to understanding and controlling concrete cracking.
Plastic shrinkage occurs in fresh concrete while it is being placed or finished. Although plastic shrinkage is responsible for some cracking, far more cracks are related to drying shrinkage. Drying shrinkage is the volume loss in hardened concrete due to either moisture gain or loss. Shrinkage becomes a cracking problem only when concrete is restrained. The extent of cracking depends on the amount of drying shrinkage of the concrete, how much restraint against movement exists, and how strong and elastic the concrete is.
Controlling or preventing shrinkage is a matter of controlling some or all of the factors discussed above. Restraint is not usually under the control of the concrete producer, but reducing the amount of drying shrinkage is. The most influential factor on drying shrinkage is the total amount of water in the concrete mix. In general, concretes that have higher strengths are more likely to withstand shrinkage. Proper curing facilitates higher initial strengths and moisture retention, both of which prevent cracking.