Why does concrete crack?
There are many reasons that concrete cracks. Concrete can crack from a super-imposed load that is greater than the tensile strength or the compressive strength of the concrete. Concrete can crack from rapid thermal changes. But the most common type of cracking, shrinkage cracking, is due to volume changes within the concrete. As concrete changes volume, usually shrinking, the tensile strength of the concrete is exceeded by the strain of the volume change, and the concrete cracks. Shrinkage cracks can occur long after the concrete has hardened, shortly after the concrete has hardened, or even before the concrete has hardened. Drying shrinkage cracking is caused by the loss in volume that is accompanied by the concrete losing water after it has sufficiently hardened and gained strength. Most concrete will shrink approximately 500 millionths, or .05%. This can be more easily expressed as 1/16th of an inch in 10 feet.
Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. When concrete is placed on supporting soil or around steel reinforcement, the concrete mass is prevented from shrinking. This restraint creates internal forces exceeding the strength of concrete. Cracks form to relieve these forces.
