Concrete has a natural tendency to crack because it's usually relatively weak in tension. When tension resulting from applied loads, restrained shrinkage, or temperature drops exceeds the strength of the concrete, a crack forms. Concrete can crack while plastic and after hardening. Cracking can be controlled during construction, however, by using quality materials, following proper construction practices, and using reinforcement and jointing.
Typical cracking of slabs on grade during construction includes surface crazing, plastic shrinkage or settlement cracking, uncontrolled effects of early volume changes, subgrade settlement, and premature loading.
Concrete beams, walls, columns, and structural slabs also gain strength and change in volume over time. Some cracking usually occurs when expected loads are applied. Cracking of these members during construction can be caused by many of the same factors that influence cracking of slabs on grade. Additional sources of cracking are due to the nature of formed or shored construction and the presence of heavier amounts of reinforcement. Extensive or severe cracking during construction may indicate a deficiency best evaluated by a specialist.