Plastic shrinkage cracks
Plastic shrinkage cracks occur within the surface of fresh concrete while it is still plastic, and are characterized by their random pattern. They are usually discontinuous and rarely extend to a free edge. The cracks are caused by the rapid drying of the surface, particularly during warm and wind-drying conditions. The potential for cracking increases with workability—particularly when inadequate curing measures are taken.

Drying shrinkage cracks
Drying shrinkage cracks occur when concrete is subjected to a lower environmental relative humidity (RH) than its internal relative humidity. The mechanism is generally accepted to be due to the development of capillary tension as water is lost from the pores and menisci are formed. Shrinkage of the cement paste of a concrete will be larger the higher the w/c ratio because the latter determines the amount of water in the cement paste and the rate at which water can move toward the surface.

Carbonation shrinkage cracks
Carbonation shrinkage cracks can occur because of atmospheric carbon dioxide diffusing through the cement matrix pore structure to react, in the presence of moisture, with calcium hydroxide. Carbonic acid is formed during the initial reactions that subsequently result in the formation of calcium carbonate. Cracks form as the result of dissolution of the calcium hydroxide crystals while the crystals are under pressure, and deposition of calcium carbonate in places where the carbonate is not under pressure.

Surface crazing and map cracksinadequate curing practices are carried out and rapid drying conditions prevail. The result is that the top surface cracks in a random manner, sometimes referred to as alligator cracking or mud-cracking, with 
the cracks usually only extending a few millimetres below the surface.

Thermal stress cracks
Thermal stress cracks occur as a result of the evolution of heat during the cement hydration process. The potential will increase with increased thickness of section and cement content. Unlike drying shrinkage or carbonation shrinkage, contraction during cooling of mass concrete is greater within the interior.