Excluding the cracks due to gravity or lateral forces, concrete will initially crack due to shrinkage. Once cured, the shrinkage cracks can open up further due to expansion/contraction due to temperature.
Just to add a thought to your answer, whyun, a concrete slab on grade will crack from shrinkage, but only if it is restrained by the subgrade. As it shrinks, the subgrade develops friction forces across the bottom of the slab that resists this movement, thus, tensile forces are developed in the concrete and once those forces exceed the rupture of the concrete.....voila! a crack.
If the slab was on a pure, frictionless surface, i.e. grease or teflon -no cracks.
very true JAE. engineers provide a control joints at desired intervals to "force" cracks to occur. Straight cracks look better than random cracks.
Earthmover...concrete is at its maximum volume just after placement. From that point onward, it continues to shrink. Thermal expansion can rarely overcome that shrinkage. This usually only occurs in long, thin strips (sidewalks, pavements, etc.) that have other actions working as well, such as impacted joints (debris, etc. fills the joint space), moisture expansion, etc.
As JAE noted, shrinkage cracks only develop from restraint. Unfortunately, almost all concrete placements have some restraint, either from friction at the subgrade interface or from geometry. In general, expansion does not cause cracking. If expansion is extensive, heaving of thin sections can occur, or in drastic cases, shearing will occur at joints in thick sections.
See FAQ in Concrete Engineering Forum for additional info.
1. Shrinkage crack occurs due to the differential shrinkage between the free and the constrained portions of concrete. Shrinkage occurs when the free water available in excess of the water consumed in hydration of cement evaporates. This shrinkage is prevented by the friction on the constrained surfaces.
2. Three is also thermal cracking occuring in thick concrete masses due to temperature gradient caused by dissipation of heat of hydration. In thick sections, the exposed surfaces cool faster and they act as insulating layers thus preventing the heat of hydration generated in the interior portions from dissipation. This causes a temperature gradient across the section, causing the cracking of concrete. This is minimized by controlling the temperature during placement by chilling of the course aggregates before placement or adding ice cubes to the aggregate so that the steep temperature gradient is avoided.
The above cracks occur during the curing period and before the concrete gets loaded.
