Shrinkage cracking can be managed by the use of control joints placed in the slab.
Shrinkage cracks in concrete slab.
They expand when they absorb the moisture and shrink when they dry.
As the concrete slab dries after placement it shrinks.
See details at concrete shrinkage cracks.
Plastic shrinkage concrete cracks when concrete is still in its plastic state before hardening it is full of water.
Concrete and mortar are porous in their structure in the form of inter molecular space.
Shrinkage cracks in a slab photo above are unlikely to be of any structural concern but can be a source of water entry or radon entry in buildings and may form a tripping hazard.
Plastic shrinkage cracks are common in slabs.
Shrinkage cracks in concrete occur due to change in moisture of concrete.
While the concrete is shrinking many things such as reinforcement or the subgrade restrain the concrete.
While shrinkage in poured concrete walls or floor slabs is a normal property of curing concrete shrinkage cracks can be controlled or where they have occurred in some cases repairs are needed.
Similar uncontrolled or random cracks sometimes due to poor joint layout meander across the surface.
Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid which looks better than random cracks but functions the same way.
Most of the water will eventually evaporate causing shrinkage of the concrete slab.
These cracks are comparatively short cracks.
When that water eventually leaves the slab it leaves behind large voids between the solid particles.
This is the main cause of concrete shrinkage cracks on drying.
Contraction joints or control joints when cracked are really just straight shrinkage cracks.
When concrete is mixed more water than is needed for hydration is mixed with the dry components such as sand cement and an aggregate.
This restraint creates tensile stresses in the concrete slab which in turn may produce drying shrinkage cracking.
In addition to reading about repairing concrete shrinkage cracks if crack repair is needed at all.