They expand when they absorb the moisture and shrink when they dry.
Shrinkage cracks in concrete slab.
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.
When that water eventually leaves the slab it leaves behind large voids between the solid particles.
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.
Shrinkage cracks in concrete occur due to change in moisture of concrete.
When concrete is mixed more water than is needed for hydration is mixed with the dry components such as sand cement and an aggregate.
These cracks are comparatively short cracks.
Plastic shrinkage cracks are common in slabs.
Plastic shrinkage concrete cracks when concrete is still in its plastic state before hardening it is full of water.
As the concrete slab dries after placement it shrinks.
This is the main cause of concrete shrinkage cracks on drying.
Concrete and mortar are porous in their structure in the form of inter molecular space.
This restraint creates tensile stresses in the concrete slab which in turn may produce drying shrinkage cracking.
Most of the water will eventually evaporate causing shrinkage of the concrete slab.
Similar uncontrolled or random cracks sometimes due to poor joint layout meander across the surface.
Most of this excess water begins to exit the concrete during placement curing and causes the concrete to shrink.
In addition to reading about repairing concrete shrinkage cracks if crack repair is needed at all.
Concrete slab drying shrinkage cracking can be the unfortunate end.
See details at concrete shrinkage cracks.
Contraction joints or control joints when cracked are really just straight shrinkage cracks.
Shrinkage cracking can be managed by the use of control joints placed in the slab.