Concrete quality control is not just during the manufacturing of the concrete. It becomes very important after proper placement. Proper curing of laid concrete is very important. If not, then water that is for concrete bonding will evaporate and henceforth concrete strength drops down from the specified value.
Shrinkage is another aspect that occurs if quality control is not assured. Water evaporates and left empty spaces behind. Empty spaces create strain on the surface. Concrete try to restrain these strains but not do so. Therefore, cracks start propagating on the surface of concrete and hence the volume of concrete also decreases.