Shrinkage in concrete is primarily due to water loss from evaporation, hydration of cement, or carbonation, leading to a reduction in volume. The volumetric contraction results in volumetric strain in concrete. However, in practice, shrinkage is measured as linear strain, which is one-third of the volumetric strain.
When concrete is still in its plastic state, it can undergo a decrease in volume by about 1% of its absolute volume. This decrease is known as plastic shrinkage. It arises either from water evaporating off the concrete's surface or being absorbed by the concrete beneath. The American Concrete Institute specifies that evaporation rates exceeding 0.25 kg/h/m² should be avoided to prevent plastic cracking. Furthermore, concrete's cement content is directly proportional to the degree of plastic shrinkage experienced; mixes with greater aggregate content exhibit less plastic shrinkage.
Even without moisture transfer from the environment, concrete can experience autogenous shrinkage, particularly in high-performance variants, although it is minimal in ordinary concrete. This is due to the internal compressive stresses exerted on hydration products due to the internal loss of water as the cement hydrates, and not due to external drying.
