When hardened concrete is exposed to air with a relative humidity of less than 100 percent, it begins to lose the free water within its capillaries. As this water evaporates, the water initially adsorbed onto the calcium silicate hydrates migrates towards these now empty spaces and eventually evaporates as well. Over time, as more water leaves, the volume of the concrete decreases, a phenomenon known as drying shrinkage.
A portion of this drying shrinkage can be reversed; if the concrete is later exposed to moisture, it can reabsorb water and expand slightly. However, not all shrinkage is reversible. Permanent changes occur as new physical and chemical bonds form between the hydrates in the cement paste when they lose water. The extent and reversibility of drying shrinkage are influenced by factors such as the duration of wetting and drying periods, the relative humidity of the air, and the composition of the concrete, along with the degree of hydration at the onset of drying shrinkage.
The behavior of concrete under drying shrinkage also varies depending on whether it can move freely. In unrestrained concrete, the material experiences deformation over time without developing stress within it. However, if movement is restricted, no deformation is observed. Instead, tensile stresses build up as the concrete tries to shrink but is held in place.
