In the reaction of cement with water, a part of the water becomes chemically bound in the hydration products. This reaction results in a chemical shrinkage, caused by the fact that both chemically and physically bound water have smaller specific volumes than free water. If no extra water is available for cement hydration, all pore water is progressively consumed. This process empties the pores in the concrete and produces a drop of the internal relative humidity (self-desiccation). Emptying of the pores and self-desiccation leads to capillary stresses in the pore water, which induce an external volume contraction, called autogenous shrinkage. Contrary to drying shrinkage, autogenous shrinkage occurs without any loss of moisture from the concrete. Autogenous shrinkage becomes important with w/c<0.45 and increases with the decrease of w/c.
Internal curing is an efficient method for reducing autogenous shrinkage (Figure 3). Saturated lightweight aggregate or superabsorbent polymers can be added to concrete to keep the capillary pores filled with water. This reduces the capillary underpressure in the pores and the autogenous shrinkage of the concrete.