In addition, the physical structure of concrete evolves with time because of the chemical hydration reaction between water and cement grains, a process which continues for as long as water is present. The regions of uncombined water which remain at the end of the hydration period lead to the formation of an interconnected porous medium. The distribution of water within concrete is not static, however, because exposure of the material to ambient environmental conditions leads to the presence of capillary and diffusive drying mechanisms