After Concrete is poured it begins to set. This process of cement resting with water provides the concrete its strength but also causes an overall reduction in volume. This change in volume leaves pores throughout the concrete. As a result, concrete has similar properties to a sponge. Initially the concrete is wet, but after it sets the water in the pores dry out, and the concrete is left like a strong dried sponge. 

Concrete deteriorates from fluid absorption, freezing and thawing when water gets into the pores, and salt crystallization. Salt crystallizes in the pores and creates internal pressure.