Krimp door drogen

When hardened concrete is exposed to unsaturated air, the free water present in its capillaries evaporates out of the pores.

This causes the movement of water adsorbed on the large surface area of calcium silicate hydrates present in the cement paste into the dried-out capillaries. This water eventually evaporates out of the concrete.

As a result, the volume of the cement paste reduces with time, causing contraction in concrete, known as drying shrinkage.

However, some drying shrinkage is reversible if the concrete is exposed to external moisture. The cement paste absorbs the moisture and undergoes wetting expansion. 

The irreversible portion of drying shrinkage results from the rearrangement of particles as gel pores collapse and the formation of  Si-O-Si bonds within the cement gel upon removing adsorbed water.

If the concrete member subjected to drying shrinkage is free to move, it will experience an increase in strain with time while the stress will remain zero.

Alternatively, if the member is restrained during drying shrinkage, the strain will be zero, and tensile stresses will develop in the member over time.