Concrete shrinks as it dries because as it dries the water that was used to create the concrete is absorbed faster than the concrete can cure. When you mix concrete you add water to a cement mixture. That cement mixture and water when cured creates concrete. Concrete is very porous so as it cures it absorbes water (the water used when creating it), causing it to lose volume and shrink.
Drying Concrete Shrinkage Prevention
As mentioned above, concrete shrinkage is a very common problem many contractors and homeowners have. One way to prevent shrinkage is to add a concentrated silicate sealer into the concrete mixture. By adding a concentrated silicate sealer to the mixture it will increase curing time and strengthen the concrete. The concentrated silicate sealer will also stop water from penetrating the concrete and stop the concrete from absorbing water.