Many designers of concrete structural elements know that concrete shrinks when it dries, but they often fail to consider this in their designs. Article describes the possible consequences of drying shrinkage, including failures at filled joints, slab curling, and excessive cracking.
Curling occurs because the top of the slab shrinks due to drying while the bottom of the slab does not dry. This causes the top of the slab to be shorter than the bottom, which makes the slab curl upward. Curling stresses can cause cracking due to loss of subgrade support under the curled portion.
Cracking can also be caused by restraint to drying shrinkage: When the slab tries to shorten, the subbase or subgrade opposes the movement. When dealing with drying shrinkage in slabs on grade, contraction-joint spacing should be a major consideration. Regardless of slab depth, space joints no farther apart than 15 feet. If the concrete is expected to shrink more than normal concrete, the joints should be spaced closer together.
