Why Concrete Shrinks During Curing & Drying

Concrete Shrinkage

• According to TK Products' concrete information page, "[d]rying shrinkage is an inherent, unavoidable property of concrete." Shrinkage occurs as the concrete dries and cures from its plastic form to its firm, dry state. The degree to which concrete will shrink depends on the type of concrete used and the relative humidity of the environment in which the concrete was poured. At 50 percent relative humidity and for standard concrete, for instance, shrinkage tends to occur at a rate of .72 inches per 100 feet.

Causes

• The cause of concrete shrinkage is quite simply the drying and curing process itself. Moisture that is integrated into the concrete mixture in its wet form needs to leave the concrete in order for the concrete to dry completely. More specifically, the rate at which water inside the concrete mixture is evaporating into air often exceeds the rate at which the concrete produces "bleed water," the small puddles of water that appear on the surface of the concrete as it dries and cures. This difference between evaportation and bleed rates is the leading cause of concrete shrinkage.

Problems

• The most common consequence of concrete shrinkage is cracking. Cracks are formed in either the inside or on the surface of the concrete structure. Two specific types of cracking are caused by concrete shrinkage: shrinkage cracks and plastic shrinkage cracks, the latter appearing only on the surface of the finished concrete. Cracking can result in the concrete's taking on an unattractive look at best and a reduction in overall structural integrity at worst.

Preventing Shrinkage

• Several specialized concrete mixtures are available specifically to help prevent shrinkage and cracking problems. When shopping for concrete, look for "shrinkage-reducing admixtures" and types of concrete similarly labeled for shrinkage control. Shielding poured concrete from the elements using sun and wind barriers, pouring in the early morning or late afternoon, and spraying an evaporation barrier over the top of the concrete before it dries also can help prevent shrinkage and cracking problems.