Does Concrete Shrink During Drying?

Shrinkage Basics

• Shrinkage of concrete occurs after hardening when the water not consumed in the cement hydration process leaves the concrete system. This excess water, also known as "water of convenience," isn't needed to hydrate cement and is instead added to the mix for the sake of workability. In general, the greater the proportion of this type of water, the higher the potential for shrinkage. Shortening or curling of concrete slabs can also be caused by temperature decreases when the concrete contracts during the cooling period that follows the heat of hydration.

Amount and Rate of Shrinkage

• The American Concrete Institute suggests that total long-term shrinkage can vary between 1/2 and 1 1/4 inches for every 100 feet. Alan Face, concrete flooring expert and founder of Alan Face & Associates, says that for every 20 feet of concrete, 1/8 to 1/4 inch of shrinkage can be expected. Because of the range of variables involved, it can take between a few weeks and several months for most of the shrinkage to occur.

Factors That Influence Shrinkage

• Several factors influence the amount of shrinkage that can occur. These include variables related to the concrete mixture, such as paste content, water content, cement composition and the cleanliness of the aggregates. The concrete's surface-to-volume ratio, any shrinkage restraints, the temperature, and the changes in temperature and ambient weather conditions also impact shrinkage.

About Shrinkage Cracking

• Shrinkage can lead to problematic cracking in the concrete. Cracks occur when the net tensile stress equals the concrete's tensile strength. Reinforcement within the poured concrete acts as a restraint to shrinkage and causes tensile stresses to develop. Often, shrinkage cracking is inevitable and control joints are strategically saw-cut into the slab in a straight line. Cracks may also be controlled by preventing or delaying concrete drying.