Shrinkage

As concrete cures, shrinkage is inevitable. This shrinkage results in a volume change or deformation of the concrete. The paste component of a concrete mix is often the target of shrinkage as it is soft, porous and absorbent. The paste component shrinks as much as 1% of its initial volume on its first day after casting.

Several factors contribute to why cement paste shrinks and the two most common types of shrinkage are known as drying shrinkage and plastic shrinkage. Plastic shrinkage occurs while concrete is in the plastic state and results from surface evaporation due to environmental conditions. Drying shrinkage forms from a loss of water that occurs when the rate of evaporation of the concrete exceeds the rate of bleeding and happens after concrete has reached its initial set. While drying shrinkage is caused by environmental factors as well, concrete material factors and size of application area (surface area to volume ration) also play a role. Typically, this type of shrinkage is more of a problem during warm weather months and causes "drying shrinkage cracks." If the concrete is reinforced, the cracks may produce a direct path for chloride ions to reach reinforcing steel which can ultimately cause further cracking and other problems such as spalling and delamination of concrete.

Whichever reaction takes place, there are measures that can be taken to help reduce the amount of shrinkage that occurs. One measure is to use a larger size of aggregate when possible, since it would require less paste component. Using wind breaks, sunshades and evaporation reducers followed by a membrane forming curing compound will also control the rate of drying of the concrete. Curing allows the concrete to retain moisture so that it continues to gain strength and delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking.

US SPEC has a line of concrete curing and sealing products available that reduce the drying rate of concrete. The US SPEC team can help answer any questions on shrinkage, and assist in recommending ways to prevent it.