Why Concrete Shrinks and What Designers Can Do to Control Cracking

When concrete is mixed, more water is added for workability than is required to hydrate the cement. This excess, known as free water, allows the concrete to mix and place more easily. Over time, this water evaporates, leaving microscopic voids within the concrete matrix. As a result, the concrete loses mass and shrinks.

This shrinkage can lead to cracking in framed floor slabs and slabs-on-grade, caused by friction between the concrete and the supporting material as the slab volume is reduced. Curling may also occur when the top (exposed) surface of the slab changes volume at a different rate than the bottom surface supported by the subgrade or structure.

Control Joints and Crack Management

Designers typically incorporate control joints into slab design to ensure cracking occurs in predetermined locations. A common example is the joint pattern seen in sidewalks.
For typical slabs-on-grade, control joints are usually spaced 20 to 30 feet on center. These joints may be tooled into fresh concrete or saw-cut after initial set. Saw-cutting should occur no later than the day after placement, as shrinkage begins as soon as the concrete starts to cure and dry.

Shrinkage, Curling, and Structural Impacts

Shrinkage is problematic not only due to crack formation, but also because of slab curling. When the top surface dries faster than the bottom, differential shrinkage causes the slab edges near control joints to lift upward.
This condition becomes critical when moving loads, such as forklifts, travel across joints, potentially displacing curled edges. Over time, this can lead to cracking and subgrade pumping, resulting in further deterioration.

Reducing Shrinkage Through Reinforcement and Mix Design

Several methods can be used to reduce the negative effects of concrete shrinkage. Polymer fibers are now commonly used in slabs-on-grade and, in some cases, permitted on metal decking as a replacement for traditional temperature steel.
Conventional temperature steel—typically welded wire fabric—is often positioned near the bottom of the slab, where it provides limited benefit. Fibers, by contrast, help bind fresh concrete, improve early-age strength, and distribute volume changes that inevitably occur as concrete cures.

Admixtures and Water Reduction Strategies

There is hope, however. Shrinkage-reducing admixtures and water-reducing admixtures can be incorporated into the concrete mix. Lower water content directly reduces shrinkage potential.
Additionally, increasing aggregate size reduces the amount of cement required to achieve strength, which in turn lowers the water demand for hydration—further limiting shrinkage.

Curing Methods and Long-Term Performance

Proper curing plays a critical role in controlling shrinkage. Applying a curing compound or using wet curing methods slows the drying process until the concrete gains sufficient strength to resist tensile shrinkage stresses.
Curing compounds must be selected carefully, as some can interfere with floor finish adhesion. An effective alternative is maintaining surface moisture using felt-backed plastic sheeting or plastic over burlap, keeping the slab continuously damp.

Concrete does not respond well to wet-dry cycling during curing, which can cause surface chalking and alligator cracking. Maintaining consistent moisture is essential.

Cost vs. Performance Considerations

While admixtures and wet curing increase initial costs, they can significantly reduce shrinkage, allowing for wider joint spacing and fewer cracks. This leads to savings in placement, maintenance, and repair costs, while also reducing visual defects in exposed concrete.
The long-held belief that “concrete will crack” is gradually being replaced by improved mix designs, placement practices, curing techniques, and the strategic use of modern admixtures.

By Robert G. Wilkin, P.E.