Q: We have a job that calls for shrinkage-compensating concrete. What is this material and why is it used?

A: Shrinkage-compensating concrete is made with expansive cement or an expansive admixture that causes the concrete to expand during early-age hardening, and thus serves to offset normal drying shrinkage. It is typically used in concrete structures, slabs, pavements, and repairs as a way to minimize cracking due to drying shrinkage. Reinforcing steel within the structure restrains the concrete and goes into tension as the concrete expands, producing a compressive stress in the concrete. As the concrete dries, the shrinkage due to the moisture loss relieves this compressive stress, rather than producing a tensile stress that would cause cracks to form. The material is proportioned, placed, and cured much like normal concrete. You can learn more about it in ACI 223R-10, “Guide for the Use of Shrinkage-Compensating Concrete.”