Q: We have a job calling for shrinkage-compensating concrete. Is this a special mix, or concrete with more reinforcement than is normally used?
A:Shrinkage-compensating concrete is made with either expansive cement, or with normal cement and an expansive component that's added separately. It's proportioned so the concrete will increase in volume after setting and during early-age hardening. The expansion must be restrained, usually by internal reinforcement, which produces a compressive stress in the concrete. When the concrete later dries, the resulting shrinkage reduces the compressive stress instead of producing a tensile stress that could cause cracking. When it's used in floor slabs, shrinkage-compensating concrete permits greater joint spacings.
We suggest you read ACI 223-98, "Standard Practice for the Use of Shrinkage-Compensating Concrete," before starting the job. Chapter 5 of this document gives information on placing, finishing, and curing of shrinkage-compensating concrete. You can order this document from the American Concrete Institute online at www.aci-int.org.