High Performance Concrete: Low Shrinkage Concrete

Concrete loses moisture as it hardens, causing it to contract or shrink. This occurrence, called Drying Shrinkage, creates internal stresses within the concrete causing it to crack.

Low shrinkage or shrinkage-compensating concrete is high performance concrete that makes use of a controlled expansive chemical reaction to compensate for drying shrinkage and minimize cracks due to internal stresses. Expansive cement causes the concrete to increase in volume after setting and during hardening thus inducing tension in the reinforcement and compression in the concrete. The normal drying shrinkage that follows will counteract the induced strains of the initial expansion, leaving the concrete with little or no internal stresses. This process is shown above (taken from ACI223).

An understanding of the role played by ettringite in the chemical reactions of concrete is necessary to understand how shrinkage-compensating concrete works. Ettringite is a mineral crystal that forms quickly when portland cement and water are mixed. The crystal structure of ettringite contributes very little to the strength, but takes up a considerably large amount of volume in concrete. Expansive cements make use of these properties of ettringite to cause the primary expansion after the concrete has set.

The composition of the expansive cement is only one of the many factors considered when using shrinkage-compensating concrete. The composition of the aggregate and water may influence amount and timing of expansion. Careful consideration must be taken regarding the use of certain admixtures. Placing, Finishing, and Curing procedures will also need to be thought out ahead of time when using expansive cements.