By its nature, concrete tends to shrink and ultimately crack as it dries. But a new liquid admixture, introduced to the North American market in March 1996, is the first of its kind available to ready mix producers and contractors for chemically altering the basic mechanism of shrinkage without adding expansive materials to the concrete. The developers of the product claim that it can reduce 28-day concrete shrinkage by 50% to 80%, with reductions in ultimate shrinkage of 25% to 50%.
HOW THE ADMIXTURE REDUCES SHRINKAGE
Concrete drying shrinkage is a complex process that has several causes. One cause is surface tension that develops in small pores within the cement-paste portion of hardened concrete. The amount of cement-paste shrinkage caused by surface tension depends primarily on water-cement ratio. It can also be affected by cement type and fineness and by any other ingredients that alter the pore-size distribution in the hardened paste. The shrinkage-reducing admixture (SRA) reduces shrinkage of this kind by reducing the surface tension of water in pores between 2.5 and 50 nanometers in diameter.
OTHER EFFECTS ON HARDENED-CONCRETE PROPERTIES
Although the primary impact of the SRA is reduced drying shrinkage, the admixture can also affect hardened concrete in other ways. It's still too early to say for sure whether the reduction in drying shrinkage reduces shrinkage cracking in large-scale concrete structures, although preliminary tests are positive. Adding the SRA to concrete can reduce compressive strength as much as 15% at 28 days. Although the SRA does not alter the thermal coefficient of concrete, it does lower the rate at which hydration generates heat. This reduces peak temperatures within the concrete, so there's less thermal contraction upon cooling.