Mineral Admixtures, Curing, and Concrete Shrinkage – An Update

Abstract

Work currently underway at the University of Kansas to evaluate free shrinkage of concrete as a function of the length of curing prior to drying, mineral admixtures as a replacement for portland cement, and aggregate type is presented. Silica fume, ground-granulated blast furnace slag (GGBFS), and fly ash at two levels of replacement are evaluated with a high-absorption coarse aggregate (2.5 to 3.0%) and a low-absorption coarse aggregate (less than 0.7%). The results show that when cast with a high-absorption coarse aggregate, the addition of either silica fume or GGBFS results in a reduction in shrinkage at all ages, while the addition of fly ash increases early-age shrinkage and does not have a significant effect on long-term shrinkage. For mixtures containing a low-absorption coarse aggregate, the addition of silica fume or GGBFS results in increased early-age shrinkage if the specimens are only cured for seven days. These same mixtures exhibit reduced shrinkage at all ages when the curing period is doubled from seven to fourteen days. In either case the addition of fly ash increases shrinkage at all ages. Based on these results, it appears that the high-absorption limestone provides internal curing water, which results in a reduction in the shrinkage of mixtures containing GGBFS or silica fume.