Performance of shrinkage-reducing admixtures at different humidities and at early ages

Shrinkage-reducing admixtures (SRAs) have been developed to reduce the risk of early-age shrinkage cracking in concrete. While substantial results are available in the literature to illustrate how SRAs influence the free shrinkage of mixtures tested in accordance with ASTM C157, fewer results have shown the influence of SRA on volume changes that occur from the time of casting until the standard shrinkage tests begin. In addition, few tests have been conducted in environments other than 50% relative humidity (RH). This paper discusses the initial shrinkage (that is, measured from the time of casting), long-term shrinkage, and residual stress development of a plain cement paste and a paste containing 5% SRA over a wide range of RH. The results indicate that pastes containing SRA demonstrate an expansion at early ages. In addition, the results provide evidence for the idea that smaller pores are emptied upon drying in specimens containing SRA. This phenomenon can substantially change the RH levels where capillary stresses are the main cause of shrinkage in concrete and can explain the reduction in drying shrinkage that occurs when SRAs are used in concrete.