Effect of internal curing with super absorbent polymers on the relative humidity of early-age concrete

High-performance concrete is used extensively in practice. However, the water-to-cement ratio of this concrete is low and causes self-desiccation. Due to moisture diffusion and this self-desiccation, internal relative humidity (IRH) decreases in concrete structures that are exposed to ambient air and induces autogenous shrinkage. Internal curing with super absorbent polymer (SAP) is used to mitigate this shrinkage in high-performance concrete; nonetheless, IRH variation influences changes in the autogenous shrinkage of concrete with SAPs. Although tests have been performed to mitigate the autogenous shrinkage of concrete that was internally cured through SAPs, experimental study on IRH variation in such concrete remains lacking. Thus, such a study was conducted to determine the effect of internal curing with SAPs on the IRH of early-age concrete under sealed and unsealed conditions, as detailed in this paper. Test results showed that: (1) the IRH of concrete that was internally cured with SAPs increased with an increase in the content of internal curing water 28 days after casting under sealed and unsealed conditions. (2) The degree of decrease in the IRH of internally cured concrete was lower under sealed conditions than under unsealed conditions 28 days after casting. (3) The critical time of the IRH of internally cured concrete increased with increases in the content of internal curing water under both sealed and unsealed conditions. (4) The decrease rate of the IRH in early-age concrete that was internally cured with SAPs dropped with an increase in the content of internal curing water under sealed and unsealed conditions. (5) A formula was proposed to calculate the IRH of early-age concrete that was internally cured with SAPs in consideration of the content of internal curing water under unsealed conditions. This formula indicates the good accuracy of the experimental results.