Shrinkage behavior and water transport kinetics of SAP-modified concrete under different curing temperatures

The shrinkage behavior and water transport kinetics of concrete incorporating superabsorbent polymers (SAPs) were investigated under three representative curing temperatures (5 °C, 20 °C, and 40 °C). It was found that curing temperature significantly influenced both shrinkage development and water migration. The SAP-modified concrete exhibited the lowest shrinkage at 20 °C, where the 28 d shrinkage deformation of the SAP-P mixture was reduced to 60.78 % of that of the reference mix (Ref-2). At a curing temperature of 5 °C, a distinct “left-shift phenomenon” in the free water peak was observed at 3 d, suggesting a progressive phase transition of water. Specifically, free water released from SAPs or large capillaries was first converted into capillary water, followed by its transformation into gel water. In contrast, at 40 °C, the elevated temperature accelerated water transport kinetics, leading to a rapid reduction or disappearance of the free water peak after 3 d. For all tested temperatures, gel water was found to dominate the non-chemically bound water content, accounting for over 96 % of the total. Among the tested SAPs, SAP-P demonstrated superior internal curing efficiency, sustaining water release for up to 28 d and effectively mitigating internal water loss under all temperature conditions. Moreover, a relational model between shrinkage and internal relative humidity was established, providing a quantitative framework for predicting the shrinkage behavior of SAP-modified concrete under variable thermal environments.