Effects of SAP characteristics on internal curing of UHPC matrix

摘要

Superabsorbent polymer (SAP) is an effective internal curing agent, and its efficiency is highly dependent on its physical and chemical characteristics. This study investigates the effect of molecular structure, size, content and pre-treatment of SAP on its performance as an internal curing agent in ultra-high performance concrete (UHPC) through the measurement of internal relative humidity, heat of hydration evolution, autogenous shrinkage, and compressive strength. Two types of SAP (ionic and non-ionic) at three contents of 0.2%, 0.4%, and 0.6% were used at the dry state. The ionic SAP was used in two sizes of 471.3 and 95.1 um to study the effect of SAP size on the efficiency of internal curing. On the other hand, selected ionic SAPs were prewetted by additional water before mixing (0.4% by mass). Results showed that non-ionic small particle size SAP exhibited a higher restraining effect on autogenous shrinkage. The incorporation of SAP can significantly increase the cumulative heat after 15 h, and thus increase the degree of hydration of cementitious materials in UHPC. However, the investigated SAP characteristics showed no significant effect on the cumulative heat of hydration within the tested period of 72 h. The addition of pre-wetted SAP increased the autogenous shrinkage and delayed the hydration, compared to the dry SAP particles with additional water. Finally, the autogenous shrinkage of UHPC was fitted with several published prediction models, and an optimized model was proposed to predict autogenous shrinkage with respect to internal relative humidity and heat of hydration. (C) 2021 Elsevier Ltd. All rights reserved.