Effects of superabsorbent polymer on shrinkage properties of ultra-high strength concrete under drying condition

In this study, effects of super-absorbent polymer (SAP) on shrinkage of ultra-high strength concrete (UHSC) under drying condition were investigated. The total shrinkage and autogenous shrinkage of UHSC specimens of two sizes with and without SAP were measured and the associated drying shrinkage was calculated. In addition, the internal relative humidity (IRH), free water and chemically bound water at different distances from the drying surface were obtained and compared to the sealed specimens in order to analyze the shrinkage mechanism of UHSC with SAP under drying condition. The results showed that SAP caused uneven moisture distribution at the inner and outer layers based on the distance from the drying surface. The release of water from SAP was faster at distances within 3 cm from the drying surface, and the IRH dropped more rapidly comparing to inner layers. At layers within 3-10 cm from the drying surface, SAP was able to maintain the IRH comparable to those under sealed condition. Accordingly, the drying environment had less influence on the performance of SAP in larger UHSC specimens, and restraining effect of SAP on shrinkage was superior to that of small ones. In addition, the moisture gradient at inner and outer layers led to different amounts of free water and chemically bound water throughout the matrix. The total shrinkage of UHSC under drying condition had a non-linear relationship with the average moisture loss due to the coupled effect of self-desiccation and migration of water to the surface due to evaporation. (C) 2019 Elsevier Ltd. All rights reserved.