The influence of superabsorbent polymers (SAPs) on autogenous shrinkage in cement paste, mortar and concrete

Due to the high amount of fines in combination with a low water-to-cement ratio (<0.35), high performance mortar and concrete are very prone to cracking as a result of autogenous shrinkage, resulting in a decreased durability, integrity and aesthetics of the structure. Superabsorbent polymers (SAPs) can be added as internal curing agents to reduce the autogenous shrinkage in cementitious materials. In this paper, the influence of SAP addition on mitigating autogenous shrinkage in cement paste, high performance mortar and high performance concrete is investigated. Two different types of SAPs were added in varying amounts: one sulfonate based SAP with specifically selected properties, and a commercially available poly-acrylate based SAP. To study the effect of the SAP addition on the mitigation of autogenous shrinkage, corrugated tube tests in case of cement paste and mortar, and restrained ring tests for concrete were performed. The poly-acrylate based SAPs reduced the autogenous shrinkage after 7 days in the mortar mixtures with 97% compared to the reference without SAPs, whereas in the cement paste, the autogenous shrinkage after 7 days was completely mitigated. Although the mixtures with sulfonate based SAPs did not show complete mitigation of the autogenous shrinkage, the shrinkage was significantly reduced for all cement pastes: increasing the amount of SAPs from 0.257 m% to 0.38 m% and 0.57 m% by weight of cement, lead to a reduction in the autogenous shrinkage after 7 days with 80%, 85% and 89% respectively. In mortar the reductions for the same amounts of SAPs were 19%, 20% and 70% respectively. Considering restrained shrinkage ring tests, the poly-acrylate based SAPs reduced the occurring strains significantly (-88%) compared to the reference and prevented the rings from cracking. For the sulfonate based SAPs, the moment of cracking was delayed and lower strains compared to the reference were observed.