Use of superabsorbent polymers to mitigate autogenous shrinkage in ultra-high performance concrete

Ultra-high performance concrete (UHPC) with low w/c-ratio is very prone to the formation of cracks due to autogenous shrinkage. These cracks can lead to a decreased durability of the concrete, resulting in higher maintenance and/or repair costs in the future. Superabsorbent polymers (SAPs) can be added to cementitious materials to provide internal curing and as a result reduce or even mitigate this autogenous shrinkage. In this paper, two different types of SAPs were added to cement paste to see their influence on mitigating autogenous shrinkage. One SAP is a commercially available SAP whereas the other SAP is especially developed within the framework of the LORCENIS project by the company ChemStream, with the aim to mitigate autogenous shrinkage and induce self-healing of cracks. The SAPs from ChemStream were based on a copolymerization of sodium vinyl sulfonate (SVS) with 2-acryloylamino-2-methyl-propane-1-sulfonate (NaAMPS) and contained 1.0 mol% N,N’-methylenebisacrylamide (MBA) with respect to the monomer as cross-linker. The commercial SAP from BASF was based on poly(acrylamide-co-acrylic acid). In case SAPs were used, an additional fixed amount of water was added to mitigate autogenous shrinkage. The amount of SAPs used was determined based on their swelling capacity in cement filtrate and in order to obtain the same workability as the reference mixture. The amount of SAPs needed was in the range of 0.2-0.26 m% of the cement weight. To see whether the size of the SAPs plays a role in the efficiency of mitigating autogenous shrinkage, two average particle sizes, namely 40 and 100 µm, were tested. With the used amount of SAPs, a reduction or even complete counteraction of autogenous shrinkage was observed for the cement pastes