Effect of pre-soaked superabsorbent polymer on shrinkage of high-strength concrete

Pre-soaked super-absorbent polymer (SAP) was incorporated into high-strength concrete (HSC) as an internal curing agent to study its effects on early-age shrinkage and mechanical properties. On the basis of the capillary stress based model for shrinkage prediction of concrete, together with the experimental results of cement hydration kinetics, evolution of internal temperature and humidity, development of pore structure and mechanical properties, the working mechanism of SAP was discussed. Results indicate that the addition of pre-soaked SAP significantly reduces the autogenous shrinkage as well as the early-age shrinkage of HSC under drying condition. In sealed HSC specimens, the drop of internal humidity caused by the self-desiccation effect is notably postponed by addition of pre-soaked SAP. The addition of pre-soaked SAP slightly reduces the compressive strength of HSCs and this effect is more pronounced in early-age concrete. Furthermore, an insightful comparison of the behaviours of the internal curing water introduced by the pre-soaked SAP and the additional free mixing water in concrete was made. Results indicate that the internal curing water behaves differently from the additional mixing water in influencing the cement hydration kinetics, pore structure of hardened cement pastes and the mechanical strength of concrete, due to the different spatial distribution of the two types of water in the concrete bodies. The shrinkage-reducing effect on HSC due to the addition of extra internal curing water incorporated by pre-soaked SAP is much stronger than that of the additional mixing water. Besides, the internal curing water shows much less strength-reducing effect than the additional mixing water. In virtue of the shrinkage prediction model, the working mechanism of pre-soaked SAP in reducing autogenous shrinkage of HSC is proposed on the basis of the following two aspects. The participation of internal curing water in cement hydration process leads to a total volume gain of the hardening cement pastes. Meanwhile, the release of internal curing water from the pre-soaked SAP postpones the drop of internal humidity. The synergistic effect of these two factors effectively reduces the autogenous shrinkage of HSC.