Coupled effect of shrinkage-mitigating admixtures and saturated lightweight sand on shrinkage of UHPC for overlay applications

This paper evaluates the efficiency of various shrinkage mitigation approaches in reducing autogenous and drying shrinkage of ultra high performance concrete (UHPC). This included the use of various contents of CaO-based and MgO-based expansive agents, shrinkage-reducing admixture, and pre-saturated lightweight sand. Workability, compressive strength development, autogenous and drying shrinkage were evaluated for UHPC mixtures subjected to moist curing periods of 1, 3, and 7 d. Test results indicate that the use of lightweight sand was shown to be more effective in mitigating shrinkage than enhancing compressive strength. The replacement of natural sand by 60% of lightweight sand, by volume, was found to reduce autogenous shrinkage from 530 to 35 µm/m at 91 d. The coupled effect of using 60% lightweight sand and either CaO-based expansive agent, MgO-based expansive agent, or shrinkage-reducing admixture can reduce autogenous shrinkage at 91 d by up to 600 µm/m and drying shrinkage by up to 700 µm/m. In some combination, the use of shrinkage mitigating admixtures was found to reduce the 91-d compressive strength ranging from 8 to 20 MPa, even when 60% lightweight sand was employed. The initial moist curing period had positive effect on mitigating total shrinkage. The incorporation of 10% CaO-based expansive agent in UHPC with 60% lightweight sand subjected to 7 d of moist curing exhibited the best overall performance with 91-d autogenous shrinkage of 110 µm/m in expansion and 91-d total shrinkage (autogenous shrinkage after 1 d plus drying shrinkage) of 580 µm/m (also in expansion).