The effect of curing regimes on the mechanical properties, nano-mechanical properties and microstructure of ultra-high performance concrete

This study addresses the effect of curing regimes on the mechanical properties, hydration and microstructure of ultra-high performance concrete (UHPC). The results demonstrate that the mechanical properties are strengthened by increasing curing temperature, but the flexural/tensile to compressive strength ratio shows an unusual increasing tendency with increasing temperature and compressive strength, which is opposite to normal concrete. The nano-mechanical properties are also enhanced by heat treatment. The ultra-high density phase is dominated hydrates. Microstructure observation indicates that heat treatment promotes the formation of additional hydrates with high-packing density and stiffness such as tobermorite and xonotlite, enhancement of transition zone around steel fiber, quartz and clinker, average chain length of hydrates and pozzolanic reaction between quartz/silica fume and Ca(OH)2. The evolution of hydrates and microstructure due to curing regimes and the presence of quartz play key roles in controlling the unusual behavior of the strength ratio and improvement of mechanical properties.