Enhancing the rate-dependent cracking resistance of UHPC under mixed tensile-shear mode by calcined bauxite aggregate

Calcined bauxite (CB) aggregate, characterized by porous microstructure and strong micromechanical property, has potential to mitigate macroscopic mechanical degradation of Ultra-high Performance Concrete (UHPC) from autogenous shrinkage microcracks. However, the rate-dependent cracking resistance of UHPC containing CB (UHPC-CB) under mixed-mode loading condition is not clear. Herein, the enhancing mechanism of CB upon rate-dependent cracking resistance of UHPC under mixed-mode loading is clarified from a multi-scale perspective. The results indicate that, at the microscale, CB not only leads to shorter microcracks due to physical constraint effects, but also results in a stronger ITZ compared to UHPC containing basalt aggregate (UHPC-BA), due to an internal curing effect thanks to its porous microstructure. At the mesoscale, the denser ITZ results in a higher fracture percentage of CB and more obviously an interlock effect in the case of shear stress condition. Consequently, at the macroscale, CB not only results in higher cracking resistance, especially in the case of shear loading, but also a higher dynamic increase factor value, attributed to the heterogenous micromechanical characteristics and stronger phases in CB.