Effect of microbially induced calcite precipitation treatment on the bonding properties of steel fiber in ultra-high performance concrete

Bonding performance between steel fiber and cement matrix is critical for the stress transformation and crack control, which significantly affects the mechanical properties of ultra-high performance concrete (UHPC). To enhance the fiber/matrix bond, this study proposed a microbiological technique to pretreat the steel fiber surface via microbially induced calcite precipitation (MICP) treatment. MICP generates calcium carbonate (CaCO3) crystals bonded on the surface of steel fiber, leading to an increase in roughness and hydrophilicity. The results obtained from single steel fiber pullout tests indicate that the bond strength and pullout energy of steel fiber are both increased by MICP treatment. The good bonding consequently resulted in more residual hydration products attached to the fiber surface after the pullout test. The tests on the UHPC samples with untreated and treated fiber confirmed the benefits of MICP treatment in improving the mechanical properties of UHPC. It is demonstrated that MICP treatment on steel fiber can increase the compressive and flexural strength of UHPC by -16% and-50%, respectively.