Fracture Behavior of Interfaces between Ultra High Performance Concrete and High Performance Concrete

Ultra high performance concrete (UHPC) is commonly used in connections between precast high-performance concrete (HPC) bridge components. Cracking along UHPC-HPC interfaces was noted in the field, raising concerns about potential long-term implications on the serviceability and durability of bridges with UHPC connections. As an intermediate step toward understanding the sources of interfacial cracking in UHPC-HPC interfaces, this study characterized the cracking propensity of HPC/UHPC interfaces using a fracture mechanics-based test method. Three-point bending tests on notched beams consisting of UHPC and HPC were conducted in a closed-loop test setup controlling crack mouth opening displacement (CMOD). Substrate hygric state (dry and saturated surface dry, or SSD) and substrate surface tortuosity (as-cast and exposed aggregate surface finish) were varied to evaluate their effects on crack growth characteristics along the interface. Experimental load-CMOD data were used to inversely compute tensile softening laws of UHPC-HPC interfaces. The results showed a reduction in fracture energy between 63% and 86% and a reduction in tensile strength between 50% and 66% in HPC/UHPC interfaces when compared to neat HPC. The substrate tortuosity was found to significantly improve the fracture toughness of UHPC-HPC interfaces—HPC/UHPC interfaces in the “as-cast substrate� group exhibited brittle failure without a pronounced softening regime. No significant effect of substrate hygric state on the fracture properties of UHPC-HPC interfaces was noted.