Experimental evaluation of the normal and tangential stiffness of the interface between high strength concrete and ultra-high-performance concrete

The association between two types of concrete at different stages is common in precast connections, composite beams, and repairs, creating an interface. High-Strength Concrete (HSC) is extensively used in precast industries. Ultra-High-Performance Concrete (UHPC) is suitable for filling the HSC-UHPC connection due to its exceptional mechanical properties and adhesion capability. This study investigated the normal and tangential stiffness of the HSC-UHPC interface using different surface preparations: Exposed coarse aggregates, Shear keys, Exposed fine aggregate, Expanded mesh, Exposed fiber and Smooth (no special treatment). The stiffness of the interface was assessed through four-point bending, splitting tension and push-off tests. The normal stiffness coefficient derived from the four-point bending test proved more suitable than the stiffness coefficient obtained from the splitting tensile test. A trend between roughness and normal stiffness was observed, indicating decreased normal stiffness as roughness increased. Although the roughness of the interface influenced the normal stiffness by affecting adhesion and mechanical interlocking, its impact on tangential stiffness was not clearly observed and depended on the interface treatment. The normal and tangential stiffness parameters obtained at the HSC-UHPC interface were significantly higher than those observed in previous studies on the Normal Strength Concrete-UHPC interface. The complete nonlinear load displacement curves were provided to represent the interface behavior between HSC-UHPC considering different types of tests and surface preparations. A range of normal and tangential stiffness parameters was obtained, which can be useful in future research aiming to modeling the interface between HSC and UHPC.