Friction and Cohesion Interface Shear Factors of Ultra-High-Performance Concrete (UHPC) Cast on Hardened Conventional Concrete

In composite structures, interface shear resistance is a critical design criterion for transferring forces between interconnected elements at the contact surface. Recently, Ultra-High-Performance Concrete (UHPC) applications in construction have been growing rapidly due to superior mechanical and durability properties; however, there is no guidance on how to predict the interface shear resistance of UHPC cast on hardened conventional concrete (CC). This paper presents the experimental and analytical investigations conducted to develop friction and cohesion factors of the shear friction theory for UHPC cast on hardened CC in composite sections. Push-off shear tests and slant shear tests were conducted to evaluate and validate the effect of interface surface texture, interface reinforcement ratio, CC and UHPC compressive strength, and fiber presence. A friction factor of 1.0 was adopted-as in the current code provisions-while a cohesion factor of 3.45 MPa (0.5 ksi) between UHPC cast on intentionally roughened hardened CC was proposed, which is significantly higher than that in the current code provisions of CC. Also, increasing the interface shear reinforcement ratio increased the interface shear resistance significantly and resulted in a more ductile failure. Neither UHPC compressive strength nor the presence of steel fibers had a significant effect on the interface shear resistance of UHPC cast on hardened CC.