Recently, ultra-high performance concrete (UHPC) has been utilized in highway bridge connections, where its superior strength and durability help to reduce joint cracking and enhance transverse load transfer. According to the load and resistance factor design (LRFD) bridge design procedure specified by AASHTO, the strength of the connections is dependent on the adhesion and friction between the connected materials. The objective of the present research is to identify the adhesion value between UHPC and high-strength concrete (HSC) with varying degrees of roughness. To this end, UHPC-HSC specimens were tested in direct tension according to ASTM protocols, and the maximum tensile stress at failure was obtained. Test results show that the average maximum tensile stress for the UHPC-HSC specimens with a smooth interface exceeds that determined from past research for any degree of roughness. Furthermore, the average maximum tensile stress increases with the degree of roughness. The results from the direct tension tests, along with those from slant-shear tests performed on similar materials in a previous study, were then used to back-calculate the friction coefficients using Mohr-Coulomb theory.
