Asmamaw, Chalachew2026-09-022026-09-022026-08-262026-09-02http://hdl.handle.net/1993/40058Ultra-high-performance concrete (UHPC) is a promising material for shear-key connections in adjacent box beam girder bridges because of its high strength, bond capacity, and durability. However, the long-term performance of UHPC cast against hardened high-strength concrete (HSC), and the effects of surface condition and transverse reinforcement on shear-key behaviour, require further investigation. This thesis addressed these issues through two independent experimental studies. This thesis used specimens cast by Tan (2021) and Erfani (2023). The strengths found in these studies were compared with the strength values obtained in this experimental program. One study Tan (2021) provided a durability-related assessment of material and interface specimens that were exposed to outdoors environment for approximately 7 years. Temperatures in Winnipeg, MB ranged from −40°C to +37°C. Compression, indirect tension, pull-off, slant-shear, and bi-shear tests were conducted to evaluate retained mechanical and bond-related performance. The HSC specimens had average compressive and indirect tensile strengths of 101.6 and 6.25 MPa, respectively. Pull-off specimens developed an average resistance of 3.96 MPa and failed within the HSC substrate. Slant-shear specimens also failed within the HSC rather than through complete interface separation, while bi-shear specimens developed an average shear strength of 9.42 MPa. All average strength results exceeded the average strength values obtained 7 years ago. These results demonstrate that the materials and UHPC–HSC systems retained measurable mechanical and interface resistance after long-term outdoor exposure. The second study investigated shear-key specimens representative of shear keys in adjacent box beam girders. The specimens that were tested in shear were cast by Erfani (2023) and the test parameters investigated included interface surface preparation and presence/absence of transverse reinforcement. Surface roughness increased the peak load by 38.7%, while the presence of reinforcement in addition to the roughness increased peak load by 61.5%. Load–displacement measurements, Digital Image Correlation, and strain instrumentation showed that the smooth, unrestrained specimen experienced localized damage and an abrupt post-peak reduction in resistance. In contrast, the rough, reinforced specimens exhibited delayed crack initiation, greater deformation capacity, and higher residual load-carrying capacity. These observations indicate that surface roughness and transverse reinforcement improved shear resistance, ductility, crack control, and residual strength of the shear key specimens. Overall, the results emphasize the importance of interface preparation, and reinforcement detailing in UHPC–HSC shear-key performance.engUltra-high-performance concrete (UHPC); high-strength concrete (HSC); shear-key connections; interface shear behaviour; surface roughness; transverse reinforcement; precast bridge systems; digital image correlation (DIC).