Estimation of bond strengths and design parameters for precast concrete-UHPC shear key interface in box beam bridge

Adjacent precast prestressed concrete box beam bridge is often the choice for short-to-medium span bridges. However, it has been reported that box beam bridges experienced reoccurring longitudinal cracks at beam-shear key interfaces. The interface experiences a complex stress state represented by tensile/compressive stress perpendicular to the interface and longitudinal and vertical shear stress parallel to it. In this study, small-scaled specimens were cast together with the box beams and UHPC shear keys of a real bridge and were tested using four different test methods that try to replicate the actual stress conditions. The interface experienced satisfactory bond strength under different stress scenarios even at early age. The cohesion and friction coefficients were determined based on analysis of the experimental data and can be used to estimate the interfacial bond capacity at early age of UHPC. The interfacial bond capacity of a typical bridge was also determined and compared with the stress due to design truck load. The results indicate that the early age shrinkage and thermal load on the interface is more critical compared to the effects of the truck load. When subjected to the design truck load specified by current code, only 4.9% of the bond capacity was used, while the interfacial bond strength during the early age was exceeded. This indicates that the load is not the critical factor that causes bond failure at beamshear key interface in box beam bridges. Furthermore, the cohesion value is sufficient to resist the applied load and the shear reinforcement can be optimized in the future application of UHPC in box beam bridges. Meanwhile, the early age tensile bond strength was exceeded and therefore the use of shear reinforcement to carry the early age tensile stress is necessary to prevent cracking.