Abstract：Ultra-high performance concrete (UHPC) is very suitable for the repair and reinforcement of damaged normal strength concrete (NC) structures due to its excellent mechanical characteristics and durability. The good shear resistance of UHPC-NC interface is important to enhance the mechanical properties and impermeability of concrete structures repaired by UHPC. The shear properties and failure modes of different UHPC-NC interface with treatments, including smoothing, chiseling, rebar exposing, grooving, drilling, and rebar bonding, were evaluated and discussed by seven sets of shear push-out tests. The influences of age of UHPC, moisture degree of NC substrate, roughness of NC surface and curing conditions of UHPC on the shear resistance of UHPC-NC interface were also investigated by using the slant shear tests. The experimental results demonstrate that the interface between UHPC overlay and NC substrate exhibits an excellent shear bonding performance, and basically their failure modes were either complete NC substrate failure or partial interface + partial NC failure without the pure interface failure being observed. The roughness of NC surface is considered to be a major factor affecting the shear bonding strength of UHPC-NC interface. The UHPC-NC interfaces with the treatments of roughness or grooving on NC surface may attain the best shear resistance, and the interfaces with grooving and rebar bonding exhibit good ductility at failures. In addition, the wetter the NC substrate is, the higher the shear strength of UHPC-NC interface shall be, most of the shear resistance of UHPC-NC interface (about 88.5%-95.0%) can be obtained during the early age of UHPC (3.7 days) and the specimens cured at normal temperature may produce a higher interfacial shear bonding strength than those of high temperature curing.
