Dic and Mesoscale Based Analyses on Damage Evolution in Uhpc-Nc Interface Under Direct Shear Loading

Interface treatments are vital for the bonding performance between ultra-high performance concrete (UHPC) and normal concrete (NC), owing to the mesoscopic nonlinear characteristics caused by interface heterogeneity. This research is devoted to providing an understanding on the mesoscopic behavior of the UHPC-NC interface using the digital image correlation (DIC) technique. Types of interface treatments, including post-installed dowel rebar, high-pressure water, and castellated key, were considered. Results demonstrated that, a three-stage damage evolution consisting of the elastic stage, yield stage, and descend stage was observed in the load-slip curves during the interfacial crack propagation. Aggregate interlock behavior plays a significant role in bonding performance for specimens treated with high-pressure water. Excellent bond strength of 5.42 MPa was obtained by high-pressure water treatment, and the shear strength was enhanced by 146.7% when compared with smooth treatment. With the increase of dowel rebar, the strength improvement of rough treatment was reduced to 73.1%. The failure was mainly observed in the interface transition zone (ITZ) between UHPC and NC. When the ratio of dowel rebar was increased, the failure changed into the ITZ failure between NC mortar and coarse aggregate, fracture of the coarse aggregate was observed near the interface. For specimens treated with a castellated key, the shear strength was 111.3% enhanced by installing dowel rebar. Post-installed dowel rebar restricted the rotation and separation of the castellated key, thus changing the NC failure into UHPC-NC ITZ failure. Meanwhile, the interfacial failure strain is less than 0.5% and 0.25% in the separation and slip direction, respectively, when the brittle failure occurred. While for ductile failure, the failure strain was generally more than 1.0%.