Experimental study on bond performance between high strength steel rebar and ultra-high performance concrete

To investigate the bonding properties of high-strength steel rebar in ultra-high performance concrete (UHPC), 69 specimens were designed and produced. The effects of UHPC strength, fiber volume ratio, fiber size and shape, concrete cover thickness, bonding length, loading condition, and bonding segment condition on bonding properties were studied through pull-out tests. The results show that the specimens fail in three modes: pull-out, rebar fracture, and splitting failure. The main factors affecting the bond strength between high strength steel bar and UHPC are UHPC compressive strength, fiber volume content, aspect ratio, and cover thickness.The incorporation of fiber has a significant effect on improving the bonding strength. When the fiber volume ratio increases from 1% to 3% and the aspect ratio increases from 35 to 100, the bonding strength increases by 23% and 16%, respectively. However, the change of fiber shape has no pronounced effect on bonding strength. The bonding strength increases significantly with the compressive strength of UHPC and concrete cover thickness but decreases with the bonding length. When the concrete cover thickness exceeds four times the rebar diameter, the increment is hardly changed. When the bonding segment is located at the loading end, the bonding strength of the specimens under pull-out loading is only 77% of that under compression loading. The closer the bonding segment is to the middle of the specimen, the less influence the loading condition has on the bonding strength. Based on the experiment results, the formula for calculating critical anchorage length was established, and the formula of bonding strength between high-strength steel rebar and UHPC interface was proposed, and the bond-slip constitutive model was established. The existing equations for the bonding strength of ordinary concrete underestimate the bonding strength of high-strength steel rebar to UHPC. The simplified equations in this paper predict the results in good agreement with the experimental results.