Experimental study on bonding behaviour of interface between UHPC and concrete substrate

The aim of this study is to investigate the effect of substrate surface roughness, fibre type, and water-to-binder ratio on the bond behaviour between ultra-high-performance concrete (UHPC) and normal-strength concrete (NSC). In addition, the suitability of different methods for evaluating the surface roughness using a sand-filling method, the height, number, and area of exposed coarse aggregates, and capillary absorption was verified. The results indicated that surface roughness was the most critical factor, followed by the water-to-binder ratio, and finally, the steel fibre type on influencing the bond strength. These methods for evaluating the surface roughness showed good consistency. The bond strength enhancement was mainly attributed to the compact transition zone of UHPC–NSC and the dowel effect of the steel fibres on the substrate surface recess. Based on the experimental bond strength values, an UHPC–NSC shear strength equation was derived, considering the repair surface roughness, and other research data verified the applicability of the proposed equation. Moreover, the NSC area fraction on the UHPC overlay side and the damage index of the composite specimen after debonding had a good linear relationship with the interface performance.