Bond Strength of Ultra-High Performance Reinforced Concrete: The Effects of Rebar on Fiber Distribution and Orientation

This study investigates the influence of steel fiber orientation and distribution, induced by rebar arrangement under single-point casting, on the bond behavior of ultra-high performance reinforced concrete (UHPRC). Three rebar configurations (I, III, and V) were designed, and the spatial characteristics of each arrangement were quantified using the ratio of the minimum clear spacing between rebars to the fiber length, denoted as SFR. Bond–slip tests and X-ray computed tomography (CT) analyses were conducted under different fiber volume fractions (0.5%–2%) and sample positions (A, B, and C). The results show that rebar presence significantly alters fiber distribution patterns. Fibers around the rebar displayed pronounced alignment along the rebar direction, and this alignment became more distinct with increasing distance from the casting point. At the tail end position (section C), the combined action of rebar induced disturbance and flow history produced an optimal fiber orientation, which led to enhanced bond performance. The SFR was also found to strongly affect fiber orientation and distribution. A moderate spacing (SFR≥1.35) promoted the formation of a continuous zone of aligned fibers, thereby improving bond behavior. In contrast, a small spacing (e.g., SFR=0.64) tended to randomize fiber orientation and create fiber free regions, both of which reduced interfacial resistance. A three-dimensional nonlinear modification model was developed to integrate casting-induced fiber orientation into the prediction of rebar–matrix bond strength. This research provides quantitative evidence for optimizing rebar layout and construction techniques of ultra-high-performance concrete (UHPC) members.