Experimental and theoretical study on the flexural behavior of NC-UHPC composite beams with embedded thin UHPC layers

Placing a thin layer of ultra-high performance concrete (UHPC) locally in critical regions, particularly in negative-moment zones, provides an effective means to improve the flexural behavior and durability of concrete bridge girders while limiting UHPC usage. However, targeted experimental evidence on locally embedded UHPC-NC beams and systematic investigations of their flexural and interfacial responses remain limited. In this study, NC-UHPC composite beams with embedded thin UHPC layers (NC-UHPC-ETUL) with different section heights and reinforcement layouts were fabricated and tested under three-point bending to evaluate flexural behavior and interfacial performance. The results show that embedding a thin UHPC layer in the NC matrix increases flexural stiffness, delays crack initiation and propagation, and enhances ultimate load capacity. Specimens with through-reinforcement across the UHPC-NC interface exhibited more uniform crack patterns and greater ductility than those without. An analytical model was developed to predict stress distributions, interface-debonding loads, and flexural capacities, explicitly accounting for the mechanical interaction between the UHPC and NC layers; its predictions agreed well with the experimental results. The validated framework provides an effective and efficient tool for the preliminary design and assessment of NC-UHPC-ETUL beams.