Influence of reinforcement ratio on fracture mechanisms and intrinsic tensile properties of UHPFRC in R-UHPFRC under direct tension

The combination of ultra-high-performance fiber-reinforced concrete (UHPFRC) with steel rebars (R-UHPFRC) offers superior strength, crack control, and durability, yet the quantitative interaction between rebars and UHPFRC remains insufficiently understood. To address this gap, direct tensile tests were performed on 4 UHPFRC and 22 R-UHPFRC specimens with varying reinforcement ratios. Digital Image Correlation (DIC) was employed to capture the full fracture process, enabling quantitative analysis of tensile response, strain-hardening capacity, crack evolution, and intrinsic UHPFRC contribution. The results show that reinforcement markedly extends the strain-hardening stage and enhances crack-width control, whereas higher reinforcement ratios generate radial stresses that trigger longitudinal splitting, and reduce elastic limit strength. Analysis of intrinsic tensile properties further confirmed that rebars reduce the effective contribution of UHPFRC near the bar due to stress concentration and partial debonding. A critical reinforcement ratio of around 2.45% was identified as the threshold for ductile tensile behavior. In addition, an extended database of 98 direct tensile tests was compiled, from which a new predictive model for tensile capacity was developed. The model achieved excellent agreement with experimental results (R² = 0.955), outperforming conventional superposition approaches. Finally, a sustainability evaluation framework was proposed, integrating carbon intensity and cost analyses. Results indicate that an optimal reinforcement ratio of around 2.75% provides both mechanical performance and environmental benefits. This study advances the mechanistic understanding, predictive modeling, and sustainability-oriented design of R-UHPFRC, supporting its broader application in durable and resilient infrastructure.