Reinforcement-ratio-based piecewise modeling of flexural capacity and deformation response in UHPC beams

Abstract

Ultra-high-performance concrete (UHPC) beams exhibit reinforcement-dependent flexural behavior governed by fiber bridging, reinforcement ratio, and compression-zone resistance. This study proposes a mechanics-informed piecewise predictive framework for modeling the flexural and deformation response of UHPC beams. A quality-controlled database of 48 beam tests was compiled from an initial dataset of 156 specimens reported in the literature. Optimal reinforcement-ratio boundaries were identified through an exhaustive-search procedure and used to classify beams into lightly, moderately, and highly reinforced regimes. Separate regression models were developed within each zone for predicting normalized flexural capacity (MomentIndex) and normalized deformation capacity (DispRatio). The proposed MomentIndex model achieved a global coefficient of determination of R² = 0.9635, demonstrating that reinforcement-ratio-based zoning effectively captures the dominant mechanisms governing UHPC flexural capacity. The DispRatio model achieved a global R² of 0.5680, reflecting the greater complexity of deformation behavior and the influence of additional parameters not explicitly considered. The proposed framework combines predictive accuracy with physical interpretability and provides a practical tool for preliminary design and assessment of UHPC flexural members.