Flexural performance of CFRP-CA-UHPC hollow beams with double holes: Experimental investigation and modeling

Research on Carbon Fiber Reinforced Polymer (CFRP)-reinforced coarse aggregates-Ultra-High Performance Concrete (CA-UHPC) beams has mainly focused on solid section, with limited investigation into double-circular-hole rectangular hollow beams, whose reduced material usage can significantly improve structural efficiency. However, the effects of steel fiber content on their flexural performance and the accuracy of existing design models remain insufficiently understood. This study experimentally investigates the flexural behavior of CFRP-reinforced CA-UHPC hollow beams with double circular holes under cyclic four-point bending, considering three steel fiber volume fractions (1.5 %, 2.0 %, and 2.5 %). Digital Image Correlation (DIC) was used to capture full-field crack development and width evolution. The results demonstrate that higher steel fiber content improves elastic modulus, flexural capacity, and crack control, with the 2.5 % fiber beam showing a 9.8 % increase in ultimate bending moment and a 300 % reduction in residual displacement compared to 1.5 %. A simplified flexural capacity model, incorporating UHPC tensile contribution and a modified ultimate compressive strain, was proposed. Based on the model predictions, an empirical correction to the ACI equivalent compressive zone coefficient β was developed for double-hole sections. The findings provide practical design guidance for lightweight, high-strength hollow beam systems in engineering applications.