Cracking behavior of hollow ultra-high-performance concrete beams reinforced with CFRP bars

This study focuses on developing cost-effective ultra-high-performance concrete (UHPC) and optimizing beam cross-sectional geometries to enhance the economic viability of UHPC applications. Six hollow UHPC beams reinforced with CFRP bars (CFRP-UHPC beams) were fabricated and subjected to cyclic loading via four-point bending tests. The research investigates the effects of varying CFRP reinforcement ratios, hollow shapes (double circular and rectangular), and hollow ratios on the flexural performance of these beams. A comprehensive analysis was conducted, evaluating failure modes, crack propagation, characteristic load, deformation, and crack width. The results indicate that the newly developed UHPC containing coarse aggregates reduced costs by 43.34 % compared to conventional types and achieved scalability beyond 400 mm. A consistent failure mode was observed across all hollow beams, characterized by concrete crushing at the beam's top, initiating from the weakest point in the compression zone and progressing towards the mid-span or loading point. Reducing the cross-sectional hollow ratio significantly enhanced the flexural performance of the beams, improving cracking load, control load, ultimate load, deformation capacity, and effectively suppressing crack generation and propagation. Conversely, increasing the CFRP reinforcement ratio had the opposite effect. The influence of the hollow ratio on crack resistance was more pronounced in beams with higher hollow ratios. Rectangular hollow beams exhibited superior deformation and crack resistance compared to circular hollow beams at the same hollow ratio. A calculation model for the maximum crack width of hollow beams was developed based on composite material mechanics theory, with predicted results showing good agreement with experimental data.