Research on flexural performance and damage evolution of pre-damaged UHPC beams under different reinforcement methods

• Identified and classified key bending failure modes in UHPC, including fiber pull-out, fiber breakage, reinforcement disbonding, interlayer shear failure, and matrix layer sliding. • Demonstrated that steel plate and CFRP sheet reinforcements significantly enhance UHPC beam performance by increasing ultimate bearing capacity by up to 34.10% and reducing mid-span deflection by 9.4%. • Utilized acoustic emission techniques with Gaussian mixture models and B-value analysis to effectively monitor and distinguish between tensile and shear cracks, elucidating the internal damage evolution in reinforced UHPC structures. . This study investigates the mechanical properties and failure mechanisms of pre-damaged UHPC beams strengthened with externally bonded carbon fiber reinforced polymer (CFRP) sheets and steel plates, aiming to address the existing research gap regarding the post-reinforcement performance of pre-damaged UHPC structures. Through three-point bending tests on pre-damaged UHPC beams with 100 mm×100 mm cross-sections, combined with real-time monitoring using digital image correlation (DIC) and acoustic emission (AE) techniques, this research systematically analyzes the impacts of different reinforcement methods on ultimate bearing capacity, stiffness, crack propagation rate, and internal damage evolution. A theoretical formula for ultimate bearing capacity incorporating the effect of steel fibers in UHPC was derived. Experimental results demonstrate that both CFRP and steel plate reinforcements significantly enhance the beam's ultimate bearing capacity, with the unreinforced beams showing 29.09% and 34.10% capacity improvements through CFRP and steel plate reinforcement, respectively. The reinforcement effectively suppresses crack development, where steel plates demonstrate superior performance in controlling crack width due to their high stiffness, while CFRP exhibits greater effectiveness in reducing tensile cracks and enhancing structural durability. Furthermore, reinforcement mitigates initial AE signals, with steel plate reinforcement notably slowing the progression of internal concrete damage from microcracks to macrocracks. AE ring-down counts accurately reflect the damage evolution process. The relative error between theoretical and experimental values remains below 10 %, validating the accuracy of the proposed theoretical formula.