Study on the Flexural Capacity of Reinforced Concrete Beams Strengthened with UHPC Thin Layers Considering Interface Slip

This study investigates the flexural behavior and practical application of Reinforced Concrete (RC) beams strengthened with Ultra-High Performance Concrete (UHPC). Flexural tests were conducted on ten beam specimens to systematically analyze the effects of steel fiber dosage, reinforcement ratio, and beam height on the failure modes, load-bearing capacity, and deformation characteristics of the strengthened beams. The results were compared with those of unstrengthened control beams (CB). Experimental observations indicated excellent interfacial bonding between the UHPC layer and the RC beam, with no debonding failure occurred. All specimens exhibited typical under-reinforced flexural failure characteristics, and their load–deformation curves displayed three distinct stages. Compared to the control beams, the ultimate load-bearing capacities of the strengthened beams increased by 9.5–15.7% with varying steel fiber dosages, 16.4–110.2% with varying reinforcement ratios, and 6.2–518.8% with varying beam heights. Furthermore, the UHPC layer significantly enhanced the flexural stiffness of the beams. Although ductility was slightly reduced, all strengthened beams demonstrated clear yield characteristics prior to failure, avoiding brittle fracture. Additionally, nonlinear numerical simulations performed using MATLAB R2020a showed high agreement with the experimental results, verifying the accuracy of the analytical procedure. Based on the validated model, a parametric study was conducted to further investigate the influence of beam height, reinforcement ratio, and interface coefficients on flexural performance. The findings confirm the reliability and effectiveness of the UHPC strengthening technique.