Abstract To enhance the strengthening effectiveness of existing reinforced concrete (RC) beams and fully utilize the material properties of ultra-high performance concrete (UHPC), this study systematically investigates the flexural strengthening mechanism of UHPC layers through mechanical property tests, four-point bending experiments, and refined finite element simulations. The research first calibrated the constitutive model of UHPC via uniaxial tension and compression tests. Subsequently, flexural tests were conducted on RC beams strengthened with a 40 mm thick UHPC layer, and the crack propagation process was observed using digital image correlation (DIC) technology. Based on experimental results, a finite element model considering material nonlinearity and interfacial bond-slip behavior was established. Parameters such as the thickness of the strengthening layer, material strength, and reinforcement ratio were analyzed. The results demonstrate that the UHPC strengthening layer significantly improves the flexural capacity, stiffness, and crack resistance of RC beams. The reinforced UHPC strengthening layer exhibited even more pronounced enhancements, with the cracking load and ultimate load increasing by up to 145.2% and 67.9%, respectively. DIC results revealed that cracks initiated in the UHPC layer and propagated across the interface into the RC beam. Surface roughening treatment ensured effective composite action without debonding failure. Finite element analysis further indicated that unreinforced strengthening layers led to stress concentration at the beam bottom, exhibiting brittle failure characteristics. In contrast, reinforced strengthening layers significantly improved stress distribution, resulting in more uniform and delayed crack development. The load–displacement curves displayed a distinct plateau after the peak load, indicating enhanced structural ductility and controlled failure. This study provides experimental evidence and theoretical support for the optimal design and engineering application of UHPC in flexural strengthening of RC beams.
Wáng et al. (Sat,) studied this question.