Experimental study on impact and post-impact performance of UHPC-RC composite beams

Ultra-High Performance Concrete (UHPC) has been widely applied to repair and strengthen reinforced concrete (RC) structures due to its excellent mechanical properties. In this study, two types of UHPC-RC composite beams with different composite layer arrangements were designed and fabricated, with conventional RC beams and full-section UHPC beams as references. Drop hammer impact tests, post-impact static loading tests, and additional finite element analyses were conducted to systematically investigate the influence of UHPC layer placement on the dynamic response, energy absorption behavior, and residual load-carrying performance of RC beams. Results indicate that the position of the UHPC layer significantly affects impact resistance. UHPC placed on the compression side effectively suppresses local impact damage, reduces peak and residual displacements, and enhances post-impact residual capacity and displacement ductility; correspondingly, the RC.CU80 specimens exhibited the highest impact-stage absorbed energy, exceeding that of the RC specimens by 19.8%. In contrast, UHPC placed on the tension side aggravates stiffness degradation and residual capacity loss due to the isolating effect of interfacial cracks. In addition, it was observed from the experimental results that the impact plateau load extracted from the load-displacement response during impact exhibits a consistent trend with the residual load-carrying capacity measured in the post-impact static stage. This observation suggests that the impact plateau load may, to some extent, reflect the overall mechanical state of the specimens after impact, and could potentially serve as a reference indicator for evaluating the residual load-carrying performance.