Coupled bond-slip and fracture modeling of reinforced concrete with degraded reinforcement

Reinforcement degradation caused by environmental corrosion or construction-induced surface damage inevitably weakens both the reinforcement itself and the reinforcement-concrete interface, thereby compromising the structural integrity of reinforced concrete (RC) members. However, existing numerical models typically simplify the interface into a unidirectional bond-slip constitutive relationship. They fail to comprehensively consider the complex coupling effect of normal separation and tangential displacement, as well as the characteristics of steel bar damage. To address this issue, this paper presents a three-dimensional finite element model based on a modified cohesive zone model (CZM) for evaluating the fracture behavior of RC structures with degraded reinforcement. The model is established using a combination of solid elements and cohesive elements. A solid model is created, followed by meshing and mesh splitting. Zero-thickness cohesive elements are inserted into the separated element interfaces to represent the potential fracture surfaces of the concrete and the interface between the steel and concrete. Additionally, the steel bar is equivalently modeled as a rectangular cross-section to avoid serious mesh distortion issues. A mixed-mode constitutive model incorporating interfacial friction is formulated for the concrete fracture surfaces, while a coupled normal-tangential bond-slip constitutive model with a degradation coefficient is developed for the reinforcement-concrete interface. The model is validated against three sets of four-point bending tests involving corroded steel-reinforced beams and locally damaged steel-basalt fiber reinforced polymer (BFRP) hybrid reinforced beams. Simulation results demonstrate that the proposed model can accurately reproduce the load-displacement responses and crack propagation patterns observed in the experiments. Parametric analyses further reveal that: (i) the structural load-bearing capacity increases with bond strength up to a saturation threshold (approximately λ s a t = 4 f c k / f t for the investigated cases), beyond which additional bond strength yields negligible improvement; (ii) the interface tangential stiffness primarily affects crack distribution rather than the global load-displacement response; and (iii) neglecting the coupling between normal separation and tangential slip leads to an overestimation of load-bearing capacity by up to 15.6% in bending-shear failure modes. It should be noted that the current model is limited to monotonic loading conditions and flexural-dominated structural configurations, and the identified thresholds are case-specific. This work provides a computational framework for assessing the residual mechanical performance of RC structures with degraded reinforcement.