Mechanical degradation of naturally corroded rebars from a 45-year-old bridge slab: 3D quantification of section loss and a new corrosion damage indicator

Corrosion of reinforcing steel is a principal cause of degradation and strength loss in reinforced concrete (RC) structures, particularly those exposed to aggressive environmental conditions over extended service periods. This study investigates the residual mechanical performance of fifty-seven naturally corroded rebars extracted from a 45-year-old bridge deck. Based on chloride exposure records and damage inspection, corrosion initiation was estimated to have occurred approximately 24 years after construction. A simplified three-dimensional reconstruction method was developed to capture local cross-sectional loss and pit depth at the fracture zone. The resulting geometric indicators were then correlated with the tensile properties—yield strength, ultimate strength, and elongation—to assess the mechanical degradation due to corrosion. Among the tested parameters, a combined index incorporating maximum pit depth and residual cross-sectional area exhibited the strongest and most consistent correlation with mechanical strength, effectively reducing the data scatter typical of single-parameter models. The proposed empirical relationships provide a practical approach for estimating the residual tensile capacity of corroded reinforcement, thereby supporting quantitative durability assessment and service-life prediction of long-term RC bridge structures.