Corrosion in concrete structures: case study of an existing I-girder bridge

The ageing infrastructure of bridges in Karachi, Pakistan, has become a pressing concern due to the combined effects of harsh environmental conditions and increased traffic loads. The focus of this study was on assessing the performance of existing bridges, with specific emphasis on bridge bents suffering from atmospheric corrosion in reinforcement. Through numerical analyses, fragility functions and incremental dynamic analysis curves, the research demonstrates that as material degradation from corrosion increases, the base shear capacity, energy dissipation and ductility of structures decrease significantly. Corroded bents exhibit reduced seismic capacity, with a marked increase in the likelihood of collapse under inelastic deformations. Five damage states, ranging from minor to critical, were identified and it was observed that corroded materials have a higher probability of reaching severe damage states at lower displacement levels. The results revealed a significant loss in bridge capacity or a backward shift in fragility functions with an increasing level of corrosion. In the transverse direction, the backward shift in fragility curves was found to be 10–30%, whereas in the longitudinal direction it was around 25–30% from slight damage to the collapse limit state. These findings highlight the urgent need for corrosion mitigation strategies to maintain the structural integrity of corroded bridge infrastructure.