Corrosion assessment of reinforced concrete structures exposed to chloride environments in underground tunnels: Theoretical insights and practical data interpretations

Field investigations were conducted to assess the corrosion status of reinforcements in the underground metro tunnels built between 1980 and 2006. The carbonation depth, half-cell potential, and concrete resistivity were measured on-site, and the chloride profile of cored concrete specimens was analyzed in the laboratory. A theoretical analysis was conducted based on Nernst equation and Butler-Volmer equation. The nature of corrosion potential (half-cell potential) and its relationship with corrosion rate in different corrosion status were revealed. The impact of galvanic coupling on localized corrosion and macrocell corrosion was also analyzed. Connecting the theoretical analysis with the test results, the drop of the half-cell potential that occurred near the NaCl crystallization on some tunnel segments can serve as an indicator of chloride-induced corrosion. However, for the segments showing damp patches and relatively low resistivity, it will be challenging to assess the corrosion status based simply on the half-cell potential due to the impact of galvanic coupling. The corrosion assessment in similar environments should be conducted carefully with the determination of corrosion sources and considering the impact of concrete mix proportion and galvanic coupling.