Effect of steel bar configurations and corroded area evolution in reinforced concrete slabs on microcell and macrocell corrosion kinetics

Abstract

Corrosion of reinforcement embedded in concrete is regarded as one of the major factors affecting performance of reinforced concrete (RC) structures. Consequences of corrosion process include cracking/spalling of concrete cover, a reduction of the steel bar cross-section and a decrease of bond strength between the reinforcement and concrete. Corrosion rate is determined based on two mechanisms: microcell corrosion and macrocell corrosion. In the microcell mechanism, the iron oxidation and oxygen reduction reactions happen at the same place or adjacent to each other on the corroded steel bar surface, while they take place distantly in the macrocell mechanism. In RC slabs, only a limited area of steel bars is corroded as commonly observed from sites, while surrounding steel bars remain uncorroded. These uncorroded steel bars will increase the corrosion rate at corroded areas through macrocell mechanism. Depending on uncorroded/corroded steel bar area ratios, steel bar arrangements, potential difference between corroded and uncorroded steel bars and concrete resistivity, the contribution of macrocell corrosion may be up to a few hundred times larger than microcell corrosion and becomes a dominant factor. Thus, it is significant to investigate the contribution of macrocell corrosion compared to microcell one in RC slabs under different steel bar arrangements and uncorroded/corroded area ratios. In this project, four RC slabs were cast with three scenarios of corrosion evolution and two steel bar configurations. One small central top steel bar area was corroded in the first sample with uncorroded/corroded area ratio of 97, while in the second sample, steel bars in a moderate central top region were corroded and the entire top mesh of steel bars were corroded in the third sample. In addition, to investigate the effect of steel bar arrangement on macrocell contribution, the fourth sample was produced with the same corroded area as the second one but only had one steel bar mesh. Under each corrosion scenario, various electrochemical corrosion kinetics were measured such as corrosion potential, polarisation resistance, microcell and macrocell current densities. Each steel bar was cut into several 50 mm segments to determine the distribution and contribution of macrocell corrosion from different locations of uncorroded steel bars. From the test results, corrosion potential could be used to detect corrosion locations regardless of corroded areas. The distribution and contribution of macrocell corrosion in each sample were determined in 3 levels: From uncorroded segments (50 mm) in one steel bar; from steel bars in one layer; from layers in one specimen. In addition, total microcell and macrocell corrosion rates of steel bars in different samples were also analysed and compared. The effect of corroded area evolution and steel bar arrangements on corrosion kinetics was concluded.