Rebar corrosion and its interaction with concrete degradation in reinforced concrete sewers.

Concrete corrosion, as a major issue in sewer management, has attracted considerable research. In comparison, the corrosion of reinforcing steel bar (rebar) is not well understood. Particularly, fundamental knowledge of rebar corrosion and its interactions with concrete corrosion/cracking is largely lacking. This study investigated rebar corrosion and concrete degradation using reinforced concrete coupons exposed in a pilot sewer system. The physical-chemical corrosion characteristics were investigated in local regions; the nature of rebar rusts was analyzed using the advanced mineral analytical techniques, including Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD); further, the interactions between rebar corrosion and concrete corrosion/cracking were elucidated by characterizing the microstructure and element distribution in interfacial areas using Mineral Liberation Analysis (MLA). The rebar corrosion products were found to be iron oxides, oxyhydroxides, chlorides, sulfides and sulfates. The predominant rebar corrosion reactions varied with exposure time and the development of concrete corrosion. When concrete corrosion reached rebar surface, the cracking of the concrete cover was influenced by multiple effects, including the macro-cracking induced by the corrosion products expansion, and the micro-cracking accelerated by the dissolution, diffusion and deposition of Fe derived from rebar rusts at the concrete corrosion front. A conceptual model elucidating rebar corrosion and the complex interactions between rebar corrosion and concrete degradation is proposed to support the development of corrosion prevention and refurbishment strategies for reinforced concrete sewers.

Corrosion of reinforcing steel in concrete sewers.

Hydrogen sulfide is a controlling factor for concrete corrosion in sewers, although its impact on sewer rebar corrosion has not been investigated to date. This study determined the corrosion mechanism of rebar in sewers by elucidating the roles of chloride ions, apart from the effects of hydrogen sulfide and biogenic sulfuric acid. The nature and distribution of rusts at the steel/concrete interface were delineated using the advanced mineral analytical techniques, including mineral liberation analysis and micro X-ray diffraction which is the first-ever use in such studies. The corrosion products were found to be mainly iron oxides or oxy-hydroxides. H2S and biogenic sulfuric acid did not directly participate in the product formation of steel partly covered by concrete or directly exposed to sewer atmosphere. Instead, chloride ions played an important role in initiating steel corrosion in sewers, supported by a thin chloride-enriched layer at the steel/rust interface. Away from the chloride-enriched layer, iron oxides accumulated on both sides of the mill-scale to form a corrosion layer and corrosion-filled paste respectively. The corrosion layer around rebar circumference was non-uniform and the rust thickness with respect to polar coordinates followed a Gaussian model. These findings support predictions of sewer service lifetime and developments of corrosion prevention strategies.

The role of iron in sulfide induced corrosion of sewer concrete.

The sulfide-induced corrosion of concrete sewer is a widespread and expensive problem for water utilities worldwide. Fundamental knowledge of the initiation and propagation of sewer corrosion, especially the interactions between chemical reactions and physical structure changes, is still largely unknown. Advanced mineral analytical techniques were applied to identify the distribution of corrosion products and the micro-cracking that developed along the corrosion boundary. It was found that sewer concrete corrosion caused by reactions with sulfuric acid progressed uniformly in the cement of concrete. In contrast to conventional knowledge, iron rust rather than gypsum and ettringite was likely the factor responsible for cracking ahead of the corrosion front. The analysis also allowed quantitative determination of the major corrosion products, i.e., gypsum and ettringite, with the latter found closer to the corrosion front. The conceptual model based on these findings clearly demonstrated the complex interactions among different chemical reactions, diffusion, and micro-structure changes.