Effect of corrosion layer on the deterioration of concrete in gravity sewers

•Mass loss of concrete exposed to sewage flushing started early within one month.•Concrete at the condition of a low flushing rate deteriorated greatly after 2–3 months.•Corrosion layer formed in the sewage was more porous and had little protective effect.•Sewage flushing could alleviate the microbial induced concrete deterioration.•The designed flow velocity of sewage should not be lower than 1.0 m/s. Fully filled sewers are always subjected to a continuous sewage flushing state, and the corrosion layer of concrete structure formed and its influence on the deterioration would be different from that of partially filled sewers. To distinguish the differences between the deterioration of concrete below and above sewage level in sewers, performance of concrete and the microstructure of the corrosion layer exposed to the mimicked sewage and sulfuric acid flushing at various flow velocities were investigated in the laboratory. The experimental results revealed that the mass loss of concrete exposed to the sewage flushing started early within one month, and it increased with the flow velocity and flushing duration. Although the mass loss of concrete was not obvious at a relatively low flushing rate in the early period, it was considerably increased with serious deterioration after 2–3 months. Moreover, subjected to the mimicked sewage condition, the corrosion layer formed on the surface of specimens was more porous and easily to be washed away, and no clear corrosion front can be found. Such corrosion layer has little protective effect on the concrete as compared with the exposure to the sulfuric acid flushing. It was also unraveled that the sewage flushing at a high flow velocity in sewers can remove the alkaline surface and lead to the formation of a dense biofilm, and both of these behaviors could alleviate the microbial induced concrete deterioration. Referring to the results in the study, the designed flow velocity of sewage is suggested not to be lower than 1.0 m/s in the long run.