Search

With the increase of sewage discharge, concrete sewage pipelines with small diameters are always under a fully flow-through operation stage and consequently suffer severe corrosion. To understand the deterioration process, concrete specimens were submerged in an accelerated sewage environment. The experimental results indicated that the main cause of the submerged concrete deterioration was corrosive media produced by a large number of acid-producing bacteria (APB) and sulfate-reducing bacteria (SRB) in the anaerobic environment of sewage. The corrosion process resulted in the increase of the Ca2+ concentration in the environment; the decomposition of hydration products; and the formation of amorphous silica gel, CaCO3 , and AFt. The deterioration evolution of concrete in sewage was a combined effect of dissolution, decomposition, and expansion. After immersion in sewage for 6 months, the surface pH of concrete decreased from an initial 12.6 to 5.0, and the mass loss of concrete was as great as 15.9%. The corroded concrete can be classified into four zones from the surface to the innermost layer. The outmost layer was a strongly deteriorated zone, which was white and porous, and was composed of calcium carbonate, gypsum, and amorphous silica gel. The inner adjacent layer was a separation zone, which was yellow, and had microcracks due to the accumulation of expansive AFt crystals. Next to this zone was a white visible transition zone, which developed across the cracks and was dominated by CaCO3 and AFt crystals. The innermost layer, called the inapparent transition zone, almost had no visible sign of deterioration; however, some decomposition and dissolution of hydration products also occurred, except calcium silicate hydrate. [ABSTRACT FROM AUTHOR]

• 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. [ABSTRACT FROM AUTHOR]