Deterioration of Fully Flow-Through Concrete Sewers Subjected to an Accelerated Sewage Environment

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 ${\mathrm{Ca}}^{2+}$ concentration in the environment; the decomposition of hydration products; and the formation of amorphous silica gel, ${\mathrm{CaCO}}_3$, 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 ${\mathrm{CaCO}}_3$ 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.