Biogenic sulfur attack in a reinforced concrete sewage treatment plant. Re-visited mechanism and rehabilitation proposal

Abstract

Sewage treatment plants are examples of structures subjected to highly aggressive industrial environments. Given this, the objective of the present work is to evaluate the degree of degradation of the concrete structure of a sewage treatment plant located in Curitiba, Brazil. This plant was exposed to an environment within the process of wastewater treatment for nearly 20 years. Samples were collected from four slabs and four walls of the drainage channels of the sewage treatment plant reactor. Tests of the corrosion potential of the reinforcement, depth of neutralization, SEM/EDS, XRD, optical microscopy and uniaxial compressive strength were performed. Several exposed and heavily corroded reinforcement extensions were identified. The pH inside the slabs was found to be alkaline (over 10). With the analysis of SEM/EDS and XRD, it was possible to identify the predominance of gypsum in samples of the external part of the core and surface of the channel walls. The mechanical properties were preserved, with compressive strength even greater than that observed in the last inspection, which was 6 years ago. Finally, the results indicate that the attack on concrete initially occurred on the surface, advancing in depth over time. This attack was caused by the decalcification of C-S-H and resulted in the formation of gypsum as the final product of the degradation mechanism.