Identification of acid attack on concrete of a sewage system

A sewage system, 300 km long, showed superficial deterioration of concrete just 2 years after construction. In order to re-habilitate the structure, it was necessary to identify the main mechanism of deterioration and to understand the heterogeneous distribution of the damage observed. The study was performed in a three stepped program: site investigation and sampling, laboratory tests and concrete petrography. During the site inspection it was recognized that there was dissolution of the concrete in some sectors of the structure, with the aggregate particles protruding in relation to the undulated surface. In some places a white to yellowish putty-like product could be excavated by hand. The main deterioration was observed above the water level. The composition of the atmosphere inside the sewer was assessed and a high content of hydrogen sulfide was detected. Sampling was performed in different structural elements which showed diverse exposition to the aggressive environment. Impregnated thin sections of concrete with fluorescent yellow dye were analyzed by optical microscope. Concrete petrography showed to be crucial for the diagnoses. The study showed that the putty-like product was composed by gypsum with small residual particles of siliceous sand which resisted to the acid attack. SEM/EDS was used to evaluate the content of sulfur in different sections of the concrete cores and also to characterize the putty-like product at the surface of the concrete. This study led to the confirmation of the presence of an extensive sulfuric acid attack with rather minor sulfate attack within the concrete due to the exposition to aggressive environment. It also showed that behind the superficial deteriorated level, the concrete was sound with no signs of internal deleterious reactions. Ammonium content in residual water might have also contributed to the superficial deterioration of the concrete sewer.