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RESUMO Em ETEs, o ataque acido (associado ao ataque por sulfatos como consequencia secundaria) afeta a pasta de cimento formando etringita e/ou gipsita, provocando expansao, fissuracao e friabilidade do material. Este mecanismo de degradacao do concreto atinge, tambem, o C-S-H, reduzindo a capacidade portante e a vida util da estrutura. O objetivo deste trabalho e apresentar resultados de uma inspecao de uma ETE em funcionamento ha aproximadamente 15 anos, onde ha visivel degradacao do concreto por ataque combinado de acido e sulfato. A analise contempla resultados de amostra coletada em regiao imediatamente acima do nivel do efluente, onde ha maior concentracao de bacterias, e de testemunho extraido da laje de cobertura do tanque de tratamento. A resistencia a compressao do testemunho atende o valor especificado no projeto, apesar de ensaios de microscopia eletronica de varredura e difracao de raios-X identificarem a formacao de gipsita e etringita no interior do concreto. Palavras chaves: ataque acido; ataque por sulfatos; durabilidade; concreto armado. ABSTRACT At sewage treatment plants, acid attack (and sulfate attack as a secondary consequence) degrades the cement paste, forming ettringite and/or gypsum, causing expansion, cracking and brittleness. The degradation mechanism of concrete in sewage treatment plants also affects the calcium silicate hydrate (C-S-H), reducing the mechanical capacity and service life. The aim of this paper is to present results of the inspection of a sewage treatment plant, which has been operating for about 15 years, where there is noticeable degradation of concrete by a combined acid and sulfate attack. The analysis includes results of a sample collected in the region immediately above the effluent, which had the highest concentration of bacteria and a specimen extracted from the treatment tank slab. Compressive strength of the specimen is consistent with the project, although scanning electron microscopy images and X-ray diffraction identified the formation of gypsum and ettringite within the concrete. Keywords: acid attack; sulphate attack; durability; reinforced concrete.

Degradação do concreto de uma Estação de Tratamento de Esgoto (ETE) por ácido sulfúrico biogênico