Large constructions, such as concrete dams, commonly use aggregates extracted from rock masses near the implantation sites, which may contain sulfide minerals in its composition. These aggregates, inside the concrete, may oxidize, forming sulfate ions (SO 2À 4 ), which can cause the internal sulfate attack. Therefore, in order to study aggregates for concrete dosages of these structures, aimed to evaluate on this paper the oxidation process of aggregate containing sulfide minerals in amounts of up to 5.0 %, by way of studies into the effects of particle size and pH of the medium (Study 1) and the pyrite content (Study 2). In Study 1, diabase aggregates containing pyrite (FeS 2 ) were separated into three particle sizes (sand, coarse aggregate 0 and coarse aggregate 2). Samples were analyzed in two groups and immersed in solutions of distilled water and calcium hydroxide, with pH and electric potential measurements. In Study 2, diabase sand from the same origin of Study 1, but in smaller particle size, and natural aggregates (sand) with the addition of pyrite in four levels (0, 0.5, 1.0 and 5.0 %) were immersed in distilled water, with measurements of pH, electric conductivity and total dissolved solids. The results of the tests, under constant oxygenation, showed that the reduced grain size and alkaline medium accelerate the oxidation reactions, in contrast to aggregates immersed in neutral solutions.Comparing the results by ANOVA test indicates the presence of around 0.5 % of sulfide mineral in the diabase aggregates, with better results from monitoring of electric conductivity and dissolved solids in contrast to pH readings.
Resumo: A presença de íons sulfatos ( 4 2− ) no interior da matriz cimentícia de concretos pode dar origem a um tipo de ataque interno, resultando na sua deterioração ao longo do tempo. Estes íons podem reagir com os álcalis do cimento, formando compostos como a gipsita, etringita e outros sais de sulfato, com características expansivas. Este ataque pode provocar novas reações químicas, alterando as propriedades físicas e mecânicas das estruturas de concreto. Assim, o presente trabalho teve como objetivo avaliar o comportamento físico e mecânico de testemunhos de concreto compactado a rolo (CCR), extraídos de uma barragem de usina hidrelétrica com sinais de manifestação patológica de ataque interno por sulfatos. Após a inspeção visual das estruturas, foram selecionados os locais para a extração dos testemunhos e a realização dos ensaios de resistividade elétrica e resistência à compressão axial. Resultados diferentes foram obtidos no bloco 22, com baixos valores de resistividade elétrica e altos valores de resistência à compressão axial, apresentando picos de thenardita e gipsita pela análise por DRX. A correlação linear R² de 0,85 entre os ensaios demonstrou a sua efetividade na detecção do ataque interno por sulfatos em testemunhos de concreto. Esse trabalho foi desenvolvido no âmbito do Programa de P&D ANEEL, projeto 6491-0301/2013. Palavras-chave: ataque interno por sulfatos, ensaios destrutivos, ensaios não destrutivos, concreto compactado a rolo (CCR)Abstract: The presence of sulfate ions ( 4 2− ) inside the cementitious matrix of concrete can originate a type of internal attack, resulting in its deterioration over time. These ions can react with alkalis of the cement, forming compounds such as gypsum, ettringite and other sulfate salts, with expansive characteristics. This attack can cause new chemical reactions, changing the physical and mechanical properties of concrete structures. Thus, the present work had the objective to evaluate the physical and mechanical behavior of rolled compacted concrete (RCC) cores, extracted from a hydroelectric power plant dam with signs of pathological manifestation of internal sulfate attack. After visual inspection of the structures, there were selected the sites for the extraction of the cores and development the tests of electrical resistivity and axial compressive strength. Different results were obtained in block 22, with low values for electrical resistivity and high values for axial compressive strength, showing thenardite and gypsum peaks by XRD analysis. The linear correlation R² of 0,85 between the tests showed its effectiveness in the detection of the internal sulfate attack in concrete cores. This work was developed under the ANEEL P&D Program, project 6491-0301/2013.
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