Role of granitic aggregates in the deterioration of a concrete dam

A concrete dam was built on the early 1970s in the Douro River about 70 km from the estuary in the Atlantic Ocean, at Porto city. According to other studies developed in Portugal and some reports in the literature, granitic aggregates can be potentially reactive in concrete. The study of the concrete was, therefore, focused on the assessment of the deterioration of concrete by alkali-silica reactions and also on the influence of the conditions of exposure on concrete deterioration processes. The site inspection permitted the identification of signs of deterioration in those structural elements of the dam which were more exposed to the environmental conditions, but not on the dam body. Concrete cores were obtained by drilling and petrographic analysis was performed in order to identify possible products resultant from internal reactions. Products of alkali-silica reaction and sulfate attack were detected on thin-sections of the concrete collected from the thinner structural elements exposed to the external environmental conditions. Their qualitative composition was assessed under a scanning electron microscope and by energy dispersive spectrometry. The study confirmed the occurrence of alkali-silica reactions with granitic aggregates in which very scarce microcrystalline silica was identified. The deterioration was influenced by the environmental conditions of exposition.

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