Composition of alkali–silica reaction products at different locations within concrete structures

Four concrete structures from Northern Portugal–three dams and one bridge–were studied. In the three dams concrete was produced with granitic aggregates. In the bridge, aggregates from an aluvial deposit were used. Pebbles show a wide range of lithologies, though dominantly siliceous. Petrographic techniques were applied in the study of these structures in which signs of deterioration were identified in order to confirm that the deterioration was caused by alkali–silica reactions.Alkali–silica gel was found in all the structures under different forms. It was verified that the texture and composition of the gel varies according to its location inside the concrete and also when compared with extruded gel. This finding is of interest in regard to the assumption that the composition of the gel is related to its expansivity.From the analyses carried out it was verified that silicon is always the main component. In some samples, calcium exists as the second most important component, but in others calcium is hardly detected. Alkalis show a low content in all the analyses. Sometimes aluminum is also detected.An attempt is made to correlate the composition of the gel to its location in concrete. The results are discussed in relation to data published by other authors.