Expansions with different origins in a concrete dam with bridge over spillway

•Comprehensive diagnosis of concrete dam reveals two different expansive reactions.•High temperatures during construction led to delayed ettringite formation.•Model based on finite differences to simulate heat transfer phenomena is developed.•Thermal modelling of construction process confirms concrete temperatures over 70 °C.•Microstructural analysis reveals presence of products of alkali silica reaction. Concrete dams with expansions from different origins are rare and scarcely documented in the literature, therefore their study gains special interest. This study presents the case of a 62-year-old concrete dam with signs of deterioration due to expansive reactions. Several tests performed on the samples extracted from the dam such as X-ray diffraction, scanning electron microscope with energy dispersive spectroscopy mode and petrography suggest that the expansions are due to a delayed ettringite formation and a subsequent alkali-silica reaction. Thermal and mechanical models developed in the study support this diagnosis.