Alkali-aggregate reaction (AAR) is a deleterious reaction that affects a large number of concrete structures, among which concrete dams, worldwide. It has been observed on dams built on each of the five continents. In France, they are about thirty affected dams, and in Cameroon AAR have been noticed on the Song Loulou hydropower dam. AAR produces concrete expansion and generally leads to a loss of both strength and stiffness due to the cracking. That creates undesirable deformations, disturbances in the equilibrium of internal forces, and affects the operability and safety of dams. In this work, we develop the real case of predicting the Song Loulou spillway's gate damage by both modeling a spillway's pier and implementing a methodology in Cast3M. The loss of functionality considered here is jamming of the spillway's gate due to the deformations induced by AAR, which is one of the most dangerous disorder impacting negatively the dam on all the other assets. First of all, we present a brief review of chemo-physical processes that control the structural behaviour of concrete dams suffering from AAR. In a second step, we make a summary of the simplified methodology for recalculating an AAR affected structure proposed by the IFSTTAR in the technical guide for internal swelling reaction affecting structures management support. It is mainly based on damage law proposed by Larive, works by Ulm and his collaborators, and model suggested by Li and Coussy. Thirdly, we indicate the collected and processed data (geometry, material properties, loads, temperature and relative humidity) for our case study. Computations performed with Cast3M predict the date of occurrence of the spillway's gate's blocking due to AAR. Although this work might be used as a maintenance management tool of the Song Loulou AAR affected hydropower dam, it would be more efficient to consider the effect of uncertainties affecting the input data and the model parameters. Probabilistic approaches, reliability analysis in particular, will be used to address this problem in a further work.