Hydroelectric dams suffer from Alkali-Silica Reaction (ASR), a concrete damage mechanism causing structure expansion and cracking. ASR involves a wide range of length and time scales: chemical reactions at the atomic scale, up to the degradation of the structure, which can exceed 200 meters in height. According to the Swiss Committee on Dams [1], the need for representative numerical models is crucial to understand and predict the deterioration of dams. Current multiscale numerical models of dams under ASR are limited to 2D due to the computational cost of conventional methods such as Finite Elements (FE) [2, 3]. However, 3D modeling is crucial for crack branching, coalescence and percolation phenomena [2]. In this work, we propose a multiscale approach that combines an FFT-based method for efficient 3D crack simulation at the mesoscopic scale and FE modeling at the dam scale. At the mesoscopic scale, the concrete microstructure is modeled by two phases: the aggregates and the cement paste. Expanding ASR gel pockets are included in the aggregates and a damage model is used to account for crack initiation and propagation. Different periodic representative volume elements are subjected to average strains, each corresponding to an element of the macroscopic mesh. At the macroscopic scale, the weight of the structure and the hydraulic pressure constitute the loading. The end goal is to compute the homogenized stiffness of the structure and the global displacements at the dam crest, where experimental field data are available. In order to validate this FE-FFT approach, the proposed model is compared with the existing 2D FE² model developed by Gallyamov et al. [3] and with experimental data from the Illsee dam in Switzerland. Preliminary results in 3D are discussed along with numerical efficiency and effects of ringing artifacts. The model shows anisotropic softening and expansion of the structure. [1] Swiss Committee on Dams. Concrete Swelling of Dams in Switzerland. 2017. [2] A. I. Cuba-Ramos. Multi-Scale Modeling of the Alkali-Silica Reaction in Concrete. EPFL, 2017. [3] E. R. Gallyamov, A. I. Cuba Ramos, M. Corrado, R. Rezakhani, and J. -F. Molinari. Multi-Scale Modelling of Concrete Structures Affected by Alkali-Silica Reaction: Coupling the Mesoscopic Damage Evolution and the Macroscopic Concrete Deterioration. International Journal of Solids and Structures 207: 262–78, 2020.