The strengthening method of bridge deck slabs by means of an Ultra High Performance Fiber Reinforced cement-based Composite material (UHPFRC) overlay has been validated by a specific experimental program which is presented in a companion contribution. This chapter presents results of a non-linear finite element analysis of the structural and cracking behavior of composite UHPFRC/concrete slabs subjected to loading similar to loading of real bridge decks. The deck slab of the Chillon Viaduct located on the Swiss National Highway on the East end of the Lake of Geneva, was strengthened by adding, on the previously hydro-jetted top surfaces of deck slabs, a 40 to 50 mm thick layer of UHPFRC which is additionally reinforced by steel rebars. Finite volume elements with 20 nodes were employed to represent the geometry of the structure, consisting of Reinforced Concrete (RC) monolithic slab elements and composite R-UHPFRC—RC slab elements, steel loading platesand supporting beams.