The addition of a layer of strain-hardening UHPFRC to an existing member in reinforced concrete (RC) has proven to be an effective method to increase the structural resistance and durability of concrete structures [1-3]. This paper is intended for structural engineers willing to use UHPFRC to strengthen existing structures. The basic idea of the UHPFRC strengthening method is introduced. The conceptual designs for the improvement of two large highway viaducts are presented and their feasibility is validated by means of simple analytical formulas.