The extremely low permeability of Ultra High Performance Fibre Reinforced Concretes (UHPFRC) associated to their outstanding mechanical properties make them especially suitable to locally "harden" reinforced concrete structures in critical zones subjected to an aggressive environment and to significant mechanical stresses. UHPFRC provide a unique and robust solution to simplify the construction process, dramatically reduce the duration of sites, and save money thanks to a long term durability. UHPFRC can be applied on existing or new structures as thin watertight overlays, replacing waterproofing membranes, or as protective layers on walls. They can also be combined to reinforcement bars to improve the stiffness and load-carrying capacity, or used as prefabricated elements such as kerbs. Composite UHPFRC-concrete structures promise a long-term durability which helps to avoid multiple interventions during service life. The project SAMARIS (Sustainable and Advanced MAterials for Road InfraStructures) of the European Community (5th Framework program) dedicated a major effort to demonstrate the applicability of UHPFRC for the rehabilitation and improvement of structures. In this context an extensive research and development program was conducted including laboratory tests as well as full-scale pilot tests of application on sites. The following paper reports on the major results achieved in this context with a focus on two main aspects: (1) the underlying more fundamental aspects related to the potential of UHPFRC in the tensile hardening domain, (2) the experiences gathered during first full scale tests of applications, in Switzerland. In conclusion, an insight will be given on ongoing or foreseen applications