The extremely low permeability of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) associated with 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 materials can be applied on new structures, or on existing ones for rehabilitation, as thin watertight overlays in replacement of waterproofing membranes, as reinforcement layers combined with reinforcement bars, or as prefabricated elements such as kerbs. The successful rehabilitation of existing structures is a major challenge for civil engineers. When existing concrete needs to be replaced, a new composite structure formed of the new material cast on the existing substrate will result from the intervention. Both the protective function and the mechanical performance of the composite system have to be guaranteed over the planned service life. These requirements can be fulfilled by a sound understanding of the origin of deteriorations, and by proper design and application. Further, to take the full benefit of the potential of UHPFRC to rehabilitate or reinforce structures, it is often needed to get better insights of the performance of composite UHPFRC-Concrete members by means of numerical models such as finite element simulations. The numerical modelling of multiple layer systems involves comprehensive models of the time dependent behaviour of the materials at early-age and long term. UHPFRC require special consideration of their tensile hardening behaviour. This document gives the theoretical background, and provides validations of a constitutive model for UHPFRC under tensile loading. This model is then applied to the inverse analysis of test results in bending, and to the simulation of various configurations of application of UHPFRC in composite structural members.