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. Composite UHPFRC-concrete structures promise a long-term durability which helps avoid multiple interventions on structures during their service life. Temperature Stress Testing Machines (TSTM) are very well suited to "experimentally simulate" the conditions encountered in composite structures in a new layer, at early age, subjected to restrained shrinkage under complex environmental conditions. They can also be used to determine intrinsic properties such as free shrinkage and creep response under well defined conditions of restraint and temperature. A new TSTM testing setup has been used to characterize the response of two types of strain hardening UHPFRC, with different binders (CEM I and CEM III), under various thermo mechanical conditions. Isotherm tests as well as test under realistic temperature conditions have been performed. Current numerical models have been applied to simulate the tests and the range of their applicability has been determined for temperatures between 10 and 30 °C. Moreover, the significant influence of non linearity in the creep response of UHPFRC, at early age, under moderate to high tensile load levels has been demonstrated. The test results showed a significant creep potential due to the high volume of paste. This beneficial effect was reflected by the linearly increasing -relationship between tensile creep and shrinkage. As expected, UHPFRC tensile creep behaviour was also sensitive to the loading level. Above 35 % of the tensile strength at the loading age, the material exhibited a non linear behaviour. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep and confirmed the observed non-linear response.