Comportement au jeune âge et différé d'un BFUP écrouissant sous les effets thermomécaniques

This thesis is dedicated to study the early age behavior and time dependant as well as the thermo-mechanical response under restraint of a hardening Ultra High Performance Fiber Reinforced Concrete (UHPFRC). The knowledge of the UHPFRC behavior is indispensable to estimate the cracking risk in composites structures. The phenomena which develop during the hydration progress of the material as the capillary network development and the physico-mechanical properties were considered. The effect of temperature curing conditions on these phenomena was characterized. The test and predicted (issue from modeling and numerical simulations) results are presented and discussed in this report. The tested UHPFRC is characterized by high mechanical performances with a faster evolution of the Young modulus with regard to the other properties. The measured mechanical properties can be predicted very well with the CEB-FIB model. The tested UHPFRC is characterized by a low degree of hydration due to the low water-binder (w/b) ratio. This experimental result was confirmed by a numerical estimation by means of the adapted Powers and Waller models as well as a numerical estimation through the heat realized during the hydration process. A linear relationship between the mechanical performances and the degree of hydration was shown and the action of the microstructure evolution on the mechanical performances development was elucidated. It was attributed to the beneficial effect of the unhydrated particles of silica fume and cement because of the pore size refinement in the microstructure. Besides, the current UHPFRC is characterized by a fast autogenous shrinkage kinetics related to the dominant self-desiccation during the first days of the hydration. Thanks to a numerical estimation with Laplace and Kelvin laws, self desiccation measurement under various temperatures were used to distinguish and to put in evidence the effect of the temperature on the porous structure, the capillary pressures and consequently on the contraction of the solid matrix. The contribution and the beneficial effect of fibres towards the autogenous shrinkage were also showed. In fact, fibres act as internal restraint and can prevent the free matrix deformation. The high temperatures improve widely the rate of hydration at early age (7 days) and so lead to increase initially the autogenous shrinkage. In the long term, an inverse effect of the temperature was observed on the evolution of the degree of hydration and the measured physico-mechanical properties, which was attributed to the influence of the temperature on the ions haste and distribution during the hydration process. The influence of the temperature on the degree of hydration and the compressive strength was characterized and modeled and the corresponding parameters were used to estimate the energy of activation of the studied UHPFRC. The dominant role of the autogenous shrinkage at early age was clarified and the associated stresses, which develop at the early age under total restraint, were quantified by means of the TSTM. These stresses remain moderate. The UHPFRC creep potential is important. UHPFRC exhibits high creep. This was attributed to the high volume of paste (88 %), which is subjected to creep. Existing models in the literature were used; certain models predict the UHPFRC behavior in a satisfactory way and with a certain precision. One notes that the early age stress predictions are dependent on the software used and especially on the creep model. In our case, the generalized Maxwell model was used.

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