Comportement vieillissant du béton en fluage : application au béton VeRCoRs

Abstract : Concrete is a material presenting basic creep (delayed strain evolving over time, under constant mechanical load, constant temperature and sealed conditions with respect to the humidity). This creep behaviour is also ageing (dependence of the evolution of the strain on the instant of application of the constant load, which does not correspond to a simple translation in time). This behaviour can be attributed to the "living" character of the material, whose microstructure evolves significantly over time at different scales. The concrete creep is critical for the prestressed containment building of nuclear power plants, since creep yields prestress relaxation. Obviously, the creep response measured on a test at less than 1 year cannot be directly transposed to a structural calculation for a load at 3 years. It is also excluded to wait several years to perform a creep test on a specimen. In order to connect these two strain responses, obtained by loading at two different instants, a material behaviour model integrating the ageing creep is necessary. To develop a model of material behaviour incorporating the ageing creep, given the multi-scale and multi-physics nature of concrete, creep experiments combined with the micro-mechanical modelling approach are attractive and are applied to the study of the VeRCoRs concrete, which is a typical mix design for the construction of the containment building. In fact, several orders of magnitude of dimensions separate the scale of physical mechanisms from the macroscopic scale. The validation will be done with regard to experiments on the VeRCoRs concrete. These micromechanical approaches are the subject of a research action in collaboration with EDF, ENPC and CEA Saclay. Homogenization of random media is a widely used practical and efficient tool to estimate the effective mechanical behavior of composite materials. However, when the microstructure evolves with respect to time, due to phase transformations, care should be taken when upscaling behaviors that themselves involve time, such as viscoelasticity. The improved approach in this thesis is able to overcome this limitation, building an equivalent composite replacing transforming phases by fictitious ageing viscoelastic phases to carry out the upscaling process. Applications to cement paste, to mortar and to the VeRCoRs concrete are proposed, referring to a simplified morphological model. Then it extends the approach developed at EDF to evolving morphological models of cement paste. Besides, we have generated a 3D numerical polycrystalline microstructure and compared the finite element simulation results with the analytical ones in the case of ageing viscoelasticity. Finally, the experimental campaign is carried out to characterize the ageing basic creep at different scales and at different ages of loading with the formulation of the VeRCoRs concrete in two ways : on concrete, complete the experimental program at different ages of loading; apply behaviour at lower scales (mortar and cement paste).These tests are carried out in the civil engineering laboratory in EDF les Renardières. A direct confrontation between the experimental results and the modelling results is performed. The ageing basic creep behaviour is compared with the application of mean-field homogenisation schemes
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Shun Huang. Comportement vieillissant du béton en fluage : application au béton VeRCoRs. Matériaux. Université Paris-Est, 2018. Français. ⟨NNT : 2018PESC1052⟩. ⟨tel-02085828⟩

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