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- Resumen es exacto "Nowadays, concrete is considered one of the most used materials in the civil construction field due to some advantages, mainly related to its resistance, durability, versatility and economy. Generally, concrete is combined with steel reinforcements to improve its mechanical behavior in tensile and shear regimes. One of the most severe actions on concrete is the long term exposure to high temperature and fire. When it is exposed to temperatures above 200_C, concrete experiences a strong degradation of its mechanical properties such as cohesion, friction, stiffness and strength, leading to significant changes of its failure mechanisms with potential irreversible damages and sudden collapses of the affected structures. In the last three decades, a significant progress was made in the experimental analysis and understanding of concrete mechanical behavior after long term exposure to high temperatures. However, there is still a need of more accurate constitutive theories to predict the complex mechanical response behavior of concrete under high temperature. There is a demand for models that consider all different relevant aspects involved, such as the dehydration, porosity, confinement, decohesion, etc, in the framework of thermodynamical consistency and material objectivity. In this thesis, a thermodynamically consistent gradient poroplastic model for concrete subjected to high temperatures is proposed. A particular and simple form of gradient-based poroplasticity is considered, where the state variables are the only ones of non-local character. The degradations of these variables due to coupled thermomechanical effects are described in the framework of the thermodynamic approach.
After describing the material formulation, the model calibration is performed with experimental data taken from literature.
The conditions for localized failure in the form of discontinuous bifurcation is evaluated for the particular constitutive equations of the proposed model. Thereby, the effects of the temperature on the performance of the localized indicator are evaluated and discussed. After the theoretical framework is proposed, a comprehensive numerical analysis is presented which demonstrates the ability of the model to capture the variation of discontinuity surfaces and the critical failure modes for different stress paths and thermal conditions. The results widely described in this thesis will contribute to improve the knowledge in the formulation of constitutive theories for porous material like concrete, subjected to high temperatures and, also, to better understand the failure behavior of concrete structures under high temperature effects. Finally, it is discussed the possibility to employ the proposed model to evaluate the residual lifetime of a concrete structure after a fire. "
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