Analyse des mécanismes d'écaillage du béton soumis à des températures élevées

Abstract : The concrete is a material which behavior was not well understood especially in spectacular accidental situations: tunnels _res. Many researches were thus undertaken to understand its behavior at high temperatures (up to 800°C) especially that there were three tunnels fires in Europe during the ten last years causing a spalling of concrete structures. This work aims to contribute in the comprehension of the mechanisms of this spalling in the heated concrete. In a first approach, we are interested to study the hydraulic spalling caused by the fluids pressures in the concrete considered as a porous medium. This study, based on a non deformable thermo-hydro-chemical model, focuses more particularly on the formation of a "plug " (zone quasi-saturated with liquid water) which is at the origin of the significant increase of pressures of gas which is "blocked". A simplified analytical approach, which is justified by numerical simulations of the complete model, made it possible to highlight the existence of a "saturation" temperature which can inform us about the "plug" depth raising then the depth of the hydraulic spalling localization. In a second approach based on a thermo-chimio-plasticity model, we seek to study the mechanical spalling. The concrete, considered this time as a continuous medium, is subjected to compressive stresses caused by restrained thermal dilatation. Its rigidity and its strength are also reduced under the chemical effect of CSH dehydration at high temperatures. The zone close to heated surface reaches then the plastic phase. An analytical simplified model, justified by numerical simulations of the complete model was useful to understand this mechanical spalling and to highlight the existence of a "plasticity" temperature which can inform us about the depth of the plastic zone raising then the depth of the mechanical spalling localization. Finally, a thermo-poro-elastic model which takes account of heat and mass transfers as well as concrete deformability made it possible to define two effective stresses: a tensile stress exerted by the fluids on the skeleton which characterizes then the hydraulic spalling and a compressive stress consequence of restrained thermal dilatation characterizing then the mechanical spalling. A plasticity criterion was used to see the contribution of each stress to reach the plastic phase. This can inform us about the "weight" of each one of these two mechanisms (hydraulic or mechanical) in the spalling process.
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Yahia Msaad. Analyse des mécanismes d'écaillage du béton soumis à des températures élevées. Sciences de l'ingénieur [physics]. Ecole des Ponts ParisTech, 2005. Français. ⟨NNT : 2005ENPC0029⟩. ⟨pastel-00001794⟩

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