Loi constitutive chimioplastique pour le beton expose aux hautes temperatures

Resume Le beton est le materiau le plus repandu dans le monde de la construction et ceci depuis des siecles. Neanmoins, il reste encore enigmatique pour les ingenieurs qui essayent de comprendre et de simuler son comportement mecanique sous differents environnements. A la lumiere des recents evenements, incluant les incidents industrielles et pyromanes, une attention speciale a ete focalisee sur la performance du beton soumis a des hautes temperatures. Succinctement, l'exposition du beton a des hautes temperatures reduit considerablement ses proprietes mecaniques, ce qui endommage significativement l'integrite structurelle ainsi que la capacite portante d'une structure. L'ecaillage du beton reste l'un des principaux problemes a resoudre dans le cas des incendies dans les bâtiments et les tunnels. Une modelisation reussie de ce phenomene depend non seulement de la precision du profile de la temperature a travers la structure en beton, mais egalement de sa reponse mecanique vis-a-vis d'un chauffage rapide, des conditions aux limites et de la migration de l'humidite associee aux gradients de pressions interstitielles. Par consequent, il est necessaire de mettre au point des formulations mathematiques fiables pour simuler le comportement du beton pendant et apres son exposition a des temperatures elevees. Il est egalement necessaire de bien evaluer les effets de la degradation thermique afin de developper des outils de prevision et de valider les codes de calcul. De nombreux problemes structurels peuvent etre formules de facon adequate par un modele elasto-plastique. Le but ultime de cette etude est le developpement d'un nouveau modele constitutif dans un cadre chimio-plastique. Pour se faire, un programme experimental a ete lance. Le but de ce programme est double. D'abord, il est primordial pour la calibration de la loi de comportement proposee dans le cadre de cette these et, secundo, pour la definition d'un probleme inverse. Des essais usuels et des essais triaxiaux entre 1,3 et 24 MPa de confinement a des temperatures allant jusqu'a 700oC permettent d'identifier les parametres d'un modele de comportement de type elasto-plastique. Celui-ci reproduit la diminution du domaine de resistance suite a une degradation d'origine exothermique. Ce programme experimental a aussi mis en evidence le caractere fragile du beton subissant un traitement thermique et la non-applicabilite de deux criteres de ruptures souvent utilises dans le calcul d'ingenieur. Une alternative est proposee et testee. En effet, exposer le beton aux hautes temperatures resulte une perte irreversible de la raideur ainsi qu'a une perte de la resistance due a la decohesion. Ces pertes s'expriment, generalement, via des relations semi-empiriques des proprietes mecaniques fonction de la temperature. Or, ces relations sont inadequates puisque l'impact direct de cette degradation, a l'echelle macroscopique, peut se traduire par une relation de dependance entre les proprietes elastiques et la masse des hydrates. Dans cet etat de fait, contrairement aux methodes traditionnelles qui utilisent les modeles conventionnels de l'elasto-plasticite et en faisant varier certains parametres en fonction----------Abstract Concrete is the most widely used construction material in the world. Even though it has been used for several centuries, its behavior to high temperature remains to be understood. In the light of recent extreme events, including accidents, and arson, special attention has been focused on the performance of concrete in the fire safety assessment of buildings and tunnels. Fire represents one of the most severe conditions encountered during the life-time of a structure. Concrete exposed to high temperature can significantly jeopardize the structural integrity and load bearing capacity of the structure. Spalling of concrete remains one of the main issues to be addressed in the case of fire in buildings and tunnels. Successful modeling of this phenomenon depends not only on the accurate prediction of the temperature distribution through structural concrete but also on its mechanical response to the heating and boundaries restrains conditions and the migration of moisture and associated pore pressures. Therefore, it is necessary to develop a reliable formulation of concrete with all required information to understand its behavior during and after exposure to elevated temperature. It is also necessary to properly assess the effects of thermal degradation in order to develop predictive tools and validate design codes. Many structural problems can be adequately worthy by an elastoplastic model. The ultimate goal of this study is the development of a new constitutive model under a chemoplastic framework. To do this, an experimental program is carried out. The purpose of this program is twofold. First, it is essential to calibrate the proposed constitutive law that will be developed, and, second, for defining an inverse a problem. Usually, uniaxial and triaxial tests, conducted with confining pressure varied between 1.3 and 24 MPa and a temperature up to 700oC, allow us to identify the constitutive law parameters. This law reproduces the reduced field strength due to degradation of exothermic origin. This experimental program puts emphasis on the fragile nature of the preheated concrete and demonstrates the non-applicability of two failure criteria often used in engineering calculation. An alternative is proposed and well-tested. Indeed, exposing the concrete to high temperature results in irreversible loss of stiffness as well as a loss of decohesion strength. These losses are, typically, expressed through semi-empirical relationships of the mechanical properties with temperature. Unfortunately, these relationships are inadequate because the direct impact of this degradation, on the macroscopic scale, can result in a dependency relationship between the elastic properties and the hydrates mass. Therefore, unlike traditional methods using conventional elasto-plastic models and adjusting certain parameters with local temperature, the proposed constitutive law that incorporates a function of dehydration similar to the softening index in chemo-plastics gives good results.

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