Lien entre la perméabilité et l'endommagement dans les bétons à haute température

Due to its technical and economical advantages, concrete is nowadays the most used building material in civil engineering. Even if its use is known since nearly two centuries, its behavior has not been yet completely explained due to the complexity of its porous microstructure. This fact is quite evident under particular conditions such as, by instance, during an elevation of temperature. This condition can mainly occur in two cases: due to a casualty (e.g. a fire) or in normal use conditions (e.g. storage of nuclear rejects). This work aims at contributing to the study of the phenomena that can be observed in concrete exposed to high temperatures and, in particular, focuses on the study of the evolution of intrinsic permeability. The characterisation of permeability (which is hardly measurable in hot conditions) is necessary for describing and modelling transport phenomena which occur in porous media. An experimental study has been made in collaboration with the CEA. A real-scale hollow cylinder has been instrumented with gauges for studying the evolution of temperature and gas pressure fields inside concrete. Later, the cylinder has been then numerically modelled by means of a thermo-hydro-chemical (THC) and a thermo-hydro-chemo-mechanical (THCM) model. The THC model, implemented by means of the finite volume method, has allowed a first, qualitative study of the behaviour of concrete submitted to high temperature. This model, which, for sake of simplicity, has neglected all mechanical effects, has allowed the description of the main phenomena occurring inside concrete: mass transport, phase changes, microstructure evolution. Later, the modelling has been completed by means of the THCM model using the Hitecosp code, implemented by means of the finite element method at the university of Padua. This code allows a very complete description of the phenomena occurring inside concrete and takes into consideration the mechanical behavior of concrete by means of an elastic constitutive law. Total damage is also taken into account. A comparison between experimental data and numerical results has allowed the proposition of a law describing the evolution of permeability as a function of total damage.Le béton est un matériau de construction qui trouve son champ d'utilisation dans pratiquement tous les domaines du génie civil. Actuellement il est sans doute le plus utilisé pour les avantages économiques et techniques qu'il présente. Cependant, bien que largement utilisé depuis presque deux siècles, il s'agit d'un matériau dont le comportement n'a pas encore été totalement compris. Sa microstructure poreuse rend son comportement difficilement prévisible, spécialement dans des conditions particulières comme, par exemple, pendant une élévation de température. Celle-ci peut se produire essentiellement dans deux cas: des conditions accidentelles (par exemple un incendie) et des conditions normales d'utilisation (par exemple un stockage de déchets nucléaires). Ce travail vise à apporter une contribution à l'étude des phénomènes mis en jeu dans une telle situation et, en particulier, vise à étudier l'évolution de la perméabilité intrinsèque en fonction de l'évolution des caractéristiques du béton. La caractérisation de la perméabilité intrinsèque (qui n'est pas facilement mesurable à chaud) est, en fait, indispensable pour décrire et modéliser de manière correcte les phénomènes de transport qui se produisent dans un milieu poreux. Une étude expérimentale en collaboration avec le CEA a été menée sur une éprouvette cylindrique à échelle réelle pour la mesure de l'évolution des températures et des pressions du gaz dans le béton. Nous avons modélisé la maquette à l'aide de deux modèles mathématiques: un modèle thermo-hydro-chimique (THC) et un modèle thermo-hydrochémo-mécanique (THCM). L'utilisation du modèle THC, implémenté à l'aide de la méthode des volumes finis, a permis une première étude, qualitative, du comportement du béton soumis à un chauffage. Cette modélisation, qui, par souci de simplicité, a négligé les effets liés à la mécanique, a permis une première description des phénomènes de transport, changement de phase, évolution de la microstructure auxquels le béton est sujet lorsqu'on élève la température. Ensuite, la modélisation a été complétée à l'aide du modèle THCM par éléments finis, implémenté dans le code de calcul Hitecosp (développé à l'université de Padoue). Ce code donne une description très complète du comportement du béton et prend en compte les effets liés à la mécanique à l'aide d'une loi de comportement du béton élastiqueendommageable. Grâce à une comparaison entre les résultats issus des analyses expérimentales et numériques, nous avons pu aboutir à la proposition d'une loi pour décrire l'évolution de la perméabilité intrinsèque en fonction de l'endommagement total

Modélisation discrète des mouvements de foule. Application à l'interaction foule-structure.

National audienceSee http://hal.archives-ouvertes.fr/docs/00/59/28/34/ANNEX/r_KWA0951Q.pd

A new crowd movement modeling for pedestrians who hold hands

International audienceThe aim of this communication is to propose a new crowd movement modeling considering a new particular subgroup behavior: pedestrians holding hands. The first step in this study is to improve our 2D crowd movement modeling [1-3] introducing the orientation of each pedestrian, rarely used in literature. The next step is to describe the particular subgroup behavior of the pedestrians who hold hands. An at-a-distance velocity of deformation allows one to control the interaction between linked pedestrians. The proposed model takes into account the effects of the subgroup as a continuous deformation within the particular subgroup of pedestrians who hold hands while existing methods in literature describe the cohesion of the "normal" subgroup with external forces. Finally, numerical simulations will be presented to study the influence of the pedestrian's orientation on his movement and to compare subgroup behavior models

FEMxDEM multi-scale modelling with second gradient regularization

The multi-scale FEMxDEM approach is an innovative numerical method for geotechnical problems, using at the same time the Finite Element Method (FEM) at the engineering macro-scale and the Discrete Element Method (DEM) at the scale of the microstructure of the material. The link between scales is made via computational homogenization. In this way, the continuum numerical constitutive law and the corresponding tangent matrix are obtained directly from the discrete response of the microstructure [1,2,3]. In the proposed paper, a variety of operators, rather than the tangent consistent for the Newton- Raphson method, is tested in a challenging attempt to improve the poor convergence performance. The independence of the DEM computations between the different elements is exploited to develop a parallelized code using an OpenMP paradigm. At the macro level, a second gradient constitutive relation is implemented in order to enrich the first gradient Cauchy relation bringing meshindependency to the model. The second gradient regularization, together with the speedup provided by the parallelization allows by first time to the FEMxDEM model to be applied to real scale problems with the desired mesh refinement. Some results are given exhibiting the above findings with emphasis on aspects related to strain localisation

A 2D discrete model for crowd-structure interaction

International audienceAn emerging problem for civil engineering in the field of dynamics is the modelling of dynamic effects due to crowd-structure interaction. We are particularly concerned with structures such as footbridges, which oscillate due to the crossing of a group of pedestrians. The objective of this study is double: to model the movement of pedestrians with consideration of pedestrian-pedestrian and pedestrian-obstacle interactions and to include a pedestrian-structure coupling in the proposed model. This 2D discrete model would be able to study the problem of synchronization between pedestrians and structure on footbridges. Our idea is to extend the modelling of particles movements to study the movements of a crowd. The non-smooth discrete model of Frémond initially proposed to simulate a granular assembly, is chosen. It applies a rigorous thermodynamic framework in which the local interactions between particles are managed by the use of dissipation pseudo-potentials. Social forces as well as a desired direction/ velocity are introduced in order to simulate the behaviour of pedestrians. Concerning the pedestrian-structure coupling, a differential equation of Kuramoto allows one to manage the evolution of the pedestrians' phase. Two cases are studied depending on the sensitivity of pedestrians to the footbridge's oscillations. Numerical simulations on the Millenium Bridge are performed and discussed

FEMxDEM Multi-scale modelling applied to geomaterials

The multi-scale FEMxDEM approach is an innovative numerical method for geotechnical problems, using at the same time the Finite Element Method (FEM) at the engineering macro-scale and the Discrete Element Method (DEM) at the scale of the microstructure of the material. The link between scales is made via computational homogenization. In this way, the continuum numerical constitutive law and the corresponding tangent matrix are obtained directly from the discrete response of the microstructure [1,2,3]. In the proposed paper, a variety of operators, rather than the tangent consistent for the Newton-Raphson method, is tested in a challenging attempt to improve the poor convergence performance. The independence of the DEM computations is exploited to develop a parallelized code using an OpenMP paradigm. At the macro level, a second gradient constitutive relation is implemented in order to enrich the first gradient Cauchy relation bringing mesh-independency to the model. The second gradient regularization, together with the speedup provided by the parallelization allows by first time to the FEMxDEM model to be applied to real scale problems. Some results are given exhibiting the above findings with emphasis on aspects related to strain localisation

Coupling by means of strong discontinuity approach between crack opening and gas permeability for concrete

Due to industrial needs, one of the key issues nowadays is to develop numerical tools that are able to predict the leakage rate through a cracked concrete structure. This paper presents a validation of a numerical modelling of leakage rate through a mortar specimen in a splitting test versus experimental results. The mechanical state of the material is described by means of an enhanced non-local damage model which takes into account the stress state and provides a realistic damage field at failure. A semi discrete method based on a Strong Discontinuity (SD) approach is then considered to study the coupling between the mechanical state of the material and its permeability. This method consists in first determining the crack opening field in the crack surface, then coupling the permeability with the crack opening by means of a modified Poiseuille's law. The assessed crack openings given by the SD method is compared to Digital Image Correlation measurements. The comparison between the two shows that the assessed crack openings given by SD are in good agreement (the maximum relative error is less than 20%). The results of the coupling compared to experimental data show a good estimation of the structural permeability for high level of cracking. Moreover, for lower levels of cracking, low differences between numerical and experimental permeabilities are observed

Outils expérimentaux et de modélisation numérique pour l'étude des transferts : du comportement des milieux poreux à celui des foules

Ce mémoire résume sept ans d'activités de recherche, depuis l'obtention de mon titre de docteur de l'Ecole Nationale des Ponts et Chaussées en 2004. Ces travaux se sont déroulés d'abord à l'ENPC et à l'Université de Padoue (Italie), ensuite au Commissariat pour l'Energie Atomique et, plus récemment, au Laboratoire Central des Ponts et Chaussées (aujourd'hui Institut Français des Sciences et Technologies des Transports de l'Aménagement et des Réseaux). Les travaux présentés dans ce mémoire couvrent plusieurs sujets relatifs aux phénomènes de transferts: on passe de la modélisation thermo-hydraulique (intégrée par les aspects mécaniques) des milieux poreux via une approche continue, à l'étude des foules humaines, où chaque individu est pris en examen via une approche discrète. Une attention particulière a été portée aux aspects numériques et chaque sujet est détaillé avec des considérations relatives à l'implémentation et aux algorithmes associés. Dans la démarche présentée, l'expérience en laboratoire joue aussi un rôle important et la symbiose qui existe entre expérience, théorie et numérique est mise en évidence

New Signage System for Improving Pedestrian Flow on Single-Exit Metro Stations: Focus on Paris Metro Line 4 Historical Stations

International audienceFor a number of years now, the use of the Metro is steadily augmenting, making it more and more difficult to ensure traffic regularity (due to a variety of technical problems, increases in exiting/boarding time and passenger accidents), which determines a loss in the system"s energy efficiency and the increase of traction current expenses. In disadvantageous conditions, such as those found in historical stations having only one access to each platform (present notably along the north-south line 4, opened roughly a century ago) and where the movements of passengers can be disorderly, it seems possible to ameliorate the quality of spaces and the overall service through well-focused low-impact improvements. In this paper, a new signage system with a view to improving flow management and providing better information to passengers is proposed. We focus on a particular moment of the voyage, namely the platform-train exchange, in the case of stations with one single exit that have, still today, a cul-de-sac at one end. Without neglecting the architectural aspects and comfort of the passengers, the objective of the proposal is controlling the regularity of traffic and optimising the resulting electricity consumption. The effectiveness of the signage system is validated via an in-house developed numerical tool allowing to explore the impact of the signage system on the modalities and times of the train-platform exchange. The analysis and comparison of several variations have upheld the validity of the improvements hypothesis in its entirety

Lien entre la perméabilité et l'endommagement dans les bétons à haute température

MARNE-LA-VALLEE-ENPC-BIBL. (774682303) / SudocSudocFranceF