Analyse du comportement d'éléments fléchis en béton renforcé de fibres métalliques: Du matériau à la structure

RÉSUMÉ L’utilisation des bétons renforcés de fibres (BRF) à titre de renforcement structural nécessite la connaissance du comportement post-fissuration du matériau. Cette thèse passe en revue les différentes méthodes et analyses utilisées pour la détermination de la résistance à la traction des BRF. L’étude se base sur un programme expérimental comprenant cinq mélanges de BRF avec des volumes de fibres variant de 0.75% à 1.25%. Ces matériaux ont servi à fabriquer les spécimens pour les essais de traction uniaxiale, de flexion sur poutre entaillée et sur dalle circulaire. Les résultats des essais de caractérisation permettent de comparer le comportement σ−w obtenu directement à partir de l’essai de traction uniaxiale avec ceux obtenus par analyse inverse des essais de flexion. La variabilité des résultats dépend notamment de l’orientation des fibres, de la taille de la section fissurée et de la présence d’une entaille. Un modèle de calcul, basé sur des hypothèses simples et communément admises, est proposé pour l’analyse et le dimensionnement d’éléments en BRF soumis à des efforts de flexion. De manière à évaluer le modèle, une large campagne expérimentale est menée. Elle comprend des essais de flexion sur 19 poutres rectangulaires et deux poutres en Té, fabriquées avec les cinq mélanges de BRF. La contribution des fibres aux différentes phases du comportement en flexion est décrite. La capacité du modèle analytique à prédire le comportement M-wmax, en conditions limites d’utilisation et de service, est vérifiée à l’aide des résultats expérimentaux. Les écarts observés peuvent être liés à la variabilité de la résistance à la traction des BRF et à la différence d’orientation des fibres entre les poutres et les spécimens de caractérisation. Une approche de dimensionnement réaliste est finalement proposée pour des poutres en ‘T’ soumises à des moments positifs et négatifs. Par la suite, le comportement en flexion des BRF est étudié par la méthode des éléments finis. D’une part, la modélisation d’essais de flexion sur poutre entaillée et sur dalle circulaire normalisés est réalisée. La qualité des résultats comparés aux résultats expérimentaux permet de valider les hypothèses utilisées dans l’analyse inverse pour déterminer la loi contrainte-ouverture du matériau. L’effet de l’orientation des fibres, la procédure d’essai et la validité des essais normalisés sont discutés.----------ABSTRACT Utilization of steel fibre reinforced concrete (SFRC) for carrying loads in structural applications requires knowledge of the post-cracking tensile response. Furthermore, applications in loadcarrying structural members have yet to gain wide acceptance in design codes. This is partly explained by the lack of a unified design philosophy adapted to this material. This thesis reviews the experimental characterization tests and subsequent analysis commonly used for determining the post-cracking tensile properties of SFRC. The experimental program supporting this investigation comprised five different SFRC mixes with fibre volumes ranging from 0.75% to 1.25% used to fabricate a set of characterization specimens for uniaxial tensile tests, notched beam tests and round panel tests. In parallel, an extensive experimental program was conducted on large scale beams: 21 rectangular and T-beams of various sizes produced with the same material. Characterization test results allowed a comparison between direct stress-crack opening measurements and the stress-crack openings retrieved from the inverse analysis of bending tests. Result discrepancies are analyzed and related mainly to test configurations, the presence of a predefined crack, support conditions, fibre orientation, and cracked surface size. Results obtained using material characterization tests are then applied to the reproduction of the structural behaviour of large scale beams. A model based on simple and widely accepted assumptions was proposed for the analysis and the design of SFRC members subjected to bending moments. The contribution of fibres at different loading phases in bending is described in detail. The accuracy of the analytical model to predict maximum crack opening applicable in service conditions and the ultimate flexural strength are compared to experimental measurements. Discrepancies observed are related to the dispersion of the material properties and the difference of fibre orientation in beams and characterization specimens. Finally, the proposed design approach is applied to the design of a realistic T-beam subjected to positive and negative bending moments

Investigation of a district heating network expansion possibility with a 60% share of renewable energy input: A case study – Sevran district heating network in France

Climate change is making energy an important matter for scientists, politics and industries. Public concerns and energy supply limitations are changing the rules of energy markets. Fossils fuels are becoming expensive and energy policy makers encourage the development of renewable energies. Every energy sector is impacted by those changes. With a significant potential in reducing greenhouse-gas emissions and fossil fuels dependency, the heating market is moving towards greener solutions. It is within this context that Dalkia is developing district heating solutions. This French company is one of the two large actors in the heating market in France and try to keep being part of the energy sector. This thesis work was realized within Dalkia and focuses on a study case: Sevran district heating network. This network provides about 50 GWh of heat with a 60% share of renewable energy (biomass). Developing this network is one way of increasing the renewable share in France. This master thesis tackles two extension possibilities. The study case starts with drawing the state of the existing district heating network. This allows to know a consumption limit in order to keep the 60% share of renewable energy. The district heating network is then modelled with a software called Termis to know hydraulic limits. Extension projects are simulated with this same model to evaluate their technical feasibility. An economical study is finally performed. The study concludes that both extensions are technically feasible, but only one is economically relevant for Dalkia. This master thesis was also the opportunity to observe the French heating market from an industrial point of view. Sevran study case is a typical example of how district heating companies are changing considering economy, energy policies and public acceptance

Finite-element modelling of SF RC members in bending

This paper is aimed at understanding the mechanics of steel-fibre-reinforced concrete (SFRC) in the context of designing for structural applications. It focuses on the testing procedures adopted to obtain the tensile response of SFRC that are used in finite-element models of structural elements submitted to bending. Modelling of standardised material test specimens enabled validating the assumptions used in inverse analysis to determine the post-cracking σ–w response from bending tests on notched beams and round panels. The effect of fibre orientation, the testing procedure and the validity of standardised test are discussed. Modelling of SFRC structural beams of different scales, shapes, with and without conventional reinforcement, emphasises the importance of using non-uniform material properties within the model to correctly predict the member stiffness and strength, and the crack opening evolution. The paper confirmed that the integration point spacing must be used as the reference length for converting σ–w post-cracking response to σ−ε material properties for carrying out finite-element analysis. Moreover this approach is not affected by the element size and member depth. </jats:p