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Apports combinés de l'expérimentation et de la modélisation à la compréhension de l'alcali-réaction et de ses effets mécaniques

Abstract : A theoretical approach has been combined with an experimental one in order to better understand the mechanical behaviour of a concrete affected by Alkali-Aggregate Reaction (AAR), and more precisely by Alkali-Silica Reaction. The interest of a model, based on the mechanics of reactive porous media, first appears with the design of the experimental program, which involves more than six hundred samples. A first simple model has affected the choice of the main parameters (external water supply, temperature and stress) and also led us to consider a link between the mechanical consequences and the internal causes of AAR. Experimental data on deformations, cracking, weight variations and mechanical properties have been gathered. Furthermore, various techniques have allowed us to study the evolution of the material microstructure: scanning electron microscopy, visualization of the reaction products by fluorescence of the uranyle ion, mineralogical calculations. The main results are as follows: - The state of stress does not affect the development of the physico-chemical mechanisms involved in AAR. A method, based on 'imposed chemical deformations', may thus be proposed for calculting the structures concerned. - Osmotic pressure (or imbibition) appears to play a secondary role on the swelling owing to AAR, which results from the location of the reaction products. - The quantity of water available when the products are formed is a major factor of the swelling amplitude and kinetics. - Swelling can reach 0.1%, using only the water still available after cement hydration. - If the loss of water reaches a critical value, AAR simply stops, waiting for further water supply.- The intrinsic swelling heterogeneity was quantified; it emphasized the interest of a probabilistic framework for modelling AAR. - Monitoring the evolution of the mechanical properties showed that the compressive and splitting tensile strengths are not affected by AAR, whereas the Young's modulus decreases by about 30%, because of cracking. - The causes of the free swelling anisotropy are related to the casting direction and concrete anisotropy, regardless of AAR. - The S-shaped deformation curves are due to the increase in the porous space while concrete swells, making diffusion easier and hence accelerating the physicochemical mechanisms. - The swelling acceleration stage greatly determines the deformation kinetics of an AAR-affected concrete. External water supply and, above all, temperature have a major influence on the duration of this stage. All these results have enabled us to identify the parameters of a more advanced model describing the free swelling of a concrete similar to that of many affected structures in Northern France.
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Catherine Larive. Apports combinés de l'expérimentation et de la modélisation à la compréhension de l'alcali-réaction et de ses effets mécaniques. Mécanique []. Ecole Nationale des Ponts et Chaussées, 1997. Français. ⟨tel-00520676⟩



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