Abstract
Two high-strength concretes (HSC) only differing in their aggregates—silico-calcareous and hematite—were heated at temperatures up to 450°C (1°C/min). The evolution of their microstructural parameters—porosity, pore structure, permeability—were analysed. Both concretes showed equivalent initial microstructural characteristics. From 60°C, heating generated a large capillary porosity characterized by pore accesses around 0.1 μm. The intensity and especially the width of the porosity peaks increased with temperature. For silico-calcareous HSC, macropores-50 to 0.3 μm—were detected by MIP studies at 250°C and especially at 450°C. They were correlated to microcracks visually observed at the surface of the probes. Up to 250°C, the intrinsic permeability increased similarly for both concretes. Between 250 and 450°C, permeability remained stable for hematite HSC while, for silico-calcareous HSC, a major change was noticed. A good correlation between permeability and total water porosity was observed. At 450°C, influence of the microcracks on permeability was greater than the impact of the increase of capillary pore size. As both concretes showed similar initial microstructural features, conclusion was reached that the differential behaviour can mainly be attributed to internal thermal gradients discrepancies related to the type of aggregates: hematite allows to limit thermal gradient and thus, thermo-mechanical stresses. It was globally observed that damage due to high temperature thermal treatments was lower for hematite HSC.
Résumé
Deux bétons à hautes performances se distinguant uniquement par leurs granulats—silico-calcaires et hématites—ont subi des traitements thermiques allant jusqu'à 450°C (1°C/min). L'évolution de leurs propriétés microstructurales —porosité, structure des pores, perméabilité—a été examinée. Initialement, les caractéristiques microstructurales des deux bétons sont identiques. Dès 60°C, le chauffage provoque l'apparition d'une importante porosité capillaire dont les accès de pore se situent au voisinage de 0,1 μm. L'intensité et surtout la largeur des pics de porosité augmentent avec la température. Pour le béton silico-calcaire, des macropores—dans la gamme 50 à 0.3 μm—ont également été mis en évidence par la porosimétrie mercure à partir de 250°C et surtout à 450°C. Ceux-ci ont été corrélés à des microfissures détectées visuellement à la surface des éprouvettes. Jusqu'à 250°C, on observe pour les deux bétons une augmentation de la perméabilité au gaz. Entre 250 et 450°C, ce paramètre n'évolue pas pour le béton d'hématite alors qu'une évolution majeure est de nouveau observée pour le béton silico-calcaire. Ces évolutions sont bien corrélées à celles de la porosité totale à l'eau. En tenant compte de la similitude microstructurale initiale de la phase cimentaire des deux bétons, cette différence de comportement a été principalement attribuée à des différences de gradients thermiques internes liées au type de granulats, l'hématite permettant de limiter les gradients thermiques et par conséquent les contraintes d'origine thermo-mécaniques. Globalement, l'endommagement lié aux traitements thermiques est moins conséquent pour le béton d'hématite.
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Gallé, C., Sercombe, J. Permeability and pore structure evolution of silicocalcareous and hematite high-strength concretes submitted to high temperatures. Mat. Struct. 34, 619–628 (2001). https://doi.org/10.1007/BF02482129
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DOI: https://doi.org/10.1007/BF02482129