Abstract
The paper describes a study carried out to explore how controlled permeability formwork (CPF) can be used within existing concrete durability specifications (mix limitations) for chloride environments. Tests were carried out to consider (i) chloride diffusion rates and, under wetting and drying conditions, (ii) rates of chloride contamination build up at cover depth and (iii) reinforcement corrosion. The effects of CPF were measured against design strength, cover depth and cement type of concrete cast against ply-wood formwork (impermeable formwork—IMF). The use of CPF liner on formwork was found to significantly enhance chloride and corrosion resistance of concrete. Moreover, the results demonstrated that CPF could be used within the BS 5328 durability framework for chloride environments to allow either a 20 mm cover reduction (50 to 30 mm) at fixed design strength (40 N/mm2), or a reduction in design strength of 10 N/mm2 (50 to 40 N/mm2) at fixed cover depth (30 mm). It was additionally found for Portland cement (PC) concrete that the use of CPF gave equivalent performance to concretes containing PFA and GGBS as constituents of cement and a ternary cement comprising both materials, cast against ply-wood formwork. This suggests that the ‘trade offs’ within BS 8500 for PC/PFA and PC/GGBS cements in chloride environments, could also be permitted for CPF concrete containing PC.
Résumé
L'article décrit une étude effectuée dans le but d'explorer la façon dont le coffrage de contrôle de perméabilité (CPF) peut être utilisé dans le cadre des caractéristiques existantes de durabilité du béton (limitations de mélange) concernant des environnements de chlorure. Des essais ont été effectués pour examiner (i) les taux de diffusion de chlorure et, dans des conditions de mouillage et de séchage, (ii) les taux de contamination des ions chlorures s'accumulant à la profondeur de couverture ainsi que (iii) la corrosion de l'armature. Les effets du CPF ont été mesurés par rapport à la résistance de conception, la profondeur de couverture et le type de ciment utilisé dans le béton moulé à l'intérieur du coffrage en contre-plaqué (coffrage imperméable—FMI). L'utilisation du recouvrement du CPF sur le coffrage s'est révélée de nature à mettre en valeur de manière significative les ions chlorure et la résistance à la corrosion du béton. D'ailleurs, les résultats ont démontré que le CPF pouvait être employé dans le cadre de durabilité de la norme BS 5328 pour des environnements de chlorure pour permettre une réduction de 20 millimètres de la couverture (de 50 à 30 millimètres) à une résistance de conception fixe (40 N/mm2), ou une réduction de la résistance de conception de 10 N/mm2 (de 50 à 40 N/mm2) à une profondeur de couverture fixe (30 millimètres). On a également trouvé, pour le béton à base de ciment Portland (PC), que l'utilisation du CPF donnait des performances équivalentes à celle des bétons composés de cendres volantes (PFA) et de laitiers de hauts fourneaux granulés (GGBS) comme constituants du ciment et d'un ciment ternaire combinant les deux matériaux, moulés à l'intérieur du coffrage en contre-plaqué. Ceci suggère que les «compromis» de la norme BS 8500 concernant les ciments PC/PFA et de PC/GGBS dans des environnements de chlorure, pourraient également être autorisés pour le PC contenant le béton du CPF.
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McCarthy, M.J., Giannakou, A. & Jones, M.R. Specifying concrete for chloride environments using controlled permeability formwork. Mat. Struct. 34, 566–576 (2001). https://doi.org/10.1007/BF02482183
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DOI: https://doi.org/10.1007/BF02482183