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Use of conditioned PFA as a fine aggregate component in concrete

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Abstract

This paper describes a study carried out to examine the use of conditioned, or moistened, pulverizedfuel ash (CPFA) as a component of fine aggregate in concrete,i.e. as a nearly inert Type 1 addition, as described in BS 3892: Part 2. A fine and coarse PFA were used, after storage at a moisture level of 10% for 1 and 12 months, to determine the influence of CPFA/sand combinations (with ratios up to 0.15) on concrete fresh properties and strength development. In addition, the activity of CPFA was examined using calorimetry and thermogravimetric analyses on mortar specimens. Inclusion of CPFA as a fine aggregate component in concrete led to reductions in workability and bleeding, particularly with longer-term stored material, but handling characteristics were otherwise unaffected. It was possible to overcome the workability loss, maintain low level bleeding and enhance the other fresh properties, through the use of a super-plasticizing admixture at reasonable dosage. For hardened concrete, all mixes containing CPFA/sand exhibited improved strength development, compared to reference sand mixes, with greatest benefits obtained with short-term stored material. These effects were confirmed by the hydration studies which indicated increased heat of hydration and lime consumption with CPFA. The practical implications of the study are also examined.

Résumé

Cet article décrit une étude relative à l'utilisation de cendres volantes humidifiées comme éléments fins dans le béton, c'est-àdire comme une addition pratiquement inerte de type 1, selon la norme BS 3892: Partie 2. Après stockage d'une durée de 1 ou 12 mois à un taux d'humidité de 10%, des cendres volantes fines et grossières ont été utilisées afin de déterminer l'incidence des combinaisons cendres/sable (dans des proportions allant jusqu'à 0,15) sur les caractéristiques du béton frais ainsi que sur l'évolution de sa résistance. Des études calorimétriques et des analyses thermogravimétriques effectuées sur des échantillons de mortier ont également permis d'observer l'activité des cendres.

L'emploi de ces cendres volantes comme élément fin dans le béton a conduit à une diminution de son ouvrabilité et du ressuage, en particulier pour les matériaux dont la durée de stockage est plus longue, sans modification des autres caractéristiques. Il a été possible de surmonter la perte d'ouvrabilité, de maintenir un faible niveau de ressuage et d'améliorer les autres propriétés du béton frais, grâce à l'emploi d'un superplastifiant à un dosage raisonnable. Pour le béton durci, tous les mélanges à base de cendres et de sable ont amélioré le développement de la résistance (par rapport aux mélanges de sable de référence), l'augmentation la plus importante ayant été obtenue avec les matériaux stockés durant une courte période. Ces effets ont été confirmés par les études portant sur l'hydratation indiquant que les cendres augmentaient la chalcur d'hydration et la consommation de chaux. Les conséquences pratiques de cette étude sont également abordées.

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Authors and Affiliations

  1. Cancrete Technology Unit, Department of Civil Engineering, University of Dundee, DD1 4HN, Dundee, Scotland, UK

    R. K. Dhir, M. J. McCarthy & P. A. J. Tittle

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  1. R. K. Dhir
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  2. M. J. McCarthy
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  3. P. A. J. Tittle
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Dhir, R.K., McCarthy, M.J. & Tittle, P.A.J. Use of conditioned PFA as a fine aggregate component in concrete. Mat. Struct. 33, 38–42 (2000). https://doi.org/10.1007/BF02481694

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  • DOI: https://doi.org/10.1007/BF02481694

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