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This paper discusses the effects of including a Western Canadian subbituminous fly ash in superplasticized, air-entrained, semi-lightweight concrete. Mixtures were proportioned with 100 ± 20mm slump and 5 ± 1% air content. Cement contents ranged from 250 to 500 kg/m3 with additions of fly ash up to 50% by weight of cement.
In the freshly mixed state, the effects of the fly ash are to reduce the initial water demand, increase the dosage of air-entraining admixture required for the specified air content, and reduce the rate of slump loss after addition of superplasticizer. The dosage of superplasticizer required remains essentially constant when expressed as a percentage of the total weight of binder (cement + fly ash).
In the hardened state, the emphasis is on establishing how the proportion of fly ash to cement affects strength, unit weight, and cost. Each mixture is evaluated in terms of strength-weight ratio and cost in terms of cement equivalent in kg/m3, using the relationship between the costs of cement and fly ash locally to convert the cost of the fly ash into an equivalent cement cost. The bar chart used for this purpose readily facilitates the identification of optimal mixture proportions from the point of view of strength-weight ratio and cost. In addition, the development of strength is examined at various ages to show how the fly ash progressively asserts its effect on relationships between strength and effective W-C ratio. Finally, resistance to freezing and thawing is reported in terms of Procedure A of ASTM Test Method for Resistance of Concrete to Rapid Freezing and Thawing (C 666-84), and performance is shown to be unsatisfactory when using presaturated lightweight aggregate with the specified 14-day moist curing regime. Performance improves substantially after drying followed by further soaking simulative of the curing of lightweight concrete in structures following construction.
Professor of civil engineering, University of Calgary, Calgary, Alberta
Head, CANMET, Energy, Mines and Resources, Ottawa,
Stock #: CCA10075J