Fly Ash as the ‘Fourth Ingredient’ in Concrete Mixtures


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Title: Fly Ash as the ‘Fourth Ingredient’ in Concrete Mixtures

Author(s): M. R. H. Dunstan

Publication: Special Publication

Volume: 91


Appears on pages(s): 171-200

Keywords: admixtures; compressive strength; concretes; fly ash; mix proportioning; water-cement ratio; workability.

Date: 2/1/1986

FlY ash is usually considered to be a partial replacement of the portland cement in concrete mixtures. This paper presents a new approach to the selection of the mixture proportions of concrete in which fly ash is considered to be the "fourth ingredient*, that is in addition to the portland cement, the aggregate and water. This method enables fly ash to be used more efficiently and generally in greater quantities. High fly ash content concrete (HFCC), as the material has become known, was originally developed as a roller-compacted concrete for dam construction. The uses for the material have now been extended into road construction, both as paver-laid bases and also as pavement-quality concrete in the surface. Pumped (and skipped) structural placements have also been completed including a post-tensioned glued segmental viaduct. In all cases the concrete has performed well both in the fresh and hardened states. The in-situ strength of high fly ash content concrete has been found to be higher than the equivalent strength of conventional portland-cement concretes of equal workability and 28-day compressive strength. The paper traces the development of high flyash content concrete and shows how flyash can contribute as much, if not more, to the strength of a concrete as the same volume of portland cement.