Consistency of Performance of Concretes With and Without Fly Ash
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CONSISTENCY OF PERFORMANCE OF CONCRETES WITH AND WITHOUT FLY ASH WESTON T. HESTER University of California, Department of Civil Engineering, Berkeley, California 94720 Received 9 November,
1986; refereed
ABSTRACT Fly ash and other pozzolans are increasingly used in concretes that must consistently achieve high compressive strength and other measures of performance. In contemporary design and construction practice, however, there is some concern as to whether or not use of commercial fly ash materials contribute to or reduce variations observed in field compressive strengths and properties of the fresh concrete. Regardless of its potential for improving the performance of the concrete, if the daily ongoing use of commercial fly ash will decrease the consistency of the concrete produced, the use of fly ash will be curtailed. In this paper, the experiences of geographically diverse, specific concrete suppliers using their respective sources of fly ash are summarized in detail. The nominal compressive strength of concretes, made with and without fly ash, are analyzed statistically. Special attention is given to moderate- and high-strength concretes. In conclusion, it is shown that concretes made with fly ash may not be more consistent than concretes with only portland cement. INTRODUCTION Pozzolans, and particularly fly ash, are increasingly used as a cement replacement and supplement in commercially produced concretes. But many designers and owners continue to express strong reservations about the use of fly ash and its effect on the consistency and performance of the concrete. Many laboratory test programs have documented that fly ash materials may be used to produce more durable, higher-strength concretes. But, these laboratory results notwithstanding, particular concerns have been raised about the allegedly greater sensitivity of fly ash concretes to curing conditions and to variations in mix materials in commercially produced concretes. Also, some design professionals assert addition of another mix material, e.g., fly ash, introduces more variables to the concrete mix and results in even more inconsistent strength data. The purpose of this paper is to review selected features of commercially produced fly ash concretes relative to plain concretes, including their relative consistency in strength, uniformity in strength gain, and for anomalies sometimes seen in current practice. RESULTS AND DISCUSSION Data Considered Laboratory tests are ordinarily based on a small number of specimens prepared under specific temperature, slump, and material conditions. In contrast, in commercial concreting practice we encounter a much broader range of conditions, including natural variations in ambient environmental conditions, mix materials, and conditions of placement. In this paper we refer to data provided by two different ready-mixed concrete producers, one of which produced otherwise comparable plain and fly ash concretes. We also make reference to comparable results observed by others. The principal characteristics of the m
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