The 2004 fly ash survey1 conducted by the American Coal Ash Association (ACAA) states that out of the 70 million tons of fly ash produced, about 40% was beneficially utilized and a large proportion (60%) was not used (land filled). The 2004 survey also shows that out of the fly ash that was beneficially used about 60% was used in cement and concrete applications. Even though ready mixed concrete forms the single largest market for fly ash it can still offer the largest potential for increased fly ash utilization. Estimated ready mixed concrete production in the US in 2004 was 430 million cubic yards.
There is a large body of research and literature on the development and use of High-Volume Fly Ash (HVFA) concrete. In spite of that, the actual use of high volumes of fly ash (>30% of total cementitious content) in ready mixed concrete is limited. Surveys and discussions suggest that the average fly ash content in all ready mixed concrete is still about 10% (of total cementitious content) even though some producers may be using fly ash contents of high as 30% in summer months in certain applications. If the average fly ash content in all ready mixed concrete were increased to 20% this would increase the overall fly ash utilization from 40% to 64% thereby far exceeding CBRC’s 2010 goal of 50% fly ash utilization! In order to achieve the average of 20% fly ash use in ready mixed concrete all year around it may be necessary to use more than 50% fly ash in certain applications during warmer weather. However, many producers cite the low rate of strength gain and delayed initial setting time as the primary reason for not using higher volumes of fly ash in concrete.
This research project suggests novel science-based approaches to address the slower rate of strength gain and delayed setting times of HVFA concrete. Based on these approaches a step by step procedure will be developed that the concrete producer and the construction team can adopt to manage the resulting properties of HVFA concrete during mixture development and construction scheduling. The use of HVFA concrete supports sustainable construction initiatives for the concrete industry.
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1 2004 American Coal Ash Association Survey.
Karthik H. Obla
National Ready Mix Concrete Association
900 Spring Street
Silver Spring, MD 20910
(240) 485-1163
(301) 585-4219
kobla@nrmca.org