Best Practices Guide for High-Volume

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A best practices guide is developed from a synthesis of recent research on high-volume fly ash (HVFA) concrete mixtures. These best practices can be applied by the concrete construction industry to achieve desired properties and to ensure the (high) performance of HVFA concrete mixtures in practice. As such, the report considers all aspects of HVFA concrete production, from the characterization of the starting materials, through mixture proportioning and curing options to achieve desired properties, to the in-place early-age and long-term performance of the concrete in its fresh and hardened states. Both mechanical and transport properties are considered in detail. Perspective is established based on a brief review of current practices being employed nationally. Each topical section is concluded with a practice-based set of recommendations for the design and construction community. The report is intended to serve as a valuable resource to these communities, providing both a research summary and a guide to practical steps that can be taken to achieve the optimum performance of these sustainable concrete mixtures. The sustainability movement of the 21st century has provided impetus to various industry sectors to identify alternatives to their energy intensive (i.e., costly) materials and processes. In the concrete construction industry, a majority of the embodied energy and cost of concrete is due to the portland cement. Although the industry has been using industrial by-product materials for decades to replace a portion of the portland cement to achieve specific desirable engineering properties, recent efforts to significantly reduce the embodied energy have produced renewed focus on far greater replacement percentages of portland cement in concrete mixtures. One approach that has received considerable attention in recent years is the production of so-called high-volume fly ash (HVFA) concrete mixtures [1], where fly ash replaces cement at proportions well above the 15 % to 25 % commonly encountered in current blended cement concrete mixtures. As a reference point, according to a 2012 survey [2], fly ash usage in 2011 was estimated at 15 % of the total cementitious binder (portland cement, fly ash, and slag) in ready-mixed concrete in the U.S., with the usage consistently trending upwards during the past ten years. With much fly ash still being diverted to landfills and existing landfills filled with fly ash that could potentially be used in concrete, there is still an important opportunity to further increase fly ash contents in concretes. The aforementioned survey conducted by Obla et al. [2] identified the two primary causes preventing increased use of fly ash in ready-mixed concrete: 1) performance issues including setting time and strength gain, and 2) specifications that restrict usage. By providing information on best practices for the production, construction, and evaluation of HVFA concretes, this report will address the most commonly identified performance issues, paving the way for increased specification of this sustainable construction material. Numerous ready-mix companies are actively pursuing the development and implementation of HVFA concrete mixtures. The following summary is a composite based on interviews with two such producers [3,4]. HVFA concrete mixture proportions will vary widely depending on locality, as governed by materials’ availability, shipping costs, and structural applications. Many specifications from state Departments of Transportation and elsewhere limit fly ash replacement levels to 25 % or less, on a mass basis.1 Conversely, for projects seeking LEED certification, such as school buildings, 50:50 and even 60:40 (fly ash: cement) mixtures have been developed and implemented without significant difficulties. In some cases, the setting time delays typical of HVFA mixtures can be used to advantage, as they permit longer hauling times and may allow for a reduction in the dosage of retarders (chemical admixtures) used in the mixtures.

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