Extending the Usage of High Volume Fly Ash in Concrete
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2014-07-01
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TRIS Online Accession Number:01532656
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Edition:Research Project
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Abstract:Concrete is the world’s most consumed man-made material. Unfortunately, the production of Portland cement, the active ingredient in concrete, generates a significant amount of carbon dioxide. For each pound of cement produced, approximately one pound of carbon dioxide is released into the atmosphere. With cement production reaching nearly 6 billion tons per year worldwide, the sustainability of concrete is a very real concern. Since the 1930’s, fly ash – a pozzolanic material – has been used as a partial replacement of Portland cement in concrete to improve the material’s strength and durability, while also limiting the amount of early heat generation. From an environmental perspective, replacing cement with fly ash reduces concrete’s overall carbon footprint and diverts an industrial by-product from the solid waste stream (currently, about 40 percent of fly ash is reclaimed for beneficial reuse and 60 percent is disposed of in landfills). Traditional specifications limit the amount of fly ash to 25 or 30 percent cement replacement. Recent studies, including those by the investigators, have shown that higher cement replacement percentages – even up to 75 percent – can result in excellent concrete in terms of both strength and durability. Referred to as high-volume fly ash (HVFA) concrete, this material offers a viable alternative to traditional Portland cement concrete and is significantly more sustainable. By nearly doubling the use of reclaimed fly ash in concrete, HVFA concrete aligns well green initiatives on recycling. However, HVFA concrete is not without its problems. At high replacement rates, HVFA concrete has shown very poor scaling resistance and mixed results with regard to other durability measures. The objective of this research was to determine the implications of increasing the amount of fly ash in concrete with regard to durability. This report consists of two parts. Part I investigates the mechanism of salt scaling in HVFA concrete, while Part II investigates the full range of durability resistance including freeze-thaw, chloride permeability, and scaling. The results indicate that fly ash replacement levels up to 50% result in concrete that possesses excellent durability.
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