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Fly ash particles can also be of different chemical characteristics. Fly ash obtained from the burning of sub bituminous coal (also known as lignite) is categorized as class C fly ash (Oweiss et al. 1998). Class C fly ash is more common in the western/Midwest part of the United States. This type of fly ash contains lime and other compounds in which self-cementing characteristics are found. According to ASTM C618, the chemical composition of class C fly ash must be such that the total content of: SiO2 + Al2O3 + Fe2O3 should be less than or equal to 50% and the sulfur trioxide should not exceed 5%. Another type of fly ash is formed by the burning of bituminous coal. This type of fly ash is identified as Class F fly ash. In class F fly ash, the chemical composition of SiO2 + Al2O3 + Fe2O3 should account for a total content less than or equal to 70% and the sulfur trioxide should not exceed 5% (ASTM, C618). This type of fly ash is used in this research investigation and can mostly be found in the eastern part of the United States. Unlike class C fly ash, this type of fly ash has very little self-cementing characteristics (Oweiss et al. 1998). Fly ash is known to be a very good and effective partial replacement for Portland cement in concrete. The rate of substitution usually varies based on the overall chemical composition of both the Portland and the fly ash itself. For instance, a minimum rate of replacement of 1 pound of fly ash for every pound of Portland cement is normally specified. If fine aggregates are used in the mix design, its total amount should be decreased in order to account for the added volume of fly ash. Furthermore, the chemical characteristics of both the fly ash and the Portland cement also play a significant role in determining the rate of substitution. Currently, up to a 15 – 25% replacement is allowed by some states (Federal Highway Administration, 2010). A large 14PDF Image | HDPE FIBER REINFORCED RECYCLED AGGREGATE CONCRETE
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