Performance of Portland cement concrete containing chemically beneficiated high loss on ignition fly ashes with air entrainment
1 online resource (187 pages) : PDF
University of North Carolina at Charlotte
The use of fly ash as a partial replacement of Portland cement in concrete production has been widely accepted and has been proven to be a successful practice that enables production of economical durable concrete. However, following the Clean Air Act Amendment of 1990 power plants have started using the technology of adding activated carbon to the combustion chamber of coal plants to remove mercury and other contaminant from the exhaust. Consequently, the ashes produced in recent decades contain high quantity of unburned carbon which adsorbs air entraining admixture in concrete. For resistance to stresses induced by freezing and thawing, concrete must be air entrained to prevent expansion of aggregate and cracks. The amount of air entraining admixture required to achieve a target air content in concrete can be difficult to predict and can vary significantly due to the presence of unburned carbon in fly ash. The main objective of this study was to evaluate the performance of off-spec fly ashes that were chemically treated with a carbon blocker because they contain too much carbon to meet concrete use specifications. For this study, off-spec fly ash is the ash that do not meet North Carolina Department of Transport (NCDOT) specification for LOI limit in fly ash. Three off-spec fly ashes from three different power plants namely BU, MU and AU were used for this study, along with one on-spec fly ash BC which meets NCDOT specification for LOI, a marketable ash was also recovered which was used as a control. The off-spec fly ashes were chemically treated to reduce impact of unburned carbon on admixture adsorption and this treated ashes were used to produce concrete specimens with varying air contents. Twenty different concrete mixtures, produced using the different treated and untreated ashes were batched and tested to provide insight on the durability performance and ability to predict the required air entraining admixture needed to achieve target air content in concrete containing treated ashes. Durability of the concrete mixtures was measured through the use of compressive strength, surface resistivity, Super Air Meter test, air void analysis, and freeze thaw testing. Analysis of covariance (ANCOVA) was also performed for the response of durability factor, spacing factor and compressive strength to determine if the treatment process had any effect on the air void network, durability factor and strength of the concrete produced.The variable dosage test showed that it was easy to predict the amount of AEA needed to achieve target air content in treated ashes and the control ash. For the untreated ashes, the relationship between AEA dosage and air content generated was scattered and inconsistent. This showed that it is difficult to predict the amount of AEA needed in untreated ashes due to the adsorptive nature of the unburned carbon. The air void analysis of the treated and untreated mixtures with air content greater than 5% meets the required specification for spacing factor to be frost resistant. Additionally, all mixtures with air content greater than 5% showed good freeze thaw performance. ANCOVA analysis demonstrated that the treatment of the fly ash did not significantly influence the durability factor, air void system and strength.
Cavalline, TaraOgunro, Vincent
Thesis (M.S.)--University of North Carolina at Charlotte, 2018.
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