Date of Graduation

5-2021

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Julian Fairey

Committee Member

Cameron Murray

Second Committee Member

Micah Hale

Keywords

Air entraining admixture dosing, Asymmetric field flow fractionation, Fly ash in concrete, Freundlich adsorption isotherm, Instability of AEA compounds, Variability of fly ash properties

Abstract

Fly ash is a coal combustion byproduct that can be used as a supplementary cementitious material (SCM) in concrete, offering benefits such as increased workability, durability, and strength. Fly ash can be used as a partial replacement to cement, reducing concrete productions costs and decreasing disposal in landfills. However, variability of fly ash properties (e.g., carbon content, adsorption site density) confounds estimates of the air entraining admixture (AEA) dose needed to achieve a specified air content in concrete. Air entrainment is needed to improve freeze-thaw resistance and limit concrete cracking. The foam index (FI) test is currently used to estimate the AEA dose for concrete containing fly ash, but concerns exist regarding its accuracy and reproducibility stemming from the subjectivity of the test. Specifically, the end point of the FI test is based on visual judgement as to the AEA dose that produces a temporary stable foam over a sufficient portion of the sample surface.

The objective of this project was to develop a reproducible and quantitative test to estimate the required AEA dose needed to achieve a specified air content in concrete containing fly ash, typically around 6 %. Fly ash samples from 12 coal-fired power plants across seven U.S. states were used to make concrete and the required AEA doses needed to achieve a 6 % air content were determined in a companion MS thesis. Here, ultraviolet (UV) absorbance assays were used to assess the sorption capacity of the fly ash for two AEAs – Daravair and TeraPAVE – and one dye selected to be an AEA surrogate. The fly ash samples were characterized chemically (e.g., loss on ignition, LOI) and physically (e.g., specific surface area and pore volume) and correlations were sought between these properties and sorption of AEA or dye. AEA and dye sorption by fly ash was poorly correlated with the required AEA dose, with correlation coefficients (R2 values) ranging from 0.05 to 0.13. However, a multilinear regression model with the ƒly ash cumulative surface area (CSA) and specific surface area (SSA), two uncorrelated independent variables, was strongly correlated to the required AEA dose (adjusted R2 = 0.85, n = 12). Additional experiments revealed the need for composite sampling in future testing, as evidenced by LOI for one fly ash sample varying by about a factor of three, from 0.57–1.71 % (n = 30). Further, the UV spectra of the AEAs was determined to decay over time, reflecting instability of the AEA mixture compounds, and highlighting the need to coordinate the concrete production and AEA–fly ash adsorption tests, which were staggered by about one year in this study.

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