Iowa State University
Alkali content, Alkali silica reactions, Cracking, Fly ash, Iowa, Potassium, Quality control, Sodium, Specifications
Sodium and potassium are the common alkalis present in fly ash. Excessive amounts of fly ash alkalis can cause efflorescence problems in concrete products and raise concern about the effectiveness of the fly ash to mitigate alkali-silica reaction (ASR). Fly ash marketing agencies occasionally provide materials that just miss the criteria for alkali content given in Iowa Department of Transportation (IowaDOT) IM 491.17. Since usage is only from an approved list (certified sources) this leads to disputes that can be difficult to resolve. This is especially problematic when the alkali content of a given source of fly ash only changes by a small amount but the change causes the source to cross a specification limit. This immediately prompts the marketing agency to question the precision of the alkali determinations and/or the validity of the specification limit. This proposed research project is aimed helping to resolve these issues. Specifically, it plans to: (1) review the existing methods of fly ash alkali measurement; (2) reviewing models used to estimate alkali content of the various sources of fly ash commonly used in Iowa; (3) ascertaining how the measured alkali content of a fly ash impacts its ability to mitigate ASR; and (4) evaluating existing pavements in Iowa that were constructed using high-alkali fly ash, for ASR-induced cracking. The scope of the project is limited to the examination of fly ash sources that are commonly used by the Iowa DOT. Special attention will be given to fly ashes obtained from power stations that utilize alkali-bearing materials in their normal operating procedures. The objectives of this project are to: (1) determine if and at what content level fly ash with soda dosing has increased potential for alkali silica reactivity (ASR) as well as any other potential performance impacts in the concrete both during mixing and placing as well as long-term (good or bad); (2) evaluate field concrete containing high-alkali fly ash and moderately reactive fine aggregate to see if ASR-related distress has occurred; (3) determine a better method for determining available alkali in fly ashes with soda dosing. The method should be relatively simple and rapid in order to provide a quality control (QC) tool for the fly ash marketer and quality assurance (QA) test for the Department of Transportation (DOT); and (4) perform a literature review and/or a survey to determine if there are other materials and methods for emission control that may impact the mid-west power plants and their fly ash chemistry. A specification content limit or performance level should be determined and recommended.