Use of fly ash in the mitigation of alkali-silica reaction in concrete

Graduation date: 2011Alkali-silica reaction (ASR) is one of the leading causes of concrete deterioration throughout the world, and affects both structural and non-structural elements. Despite strong research efforts since its discovery by Stanton in the late 1930s, this deteriorating reaction is still not clearly understood nor are proper preventative measures effectively applied to reduce the risk of ASR. Central to the debate is a lack of clear understanding about how supplementary cementing materials (SCMs), in particular fly ash, limit the progression of the reaction. One reason for avoiding the use of a certain fly ash in concrete is the alkali content of the particular fly ash. Because there is very little guidance and no standardized test to accurately measure the contribution of alkalies from a given ash in a real concrete mixture, many ashes are simply avoided. Development and standardization of a rapid test method to accurately predict the contribution of fly ash alkalies to the pore solution of concrete is the goal of a multi-phase project in the Civil Engineering department at Oregon State University. The first phase of the project included performing a series of ASTM C 1567 tests (Standard Test Method for Potential Reactivity of Aggregates (Mortar-Bar Method)) utilizing various materials and replacement levels to determine the effectiveness of different fly ashes in the mitigation of ASR expansion. Also, predictive equations introduced by Malvar et al were applied to a variety of materials to determine the efficacy of different fly ashes to control alkali silica reaction based on chemical composition. In addition, pore solution evaluation was conducted using a specialized extraction device, one of only 3 of its kind in the United States. This process, carried out on 265 paste samples made of various combinations of cements, fly ashes and fly ash replacement levels, enabled the authors to evaluate the effects of curing temperature, time and raw material chemistry on the paste pore solution. The results of the pore solution analysis were compared against the expansion tests and it was concluded that aluminum also plays a key role in the ability of a given ash to mitigate alkali-silica reaction