05-CBRC-W04
Using Class C Fly Ash to Mitigate Alkali-Silica Reactions in Concrete

High-calcium fly ashes, classified as Class C by the ASTM International (ASTM) C618 definition, are excluded as a means to mitigate alkali-silica reactions (ASR) in concrete because a relationship between high-calcium content and expansion was documented when Class C fly ash was used at a 10% to 15% replacement level in concrete. It is generally true that low replacement levels (<15%) of Class C fly ash may not offer ASR mitigation; however, it has been demonstrated that Class C fly ashes can mitigate the effects of ASR at higher replacement levels than specified. For highly reactive aggregates, the required dosage of Class C fly ash may be quite high, resulting in reduced early strengths. In some cases, the amount of Class C fly ash needed to control ASR may exceed specification limits set by state Departments of Transportation. In theses instances, combining Class C fly ash with silica fume, for example, can help to mitigate ASR and improve early-strength gain.

The University of North Dakota Energy & Environmental Research Center (EERC) and its partners, the University of Texas at Austin Concrete Materials Laboratory and the University of New Mexico Civil Engineering Department, propose to perform a multiyear investigation to evaluate the performance of several Class C fly ashes (>10% CaO) using existing predictive ASR test methods. ASTM standard methods will be applied to fly ash samples and cast specimens produced using varying levels of Class C fly ashes. In addition to these empirical tests, the EERC will evaluate specimens using advanced electron microscopy techniques to look at the mineralogy of the ash and the aggregates and, especially, the reaction products. It is anticipated that results will confirm limited unpublished work that indicates the efficacy of using higher percentages of Class C fly ash to mitigate ASR when using moderately reactive aggregates.

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