High volume fly ash (HVFA) concretes are one component of creating a more sustainable infrastructure. By replacing 50 % or more of the portland cement with fly ash, a significant reduction is achieved in the carbon footprint of the in place concrete. While HVFA mixtures can be proportioned to produce equivalent long term performance as conventional (cement-only) mixtures, performance problems are often encountered at early ages, including low early-age strengths, long delays in finishing, and potentially greater susceptibility to improper curing. In this paper, a variety of mixture proportioning options to mitigate these deficiencies are investigated. Variables examined include cement type, fly ash class, the provision of internal curing, and the addition of either calcium hydroxide or a rapid set cement to the binder. Switching from a Type II/V to a Type III cement enhanced one-day compressive strengths by over 50 %. Using a class C fly ash produced a mixture with a higher calcium-to-silicate ratio than a comparable class F ash and increased the 7-d strength. However, in this study, sulfate balance was a problem in the class C HVFA mixtures, requiring 2 % additional gypsum to maintain a proper sulfate balance. Internal curing was found to significantly reduce autogenous deformation, with a concurrent decrease in compressive strength. Excessive retardation was observed in both mixtures with the class C and the class F fly ashes; powder additions of either a rapid set cement or calcium hydroxide were found to be effective in reducing this retardation (and setting time delays).
Proceedings Title: International Conference on Sustainable Concrete Pavements: Practices, Challenges, and Directions
Conference Dates: September 15-17, 2010
Conference Location: Sacramento, CA
Pub Type: Conferences
Building technology, compressive strength, concrete, high-volume fly ash, mixture proportioning, setting.