A series of paste and mortar mixtures containing different fly ashes and replacement levels ranging from 20% to 60% with high and low alkali cement and with high and low carbon content Class F ash were evaluated. Tests included compressive strength at different ages, flow, setting time, isothermal calorimetry, and evaluating rheological properties. In general, for the same water-binder ratio and replacement level, Class C fly ash mixtures presented higher strength but a delayed setting when compared with Class F fly ash mixtures. Class C fly ash mixtures presented lower viscosity and lower yield stress, as well as higher flow. Viscosity and yield stress increased with the increase of fly ash content for both fly ashes. To address the excessive delays in setting times that are normally associated with high volume fly ash (HVFA) mixes, the authors are planning on using 5 to 10 percent replacement of fly ash with nano size to very fine limestone. The primary objective of this study is to develop performance-based specifications for incorporation of fly ash in concrete mixes and using dynamic shear rheometer (DSR) and calorimeter to characterize their behaviors. The ultimate goal is to take the guesswork out of using fly ash and encourage State departments of transportation (DOTs) to use them with more confidence in a variety of applications.
The primary objective of this study was to develop a rationale for using a dynamic shear rheometer and an isothermal calorimeter as practical, quick scanning tools for the following purposes:
- Predict and assess early-age behavior of concrete mixtures containing different types and levels of cement and fly ash.
- Identify incompatible blends.
- Verify performance.