Effect of Fly Ash Composition on Thermal Cracking in Concrete

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Title: Effect of Fly Ash Composition on Thermal Cracking in Concrete

Author(s): M. D. A. Thomas, P. K. Mukherjee, J. A. Sato, and M. F. Everitt

Publication: Special Publication

Volume: 153

Issue:

Appears on pages(s): 81-98

Keywords: calcium; cracking (fracturing); flexural strength; fly ash; heat of hydration; temperature rise; tensile stress; Materials Research

Date: 6/1/1995

Abstract:
Reports the results from a laboratory investigation of the effect of fly ash on the temperature rise and early-age tensile strain capacity of concrete. Twelve different fly ashes, with a wide range of chemical compositions, were used in various proportions (25, 40, and 56 percent) in the study. The results of conduction calorimeter tests show that the rate of heat development was strongly influenced by the composition of the ash. Generally, the rate and quantity of heat evolved increased with the calcium level of the fly ash. High-calcium ashes (>20 percent CaO) did not significantly reduce the seven-day heat of hydration when used at a replacement level of 25 percent. However, the heat of hydration decreased as the level of replacement was increased for all ashes tested, regardless of composition. Consequently, even high-calcium ashes may be effective in reducing the temperature rise in concrete, provided they are used at a sufficient level of replacement. Flexural tests were carried out on concrete prisms at early ages; the tensile strain capacity was determined as the strain (in the tensile fibers) at 90 percent of the flexural strength. The flexural strength decreased with higher levels of replacement; however, the strain capacity was similar or slightly higher in fly ash concretes (compared with control specimens) at three and seven days. These results imply that the beneficial effect of reduced temperature rise in fly ash concrete is not necessarily offset by a reduced capacity to resist thermal strains.