Effect of Fly Ash on the Air-Void Stability of Concrete


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Title: Effect of Fly Ash on the Air-Void Stability of Concrete

Author(s): Steven Gebler and Paul Klieger

Publication: Special Publication

Volume: 79


Appears on pages(s): 103-142

Keywords: admixtures; air entraining agents; concretes; fly ash; fresh concretes; hardened -- quality control; tests. air entrainment; --- concretes; pozzolans;

Date: 5/1/1983

Concretes containing both portland cement and fly ash were evaluated to determine the effect of fly ash on air-void stability. Ten fly ashes were used, they have a wide range of chemical and physical properties as well as geographical origins. Air contents of plastic concretes were determined, and both air content and air-void parameters were measured in hardened con-cretes cast at four time intervals after initial mixing. These tests indicate that air contents of concretes containing Class C fly ash appear to be more stable than those in concretes containing Class F fly ash. The higher the organic matter content of a fly ash, the higher will be the air-entraining admixture requirement for concrete in which the admixture is used. In addition, the higher the air-entraining admixture requirement, the greater is the air loss on extended mixing. Even though the air volume is reduced the spacing factor, specific surface, and number of voids are little affected. A "Foam Index" was determined for each of the ten fly ash-Portland cement combinations. Air-entraining admixture requirements of actual concretes containing both portland cement and fly ash were compared to the "Foam Index" test results. These tests indicate that the "Foam Index" could be especially useful to concrete pro-ducers as a quality control test for checking the air-entraining admixture requirements for different sources or lots of fly ash.