Fly ash concrete final report - the influence of Class C fly ash on the properties of concrete (81-03-3)

Abstract

The behavior of concrete mixes containing various amounts of fly ash conforming to ASTM C-618, class c, was investigated. Twenty to fifty percent of the port 1 and cement lvas replaced on a weight basis. Two tasks were investigated simultaneously. Task 1 investigated the freeze-thaw durability of the various mixes using Procedure A, Resistance of Concrete to Rapid Freezing and Thawing in Water (ASTM C-666). Task 2 investigated the effects of ambient temperature. Concrete was batched and mixed at 55, 70, or 90 F (13,21,or 32 c) ; the influence of temperature on the quantities of mix water and air entraining agent dosage and on the time of set was established. Slump, unit weight, temperature, and air content were measured after initial mixing. The speed of the mixer was then reduced to 2 rpm and the concrete was subjected to agitation until it was unworkable; during this time the properties of the mix were determined at 30-minute intervals. Task 1 results indicated that all mixes tested exhibited high resistance to freeze-thaw action. As the percentage of fly ash was increased it was possible to reduce the amount of mix water; however, it was necessary to increase the dosage of air entrainment admixture. The 28-day compressive strength concrete was not strongly influenced by fly ash; a modest increase in strength was normally exhibited with 30 and percent fly ash replacement. Although fly ash significantly retarded the time of set, it appeared to slightly accelerate the rate of loss of slump. The air void system in the hardened concrete appeared to be unaffected by the percentage of fly ash.