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The air contents of several concrete mixtures were measured while the material was in a plastic state, and the results were compared to corresponding values determined microscopically on the same hardened concretes. Two volumetric airmeters, a roller and a mini airmeter, as well as a pressure airmeter were employed for monitoring the air content in fresh concrete. The linear traverse method was used to determine the air content of hardened concrete. A total of 21 laboratory-prepared concrete mixtures were examined. Both 10 and 25-mm (3/8 and 1-in.) nominal size aggregates of different densities were used to produce normal-weight and lightweight concretes. The water-cementitious materials ratios for the concretes ranged between 0.23 and 0.53, and the cement content varied from 267 to 540 kg/m3 (450 to 910 lb/yd3), with silica fume incorporated in some concretes. An air-entraining agent was used to yield air contents between 0.5 and 12.5%.
For the materials used in this investigation, the test results indicate that the air content in the fresh concrete can be 0.5 to 1.5% greater than that actually present in the hardened concrete. The mean discrepancy between the air contents of fresh and hardened concretes is 22, 13, and 19%, respectively, when the pressure, roller, and mini airmeters are used. For any single airmeter, the spread between air contents of fresh and hardened concretes increases with the reduction in compressive strength. Similar decrease is observed when the nominal size of aggregate increases.
Assistant professor, Faculty of Applied Science, University of Sherbrooke, Sherbrooke, Quebec
Professor, University of Saskatchewan, Saskatoon, Saskatchewan
Stock #: CCA10545J