Further Studies on the Effect of Entrained Air on Strength and Durability of Concrete with Various Sizes of Aggregates

摘要

This paper is the second part of an extensive study of the effects of entrained air on the physical properties of freshly mixed and hardened concretes. Although researchers and specifiers at state DOTs and other agencies realized very early on that entrained air greatly improved resistance to freezing and thawing and deicer scaling, acceptance of air-entrained concrete developed slowly through the early 1950s. In part, this was due to the need for a better understanding of the effects of maximum aggregate size―No. 4 to 2-1/2 in. (63 mm)―and cement content―376 to 658 lb/yd~3 (220 to 390 kg/m~3)―on the properties of air-entrained concrete. An earlier study dealt with pavement concrete having a 2 to 3 in. (50 to 75 mm) slump, and this study used more conventional 5 to 6 in. (130 to 150 mm) slump concrete. An amazing 495 individual batches of concrete were tested for compressive and flexural strength and resistance to freezing and thawing and deicer scaling. The often-quoted rule was that the strength of mixtures with the same water-cement ratio (w/c) were reduced 5% for each 1% of entrained air added. This tended to cause considerable concern with potential users. Klieger's studies demonstrated that when mixtures were compared with the same slump and the same cement content, the reduction in water content required to maintain the slump was enough to reduce the w/c enough to actually increase the strength above that of nonair-entrained concrete. Even for moderately high-cement-content mixtures, the strength reductions at normal levels of air content were much less than suggested by the 5% rule. Additionally, the extensive freezing and thawing and deicer scaling tests led to the basic principle still used today that the concrete air content should be that which is equivalent to an air content of the mortar fraction of 9 +- 1%.