Effect of Air Entrainment on the Mechanical Properties, Chloride Migration, and Microstructure of Ordinary Concrete and Fly Ash Concrete

Air-entrained concrete has been extensively used in bridges and water conservancy projects in cold regions. The objective of this study was to investigate the effect of air entrainment on the mechanical properties, chloride migration, and microstructure of ordinary concrete and fly ash concrete. Twelve concrete mixtures were prepared with two water-to-binder ratios (0.53 and 0.35), three levels of air content (no air entrainment; moderate air entrainment, with 4–5% air content; and high air entrainment, with 7–10% air content), and two levels of fly ash (no fly ash and 30% replacement of cement). The results indicated that, compared with ordinary concrete, the addition of an air-entraining agent (AEA) to fly ash concrete resulted in a greater decrease in compressive strength. Care must be taken when introducing artificial air bubbles into fly ash concrete because the bubbles will affect the mechanical properties. With proper air entrainment (air content of 4–5%), stable, closed, and well-distributed air bubbles are introduced into concrete so that the connectivity of the pore system is interrupted and chloride migration is consequently minimized. However, if the concrete contains a large amount of entrained air, a substantial amount of large and detrimental pores is introduced, causing air bubbles to overlap and merge, as well as causing additional chloride migration. This effect is more prominent in fly ash concrete.