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Chap8 - HOME PAGE CHAPTER 8 Air-Entrained Concrete One of...

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CHAPTER 8 Air-Entrained Concrete One of the greatest advances in concrete technology was the development of air-entrained concrete in the mid- 1930s. Today air entrainment is recommended for nearly all concretes, principally to improve freeze-thaw resist- ance when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete is produced by using either an air-entraining cement or adding an air-entraining admix- ture during batching. The air-entraining admixture stabi- lizes bubbles formed during the mixing process, enhances the incorporation of bubbles of various sizes by lowering the surface tension of the mixing water, impedes bubble coalescence, and anchors bubbles to cement and aggregate particles. Anionic air-entraining admixtures are hydrophobic (repel water) and are electrically charged (nonionic or no- charge admixtures are also available). The negative elec- tric charge is attracted to positively charged cement grains, which aids in stabilizing bubbles. The air- entraining admixture forms a tough, water-repelling film, similar to a soap film, with sufficient strength and elas- ticity to contain and stabilize the air bubbles and prevent them from coalescing. The hydrophobic film also keeps water out of the bubbles. The stirring and kneading action of mechanical mixing disperses the air bubbles. The fine aggregate particles also act as a three-dimensional grid to help hold the bubbles in the mixture. Entrained air bubbles are not like entrapped air voids, which occur in all concretes as a result of mixing, han- dling, and placing and are largely a function of aggregate characteristics. Intentionally entrained air bubbles are extremely small in size, between 10 to 1000 µ m in diam- eter, while entrapped voids are usually 1000 µ m (1 mm) or larger. The majority of the entrained air voids in normal concrete are between 10 µ m and 100 µ m in diameter. As shown in Fig. 8-1, the bubbles are not interconnected; they are well dispersed and randomly distributed. Non-air- entrained concrete with a 25-mm (1-in.) maximum-size aggregate has an air content of approximately 1 1 / 2 %. This same mixture air entrained for severe frost exposure would require a total air content of about 6%, made up of both the coarser “entrapped” air voids and the finer “entrained” air voids. PROPERTIES OF AIR-ENTRAINED CONCRETE The primary concrete properties influenced by air entrain- ment are presented in the following sections. A brief sum- mary of other properties not discussed below is presented in Table 8-1. Freeze-Thaw Resistance The resistance of hardened concrete to freezing and thawing in a moist condition is significantly improved by the use of intentionally entrained air, even when various deicers are involved. Convincing proof of the improve- Fig. 8-1. Polished section of air-entrained concrete as seen through a microscope. (67840) 129 HOME PAGE
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ment in durability effected by air entrainment is shown in Figs. 8-2 and 8-3.
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