Microstructure of entrained air voids in concrete, Part I
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The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland-cement systems. Scanning electron micrographs of air voids are presented for different ages. The solidification process of portland-cement paste and mortar was frozen at different ages using a low temperature scanning electron microscope and freeze drying. At very early ages the air voids show thin shells made of very fine particles. The packing of the cement grains behind the air void varies, depending on the water-to-cement ratio. Air voids appear to have the same interface with the cement paste matrix as aggregates. No readily visible difference was found between entrapped and entrained air voids. I. INTRODUCTION It was observed in the early 1930's that some stretches of roads in the northeastern states of the United States were more durable than others when exposed to freeze-thaw conditions. An investigation showed that the more durable roads were made of less dense concrete. This concrete was made of cement that was manufactured in mills using a grinding aid. The grinding aids consisted of small quantities of substances such as stearates of lime and alumina, grease, coal, and resin. It was found that such a cement entrains air in the concrete. Jackson1 concluded that the action of airentraining agents, when properly used, improves the resistance of concrete to a freeze-thaw condition. Air-entraining agents belong to a class of chemicals called surfactants (surface-active substance),2 materials whose molecules are adsorbed strongly at the air-water or solid-water interface (i.e., cement and aggregate-water interface and air-water interface). Due to their dual nature, these molecules are partially abstracted from the water and concentrated at the interfaces. The molecules have a polar "head", which stays in the water phase, and a nonpolar "tail", which is excluded from the water and sticks out into the air phase. Air-entraining admixtures are added to the mixing water as a liquid. A sodium salt of wood resin3 is probably the most widely used type of air-entraining admixture. It is an anionic surfactant; i.e., the head is negatively charged, and it is a mixture of phenolics, carboxylic acids, and other substances. Neutralization with sodium hydroxide makes it soluble. Mielenz et al.4 found that the adsorbed film at the air-water interfaces produced by air-entraining agents decreases the tendency of bubbles to coalesce, permits bubbles to bear short-term loads, and facilitates the production of a system of bubbles of a given specific a)
Now with Testing Engineers, Inc., Oakland, California 94623.
2004
http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 9, Sep 1991
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surface. They also ascertained that the formation of the optimum void system was influenced by the following factors: (1) reduction in surface tension by the concentration of the air-entraining agent
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