Air-entraining agents improve the durability and workability of concrete in climates with frequent freezing and thawing. These agents prevent cracks by introducing small air bubbles into the mix, creating spaces accommodating water expansion when temperatures drop. The air-entraining agents lower the surface tension of water, forming stable, small air bubbles. This method is more effective than having accidental large voids, as the intentional, smaller, and evenly distributed air voids improve the concrete's resistance to freeze-thaw damage.
Common air-entraining materials include natural wood resins, animal and vegetable fats and oils, and synthetic detergents such as alkali salts. These materials either react with cement lime to form soluble resins or coat the air bubbles to prevent merging. The effectiveness of these agents is gauged by the spacing factor, which is the distance between air voids in hardened concrete, with optimal protection at about 0.01 inch.
It's crucial to note that the air bubbles introduced by these agents are different from water-filled capillary pores, which contain the hydration products of cement. The air bubbles act as additional voids that absorb the expansive forces of freezing water, enhancing the concrete's resistance to freeze-thaw cycles. This property makes air-entrained concrete indispensable for construction in colder regions.
