12.3 : 공기 유입제

Air-entraining agents improve the durability and workability of concrete in climates with frequent freezing and thawing.

The air-entraining agents prevent cracks by introducing smaller, evenly distributed air bubbles into the mix, creating spaces accommodating water expansion when temperatures drop.

Air-entraining agents lower the water's surface tension, forming stable, small air bubbles. This method is more effective than having accidental large voids due to improper concrete compaction.

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 inches.

Air bubbles in cement paste are separate from the water-filled capillary pores. The hydration products of cement are formed only in water-filled pores and not within the air bubbles. So, air bubbles act as additional voids that can accommodate the expansion of water when it freezes.