What is an air-entraining admixture used for and how does it affect finishing?

• A. It speeds up finishing by increasing surface hardness
• B. It reduces curing time
• C. It improves freeze-thaw durability and workability; it creates microscopic air voids that can slightly soften the surface, affecting surface finish if not controlled
• D. It eliminates laitance completely

Answer: (C)

Air-entraining admixtures introduce tiny, evenly distributed air voids throughout the concrete. These microscopic air pockets act as cushions that relieve pressure during freezing and thawing, which greatly improves durability in freeze-thaw conditions and also helps with workability, especially in mixes prone to segregation or bleeding. Because these voids add a bit of porosity near the surface, the surface can be slightly softer and may finish differently if finishing is done too aggressively or too early, or if the surface still has bleed water. To keep a good finish, finish after the surface has stiffened and bleed water is controlled, and use finishing practices that don’t overly close the surface. Remember, this admixture doesn’t speed up curing and it doesn’t eliminate laitance.