The term "entrained air" refers to the system of microscopic air bubbles purposefully generated in the mortar by the addition of air-entraining agents. ASTM C 270 permits air-entraining agents to be added, if specified, as long as these limits are not exceeded. This topic has stirred up considerable controversy in the industry. While some masonry professionals say that air-entrained mortars should be used under certain circumstances, others argue that they should not be added because they can be detrimental to masonry performance. Masonry specialist John Melander of Skokie, Ill.-based Portland Cement Association and masonry consultant Colin Munro of Batavia,Ill., are on opposite sides of this issue. We've asked them to explain their reasons and respond to each other's comments. Yes: Controlled Levels of Air Entrainment Are Beneficial by John Melander Controlled levels of air entrainment can enhance the performance of both freshly mixed (plastic) and hardened mortar- by improving workability, increasing water retention, providing longer board life, improving durability, and reducing water absorption. Highly plastic, workable water-retentive mortars make unit placement easier and adapt better to irregular unit surfaces, providing intimate contact between mortar and unit. Durability studies indicate that minimum air-content levels of about 10% to 16% are needed to provide hardened mortar with the greatest resistance to freeze-thaw deterioration. Never in Brick Walls: The Higher the Air Content, the Leakier the Wall by Colin Munro There is no doubt in my mind that excessive air entrainment of mortar is one of the main causes of leaky brick walls. The higher the air content, the greater the capacity for a brick wall to leak. Air causes bubbles to form in the set mortar. When the air content is high, the bubbles begin to form chains. It's the chaining of these bubbles I object to because this creates passageways for water infiltration at the interface of the brick and mortar. Another problem is that high air-entrained mortars don't establish sufficient bond with the units. The ASTM C 270 limits on air content are much too high.