Q.: Is it possible to entrain air in shotcrete when the dry-mix method is used? In 30 years experience I have never learned of a way to do it. I am interested in anything that can enhance the durability of shotcrete.

A.: We do not believe it is possible. Fortunately, it doesn't seem to be necessary, as indicated by a Portland Cement Association laboratory study of the durability of shotcretes to freezing and thawing. They found that the dry-mix shotcretes studied had excellent durability, even though the air contents were very low. The small amount of air that these mixes did contain was found to be entrapped (not entrained), so the air could not have contributed to the durability. Details are given in "Laboratory Study of Shotcrete," Symposium on Shotcreting, Publication SP-14, American Concrete Institute, 1966. This has also been reprinted as Portland Cement Association Development Department Bulletin D111.