Q. I just got a spec for underwater concrete that includes air entrainment of 5% to 7%. In the past I have found that air in underwater concrete causes slump loss problems. FYI, for every 33 feet you go down, you decrease the air volume by 50%. If you start off with 7% air and go down 33 feet, you have only 3.5% air, resulting in about a 3-inch slump loss. Of course, this air volume decrease is theoretical, since you can't easily do the air test underwater, and if you bring the concrete back to the surface to do the test, the air goes back to 7%. The slump loss is readily observable. Is there a valid reason to specify air entrainment in underwater concrete, especially if there is no freeze/thaw condition? About 4% air is typically used to reduce water, but if you are to lose 2 inches of slump placing the concrete underwater, that may not be a benefit.

A. You don't really lose the air entrainment. The head pressure from the weight of the water compresses the entrained air. If you're pumping, which we assume you are, slump loss should not be a problem. Slump is only critical for consolidation, and the head pressure will take care of that. ACI 304 R-00, “Measuring, Mixing, Transporting, and Placing Concrete” Chapter 8 gives excellent information on concrete placed underwater, including the use of admixtures to prevent segregation. The addition of an air-entraining admixture increases pumpability. ACI recommends a cement content of 600 pounds per cubic yard, a water/cement ratio of 0.40, and the use of high-range water reducers to increase the slump to 9 inches.

One contractor noted that about five years ago he did an underwater placement in Maine to a depth of about 250 feet but didn't use air. Even though the river froze during the winter, the caissons he was casting were cut off about 40 feet below the surface and probably didn't experience any freezing. The caissons were encased in steel, so they didn't have to worry about setting time, and they used a low dosage of a hydration stabilizer so the concrete didn't set for about 18 hours. Also, using a well-graded mix, there was almost no segregation. It worked beautifully.

Another respondent noted that if the structure will be in the splash zone, then air entrainment is necessary in a freeze/thaw environment. But if the structure will be completely submerged during its service life and designed at 0.40 w/c+p or less, you do not need air entrainment. Additionally since underwater concrete is impossible to vibrate, he recommends self-consolidating concrete with an anti-washout admixture such as Rheomac by Degussa.