At a committee meeting during the recent ACI convention in Denver, the use of air-entraining admixture (AEA) became a topic of discussion. I was amazed at the differences of opinion. The quandary seems to lie in the potential side effects of AEA.
The fact is that concrete with AEA bleeds much less than concrete with only entrapped air and can cause finishing problems, although air-entrained concrete also finishes easier than non-air-entrained concrete (as long as the air content is not too high). The problem with this is that due to the different bleeding rates of air-entrained concrete, the surface can be sealed too quickly by the finisher and blistering of the surface or a complete delamination of the surface can occur. Most finishers don't know whether the concrete of the day has AEA or not so they use the same finishing guidelines as always. The result is that the surface is sealed too quickly, trapping the bleed water under the finished surface. Too much air can also make the concrete “sticky” and difficult to finish, leading to surface tears.
The use of AEA in concrete makes it more resistant to the detrimental effects of freeze/thaw cycles, however, it also makes the concrete less durable to wear and abrasion due to its decreased compressive and flexural lengths. It is a fact that concrete, when exposed to numerous freeze/ thaw cycles, is far more resistant to the destruction caused by this action if air-entrained concrete is used. The questions posed then are: What is the definition of concrete exposed to freeze/thaw? Does this apply to exterior painted concrete? What if a wall is painted but the section below grade is exposed to freeze/thaw? Can interior concrete be subjected to freeze/thaw?
Most floors subjected to any type of vehicular traffic are not constructed using air-entrained concrete because of the reduced durability. Keep in mind though that most warehouses are not climate controlled and in numerous parts of the country the concrete at all of the overhead doors (through cleaning or just water from natural causes) is subjected to freeze/ thaw cycles that can occur almost daily in the winter. Also, most people do not prime or seal the section of a painted wall that falls below grade. This section of wall is continually subjected to both moisture and freeze/thaw conditions during the winter.
A third potential problem is the fact that combining admixtures in concrete can lead to unwanted side effects. When two admixtures are used together in concrete, the results can be sometimes unpredictable. Further, with each dosage increase or additional type of admixture, characteristics can continue to change. Many times these characteristics become just the opposite of the initial intended use of the product and wreak havoc on the concrete. You must be aware of the potential interactions.
AEA without question has its merits but like most things, it also has its negatives. Sometimes it's a matter of not only what is a more important quality for my concrete to possess but also what potential detriment am I willing to live with.
— Andrew McPherson, vice president Seretta Construction, Apopka, Fla.
Regarding your October issue, you have my letter on page 12 [criticizing the lack of concern about silicosis among many workers] and a photo of workers cutting concrete without masks on page 67. What message is CC trying to send on this topic?
— Terry Holland
Editor's note: Apparently a bad message! The worker in the photo on page 67 should have had a protective ventilator. We thank Mr. Holland for pointing this out.