Photo courtesy of Euclid Chemical Co.

Technology: Concrete Admixtures, Air Entrainers
Freezing and thawing causes expanding and contracting of molecules. This creates pressure that naturally finds a route to the surface, which can result in micro-cracks. Air entraining agents build a structure of bubbles that relieves that pressure.
by Mike Dawson

Unless you live in sunny vacationland, you need something to relieve the stress when you work with exterior concrete — a pressure release. Fortunately, your ready-mix supplier is probably giving you the tiny bubbles you need to make you happy. That is, a structured air system created by air entraining agents, which are admixtures used when concrete will be subject to freeze-thaw cycles.

Freezing and thawing causes expanding and contracting of molecules. This creates pressure that needs a route to the surface, which can result in micro-cracks. Air entraining agents build a structure of bubbles that relieves that pressure. "It's a whole network of escape valves," says Sean Fallon, product manager for admixtures at The Euclid Chemical Co., one of several major manufacturers of air entrainers.

Freeze-thaw is not the only consideration. Contractors in many parts of the Western U.S. know that sulfates in soils and aggregates can cause expansion due to crystal formation. Air entraining agents help remedy this problem as well, again through the bubble structure.

Ready-mix plants in freeze-thaw regions usually produce most of their concrete with entrained air. The industry standard is 6 percent air content, but hitting a specified percentage of air is difficult because of the variables involved, so there is a leeway of 2 percent.

The reason for this, Euclid's Fallon says, is the many variables that come into play when it comes to making bubbles. The surface tension of bubbles is affected by everything from the type of admixture to chemical variations in the aggregate to the relative smoothness of the mixer blades in each truck. With so many forces at work, there is no way to hit an exact percentage every time.