Wasted time on a jobsite can mean profits down the drain. How often has this happened to you? A load of concrete arrives specified for 6% entrained air. The air volume tests too high or too low. The owners reject the load while your crew waits idle for the next batch. Or worse, you might not find the air volume was out of spec until after it has cured. Either way, these potential problems can only be solved by replacing the concrete.
Terry Lavy, Lavy Concrete Construction Inc., Dayton, Ohio, tells a familiar story. “The last time we received a load where the air volume was low, we had to let a truck sit while someone from the ready-mix plant came out and dosed the truck on the jobsite to get it up to specifications,” he says. “It delayed the job and caused us some anxiety.”
Because nearly all exposed concrete in moderate and cold climates is specified to have entrained air, making sure the air volume is within spec and stable can be tricky. If air falls short of the specification, deterioration of the concrete may be faster, leading to finger-pointing and expensive repairs.
Lavy has also experienced the long-term consequences of low air. “We did a slab that suffered freeze/thaw damage. I think the concrete didn't have enough air in it, but no one tested it. It's an outdoor parking lot at a factory, and I drive by it and see how much damage has occurred to it, and I'm pretty sure it didn't have enough air.”
The common specification calls for approximately 6% of total volume to be air bubbles. Air-entrained concrete is far more durable in freeze/thaw conditions. On the flip side, entrained air above 4% lowers strength. In fact, each percentage point above this threshold reduces concrete strength by an estimated 5%. So air content elevated by only two percentage points can cut the strength of a 4000 psi mix by 400 psi. Jeff Crowell, Rodgers Builders, Charlotte, N.C. says that “When you target 6% and you get 7% to 8%, you can start seeing the impact on the strength. Above 10% and strength drops off significantly.” This can be enough to miss the strength specification, creating angry owners, delays, and expensive replacement.
Factors you can control
As a contractor, there are three factors you control that can affect air volume after the concrete arrives onsite: mixing time, pumping, and hydration.
Conventional air-entraining admixtures (AEAs) are all surfactant-based. Surfactants are basically soaps and detergents, and like the common household versions of those products, they foam up into bubbles when they are agitated with water. They can be inserted directly into a mixer where the agitation creates bubbles that disperse throughout the concrete because the surfactants are attracted to the cement. In short, they react readily with the water, the agitation, and the cement.
In mixing, air content typically increases for a few minutes as the agitation froths the AEAs, then it falls as mixing time increases. Hotter weather speeds up the loss of air. Pumping concrete compresses the air, destroying bubbles and reducing air content. Adding water in the field may seem like a good way to increase flowability, however, it creates new bubbles and usually increases the air content, decreasing the strength of the cured final product. This is on top of the reduction in strength that comes from raising the water-cement ratio.
“With conventional surfactant-based AEAs, air is a hard one to maintain,” says Kirby Justesen, Formco Foundations, West Jordan, Utah. “It gets compressed over time. When you pump it, it compresses and that takes out some of the air. Also the addition of water and higher slumps will change it.”
Mix design
In addition to the factors under your direct control, there are others affecting air entrainment that are the responsibility of the ready-mix producer. Joe Neuber, Neuber Concrete, Kimberton, Pa., notes, “If you are using a straight cement mix, the air content is fairly easy to control. When using hybrid mixes that use slag, fly ash, cement, and combinations of the bunch, we start to see that the air is more of a problem. When you use supplementary cementitious materials, they all have different properties and characteristics which affect air differently.”
