This driveway entrance pavement was placed on a commercial site in late summer, but looked like this by the following spring. Only petrographic analysis can determine the exact cause of failure, but inadequate air entrainment is a leading suspect.
This driveway entrance pavement was placed on a commercial site in late summer, but looked like this by the following spring. Only petrographic analysis can determine the exact cause of failure, but inadequate air entrainment is a leading suspect.

Air-entraining admixtures are the most problematic of all admixtures for manufacturers, ready-mix producers, and concrete contractors. When there isn't enough air entrainment in concrete placed in a freeze/thaw climate, scaling results. If there is too much air, compressive strength goes down and durability can become an issue. Air is affected by a wide range of conditions: the type of cement used, other admixtures, some types of integral color, the amount of water in a mix, the condition of the ready-mix truck, pumping, and how long the concrete is in the ready-mix truck before placement.

Entraining decorative concrete

Decorative concrete is dependent on the durability of its surface. Some scaling or damage to plain concrete is generally accepted, but not tolerated when it occurs with decorative concrete. These complaints are directed to contractors, who are the ones that usually pay for the repairs. Repairs involving insufficient air entrainment require removal and replacement because the entire thickness of a slab is affected and will eventually scale.

Some in the decorative industry recommend air entrainment be reduced when dry-shake coloring is applied. The rationale is that less entrainment will result in more bleed-water, making it easier to wet-out the color. But the ground rule to have the right bubble size and the right spacing between them must still be followed and contractors bear significant liability if they choose to do otherwise.

Integral colors do not have an effect on air entrainment, with the exception of carbon black color. Carbon particles cause significant loss of air entrainment bubbles during the mixing process. However, most black color on the market is made from mineral oxides and doesn't affect air entrainment. But carbon black provides a richer black appearance and is sometimes used.

Make sure you know which black color you are using. As a precaution, it's good to check air entrainment percentages in integrally colored concrete before placement.

Being preventative

Air-entraining admixtures are the least expensive of all admixtures, and the most expensive. So many conditions affect air entrainment and repairs can be very expensive, yet prevention is penny cheap. Test your concrete for air content at the point of placement just before you place it. If your placement is time-consuming, check the air content again after 30 minutes and again after one hour. You might think this is costly in terms of time—it is. Air meters are fairly expensive and it takes at least five minutes to perform the test. But losing one project due to scaling makes for a very bad day, to say nothing about your reputation.



Two kinds of air

Vinsol resins made from wood byproducts were the first air-entraining admixtures in the marketplace. They performed best at lower water-cement ratios, but their effectiveness diminishes as slump increases beyond 6 to 7 inches.

Today, most ready-mix producers use synthetic air-entraining admixtures with exotic names such as tall oils and fatty acids. They create smaller air bubbles in large numbers, which makes them more effective in freeze/thaw situations. Unlike Vinsol resins, synthetic air entrainment increases as the slump rises, resulting in air entrainment as high as possibly 20% or more, causing serious concrete problems.

In spite of the differences between entrainment admixtures, the requirement is always the same. For example, a specification for 6% air in a mix remains the same for all entraining admixtures, though synthetics would seem to make concrete more freeze/thaw durable due to the smaller bubble size and spacing between bubbles. You should know which product is being used for your concrete because of how they perform in relation to slump.