Luke M. Snell, P.E.
An inspector recently contacted me regarding some concrete loads that exceeded air-content specifications. The technician was concerned about the ramifications of the issue and what they could do to correct it.
Air entrainment is typically specified for concrete that will be exposed to freezing weather. All concrete has some internal water, which will expand about 10 percent when it freezes. This results in extremely high pressures in the concrete that can cause cracking, scaling, and deterioration.
The concrete industry has developed air-entrainment admixtures to solve this expansion problem to create tiny microscopic bubbles in the concrete. If the concrete is above freezing, these bubbles provide empty, voided areas. When the water in the concrete starts to freeze, it's forced into the empty bubbles and relieves the pressure and the potential damage to the concrete.
Typically, the amount of air in concrete should be between 4.5-7.5 percent but can vary depending on the size of the coarse aggregates, the specified strength of the concrete, and the weather at the job site. Air entrainment admixtures are added at the batch plant, and the concrete should have the correct air content when it arrives at the project.
The amount of air entrainment admixtures added to concrete mixtures is not an exact science. There is no formula for the batch plant to make sure they get the correct air content. The most significant variable in getting the percent air accurate is the temperature of the materials at the batch plant.
Batch plants have “rules of thumb” that they follow from their experience with local materials. When the materials (aggregates, cement, and water) cause the concrete mixture to be hot, the batch plant puts in more air entrainment admixtures. When the concrete mix is cool, they add fewer. This approach may appear to be unsophisticated, but from experience, the batch plants can usually achieve the required air content in the concrete.
To read the rest of this article, you are invited to purchase the digital issue here.
This article originally appeared in the bimonthly Arizona Contractor & Community magazine, Sep/Oct 2021 issue, Vol. 10, No. 5.