By Luke M. Snell, P.E.
A concrete precaster recently contacted me with problems from bubbles forming during casting and white streaking on the concrete surface. I learned that he was placing aluminum inserts into the fresh concrete, which were required and could not be substituted for other metals.
The problem with aluminum and fresh concrete is well understood. The alkali in the cement will react with the metal and cause hydrogen gas to form within minutes, as shown in this YouTube video: https://www.youtube.com/watch?v=nR9BtBZMF0I.
When hydrogen gasses are trapped in the concrete as it hardens, voids resulting from entrapped air are created that result in a significant loss of concrete strength. The New York Department of Transportation did a study casting concrete in aluminum molds. Their research showed a 35 percent loss in strength compared to concrete cast in standard cylinder molds.
The loss of concrete strength is further complicated if aluminum is used in reinforced concrete or concrete containing calcium chlorides. Corrosion will occur on the aluminum resulting in spalling and cracking of the concrete. For this reason, many state and federal agencies will not allow aluminum conduits or inserts to be used in concrete.
The best way to solve this problem is to provide a protective coating on aluminum. Research shows that using bitumen, lacquer, or enamel paint will provide adequate protection. However, care must be used when placing the concrete to ensure the coating is not scratched or damaged.
There is a simple experiment to show the effects of concrete cast against aluminum and the effectiveness of a protective coating. Cast a concrete mixture in an aluminum cupcake form, and repeat this time applying enamel paint from a spray can on the second cupcake form. Allow the paint to dry overnight before placing the concrete in the mold.
The concrete cast against the aluminum had several small bubbles at the contact areas caused by the hydrogen gas. In contrast, the concrete in the aluminum with a protective coating shows no distress.
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This article originally appeared in the bimonthly Arizona Contractor & Community magazine, Mar/Apr 2021 issue, Vol. 10, No. 2.
