Answer: Forty Broom, former mixer salesman and unofficial historian for THE CONCRETE PRODUCER, tells a story about a drum manufacturer that developed a new mixer drum in the late 1960s. A staff engineer had this great idea of trying to reduce weight by using aluminum. The marketing department was easily hooked on the new product's potential appeal, and a prototype machine was soon made. The unit worked very well for a few days.
But then, after being charged with a full load of concrete at a nearby ready-mix plant, the drum's bottom split at the seam and opened up, practically burying the chassis in fresh concrete.
Unfortunately for that manufacturer, someone didn't ask a question recently posted on Aggregate Research Industries' interactive industry forum page (www.aggregateresearch.com/forum).
The question of the effect of concrete contact with aluminum drew some great responses. One respondent summed it up: "Aluminum and concrete don't mix, either in the plastic or hardened state."
Aluminum reacts with the alkalis (OH) found in portland cement concrete. When these two chemicals are combined, the reaction produces hydrogen gas. This is why, when the reaction occurs in wet concrete, you'll notice tiny bubbles coming to the surface of a slab.
One responder commented, "If you add enough aluminum powder in a full load of concrete, eventually, there'll be enough gas to cause the mix to expand, and eventually the fresh concrete spills out of the drum. For a less dramatic test, take an aluminum chute on a jobsite, and fill it up with concrete. After about 45 minutes, you should see bubbles coming to the top of the concrete."
In most situations, the contact time between aluminum and fresh concrete is relatively short, so it normally doesn't cause a problem. Therefore, you can use an aluminum truck bed to haul concrete for a short duration without any adverse effect, as along as it's cleaned after use.
But serious consequences can arise in situations of long-term contact. First, a significant corrosion of aluminum embedded in concrete can occur. The corrosion can cause expansion of the concrete and subsequent cracking of hardened concrete.
Second, if the aluminum is coupled with any ferrous metals, galvanic corrosion will occur also. In both cases the presence of calcium chloride greatly accelerates the corrosion process.
Even seemingly small amounts of aluminum in concrete can be a problem. One responder offered the example of a jobsite where the contractor had used 10-gauge aluminum wire to tie rebar together. Within nine months the producer was back on the site, trying to explain areas of localized distress with spalling. When the testers chipped into the concrete, they discovered the location of each aluminum tie the crew had used.
Sometimes the reaction created by mixing aluminum with concrete can be a good thing. At least one admixture on the market uses aluminum corrosion to produce expansive cement. The cement paste expands to fill voids.
Connie Field, librarian at the Portland Cement Association (PCA)'s library in Skokie, Ill., provided a good source for information on this topic. Years ago PCA issued two reports on the subject: Corrosion of Aluminum Conduit in Concrete (RX173) and Corrosion of Embedded Material Other Than Reinforcing Steel (RX198). You'll find summaries of these reports at www.cement.org. You can download an Adobe Acrobat Reader file, or you can contact Connie at firstname.lastname@example.org.