Q: I read that aluminum reacts with concrete. I am interested to ?nd out if an aluminum bull float will affect the concrete of bridge decks. We (a state transportation department) allow the use of burlap or wooden floats to finish the concrete after the screeding. The reason for this is to create a sandy texture that will increase the skid resistance of the concrete. Will a magnesium or aluminum bull float give this kind of finish or will it be a very smooth finish?
A: While aluminum will react with concrete when embedded, that's not relevant in a tool. An aluminum float will not react with the concrete during a finishing operation since the contact time is shortùthere is no long-term change to the concrete. Therefore, you can use aluminum tools or an aluminum truck bed to haul concrete for a short duration without any adverse effect.
A magnesium or aluminum bull float could make the surface a bit smoother than a wooden bull float, but since you are probably using air-entrained concrete on a bridge deck, a wooden bull float may be difficult for the finishers due to the stickiness of the concrete. Bill Phelan, with Euclid Chemical, wonders what you mean by a "sandy" texture? He recommends that proper texture be determined from a sample slab; otherwise it is in the eye of the beholder.
Chemically, aluminum will react with the alkalis in concrete (OH) and produce hydrogen bubbles. If you add enough aluminum powder to a full load of concrete, it will spill out of the drum after a little while. Make a cylinder with this concrete and you will notice a mound on top of the cylinder the next day. There is at least one admixture available that uses this principle to produce expansive cement, and this reaction is used in the production of autoclaved aerated concrete.
Significant corrosion of aluminum embedded in concrete will occur due to this reaction. The corrosion of the aluminum will result in expansion and subsequent cracking. If coupled with ferrous metals, galvanic corrosion will also occur. The presence of calcium chloride greatly accelerates the process. Even seemingly small amounts of aluminum can be a problem. In one case, 10-gauge aluminum wire was used to tie rebar together. In less than 9 months there was significant localized distress with spalling immediately over the location of each tie.
A good resource for more information is "Corrosion of Nonferrous Metals in Contact with Concrete," publication #IS 136.04T, from the Portland Cement Association (phone 847-966-6200), or go to www.portcement.org.