Tarmac: Ready Mix Concrete

TARMAC'S SILICA FUME HELPS REYNOLDS RECYCLE

Glenn Webb, Sales Manager, Richmond Ready Mix

Reynolds Metals operates an aluminum recycling plant at their facility in Richmond, Virginia. In 1993, Reynolds was having a problem with outside paving breaking up, prompting them to contact Tarmac. You may have seen Reynolds aluminum recycling centers in your area- they're usually tractor- trailers painted to resemble a pink and blue box of Reynolds Wrap. "Smelting in the recycling process uses a lot of salt during cooling," explained Don Holder, Tarmac quality control manager for the Richmond area. "Salt draws the aluminum out of the raw material but also deteriorates concrete. The concrete in front of their 'salt shed' is a high traffic area for fork lifts, as well. Reynolds was having to replace this concrete every three years." Holder suggested using silica fume concrete as a test area.

Silica fume is the residue created by gas burning smoke stacks. It's a pozzolanic material, meaning it causes extremely high strength and density with low permeability. A coulomb value of 123 was measured in this particular silica fume mix; any rating below 1,000 indicates low permeability. This is important because salt penetration, which can break up concrete and rust steel rebar underneath, is slowed. Compressive strength results measured 8,950 pounds-per-square-inch after 56 days. This indicates a highly dense concrete, which also contributes to low permeability.

Tarmac was told to proceed when the concrete needed to be replaced in 1993. For the Reynolds project, 70 pounds of silica fume was added to 700 pounds of Portland Cement, with a low water/cement ratio of .38. So little water was used, in fact, that a super-plasticizer was added to increase the concrete's slump.

"Silica fume is more expensive than regular concrete. However, Reynolds will get three to four times the service life before it needs replacing- this saves on materials but especially labor. I looked at it three months ago and there's no deterioration," Holder said.

Reynolds was so impressed with the results of this test, they have Specified Tarmac silica fume concrete for use in several different locations in their facility.

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