Gerdau recycles concrete-preserved steel for historic Knoxville bridge
June 2012 - On Google Maps, if you zoom in on downtown Knoxville, Tenn., you might notice a missing link where Henley Street, a major thoroughfare, would straddle the Tennessee River. Knoxville drivers will tell you, in fact, it does exist—the 80-year-old Henley Street Bridge merely is under construction. And some of its original steel is being reused.
Gerdau’s Knoxville steel mill is melting, recycling and returning some of the bridge’s original steel rebar, which was preserved in concrete for decades, to reconnect the material with its modern incarnation.
Gerdau processed about 700 tons of the scrap rebar, most of which will become salable product. But Gerdau made sure about 150 tons of what was removed was reused, says Johnny Miller, vice president and general manager of Gerdau’s Knoxville steel mill.
“For historical purposes, we wanted to track some steel that we knew came out of the bridge and went straight back in,” he says.
Knoxville Iron Works, which Gerdau acquired in 2000, fabricated the original bridge’s rebar in 1930. A side-by-side comparison with modern rebar reveals how its manufacturing has evolved.
For one, the removed rebar featured the letter “K” embossed on its side, a branding or signature of the company, indicating the rebar’s source. But more noticeable is the shape. The extracted rebar is square, and the new bar is round.
Miller says after discussing the shape with Gerdau’s engineers and metallurgists, they came up with an explanation as to why round has become the standard. The square shapes used early on didn’t bond as well to concrete as round rebar. The ribbed-like contours, or deformations, bond better to concrete and are easier to form with today’s processes. The steel’s chemistry was different, too. Steel industry veteran and consultant Blair Trimble told Miller that old rebar was often made from converted rail.
“Now, we have chemical specs depending on the kind of rebar we make, and we make it exactly to the proper chemistry,” Miller says. “Back then, I think you were at the mercy of whatever the chemistry of that rail was that you rolled into that rebar.”
With modern processes, Gerdau can melt scrap down to liquid steel and tweak the chemistry as needed. “That’s the big difference,” Miller adds.
Although contractors demolished much of the 1,793-foot-long Henley Street Bridge, the project is not a complete rebuild. The Tennessee Department of Transportation was only a few years away from having to impose load restrictions due to the bridge’s deterioration, says Mark Nagi, community relations officer for TDOT.
“The arch and piers were the only salvageable components of the bridge,” he says. As of late April, the repairs were about 40 percent finished, with scheduled completion in 2013. In 1930, the cost of construction was about $1.15 million, and the rehabilitation is estimated to cost about $24 million.
During the first days of construction in January 2011, the contractor decided he would try to recycle the steel rebar and concrete, says Nagi. It wasn’t a requirement of the contract.
Miller says the contractor removed the concrete away from the rebar with a mobile shear, then fed concrete chunks into a processor that left behind rebar that, despite its age, was in good condition. It was then delivered to Gerdau.
“It’s perfectly good scrap,” he says.
The bridge deck, caps and columns down to the arch supports are being rebuilt, and the bridge deck will be about 12 feet wider. Tennessee’s State Historic Preservation Office worked with TDOT to maintain the bridge’s historical integrity, which resembles the original in sidewalks and railing, according to Nagi.
However, locals traversing the bridge, as well as those who worked directly on the construction, will know the history beyond the pavement. That Gerdau could recycle the steel is a testament to its key qualities.
“It just really tells you about the recycling aspect and the longevity and durability of steel,” Miller says. MM