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Aluminum

Counteracting collapse

By Gretchen Salois

Above: An AlumaBridge deck is installed in one day to rehabilitate a bridge in Sandisfield, Massachusetts.

Rehabilitating aging bridges inspires technology shift

November 2014 - With each set of tires carrying passengers, cargo and commerce over the roadways, inevitable deterioration ensues. The structures take a pounding as vehicles rumble over them and Mother Nature also takes her toll.

When a bridge is no longer structurally sound, methods to remedy the problem are often limited. “Replacing an entire bridge is difficult, time consuming and expensive due to traffic control during reconstruction, long detours, or the need to build a [temporary] parallel structure for traffic flow while making repairs,” says Greg Osberg, president and CEO of AlumaBridge LLC, Ann Arbor, Michigan. “That’s why an option to rehabilitate the bridge with lightweight aluminum decking to reduce dead load is a big benefit. Aluminum is one-fifth the weight of concrete, but still possesses the same look, safety and skid resistances with an epoxy/aggregate wearing surface. Aluminum is also corrosion resistant and helps minimize maintenance.”

Bridge owners have still fewer rehabilitation options once the structure approaches 50 years of its expected life span. As bridges age and traffic volume increases, needed repairs become more difficult to make. Aluminum isn’t new to bridges or decking, says Osberg, but “with the introduction of friction stir welding, it helps make this rehabilitation solution commercially viable and scalable to address larger structures.”

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Methods for welding aluminum are advancing, providing more versatility and opening up new applications. Compared with arc welding, friction stir welding introduces less heat to the process. “The material to be welded is stirred using friction and pressure, leaving you with a superior weld,” Osberg says. Friction stir welding can join metal without bringing it to a molten state. A pin tool spins to join the two surfaces by allowing the two pieces of metal to intermix at the joint. The softened metal is joined by applying mechanical force with the pin tool. By generating less heat, the joined material is subject to less deformation.

“If you put heat on aluminum with traditional fusion welding, you’ll have a larger heat-affected zone. When you’re welding 33-foot-long panels like those for the St. Ambroise River Bridge project in Quebec, Canada, the heat-affected zone does not substantially alter the material like gas/metal arc welding, which is an example of traditional fusion welding,” Osberg says. The St. Ambroise River Bridge is a multipanel bridge deck and the first bridge in Canada to use this type of friction joining process.

Comparing materials

Each winter brings with it a freeze/thaw cycle that over time breaks down the concrete used on bridges, not to mention road salt’s ability to do likewise. “With aluminum, the process of expansion and contraction doesn’t lead to cracking like concrete,” Osberg says. The aluminum deck also has a 3⁄8-inch epoxy wearing surface with aggregate that is not prone to the buckling seen on concrete bridge decks. Epoxy overlays can be used to make the wearing surface indefinitely sustainable, even on bridges with high traffic.”

AlumaBridge offers an 8-inch-deep deck that replaces concrete covering steel beams. “While concrete is cost effective, often times it isn’t an option because of its weight on a structurally deficient bridge,” he explains. According to AlumaBridge, pricing for the project cost $150 per square foot (which assumes a cost for aluminum at $1 per pound). 

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AlumaBridge also produces a 5-inch-deep bridge deck to replace an open-grid steel deck. These decks can be rehabilitated with aluminum decking in locales such as Florida, where 10 percent of U.S. steel grid bridges are located, Osberg says. Many such grid decks are on movable bridges but civil engineers “are steering away from steel grid and want a weight-neutral alternative with a solid riding surface,” Osberg says. Prices for aluminum decking are demonstrably competitive with traditional options. According to the Florida Department of Transportation, reinforced concrete flat-slab, simple-plan bridges range from $115 to $160 per square foot. Concrete deck/steel girder for simple-plan, medium- and long-span bridges range from $125 to $142 per square foot. 

Safety is a major concern due to limited skid resistance on open steel grid decks. “Not only do bridge owners want a solid skid-resistant surface, but they also want a corrosion resistant material. That’s another area where aluminum is superior to steel because it doesn’t require painting,” he says.

Where timing and location are a concern—it can be both costly and inconvenient to shut down bridges or detour traffic—AlumaBridge’s option allows for prefabricated modular decking to be rapidly deployed and installed. “The deck panels arrive ready to bolt to the beams and in some cases, are connected to brand-new beams and shipped to the site as an entirely new assembly,” Osberg says.

Rapid deployment

While weight and traffic patterns are concerns when restoring structurally deficient bridges, some structures are functionally obsolete—they are too narrow, historic, or are in a poor location to match current traffic needs. “That’s a perfect storm for aluminum decking,” says Osberg. 

In Portland, Oregon, a movable bridge with steel grid decking was in need of replacement. “They put a fiber reinforced polymer (FRP) composite deck on initially and had some issues. The bridge owner is now looking to specify and bid an aluminum deck as a solution.”

AlumaBridge uses a grade 6063 alloy tempered to a T6 condition. The aluminum deck has been tested and achieves maximum fatigue life. According to Osberg, “After loading that simulates highway conditions, fatigue properties level off, showing a pattern where additional cycling has no effect on the material.”

The company is working on a bridge deck for the Ministry of Travel administration in Quebec. “We’re in the final stages of fabricating that deck,” Osberg says, adding that the work is in the friction stir welding stage. Once the panels are welded, hole drilling for beam connections will be done in the factory to accelerate on-site installation. 

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For other applications, Osberg says friction stir welding is ideal for military bridges, rapid deployment bridges and emergency deployment bridges. These structures benefit from structural aluminum and the automated fabrication process. “If an act of nature, like say, a hurricane, wipes out major bridges, aluminum decking can be pre-attached to a superstructure, shipped over-the-road, and put into place instead of a temporary bridge,” he explains. “It can be opened right away and serve as a permanent structure.”

The company has also received inquiries for construction platforms and for commercial marine applications. “They want corrosion-resistant solid surfaces,” he says. “In the marine environment, you can drive trucks onto it with the same capacity as a U.S. highway bridge.”

Rapid bridge rehabilitation can be used to help solve what Osberg deems “the nation’s bridge crisis.” The company’s first project resulted in a single-lane bridge installation for the Massachusetts Department of Transportation. “With the steel superstructure already attached to the deck, the aluminum deck was installed from crane to bearings in 30 minutes.

“Aluminum isn’t new and neither is friction stir welding,” Osberg continues. “But the combination of the two is new and that’s what is appealing to the bridge industry, which is looking for new materials and technologies to help accelerate bridge construction.” MM

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