Building The Bridge to The 21st Century With Aluminum?
Photo 1: The Reynolds Metals Company's first aluminum bridge deck was installed on the Corbin Bridge in Huntingdon, Pa., in the fall of 1996.
The city fathers of Huntingdon, Pa., had a problem. The Corbin suspension bridge that connected the two sides of the city was, as its historical trust designation implied, aging dramatically and posed a potential threat to any heavy vehicles using it. With a posted 7-ton (6.35-metric ton) weight limit, the venerable structure was off-limits to heavy emergency vehicles, which were forced to use a circuitous 24-kilometer detour to avoid the bridge.
Desperate to save the bridge, but unable to strengthen its deck using conventional construction materials because of its weak substructure, the city opted to replace the existing superstructure with a lightweight aluminum deck that decreased the bridge's "dead load" and concomitantly increased its bearing or "live load" capacity. The result today is a stronger bridge that literally serves as a lifeline for citizens in need of prompt emergency attention.
Although not unheard of, the use of aluminum for bridge deck construction and repair remains an underused technology that at least one aluminum corporate giant _ Reynolds Metals Co. _ hopes to push into the mainstream. Working in conjunction with the Virginia Department of Transportation (VDOT) and the Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center (TFHRC), Reynolds has developed an aluminum bridge deck that early this year will replace the existing deck on a "functionally obsolete" four-lane bridge that carries Route 58 traffic over Little Buffalo Creek in Mecklenburg County, Va.
Although aluminum is more expensive than conventional decking materials, such as concrete and steel, state and federal highway officials are intrigued by the material's potential for long-term savings. So much so, in fact, that VDOT agreed to employ Reynolds' aluminum deck system in two different bridges. (The second site has yet to be selected.) For its part, FHWA is funding the program to the tune of $230,000.
First Test Limited in Scope, Complexity
Already scheduled to be upgraded and widened (to include shoulders) by VDOT, the Route 58 bridge made an ideal candidate for testing Reynolds' aluminum decking system because traffic can be detoured to its companion bridge while construction is underway. The Virginia Transportation Research Council under FHWA sponsorship has initiated a three-phase study of the Reynolds' deck system.
Phase 1 recently was completed at the TFHRC Structures Laboratory, where a deck section was subjected to a series of static load tests simulating truck wheel loads. It took two wheel loads of 77 metric tons each to produce failure in the deck section. This is equivalent to balancing four fully loaded tractor-trailers on two wheels. Clearly, the strength of the deck is more than adequate for highway use. Phase 2 involves a field evaluation of the construction of the deck system. A number of transducers will be placed at strategic locations throughout the Route 58 structure to measure strains, displacements, and rotations under a series of calibrated, vehicular tests. VDOT and FHWA engineers also will develop a finite element computer model that will be used with the field test results to further evaluate the deck system's performance. Any difficulties in the construction process will be noted, and a general assessment will be made, comparing the aluminum construction effort with similar projects using more conventional construction materials. Long-term performance tests will continue to be conducted even after the bridge is open to the public.
Phase 3 will evaluate the structural and environmental durability of a deck panel with the installed wearing surface for fatigue strength and durability. The strains and displacements measured during the field test will be the basis for static and fatigue testing in the laboratory. Prior to testing the panel to failure, non-destructive evaluation of the panel will be used to detect any damage created by the fatigue testing.
At the same time, the durability of the wearing surface will be assessed. This is a particularly important area of study because a major disadvantage of aluminum is its lack of skid-resistance. FHWA is funding research on a variety of polymer-sand aggregate surface materials that are designed to bond with the aluminum to prevent skidding. A notched coating adhesion (NCA) material has demonstrated some promise for accelerated characterization of interfacial strength and, according to FHWA tests, "appears to be well adapted to the coating geometry pertinent to the wearing surface on a bridge deck." Regardless of the material used, however, it is likely that aluminum decks will require some degree of polymer resurfacing although how often will need to be determined.
Photo 2: Phase 1 deck panel test in the Structures Lab of the FHWA's Turner-Fairbank Highway Research Center. Simulated wheel load is being applied in the center of the deck panel.
"We felt (the Route 58 bridge) was a good, simple structure with which to evaluate the aluminum deck," said Malcolm Kerley, VDOT's state structure and bridge engineer. Kerley said the old deck will be removed, new beams will be put in, and the aluminum deck will be placed on top. Afterward, a series of tests will be conducted on the bridge while traffic continues to be rerouted to the companion bridge. "Ultimately," said Kerley, "this project is designed to give us another option as a bridge decking material." Kerley said VDOT and the research council are looking for a second site for an aluminum deck, but he noted that the state would like to "consider a rapid replacement scenario" where the speed of installation is critical to the project's success. Unlike the Route 58 bridge, which is a normal construction project, many bridges need to be redecked as quickly as possible because of limited detour possibilities, cost constraints, heavy traffic, and other considerations.
Lightweight, Anti-Corrosive Properties Appear Promising
Aluminum bridge decks have a number of advantages over their concrete and steel counterparts. Aluminum, for example, offers a higher strength-to-weight ratio than concrete. Where a concrete deck weighs about 5 kilograms per square meter (120 pounds per square foot), an aluminum deck is about 80 percent lighter, usually weighing in at about 0.73 to 1 kilogram per square meter (18 to 25 pounds per square foot). This dramatic weight savings means many bridges can be strengthened without undergoing extensive (and expensive) reengineering of substructures. Additionally, an aluminum deck can accelerate the construction time frame because it isn't burdened by concrete's 28-day cure rate.
Photo 3: Strain gauge instrumentation of the top of the deck panel to record the response to the deck to wheel load.
Likewise, steel can prove problematic because it requires more welds than aluminum, and _ as any structural engineer knows _ every weld is a potential failure point. And compared to steel, aluminum offers lower initial and life-cycle costs.
"Using an aluminum deck can save you a bundle in terms of weight because you don't have to rehabilitate the bridge from a structural standpoint," noted Larry Shives, business development manager for Reynolds Metals Co.
For example, increasing traffic loads in 1934 forced an upgrade of the Smithfield Street bridge in Pittsburgh. A heavy steel replacement deck would have required significant enhancements to the bridge's foundations and girder supports. Instead, an aluminum deck was fitted to the bridge, reducing its dead load by 675 metric tons and increasing its traffic-bearing capacity from 4.5 to 18 metric tons. In fact, it has been estimated that a posted bridge could be upgraded by 30 percent to 40 percent by using an aluminum deck replacement. In 1967, the bridge was outfitted with a more corrosion-resistant aluminum alloy with a 0.95-centimeter polyester-sand wear surface versus a 3.8-centimeter asphalt surface, stripping an additional 97 metric tons from the bridge's dead weight.
An aluminum deck is also more resistant to corrosion and other environmental degradation than more conventional materials, resulting in substantial savings in long-term maintenance costs. When the aforementioned Smithfield Street bridge was rebuilt in 1994 to accommodate more lanes of traffic, Reynolds purchased a 365-meter section of the aluminum deck (which had been redecked again in 1967) to evaluate how much corrosion had taken place in the 27 intervening years.
"We conducted a forensic analysis from a corrosion and fatigue standpoint," said Shives. "We were really amazed at how little deterioration was evident. There was very little degradation after all those years."
Aluminum's lighter weight also results in speedier deck installation, an increasingly important factor given the congestion-weary public's attitude toward road construction delays. And shorter downtime equates to cost savings in the form of less traffic control, fewer accidents, and greater productivity. Aluminum panels can be constructed off-site and then delivered and placed very rapidly _ particularly in comparison to concrete. According to a study conducted by Alcoa in the 1980s, the average downtime for a bridge receiving an aluminum deck was 24 days. Compare that to 10 to 12 months for a comparable job using concrete.
Photo 4: Bubbles indicate buckling failure of the rib plates under wheel load. However, the load that caused the failure was about eight times the legal truck wheel load.
Aluminum also is more ductile than most bridge materials, making it an excellent candidate for use in areas with moderate to high seismic risks. The material is easily configurable to any number of shapes and sizes, and it boasts a high salvage value when replaced.
Plenty of Potential for Both Past and Future Bridges
Depending on which statistics you believe, there are between 250,000 and 600,000 bridges in the United States in need of significant repair. Of these, more than 65,000 suffer from structural deficiencies, and another 12,000 have weight restrictions placed on them. Repairing these bridges could drain as much as $50 billion from the nation's coffers, an enormous sum given the myriad infrastructure problems facing the country.
Eager to upgrade these bridges without the enormous cost and traffic delays associated with more conventional construction materials, state and federal highway officials are monitoring the aluminum bridge deck initiatives in Virginia with great interest. Currently, there are only nine aluminum bridges in the United States up to 50 years old, each of which is faring well.
But while the refurbishment of older bridges may be a cash cow waiting to be milked, Reynolds also has its eye on newer projects with an eye to the future. Aside from the Route 58 bridge deck replacement project in Mecklenburg, Reynolds is also interested in supplying a modified version of the Route 58 deck on a bridge along the "Smart Road" _ a 10-kilometer stretch of highway connecting the city of Blacksburg and Virginia Tech with Interstate 81 in southwestern Virginia.
The highly touted Smart Road is the first of its kind to incorporate many of the cutting-edge technologies of the intelligent transportation system. Part of this mammoth endeavor includes a high-performance materials project that will incorporate high-performance concrete, steel, and other materials in the construction of roadways, bridges, and related structures. Shives is confident the company's aluminum deck will be used on one of the bridges. "It looks like that is going to happen, and we are going to be part of the Smart Road project."
VDOT's Kerley concurred, suggesting that two of the next-generation road's smaller bridges will employ high-performance materials _ concrete on one and aluminum on the other. One of the Smart Road bridges under consideration for an aluminum deck is a three-span structure that is "a much more complicated structural development project because of load reversals and other phenomena not present" on the Route 58 project, said Kerley.
This three-span continuous bridge carries I-81 traffic over a series of railroad tracks in Montgomery County, Va. This bridge, which is longer than the Route 58 bridge, would be the first continuous structure to use an aluminum deck.
"We're very excited to be a part of such an ambitious project," said Shives. "Smart Road is about cutting-edge technologies, and we think the aluminum decking system is one of them."
With apologies to President Clinton and his 1996 campaign team, it does indeed appear aluminum will play a major role in building America's bridges to the 21st century.
William Wright is a research structural engineer in the Structures Division of FHWA's Office of Engineering Research and Development, where he has worked for the last eight years. He manages the Structures Laboratory and directs the high-performance materials research for both steel and aluminum. He received a bachelor's degree in civil engineering and a master's degree in structural engineering from the University of Maryland in College Park. He is currently completing requirements for a doctorate at Lehigh University. Wright is an active member of the American Society of Civil Engineers and a registered professional engineer in Maryland.