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|Federal Highway Administration > Publications > Public Roads > Vol. 67 · No. 3 > A Fix for Aluminum Overheads|
A Fix for Aluminum Overheads
by Paul Mooney
Using innovative fiber-reinforced polymer repairs to correct distress in welded joints on aluminum sign structure trusses.
In the early 1960s, State departments of transportation (DOTs) began using aluminum trichord overhead structures to support signs along our Nation's highways. The advantages of using aluminum were obvious. It is lightweight, costs less than steel, and is inherently resistant to rusting. In 1994, however, the American Association of State Highway and Transportation Officials (AASHTO) started to include fatigue as a design factor for overhead signs.
“This development, combined with observations of cracking and occasional structural failures in aluminum signs,” says Chris Pantelides, professor of civil engineering at the University of Utah, “alerted engineers to a potential life-threatening problem that may exist in States throughout the country.”
Over time, wind forces create stresses on an aluminum structure, eventually causing cracks to appear in the welded joints of the truss diagonals. If these cracks are not discovered and repaired, a welded joint eventually could fail and cause an aluminum diagonal to fall onto the roadway. “These problems are surfacing in large numbers now that many of these aluminum sign structures have been in service for 30 years or more,” says Pantelides.
The consequences of a failed welded joint could be tragic, especially on high volume roads where traffic moves continuously and at high speeds beneath hundreds of aluminum overhead sign structures.
Fortunately, New York State DOT (NYSDOT) engineers, working with private industry and a research team from the University of Utah, found an inexpensive way to repair problematic structures to increase their safe and useful lives. Using a fiber-reinforced polymer (FRP) composite to wrap cracked joints, workers can restore the structural integrity of a cracked joint to virtually the same strength as the original aluminum weld. The cost of the material is minimal, and the repair can be conducted in the field. In sum, this solution can be applied with minimum difficulty.
A Sizable Problem
NYSDOT engineers first became aware of a problem when maintenance crews reported some minor failures of truss diagonals. Those small failures alerted the engineers to a potentially larger problem.
“It was decided to inspect all of the aluminum overhead signs on Long Island to determine the extent of the problem,” says NYSDOT Senior Civil Engineer Harry White. “Initial inspections in one area of the State revealed that approximately 10 percent of all the overhead sign structures had some form of structural damage. We then decided to inventory and inspect every overhead sign structure in the State. If we hadn't inspected, the risk would still have been unknown. There are more than 2,000 of these types of structures in service in New York. Depending on the size and location, replacing an overhead sign structure can cost as little as $50,000 or up to $300,000 for a large structure carrying variable message signs.”
With little warning and no time to prepare for sticker shock, NYSDOT suddenly was facing a potential $25 million problem.
Finding a Low-Cost Answer
Facing the tremendous cost and logistics of replacing hundreds of overhead sign structures spanning some of the busiest roads in the State, the agency needed to find a better solution. At the time, John Neidhart was a senior civil engineer for NYSDOT and head of the overhead sign structure unit. Neidhart recalls, “We were thinking of all kinds of crazy repair ideas, and someone directed us to an FRP [fiber-reinforced polymer] contractor.” At the time, these composites already were being used to retrofit substructures in New York.
Fiber-reinforced polymers have been used successfully in many transportation applications, but this was the first time they were considered as a wrap for welded aluminum joints on overhead signs. NYSDOT and the Utah DOT organized a pooled-fund study to determine whether fiber-reinforced polymers could be used to repair cracked aluminum joints. Larry Cercone, a consultant with Air Logistics Corporation, contacted Chris Pantelides to conduct the study.
Testing a Practical Solution
The research was designed to test cracked aluminum weld connections that were either unwrapped or wrapped with fiber-reinforced polymers. Researchers wanted to see whether the cracked aluminum welds could be replaced by this composite material. The research also tested aluminum chords that had not been previously welded but were reinforced with fiber-reinforced polymers. For these specimens, a tack weld (for alignment) was applied and then wrapped with the composite material.
Testing involved placing repetitive forces simulating wind on the test specimens. A triangular load frame was built to enable the diagonal of the specimen to be positioned vertically, directly under an actuator (a device that converts hydraulic energy into mechanical energy). The actuator applied loads on the joints of specimens at the same angles that natural forces act on the structure in the field.
The results were promising. Cracked samples repaired with fiber-reinforced polymers achieved strength in excess of 115 percent of the original welded connection. In addition, samples fitted only with a tack weld and fiber-reinforced polymers reached capacities virtually equivalent to a newly welded aluminum connection.
New York Implements The Solution
The New York transportation agency watched the test results closely and implemented the solutions as early as possible. Initial results enabled NYSDOT engineers to consider the repair method effective for 2 years.
“Adding just 2 years of usable life to a structure is a tremendous benefit,” says White. “It allows the OSS [overhead sign structure] fabricator to work the replacement structure into the schedule, rather than having to stop current production to fabricate an emergency replacement structure. Having the OSS fabricated and erected under an emergency situation significantly increases the cost over typical construction. This repair eliminates those extra costs.”
As additional results continued to confirm the effectiveness of the repair technique, New York engineers upgraded their confidence in the repair. “Additional results from the study allowed us to consider the repair effective for 5 years,” says White. As long as the technology proves to be successful over time, estimates of the lifespan of the repair will continue to increase. “Once the repair is complete, we consider the overhead sign structure to have its full load-carrying capacity,” says White.
One of the most attractive features is that the repair can be made in place at a minimal cost and in about 3 hours' time (discounting time for traffic control). Once the necessary work zone is set up, trained maintenance or contractor workers can repair the structure on the roadway using a manlift.
One of the keys to successful implementation is to make certain that the surface is clean prior to wrapping, ensuring proper adhesion to the aluminum surface. One approved system requires 37 simple steps to complete the repair. “The repair itself requires care and diligence by the workers, but it is not difficult,” says White.
Larry Cercone says, “The application of the FRP-aluminum adhesion is based on standard aerospace procedures. First, absorbent pads are hung beneath the joint to capture any chemicals used.The surface is prepared using an alkaline chemical, acid, and water to clean and etch the surface to improve the mechanical bonding properties of the aluminum.Primer and an adhesive are applied prior to the FRP. Four different weaves of FRP—a fine woven wrap, a heavier weave, a tubular braid, and unidirectional tendon wrap—are applied strategically on the joint to achieve the needed strength characteristics.
“Water acts as the catalyst to start the chemical reaction that hardens the material,” Cercone continues. “The FRP achieves its strength approximately 60 to 90 minutes later. The only byproduct from the reaction is carbon dioxide, making the repair environmentally safe. Optionally, the cured FRP can be painted to match the rest of the aluminum structure so the repair will be invisible to the traveling public.”
When a contractor repairs a sign structure, NYSDOT requires an inspector to be present, or the procedure is videotaped. The agency also requires all repaired structures to be inspected annually.
According to White, the cost of the repair is far less than that of replacing the structure. The repair materials include surface cleaning supplies, an epoxy for adhesion, the composite material, and a few other low-cost materials. In total, NYSDOT spends approximately $300 on the materials needed for each joint repair. Few tools are needed, aside from a cordless drill and scissors.
Looking toward the Future
New York engineers continue to inspect repaired structures and watch for additional results from the ongoing research. They are confident in the repairs as a temporary fix until the structures can be cycled into a replacement program. Future research and experience will determine whether the repair can be considered more permanent.
The important thing is that the safety of the motoring public in New York is being held to the highest standard. State DOTs around the country strive to make highway facilities as safe as possible in light of limited budgets. This repair accomplishes both goals. It saves money, and, more importantly, it protects lives.
AASHTO's Technology Implementation Group (TIG) chose to support nine technologies that are likely to yield significant economic or qualitative benefits. The group added repairing hazardous sign structures to their technology priority list.
Paul Mooney is the technology and marketing specialist for the Federal Highway Administration's Utah Division Office.
For more information, contact Harry White II, P.E., senior civil engineer, NYSDOT, 518-485-1148; John Neidhart, P.E., senior civil engineer, NYSDOT, 518-485-5700; or Chris Pantelides, P.E., professor of civil engineering, University of Utah, 801-585-3991.
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