Composite Bridgework Expands Across America
Workers install the first balsa-cored composite deck project containing Single-Walled Carbon Nanotubes in Assumption Parish, La.
Composite materials continue to penetrate into various bridge projects across the country.
A new balsa-cored composite bridge deck opened to traffic over the Pierre Part Bayou in Assumption Parish, La. The parties involved in the project included the Louisiana Department of Transportation and Development (LA DOTD), Louisiana State University (LSU), Crescent City Composites, and Alcan Baltek. The latter company says this installation is believed to be the very first balsa-cored composite deck project containing Single-Walled Carbon Nanotubes (SWCNTs).
The bridge is intended to replace traditional steel grating structures over the state’s regional bayous. Louisiana has regulations for movable bridges over waterways, and navigable water has to have at least one removable section to accommodate heavy waterway traffic. Kurt Feichtinger, technical services manager for Baltek, says composites help meet that requirement.
“For the most part, the bridges all have one span with a grid metal deck, which allows a large ship to go up the bayou by undoing the nuts and pulling the grid metal deck plates already welded to rolled steel girders and put them back later,” he said. “But, they don’t last long, so the idea was to identify an FRP solution to equal the strength, stiffness and weight.”
Half of the six 6-feet by 25-feet plates are additionally comprised of SWCNTs. This along with installed fiber optic strain gauges in the bridge panels is intended to monitor the performance of the laminate over several years. The location was chosen because it is heavily traveled by sugar cane trucks headed south to New Orleans.
Feichtinger believes this project could advance the technology in the infrastructure sector. “We’ve developed a lot of data on characterizations of laminates with single-wall carbon nanotubes,” he said. “Part of the problem is they affect the viscosity of the resin, and since these are vacuum-infused, there’s a practical limit of 0.5 percent by weight. But even that 0.5 percent by weight in e-glass, we found a 30 to 35 percent increase in flexural strength.”
Feichtinger added that as carbon nanotubes catch on and find new applications, the higher volume will drop the price lower. It is the price performance issue that remains the biggest obstacle for the technology.
Meanwhile at Fort Bragg, N.C., Axion International Holdings, Inc., Basking Ridge, N.J., constructed two 100 percent recycled plastic bridges for the US Army. The eco-friendly structural building materials used in the bridges have been specifically engineered to allow for the crossing of armored military vehicles.
The composite bridges were commissioned by the military and designed to help facilitate troop movements. According to Axion CEO James Kerstein, the new models have been tested to last at least 50 years and are capable of holding up to 71 tons. “Rather than having a concrete and steel bridge where each pour requires a 28-day cure period, we built two bridges totaling over 90 feet in length in 11 weeks,” he said.
The bridge design was developed using Axion’s I-beam technology. “We take the material, we create a greater surface area by doing quarter angles or I-beams or T-beams and that gives us greater stiffness to go with greater strength,” said Kerstein. “We’ve designed it in a way that when we use girders or cross beams, we also use I-beams, but they’re made especially to nest into the larger I-beams.” Kerstein added that the product was designed to be similar with wood in that it can be cut with chainsaws, table saws, and take standard screws and bolts.
This is Axion’s third composite bridge designed for the military. Despite the fact that the bridge was larger (previous models were rated at five and 31 tons), Kerstein said the company didn’t have to modify its manufacturing. “It was just a question of engineering to the best properties of our material for the performance parameters that were laid out for us,” he said.
Kerstein believes the military will continue to embrace composites usage. “When you have something that’s green, that’s less expensive, that lasts longer and is easier to install and can be moved around more readily, those are all major wins. In tactical applications, when you have to move a bridge or troops from one place to another, having something with all those features is pretty cool.” Kerstein also pointed out the advantages of sending the composite material abroad, saying that while wood is not brought back to the U.S. because of pathogens or infestations like termites, composites could return with troops.
The company is also actively trying to increase the adoption of composites. “The biggest challenge in greater adoption is getting the end user to tell you what they need the project to achieve. So many contractors are used to using certain materials and specifying them by rote,” said Kerstein. “So our goal is to get them to move forward and say ‘we’re anxious to work with a new material.’ The biggest challenge is always finding someone who has the willingness and the nerve to move forward with something that’s different. Anytime you’re attempting to introduce a revolutionary product, it takes a little bit of time to gain momentum. No engineer wants to put their license on the line.”
