Plastic lumber has been drafted by the U.S. Army, and is being sent to the front lines of bridge construction.
The U.S. Army Corps of Engineers teamed up with other government agencies and plastic lumber manufacturers to build a bridge at Fort Leonard Wood in St. Robert, Mo.
The bridge, made from 13,000 pounds of mixed recycled plastics, represents a structural breakthrough for plastic lumber, according to Richard Lampo, a researcher with the U.S. Army Construction Engineering Research Laboratories.
``That was the intent of this project: to showcase the structural capabilities of these materials,'' Lampo said in a July 7 telephone interview.
Those capabilities include the ability to support light trucks as well as the bridge's usual pedestrian traffic.
Another intent was to show plastic lumber could be an economical choice for bridge construction.
``What we didn't want to do was just build a block of plastic,'' he said.
Instead the designer, Malcolm McLaren of M.G. McLaren Consulting Engineers of West Nyack, N.Y., came up with a plan that was similar to the wooden bridge that was replaced.
The 261/2-foot-by-25-foot deck is designed to support as much as 30 tons. Plastic lumber joists, decking planks and handrail components were added to six steel stringers that were part of the original wooden bridge.
The Army expects the plastic lumber deck to last a maintenance-free 50 years, as opposed to 15 years for treated wood and 5 years for untreated wood.
Because treated lumber contains toxic chemicals, it is hard to dispose of when its structural days are numbered. Plastic lumber, on the other hand, can be recycled all over again.
Such environmental factors make wooden bridges in parks and rural areas likely targets for replacement by plastic lumber, Lampo said.
Technological developments allow designers to consider plastic lumber for structural applications once considered off limits.
Joists had to be able to withstand higher loading than typical for average plastic lumber products, Lampo said. The design team chose a polystyrene-modified 3-by-12 product from Polywood Inc. of South Plainfield, N.J.
The modulus of the Polywood product is 21/2 times that of ``average'' plastic lumber, Lampo said.
That type of breakthrough broadens the horizons of plastic lumber.
``When someone like the Corps of Engineers endorses our product in a structural use, it opens any number of markets that were closed before,'' said Polywood President Jim Kerstein.
Polywood is the exclusive licensee of a patented process that combines post-consumer polystyrene foam with the recycled high density polyethylene found in most ``traditional'' plastic lumber.
``It was a very exciting project for us,'' Kerstein said. ``It points toward some very positive things for the future of plastic lumber, especially for a material notoriously known for lack of stiffness.''
While unmodified plastic lumber does not have a lot of flexural strength, it possesses some qualities that help bolster structures. Because it is a thermoplastic material, plastic lumber melts around screws when they are driven into the boards. The plastic then cools and hardens around the screw, locking it into place.
While such behavior makes it tough to correct mistakes, it ensures a long-lasting bond, said Lampo, who helped assemble the bridge over the course of a week.
``It's a very sound structure,'' he said. ``You can tell just by walking on it and going up to the rails and shaking them.''
In addition to the joists provided by Polywood, Plastic Lumber Co. of Akron, Ohio, supplied the 3-by-12 decking planks; and materials for the bridge railing were donated by Hammer's Plastic Recycling Corp. of Iowa Falls, Iowa, and Renew Plastics Inc. of Luxemburg, Wis.
The project also involved the Environmental Protection Agency, Rutgers University, Battelle Inc. of Columbus, Ohio, and the Plastic Lumber Trade Association of Akron.