A bridge made entirely of recycled high-density polyethylene milk jugs and polystyrene packing peanuts, formulated into a composite by a patented process, was completed in November over the Mullica River in Wharton State Forest in southern New Jersey. The structure, 46 ft long by 15 ft wide (14 by 4.5 m) and weighing 30,000 lb (13,608 kg), was built with I-beams and decking fabricated by a process dubbed “intrusion molding,” a form of extrusion.
The bridge is the latest example of how plastics are being applied in civil engineering. What’s notable about this structure is it is one of the few to use recycled thermoplastics as the primary material. Most composite bridges have been fabricated of thermosets.
The composite withstands 3000 lb/in2 of force, enough for the bridge to support a truck. The bridge weighs about half that of a wooden one. This, combined with its modular design and the use of interlocking I-beams and tongue-and-groove deck planking, permitted the structure to be built in substantially less time than a wooden bridge.
In the design, 12-in-deep stringer I-beams nest between 17-in-deep pile-cap I-beams. The I-beams are connected with 4-in deck screws and aluminum brackets. The deck planking rests on the stringers.
To accommodate expansion and contraction of the structure, the design required more joints than normal. The connections as well had to be more forgiving than with conventional materials, so slotted holes and expansion bearings were used.
The bridge won’t warp or chip and does not need to be painted, while the extruded texture of the deck material reportedly makes it less slippery. The scrap generated during installation was recycled.
The I-beams were made by Polywood Inc., an Edison, nj, processor that has produced railroad ties out of the same material for light-rail systems in major metropolitan areas. Polywood purchased rights to a patent secured in 1994 by Rutgers, the State University of New Jersey, after polymer scientists there developed a process for A blending hdpe and ps recyclate into a structural composite.
Polymer scientists from Rutgers and a group of student assistants helped build the bridge. The structural en-gineer for the project was McLaren Engineering Group, West Nyack, ny.
“We’ve done good materials science,” says Prof. Thomas J. Nosker, one of the patent-holders and a polymer physics scientist who works with the university’s year-old Center for Advanced Materials via Immiscible Polymer Processing, in Piscataway, nj. “These are good materials, resistant to degradation…[The bridge] can last for hundreds of years…”
The material is 65% hdpe and 35% ps. Polywood buys the recyclate from brokers prior to blending it. The composite is processed at 350 to 400°F. Using what it terms standard equipment, Polywood extrudes the material into a series of molds.
Polywood began making structural railroad ties three years ago, and doubled in size in its first year. “We went up another 50% in the following year,” says Jim Kerstein, president. “There are more than 50,000 [of our] ties in place.”
One focus of the company is working with different industries to design products for their needs. “We try to organize councils of engineers from different companies to outline what performance standards they need to achieve, and then we work toward these targets.”
Applications that Polywood and Rutgers scientists are evaluating include telephone utility poles, boardwalks, and various highway products.