Recycled Plastics Play Supporting Role in Civil Engineering
Post-consumer plastics have been around for years, providing
viable raw materials for a wide variety of new products. Among the new
uses for recycled plastics is a growing interest in structural applications.
Plastic lumber, available from several manufacturers in most of the common
structural sizes, has been gaining more widespread acceptance as an alternative
building material over more traditional wood products. The commercial
success of decks, walkways, leisure furniture and other outdoor fixtures
made from post-consumer polymers has fueled further research to develop
new products and new uses for recycled plastics in construction.
Earlier this year, the first all plastic vehicular bridge using unreinforced
I-beams and other structural components made from recycled plastics was
completed in Wharton State Park in New Jersey. The bridge is made from
a unique composite polymer material developed at nearby Rutgers University.
Recycled post-consumer plastics, such as high density polyethylene and
polystyrene from consumer packaging, were used to make the durable, inexpensive
structural materials, keeping these plastics out of our nation’s
landfills.
The Polywood Corporation fabricated all bridge members from a special
formulation of polyethylene and polystyrene. Neither of the base polymers,
found in polystyrene cups and polyethylene milk jugs, would be suitable
for structural use by themselves. A patented processing technology developed
at the Center for Advanced Materials via Immiscible Polymer Processing
(AMIPP) at Rutgers University combines the polymers to create a blended
composite material with great strength.
The process heats and blends two or more polymers together
and extrudes them to form a fine microstructure. The special properties
of the material result from the development of an oriented microstructure
that gives the material unusual strength, enabling it to be used for bridge
beams, railroad ties, boardwalk substructures and decking, and numerous
other applications.
The Wharton State Park project is the first demonstration of this new
bridge building technology. The bridge was designed by M.G. McLaren Consulting
Engineers, a civil engineering design and consulting firm, and consists
of large I-beams supported by posts with smaller I-beams spanning between
the larger structures. Three-inch thick tongue and groove decking material
provides the road surface. According to project engineers, the 42 foot
long, single lane service bridge over the Mullica River is strong enough
to support a loaded fire truck weighing 36,000 pounds.
A recycled plastic bridge at Fort Leonard Wood, Missouri is more positive
proof of the potential for recovering a solid waste while providing an
alternative to conventional construction materials. Completed in 1998,
the bridge spans a creek on the fort’s Gammon Field and represents
the reuse of some 13,000 pounds of mixed plastics that would otherwise
had gone into landfills. In five years of service, the bridge has performed
exactly as planned.
“This construction is significant in that, while larger-sized structures
have been built using recycled plastic lumber, no other known structure
has the structural capacity of this bridge,” said Richard Lampo,
a researcher at the U.S. Army Construction Engineering Research Laboratories
(USACERL), which led the project to build the bridge.
The recycled plastic bridge replaces an older wooden bridge at Fort Leonard
Wood. The structure, 25 feet long and 26 feet wide, sits on six steel
beams that supported the original bridge. It was designed to bear loads
up to those of light-vehicles.
“I drove my half-ton pickup over it,” said Stan Martin, civil
engineering technician in Fort Leonard Wood’s Directorate of Public
Works (DPW). “The bridge looks great. It just looks like a painted
wooden bridge until you get up close and see that it’s plastic.”
The bridge-building project involved USACERL, the Environmental Protection
Agency and Fort Wood’s DPW. The Battelle Company, the Plastic Lumber
Trade Association (PLTA), and Rutgers University also worked on the project.
Four companies that produce construction supplies donated materials.
McLaren designed the bridge using guidelines it developed for use of recycled
plastics in coordination with the American Society for Testing and Materials
and PLTA. The structure of the new bridge can safely support more than
30 tons. The railings of the bridge meet safety code requirements, and
the response of bridge materials to weight stresses are “well within
the accepted limits for this type of bridge,” said Malcolm McLaren,
president of M.G. McLaren.
USACERL has worked since the early 1990s with Rutgers University, the
EPA and a group of plastic lumber manufacturers to improve product quality
and develop standards and specifications for the materials. As a Corps
of Engineers laboratory, USACERL’s interest was to infuse this environmentally
friendly technology into the Corps’ military and civil works construction.
At the same time, the EPA had announced a national goal to recover 35
percent of municipal solid waste.
“Over the past 10 years, the EPA has worked very hard to encourage
recycling through the development of end-use markets for products made
from recovered materials,” said Terry Grist, environmental protection
specialist at the EPA Headquarters’ Office of Solid Waste. One such
effort has been to develop tools such as procurement guidelines that designate
items containing recovered materials that government agencies must purchase.
“The success of projects such as this one will serve to open up
new markets for these materials and provides the opportunity to increase
the overall recovery rate for plastics,” he added.
Recycled plastic lumber offers a replacement for wood products, many of
which are treated with chemicals to resist rot and insect attack. Chemically
treated wood often requires special handling and disposal, and chemicals
can leach into the environment if the wood is not disposed of properly.
Untreated wood structures are high maintenance items, requiring periodic
coating with paints and other preservatives that can release chemical
compounds into the atmosphere by way of evaporation. In contrast, plastic
lumber requires no such maintenance.
“We have wooden bridges, and they’re a maintenance problem,”
said Fort Leonard Wood’s Martin. “We have to send crews out
two or three times a year to replace deteriorated lumber and fasteners
that have worked loose. Most of our wood bridges are on running or hiking
paths, so the splinters and loose fasteners also become a safety hazard.”
Martin estimates that bridges made with chemically treated wood last about
15 years under the climate and use at Fort Leonard Wood. Untreated wood
bridges may have to be replaced as often as every 5 years. In contrast,
Lampo projects a 50-year, maintenance-free service life for the recycled
plastic lumber bridge—though he admits the steel supports may need
repainting from time to time.
USACERL and Fort Wood’s DPW will continue to monitor the bridge.
By successfully demonstrating recycled plastic lumber in a large-scale,
structural application, the project opens a world of potential for diverting
waste plastics to beneficial use—and the supply of raw materials
appears to be unlimited. At present, more than 8.4 billion pounds of plastic
containers are produced each year in the U.S., with some 6 billion pounds
sent to landfills as waste.
“It would take 87 miles of a bridge the same width as this one to
use up just one year’s landfilled plastics,” Lampo said, or
the equivalent of 462,500 bridges the size of the one on Gammon Field.
“We’re not going to run out of raw materials any time soon.”
Recycled plastics appear to have a bright future in civil engineering.
