In 2010, two recycled thermoplastic composite lumber railway bridges were built at Fort Eustis, Va., with a maximum load capacity of 130 tons. Photo: Tetaun Moffett, Ft. Eustis, Va.

Reinforced thermoplastic composite lumber (RTCL) sourced from recycled materials is an emerging technology available for heavily loaded infrastructure applications. RTCL materials offer a sustainable alternative and many advantages compared with traditional construction materials such as wood and steel. Research, development, and experimental projects during the last 15 years have culminated in several RTCL high load capacity military installations.

High-density polyethylene (HDPE)-based recycled plastic lumber (RPL) emerged in the United States marketplace in the early 1990s. The original RPL suffered from low modulus and low creep resistance. However, research efforts were made to enhance these properties by reinforcing the HDPE with post-industrial polystyrene (PS) and fiberglass reinforcement of polypropylene (PP) followed by melt-blending with HDPE to form fiberglass-reinforced PP (FRPP)/HDPE blends. The post-industrial-sourced FRPP is automobile bumper scrap.

Although these materials displayed engineering-grade properties, they were not time tested and had slow acceptance into the marketplace. The first structural application of PS/HDPE and FRPP/HDPE materials was railroad crossties. In 1998, the first vehicular bridge at Fort Leonard Wood, Mo., was built with PS/HDPE components and steel I-beams as substructure. The bridge has a 24-foot span and maximum load rating of 30 tons. While the lifetime cost was lower because of the lack of maintenance required, the initial installed cost was more expensive than traditional materials. However, the military determined that after eight years, the low lifetime cost would outweigh the higher initial costs. After eight years, the Fort Leonard Wood bridge looked and behaved as it did in 1998. This was the pivotal bridge and moment in time when the military realized the potential of RTCL materials for heavily loaded infrastructure applications.

Bridge projects
Since 1998, there have been three major bridge projects using these RTCL materials. Each installation provided a learning experience that benefited the next one. The first RTCL bridge that was cost-competitive on a first-cost basis because of its innovative design was built in Wharton State Forest, N.J. In 2009, two bridges were built at Fort Bragg, N.C., and in 2010, two railway bridges were built at Fort Eustis, Va.

Two HDPE-based RTCL materials — developed at Rutgers University and currently manufactured by Axion International Inc. (www.axionintl.com) — were used in these infrastructure projects, including PS/HDPE and FRPP/HDPE. Both RTCL materials have a specific gravity of about 0.85 and are one-eighth the density of steel, but the resulting specific strength (strength/unit weight) is greater than many steels. Degradation due to natural, ultraviolet (UV) direct sunlight does not exceed a rate of 0.003 inches per year.

The Wharton State Forest, N.J., vehicular bridge built in 2002 is the first bridge to use RTCL I-beams. All bridge components are PS/HDPE, except the wooden pilings that already were in place. The substructure is an interlocking I-beam design. The pile caps are large I-beams with small I-beams nestled in the web of the large I-beam. The top surface of the pile cap also serves as decking. Tongue-and-groove decking was laid parallel to the pile caps and across the small I-beams. The completed bridge has a 56-foot span, weighs 30,000 pounds, and has a maximum load capacity of 36 tons.

The Fort Eustis railway bridges were live-load tested with several railway vehicles and speeds, including a 120-ton GP 16 locomotive. Axion International

The two bridges built at Fort Bragg, N.C., are composed of the FRPP/HDPE RTCL material and were built with the purpose of supporting M1 Abrams tank loads (71 tons). The basic construction design includes rows of pilings, with pins holding I-beam pile caps to each row, a steel sill plate pre-drilled with holes to align girders, I-beam girders spanning the length of the bridge through-bolted to the pile caps and placed edge to edge across the bridge width, smaller I-beams nestled in the web of and perpendicular to the larger I-beam girders, deck boards affixed to the girders with standard deck screws, and curbing and a railing affixed to the edges of the bridge. The width of both bridges is 16 feet, 6 inches; spans are 42 feet and 56 feet. The maximum load capacity of both bridges is 73 tons for tracked vehicles and 88 tons for wheeled vehicles.

The two railway bridges at Fort Eustis, Va. — bridges 3 and 7 — were constructed in 2010 and are composed of the FRPP/HDPE RTCL material. Virtually all bridge components, including pilings, pile caps, girders, and crossties, are made from recycled post-consumer and industrial plastics that would otherwise be discarded into landfills. These RTCL bridges were designed and built to carry the Cooper E60 load and the 260 kips alternate live load and have a maximum load capacity of 130 tons. The span of bridges 3 and 7 are 40 feet and 75 feet, respectively. Assembly time was significantly reduced by creating pre-assembled panels in the factory.

The Fort Eustis bridges were load tested and performed well. The ultimate tensile strength of this FRPP/HDPE RTCL is 4,500 psi, and the allowable design stress is 600 psi, as determined by the Non Linear Strain Energy Equivalence Theory. This theory is a correlative method in which data from two short-term stress-strain experiments conducted at different strain rates are used to predict long-term creep strain at any stress level. The design stress is only a fraction of the ultimate tensile strength so that stresses in the bridge components are below the stress levels that would cause creep. Potentially, these bridges could be loaded for as long as 25 years at a stress below 600 psi and suffer no creep strain upon load removal.

The Fort Eustis bridges underwent live-load testing and beam deflections were measured. The sources of beam deflection are deformation of elastomeric bearing pad, pile axial deformation, and beam deflection. Bridges 3 and 7 were live-load tested with several railway vehicles and speeds, including an 80-ton switcher and a 120-ton GP 16 locomotive. The live loads passed through both bridges several times back and forth at various speeds ranging from 5 mph to 25 mph. Both bridges were constructed on tangent, but the testing speed was limited to 25 mph due to curved tracks nearby. There was no sign of visual distress observed during the live load testing. The live-load test results for bridge 3 are shown in Table 1. Note that the allowable deflection in the bridge design is 0.75 inches.

Conclusions
The construction and installation benefits of RTCL materials include efficient designs that use less material and reduced assembly time. General advantages of using RTCL include corrosion, insect, and degradation resistance; no toxic chemical treatments required to increase service life; diversion of waste plastics from landfills; and reduction of deforestation, greenhouse gases, and global warming.

Jennifer K. Lynch is a research associate, and Thomas J. Nosker, Ph.D., is assistant research professor, Department of Materials Science and Engineering, Rutgers University, New Brunswick, N.J. They can be contacted at jklynch@rci.rutgers.edu and tjnosker@rutgers.edu, respectively.