Thermoplastic Composites as Degradation-Resistant Material Systems for Timber Bridge Designs

Plastic lumber made primarily from recycled high-density polyethylene first emerged on the United States marketplace in the early 1990’s.  Plastic lumber is an attractive substitute for natural wood because it diverts waste plastic from landfills and is inherently resistant to moisture, rot, and insects without the need for chemical treatments.  However, while as strong as an equivalent-sized piece of wood, these original plastic lumber products had an elastic modulus (stiffness) at least an order of magnitude less than even the most common wood species used in construction.  Eventually manufacturers started incorporating fibers into the formulation to produce a reinforced thermoplastic composite lumber with a higher elastic modulus.  The first plastic vehicular bridge using reinforced thermoplastic composite lumber (in typical rectangular shapes) was built at a mid-west Army Installation in 1998.  This bridge has not had any maintenance done to it since its completion and still looks like new.  Due to its no-maintenance needs, when viewed on a lifecycle basis, this bridge has now more than paid for its higher initial material costs.  However, first costs are still most often the deciding factor whether these type materials are or are not used over traditional treated-wood.  Since 1998, researchers and engineers have looked at arch and I-beam designs as a means to reduce the material and installation costs for a given load capacity in order to come up with a design that is cost competitive to traditional wood designs on a first cost basis.  The latest demonstration of this sort is a thermoplastic composite I-beam bridge being constructed at an east coast Army Installation designed to cross an M-1 battle tank.  This innovative design is cost competitive to a wood timber bridge to carry the same load and virtually maintenance-free from the degradation effects of moisture, rot, insects, and weather.  This paper will describe the evolution of these materials, their durability, and the design factors that make these materials attractive sustainable alternatives to chemically treated-wood for timber bridges and many other outdoor structural applications where treated wood is traditionally used.