Thermoplastic Composite Material as Substitute for Timber Bridge Design to Mitigate Greenhouse Gas Emissions

Abstract 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. 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. The Fort Bragg, NC, plastic vehicular bridge using reinforced thermoplastic composite lumber (in typical rectangular shapes) was built in May 2009 and is used to illustrate the GHG benefits of plastic lumber structures. Due to its no-maintenance needs, when viewed on a lifecycle basis, this bridge will be more than paid for its higher initial material costs. First costs are still most often the deciding factor whether these type of materials are or are not used over traditional treated-wood. This innovative design is cost competitive to a wood timber bridge to carry the same load and virtually maintenance-free from the degradation effects. This study will describe the GHG benefits 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. The calculations developed in this study show that the substitution of recycled-plastic lumber for chemical-treated wood can have a very significant positive effect on reducing greenhouse gases. The numbers reported are impressive, especially in severe exposure applications where wood is replaced often. The exact level of the greenhouse-beneficial effect depends on how frequently the conventional wooden element is typically replaced. The benefit for replacing plastic structures for wood structures would be significant at any reasonable level of market share. Interestingly in this case, the greatest greenhouse gas benefit occurs where the financial benefit of using a long-life material is also greatest. This finding is an encouraging departure from the many cases in which the most environmentally friendly option ends up being the most costly.