Wood-plastic composites (WPCs) have emerged as a viable replacement for industrial structural applications such as waterfront structures and bridge decking due to its resistance to moisture and decay. In this study, procedures for assigning allowable design stresses were developed, including adjustments in design values for load duration, moisture, and temperature effects. The proposed procedures were applied to an extruded composite material determined by evaluating twenty-two maple and pine polypropylene formulations for mechanical and physical properties. The resulting allowable design stresses were used to determine required section properties for AASHTO loadings, resulting in the creation of span tables. The influences of coupling agents, test frequency, and stress ratio on the fatigue life were investigated. Results show that fatigue life and internal heating increased with increasing test frequency; however, strain to failure remained relatively constant. Comparing the static and fatigue test distributions indicated that the uncoupled formulation displays different mechanisms controlling short- and long-term failures, unlike those for the formulation containing co-polymer coupling agents. Finally, fatigue testing indicated that the selected WPC formulation is suitable for pedestrian bridge applications.
Wood-plastic composites (WPCs) are defined as filled thermoplastics consisting primarily of wood fiber and thermoplastic polymer (Wolcott, 2001). Thermoplastics such as polyethylene (PE), polyvinyl chloride (PVC), and polypropylene (PP) are currently being utilized for a variety of commercial products, including automotive trim, window frames, and roof singles. However, the largest and fastest growing market for WPCs is extruded residential decking and railing (Clemons, 2002; Wolcott, 2001).
When compared to timber, WPCs exhibit increased durability with minimal maintenance (Clemons, 2002). Wolcott (2001) found that the addition of 40-50% wood improved thermal stability, while the thermoplastic component improved moisture and thermal formability. When exposed to moisture, WPCs absorb less moisture at a slower rate, leading to superior fungal resistance, and dimensional stability when compared to timber (Clemons, 2002). Waterfront applications have also demonstrated that WPC materials exhibit improved durability with respect to checking, decay, termites, and marine organisms in contrast to timber (Balma and Bender, 2001).
Preservative treatment of wood to resist fungal decay has been identified as a leading problem for utilization of timber in certain applications (Smith and Cesa, 1998), thus, providing an incentive to employ WPCs as a timber replacement. Leading wood preservative treatment manufactures, in an agreement with the Environmental Protection Agency (EPA), voluntarily withdrew the use of chromated copper arsenate (CCA) for consumer applications (Southern, 2002). Consequently, next-generation treatments are now being applied at a higher cost, which has narrowed the cost gap between timber and composites.
Research of high-strength engineered plastics has been performed, and Wolcott (2001) concludes that WPCs should not be limited to nonstructural applications. Therefore, expansion of the WPC market for structural applications is appropriate, provided societal incentive exists and feasible applications are developed and accepted by industry.