Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts

Abstract

The Wisconsin Department of Transportation (WisDOT) uses wooden posts to support many types of signs along state highways. WisDOT currently uses red pine or Southern Pine posts treated with chromated copper arsenate (CCA) and has generally experienced satisfactory performance and service life. However, there are some areas of concern, as well as potential opportunities for diversifying the wood species and preservatives used. Warp is sometimes a problem with the current pine posts, and increased use of locally sourced wood species for the sign posts could decrease transportation costs and potentially benefit Wisconsin industries. Although CCA is a highly effective preservative, it may not be the optimum treatment for alternative wood species. This study reviewed the characteristics of alternative wood species and wood preservatives and evaluated their potential for use in Wisconsin highway sign posts. The evaluation concluded that WisDOT’s current practice of using red pine or Southern Pine posts treated with CCA is logical and may be the optimum combination of wood species and preservatives currently available. Red pine and Southern Pine are readily available and relatively strong compared with many other softwood species. Red pine is also an important commercial resource for Wisconsin and the upper Midwest. Other Wisconsin species to consider include eastern white pine and soft maples. However, strength may be a concern with eastern white pine, whereas cost may be a concern with soft maples. CCA is an effective preservative, readily treats red pine and Southern Pine, and is compatible with aluminum signs. Copper naphthenate in oil solvent appears to be one of the most logical alternatives to CCA, and would be a strong candidate for treatment of hardwoods. Copper naphthenate is non-corrosive to aluminum and would avoid warp associated with re-drying after treatment with water-based preservatives. WisDOT may want to consider purchasing a small volume of copper naphthenate-treated posts to evaluate the handling characteristics of this oil-based preservative.