Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts
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United States Department of Agriculture Evaluation of Wood Species and Preservatives for Use in Wisconsin Highway Sign Posts Stan T. Lebow Robert J. Ross Samuel L. Zelinka Forest Forest Products General Technical Report July Service Laboratory FPL–GTR–231 2014 In cooperation with the Wisconsin Department of Transportation. Abstract hardwoods. Copper naphthenate is non-corrosive to alu- minum and would avoid warp associated with re-drying The Wisconsin Department of Transportation (WisDOT) after treatment with water-based preservatives. WisDOT uses wooden posts to support many types of signs along may want to consider purchasing a small volume of copper state highways. WisDOT currently uses red pine or Southern naphthenate-treated posts to evaluate the handling character- Pine posts treated with chromated copper arsenate (CCA) istics of this oil-based preservative. and has generally experienced satisfactory performance and service life. However, there are some areas of concern, as Keywords: wood preservatives, wood species, sign posts, well as potential opportunities for diversifying the wood highway, Wisconsin species and preservatives used. Warp is sometimes a prob- lem with the current pine posts, and increased use of locally Acknowledgment sourced wood species for the sign posts could decrease The authors acknowledge the valuable input received from transportation costs and potentially benefit Wisconsin in- Matt Rauch of WisDOT in refining the study problem areas, dustries. Although CCA is a highly effective preservative, it relating field experiences, and describing WisDOT practices may not be the optimum treatment for alternative wood spe- and procedures. cies. 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 June 2014 Lebow, Stan; Ross, Robert; Zelinka, Samuel. 2014. Evaluation of wood species and preservatives for use in Wisconsin highway sign posts. General Technical Report FPL-GTR-231. Madison, WI: U.S. Department of Agri- culture, Forest Service, Forest Products Laboratory. 45 p. A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726–2398. This publication is also available online at www.fpl.fs.fed.us. Laboratory publications are sent to hundreds of libraries in the United States and elsewhere. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the United States Department of Agriculture (USDA) of any product or service. 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Contents Introduction ..................................................................................................................................................................... 1 Wisconsin’s Current Sign Post Standard Specifications ............................................................................................. 1 Wood Species Considerations ..................................................................................................................................... 1 Wood Preservative Considerations ............................................................................................................................. 2 Objectives ....................................................................................................................................................................... 2 Wood Preservative Background ...................................................................................................................................... 2 Pressure-Treatment Preservatives and Pressure-Treated Wood .................................................................................. 2 AWPA vs ICC-ES and AASHTO Preservative Listings ............................................................................................. 3 Evaluating Preservative Efficacy ................................................................................................................................ 3 Role of Solvent Characteristics in Durability ............................................................................................................. 4 Corrosion Testing ........................................................................................................................................................ 4 AWPA Use Category System ...................................................................................................................................... 5 Wood Preservative Characteristics .................................................................................................................................. 5 Role of Copper in Wood Preservatives ....................................................................................................................... 5 Current Ground-Contact Preservatives ....................................................................................................................... 6 Corrosion Aspects of Selecting Preservatives for Sign Posts ........................................................................................11 Mechanism of Corrosion in Treated Wood ................................................................................................................11 Review of Corrosion Data from New Wood Preservatives .......................................................................................12 Special Corrosion Considerations for Sign Posts ......................................................................................................14 Corrosion Recommendations .....................................................................................................................................14 Summary of Preservatives’ Potential for Sign Post Treatments ....................................................................................14 Water-Based Preservatives .........................................................................................................................................15 Oil-Based Preservatives .............................................................................................................................................16 Wood Species Options for Wisconsin DOT Signposts ..................................................................................................17 Wood Species Currently Listed in WisDOT Signpost Specifications .......................................................................17 Hem-Fir ......................................................................................................................................................................21 Treatability and Durability of Other Wisconsin Wood Species .................................................................................23 Ash .............................................................................................................................................................................23 Aspen..........................................................................................................................................................................24 Balsam Fir ..................................................................................................................................................................24 Basswood ...................................................................................................................................................................25 Birch—Paper and Yellow ...........................................................................................................................................25