Residual Strength and Stiffness of Lumber from Decommissioned Chromated Copper Arsenate–Treated Southern Pine Utility Poles

The reusability of decommissioned treated wood is primarily dependent on the residual strength of the wood after service. Determining the residual strength can provide useful information for structural design and reuse of the decommissioned treated wood. This study evaluated the residual strength of decommissioned chromated copper arsenate–treated utility pole wood. Eleven decommissioned southern pine (Pinus spp.) distribution poles and pole sections were evaluated, using small clear samples, for bending strength and stiffness across and along each pole. Results showed that the strength of the decommissioned treated wood varied across and along each pole and among the poles. Average modulus of rupture (MOR) was 80.9 percent of the typical MOR of longleaf pine (Pinus palustris) virgin wood, and average modulus of elasticity (MOE) was 83.9 percent of the typical MOE. Average MOR of the samples in the outer surface (first test zone) was 7.5 percent lower than the average MOR of the adjacent samples toward the pith (second test zone) on each side of the pole surfaces, but average MOE showed no significant difference between the two zones. Older poles lost more strength in the first test zone. Results demonstrated that spiral grain substantially reduced the strength of utility pole wood. Wood utility poles are removed from service primarily because of system revisions, mechanical damage, decay and insect attacks, wildfires, and/or damage from adverse weather conditions. Preservatives protect the poles while in service; therefore, a large portion of decommissioned poles are still mechanically sound and reusable for other purposes (Smith and Morrell 1989, Stewart and Goodman 1990, Huhnke et al. 1994, Cooper et al. 1996, Falk et al. 2000, King and Lewis 2000, Shi et al. 2001, Wang et al. 2001, Leichti et al. 2005). However, because wood poles deteriorate with time (Stewart and Goodman 1990), the reusability of decommissioned wood utility poles is primarily dependent on the residual strength of the timber, which is affected by service age, environmental conditions, and preservative type and treatment quality, and on the variability inherent in wood, such as species, age and growth rate, juvenile wood, etc. Determining the residual strength of poles can provide a useful reference for reuse and recycling of quality decommissioned treated wood. Cooper et al. (1996) studied the potential of 456 poles and pole sections for reuse as round poles, posts, sawn posts, timber, lumber, or cedar roof shingles. Pole species included cedar (western red cedar [Thuja plicata] and northern white cedar [Thuja occidentalis]), red pine (Pinus resinosa), jack pine (Pinus banksiana), southern pine (Pinus spp.), Douglas-fir (Pseudotsuga menziesii), and lodgepole pine (Pinus contorta). Treatment chemicals included all three major preservatives (i.e., creosote, pentachlorophenol [penta], and chromated copper arsenate [CCA]). The authors found that about 50 percent of the pole volume could be converted to sawn products and shingles and that 8 percent of the poles could be reused without reprocessing. Their study also demonstrated that the modulus of rupture (MOR) and modulus of elasticity (MOE) of the decommissioned treated wood were comparable to the average MOR and MOE of untreated virgin wood of the same species. Wang et al. (2001) and Shi et al. (2001) evaluated the residual strength of Douglas-fir and southern pine sawn timber that The authors are, respectively, Assistant Professor, Calhoun Research Sta., Louisiana State Univ. Agric. Center, Calhoun ([email protected]); and Project Leader, USDA Forest Serv., Southern Research Sta., Pineville, Louisiana ([email protected]). This paper (2009-255-4053) is published with the approval of the Director of the Louisiana Agricultural Experiment Station. This paper was received for publication in November 2009. Article no. 10708. Forest Products Society 2010. Forest Prod. J. 60(2):166–172.