Abstract

Improved estimates of the long-term durability of treated wood products are needed to guide choices about construction materials and allow estimates of design life. This report summarizes the long-term decay and insect resistance of treated wood post and lumber specimens placed in ground contact at a test site of the U.S. Department of Agriculture, Forest Service, Forest Product Laboratory in southern Mississippi. Posts treated with relatively low levels of creosote had an estimated durability of 54 years, and slightly less durability was observed in creosote-treated lumber. Pentachlorophenol-treated posts exhibited durability in excess of 60 years, whereas lumber specimens treated to standard ground-contact retentions had no failures after 39 or 45 years. Posts treated with low retentions of copper naphthenate had an estimated 65-year longevity, but lumber specimens treated to higher retentions of copper napththenate had lower average lives of 27 to 30 years. Low-retention ammoniacal copper arsenate (ACA) posts had an estimated durability of 60 years, whereas stakes treated to retentions of 8 kg/m3 (0.5 lb/ft3) or greater with ammoniacal copper zinc arsenate (ACZA) or ACA have had no failures after 30 and 60 years, respectively. Posts treated with a range of retentions of chromated copper arsenate (CCA-C) have had no failures after 35 years, and stakes treated with CCA-A, CCA-B, or CCA-C to retentions above 7 0 kg/m3 (0.43 lb/ft3) have had no failures after 60, 61, and 40 years, respectively. As a whole, the post and lumber specimens indicate an expected durability of over 50 years for creosote-treated wood and over 60 years for wood treated with pentachlorophenol, copper naphthenate, ACZA, or CCA. Comparison of the results from this site to reports from other locations suggests that these results might underestimate the potential durability in more moderate exposures. In relating these findings to treated commodities, it should be noted that these test specimens have not been subjected to the same mechanical loads or wear and tear associated with in-service structures.

Issue Section:
Research Papers
Keywords:
pressure-treated wood, long-term, durability, posts, stakes

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