Long-term strength of CCA-treated lumber

The effect of chromated copper arsenate (CCA) treatment on the bending duration-of-load (DOL) characteristics of 2 by 4 southern pine lumber was studied. Previous DOL studies have used untreated lumber. The effects of treatment on material properties are needed since much treated lumber is used in engineered structures, such as permanent wood foundations. We tested 200 untreated and 200 treated specimens. One-half of each group was tested for static bending strength, and the other half was tested under constant load at the 10th and 40th percentile (50 specimens at each percentile) of their respective static strengths for 12 weeks. The effect of CCA treatment on both static strength and DOL characteristics was related to the percentile level within the strength distribution. The effect on static strength was negligible below the 40th percentile, but average static strength was reduced. Changes in the DOL factor occurred at or above the 40th percentile of static strength. Since design allowable stresses are based on 5th percentile values, we conclude that preservative treatment has a negligible effect on DOL strength for a 12-week duration at this stress level.

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