Influence of time-to-failure on strength of CCA-treated lumber

Recent studies on the effects of chromated copper arsenate (CCA) treatment on lumber design properties have primarily evaluated the effects of such treatment at times to failure of 1 to 10 minutes and at 12 percent moisture content (MC). The influence of faster loading rates and different MCs on treated lumber is unknown. This report discusses the influence of time-to-failure under ramp loading conditions and at various MC levels on the bending strength of CCA-treated (6.4 kg/m3 (0.4 lb./in.3)) lumber. The factors studied were failure times in bending of 3 to 6, 30 to 60, and 300 to 600 seconds and MC levels of 10, 15, and 23 percent (green lumber). Results show important differences in bending strength related to time-to-failure between untreated and CCA-treated southern pine lumber. The bending strength of CCA-treated lumber did not increase at faster loading rates compared to untreated lumber. Most importantly, specimens in the lower quarter of the CCA-treated strength distribution did not show any time-dependent strength increases compared with matched untreated material. These results imply that existing North American design guidelines for short-term duration-of-load adjustments for CCA-treated lumber, which are based on untreated lumber, should not be applied. Accordingly, this paper offers a revised adjustment model.

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