INFLUENCE OF TIME-TO-FAILURE ON STRENGTH OF CCA-TREATED LUMBER

Recent studies on the effects of chromated copper arsenate (CCA) treat ment on lumber design properties have primarily evaluated the effects of such treatment at times to failure of 1 to 10 minutes and at 12 per cent 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 vari ous MC levels on the bending strength of CCA-treated (6.4 kg/m3 (0.4 l b./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 bend ing strength related to time-to-failure between untreated and CCA-trea ted southern pine lumber. The bending strength of CCA-treated lumber d id not increase at faster loading rates compared to untreated lumber. Most importantly, specimens in the lower quarter of the CCA-treated st rength distribution did not show any time-dependent strength increases compared with matched untreated material. These results imply that ex isting North American design guidelines for short-term duration-of-loa d adjustments for CCA-treated lumber, which are based on untreated lum ber, should not be applied. Accordingly, this paper offers a revised a djustment model.