Development of decay in untreated, second-growth Douglas-Fir using two exposure techniques in North Queensland
J Norton, S Kleinschmidt, R C De Groot, D Crawford
The results of two exposure techniques for evaluating the development of wood decay in untreated, mill-run lumber from second-growth Douglas-fir containing both sapwood and heartwood are presented. Nominal 50 mm by 100 mm by 2.5 m (2 in. by 4 in. by 8 ft) lumber, No 2 and better, was obtained from a production run in a mill that was processing second growth, Coastal Douglas-fir in western Oregon, USA. Untreated wood members were kiln dried, then shipped to Queensland. At the Timber Research Laboratory, Indooroopilly, Queensland, two units for above-ground exposure were fabricated from each 2.5-m (8-ft) untreated member. A 600-mm (2-ft) length was cut from each end of each member. One 600-mm section was transversely bisected, forming two, 300-mm (12in.) units; the other 600-mm section was used in a separate study. The middle 1200-mm (4-ft) length was used as one exposure unit. These shorter members were then milled to form a joint with a 100-mm (4-in.) overlap. Two holes were drilled through the overlapping portions, and the two sections were bolted together. The identity of the 1200mm length and the end-matched lapped jointed section was maintained throughout the exposure. Both units were positioned on a horizontal support approximately 1 m above ground in an open field near Innisfail, Queensland. As units were installed in the field, all cut surfaces inside the joint and at the ends were brush coated with a commercially available (in Australia) copper naphthenate emulsion containing 1% copper. Decay was first detected after 2 years of exposure and it advanced rapidly during the third year. The pattern of results suggests that the weathering of the upper, exterior surface and the retention of a high moisture content by the 50-mm-thick wood, when wetted, is more important in predisposing wood to decay than is end-grain absorption of moisture. Decay ratings for the 1200-mm members were equivalent to those observed with the lapped joints. The two types of units are of equal utility in demonstrating the potential for decay in a wood material that has natural susceptibility to decay.
Keywords: DECAY; UNTREATED; DOUGLAS-FIR; FIELD EXPOSURE
Conference: 97-05-25/30 Whistler, British Columbia, Canada