Relationship Between Early Decay and Radial Compression Strength of Douglas-Fir

The ability of radial compression strength (RCS) to indicate early decay in Douglas-fir was examined. Plugs 3/8 inch in diameter and 3/4 inch long, cut from the sapwood and heartwood of a sound Douglas-fir pole, were decayed to weight losses up to 20 percent by the brown-rot fungus Poria placenta. Decayed and matched sound plugs were tested at 13 percent moisture content for RCS measured at stress at 5 percent compression strain and at the first load-inflection point. RCS losses were compared with weight losses caused by decay. Sound heartwood was stronger in radial compression than sound sapwood. RCS losses were highly correlated with weight losses, especially at 5 percent compression (R2=0.80). RCS losses were evident before weight losses, reaching 55 percent at a 10 percent weight loss, and before decay could be detected microscopically. Sapwood decayed faster than heartwood but suffered similar strength losses at the same weight losses. Microscopic examination of the specimens revealed that decay was first detected in earlywood. RCS was not proportional to specific gravity and may be affected by heartwood extractives.

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