Shear Strength of High-Temperature Heat-Treated Douglas-Fir Lumber Laminated with Phenol-Resorcinol Adhesives

Ultimate shear strengths of Douglas-fir lumber glued with two different phenol-resorcinol adhesives were obtained. The first adhesive was formulated for high-temperatures; the second, for room temperature. Specimens were prepared from heat-treated billets of clear dimension lumber glued into a block of two laminations. For one set of laminations, the billets were heat treated, cooled, spread with adhesive and glued at room temperature. For the other group of laminations, the billets were heat treated after a pretreatment of resorcinol was applied, then spread with the adhesive and laminated using the residual heat in the wood. Compression shear specimens were cut from the laminated billets, conditioned in “dry,” “soak,” ??F (54?C) temperature, and cyclic environments; then loaded to failure. Over the wood surface heat-treatment temperature range of 380?F (193?C) to 500?F (260?C), in all test environments, shear strength decreased rapidly with an increase in temperature. Evidence is presented which suggests that wood fiber degradation and wood surface inactivation contribute to the loss of strength. In view of weakening of wood fibers at temperatures of 420?F (216?C) and higher, Douglas-fir with a moisture content of 9 percent should not be glued with any adhesive when the wood surface has been heat treated at 420?F(216?C) or higher for one minute or longer at a pressure of 150 psi.

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