Abstract:

Objective: The mechanical properties of full-scale glued laminated timber(glulam) beams made of thermal treated laminas were performed. The effects of thermal modification and humidity in the surrounding environment on wood moisture content, longitudinal shear strength and tensile strength were determined. The study can provide a reference for thermal treatment technology in the structural applications. Method: The high-grade Larix gmelinii was used in the tests. A total of 12 full-scale glulam beams were fabricated with modified laminas using industrial thermal modification technology. The effects of humidity in the surrounding environment and industrial thermal treatment on bending properties were determined, including bending strength, modulus of elasticity(MOE), failure mode, the relationships between load and deformation and the strain profile of full-scale glulam beams. All beams were tested using the four-point bending method according to BS EN 408 standard. Result: The results showed that, thermal modification improved MOE significantly in high humidity with a slight reduction in bending strength. In 90% relative humidity, bending strength of thermally treated glulam beams decreased by 29.79% than that of the untreated. MOE of thermally treated glulam beams increased by 23.71% than that of the untreated. MOE of the untreated glulam beams under the two humidity conditions was 23.27% difference, and the thermally treated glulam beams under the two humidity conditions was only 7.55% difference. The curves of bending load with displacement and strain profile curves of the glulam beams in different relative humidity showed that, the untreated glulam beams exhibited a more nonlinearity than the thermally treated glulam beams, especially at a high moisture in the surrounding environment. The relationships between moisture in the surrounding environment and mechanical properties were negatively correlated. Compared with the untreated control at 60% relative humidity, bending strength and modulus of elasticity of glulam beams at 90% relative humidity were 43.98 MPa and 12.191 GPa, and low ered by 17.07% and 23.27%, respectively. The effects of humidity in the surrounding environment on equilibrium moisture content of wood specimens were significant, and equilibrium moisture content of wood specimens can be deduced obviously by industrial thermal treatment. Thermal treatment decreased equilibrium moisture content from 10.74% to 4.76% and from 20.62% to 11.18% at 60% and 90% relative humidity, respectively. Conclusion: The untreated glulam beams at 60% and 90% relative humidity both failed in tension mode, while the series of the thermally treated glulam beams all failed in combined flexure and shear mode. Longitudinal tensile strength and shear strength of wood specimens both decreased after thermally treated, and led to the decrease of bending strength of the glulam beams. Industrial thermal treatment technology could improve modulus of elasticity of glulam beams significantly in a high relative humidity with a reduction in bending strength, so thermally treated wood materials could be used in construction after checked in bending strength.

Key words: full-scale glued laminated timber beam, thermal modification, relative humidity, bending performance