Experimental analysis of thermally-treated Chinese poplar wood with focus on structural application

The primary issues restricting fast-growing wood utilization in buildings are its low dimensional stability and lack of strength grading. Thermal treatment has been shown to improve wood dimensional stability while decreasing certain mechanical parameters. This study was aimed to optimize the thermal treatment for fast-growing poplar wood to determine its impact on the wood strength class from the perspective of structural applications. Seven temperature levels between 20 and 210 °C and 2 h heating duration were used to thermally treat fast-growing poplar wood, which was then subsequently investigated for chemical composition and color change, as well as mechanical properties, such as parallel-to-grain bending, tensile, compressive (fc,0), shear strength (fv), modulus of elasticity (M0), and perpendicular-to-grain tensile strength. In addition, characteristic values were derived to provide an initial indication of design values. Thermal treatment resulted in hemicelluloses degradation, which darkened the color and reduced strength according to chemical composition and infrared spectroscopic analyses. In general, mechanical properties decreased with temperature, except for fc,0 and M0, which increased with temperature (≤180 °C) and followed by a decrease at ≥ 190 °C. The strength class of heat-treated wood depended on the smaller values of M0 and fv. M0 determined the strength class of native and heated wood to be ≤ 180 °C and fv at ≥ 190 °C. Thermal treatment in the region of 170–180 °C was a practical approach to improving wood properties in view of their structural use. Results of this study provided a basis for developing a design guide for structural uses of thermally-treated poplar wood.