Thermal-hydro-mechanical (THM) densification was performed on Picea abies (spruce), Abies alba (European fir), Populus nigra (poplar), and Fagus sylvatica (beech). Prior to THM treatment, the wood was impregnated with an aqueous solution containing 1%, 2.5%, and 5% biochar. Mechanical characteristics (modulus of elasticity, modulus of rupture), set-recovery, and contact angle were measured and compared for all samples in relation to the percentage of biochar present in the mixture. Impregnation with biochar/water mixture in combination with THM treatment resulted in densified wood with improved properties. Spruce showed the most notable improvement in water resistance, in addition to having the largest reduction in set-recovery at 57% and the highest contact angle value at 120 degrees. There was also significant improvement in the mechanical properties of spruce wood, especially when impregnated with 2.5% and 5% of biochar, with MOE values of 17.09 and 18.61 GPa and MOR values of 190.47 and 176.41 MPa, respectively. The only comparable values were observed in beech impregnated with 2.5% biochar mixture, with a MOR value of 204.6 MPa. These findings highlight the effectiveness of biochar-enhanced impregnation together with THM treatment in improving the structural and mechanical properties of wood, with distinct improvements observed in spruce and a few improvements in beech.
