In this study, the effects of hydrothermal treatment on the physical and mechanical properties of beech wood were investigated, with an emphasis on the differences between sapwood and red heartwood. Test specimens were prepared from air-dried wood and conditioned at three different relative humidity levels (65%, 75%, and 88%) at a temperature of 20 °C. Hydrothermal treatment was carried out in a steam chamber at temperatures of 70 °C and 90 °C for durations of 3 and 10 minutes. After treatment, equilibrium moisture content and density were determined, followed by three-point bending and compression tests. The results showed that increasing temperature and treatment time led to a decrease in wood density, with an overall reduction of approximately 5% compared to control samples. The modulus of elasticity decreased by up to 38%, while bending strength decreased by up to 26%, with the most significant changes already occurring at shorter treatment times. At the higher temperature (90 °C), a partial improvement of dimensional stability of the material was observed. Compressive strength decreased by up to approximately 40%, with red heartwood generally exhibiting lower values than sapwood. The results confirm that hydrothermal treatment significantly affects the mechanical properties of wood and increases its deformability, while anatomical characteristics and moisture content play a key role in the mechanical response of the material.
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