Effect of moisture on the bending properties of thermally modified beech and spruce

For appropriate and successful applications of thermally modified wood, a detailed knowledge of its distinct properties is essential. A thermal modification leads to structural and chemical changes in the wood constituents, which may significantly alter the material properties as compared to untreated solid wood. As contribution to a comprehensive material characterisation, moisture-mechanical property relationships were studied for selected bending properties of untreated and thermally modified beech and spruce. Static bending tests were conducted on small clear specimens at three treatment and five moisture levels. Bending strength at standard (dry) climate conditions was reduced by the thermal modification, while stiffness tended to show some increase. Furthermore, both properties decreased with increasing moisture content in untreated as well as thermally modified wood. However, because of the lower moisture sensitivity of thermally modified wood, the moisture dependence of its bending properties was considerably reduced. Therefore, in moist environments, equal or even better stiffness and strength values may be expected for thermally modified wood as compared to untreated solid wood. On the other hand, the changed fracture behaviour of thermally modified wood related to its increased brittleness, which was present also in wet conditions, has to be taken into account for potential structural applications.