Mechanical Characterisation of Densified Hardwood with Regard to Structural Applications

The demand for high-performance, yet eco-friendly materials is increasing on all scales from small applications in the car industry, instrument or furniture manufacturing to greater dimensions like floorings, balcony furnishings and even construction. Wood offers a good choice on all of these scales and can be modified and improved in many different ways. In this study, two common European hardwood species, Beech (Fagus sylvatica L.) and Ash (Fraxinus excelsior L.) were densified in radial direction by thermo-mechanical treatment and the densified product was investigated in an extensive characterisation series to determine all relevant mechanical properties. Compression in the three main directions (longitudinal, tangential, radial) and tension perpendicular to the grain (tangential, radial) were tested and compared to reference specimens with native density. Strength and modulus of elasticity were determined in all tests. In addition, a Life Cycle Assessment was carried out to evaluate the environmental impact associated to the densification process. The experimental investigations showed that strength and stiffness of hardwood in the longitudinal and tangential directions improve significantly by radial densification, whereas some properties in the radial direction decrease. The Life Cycle Assessment showed that artificial wood drying has higher impact than wood densification. Furthermore, the transport distance of the raw material highly influences the environmental impact of the final densified product. The paper then also offers an overview of possible applications in structural timber construction. Densified hardwood is a viable option as local reinforcement, where high compressive or tensile strength is needed. The wood densification process offers an alternative to the use of carbon-intense steel components or hardwoods from tropical forests.