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Abstract: Conventional wooden dowel connections in timber structures rely on tight press-fit installation, which requires high insertion forces and often loosens over time due to stress relaxation. This study investigates an alternative approach that exploits the moisture-activated set-recovery of thermo-hydro-mechanically (THM) densified hardwood dowels to enable slip-fit assembly followed by self-tightening in service. To this end, European beech and black poplar were densified radially and tangentially at different compression ratios. They were then evaluated for swelling kinetics, swelling pressure, bending performance, and moisture-activated expansion using in-situ X-ray CT in water at 20 °C and 100 °C. Results show that activation kinetics can be controlled by temperature. Expansion was rapid within minutes in hot water and slower but equivalent in magnitude at room temperature. Beech outperformed poplar, with radial densification at 35 % compression ratio producing a peak swelling pressure of 5.7 MPa and a modulus of rupture of 268 MPa after activation. Poplar generated higher free expansion but significantly lower pressure due to its lower stiffness. Radial densification was consistently more effective than tangential, enhancing both expansion magnitude and pressure generation. Capillary uptake triggered expansion along the dowel length (∼30 mm in 1 h) and produced an elliptical expansion profile. Importantly, mechanical strength was retained post-activation, which confirms structural suitability. These results demonstrate that THM-densified beech dowels can offer a robust self-tightening mechanism, combining low-force installation with durable pressure generation and stable mechanical performance. This provides a viable path toward adhesive-free, metal-free, high-tolerance timber connections.
Keywords: THM densification, dowel laminated timber, CT scanning
Published in RUP: 28.01.2026; Views: 284; Downloads: 0
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