Bending performance improvement of spruce LVL using adhesive-bonded flax fiber composites

Abstract Increasing pressure on high-quality timber resources like Norway spruce necessitates enhanced material efficiency, driving interest in hybrid wood products. This study evaluated flax natural fiber-reinforced polymer reinforcement for spruce laminated veneer lumber. Reinforcement composites using conventional epoxy and bio-based polyfurfuryl alcohol matrices were bonded to the lumber with structural wood adhesives (melamine-urea-formaldehyde and polyurethane); direct reinforcement using these adhesives was also tested. Mechanical properties (tensile, shear, 4-point bending, impact) were assessed, including moisture-dependent behavior at 35–85% relative humidity. Results demonstrated effective bonding of prefabricated fiber composites using both wood adhesives, yielding significant increases in the lumber’s bending stiffness and strength. Notably, the polyfurfuryl-alcohol-based composites provided the highest stiffness gains, validating its bio-based potential, while epoxy-based composites exhibited superior toughness and impact resistance. Both systems maintained significant reinforcement despite expected moisture sensitivity. Direct reinforcement attempts using the wood adhesives revealed processing limitations. Findings confirm natural fiber reinforcements as a viable strategy for enhancing wood performance, enabling advanced hybrid wood products, contributing to more efficient use of wood resources.