Quantitative assessment of adhesive effects on partial and full compressive strength of LVL in the edge-wise direction

Laminated wood-based materials have been widely developed, and the laminating process and adhesive itself have been reported to enhance performance beyond the sum of the individual layers' performance. This phenomenon is particularly notable under loads applied in the "edge-wise direction", where each layer bears stress collectively. These combined effects are referred to as the "adhesive effect". Strength under partial compressive loads is critical in timber engineering, as partial compressive stress generates complex stress distributions influenced by boundary conditions. The adhesive effect may also be impacted by these conditions. The aim of this study was to quantitatively and directly evaluate the adhesive effect under partial and full compressive loads using various parameters. The strength of laminated veneer lumber (LVL) with adhesive was compared to that of simply layered veneers without adhesive to assess the adhesive effect. Three mechanisms contributing to the adhesive effect were proposed: Mechanism I, caused by the deformation of the adhesive layer independently from the veneers; Mechanism II, resulting from the adhesive impregnating the veneers; and Mechanism III, arising from the reinforcement provided by adjacent veneers. The results suggested the following: (i) Mechanism I had minimal impact, as the fiber direction and the presence of additional length showed strong and slight effects on the adhesive effect, respectively; (ii) Mechanism II contributed to preventing crack propagation and altering the relationships among mechanical properties, with its effectiveness increasing as the adhesive weight increased; and (iii) Mechanism III functioned as a crossband effect, reinforcing weaknesses caused by the slope of the grain and the angle of the annual rings.