Gap-filling properties of adhesives used for cross-laminated timber

Cross-laminated timber (CLT) laminates layers of laminae side by side, with the fiber direction of each layer orthogonal to each other. Therefore, any error in the thickness of the laminae may cause gaps in the glue lines, leading to adhesion failure. In this study, a three-layer CLT with a thin lamina inserted in the inner layer was manufactured, and the gap-filling performance of adhesives and the allowable error for lamina thickness were investigated. The outer layer of CLT was arranged with 30.0 mm-thick laminae, and the inner layer alternated between 30.0 mm-thick lamina and thinner laminae (29.9, 29.7, and 29.5 mm). One-component polyurethane adhesives (PUR), emulsion polymer-isocyanate (EPI) adhesives, and phenol-resorcinol–formaldehyde resin (PRF) were used. The glue line thickened when the lamina thinned down in PUR and EPI. By contrast, the glue line thickness at the lamina thickness of 29.5 mm in PRF was almost the same as the glue line thickness without gaps. PUR demonstrated low delamination and a high wood failure rate (WF) even at 0.5 mm lamina thickness error, meeting the criteria set by the Japanese Agricultural Standard (JAS). Conversely, EPI failed to meet the JAS requirements for a lamina thickness error exceeding 0.3 mm. On the other hand, PRF exhibited favorable adhesive performance within a 0.3 mm lamina thickness error, with delamination and WF meeting the criteria of the standard.