Bonding performance evaluation of mixed-species palm CLT: limitations of wood failure percentage in assessing bondline integrity

This study evaluated the bonding performance of mixed species cross-laminated timber (CLT) panels composed of high-density coconut wood (600–900 kg/m 3 ) as face layers and either low (200–300 kg/m 3 ) or medium (400–500 kg/m 3 ) density oil palm wood as core layers. The hybrid CLTs were consolidated using one-component polyurethane (PUR), resorcinol formaldehyde (RF), and melamine urea formaldehyde (MUF) adhesives, considering the maximum and minimum spreading rates and clamping pressure levels specified by the adhesive manufacturers for softwood and hardwood species. Block shear and delamination tests were conducted to evaluate the bonding performance based on the American CLT product standard, ANSI/APA PRG 320. Results showed that block shear strength (BSS) and wood failure percentage (WFP) were primarily governed by oil palm core density. The delamination rates of the PUR specimens were larger than those of MUF and RF, regardless of spreading rate and clamping pressure. For RF and MUF adhesives, the application parameter ranges specified in the manufacturer’s guidelines can be adopted for the manufacturing of this hybrid CLT product. Although PUR adhesive shows potential for use in this CLT product, its application parameters require further optimization to achieve satisfactory adhesion. For the PUR-bonded mixed-species CLT examined in this study, WFP and delamination rate did not consistently coincide, supporting the use of both block shear and delamination tests for bond-quality evaluation.