Degradation mechanism of block shear strength and microstructure at the bonding interface of hybrid cross laminated timber after thermal treatment

Abstract Cross laminated timber (CLT) is recognized as alternative to traditional construction materials, which is composed of orthogonally bonded layers of solid sawn timber or structural composite materials (e.g., oriented strand board) using adhesive. The bonding performance of CLT is an important parameter to determine the structural safety at a high temperature environment. In this study, it was performed to clarify the deterioration mechanism of bonding interface of CLT and hybrid CLT (HCLT) after thermal treatment. CLT was prepared by the plantation Chinese fir using one component polyurethane, and HCLT was the use of oriented strand board (OSB) to replace the lateral layer of Chinese fir laminate. The block shear strength and the delamination of vacuum pressure impregnating were carried out on CLT and HCLT after thermal treatment. The changes of chemical components of Chinese fir, OSB and adhesives were analyzed, and the damage microstructure of bonding interface of CLT and HCLT were studied. The results showed that the qualified rate of delamination of vacuum pressure impregnating for HCLT still reached 90% at thermal temperature of 200 °C, which was higher than the qualified rate of CLT reached 40%. The block shear strength of HCLT was significantly higher than that of CLT. When in early-stage temperature from 20 to 80 °C, the main failure mode of CLT transitioned from sawtooth failures within the wood towards glue layer fractures and gradually extending to the direction of wood-ray. And the block shear strength decreased by 47.14%, the wood failure ratio decreased to 60%. The primary failure mode of HCLT transitioning from wood failures into OSB core layer failures. When treatment temperature rose to 80 °C–100 °C, PUR adhesive began to pyrolysis and formed –NCO group formation. Meanwhile, the small cracks occurred gradually at the bonding interface. During later stages where temperatures increased from 100 to 200 °C, the tracheid occurred deformation and intercellular layer was split of the bonding interface for CLT and HCLT.