Durability and mechanical performance of copper azole-treated cross-laminated timber (CLT) in-ground-contact exposure for 6 months

Cross-laminated timber (CLT) is a sustainable engineered wood product increasingly used in outdoor and structural applications. However, its durability in-ground-contact environments remains a concern due to moisture and biological degradation risks. This study investigated the mechanical performance and durability of copper azole (CuAz)-treated CLT after six months of open-field ground-contact exposure. Scots pine timbers were impregnated with CuAz at three different retention levels (7.4, 8.1, and 9.4 kg/m³) and used to manufacture CLT. Both destructive (bending strength, modulus of elasticity, shear strength) and non-destructive (screw withdrawal resistance, dynamic modulus of elasticity) tests were applied before and after exposure. Results indicated that CuAz treatment enhanced biological durability but negatively affected mechanical properties, with greater strength losses at higher CuAz retention levels. Significant copper leaching was observed, especially in below-ground sections. The findings emphasize the need to balance preservative retention and mechanical performance and highlight the importance of long-term field evaluations for treated CLT in ground-contact applications. These findings contribute toward closing the knowledge gap in the field performance of preservative-treated CLT and provide foundational data to inform future standardization efforts.