Utilization of Pressure Treated Lumber in Cross Laminated Timber Manufacture and its Impact on Bondline
G Presley, C Wainscott, J Valenti, S Noble
Mass timber building construction is increasing dramatically across North America which presents challenges to these structures not seen in Northern Europe. Large parts of the United States harbor termites and existing mass timber building technologies that rely solely on moisture exclusion to increase durability are insufficient in these areas. The successful expansion of mass timber construction in the United States will require the incorporation of pressure treatments into panel fabrication, but chemical presence introduces difficulties with resin compatibility. This work examines the impact of preservative treatment on cross laminated timber (CLT) bondline integrity by constructing small-scale panels using treated lumber or treating panels after layup. Douglas-fir 2 x 6-inch lumber or untreated CLT panel sections were treated with one of three different preservative systems, pressure treatment with borates, pressure treatment with propiconazole, tebuconazole, imidacloprid, permethrin and iodopropyl butyl carbamate (PTIP+IPBC), or dip treatment with propiconazole, tebuconazole and imidacloprid + borate (PTI). Panels were made with one of two resins, melamine formaldehyde (MF) or Polyurethane (PUR). Delamination, block shear and wood failure were tested according to the PRG-320 standard. The all-organic preservative treatment passed delamination test standards when pressure treated lumber was used to layup panels, while utilizing borate-treated lumber for panel manufacture resulted in failure. Bondline characterization was done microscopically to measure depth of resin impregnation into the cross section of CLT laminae. This work provides useful information on preservative-resin compatibility for CLT manufacture and can help improve the durability of mass timber structures.