Characterization of bondlines in cross-laminated timber made with preservative-treated lumber
Abstract
The number of mass timber construction projects is rapidly increasing in North America but this technology encounters durability issues where termites are present. One method for minimizing this risk is to incorporate termiticidal treatments into mass timber elements. This study examined the impact of pre and post layup treatment of cross laminated timber (CLT) on bond line integrity. Douglas-fir 2 × 6-inch lumber or CLT panel sections were pressure treated with 1) borates or 2) (PTIP), or 3) dip treated with a mixture of propiconazole, tebuconazole and imidacloprid + borate (PTIB). CLT panels were manufactured using melamine formaldehyde (MF) or polyurethane (PUR) resins. The impact of preservative treatment on bondline integrity was tested by delamination and block shear tests. Adhesive penetration was also measured using fluorescence microscopy and surface wettability was measured using a contact angle analyzer. Planing treated lumber prior to use in CLT panel assembly reduced actives by 57—94% compared to unplaned lumber containing the same treatment. Panels made with borate treated lumber were more easily delaminated than panels composed of PTIP treated wood. Microscopic evaluation of CLT bondlines showed greater resin penetration in panels made with PTIP-treated wood; however, penetration was highly variable across specimens. Borate-containing treatments increased surface wettability which may have contributed to reduced treated panel performance. The results help define the challenges associated with incorporating biocidal treatments into panels and identify some mechanisms by which they reduce performance.
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