Enhanced durability of structural bonded timber joints. Effectiveness of adhesion promotion techniques

Abstract

Long-term durability of a structural adhesive joint is an important requirement, because it has to be able to support the required design loads, under service conditions, for the planned lifetime of the structure. Epoxy adhesives, whilst not ideal, are currently the best family of adhesives for in situ repair operations. As long as the bonded joint remains dry and unexposed to high service temperatures, epoxy adhesives produce strong bonds to timber. However, once they are exposed to severe stresses as a result of repeated water soaking and drying cycles, the bonded joint delaminates and does not fulfil the requirements for structural timber adhesives intended for exterior exposure. One way of improving bond durability is through the use of surface treatments prior to bonding. In this study, the effects of four surface treatments on three timbers (maritime pine, iroko and European oak) were evaluated through contact angle measurements (Dynamic Contact Angle Analysis using the Wilhelmy plate method) and compressive lap/shear tests involving weathered and non-weathered epoxy-bonded specimens. The results proved that surface modification methods for adhesion promotion can be adapted to cellulosic substrates with significant improvements in bonded joint durability.