Comparison of different bonding parameters in the production of beech and combined beech-spruce CLT by standard and optimized tests methods

Interest in the use of beech wood in construction is growing steadily. Considering its high mechanical performance, it has a large potential to be used for the production of glued timber structural products. However, the issue of structural bonding remains to be solved before effective production and safe use of beech wood in these products can be achieved. Three adhesives (one-component polyurethane - PUR, PUR + primer and melamine-urea-formaldehyde - MUF) and two press systems (hydraulic and vacuum press) were explored in the production of pure beech and combined beech-spruce-beech cross laminated timber (CLT). An evaluation of the bonding quality was performed with both standardized and optimized (the specimen layers were oriented with the wood grain forming an angle of 45° with respect to the load application) delamination and shear test methods. None of the adhesives that were tested met the requirements for delamination tests provided by the current standard for softwood. Specifically, PUR yielded the poorest performance in the production of the CLT panels entirely made of beech; the addition of a primer improved the bonding, achieving results comparable to those observed for MUF. On the contrary, in the production of beech-spruce panels, the three adhesives showed similar results. The press system was not a relevant factor in terms of bonding quality. In regard to testing, a size effect was noticed in the delamination test: the larger the specimen the greater the delaminations that were observed. The shear tests on dry specimens proved to be less efficient in determining the influence of the several bonding parameters, even if a 45° grain orientation seemed to reduce the rolling shear during the shear test and to improve the response of the test in this regard. Combining delamination pretreatment and shear made the test more sensitive and objective.