BONDING OF FIRE-RETARDANT TREATED SPRUCE LAMELLAE FOR USE IN CROSS LAMINATED TIMBER (CLT)

Abstract

This paper presents results from bond line testing to investigate the influence of fire-retardant (FR) treatment on the bondability of spruce, tested in glued laminated timber (GLT) specimens as requested by standards for testing of the actual adhesive systems, and specimens of cross-laminated timber (CLT) produced under laboratory conditions to investigate a product bonded with the adhesives. One MUF- and two PUR adhesives were chosen for the investigations, selected on the basis of their use in CLT-production today as this product was the main focus. They were tested in five different material combinations: lamellae with high uptake of fire-retardant against lamellae with high uptake of fire-retardant, high uptake against untreated lamellae, low uptake against low uptake, low uptake against untreated lamellae and untreated against untreated lamellae. All 22 GLT specimens fulfilled the requirements for resistance to delamination according to EN 302-2:2017/EN 301:2017. The requirements for compressive shear strength according to EN 14080:2013 were fulfilled by all 24 specimens cut from the GLT samples. 73 of 75 CLT specimens fulfilled the requirements for resistance to delamination according to EN 16351:2021. The results do not show any influence of the FR treatment on the bondability of the FR treated spruce lamellae. Potential explanations for this might be that a) the adhesives are fully compatible with the FR, or b) the FR-treated wood was removed by the slight planing of the lamellae prior to bonding, leaving untreated but insufficiently planed surfaces for bonding.