European assessment and creep behaviour of load bearing timber structure PUR adhesive

For many years several types of adhesives for load bearing structures have been designed and developed, but the admittance of these adhesives has been hindered. The reason for this is a lack of demonstrated long-time performance and the lack of a testing, approval and classification system. Two new European standards on glue laminated timber and cross laminated timber establish the use of phenolic (PF) and aminoplastic (MF) resins, one component polyurethane (PUR) and emulsion polymer isocyanate (EPI) adhesives. According to the standards, the adhesives have to fulfill many requirements, among others they must not unduly reduce strength and stiffness of engineered timber elements. Also the creep properties and performance at elevated temperatures must be taken into account. The purpose of this study is to explain the European assessment of a new structural adhesive and to analyze the bonding performance of a new PUR adhesive focusing on creep test. Creep performance frequently is a weak point for the PUR adhesives and no information is provided in the literature. Creep deformation test is performed as four point bending tests on specimens 50x50x600 mm that are subjected to a constant load of 4 000 N at weekly varying climate conditions. Specimens bonded with the tested adhesive are compared with specimens bonded with a PRF adhesive. Results show the behavior of the adhesive over one year trial and a statistic discussion is presented. The high bonding strength and particularly the good creep performance that can be achieved indicate that one component polyurethane adhesive has great potential in load bearing timber structures.