This testing report summarises the experimental investigations on finger-jointed timber specimens, glued with different types of adhesives, loaded in tension and exposed to standard ISO-fire. The tests were performed as part of the project entitled “Fire safety of bonded structural timber elements” in the frame of a CTI-project (Commission for Technology and Innovation). The extensive testing programme on finger-jointed timber specimens was performed in cooperation with industry partners at the Swiss Federal Institute of Technology Zurich (ETH Zurich). The main aim of this research project is to clarify if the currently used design model for the fire resistance of bonded structural timber elements, such as glued-laminated timber, should consider the behaviour of adhesives at elevated temperatures.
In this experimental study, different adhesives available on the market from adhesive manufacturer from Europe (such as Casco AG, Dynea AG, Jowat AG, Türmerleim AG, Purbond AG) were tested. Adhesives being used for structural applications as well as adhesives not certified according to current European testing standards for the use in structural applications were tested. The fire performance of 12 different adhesives - of type 1C PUR, MUF, PRF, EPI, PVAc, UF - were tested in a finger-jointed connection for cross-sections with a width of 80, 140 and 200 mm.
In total, 49 fire tests were performed under ISO-fire exposure at the Swiss Federal Laboratories for Materials Testing and Research (EMPA) in Duebendorf/ Switzerland. Two tests were conducted with specimens equipped with thermocouples to determine the temperature distribution along the cross-section width. In the other tests, different parameters and their influence on the fire resistance were varied, such as the adhesive in the finger joint, the width of the specimen, the load level and the type of fire exposure on the testing lamella.
The tests were performed in two test series in March and April, 2011 as well as in July and August, 2012. The second test series was extended by five additional tests with higher graded timber in August 2013. The main result from the first test series can be concluded as follows: The adhesives tested (2 x PUR, 1 x MUF) fulfil current approval criteria according to EN 301 (2013c) and EN 15425 (2008) for the use in load-bearing timber components in Europe. The adhesives fulfil at least the A7 test at 70°C according to EN 302-1 (2013a). Taking into account the failure pattern, no significant difference was observed between these adhesives. It could be shown that the higher loss of strength for some adhesives tested at elevated temperature does not necessarily lead to the same loss of strength in fire, since defects like knots may be dominant - depending on the strength class (grading). The main result from the second test series can be concluded as follows: No substantial difference was obtained for finger-jointed specimens glued with PRF and other structural adhesives. The PUR adhesive fulfilling the ASTM D7247 (2007) standard test at temperatures higher than 200 C did not reach a higher fire resistance than PUR adhesives which do not fulfil this standard. It was found that adhesives, which are used in structural timber members such as glued-laminated timber beams, need sufficient strength at lower temperatures than 200 C.This is especially explained by the steep temperature gradient typical for timber members such as glued-laminated timber.
In addition to the fire tests, about 120 tensile tests on finger-jointed lamellas were performed at normal temperature. These lamellas were produced with the same types of adhesives as studied in the fire tests. The results of the whole investigation are summarised in this test report.