The effects of timber species and adhesive type on the behavior of finger joints in tension under fire conditions

In this work the bonding behaviour of finger joints made of three timber species (chestnut, beech and spruce) and glued with two different structural adhesives (melamine (MUF) and phenolic and amino-plastic (PRF) adhesives) was investigated using small-scale fire tests. An experimental campaign was carried on by testing the specimens under the cone heater at a constant heat flux and tensile load. The time to failure (TTF) as well the residual cross-section and the charring depth were measured. Overall, the specimens bonded with MUF experienced slightly higher time-to-failure (TTFs) than those bonded with PRF. Beech experienced high time-to-failure and failures on timber sided rather than on the bonded joint. A characteristic behaviour was evidenced for chestnut specimens that reached lower TTFs compared to beech and even to spruce, with the same charring rates. The singular outcomes from chestnut tests were explained as the combination of different factors: its low tensile strength even at ambient temperature, the development of overpressures in gas linked to the heat conditions during the tests and the presence of abundant extractives that char over 700°C.