Comparing timber adhesive shear strength properties after fire damage

Due to the rapid growth of the engineered timber industry worldwide, a deeper knowledge of the high temperature material properties and behaviour in fire is required. The primary difference between mass timber and engineered timber is the addition of an adhesive through which several laminates of timber are built up into larger sections. Adhesives may degrade at lower temperatures than timber which can lead to delamination during or after a fire. In order to fully understand the performance of engineered timber materials after extreme heating, it is necessary to study the behaviour of these adhesive layers during infire and post-fire exposure, the latter being the primary focus of this paper. Current standards do not look at the performance of wood adhesives beneath the char layer in burned engineered timber, but previous tests have shown that this may introduce complex failure mechanisms. To study this issue, samples with two different types of adhesives were heated with various incident heat flux exposures and heating durations followed by mechanical shear testing. A cone calorimeter was used for controlled heating exposure, and the residual shear strength of the glue lines was measured through an innovative mechanical apparatus. Digital image correlation technology was used to assess the deformation (slip) of the adhesive layer as load was applied. It was found that the residual shear strengths were highly variable but did show a slight correlation with the heating duration. Additionally, the slip measurements of the shear line showed that generally, the unheated adhesives were more ductile than the heated samples which exhibited brittle failure