In this episode of the Fire Science Show we invite dr. Antonela Čolić from the OFR Consultants, to break down the performance of adhesives used in CLT in fire, what differences between the glues are observable at the microscale  and how they show up in real structure fires.

We compare common polyurethane adhesives: one that softens near 200–220 C and one that resists softening, crosslinks, and ultimately chars. Through thermogravimetric and calorimetric testing, we map pivotal transitions like glass transition and softening. Then we scale up. With small shear-lap coupons and meter-long cantilevers under controlled heat flux, we see how mechanical load amplifies normal strains at the bond line—especially in cross-laminated elements where grain orientation concentrates stress. The result is a clear picture of when heat-induced delamination begins, how it differs from char fall-off, and why heat flux often dominates the story.

Moisture emerges as a powerful, often overlooked driver. Using neutron imaging, we visualize vapor moving toward and across the bond line, slowing as it crosses the interface. That temporary moisture retention can make an adhesive appear to “fail at a lower temperature,” not from chemistry alone but from local pore pressure and hydration dynamics. We translate these findings into actionable guidance: specify adhesives that char rather than soften, control lamella thickness, consider parallel lamellas to preserve capacity after a ply loss, and model realistic heat flux and shear demands instead of relying on a single critical temperature.

If you design or review mass timber, this conversation gives you the tools to ask better questions: Which adhesive? What heat flux history? How much shear at the bond line? And how will moisture in use and during fire shift the thresholds you’re counting on? 

Interested in further reading? Got your back.

• Paper on microscale experiments on adhesives
• Book Chapter on Compartment Fire Dynamics with Timber
• Full scale experiments on CLT with different adhesives

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The Fire Science Show is produced by the Fire Science Media in collaboration with OFR Consultants. Thank you to the podcast sponsor for their continuous support towards our mission.