Bond line integrity in fire of glued laminated timber predicted by high temperature block shear tests

The paper reports on a first approach to correlate charring results from three-sided fire tests on unloaded glued laminated timber (GLT) beams with high temperature shear strengths of bond lines manufactured with twelve adhesives from four different adhesive families. The investigation comprised i) six polycondensation (pcon) adhesives of types phenol resorcinol, melamine and melamine-urea formaldehyde and ii) polyaddition (padd) adhesives represented by five one-component polyurethanes and one emulsion polymer isocyanate brand. Shear strengths were investigated in the temperature range of 20–270 °C in two test series with specimens obtained from i) the industrially manufactured fire test GLT beams and ii) laboratory bonded spruce scantlings. Fire and high temperature shear strength tests showed alike an expressed difference between pcon and padd adhesives. The vertical charring rates of the pcon beams conformed throughout to the notional charring rate stipulated in the European fire design code for timber structures, EN 1995–1–2, while padd adhesives revealed up to 60 % higher values. Horizontal charring rates were not affected by adhesives. Pcon adhesives showed a rather linear shear strength decrease well conforming to solid wood up to 270 °C, whereas all padd adhesives displayed extreme strength decreases to almost zero beyond 232 °C. The charring depths and shear strength results at 270 °C can be well correlated ( R ² = 0,88) by a power type equation. Maintenance of bond line integrity of GLT in fire can be safely predicted without fire tests for adhesives conforming to temperature resistance class T270 acc. to EN 18070. • Bond line degradation in fire of GLT beams depends decisively on adhesive type. • Polycondensation adhesives exclusively show charring behavior similar to solid wood. • Polyaddition adhesives show up to 60 % higher charring rates than solid wood. • Bond line integrity in fire can reliably be predicted by adhesive shear strengths at 270 °C. • High residual shear strengths were correlated throughout with low charring rates in fire.