The effect of long and short term elevated temperatures on the behaviour of lap glued joints of load-bearing spruce elements

The paper deals with the behaviour of glued joints of wooden sections exposed to long and short term elevated temperatures. If the adhesive is not sufficiently temperature resistant, the glued joint may lose adhesion, which could result in a change in behaviour of the glued element cross-section when exposed to fire. Moreover, detailed knowledge of the behaviour of bonded joints under such long and short term thermal exposures will enable us to make informed decisions about the need for major structural intervention in structures post exposure. Three types of adhesives: phenol-resorcinol-formaldehyde (PRF), polyurethane (PUR) and emulsion polyisocyanate (EPI), commonly used in load bearing wooden structures, were chosen for the experimental work. Thermal analysis showed that the PRF and PUR were not sufficiently cured, especially the PRF. Calorimetric analysis showed the point at which thermal degradation of all three adhesives began, at 135 °C for EPI and at about 160 °C for PRF and PUR. Spruce lap joints, with a cross section of 40 × 48 mm, bonded using these adhesives were exposed to temperatures of 140 and 170 °C. To properly distinguish the effect of thermal exposure duration on the lap joints, different thermal durations were used: 20, 40, 60 and 80 min, plus 3 h and 1 day. The thermal resistance at each of the elevated temperatures was investigated through measurement of the reduction in tensile shear strength. At an elevated temperature of 140 °C there was no significant strength reduction in all of the tested adhesives, it was only a longer duration (1 day) that produced a reduction in the tensile shear strength properties (31.0% for PRF, 16.2% for PUR and 34.1% for EPI). A temperature of 170 °C produced a greater decrease in strength, a significant drop (more than 15%) was found for PRF and EPI after an exposure of 40 min; for PUR after 60 min. Moreover, the longer duration exposures to a temperature of 170 °C resulted in a total loss in strength for PUR and a half loss of strength for EPI; for both these isocyanates adhesives this was confirmed by the total loss of wood failure criterium. On the contrary, a strength reduction of 50.0% and a sufficient wood failure criterium for a 1day temperature exposure at 170 °C was found for PRF.