Creep performance of glued-in rod joints in controlled and variable climate conditions

Abstract Presented in this study are the results of creep tests on joints with single glued-in rods performed in controlled and variable climate conditions. The joints were prepared using steel threaded rods of 8 mm in diameter, wood of two species: Black spruce (Picea mariana Mill) and Norway spruce (Picea Abies L.), and two adhesives: polyurethane (PUR) and epoxy (EXP). The first test campaign was performed under two constant climate conditions: 20 °C / 65% relative humidity (RH) and at 50 °C / 72% RH. The applied load corresponded to a stress level of 50% of the static breaking load. The moisture content of wood remained constant during the experiments. Results revealed that the joints were subject to creep with major differences between the adhesives. Low data variability was observed for specimens with the EPX and strong correlations were found between the initial stiffness and the stiffness at 10 days and 25 days. The EPX joints developed lower creep than the PUR joints at 20 °C. At the temperature of 50 °C, the creep of the EPX joints increased but there were no failures observed within 60 days, whereas all PUR joints reached failure at the same stress level within days. The second test campaign was launched using the EPX to study the creep of the joints in variable climate conditions at the stress ratios between 60% and 78% of the static strength. The variation in ambient conditions appeared to affect the creep: humidification phase preceded by drying generated significantly higher creep. The humid and dry cycles may govern the load duration and cause the rupture. In addition, this study revealed that the wood drying from 18% to 10% created some damaging cracks at the wood-adhesive interface. The propagation of stable cracks induced an increase of the slippage in the joints and reduction of the residual strength.