Durability of epoxy and polyurethane bonded timber-hybrid FRP joints under hygrothermal and weathering conditions

This paper investigates the bond between laminated veneer lumber (LVL) and flax-glass hybrid fibre reinforced polymer (HFRP) with different fabric stacking sequences in the LVL-HFRP joints exposed to (1) room condition at the temperature of 20 °C and relative humidity (RH) of 57% (as reference condition), (2) hygrothermal condition (50 °C and 95% RH), and (3) weathering environment (a dry-wet-ultraviolet radiation cyclic condition). Epoxy (EP) and polyurethane (PUR) adhesives were used to manufacture the HFRP composites and bond them to the LVL as LVL-HFRP joints. The joints were tested under block shear tests to evaluate the bond behaviour. The LVL block shear specimens were also tested as the reference for the shear strength change of the LVL in the joints. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) tests were conducted to study the influence of the hygrothermal and weathering conditions on the functional groups and the glass transition temperature (Tg) of the LVL-HFRP joints. After the hygrothermal and weathering exposures, the results showed that the Tg of the EP-based and PUR-based HFRP in the LVL-HFRP joints increased due to the post-curing and the arrangement of the disordered urea groups. The average shear strength values of the LVL-HFRP joints bonded with the EP adhesive decreased by 4.6-35.2% and the reduction of the shear strength of the LVL part in the joints accounted for the decreased shear strength at the joint level. The average shear strength values of the LVL-HFRP joints bonded with the PUR adhesive decreased by 27.9-57.0% and the reduction of the shear strength at the joint level was controlled by the glass fibre layer of the PUR-based HFRP, regardless of the fabric stacking sequence in the PUR-based HFRP.