The effect of adhesive amount and type on failure mode and shear strength of glued timber-concrete joints

Wet and dry bonding techniques were used to bond the spruce timber and concrete with the adhesive as a timber-concrete joint (TCJ) in this study. This paper discusses the effect of adhesive amount and type, i.e., polyurethane and epoxy, on the bond strength of wet and dry TCJs. Adhesive amounts (i.e., quantified with equivalent thicknesses) for the polyurethane were 0.25-, 1- and 3-mm, and for epoxy 1-, 2- and 3-mm. For the wet TCJ, the hardener and resin of two-component polyurethane and epoxy were blended and applied to the spruce timber. Afterward, cast-in-place concrete was cast on the timber substrates with the delayed time (waiting time) of T-0, T-GEL/2, and T-GEL. According to the oscillation test, the gel time of adhesive (T-GEL) was 72 and 221 min for the polyurethane and epoxy, respectively. A double lap shear test was run to the wet and dry timber-concrete joints to analyze the influence of adhesive amount and type on their failure modes and bond strength. Experiments indicated that increasing the adhesive thickness (or amount) of wet TCJs resulted in larger shear strength of the joints. A longer delayed time improved the bond strength of wet TCJ glued with 1- and 3-mm polyurethane equivalent thicknesses. However, with a similar amount of adhesive, the bond strength of wet TCJ glued with epoxy decreased with a longer delayed time. This behavior was attributed to differences in shear storage modulus of used polyurethane (G'=1.2 kPa) and epoxy (G'=229.7 kPa) adhesives at T-0 delayed time. It was found that the wet bonding technique used to produce TCJs tended to have higher shear strength than the dry bonding technique for the given same adhesive amount and type.