Bonding of beech wood to mortar with a novel epoxy hybrid-adhesive: Performance in dry and wet conditions

Composites made of timber and cementitious materials require a rigid connection to exploit their full composite action, which can be achieved by using full-surface adhesive bonding. In this work, we investigated a novel hybrid-adhesive system consisting of a silane-terminated polyurethane (STP) and epoxy resin for the bonding of beech wood timber to fresh mortar for use in timber-mortar composites (TMC). The mechanical performance and the influence of moisture on TMC produced by the wet-in-wet process (fresh mortar) was investigated and compared to the bonding of prefabricated mortar (prefab process). The STP-epoxy hybrid-adhesive showed a suitable bonding performance of beech wood to both, fresh mortar and precured mortar with median compression shear strengths of 4.57 MPa and 6.07 MPa, respectively. The fracture pattern showed the strength of the near-surface layer in the mortar, close to the adhesive, being often decisive for the bond performance. The same failure mode predominated in TMC beams after 3-point bending tests. The stability of the composite upon the influence of moisture is especially challenging when using beech wood due to its low dimensional stability. Thus, the moisture stability of the bond was investigated by compression shear tests after water immersion. It showed an improved water stability compared to composites bonded with an epoxy resin. Nonetheless, a clear reduction in bond strength compared to the dry state was observed, with delamination of 25 % of the wet-in-wet and 17 % of the prefab specimens during water immersion. Furthermore, it was seen that the adhesive open laying time played a decisive role in the wet-in-wet produced specimens influencing both, dry and wet shear strength.