Effects of concrete type, concrete surface conditions and wood species on interfacial properties of adhesively-bonded timber

To optimize the bonding performance between prefabricated concrete and wood in adhesive-bonded timber-concrete composite (TCC) system, the authors conducted quasi-static shear tests on several types of TCC joints bonded with epoxy resin in this study. The experimental variables considered were (1) concrete surface conditions (i.e. sandblasted, sanded and non-treated), (2) concrete type (i.e. normal strength concrete (NSC), high strength concrete (HSC) and ultra-high-performance concrete (UHPC)), and (3) wood species (i.e. glulam GL24 made of spruce and beech wood). The test results showed that the surface treatment on NSC (sandblasted) and HSC (sanded) significantly increased the shear bond strength of the TCC joints and changed the predominant failure mode from tearing-off of mortar-facial covers into failures in concrete substrates. Upon the surface treatment, using HSC increased the shear bond strength compared with the TCC joints using NSC. The joints with UHPC predominantly failed in the adhesive/concrete interface, and adhesion between adhesive and concrete, due to that the low porosity of UHPC impeded the penetration of epoxy resin into the substrate. When being bonded with UHPC, compared with the GL24 made of spruce, the beech wood increased the shear bond strength, since the wood-failure percentage decreased in the joints using beech wood. Furthermore, bond-line areas of all types of joints were examined under an optical microscope, to reveal the morphology (i.e. profile of a glue line, glue-line thickness, and penetration of resin into substrates). It was found that epoxy used in this study with a high initial viscosity (i.e. 6000 mPa·s at 23 °C) is suitable to bond surface-treated concrete and engineered wood, from the perspectives of penetration into substrates, the thickness of glue lines, and workability during gluing processes.