Interfacial bond strength of epoxy and PUR adhesively bonded timber-concrete composite joints manufactured in dry and wet processes

Abstract

This paper investigated the shear bond strength of adhesively bonded timber-concrete composite (TCC) joints between spruce wood and cast-in-situ concrete (i.e., TCC joint produced in wet bond process), and between spruce wood and prefabricated concrete (i.e., produced in dry bond process). Two types of adhesive including two-component polyurethane (PUR) and two-component epoxy were used for bonding wood and the concrete blocks. In fabrication of the wet bond TCC joints, three waiting times defined as the interval times between the starting time of adhesive cross-linking process and starting time for pouring fresh concrete were considered for both PUR and epoxy adhesives. These three waiting times were 0, 0.5tgel and tgel in which tgel represents the gelation time of the corresponding PUR and epoxy measured by oscillatory test. The wet TCC joints manufactured at 0, 0.5tgel and tgel waiting times and the dry TCC joints were tested under shear load to obtain their shear strength. The values of the shear strength of the wet and dry bond TCC joints were discussed, compared and correlated with respect to the failure modes. The experimental results showed that the average shear strength of wet bond TCC joints bonded with PUR and epoxy both reduced by around 47% when increasing the waiting time from 0 to tgel. It should be attributed to that the adhesives tended to have a weaker bond with cast-in-situ concrete at a longer waiting time, based on optical microscopy observations. Both PUR and epoxy bonded dry TCC joints possessed similar average shear strength and identical failure modes, i.e., failure in adhesive area and small portion of concrete fracture. Moreover, the use of epoxy in wet bond TCC joints led to higher shear strength than that in the dry bond TCC joints, while this trend was conversely observed in PUR bonded wet and dry TCC joints.