Moisture-induced stresses and damage in adhesively bonded timber-concrete composite connection

An experimental program was conducted to clarify the wood swelling effect on the shear strength and failure modes of adhesively bonded timber-concrete composite (TCC) joints. A total of 150 TCC joints were fabricated and cured in the condition of 20 °C & 55% relative humidity (RH) for one month, then the joints were divided into three groups (50 each group) and treated at 20 °C and 55%, 75%, and 95%RH, respectively. After about 30-, 100-, 200-, 300-, and 390-day treatment, ten samples from each group were tested to determine their shear strength and failure modes. It was found that an increase in moisture content (e.g., by 10.8%) caused a reduction of shear bond strength (e.g., by 25%). Concrete failure was found to be the dominant failure mode for all tested joints. Finite element (FE) analyses were performed to facilitate understanding of the underlying mechanisms of wood swelling effects. When the moisture content of wood increased by 10.8%, the FE analyses showed that the wood swelling resulted in significant internal stresses and concrete tensile damage at the wood/concrete bonding interface, which was the main reason for the reduction of the shear bond strength. In addition, the effects of essential factors, such as material properties of adherents, orientation of wood components relative to the bonding surface, and bonding width perpendicular to the grain direction of wood, on the moisture-induced stresses were studied. Concrete grades hardly had influence. For the bonding process of TCC structure in practice, a smaller orientation angle (i.e., the angle between the radial direction and the bonding surface) is recommended to avoid moisture-induced damage in wood and concrete at their interface. In other words, the surface of a Glulam or CLT product designed for bonding should be as close as possible to R-L planes of the constituent wood blocks and avoid T-L planes. Furthermore, a larger bonding width resulted in higher moisture-induced stresses.