Long-term bending and bond integrity of adhesively bonded glulam-concrete panels under outdoor exposure and sustained loads: Impact of manufacturing techniques and adhesive types

Adhesive bonding has been proposed for timber–concrete composites because it provides a rigid connection, unlike mechanical fasteners that allow partial slip and consequently increase deflection while reducing stiffness and load-bearing capacity. However, the long-term behavior of adhesively bonded timber–concrete systems, particularly under outdoor conditions relevant to bridge applications, remains insufficiently understood. This study investigates the 86-week bending behavior and bond integrity of glulam–concrete panels manufactured using wet or dry bonding with ductile polyurethane (PUR) or brittle epoxy adhesives. The panels were subjected to unsheltered outdoor exposure under sustained bending, including direct exposure to precipitation (rain and snow), ambient temperature and humidity variations, as well as solar radiation (direct and reflected). Subsequently, the panels were tested in four-point bending. Mid-span creep deflection increased gradually from zero at the start of exposure (21 October 2022) to approximately 20 mm by late autumn 2023. RMSE values ranged from 0.34 to 2.98 mm, indicating comparable mid-span deflection trends across all configurations. Destructive testing showed that 7 of 12 panels achieved comparable ultimate bending capacities, while 11 of 12 exhibited effective bending stiffness, with maximum deviations of 13.0% and 13.3% from design values, respectively. In the remaining five panels, lower ultimate bending capacities were attributed to the presence and severity of natural wood defects, such as unfavorably located knots, dense knot clusters, weaker finger joints, or cracks that developed during outdoor exposure, which are known to govern bending failure in glulam members. Adhesive bond integrity remained intact in all but one specimen, the long-term PUR-bonded dry panel, likely due to weaker mechanical interlocking at the adhesive–concrete interface in dry bonding compared to wet bonding. Mechanical properties of exposed spruce (shear strength, tensile strength, and tensile stiffness) and concrete (compressive strength and stiffness) showed no notable degradation compared with unconditioned material. Overall, wet bonding exhibited higher durability than dry bonding under combined outdoor exposure and sustained bending.