Enhancement of Interface Performance in Steel–Concrete Composite Structures by Humidity-Cured Epoxy Bonding: Mechanisms and Structural Implications

This study evaluates a route in which a humidity-cured epoxy is applied to a cleaned steel surface and concrete is cast directly onto the still-tacky adhesive, eliminating welding and pre-drying. As a preliminary study, six push-out specimens in three configurations—studs only (G1), pure bonding (G2), and studs combined with bonding (G3)—were tested under monotonic load control, with two specimens loaded statically in each group. Mean ultimate capacities were 474.5, 680 and 720 kN for G1, G2 and G3. Bonded specimens failed within the concrete matrix, leaving concrete on the steel plate and indicating that the wet-cast bond line exceeded the near-surface strength of the concrete. Interface slip in G2 and G3 remained below the measurement resolution (±0.001 mm), consistent with elastic shear of the 1–2 mm epoxy layer. Concrete strains remained compressive throughout loading in bonded specimens, whereas stud specimens developed bottom-surface tensile strains of about 220 με, indicating that discrete connectors induce a local bending field that continuous bonding removes. However, bonded failure was abrupt and non-ductile; the hybrid configuration retained a post-debonding load path that provided a residual capacity of about 68% of the stud-only residual (reconstructed numerically in “Finite Element Verification”), converting a brittle mechanism into one with a secondary, damage-tolerant reserve.