Experimental Investigation of Curvature Effects on the Bond–Slip Behavior of ECC–Steel Tube Interfaces

To quantify the effect of steel tube curvature on bond–slip behavior at the ECC–steel tube interface, 12 push-out specimens were tested in four groups. Steel–tube curvature was varied as K = 0, 0.01, 0.012, and 0.02, while the ECC cover thickness (20 mm), tube wall thickness (6 mm), and bond length were held constant. Monotonic push-out tests were used to obtain interfacial bond stress–slip curves and to assess failure modes, bond strength, and characteristic slip. A curvature-dependent piecewise bond–slip model was then calibrated to the test data. Within the investigated range, increasing curvature improved the measured interfacial response. As K increased from 0 to 0.02, the peak bond stress rose from 0.236 to 0.644 MPa (172.9%), while the characteristic slip at the onset of the residual stage increased from 2.696 to 4.630 mm (71.7%). The proposed model reproduced the measured curves with coefficients of determination R2 ≥ 0.96 and an average fitting error below 4.1%. These findings indicate that steel–tube curvature can enhance interfacial load transfer and deformation capacity within the tested parameter range and provide a basis for interface design and numerical modeling of ECC–steel tube composite members.