This study investigates the effect of concrete-steel debonding on the modal dynamic properties, i.e. frequency values and vibration mode shapes, and deals with the identification of interfacial debonding zones in CFST columns. In this context, debonding, defined as a decrease in concrete stiffness at the connection with the steel tube by a depth of 3 mm, was analyzed through the frequency analysis of the column. The findings indicate that debonding results in a discernible disparity between the primary and secondary mode shapes, as assessed by the Modal Assurance Criterion (MAC). Furthermore, there is a reduction in the frequency values across all modes. This paper proposes the irregularity detection indices of the Modal Curvature Index (MCI), Modal Curvature-Slope Index (MCSI) and Modal Curvature-Curvature Index (MCCI) using the concept of curvature to detect the debonding zones. These indices indicate the relative extreme values of irregularities in debonding zones for all modes, such that the MCSI and MCCI show an increase in the height of irregularities regardless of negative or positive signs in a debonding zone as the damage severity in that zone increases.
