Field and Numerical Investigation of Impact-Echo Responses for Internal Defect Assessment in Cylindrical Concrete Bridge Piers

Impact-Echo (IE) testing is a promising nondestructive evaluation technique for concrete structures, but interpreting its frequency response in cylindrical bridge piers remains challenging because structural geometry, propagation paths, and internal discontinuities can produce overlapping spectral characteristics. This study investigates IE responses of cylindrical concrete bridge piers through field measurements and three-dimensional finite-element simulations. Field tests were conducted on two in-service bridges to characterize dominant-frequency responses and measurement repeatability under practical conditions. The numerical investigation considered hollow voids and segregation-like low-stiffness regions with different sizes and spatial configurations. The effects of defect type, size, and location on dominant frequency, waveform timing, and transmission characteristics were examined to interpret the underlying response mechanisms. The simulations showed that hollow voids generally produced stronger transmission blockage and more pronounced temporal shifts in the backside response than segregation-like regions in the representative cases, although their frequency responses were not uniquely distinguishable. A normalized dominant-frequency-shift index was subsequently evaluated as a candidate measure of relative spectral deviation. Within the deduplicated numerical parameter set, hollow-void cases generally exhibited larger dominant-frequency shifts than segregation cases. However, the distributions overlapped, and one segregation case was highly sensitive to dominant-peak selection because of two nearly equal spectral peaks. The field measurements demonstrated repeatable dominant-frequency responses under the investigated conditions, while spatial variations and measurement geometry highlighted the limitations of single-frequency interpretation. The findings support comparative interpretation of IE responses in cylindrical members but also highlight the need for further validation using independently confirmed defect conditions.