Radial crack patterns in the arch ring were detected by modal frequency shifts aided by analysis of the auto-spectra. Crack depths of between 10% and 50% of the arch ring thickness were cut at various points around the span and across its entire width. A-scan plots showed crack patterns pertaining to both single and multiple cracks could be detected but less readily sized than backfill void type defects. The NDT method used is well established as a rapid, cost effective, and reasonably accurate technique. This study has shown it to be applicable to the condition

monitoring of masonry arch bridges.