POST-TENSION SPECIAL INSPECTION OF A BRIDGE SOFFIT

OBJECTIVES

TRACE was instructed to undertake a non-intrusive structural investigation of nominated soffit areas on two bridges to accurately locate ducts and anchorages, and to identify any potential voids or defects. All works were completed during nightshifts under possession.

SOLUTIONS

TRACE deployed a two-stage non-intrusive workflow combining ground-penetrating radar (GPR) for precise duct location, and ultrasonic pulse echo (UPE) testing to assess internal duct condition. GPR was used to identify the location of the ducts to allow for UPE scans to be conducted along the ducts. UPE testing was then undertaken along the identified duct alignments to assess their condition and detect any potential voids or internal defects. The data was collected over a series of swathes running along the ducts at each nominated duct. The swathes were split into two 3m sections and conducted on either side if the bridge centrelines.

KEY FINDINGS

All 17 post-tensioning ducts across both bridges were successfully located and marked on site, with ducts identified at depths of approximately 60–100 mm from the soffit surface. Most sections showed little or no unusual response, indicating no clear evidence of voiding, while a small number of locations showed stronger or weaker signals that could indicate possible voids but may also be influenced by other factors within the structure. The results are indicative rather than definitive and are intended to help guide where, if necessary, more targeted follow-up investigations could be considered.

BENEFITS

The investigation enabled accurate, non-intrusive location of post-tensioning ducts under live infrastructure, reducing the risk of accidental tendon damage during future intrusive works. By combining GPR and ultrasonic testing, TRACE supported targeted identification of localised anomalies indicative of potential voiding, rather than assumption-led intervention, with clear communication of confidence levels and limitations. The works were delivered efficiently during night-time possessions, supporting proportionate and informed decision-making without unnecessary disruption.