Seeing inside a 1950s post-tensioned motorway bridge without breaking concrete
Project background
GScan used muon tomography to see inside a 1950s post-tensioned motorway bridge in England, revealing its as-built steel and tendon condition without breaking any concrete.
Post-tensioned bridges from this era carry a well-known risk. If the ducts around the tendons were never fully grouted, the steel inside can corrode or snap unseen. This bridge had a history of exactly that problem.
The owner needed answers to two questions. What steel is really inside the beams, and does it match the drawings? And what condition are the tendons in today? Answering them the traditional way means opening up the concrete, one location at a time, on a live motorway.
Why muon tomography
Muon tomography uses naturally occurring cosmic-ray muons to image the inside of dense structures. It needs no drilling, no coring and no radiation source. That made it a fit for this bridge for three reasons:
• Non-destructive. The owner could see the steel and ducts without cutting into a structure already suspected of hidden damage.
• No disruption. Detectors sit outside the beam, so the motorway stayed open for the whole measurement.
• Full volume, not spot checks. One scan covers a whole length of beam in 3D, rather than the few points an intrusive survey can open.
Deployment
All four detectors were installed in a single night, then measured for 18 days while motorway traffic kept running.
A 4.3 m length of one beam was scanned using two pairs of XL detectors. One pair went on the hard shoulder above the beam and the other on a scaffold table beneath it.
The setup was plug-and-play. A battery pack powered the detectors, and data streamed live to GScan's platform over 4G/5G. After 18 days, the units were lifted out in reverse order.
Install | Measurement | Muon tracks recorded | Reconstruction resolution |
1 night | 18 days under live traffic | 203 million | 3 mm voxels |
Measurement and key findings
The scan produced a full 3D model of the beam's reinforcement and post-tensioning, and found things the drawings did not show.
Every bar and tendon, in 3D
Machine learning-based object detection turned 203 million muon tracks into a 3D model of every bar and tendon. After calibration, bar diameters were measured to within ±1.7 mm. Tendon spacing in the top flange matched the original drawings to within 3 mm. The model exports to IFC, IGES and OBJ, so engineers can load it straight into their own software.
What the drawings didn't show
• Fewer wires per tendon. The drawings specify 12 wires per tendon. The scan suggests each tendon may hold only 9. If confirmed, the beam has 25% less prestressing steel than assumed.
• Later works, now on record. The scan revealed dowel bars added in the 1990s, and a tie bar that had been cut during later works.
Defects located, not guessed
The scan identified 4 high-confidence wire break indications and 3 full-length grout voids. Each was confirmed by two independent detection methods, and each is pinned to a precise location in the beam.
That gives the owner a short list of exact places to investigate, instead of opening up the beam at random.
Results are from a 2026 validation trial on an in-service bridge in England. Defect indications are pending confirmation by intrusive investigation.
Is muon tomography a good choice for your next infrastructure assessment project? Take this short quiz to find out...
