Post-tensioning investigation
AQUILA recently completed an advanced non-destructive testing (NDT) investigation of a post-tensioned road bridge in Switzerland, demonstrating how modern inspection technologies can support safer and more efficient asset management.
Built in the 1960s, the bridge near Zurich required an assessment of its post-tensioning (PT) system to support informed maintenance and long-term asset management decisions. AQUILA applied a combination of Ground Penetrating Radar (GPR) and Ultrasonic Pulse Echo (UPE) technologies to investigate the condition of the bridge’s post-tensioning ducts and anchorage zones.
NDT investigations using GPR and UPE
Using advanced subsurface mapping technologies, AQUILA first identified the location of reinforcement and PT ducts before conducting targeted UPE investigations to detect anomalies that could potentially indicate voids or corrosion risks. The data enabled the team to pinpoint specific areas for further verification, minimizing the need for extensive destructive investigations.
Targeted openings at high-risk locations
Selected locations showing the highest probability of defects were subsequently opened and inspected. The findings confirmed that the post-tensioning system was in good condition, including in areas where scan results had suggested elevated risk. This provided confidence in the overall integrity of the structure while demonstrating the value of combining advanced NDT methods with targeted verification.
Recording results for future reference
The project also incorporated AQUILA’s digital twin technology, enabling inspection results, photographs and location-based data to be securely recorded for future reference. The approach supports safer decision-making, reduces unnecessary intervention, minimizes disruption to users and helps extend the service life of critical infrastructure assets.
Hidden risks in post-tensioned bridges
Post-tensioning (PT) tendons are critical load-carrying elements in many concrete bridges but are typically hidden within ducts and inaccessible to visual inspection. Bridge failures worldwide have shown that tendon deterioration can remain undetected for decades. The 2024 collapse of the Carola Bridge in Dresden underscored the need to assess concealed prestressing systems. Advanced non-destructive testing enables owners to detect hidden defects, evaluate tendon condition and plan timely maintenance interventions.
Evolution of modern grouting standards
Proper grouting of tendon ducts is essential to protect prestressing steel from moisture ingress and corrosion. However, comprehensive grouting standards and quality-control procedures were not widely implemented until the mid-1990s. Today, grouting in Europe is governed by EN 445, EN 446 and EN 447, while in North America, corresponding requirements are defined by PTI and AASHTO specifications. These standards have significantly improved grouting quality, durability and corrosion protection of post-tensioning systems.
Why bridges built before 1996 require special attention
Bridges and other prestressed concrete structures constructed before 1996 carry an elevated risk of hidden tendon deterioration due to potentially inadequate grouting practices. Incomplete grout filling, bleed water accumulation, grout segregation and void formation were more common before modern standards became widely adopted. These defects can allow moisture and chlorides to reach the prestressing steel, leading to corrosion that remains concealed for many years. Owners of post-tensioned structures built before 1996 should therefore consider targeted inspections using advanced non-destructive testing approach to identify hidden risks, improve safety and extend service life.
For all enquiries, please contact Gianni Moor:
info@aquilabuilthealth.com
Use of a Proceq GS9000 ground-penetrating radar (GPR) device to map the layout of post-tensioning tendons/ducts and deck reinforcement steel.
A Proceq Pundit PD8050 ultrasonic pulse echo (UPE) device was used to investigate where the presence of air voids in the bridge’s post-tensioning ducts appears to be most likely.
Based on the findings of the UPE investigation, locations were identified where the presence of air voids in the PT ducts appears to be most likely.
The good condition of the PT system was confirmed at openings of the concrete and the PT duct at locations where the UPE testing indicated the highest likelihood of air voids being present.
Grouting defects in post-tensioning systems can result in severe corrosion if water finds its way into the ducts.
Major international standards governing the grouting of post-tensioning systems have only appeared in recent decades – so older structures are at greater risk of poor grouting.
The collapse of the Carola Bridge in Dresden in 2024 highlighted the importance of understanding the condition of hidden prestressing systems.
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