Post-tensioning Special Inspection (PTSI) – Pinnels Farm Bridge, Wiltshire

Freyssinet was engaged to carry out:

• Duct and tendon exposures DTEs and end anchorages exposures EAEs. A localised area of concrete was carefully removed to expose the external sheath of the tendon duct or the end anchorage assembly. The technique involved controlled breaking out of the concrete using hand tools to prevent damage to the duct or tendons.
• GPR and tomography. Ground penetrating radar was used to locate the tendons.
• Concrete testing which including sampling of concrete and grout for chloride, cement and sulphate content, carbonation depth tests,
  half-cell potential and resistivity and cover survey.

In total, there was 35 DTEs, 5 EAEs and 18 concrete test areas.

The works were delivered in two phases. The first phase was carried out during night shifts, with test areas located on the external sections of the bridge. Traffic management was implemented on the M4, and mobile access platforms were used to facilitate inspections and testing. This phase was completed in approximately two and a half weeks. In mid-December, the works progressed to the bridge deck and operations transitioned to daytime working.

Delivery

The works were delivered in two phases. The first phase was carried out during night shifts, with test areas located on the external sections of the bridge. Traffic management was implemented on the M4, and mobile access platforms were used to facilitate inspections and testing. This phase was completed in approximately two and a half weeks. In mid-December, the works progressed to the bridge deck and operations transitioned to daytime working. Concrete exposures were undertaken using carefully controlled breakout methods with handheld electric tools. For post-tensioned structure investigations (PTSI), hydro-demolition is not permitted.

While coring can sometimes be used, the dense reinforcement within the structure made this impractical. All works undertaken in the vicinity of tendons and anchorages required exceptional care, as damage to a single strand could have serious structural consequences. Throughout the project, the works were closely supervised by National Highways.

Collaboration

The project was delivered in close collaboration with National Highways, whose representative remained on site throughout the works to witness all DTE and EAE activities.

A positive working relationship was also developed with the local landowner. Trust was established through regular communication and a collaborative approach. This included temporarily pausing operations when required to allow his cattle to cross the bridge and access grazing land on the opposite side of the M4, helping to minimise disruption to his farming activities.

Overcoming challenges

One of the primary challenges was the exceptionally dense reinforcement surrounding the tendon ducts. As a result, coring was not a viable option due to the risk of damaging the reinforcement, ducts or tendons.

The dense reinforcement also significantly reduced the effectiveness of scanning techniques. Ferroscan surveys produced unclear results, with substantial signal interference limiting the reliability of the data obtained.

To overcome these challenges, all exposures were undertaken using handheld electric breakers, applying a highly controlled and cautious approach. Given the proximity to critical post-tensioning elements, the works required exceptional care and precision throughout.

To further mitigate risk, low-vibration tools were specifically selected for the contract, supported by strict control measures including regular breaks and operative rotation to minimise vibration exposure.

Ways of working

• All inspections were completed in full compliance with National Highways standards and Post-Tensioned Structure Investigation
  (PTSI) guidance.
• Detailed inspection plans, methodologies and hold points were agreed in advance with National Highways to ensure safe and controlled delivery.
• All physical exposures and non-destructive testing (NDT) results were independently verified and witnessed by the client’s representative (DTE/EAE).
• Robust controls were implemented when working in close proximity to live post-tensioned tendons, with any unexpected findings escalated immediately for review.
• Investigations were undertaken by competent, specially trained PTSI operatives, supported by regular toolbox talks covering tendon strike risk, working at height and night working.
• Low-vibration equipment was used wherever practicable, with operative rotation and planned breaks introduced to minimise vibration exposure.
• National Highways maintained a continuous on-site presence throughout the works, providing ongoing quality and safety oversight.
• Close collaboration with the local landowner ensured that farming operations could continue with minimal disruption, including accommodating livestock movements when required.