British Sugar engaged Freyssinet to undertake strengthening works on Silo 3 and Silo 4 at the Bardney British Sugar Factory.

Constructed in 1960, the silos comprise cast in‑situ reinforced concrete walls incorporating an internal bonded post‑tensioned system. Condition surveys identified widespread areas of concrete spalling and defective tendons. Investigations attributed the deterioration of the tendons to insufficient duct cover, measured at approximately 25 mm.

  • Client
    British Sugar
  • Scope
    Post-tensioning and concrete repairs
  • Principal Designer
    Atkins
  • Designer
    Freyssinet
  • Works completed
    July 2024

The scope

The limited concrete cover allowed moisture to penetrate the galvanised ducts, leading to progressive corrosion. As the ducts corroded and expanded, they induced cracking and spalling of the surrounding concrete. This further exposure accelerated moisture ingress, resulting in continued tendon degradation. In several areas, corrosion of the strands led to a loss of prestressing force within the silo walls.

Freyssinet proposed a solution involving the installation of an external posttensioning system applied to the affected areas. The system comprised of unbonded 15.7mm strands placed within grouted HDPE ducts. Each tendon consisted of two half-loop tendons connected with X-anchors installed on opposite faces of the silo wall. To restore the pre-stress, force each tendon was stressed to 209KN which was equivalent of 75% UTS (279kN).

Silo 3 required 2 external tendons, while Silo 4 required 30 new tendons installed at heights ranging from 20.2m up to 35.0m above ground level. The works commenced in March 2024 and were successfully completed by the end of July.

Challenges and solutions

During planning, Freyssinet identified that gaining access to Silo 4 would be particularly challenging. Mast climbers were selected to provide safe and stable working platforms, with the units fixed directly to the silo wall. However, the presence of surrounding ventilation pipework and adjacent outbuildings created congestion that complicated installation.

To overcome these constraints, Freyssinet designed and installed a bespoke scaffold over the obstructed areas, enabling the mast climbers to be safely erected on the scaffold. This temporary structure provided access to approximately 60% of the silo’s perimeter.

In addition, a full‑height scaffold was installed at the rear pilaster to accommodate ventilation pipes running from the roof to ground level, ensuring safe and uninterrupted access throughout the works.

Another major challenge was the extent of the coring works required. The scope included coring 240 holes, each 120 mm in diameter and up to 800 mm deep, through the curved pilasters to accommodate the installation of new tendons.

To create a single continuous duct, operatives had to core from both sides of each pilaster at precise, pre‑defined angles. This level of accuracy demanded meticulous setting‑out and execution. Freyssinet therefore appointed a specialist coring contractor to deliver the works safely and to the required tolerances.