More than 100 years after its construction, the 230-meter long viaduct supporting the only railway in eastern Madagascar, experienced foundations settlement, arches weakening, concrete and masonry defects and a corrosion issue. With no possible track closure, the masonry structure strengthening was the best option.

  • Name of the owner
    Ministry of Public Works
  • Name of the client
    ARM (Madagascan Highways Agency)
  • Delivery date of the project
    June 2020
  • Partners of the project
    Severally liable consortium (FIC DGP / SOGEA Satom, Madagascan branch - lead organisation)
Sahasinaka viaduct - Reinforced shotcrete counter-arches - Masonry structure strengthening

Masonry structure strengthening – Reinforced shotcrete counter-arches

In 2016, Freyssinet’s partner, Sixense, was contracted to inspect and examine the combined concrete and masonry structure of the Sahasinaka viaduct. During its investigations, Sixense detected signs of erosion around the foundations for the central pier (the viaduct’s highest pier at 39 metres high), causing subsidence. Freyssinet’s teams were urgently appointed to inject cement grout in a bid to consolidate the foundations, which helped stabilize the structure and prevent any further settlement. But the concrete arches on either side of the damaged pier also needed reinforcing, since the differential movement had fractured the concrete. A second contract was therefore awarded to strengthen the arches, and work began in May 2019. The main aim was to structurally reinforce the underside of the two arches by creating reinforced shotcrete counter-arches supported by concrete beams (corbels) fixed to the piers with Freyssibar prestressing bars.

Masonry structure strengthening – Flying scaffolding from the top of the piers

There were initially plans to access the intrados of the arches by erecting scaffolding from the bottom of the valley. Since there were no paths leading to the viaduct’s pier bases (in the middle of the countryside), the logistics required to bring in the equipment would have proven too complicated and expensive. Freyssinet suggested fitting the scaffolding to brackets lashed directly to the viaduct’s piers at a height of 25 metres. Once the brackets were mounted, metal beams were fitted between the piers to create a platform to support the scaffolding. A secure stairway tower gave technicians access from the viaduct’s deck.

Flying scaffolding to access the viaduct's intrados - Masonry structure strengthening
Key figures

Strengthening of the access viaducts

Work also focused on strengthening the reinforced concrete access viaducts, spanning a total length of 69 metres. The repair programme included injecting epoxy resin into the cracks, rebuilding the damaged sections with shotcrete, applying a corrosion inhibitor to protect the existing reinforcements and applying a thin layer of modified hydraulic binder (MHB) to prevent the ingress of external corrosives. Finally, the structure’s 9,000 m² of facing panels were power-washed with a helping hand from rope access technicians.

  • 550
    Surface area of the structural concrete treated and repaired (m²)
  • 300
    Surface area of the 10 cm-thick counter-arches installed (m²)
  • 9,000
    Surface area of the facing panels power-washed (m²)

We stood out from the competition by advising the client to hang the scaffolding from the viaduct’s piers, which considerably reduced equipment supply costs and improved safety for workers… and the client was absolutely delighted!

Pierre Quinot -  
Project manager, Freyssinet

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