As part of the extension projects of the Nîmes Ouest Wastewater Treatment Plant (WWTP), the Freyssinet France teams were asked  to propose a variant in prestressed reinforced concrete during the negotiation phase of the call for tenders for the works contract.

Objective: to optimise the sizing of the structure of a tall digester. They then provided support to the incumbent’s teams for the installation and tensioning of the prestressing tendons.

  • Owner
    Nîmes Métropole
  • Client
    RIVASI
  • Delivery date
    2020

Using prestressing to ensure sustainable waterproofing and reduce the carbon footprint of the structure

Due to the high hydraulic thrusts to which they are usually subjected, reinforced concrete tanks are particularly sensitive to micro-cracking, a phenomenon that is detrimental to their waterproofing.

This is why RIVASI asked our design office, which specialises in prestressing and water engineering, to optimise the sizing of a reinforced concrete digester structure with a height and diameter of twenty metres. The objective: to define an economically advantageous technical variant.

Our teams proposed to encircle the structure with 40 single strand F15 cables. Indeed, the use of prestressing by post-tensioning devices on tanks improves the behaviour of the concrete with respect to the taking up of tensile stresses and reduces deformations within the structure. Our offer also enabled to reduce the carbon footprint of the construction of the structure while ensuring its sustainability.

As the cables cannot be tensioned until after the concrete has set, they are laid in recesses made up of sheaths that will be embedded in the cast wall when the tank is built.

  • 20m
    Height and diameter of the tank (metres)
  • 40
    Number of rows of prestressing tendons installed
  • 20%
    Volume of concrete and tonnage of steel saved through the use of prestressed tendons
Nîmes tank - France / prestressing

Seamless coordination with the concreting teams

For our customer, the Rivasi company, this was the first project with sliding formwork and prestressing.

The technique of constructing the digester using a continuously rising sliding formwork required the mobilisation of shift work (3x8h) over a period of seven days to ensure the continuity of the concreting operations.

The Freyssinet teams worked in parallel with the reinforcing steel erector and concrete builder teams, requiring seamless coordination throughout the operations. This concurrent work implied a very fast pace of laying and a rigorous self-inspection of the operations carried out (installation of rows of cables, bands and anchor boxes) before being covered by the next concrete lift.

Final tensioning of prestressing tendons

Our teams returned after 28 days of setting for the final tensioning of cables. In order to avoid any risk of cracking or concrete spalling during this sensitive operation, a precise phasing was defined. The cables were tensioned progressively from the base to the top of the structure. The anchor boxes were arranged in three vertical rows oriented at 120° around the perimeter of the structure. The various rows were tensioned by successive intervals in three phases in order to ensure stress distribution and homogeneous compression of the structure. After this last step, the recesses for anchorage were filled in with shotcrete and a wooden cladding was applied to cover the structure.

Nîmes tank - France-post-tensioning

Ultimately, the addition of prestressing devices for the construction of the digester structure enabled an improvement in the carbon footprint of the operation and substantial savings owing to the reduction of concrete volumes and steel tonnages by around 20% compared to the initially defined solution.

Sami  
Water engineering sales manager - Freyssinet France

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