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  1. Post-Tensioning execution procedures

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Installation of bearing plates and ducts

The bearing plates are fixed to the formwork structure with boxes, in order to guarantee the correct alignment of active frameworks during start-up, leaving the minimum clearance dimensions needed to insert the hydraulic jack after the formwork removal operations.

Ducts and steel fixings are installed at the same time.

Said operations comply with the specifications described on the plans, supporting these elements on the passive framework every 1 or 2 metres, depending on the complexity of the layout and the unit used.

Socket joints are used to fix ducts. Said joints will be sealed with tape to prevent grout from being poured inside during the concrete pouring operations.

Special parts (vents) are installed on the highest points, which include a plastic tube that allows the discharge of air to the exterior during the injection operations, as required to make sure that there are no air-locks inside the duct.


Strand alignment

This operation is carried out before the concrete pouring operations, in order to prevent the risk of potential dents or cracks on the duct during such operations, which would prevent the execution of the alignment operations.

After assembling the coil on the strand, the reel will be aligned with the alignment unit.

A bullet-shaped metallic piece will be installed on the edge of each wire to avoid any deflection effects and from damaging the duct.

Tecpresa's expert will align the strands used in the project along the length, as required for the subsequent tensioning operations.


Tensioning of the tendon

After the concrete pouring and formwork removal operations, one of Tecpresas experts will install the anchor heads and wedges.

The tensioning process starts when the adequate concrete resistance value has been reached, with the corresponding cracking of test pieces. Said situation will be reported by the Site Management Area to Tecpresas expert.

The tensioning process will be carried out in compliance with the instructions of the tensioning program that has been designed and drafted at the offices of Tecpresa, including the following:

  • Partial tensioning stages
  • Tensioning order in each stage
  • Anchor tension in each tendon

During the tensioning operations, the expert will record the different load steps and elongation that has taken place during the operation on the corresponding report.

The measurement equipment will be adequately calibrated. Likewise, elongations will be measured with a minimum precision of 2% of the total length.


Cutting edges and sealing boxes

After the completion of the tensioning operations and once the elongations have been checked, the corresponding supervisor will give the instructions to cut the tensioning edges with a radial cutting device, 3 cm from the edge of the wedges. The boxes will be then sealed with non-retractable mortar.

The tubes used for the injection will be placed before sealing the boxes, connecting them to the openings on the bearing plates.


Injection

The conduits are washed with water before the injection process, which will eliminate all water-soluble oils during the alignment operations and it will also discharge the remaining water injected with compressed air. In case no oils have been used, the cleaning operations can be carried out with injected air.

In addition, the cleaning operations can be used to detect obstructions and communications between ducts or between ducts and recesses.

The grout injection process requires the use of an injection device with a flow of 3 to 5 m3 per hour, with a double mixer, designed as such to prevent any interruptions during the operations.

The injected grout has the following composition: 100 Kg of cement, TYPE I, 40 to 43 l of water and 450 gr of additives.

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