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Compact facts

Bridges

Self compacting concrete has been the key to the successful installation of the new continuous deviator with its ducts and anchorages, according to Graham contracts director Leo Martin.

Working from platforms under the cantilever suspended from the mobile gantry, Core Cut installed anchorage tie bars into the existing box girder, making sure to miss the dense reinforcement and tendons.

Hydrodemolition was used to form shear keys, then the deviators and anchorages could be poured.

Lifting was due to start in late August. Column NF13 is the joker in the pack, according to Mabey Support Systems senior project engineer Peter Ellarby. This is founded in the narrow gap between the two elevated carriageways of the M8.

'On the other four columns we can clamp on four support systems each and install four vertical and two horizontal jacks.

On NF13 there's only room for two support frames, which means we have to mount the vertical jacks in pairs, ' says Ellarby.

This effectively means this column is topped by two 600t jacks, compared to four 300t jacks on one other column and four 200 tonners on the rest.

Four jacks allow for more fl exibility than two. The horizontal jacks are all 50t capacity, mounted at right angles to each other.

All this power is deployed initially just to tilt the deck back, up and away from the column heads. Then, the loads in the jacks will be balanced, and the entire deck lifted 100mm straight up into the air.

By this time, VSL Systems (UK) will have installed the preformed bundles of HDPE coated strands that make up the prestressing tendons, two along the entire length of the curved section, two more running alongside across the two central spans.

'Plastic oated strands are used to aid replaceability, ' says VSL general manager Steve Cardwell. 'We also use plastic spacers in the tendons to keep the strands 5mm apart, to avoid bunching around the deviators and making full grout penetration easier.' After installation, strands are jacked to 10% of ultimate tensile stress to straighten them, before they are grouted. The final stage is a complex, carefully planned 22 phase sequence of stressing, during which all changes in the deck's alignment will be carefully monitored.

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