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High fibre diet

Concrete RailwaysPushing fibre reinforced concrete design to its limit is risky, especially under the pressure of a railway possession, discovers Ruby Kitching.

While most of Britain was tucking into its Christmas turkey, a team of engineers in Clerkenwell, central London, were working round the clock to replace 260m of rail track and slab.

The job to upgrade a 1980s reinforced concrete trackslab with a new crossover alignment was carried out during an 11 day possession on the busy Thameslink commuter line, which runs north-south across the capital. Designed by consultant Mott Macdonald and constructed by Balfour Beatty, it is needed in advance of a six-month blockade, starting in September, for construction of the Channel Tunnel Rail Link (CTRL): on its way from Kent into London's St Pancras station, CTRL will cross Thameslink.

Thameslink trains usually run straight though King's Cross station, but during the blockade they will be forced to turn around either side of the CTRL works.

This will be at St Pancras station to the north, and the Thameslink station in Clerkenwell to the south.

Turning trains back at St Pancras is relatively straightforward, but on the southern side there is a narrow tunnel with a reinforced concrete track bed.

'The Thameslink tunnels are too small by current standards so we were right on the limit for the tunnel envelope, ' says Balfour Beatty project director Paul Holland. To maintain sufficient clearance for trains, the slab thickness would have to be reduced to between 80mm and 150mm thick, taking concrete slab design to its limit.

Only fibre reinforced concrete was able to meet the requirement for a high strength shallow slab, and even then it would be on the boundary of normal application. A massive 50kg of Dramix steel fibre reinforcement was required in every cubic metre of concrete to achieve the required 4.5N/mm 2flexural strength. The slab design was based on Eurocode 2 and designed with 0.75mm diameter and 60mm long fibres to fibre manufacturer Bekaert's guidelines.

Mott tested flexural strength in accordance with the Japanese JSCE SF4 code, which takes railway traffic into account.

Such high concentrations of fibre reinforcement raised serious doubts about workability of the mix, however.

And achieving a 50N/mm 228day compressive strength within the 11 day possession was an additional, major challenge.

Polypropylene fibres were added to the mix to help hold the concrete together and to prevent early age thermal cracking. Admixtures were also used to reduce shrinking since the slab would be placed in one continuous pour.

Extensive tests were carried out on various concrete mixes at Bekaert's headquarters in Belgium and at Greenwich University, while the months preceding the blockade were filled with testing and trialling the concrete mix to ensure it met strength and pumping requirements. Two 70m long trial slabs were poured.

'We were beyond the normal working boundary of fibre reinforcement. There was no one in the country with experience of achieving the strengths we needed with such a thin slab and such big fibres, ' says Mott project manager James Howe.

'Because the work was being carried out in an 11-day blockade, we had to make sure everything would work first time. There was no room for mistakes, ' adds Holland.

There were fears that plummeting winter temperatures could retard concrete cure times.

Accordingly the entire length of track slab was covered, insulated and heated using fans to ensure the temperature did not drop below 10infinityC. This ensured the concrete achieved 6N/mm 2compressive strength within 24 hours, allowing construction traffic to pass over.

On 6 January, a day after the possession came to an end, the slab was ready to take trains packed with commuters.

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