Citytunneln is a major project where most things are counted in large numbers. All the concrete used in the Citytunneln project weigh about as much as 159,000 fully-grown African elephants.
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MCG Batching plant in Holma |
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Segment factory in Holma |
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The segment depot |
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Casting the vaults at Hyllie station |
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Casting pillars in the rock chamber at Triangeln |
In total it is estimated that 400,000 cubic metres of concrete will be used in the two tunnel tubes, the stations, in bridges, ramps and other constructions. This is more concrete than was used on the Öresund Bridge. One of the Citytunnel project’s contractors, MCG (Malmö Citytunnel Group) has erected two concrete stations and a segment factory at Holma. The project’s other contractors buy concrete from external producers.
40,000 segments
The tunnel tubes are built inside the actual machines at the same speed as the boring machines progress below Malmö. The tubes are made up of tunnel rings. A tunnel ring comprises eight reinforced concrete segments of which one acts as a lock. The segments are 1.8 m wide and each weighs around 6 tonnes. A total of approximately 40,000 segments will be cast and this corresponds to around 100,000 cubic metres of concrete. Around 7,500 tonnes of reinforcement is used in the segments.
Traditional skills
Around 80 tunnel segments a day are manufactured at the special factory in Holma. The prefabricated and pre-assembled reinforcement is positioned in the casting forms before the concrete is poured in. The forms are vibrated and the concrete distributed in the correct way. After three hours, the segments will have cured and the surface is checked manually.
The next stage in the process is hardening in an oven at approximately 45°C. The segments stay here for 7.5 hours. The concreted segments are then stacked in storage while waiting for the tunnel to advance. The final hardening process takes place during this time. The segments need to mature for at least 28 days before they can be used. When the boring machines begin their work, the objective is for 30 per cent of the segments to be cast and be set up at the storage site. A buffer stock is created in this way that means that the production of segments is not dependent on the progress of the boring machines.
High tech
Some of the concrete used at Hyllie station, Malmö C and the cut and cover tunnels at Holma is unique. This is the first time that anybody has systematically developed a formula that produces a concrete that is frost-resistant at the same time as it resists fire or extreme heat.
Extensive testing has been conducted to satisfy both the extremes. Among those who participated in the testing was the Swedish Testing and Research Institute. In the fire test the concrete needs to resist a temperature of 1300°C. Concrete does not burn. But it does contain water and during a fire ore high temperature the water in the concrete is vaporised. Steam takes up a greater space than water and can cause the concrete to break and disintegrate.
A new technology has been developed which uses very small polypropylene fibres which are mixed in the concrete mass to prevent the concrete expanding at extreme heat. The fibres melt at high temperatures and leave space for the vaporized water. This also prevents the concrete from cracking. The fibres are six millimetres long and 18 thousandth of a millimetre in diameter.
By employing this technology, a tunnel is built in the safest way possible today. The technology also means that the tunnel is, in principle, maintenance-free and the concrete structure is expected to remain intact for more than 120 years.
Concrete for stations and bridges
The purpose of the concrete stations is to provide the project with concrete to be used for the constructions that are cast on site. Depending on the properties that are demanded, the concrete is produced in special mixes according to special recipes. Each individual mix is checked in a special laboratory before it progresses to the relevant workplace for use.