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Concrete conservation

Deep in the bowels of Swedish construction giant NCC's Stockholm HQ a dragon is roaring. More than 500KW of diesel power is forcing 200l/min of water through a hardened steel nozzle only 3mm in diameter.

Water pressure tops 1,200bar, the jet emerges at supersonic speed, smashes into the chloride ridden 180mm thick concrete floor slab of the basement multi-storey car park - and blasts right through.

Better known as a Conjet 362 hydrodemolition robot, the dragon is breaking out deteriorated patches at a rate of around 0.5m3 /hour.

All it leaves behind is the steel reinforcement; clean, shiny and ready to be reused.

All that needs to be done now is for a soffit shutter to be erected and new concrete poured into the treated area.

'Our machines can remove up to 500mm of concrete in one pass, but usually some of the slab is left in place to act as a permanent shutter, ' explains Conjet marketing and sales manager András Sárkány.

'But with no access problems here it makes sense to go right through.

This option leaves no debris behind to remove later.' This particular Conjet robot can remove a 2m wide strip of floor in one pass.

Mats Holm, hydrodemolition supervisor at specialist contractor ISS, says there is an estimated 50m 3 to be removed over a six month period from a large number of individual areas of deteriorating concrete.

Water that has done its job is collected and passed through settling tanks before being discharged into the sewage system. Sárkány says that in some situations the high alkalinity the water acquires from the concrete has to be neutralised with acid before it can be discharged.

And what about the noise levels, and the comfort of the office staff still working above- 'It's a lot quieter than jackhammers, and there's no vibration, ' Sárkány retorts. 'The pumps are in soundproofed containers outside the building. The only problem for the people working here is they lose a lot of parking space while the work is in progress.'

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