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Slipform survivor

Reports of Bierrum's death have been greatly exaggerated.Christina Taylor finds the UK slipform specialist alive and well, and working in Spain.

It is over a year since concrete specialist Bierrum was forced to prematurely abandon work on Portsmouth's Spinnaker Tower. The firm went into administration just as work on the slipformed A-frame was finishing (NCE 4 December 2003). Although the tower was finished using freelance Bierrum expertise, fears were that one of the UK's most established slipform specialists had been lost forever.

But 13 months on, the reincarnated, streamlined Bierrum International looks to be in rude health working on a 130m high slipformed chimney in the Port of Cartagena, south east Spain.

Bierrum beat Spanish contractors to win the £3.5M design and build contract for a 17m diameter 'windshield', containing three steel flues. The project is part of a £300M three turbine, 1,200MW combined cycle gas fired power plant for Spanish client Gas Natural. Main contractor is Alstom, with consultant Jacobs Babtie providing design services.

The firm clinched the job not on cost but 'for the innovative design', says Alstom project manager, Mike Smeaton. 'And we wanted a specialist.'

Considerable ground improvement has been needed as the site lies on an alluvial plain near a river, with 5m to 8m of soft soil overlying competent ground.

The local arm of UK piling contractor Keller, Keller Terra, was brought in to install 14,000 vibro floatation columns to improve drainage and consolidate the soil.

And to lift the facility out of the reach of floods, ground level has been raised by 2m using 200,000m 3of granular material from a local quarry.

The ground improvements have also had the effect of minimising potential seismic loading which has to comply with recent revisions to the Spanish code .

'The windshield and liners were considered as a coupled system for the purposes of assessing the loading.

As wind load isn't that high, the moderate earthquake load determined the design, ' says Bierrum chief engineer, Charles Goddard.

Foundations for the windshield itself consist of two rings of precast driven piles outside its footprint, collected together by a reinforced concrete pile cap.

Slipforming is now the standard method for constructing tall reinforced concrete chimneys.

At Cartagena the continuous process has been performed by two teams working 12 hour shifts, each made up of five steel fixers, a seven strong concrete gang, three concrete finishers, one joiner and a hoist rider. Bierrum hired its rig, comprising a circular steel formwork with two platforms supported on 32mm jack hoists, from Austrian firm Gleitbau.

The constant demand for ready mixed concrete is met in batches of 0.67m 3, via a Bierrum designed Alimak concrete hoist skip, which charges a receiving hopper on the rig's upper platform.

The 1,150mm deep shuttering has been raised in 25mm increments, with a manual 'rodding test' before each lift. To do this, a rebar marked with the shutter depth is pushed into the pour until it hits cured concrete.

On the lower platform, three 'finishers' brush the curing concrete to smooth out any blow holes and to improve the exterior finish.

Works were complicated by the three equidistant 11.85m high by 7.86m duct entry points near the chimney's base.

The larger north side entry point is open to the ground to allow the steel flues to be fed into the chimney in 8m lengths and strand jacked to full height.

Structural strength around the openings was achieved by increasing wall thickness to 850mm for the first 30m of the chimney, decreasing to 300mm (see diagram) up to the top, and putting in extra steel reinforcement.

'All the steel reinforcement which would have been in the opening areas was displaced to one side, ' explains Nodder.

Concrete mix was designed to aid flowability and compaction in the densely reinforced base. Initially a 250ml/m 3retarding agent with a six to eight hour cure time was added to the standard C35/45 concrete mix. The retardant was gradually reduced and changed for a super plasticiser with a setting time of three to four hours, increasing the climb rate to an average of 2m per 12 hour shift towards the top.

'At 20m we had the concrete running at a slump of 90mm to keep the workability all the way to the top of the rig, ' explains Bierrum operations manager Colin Nodder.

Boxouts were used to shape and support the openings. But in the larger north facing opening temporary concrete columns were used to provide support during slipforming and support the jack rods.

'The temporary columns made it hard to control the verticality - so we tended to go slower, about 3m in 24 hours lower down, ' explains Goulden. 'But at a height of about 65m we completed just under 6m in 24 hours' he adds.

The chimney was topped out early this month and the plant is expected to be operational by March 2006.

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