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A prize that's cement for all

Concrete Restoration

This week a milestone in the history of concrete construction will fight for its future on peak time TV. Dave Parker looks at the Concrete Castle.

BBC TV's Restoration programme has been an unlikely hit. Its simple format and viewer involvement attracted large audiences to the first series. Crumbling historic buildings in urgent need of funds for major restoration work competed for viewers' telephone votes, each 50p call contributed 34p to the pot, and the winner scooped the lot.

In 2003 Manchester's Victoria Baths came out top and muchneeded work is to begin next year.

The latest series started on 13 July, with 21 decaying structures lined up for the start. In the south west heat, next Sunday 25 July at 9pm, one of the contenders is a building that marks an influential moment in the history of construction technology - Bridgewater's Castle House.

Originally known as Portland House, the Grade II* listed 'Concrete Castle' was built in 1851 and needs around £500,000 to rescue it from demolition. NCE first reported its plight nearly 18 years ago (NCE 30 October 1986).

Since then further deterioration has taken its toll.

'There have been a couple of arson attacks, most of the roof has gone and the centre section is now completely burnt out, ' explains Adam Wilkinson, secretary of conservation charity the SAVE Trust. 'After a period of dereliction it was bought by a property developer who had no real concept of the building's value or how much it would cost to restore it.

'It was acquired by the local authority, who passed it on to us.

We need the money to reroof, install services and convert it into three housing association flats.'

What makes Castle House unique is its large scale use of what was then a very new technology: precast and structural reinforced concrete (see box).

The two storey building's distinctive precast faþade, with its statues and mouldings, conceals and embellishes conventional brickwork on the ground floor and solid concrete blocks above.

Originally designed as a private home, it soon became a showroom for the products of local cement maker Board & Co.

'Given how primitive the cement used was, most of the precast facade is in remarkably good condition, ' reports concrete conservator Simon Swann. 'The projecting elements have suffered most, mainly from expansion of sulphates in the aggregate.'

Precasting was taken even further. Window frames, architraves, staircases - including the handrails - even the skirting boards were formed this way. But when structural engineer Kenneth Brown & Partners carried out the first detailed survey in 1986 it was the floors that caused all the excitement.

High technology buildings of the time featured 'fireproof' construction, with the floors usually formed from cementitious screeds supported by brick jack arches spanning between the lower flanges of cast iron beams. These were hardly practical for domestic applications, but a wood free flooring system was seen as desirable, given the still universal use of open fires, oil lamps and candles. Castle House's builders evolved their own distinctive solution.

'Marc Brunel had already discovered that mortar made with Roman cement had significant tensile and bond strengths, ' explains Swann. 'On the Thames Tunnel project he had developed a test for cement quality that involved building a brick cantilever.

A 30 brick cantilever was possible if each joint was allowed to set for an hour before the next brick was added.'

At Castle House the builders used 'hoop iron' - the basis of barrel hoops - to reinforce some of the larger cladding units. For the floors, however, they combined brick, Roman cement and iron in a very innovative form.

In the latest structural investigation report from the Brian Morton Partnership the floor design is described as a brick diaphragm supported by pairs of 4.5m long reinforced concrete beams. The beam designs show that the builders were aware of the need to hold the reinforcement firmly in place during concreting.

A 25mm diameter iron bar forms the lowest reinforcing element.

This supports clay tiles, which in turn carry parallel vertical steel rectangular bars (see diagram).

Clay saddles sit on the rectangular bars, their tops level with the soffit of the floor. Brian Morton believes the concrete or mortar would have been poured into the beam formwork up to the level of the saddles, then a brick floor laid with tight joints on falsework would have been constructed right over the beams. Finally the concrete screed would be laid and allowed to harden before the falsework was dismantled.

Iron shoes link the ends of the reinforcement together and tie the beams into the wall.

Diaphragm action in the floors could be further enhanced if, as suspected, tensile reinforcement is built into the supporting walls.

The Morton report makes no bones about describing these floors at Castle House as 'unique'.

Much of the rest of the building could be similarly described.

The SAVE Trust hopes that civil engineers will show their appreciation of such a landmark in the history of their profession by voting early and often on Sunday.

INFOPLUS More information can be found on: www. concretecastle. com.

Roman road to success

In the early 19th century there was a race on to be the first to re-invent 'Roman cement'.

With the industrial revolution gathering pace, and the demand for complex multistorey mills and industrial facilities booming, something stronger and easier to use than the lime mortars and plasters that had been the norm for centuries was needed.

These gained strength by a slow reaction with atmospheric carbon dioxide. The fabled 'hydraulic' Roman cement was known to harden even under water, and its strength gain was reputed to be much quicker. Nowadays we know the secret: a blend of slaked lime, brick dust and a fine volcanic ash rich in reactive amorphous silica. At the start of the Industrial Revolution, however, the secret had been lost for more than a millennium.

After various blind alleys, Joseph Aspin patented what he dubbed Portland Cement in 1824. The name gives away his driving priority - not to invent a new structural material but to produce an artificial Portland stone, the luxury cladding material of the time. Aspin used a mixture of raw materials, usually limestone and clay, which produced a hydraulic cement with much the same basic chemical composition as the Roman blend.

In some places there existed natural deposits of rocks or clays that also contained calcium, aluminium, silicon and iron oxide in the right proportions. These could be simply dug out of the ground and calcined in kilns similar to those used for ceramic production. If the raw material had a high organic content, so much the better: kiln temperatures would be higher and fuel consumption less.

Ground to a fine powder and mixed with local aggregates, the 'natural cement' could produce a reasonably convincing artificial stone.

One such source was at Dunball, near Bridgewater in Somerset, where Board & Co started production of what was sold in the 1840s as Roman cement. And one of the sole surviving examples of the early uses of artificial stone cladding can be found in right in the centre of Bridgewater itself.

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