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Graphic element Research > Growth > Research projects > Previous projects > Industrial Processes > Keeping your marbles
Graphic element Keeping your marbles
     
 
Cladding buildings with marble sheets enhances their appearance enormously. Unfortunately, these sheets can deform due to internal stresses brought on by temperature changes, causing the sheets to wrench themselves from the underlying surface. Partners in a CRAFT project have developed a new instrument - a type of dilatometer - that permits quarriers to extract marble slabs in such a way that the crystal structure allows stonemasons to produce sheets that do not suffer from this problem.
 

Marble has been valued by architects and builders since ancient times. Cladding a building with thin sheets of marble can enhance its aesthetic value at relatively low cost. There is just one problem: it occasionally falls off. And when it does, the expense of repairing a structure - not to mention the threat to passers-by - is considerable. Spectacular examples of marble cladding falling to earth include the Amoco tower in Chicago, the Grande Arche de la Défense in Paris and, more recently, Helsinki's Finlandia Hall.


Blame it on the weather

Like most solid materials, marble expands during periods of high temperature, and contracts when the mercury falls. The marble itself is anisotropic, meaning that the expansion and contraction characteristics vary depending on the relative direction of the material's crystal orientation. In simple terms, like wood, marble has a 'grain'. It is this differential coefficient of thermal expansion which leads to panels becoming warped, and in severe cases, a build-up of stress in the structure, which can eventually rip the panels right off the wall.
SME stonemasons Bamberger, near Vienna, were concerned about the problem and had strong contacts with the Vienna Technical University, which had carried out considerable work in this area. Together, they decided to form a consortium to look into it further. They eventually found SME partners in Austria, Germany and Italy, and successfully applied for CRAFT funding.
"This was perhaps the hardest part of the project," says Franz Bamberger, the company's head. "Italy is the world's leading source of marble slabs. They have been quarrying it for hundreds of years, and producers did not see the point of embarking on a project that could end up changing things." But, reluctant or not, Mr Bamberger finally convinced quarry owners to come on board.

A suitable case for new equipment

The technical effort was spearheaded by the university, which embarked on a series of attempts to analyse the polarity of marble's thermal expansion coefficient. They soon ran into a problem: no suitable equipment existed. The dimensional changes are too small to be observed with the naked eye. A special monitoring instrument known as a dilatometer was needed to analyse the microscopic dimensional changes during periods of temperature change - so project partners designed and made one.
As Mr Bamberger points out, this was no easy task. "If you think of a human hair, it has a diameter of around 100 micrometres," he explains. "We were trying to measure deformations of ten micrometres or less. The new equipment needed to be sophisticated and accurate, but it also had to be rugged." But measure it they did, with very interesting results.
With their new instrument, the partners were able to demonstrate that the crystal orientation within a sheet of marble is much more important than was previously thought. Indeed, it is crucial. Quite simply, if the microstructure is correctly aligned, marble cladding will stay put. If not, it is likely to deform and detach from the structure.
The dilatometry measurements should be carried out at the quarry, Mr Bamberger stresses. "As stonemasons, if we buy a rectangular block of marble that is to be cut into sheets, the shape of the block predetermines how we can cut it," he explains. "If the alignment is wrong when we receive it, there is nothing we can do. The new instrument allows quarriers to determine the crystal alignment before the stone is extracted, and to cut slabs accordingly."

Less is more

The payback from the project could be significant. "Take the Finlandia Hall as an example," says Mr Bamberger. "After they had problems with the cladding, they removed it all and replaced it with five-centimetre-thick panels. If they had been able to use the new dilatometer, they would have been able to use half that thickness."
Repairing buildings, though, is only one of the benefits brought by this project. Because marble is now a much better understood building material, its use could be broadened, while preserving precious, non-replaceable stocks. Correctly aligned marble cladding can be applied in much thinner layers than is necessary when using randomly-orientated material. "It's also a question of cost," says Mr Bamberger. "Marble is not a cheap building material. If it can be used in much thinner layers, architects will be able to specify it more widely and still remain within budget."

Cordis RCN: 29277
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