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
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
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."