David West is Technical Director at Inhabit, part of Egis Group. He spends a good deal of his time examining the condition, performance and future life of façades on existing buildings.
For facade specialists like David, stone is highly prized for its performance and character. The right choice of stone can anchor a building in its local context, or make a striking statement. David’s appreciation goes beyond his own specialism: he believes it is ripe for a return as a structural material, decades after it was supplanted by concrete and steel.
Welcome to Engineering Matters. I’m Rhian Owen, and I’m Alex Conacher. In this episode, part of a mini-series produced in partnership with Egis, we will be looking at the benefits of giving new life to stone.
From Stonehenge and the Pyramids, to the temples of Angkor Wat and Europe’s great cathedrals, stone has been the material of choice for humanity’s greatest builders.
But it became almost obsolete in the 19th century. Its natural properties had become a constraint as building designs got taller and new, cheaper materials could help achieve modern architectural ambitions.
DW : Historically, stone masonry walls were load bearing and they provided both structure and enclosure. But as buildings started to get taller in the last few decades of the 19th century, the thickness of masonry required to carry the loads became excessive. And so engineers developed iron and steel structures initially and then later reinforced concrete structural frames as well.
DW : Stone was initially kept as external walling, primarily because it was seen as a very durable material and there was a substantial industry available to supply it.
DW : But then after the Second World War, we got aluminium.
DW : And there was a glut of aluminium after the war. So industry was motivated to find a market for it.
DW : And we got aluminium curtain walls on buildings. And all of a sudden, we could build the enclosure on buildings, the facade on buildings so much more cheaply.
DW : It was also much lighter and that, of course, had structural benefits as well.
Some architects refused to give up on stone. With the application of tensile capacity, it could offer huge potential. But how could that be achieved?
Ironically, an answer was suggested in one of the most celebrated pieces of concrete architecture
DW : So obviously there's fantastic stories around how the roof shells to the Sydney Opera House were developed and the geometry and all of the rest of those stories.
DW : But there's also a fascinating story that's just kind of been rediscovered discovered in the last few years by some researchers at Sydney University around the input of the main contractor Hornibrook to the development of the pre-casting process—you've got these very complicated shapes that changed along the length of roof ribs—and about how they were put into place and ultimately post-tensioned to achieve the required capacity of the roof rib, those curved elements that repeat along the roof shells and support the roof
DW : One of the advantages with post-tensioning is that it can be done on individual elements like a beam or a slab, or it can be used to pull together a number of precast elements and to therefore create a post-tensioned member out of multiple precast components.
DW : Fascinating, amazing, story but really pioneering the potential of combining precast concrete and post-tensioning within this really complex and you know groundbreaking structure. And for me that really creates this I guess demonstration that if we can do it there, we can do it with all manner of things: we can do it with long bridges, we can do it with curved bridges, we can do it with thin slabs, and all of a sudden the opportunities for using post-tensioning are kind of liberated because we're doing it with something so complicated
Hornibrook’s work on the Sydney Opera House proved that complex shapes could be post-tensioned, the steel cables giving the concrete tensile strength. If this can be done with concrete, why couldn’t it also be done with stone?
It already is.
Proof of post-tensioned stone’s versatility and utility has been amply demonstrated in projects built over the past three decades..
DW : Probably the most eye-catching example of those would be the Pavilion of the Future at the Seville Expo in 1992, designed by Peter Rice of Ove Arup partners.
Notably, Rice had worked on the Sydney Opera House, before applying similar techniques to stone in Seville.
DW : This is blocks of granite that are post-tensioned together to form column elements, and the column elements themselves are then subsequently also tensioned up by a steel—I'm going to call it a superstructure, but let's think about it as a bracing system.
DW : So not only was there significant work done to develop that technology at the time, we've got the validation of over 30 years of performance telling us that it's feasible.
DW : And at a more, I'm going to say esoteric scale, a whole series of post-tensioned internal staircases out of solid stone that have been designed or engineered by Steve Webb of Webb Yates and fabricated by the Stone Masonry Company in both the UK and the US.
The Sagrada Familia in Barcelona, too, demonstrates the feasibility of the material’s longevity and wider use
DW : And there can be very few people who aren't aware that the Sagrada Familia is one complicated piece of architecture with some amazing curvilinear shapes.
DW : And so the complexity of engineering design that must have gone into developing those units, to me, suggests that we've got a similar sort of catalyst now to see an expansion and spread of post-tensioned stone structural members into potentially much simpler applications, but quite possibly on a much larger scale than we might ever have conceived possible.
A key benefit of stone is its superior sustainability performance over concrete, which scores poorly on many counts.
DW : What's much more problematic for the production of concrete and reduction of carbon impacts is the actual manufacture of cement, which currently requires heating the raw materials to around 1500 degrees Celsius in the kiln, which requires the use of very large quantities of fossil fuel.
And even with net zero approaches to firing kilns, the chemical process of cementation naturally generates carbon dioxide.
DW : So far, nobody's come up with an alternative manufacturing process.
DW : Stone, on the other hand, used as a fabricated unit, has the potential to be produced with either very low or zero carbon emissions.
And this can be done with less steel as well
DW : What we don't need to put into stone is the reinforcement that we put into concrete when we create reinforced concrete.
DW : We don't have to construct a reinforcement cage before we pour the concrete.
But concrete has one great advantage. Wherever you are in the world, you can specify the concrete mix you need, referring to long established standards, and be sure how it will perform.
David argues that standardisation of stone use is possible, and can help popularise its wider adoption
DW : There's absolutely the potential to leverage the standards that have been developed for reinforced and post-tensioned concrete because at the end of the day, they really give us guidance on what the loads are required to be, what the performance requirements might look like, what are our durability expectations, where are the problems that have occurred with post-tensioned concrete, how can we avoid those problems in post-tensioned stone.
DW : I think there's some really interesting work that's happening to that regard and a logical place for changes like this has often been the housing market where you can build small scale structures and you can start off with a few stone beams that might span 6, 8, 10 metres and work out how you support floors with them and develop systems that solve the problems a small scale. Once you've got that sort of track record then it's much easier to upscale it into a larger building.
He is even optimistic that support could come from some unexpected quarters
DW : And I think it's fair to say that no matter what type of system or material we're talking about, the incumbent players in the marketplace obviously have a vested interest in protecting their portion of the market.
DW : And maybe that will be one of the catalysts for the growth of a post-tension stone industry in the years to come is that we will see investment from some of the big players in the concrete industry who are looking to find supplementary ways, shall we put it, of improving the performance of, you know, at the end of the day, they're ultimately selling a material that's being used to produce structural members so if they can find a way to sell an alternative material that has lower carbon impacts and the market is you looking desperately for such um products then they may well go here's an opportunity for us to improve our performance.
Spreading the gospel of stone helps to raise its profile as a construction material.
But David believes the industry could create more momentum if its constituent parts committed to the cause.
DW : The other key aspect is really collaboration within the existing stone industry so that the incumbent players in quarrying dimension stone can leverage this opportunity and develop their businesses in the space, but also so the people who know about stone can collaborate with the people who know about post-tensioning, and the people who are interested in low-carbon structural systems, and getting that cross fertilization of ideas and using that to develop standards that will provide the framework for a post-tensioned stone industry to grow rapidly
