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Why the time is ripe for a return to stone as a structural material

Words:
Steve Webb

If you’re looking for a low carbon, reusable material that is strong, robust and beautiful, stone is ready for a revival

Abandoned quarry in Estremoz Aletejo Portugal.
Abandoned quarry in Estremoz Aletejo Portugal. Credit: Christina Godiksen

One hot afternoon in the UAE, Andy Yates and I stood by a four-wheel drive in a remote, arid wadi in the Al Hajar Mountains in the UAE. I love this dry rocky desert and I wanted to get out of the air conditioned car and smell the goat dungy air, feel the early evening warmth and listen to the silence. I picked up one of the red, sun burned rocks and handed it to him. ‘Heavy’, he said. ‘Iron content, basalt’. Later I looked into it. Actually it was an ophiolitic gabbro, another volcanic rock (similar to basalt), indeed rich in iron and heavy. Moreover, it is very, very strong. In compression a piece of gabbro might be as strong as 230N/mm2, in the same order as steel. 

Dubai and Sharjah are just 45 minutes from this dusty gorge. In those places a frenetic construction boom grinds on with an unending race to build the tallest and the strangest things. Despite the immeasurable pile of gabbro at our feet buildings in the UAE are made with concrete. Cement from gas powered cement works, (unbelievably) imported sand and desalinated sea water all produced using cheap, apparently limitless, energy from oil and gas. Limitless energy is what we imagined when we invented steel and concrete and that idea is where we went wrong.

Ground down to stone
The world over we crush rocks (like gabbro) into gravel. We dig up and pulverise limestone. In gas powered factories we burn the limestone to make cement. We dig up a lot of sharp sand. At state-of-the-art batching plants we mix them with water to make liquid concrete. With sophisticated algorithms and just in time management principals a fleet of wagons distributes it to building sites. Meanwhile we’ve laid out steel falsework and timber formwork. We’ve sprayed on releasing agent and laid rebar on spacer blocks. We pour on the concrete. We vibrate, level and float it and then wait. A week later we strip away the formwork; after 28 days we remove the falsework. Hey presto! Stone again! In two months we’ve turned a lump of 230N stone into a lump of 40N concrete.

The New Stone Age exhibition at the Building Centre opened on 27 February. Curated by me, Amin Taha + Groupwork and Pierre Bidaud of the Stone Masonry Company, it aims to examine the modern use of stone as a structural material and pulls together various threads of work from around the world and presents them as a nascent movement. The suggestion is made that stone, with modern design and fabrication methods, could come back to the mainstream in the way timber has through the development of CLT. Like timber, the reason for bringing it back is avoiding a climate disaster. 

The environmental argument for stone is counter intuitive. On the one hand it’s not exactly lightweight and it has a finite availability. On the other, it’s an abundant, available, high strength material that can be chopped up and built with immediately. 

Is there enough stone? According to the Global Cement and Concrete Association, annual worldwide concrete production is roughly 1.6 km3. Due to its higher strength its equivalent in stone would be about one quarter of that volume. To put this into context, the volume of a small, Ben Nevis-ish mountain is about 30km3; all the world’s buildings* would only make a 56km3 or two Nevis, the Earth’s crust (rock) has a volume of 10 bn km3.  (*This is my back of a fag packet calculation: 7bn people living in threes in 120m2 live work units made of 200mm slabs.)

The Inventory of Embodied Carbon and Energy 2019 says ‘general stone’ has a carbon footprint of 0.079kg carbon per kg of stone. Concrete’s is 0.15kg/kg and steel’s 2.8kg/kg. These materials have different strengths so how can we compare apples with apples? 

 

‘General stone’ has a carbon footprint of 0.079kg/kg. Concrete’s is 0.15kg/kg and steel 2.8kg/kg

Experiments in a long span stone floor slab – post-tensioned of course.
Experiments in a long span stone floor slab – post-tensioned of course. Credit: John MacLean

Carbon winner
The graphic above shows a series of different types of beam of the same depth that are all doing the same duty. On a simple carbon basis, timber wins by a long way because it sequesters quite a lot of CO2 when it grows. There is certainly an immediate imperative in locking away timber that would otherwise have rotted in the forest, releasing methane (worse for global warming than carbon), but ultimately when a timber building reaches the end of its life, it burns or gets buried and rots so that carbon escapes. In production terms stone has an even lower carbon footprint than timber; over 70% less CO2/m2 in that example than steel or concrete. 

Using stone in construction is obviously nothing new and no one is in any doubt that it lasts for a long time. Its expression as an architectural component from the ubiquitous beige-grey limestone to the kaleidoscopic, multicolour book matched marbles of Westminster cathedral shows its stylistic scope. But what can really be achieved with stone? Its compressive strength means that it can supplant many concrete and steel applications. Its flexural strength, while not in the same league as steel or even timber, is still substantially higher than concrete which means it requires far less reinforcement and can be used in shear and torsion. Pierre Bidault and The Stone Masonry Company have been developing ways to make post tensioned stone structures for years; moving from building traditional Georgian cantilever stairs to developing daring reinforced helixes, and recently completing 330˚ helix staircase, designed by Foster and Partners for the Dolunay Villa in highly seismic Turkey.

Strong and stylish
Our exhibition postulates a 30-storey tower with a 12m open plan floors entirely made from stone. A section of floor was erected outside the Building Centre to demonstrate the 12m span. Measuring 450mm in depth and weighing 7t, it is shallower and lighter than its concrete equivalent. It is formed with small blocks threaded together with 16mm diameter cables. The cables are pre-stressed. The stone blocks that form the beams are drilled and laid out on the floor of a workshop, a pair of cables is threaded through and a small hydraulic jack is used to pull them tight. 

This is a very simple technique, borrowed from the concrete industry, but is very effective with stone because, being stiffer and stronger, stone doesn’t require supplementary reinforcement. Reinforcing stone isn’t new. Soufflot used it in the construction of the Pantheon in Paris in 1770. Hopkins and Buro Happold used it for their Emmanuel College building in Cambridge. Portcullis House uses it. Jürg Conzett’s beautiful stressed ribbon bridge in the Swiss Alps – Punt da Sarasuns, is post tensioned stone and Arup is in construction with the Sagrada Familia Jesus Tower that will reach 172.5m in height with post tensioned stone. 

Used in the right way, stone has the capacity to be very slender, durable and elegant. While timber is a great substitute for concrete and steel structures in many instances, it isn’t in others. No one is proposing the first CLT nuclear reactor, or dam, but stone is great for infrastructure and has form in tunnels and bridges. In fact the Highways Agency is positively pleading for it. This is an extract of Highways agency BD91/04: ‘Experience has shown that arch bridges are very durable structures requiring little maintenance in comparison to other bridge forms. BD 57 (DMRB 1.3.7) says their use should be considered. However, there has not previously been a standard for the design of new unreinforced arch bridges. The objective of this Standard is to encourage a renaissance in arch building using unreinforced masonry materials.’

So why isn’t everyone building with stone already? The stone industry has been left behind, while billions has been invested in steel and concrete production. Relatively unsophisticated quarries produce stone with limited or no strength testing or certification. There are many stone cutters and installers who are geared up to provide decorative or highly aesthetic stone structures but are not mechanised for the large-scale production of structural stone components.

Engineers are hesitant to use stone because they simply don’t know how strong it is and they don’t have design codes or training to apply. Fire resistance is hard to prove and requires testing. What stops architects from using stone? Architect Amin Taha, who has used stone extensively on his buildings, writes: ‘While in the early years of modernism stone will have been rejected as part of the past, unliberated from the ground like steel or concrete frames, if used at all it will have been as one of many hung facade materials. The possibility that it, like bricks, could support the entire structure let alone its own weight has not been taught because it was at first rejected, then forgotten.’ 

Used in the right way, stone has the capacity to be very slender, durable and elegant

Deep working marble quarry in Estremoz, Aletejo Portugal.
Deep working marble quarry in Estremoz, Aletejo Portugal. Credit: Christina Godiksen

Even quarries look lovely
Is it that quarrying puts people off? It is clearly highly impactful and difficult to get permission for. But quarries can be backfilled with excavated spoil or simply returned to nature, unpolluted. Old quarries form new ecosytems for wildlife and play. Christina Godiksen, senior lecturer in architecture at Oxford Brookes, has spent many years study­ing the quarries of Alentjo in Portugal: ‘These beautiful landscapes are breathtaking, they are full of wild flowers, herbs, flowers, olive trees, cork oaks and marble sometimes filled with the most turquoise water. Some have been in use since Roman times, history is cut in marble, evidence of every technique, tool and hand. Each quarry unique and specific.’ 

What could the future be? Imagine a quarry extracting a strong gabbro in the mountains of the UAE or granite in the fields of Leicester. Huge blocks are wire cut from the ground using renewable electrical power and lifted to the door of a shiny factory, there on an automated production line chopped up into standard blocks, drilled, X-rayed for flaws, sonically strength graded, sorted, tagged or chipped with their individual data. They are wired with post tensioning cables, the blocks being distributed according to their strength to reduce wastage, made specifically to fulfil orders submitted by email, length dimensions, load, finish when needed. Stone beams and planks emerge from the quarry on delivery trucks ready for erection. No shuttering, no curing time. When the trucks return they carry recovered blocks from disassembled buildings, yielded whole back to the plant to be re wired, again and again. 

Let’s start small. Stairs: Obviously. You’re about to put a boring steel portal frame in an old house: Save on decoration, make it stone. Your concrete frame: swap the columns  for stone and leave them on show. You have a small canopy over a café: Build a little stone vault. Building five units of residential – how about a timber and stone hybrid? Production starts with demand; if you want it, demand it! 


Steve Webb is  a founding partner at Webb Yates

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