by Roma Agrawal
Concrete is, however, a fussy material. It loves compression, and for millennia it was used this way, being squashed in foundations or walls. But it dislikes being pulled apart. Its resistance to tension is minimal; in fact it cracks if tested at loads less than one-tenth of what it can resist in compression. This is another reason why the Pantheon impresses me so much. The Romans really understood how concrete works, and how domes work, and even though concrete wasn’t the ideal material to use to build this immense structure, they still used it – and used it well.
Fussy concrete prefers to be in compression. At even relatively low loads of tension, concrete will crack.
To understand why making a dome from concrete is challenging, start by making an arch. If you bend a long, thin rectangular strip of card and place it on a table, you’ll find it won’t hold that curve on its own. It simply collapses. To make your arch stand up, position an eraser on the table against each of the outer edges of the curved card. The ends of the original, unsupported card arch pushed outwards, collapsing the structure; this time, however, although the arch still pushes outwards, the sideways friction between the eraser and the table reacts against the push from the base of the arch. This is Newton’s third law of motion: every action has an equal and opposite reaction. The base of the arch exerts a pushing ‘action’ on its support – and its support keeps it stable by ‘reacting’ against this force.
Forces flow around an arch, and then push out at the base.
Domes are similar to arches, but in three dimensions; the third dimension adds a layer of complexity. If instead of having one card you cut many long thin strips, then stack them one on top of the other and stuck a pin through the centre of the stack, you could still curve them downwards to create an arch. But you could also fan them out through 360 degrees (so that they form lines a bit like the longitudes of the Earth), thereby creating the shape of a hemisphere or dome. This dome, though, will be no more stable than your original, unanchored arch: it won’t retain that hemispherical shape on its own. To hold it in place you could arrange a ring of erasers on the table, one at the base of each strip. Or you could try something smarter, such as using rubber bands, arranged like the latitudes of the Earth, to tie the dome together. With rubber bands in place, you can remove the erasers and the dome still stands.
When ‘tied’ sufficiently, the forces that flow around a dome do not push out at the base.
What this means is that the supports for a dome do not feel any horizontal pushing force on them (unlike the arch). But you’ll notice that the rubber bands are in tension: they are stretched and resist the push of the card strips. So yes, each of the strips is in compression individually along their ‘longitudes’, but you need tension to hold the strips together in the ‘latitudes’.
The difference between where forces flow in an arch as opposed to a dome.
Viewed from the piazza, the Pantheon looks quite shallow, but in fact the inside is almost perfectly hemispherical. It appears shallow from the outside because the base is much thicker than the crown: the concrete at the top of the dome is only 1.2m thick, but by the time it reaches the base it has increased to more than 6m. Making it thicker towards the base meant the dome could resist higher tension forces – more material, more resistance.
The widening stepped rings around its base act to strengthen the dome of the Pantheon.
But the Romans went even further, adding more stability in the form of seven stepped concentric rings (which you can see from the outside, a little below the oculus, if you’re somehow airborne). These rings act in a similar way to the rubber bands from our demonstration, helping to resist some of the tension forces and make the dome stable. This ingenious design ensured that, even though concrete isn’t great at resisting tension, the Romans succeeded in making it work.
While thicker concrete might solve some problems in resisting tension, it also creates problems of its own. The thicker the dome, the more the cement content – which means it generates more heat, and the more it shrinks when it cools. As it shrinks, it pulls itself apart and, since concrete can’t resist this tension, it cracks. The Romans were worried that the base of the Pantheon’s dome would suffer extensive cracking. It’s believed that the series of squares which are inset all around the inside of the dome, which are part of its unique visual aesthetic, are there to allow the concrete to cool down more quickly and evenly, minimising cracking. Even so, engineers studying the Pantheon have found cracks in the base of the dome (ancient ones that occurred while it was being built) – though they haven’t undermined the integrity of this ancient building.
The first time I visited I was a teenager, and I loved this building for its beauty and sense of peace. The second time, as a trained engineer, I gazed – no less lovingly – at the recesses in its surface and searched for the fine cracks at its base. For a long time I watched the shaft of light coming through the oculus at the top of this amazing structure. I left astounded by the dome’s scale and apparent simplicity of form, but conscious of how complex it must have been to construct it so many years ago. I often wonder whether, like the Pantheon, the structures we design and build today will still be around, and in such good condition, in 2,000 years. It seems inconceivable.
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After the collapse of the Roman Empire in the fifth century, the Dark Ages – or as I like to call them, the Crumbly Ages – began, as the Roman recipe for concrete was lost for almost 1,000 years. We reverted to a move primitive way of life and concrete only re-emerged in the 1300s. Even then, engineers continued to struggle with the fundamental problem of concrete cracking in tension. It was only centuries later that the true magic of concrete was discovered, by an unlikely hero, in the most unexpected of places.
In the 1860s, French gardener Joseph Monier became fed up with the fact that his clay pots would constantly crack. He tried making pots out of concrete instead but found that they fractured just as much. Randomly, he decided to reinforce the concrete by embedding a grillage of metal wires within it. This experiment could have failed for two key reasons: first, the concrete might not have actually bound to the metal reinforcement (there was no reason to think that it would), so the metal would only create more weak points in the pot. Second, during the change in seasons, the metal and concrete would expand and contract at different rates, creating yet more fissures. Unwittingly, Monier created a revolutionary pot that remained solid and barely cracked.
Like most metals, iron and steel (as we’ve seen) are elastic and ductile, and they’re good in tension: they don’t crack when pulled. Metals aren’t brittle like brick or concrete. So by combining concrete (which breaks in tension) with iron (which can absorb tension loads), Monier had created a perfect marriage of materials. In fact, an ancient version of this principle can be found in Morocco, where the walls of some Berber cities were made of mud with straw mixed in: a mixture known as adobe, also used by the Egyptians, Babylonians and Native Americans, among others. Straw fulfils a similar function to metal in concrete; it binds mud and plaster together and stops it from cracking too much because the straw resists tension forces. The plaster on the walls of my Victorian flat has horse hair mixed into it for the same reason.
Having exhibited his new material at the Paris Exposition in 1867, Monier then expanded its application to pipes and beams. Civil engineer Gustav Adolf Wayss from Germany saw the material and had visions of building entire structures with it. After buying the rights to use Monier’s patent in 1879, he conducted research into concrete’s use as a building material, and went on to build pioneering reinforced concrete buildings and bridges across Europe.
The marriage of steel (which replaced iron once the use of the Bessemer process spread) and concrete appears so obvious today that it seems almost inconceivable to me that the two weren’t always used together in this way. In every concrete structure I design, I use steel reinforcement bars – long, textured rods between 8mm and 40mm in diameter that are bent into different shapes and tied toget
her to form a grid or mesh to bind the concrete. My calculations tell me where the concrete will be in tension and where it will be in compression, and I distribute steel bars within it accordingly.
A perfect marriage of construction materials: a steel cage provides reinforcement for concrete, resisting tension and restricting cracking.
Contractors take my drawings and set the dimensions and shapes of every single steel bar in the project, and calculate their weight. These schedules are sent to a factory, and a few weeks later real bars appear, which are fixed into shape before the concrete is poured around them.
As the chemical reaction in the concrete mixture progresses, steel and concrete form a strong bond. Just as cement paste binds strongly to aggregates in the mix, it also sticks to the steel. And once intertwined, steel and concrete are very difficult to separate. They have near-identical thermal coefficients – which is to say that they expand and contract by almost identical amounts under the same changes in temperature. When a concrete beam bends under gravity and is squashed at the top but pulled apart at the bottom, the concrete cracks at the bottom. These cracks are fractions of a millimetre wide and often not visible to the human eye – but they are there. Once this happens, the steel bars in the base of the beam are activated, and resist the tension loads keeping the beam stable.
Steel reinforcement is now part of the DNA of how we build modern structures. Many construction sites around London have small windows in the protective hoardings that surround them. As you can imagine, whenever I walk past one I can’t resist taking a peek, curious to see what’s going on inside. No matter what the site, I always see big piles of steel reinforcement bars ready to be tied together, or steel cages already made up inside wooden moulds. When the trucks with rotating drums appear, they pour a thick stream of concrete into the moulds, after which workers use short poles attached to a power supply to vibrate the concrete, mixing it to make sure that the different-sized aggregates are well-distributed throughout. Engineers like me have made sure that the gap between the steel bars is big enough to allow the concrete to flow easily around them. As a young engineer, my first boss John told me, ‘If a canary can fly out of your steel cage, the bars are too far apart. If it suffocates, they’re too close together.’ It’s a lesson I’ve never forgotten. (No canaries were harmed in this thought experiment.)
Once all the concrete has been poured and mixed thoroughly, the workers flatten the top of it with huge rakes and leave it to solidify. But this incredible material has one more secret in store. Over the next few weeks, the bulk of the chemical reaction will finish, it’s tested, and results show that it has reached its target strength. In fact, its strength continues to grow – very slowly – over months, and even years, plateauing to a steady number far into the future.
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Nowadays, we use concrete for many structures, creating skyscrapers, apartment blocks, tunnels, mines, roads, dams and countless others. In ancient times, different civilisations employed different materials and techniques that were suited to their indigenous skills, climate and surroundings. Today, concrete is universal.
Scientists and engineers are constantly innovating, trying to make concrete even stronger and longer-lasting than it already is. One recent invention has been ‘self-healing’ concrete, which contains tiny capsules with calcium lactate. These are mixed with the liquid concrete, but the capsules have a fascinating secret. Inside is a type of bacteria (normally found in highly alkaline lakes near volcanoes) that can survive without oxygen or food for 50 years. The concrete, mixed with these bacteria-filled capsules, hardens. If cracks form in the material and water seeps in, the water activates the capsules, releasing the bacteria. Habituated as they are to alkaline environments, these escapees don’t die when they encounter the highly alkaline concrete. Instead, they feed on the capsules, combining the calcium with oxygen and carbon dioxide to form calcite, essentially pure limestone. With calcite filling the cracks in the concrete, the structure repairs itself.
There are other challenges. Five per cent of human-created carbon dioxide comes from making concrete. Using concrete in small amounts is not particularly unfriendly to the environment, but we use so much of it that the emissions quickly add up. Some of the CO₂ comes from the firing of limestone to create the cement, but the rest comes from the hydration reaction. The amount of cement being used in the mix can be reduced by replacing a proportion of the cement with suitable waste materials from other industrial processes, such as ‘ground granulated blast furnace slag’ (GGBS), which is created during the manufacture of steel. Using these waste materials doesn’t affect concrete’s strength too much but can save tonnes of carbon. You can’t use them for all types of construction, because these ingredients have other effects on the mix. They can make the concrete take longer to solidify, or make it stickier, and hence harder to pump up many storeys, which is definitely a challenge when constructing skyscrapers.
‘My’ skyscraper, The Shard, uses concrete and steel in a really clever way that neatly reconciles the different requirements of office and residential areas. In typical office buildings, the aim is to create large, open spaces with few columns. Steel is often the material of choice because it behaves well in both tension and compression meaning that steel beams can span further than concrete ones of the same depth. Moreover, compared to apartment buildings, offices need a lot of air-conditioning machines, ducts, water pipes and cables. The I-shaped construction of steel beams, and the regular gaps between adjacent beams, leave plenty of space to hide these away. Steel structures are also lighter than their concrete equivalents, so the foundations can be smaller as well.
Arranging the steel beams and concrete floors for an office building.
On the other hand, residential buildings and hotels have floors that are sub-divided into flats and rooms, so you’re not under as much pressure to create huge open spaces. You can hide concrete columns in walls to support flat concrete slabs. Concrete floors are thinner than steel ones, so you can fit more storeys into a concrete building of the same height. There are fewer cables and smaller ducts to run, and these can be attached to the bottom of the slabs. Concrete also absorbs sound better, so you get less noise transfer between floors – this doesn’t matter so much in an office where you, hopefully, don’t sleep.
Arranging the concrete floors for a residential building.
Since The Shard has offices on its lower storeys and a hotel and apartments on its higher ones, we used different materials in different places. The lower storeys are made from steel columns and beams to create space in the offices; the higher storeys from concrete to create privacy. While it may seem obvious to use the right material in the right place, it’s actually quite an unusual thing to do, and only a handful of structures globally have so far adopted this design. One possible reason is that it’s arguably logistically easier (and possibly cheaper) to use the same material throughout, but I’d counter that by saying you achieve a better design for the long term, and it’s more sustainable because you use less material. Another reason is that multi-use buildings simply aren’t as widespread as single-use buildings. But with the construction of more and more multi-use buildings, I expect the multi-material method will become more common.
Using the materials we have in an efficient way is good engineering. We often think of concrete as being old-fashioned because of its ancient roots, but it’s still very much part of the future too. Scientists and engineers are working on new super-strong mixes, and trying to figure out how to make concrete more eco-friendly. Perhaps one day we may find a new material that replaces concrete completely. But in the meantime, cities are being built at breakneck speed to cope with the demands of an ever-expanding, global population. So concrete buildings will grace our horizons for a long time to come. Which means more concrete for me to stroke.
SKY
Over the years I’ve worked on a range of projects, from the steel footbridge in Newcastle and concrete apartment blocks in London, to the
refurbishment of the brick railway station at Crystal Palace. But skyscrapers have become one of my specialities – which is ironic, because I have no head for heights.
Don’t get me wrong: I won’t freeze up and go bulgy-eyed, like James Stewart at the beginning of Vertigo. I don’t collapse into a blubbering mess when I look down from a great height, even if my legs have turned to jelly. But there’s no doubt that it makes for some uncomfortable moments at work. Most days, I’m safely sat at a desk inside an office (reassuringly low down on the ninth floor). But sometimes I have to don the classic clobber of my profession – hard hat, hi-vis jacket, steel-toed boots – and climb up a structure I’ve been designing.
So it was with a mixture of excitement and anxiety that I got off the train at London Bridge in May 2012, took a right out of the station and walked up the street towards a plyboard door painted bright blue – a part of the site hoarding, ignored by the thousands of commuters on their way to work. This was once the entrance to The Shard: a sharp contrast to the gleaming glass and white steel construction that welcomes you today.