THE CONCRETE – The material that made urbanization possible_ PART I (How the steel reinforced concrete has become the most multipurpose building material of all time?)

For its entire existence human race always needed and will forever need 2 essential things in order to survive: Food and Shelter. The first humans were hunter-gatherers, their only source of food was obtained through hunting and fishing.

Then humans learned that food can also be produced, so they became farmers. Sometime about 10,000 years ago, the earliest farmers put down their roots—literally and figuratively and from here slowly but for sure humans settlements started to grow. The biggest next concern humankind needed after food was how to build a shelter. The hunter-gatherers used to live in caves but after they became farmers they started to learn how to build their own shelters, they build permanent dwellings that eventually morphed into complex societies in many parts of the world. And here is how the urbanization very slowly started to grow.

Yet, the problems started to grow too. To build a solid dwelling humans had to figure out from which material they must do this. The dwellling must really serve its purpose and keep standing regardless of weather conditions, otherwise the only option would have been to go back to the caves. So indeed this was a challange for human race. Humans indeed started to build dwellings from different materials but it took thousands of years of evolution until to really find the best material to build a solid and long lasting dwelling which happened just 2 centuries ago when the Industrial revolution took place mostly by the mid of 19th century.

The magic material discovered back then was the steel reinforced concrete. This is an incredible story, such material was discovered absolutely unexpected. But before to tell you about how that happened, I wish to tell you a little bit about concrete like I learned during my years as student in materials science.

Long before the steel reinforced concrete was discovered (in 1867), the concrete itself was already known to be a useful material. The Romans built an empire with it. Opus Caementicium went into aqueducts, the Colosseum, a European network of roads spanning some 85,000 kilometres and, of course, the Pantheon in Rome, which is still the world’s largest dome made of non-reinforced concrete at 43 metres across. 


Concrete is a composite material made of 60-75% sand and gravel, 15-20% water and 10-15% binding agent – the crucial ingredient holding it all together. The Romans used a volcanic ash known as pozzolana. In modern concrete, that binding agent is called “Portland cement”. (Concrete has tremendous compressive strength but very low tensile strength. This makes it great for building the support pillars of a bridge, for example, but not the connecting elements between the pillars. Gravity would stress the spans to the breaking point, causing the concrete to crack. That’s why a Roman aqueduct has so many arches: they transfer the tensile forces on the span, transforming them into a compressive force on the pillars. In a modern bridge, this is unnecessary; with the addition of metal reinforcing elements (now made primarily of steel instead of iron), concrete can handle both compressive force and tensile force. ) This combination of qualities is what makes reinforced concrete an ideal building material.

Still, there was a healthy degree of scepticism in 1903 when the first “skyscraper”constructed from reinforced concrete – the 16-storey Ingalls Building in Cincinnati, Ohio – went up. One year earlier, the building permit application for the Ingalls had been rejected; the building inspector found the design too experimental and had concerns about the building’s safety. Many were convinced that the building wouldn’t last a day after the construction supports were removed. The story goes that one reporter even camped out during construction, watching the building the whole night through, hoping to document the moment of collapse. But it didn’t collapse. When the building was topped out and the flag ceremoniously raised on it, The Cincinnati Enquirer wrote:“It is now assured that the building is a success.” It’s still standing to this day. 

Since then the history of tall buildings has started. Mainly in the North America, a lot of such buildings started to grow to the sky. In Europe not so much, until very late, even later than the year 2000. Currently few tall buildings exist in Europe as well. But only 10 of them are higher than 300m, most of these are located in Russia in Moscow and Saint Petersbourg. The list of these 10 European skyscrapers in the order of their height is this:

  • 1st- Lakha Center = 462,5 m tall (The tallest building in Europe) in St. Petersbourg_Russia (completed in 2019);
  • 2nd – Federation: East Tower = 373,7m tall _ Morcow_ Russia (completed in 2016)
  • 3rd – OKO: South Tower = 354,1 m tall _ Morcow_Russia (completed in 2015)
  • 4th – Neva Tower 2 = 345m tall _ Moscow _Russia (completed in 2019)
  • 5th – Mercury City Tower = 338,8 m tall _ Moscow_ Russia (completed in 2013)
  • 6th – Varso Tower = 310 m tall _ Warsaw _ Poland (still in construction, estimated completion 2021)
  • 7th – The Shard = 309,6 m tall _ London _ United Kingdom (completed in 2012)
  • 8th – Eurasia = 308,9 m tall _ Moscow _Russia (completed in 2014)
  • 9th – Neva Tower 1 = 302 m tall _ Moscow_ Russsia (completed in 2019)
  • 10th – City of Capitals: Moscow Tower = 301,6 m tall_ Moscow _ Russia (completed in 2010)

Such buildings are only possible because of steel reinforced concrete. Due their huge mass it´s extremely important have a stable ground beneath and very serious fesability studies must be with high accuracy done. When water is nearby the things could turn in a massive disaster, so no error allowed when the feasibly studies are done. Things really must evolve perfectly. It´s the case for any tall buildings not only for those close to water, but water can create additional issues.

For example, when The Shard was build in London they had to consider the fact that Thames is just nearby. They had to be perfectly sure that everything will be ok to build it there. After the feasibility studies were completed, they started by digging a huge hole. And when I say huge, I mean enormous. Gigantic machines were scooping out the dirt, digging ever deeper, as if they were mining for something. But what they were digging out was clay – clay that had been deposited there for hundreds thousands of years by the Thames river. It was the same thick clay that has always been used to fire the bricks to make the houses and warehouses from which the city of London is built. But this clay was not going to be used to build the Shard.

One day, once all of this clay had been removed, they poured 700 truckloads of concrete into the hole. This would make the foundations that would hold up the enormous skyscraper and prevent the 72 floors above, and the 20,000 people who would inhabit them, from sinking into the clay. They filled the enormous hole with concrete, layer after layer, building up floor after subterranean floor, until there was no gigantic hole any more, just an underground cathedral of poured concrete, which was now slowly becoming fully-solid, It was nicely done and impressively speedy, which was important because for cost reasons they had already started to build the tower before they finished its foundations.

Most people seeing such masterpiece will surely will ask: how long will the concrete take to dry? Well…concrete doesn’t dry out. Quite the opposite, water is an ingredient of concrete. When concrete sets, it is reacting with the water, initiating a chain chemical reactions to form a complex microstructure deep within the material, so that this material, despite having a lot of water locked up inside it, is not just dry but waterproof.

The setting of concrete is, at its heart, an ingenious piece of chemistry, which has powdered rock as its active ingredient. Not every type of rock will work. If you want to make your own concrete you need some calcium carbonate (CaCO3), which is the main constituent of limestone, a rock formed from the compressed layers of living organisms over millions of years and then fused together by the heat and pressure of the movement of the Earth’s crust. You also need some rock containing silicate -silicate being a compound containing silicon (Si) and oxygen (O), and constituting roughly 90% of the Earth’s crust – for which some form of clay will do. Grinding these ingredients up and mixing them together with water won’t get you anywhere, unless you want to create a sludgy mud. In order to create within them the essential ingredient that will react with the water, you need to free them from their current chemical bonds.

This is not easy. These bonds are extremely stable, which is why rocks do not easily dissolve or react with many things: on contrary they last in fine climates or foul for millions of years. The trick is to heat them to an extremely high temperature of about 1450°C. This is a temperature far exceeding that of average wood or coal fire, which is between 600 and 800°C if glowing red or yellow hot. At temperatures of 1450°C a fire will glow white hot, with no tinge of red or even yellow in the flames but instead a hint of blue: it is so bright it is unnerving, and almost painful to look at.

At these temperatures, rock starts to fall apart and re-form to create a family of compounds called calcium silicates. It’s a family because there are lots of minor impurities that can change the outcome of what you get. To make concrete, aluminium (Al)– and iron (Fe)-rich rocks are the magic ingredients, but only in the correct proportions. Once it has all cooled down, the result is a very special powder the grey-white colour of the moon. If you put your hands through it you find that it has the silky texture of ash – there is something atavistic about it – but your hands soon feel dried out as if under a subtle type of attack. This is a very special material with a very dull name: cement.

If you now add water to this powder it sucks it up with ease and darkens. But instead of forming a slushy mud, which is what happens if you add water to most powdered rock, a series of chemical reactions takes place to form a gel. Gels are semi-solid and wobbly types of matter – the jelly served at children´s parties is a gel, and so too is a lot of toothpaste. It doesn’t slosh around like a liquid because it has an internal skeleton that prevents the liquid moving. In the case of jelly this is created by the gelatin. In the case of cement, the skeleton is made up of calcium silicate hydrate fibrils, which are crystal-like entities that grow from the calcium and silicate molecules, now dissolved in the water, in a way that appears almost organic (see picture below). So the gel that forms inside cement is constantly changing as the solid internal skeleton grows and further chemical reactions take place.

A sketch of calcium silicate fibrils growing Inside the setting cement.

As the fibrils grow and meet, they mesh together, forming bonds and locking in more and more of the water, until the whole mass transforms from a gel to a solid rock. These fibrils will bond not only to each other but also to other rocks and stones, and this is how cement turns into concrete. Cement is used to bond together bricks to make houses, stones to make monuments, but in both these cases it is wedged between the cracks as the minority component, an urban glue. When it is made into concrete by mixing it with small stones, which play the role of tiny bricks, it fulfills its potential to become a structural material.

As with any chemical reaction, if you get the ratio of the ingredients wrong, then you get a mess. In the case of concrete, if you add too much water there won’t be enough calcium silicate (Ca₂O₄Si) from the cement powder to react with, and so water will be left over within the structure, which makes it weak. Similarly, if you add too little water there will be unreacted cement left over, which again weakens the structure. It is usually human error of this sort that proves the undoing of concrete. Such poor concrete can go undiscovered but then lead to catastrophe many years after the builders have departed. The extent of the devastation due to the 2010 earthquake in Haiti was blamed on shoddy construction and poor-quality concrete: an estimated 250,000 buildings collapsed, killing more than 300,000 people, and making a million more homeless. What is worse is that Haiti is by no means unusual. Such concrete time bombs are scattered throughout the world.

Tracking down the origin of such human errors can be tricky since, from the exterior, the concrete looks fine. For example, the supervising engineer of the building of JFK Airport in New York, noticed through routine tests that the concrete arriving on trucks before noon had good strength when it set, but that arriving just after noon was substantially weaker. Puzzled, he investigated all possible reasons for this but was unable to find the answer until he resorted to following the truck delivering the concrete on its journey to the airport. He found that around noon the driver was in the habit of taking a break for lunch and would hose the concrete with water before doing so in the belief that adding extra water would keep the concrete liquid for longer.

Or as another example, while digging the foundations for the Shard building in London and its supporting structures, the engineers found evidence of a type concrete that predates the modern stuff: Roman concrete. It was holding together the remains of a Roman bathhouse they found when they demolished the local fish and chip shop, which had resided next to the now ex-Southwark Towers.

Remains of a Roman bathhouse found by the Shard engineers.

The Romans got lucky with concrete. Instead of having experiment with heating up different combinations of ground-up rock to white-hot temperatures, they found ready-made cement (a.k.a. pozzolana) in a place called Pozzuoli just outside Naples.

Pozzuoli stinks – literally. It took its name from the Latin putere (to stink), the smell coming from the sulphur in the volcanic sands nearby. The upside of the smell was that the region had been the recipient of lava and eruptions of ash and pumice for millions of years. This volcanic ash resulted from the super-heating of silicate rock, which was then spewed out of a volcanic vent – a process suspiciously similar to that of making modern cement. All the Romans had to do was put up with the smell and mine the rock powder that had been accumulating for millions of years. This naturally made cement is slightly different from modern (‘Portland’) cement and requires the addition of lime to make it set. But once they had worked this out and added stones for strength, they had in their hands for the first time in human history the fundamentally unique building material that is concrete.


The composite nature of a brick building is part of its appeal. The brick itself is a unit of construction that is designed to fit in the hand, giving the whole a human scale. Concrete is fundamentally different from this building material, because it starts as a liquid. This means that buildings made from concrete can be poured, and what is created is a continuous structure, from the foundations to the roof, without any joins. The mantra of a concrete engineer is:

You want foundations, we will pour you foundations; you want pillars, we will pour your pillars; you want a floor, we will pour you a floor; you want it twice the size? -no problem; you want it curved? – no problem.

With concrete, if you can build the mould, you can create the structure. The power of the stuff is palpable, and addictive to anyone who visits the building sites where this stuff is being made.

If you ever have the chance to watch and follow how skyscrapers are build you will see that how a building is growing out of the foundations; it is being poured into existence by human ants. Powdered rock and stones will ‘arrive on the site and will be transformed by the simple addition of water into rock again. It is a philosophy as much as it is an engineering technique, completing a cycle that starts when the Earth’s mantle creates rock and stone through mountain building, which is then mined by humans and transformed back into our own artificial mountains of rock, made to our own design, where we live and work.

The existence of concrete feeds the ambition of engineers. Once the Romans had invented it, they realized that it would allow them to build the infrastructure of their empire. It allowed them to build ports wherever they wanted, because their concrete could set under water. They could build aqueducts and bridges, too – the very infrastructure needed to transport the raw ingredients for concrete to wherever it was needed, instead of relying on local stone or clay. In this sense, concrete is ideally, suited to empire building. The most impressive piece of Roman concrete engineering, however, is in its capital: the dome of Pantheon in Rome. Still standing today, it is 2000 years old but still the largest unreinforced concrete dome in the world.

Although the Pantheon survived the fall of the Roman Empire concrete as a material did not. There were no concrete structures for more than a thousand years after the Romans stopped making it. The reason for this loss of the materials technology remains a mystery. Perhaps the material was lost because it was industrial in nature and needed an industrial empire to support it. Perhaps it was lost because it was not associated with a particular skill or craft, such as ironmongery, stonemasonry or carpentry, and so was not handed down as a family trade. Or perhaps it was lost because Roman concrete, good as it was, did have one crucial flaw, a flaw that the Romans knew all about but could not solve.

There are two ways of breaking a material.

One is to break it plastically, which is what happens when, for example, you pull a piece of chewing gum apart: the material, is able to rearrange itself and so flows and gets thinner in the middle until eventually it is separated into two pieces. This is what you need to do to break most metals, but it takes a lot of energy to get metals to flow like this (because you have to move a lot of dislocations), which is why they are such strong and tough materials.

The other way of breaking a material is to create a crack through it, which is how a glass or a tea cup breaks: unable to flow in order accommodate the stress that is pulling it apart, a single weakness in this type of material compromises the integrity of the whole, and it splits or shatters. This is how concrete breaks, which was a big headache for the Romans.

The Romans never solved this problem and so only used concrete in situations where it was being compressed rather than stretched, such as in a column, dome or the foundations of a building, where every part of the concrete was being squeezed together by the weight of the structure. Under such compression, concrete remains strong even when cracks form. If you visit the 2000-year-old concrete Pantheon dome, you will see that over the years it has developed cracks, perhaps as the result of earthquakes or subsidence, but these cracks do not endanger the structure because the whole dome is under compression. When the Romans tried to make suspended floors or beams of concrete, which had to withstand a bending stress, they would have found that even the slightest crack causes the structure to collapse. When the material on either side of the crack is being pulled apart by its own weight and the weight of the building, it has no way to resist. So to use concrete to its full potential, as we do today, to build walls, floors, bridges, tunnels and dams, this problem had to be solved. This didn’t happen until the European Industrial Revolution hove into view, and even then the solution came from a very unexpected source.

A Parisian gardener, Joseph Monier, had a liking for making his own plant pots. At the time, in 1867, these were made of fired clay, meaning they were weak, brittle and expensive to make especially if one wanted large pots to accommodate the craze for growing tropical plants in glasshouses. Concrete seemed to offer the solution. It could be used to make huge pots much more easily than clay because it didn’t need to be fired in a furnace. It was also cheaper for the same reason. But it was still weak in tension, so in the end his concrete pots cracked just the same as terracotta.

Joseph’s solution was to embed loops of steel inside the concrete. He couldn’t have known that cement bonds very well to steel. It could easily have turned out that the steel was like oil in the vinaigrette of concrete, preferring to keep to itself. But no, the calcium silicate fibrils inside concrete stick not just to stone but also to metal.

Concrete is essentially a simulacrum of stone: it is derived from it and is similar in appearance, composition and properties. Concrete reinforced with steel is fundamentally different: there is no naturally occurring material like it. When concrete reinforced with steel comes under bending stresses, the inner skeleton of steel soaks up the stress and protects it from the formation of large cracks. It is two materials in one, and it transforms concrete from a specialist material to the most multipurpose building material of all time.

Something else Joseph couldn’t have known at the time turns out to be one of the keys to the success of his reinforced concrete. Materials are not static things: they respond to their environment and especially temperature. Most materials expand when they get hotter and contract when they get cooler. Our buildings, roads and bridges are all expanding and contracting like this, observing day and night temperature cycles, as if they are breathing. It is this expansion and contraction that causes a lot of the cracks in roads and buildings, and if it is not taken into account in their design then the stresses that build up can destroy the structure. Any engineer guessing the outcome of Joseph’s experiments might have assumed that concrete and steel, being so different, would expand and contract at such different rates that they would tear each other apart; that in the heat of the summer or in the depths of winter in Joseph’s garden, the steel would break out of the concrete, causing the pots to rupture. Perhaps this is why it took a gardener to try the experiment at all- it just seems so obvious that it would not work. But, as luck would have it, steel and concrete have almost identical coefficients of expansion. In other words, they expand and contract at almost the same rate. This is a minor miracle, but Joseph was not the only to notice it. An Englishman called William Wilkinson had also happened upon this magic combination of materials: reinforced concrete’s time had come.

Go to any of the world’s many developing countries and you will find that millions of the poorest people live in shanty towns built of mud, wood and corrugated steel roofs. These dwellings are very vulnerable to the elements. They are oppressively hot in the sun, and leaky and unstable in the rain. They are regularly destroyed by storms, washed away by floods or cleared by the bulldozers in the service of the police and the powerful. To
build a stable defense against the elements and those who would oppress you requires a material that is not just strong but also fire-, storm- and waterproof, and, crucially, cheap enough for everyone in the world to afford it. Reinforced concrete is this material.

At around €100 per tonne, concrete is by a long way the cheapest building material in the world. But it also lends itself to mechanization of construction and so allows further cost reductions. One person and a concrete mixer can build the foundations, walls, floors and roof of a house in a few weeks. Because all of these elements are part of the same structure it can easily last a hundred years in all weathers. The foundations protect it from water infiltration and are impervious to insect and mould attack. The walls will resist collapse and support glass windows securely. It needs very little maintenance: the tiles will not be blown off it, because it has none; the roof is an integral part of the structure, and vines, plants and even grass can be grown on top of it for sustenance and to thermally insulate the building. (The fact that such flat-roof gardens are only possible thanks to concrete’s reinforcing steel internal skeleton – only domes, like the Pantheon, would be possible otherwise – is a pleasing salute to one of its gardener inventors.)

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