In the Part 1 of The Concrete Story I told you about how the steel reinforced concrete has become the most multipurpose building material of all time: And it it is, but with one very important condition:
**Once the building is complete, it must regularly maintained. If not, well… then you get what you pay for.**
In this post I will tell you the more about at what you must consider when you intend to build something with concrete and even to replace it with something much better.
It´s a proven fact that the concrete buildings of the 20th century are gradually starting to crumble away. Alas, it seems the initial promise of eternal stability that had kept Roman construction works standing after 2000 years does not hold up for modern reinforced concrete. Hmmm….This is serious matter, it´s important to understand reinforced concrete very well.
It turns out that reinforcing concrete with steel makes it strong in the short term but susceptible to concrete rot as time goes on because iron or steel reinforcement rusts, even if it’s inside the concrete. There the corrosion grows and gradually breaks the concrete apart from within. So, unlike concrete without reinforcement, reinforced concrete needs continual maintenance. This means primarily sealing cracks and gaps to keep water and air away from the metal. Once concrete rot starts to occur, maintenance becomes much more difficult and may require partial replacement of the reinforcement. And that maintenance will exceed the cost of the building itself many times over. Do nothing, and you can expect your reinforced concrete to last 30, 40, maybe 50 years.
In the United States, virtually every piece of infrastructure containing reinforced concrete was given poor to failing marks for their current state of maintenance. Bridges came out best, scraping by with a score of 60%, while dams, irrigation works, schools, airports and wastewater facilities all failed miserably at 40% – and that’s with maintenance costs already in the billions of dollars. Expect China to start suffering from the same problems, only much worse, after 2030
Europe, too, got a wake-up call in 2018. In 1967, the Morandi Bridge in Italy, near Genoa, was one of the longest reinforced concrete bridges in the world. 51 years later, on 14 August 2018, that bridge suddenly collapsed. The cause: poor maintenance.
If not permanently maintained this can happen to any skyscraper as well.
At the Shard in London, the engineers were adding approximately a whole floor every few days. What made this possible was that the concrete was being continually cast. It arrived by truck at the bottom of the building and was pumped up into a mould at the top. Meanwhile, the mould, which was the size and shape of a floor of the building, was fitted with steel rods that would become the internal skeleton of the concrete tower. Once a floor had been cast, it was then used to support the mould, which was moved up a storey ready for the next floor to be cast. And so the process was repeated; this building was growing. Growing, at a rate of 3 meters a day.
What is more staggering to me, is that this boot-strapping process could seemingly continue for as long as you cared to move the mould up another floor and pour in more concrete. It is like a bud on a growing sapling of a tree. In reality, though, there are currently limits to the process. The engineers of the Burj Khalifa in Dubai, which is almost 3 times taller than the Shard, found that the capacity of the machinery to pump concrete vertically to the top of that tower proved to be a severe problem.
Nevertheless, the method is ingenious. This mechanization of the process of building is what makes concrete such a modern material. It lends itself to pouring and moulding, to the rapid building of vast structures. The big structures of old, such as the stones cathedrals of Europe or the great Wall of China, took decades to build. The central core of the Shard, one of the tallest buldings in Europe, took less than 6 months. The material enables you to think big, to dream. It is the material that has allowed the ambition of civil engineers to be realized. It is from reinforced concrete that the Hoover dam is built, the Millau viaduct is built, Spaghetti Junction is built.
One day, the Shard stopped growing, and then over a matter of days the paraphernalia of the concrete mould disappeared. What was left was a concrete tower seventy-two storeys high: it was grey, raw and wrinkly like a newborn. Work began at the bottom again, while the newest concrete tower in London swayed quietly in the wind, with seemingly nothing to do but watch while human ants swarmed at its base. But it was not idle. Inside the material, the fibrils of calcium silicate hydrate were growing, meshing together and bonding with the stones and steel. The tower, in doing so, was getting stronger. Although concrete reacts with water to harden to a reasonable strength within 24 hours, the process by which this artificial rock develops its internal architecture and so its full strength takes years to develop.
Once at full strength, the concrete structure will take the weight of the 20,000 people who will be inhabiting it by day. It will take the weight of all their thousands of desks and chairs, all the furniture and computers, as well as tonnes and tonnes of water. It will do this day in, day out, without visibly deforming. The floors will remain rigid and solid. And it is capable of supporting the building’s occupants and protecting them from the elements without complaint for thousands of years. If the concrete is looked after, that is.
Because despite reinforced concrete’s impressive credentials as a building material, it does need care. In fact its vulnerability has the same origin as its strength: its internal structure.
ln ordinary circumstances, exposed to the elements, the steel that is used to reinforce concrete is prone to rusting. But when that steel is encased within concrete, the alkaline conditions create a layer of iron hydroxide on top of the steel, which acts as a protective skin. But during a building’s lifetime, arising from normal wear and tear and the expansion and contraction that takes place during winters and summers, small cracks will appear in the concrete. These cracks can allow water inside, water that can freeze, expanding and creating a deeper crack. This type of attrition and erosion is what all stone buildings have to put up with. It is also what mountains have to put up with, which is how they get eroded. To prevent stone or concrete structures being similarly afflicted, maintenance of their fabric needs to be carried out every 50 years or so.
But concrete can suffer from a more pernicious type of damage. This occurs when lots of water gets into concrete and starts to eat away at the steel reinforcement. The rust expands inside the structure, creating further cracking, and the whole internal steel skeleton can be compromised. It is particularly likely to happpen in the presence of salt water, which destroys the iron hydroxide protection and rusts the steel aggressively. Concrete bridges and roads in cold countries and which are regularly exposed to salt (such as is used to clear snow and ice) are vulnerable to this type of chronic deterioration. Recently London’s Hammersmith flyover was shown to be suffering from concrete decay of this kind.
Given that literally half of the world’s structures are made from concrete, the upkeep of concrete structures represents a huge and growing effort. To make matters more difficult, many of these structures are in environments that we don’t want to have to revisit on a regular basis, such as the Øresund bridge connecting Sweden and Denmark, or the inner core of a nuclear power station.
In these situations it would be ideal to find a way to allow concrete to look after itself, to engineer concrete to be self-healing. Such a concrete does now exist, and although it is in its infancy it has already been shown to work.
The story of these self-healing concretes started when scientists began to investigare the types of life forms that can survive extreme conditions. They found a type of bacterium that lives in the bottom of highly alkaline lakes formed by volcanic activity.
These lakes have pH values of 9-11, which will cause burns to human skin, Previously it had been thought, not unreasonably that no life could exist in these sulphurous ponds. But careful study revealed life to be much more tenacious than we thought. Alkaliphilic bacteria were found to be able to survive in these conditions. And it was discovered that one particular type called B. pasteurii could excrete the mineral calcite, a constituent of concrete.
These bacteria were also found to be extremely tough and able to survive dormant, encased in rock, for decades. Self-healing concrete has these bacteria embedded inside it along with a form of starch, which acts as food for the bacteria. Under normal circumstances these bacteria remain dormant, encased by the calcium silicate hydrate fibrils. But if a crack forms, the bacteria are released from their bonds, and in the presence of water they wake up and start to look around for food. They find the starch that has been added to the concrete, and this allows them to grow and replicate. In the process they excrete the mineral calcite, a form of calcium carbonate. This calcite bonds to the concrete and starts to build up a mineral structure that spans the crack, stopping further growth of the crack and sealing it up.
It’s the sort of idea that might sound good in theory but never work in practice. But it does work. Research now shows that cracked concrete that has been prepared in this way can recover 90% of its strength thanks to these bacteria. This selfiling healing concrete is now being developed for use in real engineering structures. Another type of concrete with a living component is called filtercrete. This is a concrete that has very particular porosity, such that it allows naturally occurring bacteria to colonize it. The pores in the concrete also allow water to flow through it, reducing the need for drains, while the bacteria inside the concrete purify the water by decomposing oils and other contaminants.
And there is also now a textile version of concrete called concrete cloth. This material comes in a roll and needs only water to be added for it to harden into any shape you like. Although this material has great sculptural potential, perhaps its biggest application may be in disaster zones, where tents made in situ from rolls of concrete dropped from the air can create a temporary city in a matter of days, one that will keep out the rain, wind and sun for years while rebuilding efforts continue.
What happened next at the Shard, though, was no such celebration of concrete’s potential. Instead they were slowly but systematically cladding the outside of the Shard with steel and glass to remove all traces of the concrete core of the building. The implication. was stark: they were ashamed of concrete. It had no place in how this building was to face the outside world or its inhabitants. This attitude is shared by most people. Concrete is perceived as fine for the construction of a motorway bridge or for a hydro-electric dam, but it is not deemed a suitable material for building within a city. The use of concrete to express a sense of freedom and liberty, as in London’s Southbank Centre in the 1960s, is now unthinkable.
The 1960s were heady days for concrete. It was used boldly to reinvent city centres, to build a modern world. But somewhere along the way this association was lost, and people decided it wasn’t the material of the future after all. Perhaps too many poor-quality concrete multistorey car parks were built, or perhaps too many people were mugged in graffiti-covered concrete underpasses, or perhaps too many families felt dehumanized by living in a concrete high-rise estate. These days concrete is regarded as necessary, cheap, functional, grey, dreary, stained, inhuman, but most of all ugly. But the truth is that cheap design is cheap design whatever the material. Steel can be used in good or bad design, as can wood or bricks, but it is only with concrete that the epithet of ‘ugly’ has stuck. There is nothing intrinsically poor about t the aesthetics of concrete.
You only have to look at the Sydney Opera House, whose iconic shell enclosures are made of concrete, or the interiors of London’ s Barbican Centre to realize that the material is capable of – and in fact makes possible – the
greatest and most extraordinary architecture. This has not changed since the 1960s. It is the look of concrete that is now felt to be unacceptable, which means that concrete is now routinely hidden away from sight, providing the core and foundations but not allowed to be visible.
Many new versions of concrete have been invented to refresh its aesthetic appeal. The latest is self-cleaning concrete, which contains titanium dioxide particles. These sit on its surface but are microscopic and transparent, so it looks no different. However, when they absorb UV light from the sun, the particles create free radical ions, which break down any organic dirt that comes into contact with them. The remains are washed away by the rain or blown away by the wind. A church in Rome called Dives in Misericordia has been constructed with such self-cleaning concrete.
In fact, the titanium dioxide does more than clean the concrete it can also reduce the level of nitrogen oxide in the air, produced by cars, like a catalytic converter. Several studies have shown that this works, and open up the possibility that the buildings and roads in the city environment may not be purely passive in the future: they may purify the air much like plants. Now that the Shard is complete, all the concrete is hidden from sight, encased in more acceptable materials. But our ugly secret, and that of the Shard, is that concrete is literally the foundarion of our whole society: it is the basis of our cities, our roads, our bridges, our power stations – it is 50% of everything we make. But like bone we prefer it on the inside; when it sticks out we are repulsed. This may not be a permanent situation. Maybe it is just the end of the second wave of enthusiasm for concrete. The first was started by the Romans and ended for mysterious reasons. The new concrete that is coming along is more sophisticated and may yet reverse our tastes again, igniting a third wave of enthusiasm, this time for smart concrete with embedded bacteria that allow it to create living, breathing architecture, thus changing our relationship with this most fundamental of materials.
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