More than 70 per cent of the world's population is expected to be living in cities by 2050, raising the prospect of an unprecedented, and environmentally damaging, building boom. Sustainable construction, which harnesses technologies such as 3D printing, self-repairing biological concrete and recycled plastics, could make urbanisation far less threatening.
Building for billions without destroying the planet
Building for billions without destroying the planet
A time-lapse video chronicling Shanghai’s metamorphosis from decaying city into global financial hub shows a skyline transformed beyond recognition.
Now an internet sensation, the film captures in graphic detail how the dilapidated low rise residences that once huddled along and around the Yangtze river have been supplanted by more than 200 of the world’s tallest skyscrapers. Other Chinese cities have been through a similar experience. Indeed, the statistics of China’s evolution from agrarian economy into urban powerhouse as are dizzying as Shanghai’s imposing edifices. As Professor Vaclav Smil explains in his recent book ‘Making the Modern World: Materials and Dematerialisation’, China used 50 per cent more concrete between 2011 and 2013 than the US did over the entire 20th century. That is 6.6 gigatons.
China used 50 per cent more concrete between 2011 and 2013 than the US did over the entire 20th century.
Urbanisation is not just a Chinese phenomenon. According to the United Nations’ Department of Economic and Social Affairs, for the first time in human history, more than half of the global population now lives in urban areas. By 2050, the proportion is expected to rise to 70 per cent.
This demographic shift will be accompanied by an unprecedented building boom. A report by Global Construction Perspectives and Oxford Economics estimates that, in the next decade alone, world construction output will grow more than 70 per cent.
There is an irony to all this. Urbanisation could potentially mark a dynamic new phase in the evolution of human society. Yet, badly managed, it might turn out to stall our progress.
“Megacities, cities and urban areas are at the heart of economic, social and environmental processes that impinge on sustainability,” a United Nations report warns. “Achieving sustainable development will therefore require well-planned urbanisation, taking into account the growth of cities of all sizes.”
A big problem is that producing building materials in such huge volumes places a massive strain on the earth’s natural resources. Take cement, that most mundane yet ubiquitous of building aggregates.
It is the key ingredient of concrete, the single most widely used material in the world after water. It is estimated that three tons of concrete is produced for every person living on the planet.
Production is rising at 2.5 per cent per year, which means the world’s consumption of concrete could climb above 4 billion tonnes by 2050.
From a certain perspective, concrete has impressive sustainability credentials. It can absorb heat, storing it before emitting it. This means it can be used as a climate modifier, keeping out the cold in winter, and regulating heat in summer.
However, the production process is responsible for between 5-10 per cent of all the world’s annual greenhouse gas emissions. To turn limestone into cement, kilns need to be fired up to a temperature of 1500 degrees Celsius. That requires some 4.7 million BTUs of energy per tonne, the equivalent of 400 pounds of coal. So for every tonne of cement produced, almost as much CO2 is emitted into the atmosphere.
Furthermore, much of the concrete used in construction is reinforced with steel, whose environmental credentials are also poor. Not only does steel require the extraction of iron ore and the use of fossil fuels in blast furnaces but its production has damaging side-effects such as water contamination and air pollution.
One way of making concrete more environmentally friendly is to replace the steel meshes that reinforce it with something more sustainable. A team of scientists at James Cook University (JCU) in Australia, for example, has developed a type of concrete reinforced with plastic waste.
“Using recycled plastic, we were able to get more than a 90 per cent saving on CO2 emissions and fossil fuel usage compared to using the traditional steel mesh reinforcing,” says Rabin Tuladhar, the lead researcher from JCU. “The recycled plastic also has obvious environmental advantages over using virgin plastic fibres.”
The concrete was reinforced using recycled polypropylene plastic instead, and strength and durability tests show that the end result could be used to build footpaths and precast structures such as drainage pits and concrete sleepers.
Tuladhar and his team are also working on making concrete more sustainable in other ways. One option they are investigating is the replacement of natural sand with so-called ‘crusher dust’, a by-product of stone quarries, and incorporating mining waste into cement production.
Self-repairing biological concrete
Another problem with concrete is its tendency to degrade and crack, which means it must be regularly patched up.
Scientists are addressing this shortcoming by trying to develop self-repairing “biological concrete”, which involves infusing the material with bacteria. When the concrete is dry, the bacteria are dormant. But when water seeps in, the bacteria activate and multiply, producing a limestone-like substance that fills in the damage. In short – self-healing concrete with organic qualities.
Meanwhile, Swedish construction conglomerate Skanska has teamed up with Loughborough University in the UK to develop the use of 3D printing in construction. Under this process, successive layers of concrete are placed on top of one another to create an object which could be a complex structural component. The scientists involved say that, in theory at least, 3D printing could eventually be used to construct an entire building.
“3D concrete printing … has the potential to reduce the time needed to create complex elements of buildings from weeks to hours,” says Rob Francis, Skanska’s director of innovation and business improvement.
Much of the innovation in boosting the efficiency and sustainability of construction is still at the developmental lab-based stage. But mitigating the environmental impact of urbanisation and the corresponding global construction boom is about more than using fewer unsustainable materials.
Much can be achieved through intelligent building design.
A conventional skyscraper, for example, is hugely damaging to the environment both in its construction and subsequent operation. It is often designed with no natural ventilation, making it dependent on air-conditioning that has to run constantly – with all the pollution and energy use that this entails. Recent designs have sought to incorporate greater sustainability through the use of wind turbines, solar panels and rainwater recycling mechanisms.
In 2009 the Taipei 101 skyscraper became the first tall building to obtain the top level of certification under the LEED (Leadership in Energy and Environmental Design) standard – an international certification system. It achieved this largely through improvements in the efficiency of its air conditioning systems.
A new generation of building could be far more ambitious, as illustrated by some of the projects currently being proposed by engineers and designers. One, designed by Luca Curci Architects, envisages a “vertical city” that “combines sustainability with population density”.
The proposed structure would be located in the sea, linked to the mainland via a bridge but also accessible by helicopter and boat. At 750 metres high and housing up to 25,000 people, each of its 180 floors would have a garden-like “green zone” acting as communal space and providing natural ventilation while a photovoltaic glass membrane coating the entire building would provide electricity. At the top would be a further garden square.
Ambitious? Perhaps. But, as with other attempts at making construction more sustainable, possibly within reach.