Words by Tony Whitehead
The materials scientist is the designer’s friend. These questing chemists, using ever more sophisticated techniques, produce a steady flow of new and improved substances with which to create the built environment.
But despite their best efforts, game-changing breakthroughs in materials are rare. Brick, stone and timber have been used in construction for at least 7,000 years. Kiln-fired bricks have been around for 4,000 years; concrete since Roman times. Even the most recent quantum leap — the advent of structural steel — occurred more than a century ago.
So the accelerating urbanization of the world, and the spectacular architecture mushrooming across all inhabited continents, has been achieved using only these basic elements. And improvements in building safety, usability and durability, as well as speed, efficiency and economy have, like the height of skyscrapers, been made possible not by astonishing leaps in materials science, but by incremental advances, subtle adjustments and improved understanding.
The psi factor
Only 15 years ago, 8,000psi concrete was considered high strength. Now most tall projects in the US use concrete of between 10,000 to 14,000psi for at least part of their structure, thanks to the development of reliable water-reducing admixtures. The effect on the skyline has been dramatic.
The tallest building in the US, One World Trade Center, completed in 2013, used a 14,000psi mix to facilitate a less bulky core. As a result, the building has more lettable space, there is a lighter gravitational load and, therefore, less concrete was required for the foundations. And because a significant amount of carbon is used and released in cement manufacture, reducing the amount of concrete meant a smaller carbon footprint overall.
High-strength concrete mixes like this have also played an enabling role in the recent trend for super-skinny residential skyscrapers. Several in the US, Dubai and the Far East are over 1,000ft (305m) high, their slender designs offering spectacular views, along with impressive footprint-to-floorspace ratios.
A lot of benefits, then, from just a tweak to the mix, and a huge impact environmentally and architecturally. But talk to experts in this field and it quickly becomes apparent that there are plenty more developments in the pipeline.
It is now possible, for example, to specify concrete that heals its own cracks by means of limestone-attracting bacteria; concrete that uses magnesium oxide to absorb carbon dioxide from the atmosphere; and even concrete that glows in the dark. Time will tell which of these reaches the mainstream, but one variation is already attracting serious attention: geopolymer concrete.
“The desire for lower carbon alternatives to traditional materials is a real driver in the market,” says Robert Kilgour, principal engineer in materials technology for WSP in Perth, Western Australia. “Geopolymer concrete is not exactly new, but it’s only in the past three years that it has been made in commercial quantities. I think we’ll be seeing a lot more of it in the near future.”
The key benefit of geopolymer concrete is that it does not contain any Portland cement at all, and therefore has a much lower carbon footprint than traditional concrete. Its availability in Australia has seen it specified for a range of applications, though its adoption is as yet relatively limited.
But geopolymer alone will not be sufficient to address the global construction sector’s heavy carbon usage. “On average every person on the planet consumes roughly 2m3 of concrete every year, so it is vital we do something to limit the environmental cost,” says Franz-Josef Ulm, faculty director of the Concrete Sustainability Hub at the Massachusetts Institute of Technology (MIT). “Geopolymer has a part to play, but there is simply not enough of it around to replace that huge dependence on cement. Fortunately, we are developing ways of using cement more efficiently — of making it work harder.”
In most concrete, explains Ulm, the carbon-expensive calcium content is not fully utilized. By nano-engineering the cement, with the addition of silica fume (and other industrial waste products), the calcium can be more comprehensively activated, making the cement much stronger. “The same approach was taken to create the Gorilla Glass in an iPhone,” he says. “By putting calcium and silica at exactly the positions where we need them, much more of the calcium contributes to the strength and durability of the cement and, therefore, the concrete. And if the concrete is twice as strong you have the potential to use half as much, and decrease the carbon footprint by up to 50%.”
Nano-engineered concrete is certainly strong enough to change the way large structures are designed, Ulm says. “Ordinary concrete is measured at 30 megapascals of strength, and the high-strength concrete used in major civil engineering projects such as the Channel Tunnel is 80MPa. This nano-engineered stuff is 200MPa (roughly 29,000psi). It has the strength of mild steel, flows like honey and hardens at room temperature. Add fibres and you can even do away with reinforcement.”
It sounds amazing, but at the moment few plants can produce it and use has been limited to one-off designs and a small number of bridges, some of which Ulm has helped design. It has not yet been used for skyscrapers. “The product design is ready, but to make it widely available in commercial qualities would require investment in plant,” says Ulm, “A carbon tax on concrete, for example, would immediately make it viable for producers to do that.”
The almost endless adjustments that can be made to concrete will provide many more options for designers over the next few years. Ulm says that an improved understanding of concrete’s rheology — the way it flows and sets — will open the door to specialist concretes for more effective 3D printing, or even the ability to extrude columns from moving forms.
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