Marvellous Mutations: Adventures in Materials Science

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.

Reinventing the steel?

There have been advances in steel too, in its weight, strength, workability and resistance to corrosion. “Low-alloy steels, such as those made with a small amount of niobium, can be very strong,” says Mark O’Connor, director of building structures at WSP in London. “If an architect wants slim columns, they can be useful. Extra-strong rebar is useful too as it can be slimmer and fits better in congested areas.”

But such materials are hardly new, and stronger steel is not necessarily what engineers need, says O’Connor: “It’s not always going to help: sometimes area is part of the structural property you need. And although stronger steel is lighter and easier to handle on site, it does not help with building stiffness.”

In essence, structural steel’s range of properties has already been largely assimilated and deployed in building design, and the impact of any incremental improvements is not likely to have much effect on what buildings look like or how they are built.

Plywood on steroids

More revolutionary is a renewed interest in timber as a structural material, and particularly in engineered products such as cross-laminated timber. CLT was developed around 15 years ago, and although there have been small improvements in the machinery and glues used in the laminating process, the basic product remains the same.

“The difference now,” says Viktor Rönnblom, structural engineer and timber specialist in WSP’s Skellefteå office in northern Sweden, “is that we know much more about how it can be used and how it performs.” This also applies to other engineered timber products, he says, mainly glulam (glued laminated timber) and laminated veneer lumber, he adds.

CLT has been described as “plywood on steroids” and it is formed in a similar way, from alternately oriented sheets of timber. Glulam, used more for beams and columns, is again similar but the grain in the strips is aligned. LVL uses thinner, veneer cuts with occasional cross layers.

“Engineering timber in this way removes natural variations and makes its properties much more consistent and predictable,” explains Rönnblom. “And because we have had substantial buildings to monitor for over a decade now, we know how engineered timber structures perform in wind, in fire, and we know about their stiffness and their dynamic properties.” This is enabling the design of taller timber structures, although the tallest complete pure timber structure is still only 14 storeys.

Further increases in height will inevitably be incremental, says Rönnblom. “The way to go is to build, evaluate, and then maybe add five storeys to the next design, and so on. I don’t think you should jump from 15 to 40 storeys. The effects of any miscalculation could grow exponentially.”

In any case, it is likely that building regulations will stymie any sudden leap in the height of timber structures. Worldwide, these tend to forbid buildings higher than six or so storeys because of fears over how timber structures will perform in fire. Tallwood House in Vancouver, which has a 17-storey hybrid CLT structure, had to win special dispensation from British Columbia’s Building and Safety Standards Branch — and that was only granted after the designers agreed to enhanced fire and seismic standards that exceeded those for a concrete or steel building. This involved complete encapsulation of most of the CLT and glulam components with three or four layers of fire-rated Type X gypsum board.

In fact, says Rönnblom, fears over fire safety are more to do with perception than fact. “Put glulam on a fire and it is quite difficult to get it to burn out completely,” he says. “It is also very predictable, charring at 1mm per minute, with the strength of the timber behind largely unaffected. It makes it predictable, easy to calculate and in many ways it performs better than concrete or steel.”

Height is not everything. Rönnblom points out that engineered timber can play different roles in different buildings: “For example, you can use composite CLT-concrete structures. In that case, the CLT is both formwork and part of the load-bearing structure.”

One of timber’s most important qualities, Rönnblom says, is that it is light, so foundations can be smaller and therefore cheaper and quicker to lay. “You can also put timber buildings in places where concrete would be unsuitable — in soft ground, for example. Timber lends itself to prefabrication, and it is easy to handle on site, so you can build quickly and efficiently.”

There is one market in particular for which engineered timber would seem well placed, as WSP‘s Robert Kilgour points out: “The trend in big cities is towards densification — squeezing more and more accommodation onto land that is already built.” The lightweight nature of CLT makes it ideal for building extensions because it reduces the modifications you have to make to the existing structure.”

WSP is currently advising an Australian client on adding a ten-storey extension to a six-storey block, he adds. “We are looking at how the timber will perform structurally and what would have to be done to connect it effectively to the existing structure. In this sort of situation, the lightness of engineered timber, along with the potential for rapid construction, makes it an attractive option as cities continue to densify.”

Last but not least, Rönnblom says, people enjoy inhabiting timber buildings: “They feel nice, and have pleasant acoustics.” This makes them a desirable proposition for housing, and also for commercial projects where a growing body of research is linking the productivity of employees to their comfort and contentment with the working environment.

His last point is an important one, for no matter how technically advanced a new material is, it will struggle to catch on if clients do not like it, or if it does not sit well with contemporary architectural tastes. Engineered timber is attracting attention as much due to its perceived ecological credentials as its structural capabilities — even though the carbon used in the manufacturing process shouldn’t be overlooked. But no matter. Progress in materials science is never wasted, and an designer can never have too many options.