Building Stronger Concrete

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-By John Boley

Sea urchins are common enough in all oceans of the world and particularly plentiful on the many reefs around the shores of Australia. They are strange and rather unambitious animals whose idea of a good time is to lie half-hidden in sand at the bottom of the sea and sleep during daylight. In some parts of the world – the Mediterranean is an example – they form the basis of culinary recipes as diverse as Sea Urchin Bruschetta and Sea Urchin Risotto.

At night they feed, but you would not call them gregarious. You won’t get anything out of them beyond a wave of their spines. If something sharp touches its shell, a sea urchin will point all of its spines towards the area being poked. Sensitive to light, which prompts them to be nocturnal, they move their spines in reaction to shadows. Like starfish, they have a certain regenerative ability – if a spine is damaged or lost, a sea urchin can re-build it.

Which brings us to the reason why sea urchins are being examined very closely by scientists looking for more effective ways to put buildings together and make them more resistant to outside forces such as earthquakes. It appears these spines, already known to be made from calcium carbonate, are made (‘grown’ would be more strictly accurate but in this case we are interested less in biology and more in chemistry), not as previously thought, of single crystals, but of what might be called bricks of calcite – another crystal – which are cemented together with a non-crystalline mortar. The result is a chemical structure which is enormously strong, hard – and with amazing properties of shock absorption.

This work was written up recently by a team of scientists from Konstanz university in Germany, led by Helmut Coelfen, and published in Proceedings of the National Academy of Sciences (PNAS March 6, 2012 vol. 109 no. 10 3699-3704).

Other scientists are looking at a more conventional method of making concrete stronger – using the usual sand and cement but adding pure powdered quartz and a variety of reinforcing fibres or metals. The result is UHPC – ultra high performance concrete and some forms are already in use – one French product, Ductal, is an example. As The Economist reported recently, UHPC can not only withstand far higher pressure than conventional concrete but is also more durable and flexible. “It can therefore be used to make lighter and more slender structures” or, going the other way, can be used in conventional portions to build stronger structures.

The interest of The Economist was less in earthquake-proofing office blocks or apartments and more in whether buildings constructed using UHPC might withstand so-called ‘bunker-busting bombs’. The report centred on a call for the Pentagon to upgrade the power of the United States’ Massive Ordnance Penetrator MOP) if it is to be effective against – for example – the best of civil engineering in Iran.

“Iranian civil engineers are interested in using [UHPC] in structures as diverse as dams and sewage pipes and are working on improving it. Mahmoud Nili of Bu-Ali Sina University in Hamadan for example, is using polypropylene fibres and quartz flour, known as fume, in his mix. It has the flexibility to absorb far heavier blows than regular concrete.” Another Iranian expert is currently engaged at Ottawa University in Canada, working on the molecular structure of cement. But “this research is for civilian purposes and could pave the way for a new generation of UHPC with precisely engineered properties and outstanding performance.”

Similar thoughts have occupied the German team studying the sea urchin’s spines. Professor Coelfen told the BBC: “The most obvious application… is building materials, to get fracture-resistant materials by just copying or trying to copy that building principle. We are already working with two major international companies trying to improve the properties of concrete by trying to order the little nanoparticles in concrete to make it tougher and more fracture-resistant.”

The sea urchin spine “can be seen either as a composite material in which disordered inclusions are embedded in a single crystal matrix, or as an assembly of crystalline nanoparticle domains which are separated by disordered layers and which exhibit a high degree of crystallographic registration. The latter structure is described as ‘mesocrystalline’, where a mesocrystal ideally comprises a 3D array of iso-oriented single crystal particles of size 1–1,000 nanometres.” The Konstanz team seems to have settled on the latter view after the most intense X-ray scrutiny.

The fracture properties of the spine and the way in which the animal naturally repairs it after damage are covered by two separate but interlinked phases. The two-phase structure is a “mechanically optimised composite”, in very basic terms calcite nanoparticles coated with a layer of amorphous calcium carbonate which with macromolecules “modulates the mechanical properties of the hard, but brittle calcite. The nanoparticulate structure also enhances the mechanical properties in that fracture is driven to follow a tortuous path, resulting in the observed chonchoidal fracture surface. The larger, facetted, micron-sized regions observed on the fracture surface correspond to larger single crystal particles within the spine, which can arise due to the non-uniform distribution of macromolecules in the ACC precursor phase” [from the PNAS report].

The PNAS report concludes that: “Structuring over many length scales is a design strategy widely used in Nature to create materials with unique functional properties… Nature fabricates a material which diffracts as a single crystal of calcite and yet fractures as a glassy material… Nature’s demonstration of how crystallisation of an amorphous precursor phase can create a crystalline material with remarkable properties therefore provides inspiration for a novel approach to the design and synthesis of synthetic composite materials.”

Those wishing to build something that can destroy stronger buildings once constructed carried out tests at Woomera some six to eight years ago. The tests “involved a charge equivalent to six tonnes of TNT. This fractured panels made of UHPC, but did not shatter them. Nor did it shake free and throw out fragments, as would have happened had the test been carried out on normal concrete.” The MOP was planned to be able to break through some 60 metres of traditional concrete and has been shown to break through only eight metres of concrete which is double the conventional strength. The military is now worried that it may not work at all if you were to point it at UHPC.

Iranian scientists at Islamic Azad University in Saveh have published several papers on how to make use of nanoparticles to “tamper with the internal structure of concrete” and have experimented with a variety of types of metal-oxide nanoparticles. They have worked with oxides of iron, aluminium, zirconium, titanium and copper. “At the nanoscale materials can take on extraordinary properties. Although it has been demonstrated only in small samples, it might be possible, using such nanoparticles, to produce concrete that is four times stronger than Ductal.”

So whether you want to build it stronger or knock it down harder, nanoparticles appear to be the way forward. The results could include buildings that are lighter, smaller and faster to build – though not necessarily cheaper – as well as critical structures – dams, say, or nuclear power plants that have a far better chance of surviving major impacts. It could soon be time for the humble sea urchin to become famous, thanks to its extraordinary spines.

Home Automation

Call it ‘domotics,’ and you are likely to receive a blank stare, but refer to it as ‘smart home’ or ‘home automation,’ and you will get a nod of acknowledgement. For the past few years, consumers have heard the word ‘smart’ attached to countless products and services, from food and drink to snacks like popcorn and mobile phones, which no one seems to refer to as a ‘cellphone’ anymore. Yet what, exactly, constitutes ‘smart’?

July 14, 2020, 7:33 AM AEST