2012年3月1日 星期四

Smart concrete: “ultra-high performance concrete” (UHPC).


Smart concrete

Iran makes some of the world’s toughest concrete. It can cope with earthquakes and, perhaps, bunker-busting bombs

A DUAL-USE technology is one that has both civilian and military applications. Enriching uranium is a good example. A country may legitimately do so to fuel power stations. Or it may do so illegitimately to arm undeclared nuclear weapons. Few, however, would think of concrete as a dual-use technology. But it can be. And one country—as it happens, one that is very interested in enriching uranium—is also good at making what is known as “ultra-high performance concrete” (UHPC).

Iran is an earthquake zone, so its engineers have developed some of the toughest building materials in the world. Such materials could also be used to protect hidden nuclear installations from the artificial equivalent of small earthquakes, namely bunker-busting bombs.

To a man with a hammer…

Leon Panetta, America’s defence secretary, seems worried. He recently admitted that his own country’s new bunker-busting bomb, the Massive Ordnance Penetrator (MOP, pictured above being dropped from a B-52), needs an upgrade to take on the deepest Iranian bunkers. But even that may not be enough, thanks to Iran’s mastery of smart concrete.

UHPC is based—like its quotidian cousins—on sand and cement. In addition, though, it is doped with powdered quartz (the pure stuff, rather than the tainted variety that makes up most sand) and various reinforcing metals and fibres.

UHPC can withstand more compression than other forms of concrete. Ductal, a French version of the material which is commercially available, can withstand pressure many times higher than normal concrete can. UHPC is also more flexible and durable than conventional concrete. It can therefore be used to make lighter and more slender structures.

For this reason, Iranian civil engineers are interested in using it 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. Rouhollah Alizadeh of the University of Tehran may do better still. Dr Alizadeh, a graduate of the University of Tehran, is currently working at Ottawa University in Canada on the molecular structure of cement. That could pave the way for a new generation of UHPC with precisely engineered properties and outstanding performance.

One way to tamper with the internal structure of concrete is to use nanoparticles. Ali Nazari and his colleagues at Islamic Azad University in Saveh have published several papers on how to do that with different 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.

All of which is fine and dandy for safer dams and better sewers, which threaten no one. But UHPC’s potential military applications are more intriguing—and for many, more worrying. A study published by the University of Tehran in 2008 looked at the ability of UHPC to withstand the impact of steel projectiles. These are not normally a problem during earthquakes. This study found that concrete which contained a high proportion of long steel fibres in its structure worked best. Another study, published back in 1995, showed that although the compressive strength of concrete was enhanced only slightly by the addition of polymer fibres, its impact resistance improved sevenfold.

Western countries, too, have been looking at the military uses of UHPC. An Australian study carried out between 2004 and 2006 confirmed that UHPC resists blasts as well as direct hits. The tests, carried out at Woomera (once the British empire’s equivalent of Cape Canaveral), 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. In a military context, such shards flying around inside a bunker are a definite plus from the attackers’ point of view, but obviously not from the defenders’.

Those people who design bunker-busters no doubt understand these points and have their own secret data to work with. Nevertheless, during the Gulf war in 1991 the American air force found that its 2,000lb (about a tonne) bunker-busters were incapable of piercing some Iraqi bunkers. The bomb designers went back to the drawing board and after two generations of development the result, all 13 tonnes of it, is the MOP. So heavy is it that the weapon bays of B-2 stealth bombers have had to be strengthened to carry it. It can, reportedly, break through over 60 metres of ordinary concrete. However, the bomb it is less effective against harder stuff, penetrating only eight metres into concrete that is just twice as strong. It is therefore anyone’s guess (at least, anyone without access to classified information) how the MOP might perform against one of Iran’s ultra-strong concretes.

America’s Defence Threat Reduction Agency (DTRA), the organisation that developed the MOP, has been investigating UHPC since 2008. This investigation has involved computer modelling and penetration testing. The agency’s focus appears to be on the idea of chipping away at a target with multiple hits. However, this approach requires great precision; and the air force is ordering only 20 MOPs, so there is little room for error.

Deep bunkers can be tackled in other ways. The DTRA has looked at what is known in the jargon as functional defeat, in other words bombing their entrances shut or destroying their electrical systems with electromagnetic pulses. They are also working on active penetrators—bombs which can tunnel through hundreds of metres of earth, rock and concrete. Development work is also under way on esoteric devices such as robot snakes, carrying warheads, which can infiltrate via air ducts and cable runs.

In the meantime, though, the Pentagon is stuck with the “big hammer” approach. The question is how reliably that hammer would work if the order were given to attack Iran’s underground nuclear facilities. It would be embarrassing if the bunkers were still intact when the smoke cleared.