After 50 years, the dream of a new generation of high performance materials based on a very old technology has finally produced practical ultra high strength concretes. David Bennett reports.
Spanning 120m across the River Han in Seoul, South Korea, the Footbridge of Peace is just one of the many infrastructure projects spawned by this summer's World Cup. With a 4.3m wide, 1.3m deep box girder deck the crossing is deceptively innovative. Its dimensions and wall thickness would be unremarkable in steel - but in fact the Footbridge of Peace marks the first major use of a Portland Cement-based material with a characteristic compressive strength of up to 800N/mm 2.Back in the 1950s, when the idea of a cement based alternative to steel was first conceived, structural concretes were rarely specified much above a minimum compressive strength of 25N/mm 2or so. But, with postwar shortages of other high strength materials still fresh in the memory and cement relatively cheap and plentiful, the attraction was obvious.
Two decades of research produced the first real breakthrough By then the focus was not so much on developing an alternative to steel as on producing a high performance inorganic alternative to organic polymers and certain metals such as aluminium. A Runcorn-based Imperial Chemical Industries team lead by Derek Birchall were spurred on by the growing emphasis on non-combustibility of raw materials, the rise in cost of hydrocarbons and the high energy cost of production of organic plastics.
The major attraction of cement was simply the energy cost savings in manufacture. To produce one cubic metre of cement, organic polymer or aluminium requires 10, 100 and 1000 GJoules respectively of energy.
Clearly cement has an advantage in energy saving over aluminium and organic polymers because it hydrates with the addition of water under normal air temperatures. Cement's big drawback, however, was its low tensile and bending strength, and low fracture toughness compared to plastics and metals.
These flaws are largely down to macro defects - the large amount of voids in the material caused by air entrapped during mixing, and in the pores and capillaries formed within the material, when water is desiccated during cement hydration.
Removal of these large flaws from the cement paste was possible by better particle packing in the wet and dry state. The ICI researchers found that by introducing a small proportion of water soluble polymers into the cement and water mix, inter-particle friction and surface tension was greatly reduced. This allowed particles to pack more closely and significantly increase the tensile strength.
Using such techniques a macro defect free (MDF) cement polymer paste was created by mixing it virtually earth dry under high shear and turning it into a dough-like putty. The deformable dough was then rolled to eliminate entrapped air bubbles and then moulded into the desired shapes by extrusion, pressing or other conventional plastic pressing operations.
Small residual voids that remain are filled by the chemical products of hydration which contract or collapse by desiccation. In this way it proved possible to reduce the voids in MDF cement to less than 0.1%.
Performance of MDF cement paste compared favourably with aluminium and exceed the properties of ordinary concrete by significant margins. If MDF cement is reinforced by fibres such as glass, carbon or Kevlar, a composite material can be formed having a fracture toughness exceeding that of aluminium.
Processes used in fabricating products from MDF cement are quite unlike those used in conventional concrete technology.
They are allied to organic polymer technology as MDF products have to be extruded or injection moulded. Utility and luxury articles in MDF cement have been made that would usually be formed in metal or reinforced plastic, for example in door handles , a guitar and even a suspension spring for a car.
ICI eventually sold the MDF licence to a Japanese conglomerate in 1984 which, after some years of further development work, were unsuccessful in its attempts to market a new loudspeaker cabinet made with MDF and abandoned the technology.
However, the flame still burned, the search for ultra high strength concrete had not been abandoned. Inspired by the pioneering work of Birchall and his team, in 1981 French researchers conceived dense silica particle cement (DSP).
In practice DSP is quite different to MDF, since the cement paste is workable and can be cast like ordinary concrete. High strength is achieved by controlling the particle- size distribution of the cement to minimise the void spaces between cement grains. Bache added condensed silica fume - otherwise known as microsilica - to the mix and eliminated the coarse aggregates. A by-product of the manufacture of silicon metals and its alloys, microsilica is made up of very small (particle size 0.1um) spheres of reactive silica to fill the voids between the cement particles.
Microsilica has the added advantage of reacting chemically with the cement paste to become an integral part of the matrix. A dispersion surfactant (superplasticiser) is also necessary to achieve workability so tha DSP composite cement when mixed with the minimum amount of water can be poured into forms and moulds. The properties of DSP are comparable with MDF in compression, but can be more brittle without steel fibre being added, and therefore can have a lower tensile strength.
In the last decade of the 20th Century came the ultimate -for the moment - pinnacle of the DSP concept. Ductal was developed jointly by Bouyges and Lafarge to achieve high ultimate load carrying capacity and enhanced durability. By eliminating the coarse aggregate fraction, incorporating steel fibres, applying pressure to the high strength mix before and after setting, and then heating the hardened cement for up three days at 90 0C; very high compressive strengths are obtained, ranging between 200N/mm 2and 800N/mm 2. For site applications Ductal is mixed in a conventional batching plant and placed in formwork without the application of pressure. Two days of steam curing at 90 0C follows. The concrete exhibits very little sensitivity to creep and shrinkage. Moderate shrinkage is observed during the heat treatment, but thereafter there is absolutely no further shrinkage movement. It is evident that Ductal when prestressed, closely matches steel for economy of section depth and mass, and reduces the self weight of conventional prestressed concrete beams by a considerable margin.
The first major structure to use 200N/mm 2Ductal was a 60m span truss footbridge in Sherbrooke, Canada in 1995. The 30mm thick by 3.3m wide walkway slab acts as the stiffening top chord of the 3m deep truss. The struts and tension ties that connect the top and bottom chord members, are thin walled stainless steel tubes filled with Ductal Confining the concrete in external tubing enhances the characteristic strength to 350 2. The bottom chord comprise twin Ductal beams 320mm wide by 380mm deep. External post tensioning tendons placed longitudinally, pass through the bottom chord beams while the 30mm top slab is transversely prestressed. No rebar was used for any part of the structure, not even for distribution steel.
One day, the experts believe, structures like the Sherbrooke crossing and the Footbridge of Peace will seem tame and positively conservative. One day, they say, we will see concrete reinforced structures with MDF reinforcing bars and slender I beams the size of steel sections being cast on site with DSP concrete.
Fantasy? And will we have to wait until the next Jubilee to see them?