Innovation: Secrets of a sandwich: Concrete wrapped in steel is a tough option

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CONCRETE structures may soon be reinforced with steel on the outside rather than the inside. Engineers at the University of Wales in Cardiff are developing steel-concrete sandwiches which will provide greater impermeability and higher impact resistance for tunnels, offshore oil platforms and tall buildings.

Instead of casting concrete with internal reinforcement, a layer of unreinforced concrete is cast as the filling in a sandwich formed by two layers of thin steel plate, which are fastened to the concrete by welded stud connectors.

'Not only do the steel skins help to form the cast section, they also provide the primary reinforcement for the concrete and protect it by being both impermeable and resistant to impact,' explains Terry Roberts of the university's School of Engineering.

The welded stud connectors between the steel plates transfer the normal and shearing forces between the concrete and the steel plates, and also provide reinforcement against transverse shear stresses.

The steel sandwich concept was originally proposed by two firms of consulting engineers, the Tomlinson Partnership and Sir Alexander Gibb and Partners, for constructing the tunnel which carries the A55 coast route under the Conwy Estuary in North Wales. This tunnel, opened in 1991, was the first to be built in Britain using the immersed tube technique.

The tunnel sections were cast in a basin close to the site, and the basin was then flooded, allowing the sections to be floated into position and sunk into a trench dredged across the estuary.

In the event, the tunnel sections were made from conventional reinforced concrete. However, development and testing of the steel sandwich has continued at the university with funding from the Steel Construction Institute (an industry organisation which promotes new applications for steel), and has demonstrated several advantages of the technique.

Conventional reinforced concrete is cast into formwork - the mould which contains it while it is setting. With steel- concrete sandwiches the steel acts as the mould. Since a tunnel section typically measures 100m long by 25m wide and 10m high, producing the formwork is far from trivial. The steel provides a semi-permeable barrier, protecting the concrete once the tunnel section is buried.

'On a motorway bridge, steel would deterioriate faster than concrete because oxygen in the air makes it rust. When it is buried, the steel is in anaerobic conditions and protects the concrete from moisture,' Dr Roberts says (stainless steel is too expensive for such large-scale applications).

Although steel deteriorates faster than concrete when exposed to the atmosphere, the steel-concrete sandwich does provide greater impact resistance than conventional concrete. If necessary, the steel can be thickened, providing a 'sacrificial layer' that can be allowed to rust without affecting the strength of the structure. In the US, some concrete-built bridges have been encased in steel to strengthen them. The air holes between the bridge and its new steel coat are filled by high-pressure grouting, which has the added advantage of putting a confining pressure on the concrete, making it even more impact-resistant. The researchers at Cardiff have tested a number of components made from steel-concrete sandwiches, ranging from simple beams to complete one-fifth-scale tunnel sections. A range of theoretical models have been developed for predicting the strength and stiffness of structures designed using this reinforcement technique.

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