No picnic for the sandwich

It was back in 1997 when British Steel (now Corus), having spent £5m and five years on Bi-Steel's development, launched its new material. Bi-Steel's product marketing manager at the time was hopeful that it would soon be replacing stiffened steel plates or heavily reinforced concrete on a variety of structures.

Since then it has been used in specialist defence projects, one bridge-strengthening at a Corus works and a building in the city of London, which is the proud possessor of a Bi-Steel blast-resistant basement. But despite the claims of shorter build times and reduced risks, no one has used it for mainstream building.

The problem has been cost. "It has got to be competitive with reinforced concrete," says Corus' European commerical manager for Bi-Steel, Jurek Tolloczko. "Whatever the benefits, it has got to be competitive with the bottom line."

So what has changed? The development which Tolloczko claims has turned Bi-Steel from specialist blast-resister to sure and fast commercial building component is the way it is fixed together. Formerly, the panels had to be welded together to hold them in place until the concrete was poured in, costing time and money. Now a new connection system (see diagram) allows the massive panels to be slotted into one another and locked in minutes.

Corus claims that the reduced time on site can make it competitive with reinforced concrete. It says a US contractor carried out an on-paper cost comparison between Surefast and reinforced concrete for a 10-storey building core and found that a staggering three months could be shaved off the build programme.

SSimplicity works
Steel-concrete-steel sandwiches have been around for a long time (see timeline). This form of construction was probably first considered for parts of the Mulberry Harbours, towed across the channel for the Normandy landing at the end of World War II. But a shortage of steel meant that reinforced concrete was used.

When the idea for Bi-Steel popped into researcher Hugh Bowerman's head in 1992, colleagues at the Steel Construction Institute (SCI) were well-advanced on a project looking at double skin steel-concrete panels with studs welded onto the inside surfaces of the steel.

They published design guidance for this form of panel in 1995, but it was never to be manufactured commercially. The main problem was that the hydrostatic pressure of the concrete tends to force the skins apart so that ways of holding them together are needed, adding to the complexity, speed and cost of construction.

There had been great hopes that it would be used for the Conwy Crossing in North Wales, an immersed tube tunnel structure where the tunnel segments are built on-shore, floated out and then sunk into position. The design-build contractors rejected it because of the hydrostatic pressure and the fact that it was unproven.

Bowerman's brainwave was to connect the two parallel steel plates with bars, welded at each end. Bi-Steel seemed to be one of those ideas which are so straightforward that it seems bizarre no-one else has come up with it.

British Steel (Corus), which sponsored Bowerman's research, was initially interested in the application of steel-concrete-steel composite panels for offshore structures. However, to date Bi-Steel has not been used for any offshore structures. Bowerman, now Corus' Bi-Steel technical manager, says this is because defence took over as the primary market when its blast resistant properties became apparent.

Whatever the benefits, it has got to be cost competitive

Jurek Tolloczko, Corus

Bi-Steel has been used for both military and civilian test facilities and blast walls, although Corus is not allowed to talk about the specifics of the latter. Its superiority to reinforced concrete under such conditions is mainly due to the fact that when a blast force hits the front side of the wall, nothing flies off the back. With reinforced concrete this is not the case.

Bridge-strengthening, for which use Corus was pushing Bi-Steel heavily in 2000, has never taken off due to design regulations for such works. Structural engineers will not spend time designing with a new product if it is not approved; the authorities will not consider and review a new product until they are presented with a design incorporating it. Catch 22.

Sure thing?
This time, however, there are signs that Bowerman and Tolloczko, who was also involved in the original research, are making progress. The industry is seriously considering Surefast.

Tolloczko is pushing Surefast for the core of three upcoming buildings with three contractors. One is Mace, which confirmed that it is comparing Surefast with traditional methods for one specific project.

A steelwork contractor for one of the buildings, who does not wish to be named for commercial reasons, said that the use of Surefast would cut costs, as the connections could simply be welded onto the outside of the Surefast core, rather than breaking through the concrete to fasten to the main rebar.

The additional benefit for a steelwork specialist – and a disadvantage for the structural concrete contractors – is that the steel firm will be installing the Surefast panels, an extra package of work.

But there is a concrete contender to Surefast. Structural concrete specialist John Doyle has used a precast concrete system, developed in Germany, on a housing scheme in Battersea.

The two precast concrete skins are connected by lattice girders; connections for the steel frame can be cast in at the factory. Technical director Peter Goring believes this system will be more cost-effective than Surefast. What it doesn't have, he admits, is the blast resistance.

Goring is sceptical about the claim's of three month time-savings for a 10-storey building. He explains that concrete cores are generally constructed six storeys ahead of a steel frame. With one storey of steel taking around a week to erect the start of steel erection lags six weeks behind the start of the core. Erecting a prefabricated system core reduces that lag; he reckons three weeks would be more realistic.

What with a cheaper concrete competitor, and a launch which coincides with virtually all major commercial office development in the UK ceasing, could this be yet another non-event for steel-concrete-steel panels? And with its looming job cuts and rock-bottom share price Corus isn't in a position to subsidise Surefast.

Perhaps contractors and designers will not be buying the blast-resistance qualities in the UK, except for perhaps the most sensitive of cases. But Tolloczko says that there is interest in the US.