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Structures Concrete sprouts in Brussels

New Concrete EngineeringBrussels is in the middle of an office building boom - but most of the multistorey towers have precast concrete frames. Dave Parker finds out why.

With the accession of 10 new states into the European Union this year came an inevitable increase in demand for high quality office space in the EU's administrative centre.

In London such demand would have been satisfied mainly by steel framed structures with composite floors. Brussels is different.

Jean Tisseghem, managing director of specialist structural engineer Waterman TCA, says two factors have inclined developers and designers towards precast solutions. 'Height restrictions in Brussels are very tough, so we need to squeeze in as many floors as possible.

'And Belgian fire regulations have always favoured the concrete option.'

Waterman TCA is well-placed to understand Belgium's building market, having been formed in July last year following Waterman's acquisition of specialist Brussels-based structural consultancy TCA (see box).

'Buildings here are significantly cheaper than in the UK, ' adds Waterman International managing director Simon Harden. 'When the UK turned away from concrete and adopted steel frames as its preferred solution, Belgium kept developing precast technology until it could outperform steel frames.'

A lot of this development aimed to eliminate the Achilles' heel of precast concrete construction - connection details. In earlier days joint details were clumsy and difficult to complete properly on site. Beams and floor units had to be designed as simply supported elements, reducing structural efficiency. If precast concrete was to be competitive in the 21st century, some big technological steps had to be taken.

'A major step was the move to higher strength concretes for the columns, ' says Tisseghem.

'Going from C50/60 up to C85/90 cut down the steel content and made it much easier to form more efficient connections.

Then we started taking account of the diaphragm effect of the floor toppings - usually 50mm to 60mm thick when normally reinforced - to produce a more efficient design.'

Fair faced circular columns with integral corbels became something of a 'Brussels style' trademark. These were up to three storeys high, speeding construction. Pretensioned prestressed factory produced beams and floors were economical and effective - post-tensioned elements may be more structurally efficient in theory, but in most applications, pretensioned elements have the edge in terms of price and speed of erection.

Really efficient and easy to assemble joints had to wait until the advent of self compacting concrete (SCC) and non-shrink grouts. High strength SCC reduced the congestion of steel reinforcement at corbels, nonshrink grouts locked column sections together with the minimum of fuss (see diagram).

However, as in the UK, the number of precast concrete producers capable of working with high strength SCC and producing long columns is limited.

Ergon, based between Antwerp and Brussels on the bank of the Nete Canal, is one that can and has been producing high technology precast concrete since 1963.

Ergon's Paul de Meyst says the biggest problem trying to improve precast concrete technology was getting the market to accept significantly higher strength concretes.

'It took us six months to persuade consultants of the advantages of moving from C60 to C80, ' he adds. 'Now we're comfortable working with C90/105, which we can achieve with our normal limestone aggregates and without additions like microsilica.'

Ergon produces all the normal range of precast concrete elements, including hollowcore flooring up to 400mm deep. But it is its investment in SCC technology and the complex moulds needed to produce fair faced corbelled multistorey height circular columns that mark it out from many of its competitors.

It also has to satisfy customer worries about the legendary explosive spalling of very high strength concretes in fires - which it does by adding a percentage of polypropylene fibres to the mix.

And right now Ergon and its customers are seeing a special benefit from the use of high strength SCC. Steel content is much lower, so the effect of rocketing world steel prices on frame price is significantly less.

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