Euro concrete takes to the skies

Apart from height restrictions, Belgian fire regulations have always leaned towards total concrete solutions. Building costs in Belgium are significantly cheaper than the UK. Whilst the UK turned away from concrete and adopted steel frame construction as its preferred solution the Belgians aimed to eliminate the Achilles' heel of precast concrete construction, namely, construction joints. In the past, joint details were clumsy and caused problems for on-site fabrication.

Technological breakthrough

Jean Tisseghem, Managing Director of specialist structural engineering firm Waterman-TCA says that a major technological breakthrough was necessary. "When higher strength concrete for columns went up from C50/60 to that of C85/90 the steel content was cut and it made it easier to form more efficient connections. It was from this starting point that we took into account the diaphragm effect of the floor toppings, usually 50-60mm thick when normally reinforced, to produce a more efficient design."

Factory made, three-storey, fair faced circular columns with integrated corbels have become something of a specific Brussels trademark. Facilitating faster and cheaper construction time these pre-tensioned, prestressed concrete columns and their attendant beams and floors are now being found in abundance across the Benelux countries. High-strength, self-compacting concrete (SCC) reduces the congestion of steel reinforcement at corbels, whilst non-shrink grouts lock column sections together with the minimum of fuss.

As with the UK, the number of precast concrete producers specialising in working with high-strength SCC and producing long columns is limited. Ergon, the Brabanse-based company close to Antwerp has been producing high-technology precast concrete structural components since 1963. It took the market some time to accept Ergon's belief that advantages would result by moving from C60 to C80 and even further with C90/105, which, as Ergon's Paul de Meyst points out, can be achieved with normal limestone aggregates without the addition of substances such as silica fume.

Paul de Meyst provided further evidence of the benefits of the products produced by Ergon when he said, "We have approached the concern about the legendary explosive spalling of very high-strength concrete in fires, which it does, by adding a percentage of polypropylene fibres to the mix. Currently, with world steel prices shooting up on a daily basis, the lower steel content of SCC concrete components is proving very economic."

Logistical challenges

One important example of concrete construction in Belgium is the Lex 2000 tower block close to the European Commission's Berlaymont building. The new home for European parliamentarians and their supportive staff has a completion date of 2006. Like many other major city centre locations its triangular site plot is constrained by a busy road on one side and an equally busy suburban rail line on another. On the third boundary is a high level highway under which travels a Metro tunnel.

In order to maintain the alignment of the main entrance level with the Berlaymont building, five floors of basement car park above a two-storey entrance foyer topped by twelve floors of offices and conference facilities have been required. Exceptionally difficult ground conditions meant that steel plunge columns had to be combined with both in-situ and precast concrete to top down the construction sequence.

Concluding remarks

Jean Tisseghem of Waterman-TCA, the designer and mastermind of the project, says, "Getting down to level 0 will take 15 months. Once this is completed the rest of the structure will rise quickly." Connection details for this prestigious building will have a substantial continuity of reinforcement to minimise the risk of disproportionate collapse. Cores will be jump-formed; floor-to-floor height is a tight 3.42m. With a floor to be completed every three weeks, the building is due for final handover in 2006.