SWITZERLAND'S TALLEST BUILDING WAS PUT UP IN RECORD TIME

The area to the west of the Hardbrücke bridge in Zurich was formerly a bastion of mechanical engineering. It was home to shipbuilder and turbine manufacturer Sulzer-Escher-Wyss, whose immediate neighbor Maag produced cogwheels. Economic shifts have since brought about a transformation: actors now tread the boards in Sulzer-Escher-Wyss’s shipbuilding hall while, on the Maag site, blue-collar workers have been ousted by lawyers and managers.

Visible from afar, the 126 meter high Prime Tower stands as a beacon of change. Incorporating some 34,000 cubic meters of concrete and 6,000 tons of steel reinforcement, Switzerland’s tallest building was erected in only 40 months. The first tenants took up residence in August 2011 and, since mid-December, visitors to the “Clouds” restaurant on the 36th story can enjoy a spectacular view of the city during dinner.

FIRST STEP: TO CONCRETE THE CORE ZONE

The use of sophisticated technology and logistics to meet the tight construction schedule had been one of the key requirements in the bidding procedure. The contract was awarded to the ARGE Prime Tower consortium made up of design-build contractors Losinger-Marazzi and Steiner AG. “We were able to capitalize on the experience of our parent company, Bouygues, which has notched up a number of high-rise projects,” points out Alain Capt, Prime Tower’s Senior Project Manager.

The consortium proposed the use of climbing formwork to build the high-rise core.
A preliminary analysis indicated that a rate of one story per week was feasible. The first step was to concrete the core zone.

BEST CONCRETE MIX DESIGN

For the engineers, this core section posed a particular challenge. Not only did it need to house stairways, elevators, and mechanical and electrical installations, it also constituted the building’s structural backbone. In other words, the bulk of the loads acting on the high-rise had to be accommodated by this core. Accordingly high demands were placed on the quality of the concrete.

The required properties were precisely specified by the engineering team and the best concrete mix design was identified by means of laboratory tests. For example, the core concrete was required to exhibit a maximum shrinkage of 0.35 per mil after 90 days. “Otherwise the building’s structural deformation behavior would have been impaired,” explains Lukas Reichmuth, Project Engineer at Zurich-based engineering practice Walt + Galmarini AG.

ADMIXTURES GUARANTEE CONSISTENT QUALITY

While the required performance was easy to achieve under laboratory conditions, the on-site realities posed far stiffer tests. One of these was the extreme range of temperatures – from -10°C to +30°C – during the construction period. Further difficulties arose from the need to pump the concrete to a height of over 100 meters in some cases. The long distances both lengthened pumping times and raised the temperature of the concrete due to friction with the pipes.

SIKA SUPERPLASTICIZERS AND SHRINKAGE-REDUCING ADMIXTURES

To guarantee the consistent quality of the concrete placed throughout the contract period, the management team specified Sika superplasticizers and shrinkage-reducing admixtures. These allowed fine-tuning of the concrete formulation in line with temperature and season. This investment paid dividends: concreting proceeded without any notable delays and, bang on time after 60 weeks, the core reached to its definitive height of 126 meters.