The conversion of the historic office building Du Pont in the heart of Zurich is one of the most complex renovation projects which is currently being undertaken by our engineering team. In particular, the retrofitting of the protected ribbed slabs is one of the main challenges which requires special strengthening provisions. These slender structural elements, featuring smooth reinforcing bars with end hooks and no stirrups, show several deficiencies regarding their structural and fire safety. Since the ribbed slabs cover an area of approximately 4’300 m2 of the building floors– with a total of about 1’200 ribs– an individual strengthening of each rib was not economic and time-efficient and therefore out of question. Instead, a novel reinforcing solution with Ultra-High-Performance Fibre Reinforced Concrete will be applied. A thin 40 mm overlay of UHPFRC above the existing ribbed slab at the critical areas is enough to fulfil the current requirements of the Swiss Standards.
The on-site mixing and casting works began in the past weeks. To our knowledge, this is the first on site UHPFRC application in Zurich-City. This strengthening technique was previously verified within the scope of several experimental tests at the ETH Zurich, confirming the suitability of this reinforcing method before its final implementation on the building floors.
Among the first people to witness the on-site application of this technique was Professor Eugen Brühwiler, who we were delighted to welcome at the construction site. For many years, the head of the Laboratory for Maintenance and Safety of Structures (MCS) at the EPFL has undertaken extensive research on various UHPFRC topics and encouraged its application for the restoration and retrofitting of existing structures.