The Chillon Viaducts, Geneva, Switzerland - Overlay

Opened to traffic in 1969, the two 2.2 km long Chillon Viaducts were named for the Château de Chillon, on which they tower above on the eastern slopes of Lake Geneva. As an official Swiss heritage site of national significance, the viaducts are truly one of the most spectacular structures of the Swiss highway system. Construction of precast elements for this twin structure required the use of post tensioning techniques. Used by more than 50,000 vehicles per day, traffic volumes have increased significantly in recent years.

An inspection conducted in 2009 revealed that significant amounts of water had penetrated the structure in multiple locations and corroded its reinforcing bars. Furthermore, since it no longer complied with current earthquake resistance standards, the structure required major repair work that would involve changing the static system and supports beneath some of the piers. Upon commencement of work on site, hydrodemolition tests were conducted after it was determined that the structure was also affected by Alkali-Aggregate Reactions (AAR) which had compromised the mechanical properties of the concrete. If that discovery had not been made, it would probably have been necessary to completely rebuild the structure within the next 15 years.

"The main goal of taking action was to mitigate the damaging effects of the AAR," explains Stéphane Cuennet, Structural Technical Specialist from the Office Fédéral des Routes Suisses (OFROU), the Federal government agency responsible for Swiss highways. "It involved strengthening and waterproofing the road slab to remove any possibility of water penetrating the concrete and reducing the extent of stresses imposed by road traffic by making the slab more rigid, thereby increasing its ultimate strength, and limiting longitudinal distortion of the bridge decks."

To meet these objectives, a UHPC solution was chosen by a group of specialists, including engineers from the École Polytechnique Fédérale de Lausanne (EPFL): casting a 45 mm layer of Ductal^® on the existing deck in order to weatherproof it and improve the structure's overall structural behavior. “This really was the only plausible solution to guarantee a reliable rehabilitation in view of the uncertainties surrounding changes in the mechanical properties of concrete,” argues Cuennet.

With this knowledge, the Ductal^® team at LafargeHolcim put into place the technical resources required to define a formulation that would respond to the specific constraints of the project in terms of tensile strength, slope performance (gradients of up to 7% on the existing deck) and ease of implementation. Throughout the project, Damien Jacomo, the Ductal^® Business Engineer responsible for Civil Engineering projects, and the Lafarge Ductal^® team worked alongside the contractor and project owner to validate the Ductal^® solution, its implementation and onsite quality monitoring. Eugen Brühwiler, a global authority on UHPC, lecturer at EPFL and director of the structures maintenance, construction and safety laboratory, was appointed as expert consultant for the project by the OFROU: “The use of UHPC for the Chillon Viaducts is a world first, not only in terms of the area to be covered, but also the volumes to be produced and installed in such a short period of time”.

One of the remarkable benefits of the ultra-high performance concrete layer cast on the road slab is its ability to strengthen the structure against longitudinal shear forces and provide additional longitudinal rigidity with a thickness of just 45 mm, rather than the 20 cm of traditional reinforced concrete that would otherwise have been required. This UHPC layer also makes the deck more weatherproof and therefore stops the ingress of water that could trigger a deterioration of the AAR. “The UHPC provides the perfect response to the weaknesses of ordinary concrete”, continues Brühwiler. “Its composition ensures that all the spaces inside the material are occupied by fine particles. It is an extremely compact material that is very dense and impermeable to water. Thanks to the large quantity of fine steel fibers it contains, it never cracks in service."

This durable, robust solution considerably reduces the load on existing infrastructures, and is much faster to implement. Brühwiler also believes that it offers significant financial benefits. “Contrary to what many people think, the cost of using UHPC to reinforce structures is ultimately good value, given the number of requirements it meets. The performance of this technique offers undeniable benefits but, in the end, it was purely for financial reasons that it was adopted in this instance."