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.