ABSTRACT
Chillon viaducts are two parallel posttensioned concrete highway bridges, each carrying one direction, opened to traffic in 1969 and located on the shores of Lake Geneva near Montreux in Switzerland (Figure 1). They consist of variable height box girders built by posttensioned segmental construction with epoxyglued joints, and spanning between 92 m and 104 m over a total length of 2,120 m. Chillon viaducts along Lake Geneva. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig63_1.jpg"/>
These structures have a high cultural value and an economic importance for the region as they are the main road link between the shores of Lake Geneva and the mountainous Canton of Wallis, carrying approximately 50,000 vehicles per day.
In 2012, during rehabilitation works on the viaducts, early signs of the alkali-aggregate reaction (AAR) were discovered in the concrete. In later stages, this reaction could lead to the deterioration of the concrete compressive strength. This would mean an insufficient structural safety of the bridge at Ultimate Limit State (ULS) as well as unacceptable performance under service loads.
It was thus necessary to develop a concept for rehabilitation and strengthening to slow down the rate of AAR by protecting the concrete from water ingress and increase the ultimate resistance and stiffness of the deck slab and girder in view of potentially reduced strength and modulus of elasticity of the existing concrete. Such an intervention had to be cost-effective in terms of direct construction costs and user costs, and it also had to be non-invasive in order to respect the cultural value of the bridges.
The application of a layer of an Ultra High Performance Fiber Reinforced cement-based Composite (UHPFRC) material reinforced with rebars (R-UHPFRC) was quickly found to be the most efficient technique to do so. With its outstanding properties, a layer of strain-hardening UHPFRC combines waterproofing and reinforcement of the slab. Moreover, casting of UHPFRC on the bridges could be done in a short time-frame, thus reducing user-costs.
The development of the AAR in the deck slab of the viaducts was shown to be in its early stages. No surface crazing, typical of this problem, was observed on the concrete surfaces. The objective of the intervention was thus to reduce the rate of the reaction and anticipate the loss in strength of the concrete.
By casting one layer of R-UHPFRC on the deck slab (with rebars in the transverse direction) of the Chillon viaducts the following beneficial effects had to be achieved:
– increase the deck slab’s ultimate resistance in the transverse direction in bending and shear;
– increase the deck slab’s stiffness to improve the serviceability of the slab and the fatigue safety of the existing RC elements in view of future higher axle loads and number of vehicles;
– increase the hogging bending moment resistance and the stiffness in the longitudinal direction of the box girder;
– 213provide waterproofing to protect the existing concrete of the slab from water ingress and thus limit further development of the AAR;
– limit duration and cost of the intervention by realizing all above listed requirements and structural functions by the casting of just one layer of R-UHPFRC. For this purpose, a machine was developed specifically for the casting application of the large volume of fresh UHPFRC on the 2,120 m long viaduct.
The concept necessarily leads to composite structural elements combining conventional RC and UHPFRC. The protective and mechanical properties of UHPFRC combined with steel reinforcing bars (R-UHPFRC) provides a simple and efficient way of increasing the stiffness and structural resistance while keeping compact cross sections (Figure 2). Typical composite cross section [4]. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig63_2.tif"/>
The UHPFRC layer was cast over one 2,120 m long viaduct in less than 30 working days. The two viaducts were strengthened respectively during summers 2014 and 2015.
This very fast execution was made possible by the use of a casting machine specially developed by the contractor for the placement of fresh UHPFRC (Figure 3). UHPFRC casting machine. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig63_3.jpg"/>
This machine was conceived from a paving machine. It places the UHPFRC over varying widths and levels it at the right height. Dumpers, filled at an onsite UHPFRC mixing facility, supplied regularly the machine with the fresh UHPFRC.
The overall cost of the intervention was 230 CHF/m2 (or about 200 Euro/m2) which is a considerably lower cost than the estimated costs of conventional strengthening methods using reinforced concrete or carbon fiber lamellas. Also, the duration of the construction intervention was much shorter than for conventional technologies.
As was demonstrated by the application on the Chillon viaducts, strengthening with UHPFRC is a technique suitable also for large scale structures. Many typical problems of rehabilitation projects can be solved by the smart and economic R-UHPFRC strengthening method.
