PSI - Issue 64

Giovanni Pietro Terrasi et al. / Procedia Structural Integrity 64 (2024) 1347–1359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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of 3.8 m. It's tensile flange was built as a tubular space-truss ending in a longitudinal steel channel that contained two prestressed CFRP parallel wire cables with a length of 47.75 m exerting a total prestressing force of 2400 kN each (Figure 1). These post tensioning cables were built of 91 single pultruded CFRP wires of diameter 5.08 mm organised in a hexagonal pattern. The wires were prestressed to 1'350 MPa each, corresponding to 45% of the CFRP design tensile strength. At the time a novel CFRP anchorage technology developed by Empa (Meier at al. (1995)) was implemented to anchor the CFRP cables in the steel sleeves. The four cable anchorage heads where built by zinc coated steel sleeves with an inner conical profile in which the 91 CFRP wires where grouted using an epoxy with gradually increasing elastic modulus, which was soft at the load entrance and filled by very stiff alumina particles towards the anchor end (Empa's gradient anchorage system, Meier et al. 1995). In order to monitor the CFRP wire strains (i.e. the cable load) a combination of resistance strain gauges and integrated fibre optical sensors was implemented. This allowed to monitor the total cable loads over the 17.5 years of service life of the bridge: The average tensile strain of the CFRP wires was 8400 µm/m for an air temperature of 7°C and the strains varied by approximately 150 µm/m for a temperature variation of 10°C due to thermal expansion of the bridge (Nellen et al. (1999), Meier et al. (2013)). This bridge was a world premiere with respect to the implementation of three technologies: The use of CFRP parallel wire cables to post tension a steel-concrete composite bridge, the efficient anchoring of 91 CFRP parallel wires in a so called gradient Load Transfer Media (LTM) anchor system (implementing the Empa-patent by Meier et. al 1995), and the monitoring of CFRP wires in CFRP bridge cables by integrated FBG sensors in the wires during pultrusion (Nellen et al. (1999). In April 2016, after 17.5 years, the bridge had to be dismantled and was substituted with a longer span prestressed concrete bridge due to new flooding protection scheme in the city of Lucerne. 8.7 km of pultruded CFRP wires (market value approx. 45'000 Euro) came back to Empa after decommissioning of the bridge.

Fig. 1: Former pedestrian bridge over the Kleine Emme (April 2016, shortly before demolition, Meier et al. 2013)

2. Materials and Methods 2.1. CFRP wire materials and strength assessment

The fibers used in 1997/98 for the CFRP wire production in pultrusion by Stesalit Ltd in Zullwil Switzerland were Toray T700 SC 24k (www.toray.jp) embedded in an epoxy matrix LY556/HY917/DY070 supplied by former Ciba Geigy of Basel in Switzerland (Ciba-Geigy 1998). The CFRP wires (  5.08 mm) were characterized by a fiber volume amount between 70%-72% leading to a design tensile strength of 3000 MPa and a design modulus of 160 GPa in fiber direction. Several wire lots tested by Empa in 1997 and 1998 consistently fulfilled the strength and modulus criteria, and the Stesalit production lot of summer 1998 to be used for the two parallel wire cables for post tensioning the Kleine Emme bridge was assessed for mechanical properties which are reported in Table 1. An extensive investigation of the residual mechanical and thermal properties of the CFRP wires used in Kleine Emme bridge was performed during 2023 by Boloux at al. (2024). For the re-use of the CFRP wires it was of paramount importance to assess the axial strength, modulus, failure strain and T g in order to quantify the remnant properties and the degradation after 25 years. The T g of the recovered CFRP wires was 129.6°C measured by differential scanning calorimetry (DSC) in 2023 using a Perkin-Elmer DSC 7 device and applying a heating rate of 20°C/min and following