PSI - Issue 64

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

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pultruded CFRP profiles are strips (lamellae) that are used as Externally Bonded flexural and shear Reinforcements (EBR) on concrete, steel or masonry substrates or for Near Surface Mounted (NSM) reinforcements embedded in grooves cut into the concrete cover of beams and slabs to be strengthened. Fib bulletin (2019) summarizes the state of the art for structural strengthening using CFRP EBR and NSM reinforcements for concrete structures. Another widely used CFRP pultrusion profile are round CFRP wires which are mostly used as internal rebar reinforcements, as prestressing reinforcements (pre-tensioning method, see Terrasi 2013) and for assembling CFRP parallel wire bundle cables (PWB, mostly for bridge applications, cf. Meier et al. (2013)). The question addressed in this project is if the CFRP pultrusions are suitable for re-use with regard to their residual strength and durability, after a CFRP strengthened structure has reached the end of its lifetime. This needs to include the technological challenges given by the strength anisotropy of pultrusions (i.e. generally unidirectional) which makes the extraction of e.g. a CFRP post tensioning cable from a bridge before demolition a delicate affair. After having assessed the integrity of the regained CFRP pultrusions, new design strength and modulus for the serviced CFRP need to be established experimentally and new maximum service stresses need to be defined for the materials before re-use, especially if CFRP is re-used as a prestressing reinforcement. Design guidance on the maximum allowable long-term tensile service stresses of CFRP pultrusions in contact with concrete or in civil structures is obviously rather conservative, CFRP having a successful structural track record of only 30 years. Fib bulletin 90 (2019) states that CFRP can withstand sustained tensile stress levels up to an indicative value of 80% of its short term characteristic strength and refers to the Fib Model Code for concrete structures 2010 (Fib 2013). This code informs that the permissible stress level against stress rupture of unidirectional CFRP corresponds to at least 80% of its short term strength on a 50 years basis while the relaxation of CFRP prestressing elements after 50 years of loading is estimated at between 2% to 10% depending on the stress level and environmental influence. The higher the carbon fiber volume amount, the lower the assumed relaxation. The former Fib bulletin 90 also informs that a more precise upper sustained stress limit for a specific CFRP should be determined through appropriate durability experiments. However, on-going updates of this report recommend a tensile in-service stress limit of 0.56 of the characteristic tensile strength of a CFRP pultruded tensile element (Boloux et al. 2024). Moreover, it is suggested to apply a 21% reduction of the elastic modulus of the FRP material due to viscoelastic creep for a 50 year service life. ACI 2015 guidelines 440-1R (American Concrete Inst. 2015, Table 8.3 on FRP creep rupture stress limits), recommend an in-service sustained stress limit of 0.55 for CFRP bars to account for creep when CFRP is embedded in concrete (not directly exposed to earth and weather). In an early and comprehensive investigation Yamaguchi et al. (1997) tested 6 mm diameter carbon fiber reinforced epoxy resin pultruded tendons in creep at room temperature using wedge anchors at various load ratios. Their probabilistic analysis of the specimens' failure times for different sustained tensile load levels, allowed to discover a linear proportionality between the creep strength and the logarithm of the rupture time. Therewith Yamaguchi et al. (1997) could estimate the average reduction of the creep rupture strength with respect to the initial quasi-static strength and reported an extrapolated 7% strength reduction due to creep for CFRP after 57 years of service for a survival probability of 50%. Few studies have experimentally investigated pultrusions under sustained tensile creep loading for more than one year. For example Yang et al. (2018) tested 8 mm diameter pultruded CFRP tendons with a fibre content of 65% and an epoxy matrix in creep for 1000 hours. Their findings were a CFRP tendon’s residual strength which was 4.54% lower than the quasi-static tensile strength, while the elastic modulus of the tendons increased by 6.99 % compared to the short term modulus due to fiber straightening during tensile creep. Hence, available design guidelines must be considered as conservative when compared against available experiments, which was also recently concluded by Boloux et al 2024. This paper describes the re-use of a unique lot of CFRP pultruded wires that were originally used to build two 91 parallel-wire CFRP cables (of 47.75 m length) for bridge post tensioning and that were in service for 17.5 years at a sustained tensile stress of 1350 MPa and outdoors weather conditions. The material was comprehensively assessed after the two post-tensioning cables were dismounted in April 2016. The PWB cables were stored for additional 7.5 years outdoors at Empa. In 2023 two 37-wire CFRP cables in lab scale were investigated before the re-use of 2.2 km of wire to produce two new bridge post tensioning cables that were installed on 23 August 2023 to prestress and reinforce Ilfis Bridge in the Canton of Lucerne in the central Swiss Alps. Kleine Emme bridge was built in the city of Lucerne in October 1998 as a steel composite bridge for pedestrians and bicycles and was also designed to be able to carry an ambulance in case of need (Figure 1). The bridge had a reinforced concrete deck supported by a steel space truss, was totally 56 m long (span was 46.8 m) and had a width