Experimental and Theoretical Characterization of Fretting of CFRP Prestressing Strands and Effect on Fatigue Life of Partially Post-Tensioned Girders with Unbonded Tendons

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Owing to its high strength to weight ratio, carbon fibre reinforced polymer (CFRP) components are emerging as a possible replacement for steel in various civil engineering applications. The most common application is the usage of CFRP strands as prestressing tendon in long-span concrete bridges. Among several other options, the most popular and efficient variant is partial post-tensioning with unbonded tendons. This can be attributed to the design flexibility, accelerated construction, better usage of the ultimate capacity of all reinforcing elements, higher ultimate deflection etc associated with partial post-tensioning. In such bridges, the critical design criterion to be considered is the fatigue of prestressing element occurring due to the vehicular moment. The fatigue life of such prestressing elements can be considerably reduced, when there is a relative motion between the element and other element applying normal force. This phenomenon is termed as fretting. During the life of the PPTGUT, continuous to and fro relative motion occurs between the duct and the tendon. This may lead to fretting of CFRP tendon and reduction in fatigue life. When the location of relative motion overlaps with the location of high tendon curvature and the flexural cracks, the effect might be more detrimental. The relatively new material of CFRP is seldom tested against this phenomenon of fretting fatigue and the other complex situations occurring inside a CFRP PPTGUT. In this context, the proposed project is invoked, that aims to theoretically and experimentally capture the microscopic phenomenon of fretting in CFRP prestressing strands and study its macroscopic effect on the fatigue life of CFRP.For more than a decade the scientists at the Chair of Conceptual and Structural Design-Concrete Structures, TU Berlin have been involved in investigating the fretting fatigue of steel strands and the application of CFRP as tension elements. With this previously conducted research, the present project is proposed, wherein, the fretting fatigue characteristics of individual CFRP strand will be tested via. in-air fatigue test with lateral press and fatigue tests on CFRP strands bent over the saddle. In the next part, the fretting fatigue will be studied by experimentally testing the reduced scale CFRP PPTGUT. Next up, the experimental results will be utilized to setup numerical finite element (FE) models, in order to predict the fatigue life of the 20 m long CFRP PPTGUT viz. the demonstrator (built and available at the Chair’s laboratory). Also, few simplified analytical models will be developed for the same. After the theoretical prediction of fatigue life of the demonstrator, it will be tested till it fails in fatigue (or 2.5 million cycles). After these exercises, the results from all the studies will be homogenized and once again systematically compared to suggest concrete guidelines for the design and application of CFRP PPTGUT.