Behaviour of deviated CFRP-Strips

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1 Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland Behaviour of deviated CFRP-Strips M. Hwash 1, J. Knippers 1 & F. Saad 2 1 Institut of Building Structures and Structural Design (ITKE), Univ. of Stuttgart, Stuttgart, Germany 2 Institut of Structural Design, Univ. of Ain Shams, Cairo, Egypt ABSTRACT: This research work presents the application of CFRP-Strips as prestressing elements for bridges, e.g. extradosed bridges and externally prestressed box girder concrete bridges. The application of CFRP-Strips as external pre-tensioned elements is not only governed by the general structural and dynamic behaviour of the hybrid system but also by the right concept for the construction joints, i.e. the anchorage of the CFRP-Strips at the deviation saddle and the deck structure. The load bearing capacity and the deformational behaviour of deviated CFRP-Strips is influenced mainly by the contact surface, the deviation angle and the diameter of the deviation saddle. This paper introduces the results of an Experimental program of deviated CFRP-Strips at deviation saddle. The effect of the different parameters on the load bearing capacity, strain responses up to failure and the mode of failure of the CFRP-Strip at the deviation saddle will be investigated. 1 INTRODUCTION CFRP-Materials have been the focus of much attention in the engineering community since the development of light weight and high stiffness fibres in the 1940 s. In the last two decades, the use of CFRP composites as a reinforcement for concrete members has emerged as one of the most exciting and promising technologies in materials / structural engineering. Due to the high price of CFRP-Strips its application today is mostly found in buildings, where its strength is highly utilized and the strips are needed in small quantities, e.g. for the strengthening and rehabilitation of reinforced concrete structures. In the meantime the new developments/researches concentrate on direct gluing of prestressed CFRP-Strips on reinforced concrete constructions in order to improve their stability. The applied pre-stress rises the degree of utilization for the CFRP-Strip, at the same time the pre-stressed CFRP-Strips need to anchor their force locally into the concrete. This is not easy due to the sensitivity of the carbon fibers to transverse loads (Fig. 1). The development for CFRP-Strip anchorage is presently under investigation in many research institutions and universities. Applications of pretensioned CFRP-Strips as external cable elements for supporting concrete bridges, by which the full strength of the FRP-material can be utilised, was introduced in [1]. Figure 2 shows different bridge categories, where CFRP-Strips are applied [7]. This system enables inspection and exchange of CFRP-Strips easily. In all cases, CFRP-Strips should be deviated. For the construction of the end anchorages there are already some applications. Because of the flat and broad cross section of a CFRP-Strip, the lateral strain at the bearing saddle and the end anchorages are smaller than induced by circular tension members, this may be of advantage

2 At present knowledge of the deviated CFRP-Strips is still missing. How does load bearing capacity change, if the CFRP-Strip is deviated on a saddle and stressed? Figure 1: Körschtal bridge, Stuttgart-Germany Figure 2: Externally prestressed bridges using CFRP- Strips [7] 2 EXPERIMENTAL WORK The following section shows part of a comprehensive experimental program which is running on CFRP-Strips on a deviation saddle. The effect of the deviation angle and the contact surface between the CFRP-Strip and the deviation saddle on the load bearing capacity and the deformational behaviour of the CFRP-Strips will be investigated. 2.1 Program and test-setup A bending test with different deviation angles will be carried out to estimate the relationship between the breaking load and the angle of deviation on the saddle. CFRP-Strips of dimensions 50x1.4mm and its multiple will be tested. The CFRP-Strip will be fixed on two sides with hinged joints, so that it can rotate freely at any loading level without sharp bending of the strip (Fig. 3, 4). The specimens were tested under single vertical load. A hydraulic actuator with a maximum capacity of 500 kn and a maximum stroke of 250 mm was used to apply monotonic loads. The CFRP-Strip width was increased inside of the anchorage up to 90 mm to achieve full anchorage and avoid any slip (Fig. 3). Table 1: Test program Test- Group Number Strip Saddle surface Angle α G0 110 o 1 90*1,4 Steel A 110 o 3 50*1,4 Steel B 30 o 3 50*1,4 Steel C 110 o 3 50*1,4 CFRP-Strip* D 110 o 3 50*1,4 Polymer sheath* F 0 o 3 50*1,4 - * are not presented in this paper Table 2: Mechanical properties of CFRP- Strip (manufacturers data) Dimension Ideal value thickness ,43 mm width 90 ± 0,5 Tensile modulus A >160 kn/mm2 Tensile strength A >2900 N/mm2 breaking elongation >1.7 2,00% The strain in the CFRP strip was measured at five positions along the Strips using electrical strain gauges [DMS] (Fig. 3). The relative displacements (slip) between the anchorage and the CFRP-Strips were measured using linear displacement transducers. All measurements were recorded automatically at each loading level. The automatic program control of the measurements was carried out with measuring instrument connected to a computer system for data acquisition

3 Figure 3: Test set-up for deviated angle of 110 o α Table 1 summarizes the test program. Altogether sixteen tests shall be conducted with different deviation angles. The properties of the examined CFRP-Strips are summarized in table 2. Figure 3 shows the test setup of the CFRP-Strip for a deviation angle of 110 o as an example. The radius of the deviation saddle is 150 mm. 2.2 Anchorage of CFRP-Strip and Prestressing Method The adapted anchorage considered the lack of the isotropic, plasticity and the poor physical properties in the transverse direction to the long fiber direction. The available ratio of surface area to cross-section area (U/A ratio) of the Strip seems to be suitable for a clamping anchorage which is based on load transfer by friction. The procedure is based on the basic principles of the system Leoba CarboDur LC-II [3]. The efficiency of the temporary friction anchors was increased by the arrangement of clamping cross plates (Höckerbar) which provide a uniform clamping pressure along the width of the strip. The use of special braking pads guarantees a reproducible and uniform friction coefficient. An additional gluing of the permanent clamping plate onto the Strip increases permits a tension force up to the breaking load of the CFRP-Strip without breaking the anchorage (Fig. 4). Figure 5 shows the prestressing process of the CFRP- Strip during the hardening phase for a deviation angle of 30 as an example [2, 4]. It can be recognized that there is no stress loss in the CFRP- Strip during the hardening phase. The constant strain value indicates that there is no remarkable slip between the CFRP-Strip and the anchorage, i.e. perfect anchorage

4 DMS 1 DMS 2 DMS 4 DMS 5 Strain [mm/m] Figure 4: Anchorage of the CFRP-Strip; D Time (sec.) Figure 5: prestressing of the CFRP-Strip during the hardening process for deviation angel 30 o 3 EXPERIMENTAL RESULTS 3.1 Load-deformational Behaviour In order to have a basic idea of the ultimate load of the used anchorages and the deviated CFRP-Strip, an initial test was carried out on a CFRP-Strip 90*1.4 mm with a deviation angle of 110 o. After the hardening phase, the CFRP-Strip is loaded and released up to certain loadlevel. Then the CFRP-Strip is loaded gradually to 400 kn (max. load capacity of the testing machine). So that approximately 55% of the ultimate load (characteristic) is applied. Figure 6 shows the load-strain diagram. From these figures, one can observe that the maximum strain occurred like expected at the gauge of DMS 2 located at the free end of the deviated saddle. Due to the friction between the Strip and the deviation saddle, the lowest strain in the strip is measured at DMS 4 (inside the deviation saddle). The maximum measured strain was 1.1%, which is 55% of the rupture strain of the CFRP-Strip. As the result of this test, it was recognized that the deviated CFRP-Strips can be loaded up to and beyond 55% of their characteristic breaking load without reaching failure mode F [kn] 200 DMS 1 DMS 2 DMS 3 DMS 4 DMS ,2 0,4 0,6 0,8 1 1,2 Strain [ ] Figure 6: Load-strain curves on Strip 90*1.4 for test G0 Figures 7 and 8 show the load-strain diagrams for both groups A (with angle 110 o ) and B (with angle 30 o ). By analysing the test results, one should differentiate between both groups, based on the contact lengths and therefore the contact area between the strip and the saddle. For all specimens of Group A and B, it was recognized that the maximum strain occurred at either edges of the deviation saddle or inside the deviation saddle. For Group A, this was observed at - 4 -

5 gauge 5 (at the ends of deviation saddle). For the same group the lowest strain was measured at gauge 4 due to friction restraining between saddle and CFRP-Strips. On the contrary, the maximum strain for Group B was measured at gauge 4 (within the saddle). This indicates that for Group B, with a small deviation angle (α=30 o ), the friction has no significant effect on the strain behaviour of the strip due to its small contact area DMS 1 DMS 2 DMS 3 DMS 4 DMS DMS 1 DMS 2 DMS 4 DMS F [kn] 200 F [kn] ,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60 Strain [ε %] Figure 7: Load-strain curves for CFRP- Strip (group A) F = Force of the testing device 0 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 Strain [ε %] Figure 8: Load-strain curves for CFRP-Strip (group B) F = Force of the testing device 3.2 Failure Modes and Ultimate Loads In all performed experiments of deviated CFRP-Strips, they failed due to their rupture at the deviation saddle. For Group A, the fracture of the CFRP-Strip occurred at the points where the CFRP-Strip is deviated at the saddle (positions 2 and 5). For Group B their fracture occurred within the deviation saddle (near the symmetry axis of the strip, at point 4). The contact pressure between the CFRP-Strip and the saddle can be calculated as follows: T = r * b kn/ m σ where; σ = contact pressure; T = Force in the strip due to F; r = deviation saddle radius =150 mm and b = width of the Strip The load bearing capacity and the contact pressure are shown in Table 3. One can see that the CFRP-Strip failed at a contact pressure of approximately 25 N/mm² for Group A (α=110 o ) and 31 N/mm² for Group B (α=30 o ). It was recognized that larger the deviation angles lead to higher friction forces between the strip and the saddle, which causes more restraining of the CFRP-Strips and consequentially earlier failure. According to the Test group F, the ultimate strength of straight strip is about 258 kn. This indicates that for group A, the ultimate strength of deviated strips was reduced up to about 75 % of a straight CFRP strip respectively. For Group B, the reduction factor is about 86%. The same test was carried out for an angel of 30 o r σ Figure 9: Fracture of CFRP-Strip in Test C2 Figure 10: schematically representation for contact pressure - 5 -

6 and deviation saddle radius of 1000 mm. Here the reduction factor was increased up to 95% of the ultimate strength of a straight strip. With these results the acceptance criteria for mandatory requirements, which are listed in the Guideline for European Technical Approval of post- Tensioning Kits for Prestressing of Structures, are satisfied [6]. Table 3: Strip contact pressure & reduction factor for Saddle radius 150 mm F T T Group α specimen average σ average [kn] [kn] [kn] [N/mm²] C F 0 o C C Reduction factor C A 110 o C C C B 30 o C C CONCLUSIONS The use of CFRP-Strips as freestanding element for a bridge construction seems to have promise: high tensile strength of carbon fibre reinforced polymers high compression strength of the concrete and at the same time durable construction systems. Therefore very slim structural elements are possible which are architecturally desired and which are of low weight, allowing practical installation. The results of the current experimental work showed that the ultimate strength of the deviated CFRP-Strips is significantly affected by the bending radius and the deviation angle, so that the following recommendations can be made. For deviation angles smaller than 30 o and by using a bending radius of 1000 mm the acceptance criteria for mandatory requirements, which are listed in the Guideline for European Technical Approval of post-tensioning Kits for Prestressing of Structures, are satisfied, i.e. reduction factor up to 95% For deviation angles between 30 o and 110 o an ultimate strength reduction factor up to 0.86 should be considered in the design 5 REFERENCES [1] Knippers, J., Saad, F., & Hwash, M.: Application of Pre-Stressed CFRP-Strips in Bridges, Concept, Analysis and Constrution Details, the Fourth Middle East Symposium on Structural Composites For Infrastructure Applications MESC-4, Alexandria, Egypt, 2005 [2] Hwash, M., Knippers, J. & Saad, F.: Experiments on deviated CFRP-Strips for external prestressing, COBRAE Conference, Benefits on Civil Engineering, Stuttgart, Germany, 2007 [3] Andrä, H.P., König, G. & Maier, M.: First Applications of CFRP Tendons in Germany, Congress Proceedings of IABSE Symposium, Melbourne, Australia, 2002 [4] Knippers, J., Saad, F., & Hwash, M.: Analysis and Behaviour of CFRP-Strips at Deviation Saddle for CFRP Prestressed Bridges, IABSE Symposium, Weimar, germany, pp , 2007 [5] Knippers, J.: Innovative design concepts for composite bridges in Germany, Cobrae Conference, Bridge Engineering with Polymer Composites, Dübendorf, Switzerland, 2005 [6] ETAG 013: Guideline for European Technical Approval of post-tensioning Kits for Prestressing of Structures, EOTA, Brüssel, Edition June 2002 [7] Saad, F.: Innovative Development in Bridge Engineering, Concept, Structural Details and Construction, Bridge conference in Housing & Building Research Centre, Cairo, Egypt,