carbon fibre composites as stay cables for bridges

Applied Composite Materials7: 139–150, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

139

Carbon Fibre Composites as Stay Cables forBridges

JOHANNES FRITZ NOISTERNIGDYWIDAG-Systems International GmbH, P.O. Box 81 02 68, D-81902 Munich, Germany

(Received 20 January 1999; accepted 27 April 1999)

Abstract. High tensile strength and stiffness as well as high fatigue life, low weight and excellentchemical resistance are material properties of carbon fibre composites (CFRP) which make thesematerials interesting for stay cable systems. The key problem to which the application of stay cablesas well as tendons is faced is the anchoring. This paper describes the properties of CFRP-wires, therequirements to stay cables or tendons and the development of such a system through calculationsand experiments along with a successful field stress test of a CFRP based stay cable.

Key words: carbon fibre composite, stay cable, construction industry, requirements, properties,anchorage, static tests, fatigue tests, field test.

1. Introduction

Despite the widespread use of carbon fibre composites in the aerospace and defenseindustry, applications in the construction industry were limited for many yearsprimarily due to economic reasons. Key advantages of advanced composites, suchas design freedom and tailored characteristics, high strength/weight ratios (whichsignificantly exceed those of conventional civil engineering materials) and a highchemical resistance in most civil engineering environments, are lost in high mate-rial and manufacturing costs (particularly in direct comparison with conventionalstructural materials such as steel, concrete, or masonry). Furthermore, the so farpracticed direct one-to-one component replacement of elements in conventionalstructural systems by CFRP or other advanced composite components have shownthat not only economically, but also structurally, it is difficult to justify the use ofcomposites in civil engineering.

Several developments have changed this scenario over the past few years: ad-vances in manufacturing, reduced demand of these materials in the high priceddefense industry, the prospects for large volume applications in the construction in-dustry and designs of advanced composites in conjunction with conventional struc-tural materials rather than individual component replacement have shown that tech-nical efficiency can be achieved within competitive economical constraints. Partic-ularly strengthening and rehabilitation of existing structures with CFRP-laminatesor plates are the state of the art today [1].

140 JOHANNES FRITZ NOISTERNIG

Figure 1. Alamillo cable stayed bridge in Sevilla/Spain.

First considerations on CFRP-elements as stay cables were made in the early80’s, when discussions about the possibility of a stay cable bridge over the strait ofGibraltar, which is not possible if constructed in steel, where reported. At the sametime, however, the key problem of anchoring these CFRP-elements is pointed out[2]. Until now especially in Japan and USA first applications have been realized[3]. In the last years also in Europe intensive developments have been startedand lead to a first application of two CFRP-stay cables in the Stork Bridge inWinterthur/Switzerland in 1996 [4].

Besides the development in Switzerland also the author’s company started adevelopment project on CFRP-stay cables or tendons with a suitable anchorage inthe past three years. Today, the developed CFRP-system DYWICARB is readyfor first applications as stay cable in bridges or as tendon in other civil engi-neering construction projects. Furthermore, new structural concepts and systemswhich combine the superior mechanical characteristics of directional strength intension in the direction of carbon fibres with the dominant characteristics of con-crete in compression and steel in inelastic deformation capacity can be developedwith CFRP-products. It is possible that these new civil engineering structures caneven exceed the aesthetics of the Alamillo cable stayed bridge in Sevilla/Spain forexample, as shown in Figure 1.

2. Carbon Fibre Composites

On a first glance fibre reinforced plastic materials consisting of fibres and matrixseem to be very sophisticated. Basically, composites should be ideal constructionmaterials, consisting of chemical elements mainly positioned in the centre of the

CARBON FIBRE COMPOSITES 141

Table I. Properties of Carbon-Stress wires for DY-WICARB stay cables or tendons

Diameter 5 mm

Fibre volume content 65 Vol.-%

Tensile strength 2700 N/mm2

Modulus of elasticity 160 kN/mm2

Elongation at break 1.6%

Density 1.6 g/cm3

Therm. coefficient of expansion 0.2× 10−6 K−1

Relaxation after 1000 hours 0.8%

Creep after 3000 hours 0.01%

main group of the classification of elements. These elements form compositeswhere the atoms are linked by stable bond. Materials based on such compositesare stiff, strong and resistant against relatively high temperatures as well as manyaggressive media and have a relatively low density.

Carbon – in the form of graphite – however, is very brittle and thus has hardlybeen used as a construction material in the past. A small notch in the surface ora defect of micro millimetre dimension in the inside of a homogenous componentconsisting of such a material, may cause a sudden failure. It can not be avoidedthat such defects are present in larger components. If, however, the graphite ismanufactured to fibres, this situation changes considerably because the strengthand stiffness increase decisively. This is on the one hand due to structural and onthe other hand due to statical reasons. If an individual fibre of a bundle of fibresfractures, the fracture can not extend in contrast to a solid body. If the fibre bundleadditionally is embedded in a matrix, the fibre can take up loading again at bothsides of the fracture point. This is the reason why the tensile strength of CFRP-wires displays low scatter.

Carbon fibres are high-grade fibres, the properties of which may vary in a widerange depending on the conditions of manufacturing, so that a whole class of fibresis available. The tensile strength ranges from 2000 MPa to 4500 MPa and themodulus of elasticity from 200 GPa to 650 GPa. Carbon fibres are mainly producedfrom Polyacrylonitril (PAN) precursor-fibres in a multiple process of heating andstressing. Fibres of high tenacity (HT) are heated to 1600◦C, while high-modulusfibres (HM) have to be graphitized at temperatures up to 3000◦C. The diameters ofthe carbon fibres are between 7 and 9µm.

CFRP-wires are produced by pultrusion, where the fibres are embedded in amatrix material. In most cases epoxy resins are used as matrix system with a fibrevolume content of about 65%. The CFRP-wires (product name Carbon-Stress) usedin the DYWICARB system have a smooth surface with a diameter of 5 mm and are


produced by the company Nedri. Table I lists the material properties of Carbon-Stress wires.

3. Requirements to CFRP-Stay Cables or Tendons

FRP- as well as CFRP-systems have in no way been standardized nationally orinternationally up to now. Therefore it is very difficult to work out a valid table ofrequirements for CFRP-stay cables or tendons. However working groups in Japan(JSCE), USA (ACI 440) and Canada (CSA S806) as well as Europe (fib task group9.3) are striving to standardize materials, application and calculation methods. Thefib task group 9.3 is preparing progress reports for 1999 for a public discussion ofsuch requirements.

Standardization and characterization of the material of CFRP-elements canroughly lean to the recommendations known from steel. However, it has to beclear that in contrast to steel, CFRP is no homogenous material and thus differentCFRP-elements also possess different properties. For general characterization ofthe material, not only the mechanical properties under static and dynamic load-ing, also durability (long-term behaviour) as well as the behaviour under differentinfluences of media are of importance.

Concerning the requirements to CFRP-stay cables or tendons, it is reasonable toadopt the requirements of systems consisting of prestressing steel (PTI-recommen-dations for cables and similar recommendations). With regard to the anchorage forthe application of CFRP-tendons generally a high static and dynamic capacity (fortendons acc. to FIP the upper load range is 0.65 of the failure load with an ampli-tude of 80 MPa; for cables acc. to PTI the upper load range is 0.45 of failure loadwith an amplitude of 160 MPa; for external tendons the upper load range is 0.7 ofthe failure load with an amplitude of 35 MPa) must be achieved to exploit the mate-rial as far as possible (a.o. also an economic aspect). For evaluation of the durabilityof CFRP-systems, especially of anchorages, the lifetime under permanent load aswell as the behaviour under chemical influence have to be determined [5]. In pilotprojects CFRP-systems have to be controlled by optic sensors or similar.

4. Materials and Assembly of DYWICARB

The DYWICARB system in the form in which it is intended to be used in futureas stay cable or tendon is shown in Figure 2. DYWICARB consists of CFRP-wires from the company NEDRI (the properties are shown in Table I). Thesewires are arranged in parallel over the whole length. To protect the CFRP-wiresagainst ultraviolet radiation and wind erosion, which in combination over a longperiod of time may cause degradation of the epoxy resin of the CFRP-wires, theyare covered by a polyethylene (PE) or polypropylene (PP) sheath along their freelength. The number of CFRP-wires depends on the necessary load bearing capacity


Figure 2. Assembly of the DYWICARB system.

of the stay cable or tendon. Up to now DYWICARB systems consisting of 7, 19and 91 CFRP-wires have been tested under static and dynamic loading.

The CFRP-wires are potted in a conically shaped steel hull which is supportedon the structure over a steel ring nut. A cap protects the ring nut and the steel hullfrom corrosion. As a potting material DYWIPOX CBV from i.m.b. is used. Thisis a two-component epoxy resin system (conventional resin/hardener system) filledwith aluminum oxide. In the development the processing/injection properties ofthe potting material have been adapted to site conditions in filling studies. Here,different unfilled and filled two-component epoxy resin systems have been inves-tigated. Special attention has been paid to inject the potting material into the steelhulls with a negligible porosity. In subsequent static tensile tests the load bearingcapacity of this anchoring system has been determined.

5. Mathematical Investigation of DYWICARB

The possibility to develop and optimize anchoring systems through experimentalinvestigations is restricted. The reasons for this are the insufficient testing andmeasuring methods, to measure the deformation and stress propagation in such an-choring systems. Furthermore, experimental optimization is always very expensiveand time-consuming. Numerical calculations however, permit speedy and todaymuch less expensive solutions. Numerical calculations of potting systems with theFinite Element Method (FEM) in fact still require simplifications, however, theyalso permit exact description of the dimensions. Nevertheless, up to now they havenearly not been made, except in [6, 7].


(a)

(b)Figure 3. Test setup (a) and failure behaviour (b) of the tendon with 7 CFRP-wires forcalibration and verification of the FE-model.


Figure 4. Comparison of measured and calculated (by FEM) stresses for a tendon with 7CFRP-wires.

With the FEM (FE-program MARC) it was not only tried to develop a modelto display the loading state of the complete anchoring system but also to carryout a parametric investigation with the aim of reducing the critical stresses in theanchorage. The FE model was calibrated and verified through a comparison withthe stresses measured in tests under static loading and the pull-out behaviour. Forthis purpose a CFRP-tendon with 7 CFRP-wires was produced (unfilled epoxyresin was used as potting material). The test setup with measuring equipment aswell as the failure behaviour of the tendon are shown in Figure 3.

The calibration of the stresses and the pull-out behaviour in the tests and in thecalculations could be achieved through optimization of the material properties andboundary conditions of displacement and forces (Figure 4 shows the comparison ofcalculated and measured stresses). A reduction of the critical stresses was possibleby the subsequent numerical parameter study. These results were confirmed intests, as explained below.

6. Experimental Investigations of DYWICARB

Considerable care is necessary for manufacturing of the potting anchorage (see Fig-ure 5), as the load bearing capacity of the complete stay cable/tendon is determinedby the anchorage. Above all, injection becomes more difficult with increasing num-ber of CFRP-wires, as proper filling of the potting material must be guaranteedeven in case of a high number of wires. With the developed DYWIPOX CBV,which is injected with pressure, a pore-free filling as well as a high load bearingcapacity under static and dynamic loading can be guaranteed. The first tests on the


(a)

(b)Figure 5. Cleaning of the CFRP-wires (a) and pressure-injection of the potting material (b).


Table II. Failure loads of DYWICARB staycables/tendons with 7, 19 and 91 CFRP-wires

Number of CFRP-wires Failure load [kN]

7 370

19 1020

91 3600

load bearing behaviour of the potting anchorage were carried out on tendons with7 CFRP-wires. Thus, the results of the calculation could immediately be realizedin practice in the tests, making it possible to quickly achieve a high load bearingcapacity of the anchorage for 7 CFRP-wires under static loading. On the basis ofthe results for a tendon with 7 CFRP-wires, the number of wires was increased to 19and finally to 91 CFRP-wires. Table II lists the failure loads determined in staticaltensile tests with DYWICARB stay cables/tendons with 7, 19 and 91 CFRP-wires.With the developed potting material DYWIPOX CBV a maximum load bearingcapacity of over 95% could be reached for DYWICARB systems with 7 and 19CFRP-wires. The failure load of the DYWICARB stay cable with 91 CFRP-wireswas determined after a test under dynamic loading.

The CFRP-wires mainly failed through tensile fracture in the free length be-ginning in the load-near area of the anchorage. Besides the static tests also dy-namic tests were carried out with DYWICARB systems. The most extensive testswere carried out on tendons with 7 CFRP-wires. The loading profile was as fol-lows: upper load range 60% to 65% of the theoretical failure load with amplitudesof 30 N/mm2 up to 200 N/mm2. In all tests two million load cycles could beachieved without failure or reduction of the load bearing capacity. The failureloads achieved in these tests partly were only slightly below those of tendon testedwithout pre-loading. Furthermore, a very good dynamic load bearing behaviourcould be observed at a tendon with 19 CFRP-wires.

The DYWICARB stay cable with 91 CFRP-wires was subjected to dynamicloading with a subsequent failure loading. The following loading profile (corre-sponding to PTI cable tests) was selected: upper load range 2252 kN= 45% oftheoretical failure load with an amplitude of 160 N/mm2. Two million load cyclescould be achieved without failure of the stay cable. In the test upper and bottomloads were more uniform than at steel tendons. After completion of the dynamictest the DYWICARB stay cable was loaded to failure in a statical tensile test. Thefirst CFRP-wire failed at a tensile load of 3500 kN, beginning in the load-neararea of the anchorage, this was followed by failure of further five CFRP-wires (seeFigure 6). The test was ended at a maximum load of 3600 kN.

Besides high load bearing capacity under static and dynamic loading as wellas manufacturing suitable to site conditions, handling as well as stressing of the


Figure 6. Failure behaviour of the DYWICARB stay cable with 91 CFRP-wires.

DYWICARB stay cable are of utmost importance for a fist application. If neces-sary, steel stay cables can be assembled directly in the structure and stressed as acomplete tendon or strand by strand. If the cable can be stressed strand by strand,small jacks easy to handle may be used. A DYWICARB stay cable can only bestressed as a whole.

Handling and the stressing process itself were successfully tested on a DY-WICARB stay cable with 19 CFRP-wires in a special tensile test body. The appliedstressing force of 500 kN corresponds to approx. 50% of failure load (1020 kN).The stay cable was stressed at the active anchorage side via a bar with the aid ofa jack supported on a stressing chair. Figure 7 shows the stressing process of theDYWICARB stay cable.

7. Conclusion and Outlook

Carbon fibre reinforced plastics offer a great potential to the construction industrydue to the outstanding axial properties in combination with low weight and cor-rosion resistance. In civil engineering CFRP has excellent chances for the futurein certain niches and special applications, not only under the aspect of excellentmaterial properties but in fact also under economical aspects. This can be seen onthe example of CFRP-laminates for subsequent strengthening and rehabilitationof structures. The high price is the primary reason for the presently somewhatreluctant application of these materials except for CFRP-laminates. Furthermore,there is still very few experience in construction practice with these new materials.


Figure 7. Stressing of a DYWICARB stay cable with 19 CFRP-wires.

Figure 8. DYWICARB stay cable with 91 CFRP-wires.

The DYWICARB system with 91 CFRP-wires shown in Figure 8 has beendeveloped by DSI in the past three years. Its high load bearing capacity understatic and dynamic loading, manufacturing suitable for site application as well asthe successful handling and stressing test of a DYWICARB stay cable point to firstapplications in future. These applications, which have to offer high potential foracceptance, must utilize the advantages of corrosion resistance and low weight tooffset the present disadvantages of high cost and lack of existing specifications.


References

1. Meier, U., Deuring, M., Meier, H., and Schwegler, G., ‘Strengthening of Structures withCFRP-Laminates: Research and Application in Switzerland’,Advanced Composite Materialsin Bridges and Structures, 1992, 243–251.

2. Meier, U., ‘Proposal for a Carbon Fibre Reinforced Composite Bridge Across the Strait ofGibraltar at its Narrowest Site’,Proc. Inst. Mech. Eng.201(B2), 1987, 73–78.

3. Saadatmanesh, H., and Ehsani, M. R., ‘Fiber Composites in Infrastructure’, in2nd InternationalConference ICCI, 05–07 January 1998, Tucson, Arizona.

4. Schurter, U., and Meier, B., ‘Stork Bridge Winthertur’,Schweizer Ingenieur und Architekt44,Oktober 1996, 976–979 (in German).

5. Noisternig, J. F., and Jungwirth, D., ‘CFRP-Tendons for Structural Application – Require-ments and Developments’, in2nd International Conference ICCI, 05–07 January 1998, Tucson,Arizona, pp. 115–127.

6. Lutz, E., Design and Analysis of a Composite Wire-Socket Attachment, PhD Thesis, VirginiaPolytechnic Institute and State University, 1994.

7. Noisternig, J. F.,Investigations of the Load Bearing Behaviour of Anchoring Systems for aCFRP-Strand(in German), Fortschritt-Berichte VDI-Reihe 4 Nr.133, VDI-Verlag, Düsseldorf,1996.

carbon fibre composites as stay cables for bridges

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