c5-schwegler_berset

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The use of prestressedCFRP-Laminates

as post-strengthening

Gregor SCHWEGLERPlssMeyerPartner AGLuzern, Switzerland

Thierry BERSETETH ZrichZrich, Switzerland

Paper presented at the 16th Congress of IABSESeptember 18-21, Luzern, Switzerland


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The use of prestressed CFRP-Laminates as post-strengthening

Gregor SCHWEGLER

Dr. sc. techn.

dipl. Ing. ETHPlssMeyerPartner AGLuzern, Switzerland

Gregor Schwegler, born 1963,received his civil engineeringdegree from the ETH Zurich in1990. His PhD was conferred onhim in 1994 by the sameuniversity and the EMPA

Thierry BERSET

dipl. Ing. ETH

Institute of StructuralEngineering, ETH ZurichZurich, Switzerland

Thierry Berset, born 1973,received his civil engineeringdegree from the ETH Zurichin1998

Summary

This article describes the use of prestressed externally glued Carbon Fibre Reinforced Plastic(CFRP) - laminates as post-strengthening of existing structures. The advantages of CFRP-laminatescompared to steel are discussed. Prestressing the CFRP-laminates results in varying positive effectson the strengthened section and allows to increase the utilisation factor of the high tensile strengthof CFRP. A system for prestressing and anchoring CFRP-laminates under on-site conditions ispresented. A series of laboratory tests was carried out. As a first test-application a transverse girderof a highway bridge was strengthened by a prestressed CFRP-plate.

Keywords: Carbon Fibre Reinforced Plastics, CFRP-laminates, Prestressing, post-strengthening

1. Introduction

There exist many structures such as bridges, storage-buildings or parking-decks with a insufficient

bending resistance that have to be replaced or reinforced. The reason can be a change in use, newstandards, changes in the structure itself due to conversion, ageing (corrosion and fatigue), damage(impact or fire) or mistake in design or realisation. In many of these cases strengthening allows thereuse of major parts of the existing structure instead of a complete replacement. Prestressed CarbonFibre Reinforced Plastic (CFRP) - laminates offer new possibilities for reinforcing existingstructures.

Externally bonded steel plates are used successfully for reinforcing since more than 30 years. Longterm studies on weather exposed beams with external steel-plate reinforcement showed thatcorrosion below the steel plates appears even without the influence of thawing salt. Also fatigue dueto friction in the zone of cracks in the concrete can be a problem. On the other hand, CFRP-laminates are non corroding in most environmental conditions and fatigue is normally no problem.

Another disadvantage of steel plates is their heavy weight which complicates the handling on site.By contrast, CFRP-laminates are extremely light weight and can be delivered on coils in lengths upto 300 m or more. They can be glued without the necessity of a temporary fixation.

A major disadvantage of CFRP-laminates was until recently the high cost of the material. But it isto be expected that the cost of CFRP will decrease to a level that is not higher than that of steel, ifthe tensile strength is compared. Considering the easy application and the excellent durability, theoverall cost of a CFRP-strengthening is already now cheaper than strengthening with steel plates.

2. The effect of prestressing

Prestressing the CFRP results in later yielding of the internal steel reinforcement and a higherbending resistance. The cost of prestressing can partially be compensated by a reduction of thenumber of CFRP-laminates.

Prestressed CFRP-laminates decrease the stress of the internal steel reinforcement even under deadloads. This results in a reduced deflection and overall crack width. A good crack distribution results


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from the stiff bond between CFRP and concrete. These effects increase the durability and improvethe fatigue behaviour.

In most cases the utilisation factor of the tensile strength of the CFRP is limited to a low levelbecause delamination between CFRP and concrete at the rim of shear cracks appears. Prestressingresults in a reduction of the displacement of the rims of shear cracks and therefore reduces the

danger of delamination.The main problem of prestressing CFRP-laminates is how to anchor the high tensile force at the endof the CFRP-plate. The ultimate shear stress between CFRP and concrete is too low and has a widerange that is influenced mainly by the material properties of the concrete and the crack distribution.

3. The Prestressing System

The presented prestressing system is based on the idea to anchor the end of the CFRP-plate in anon-metallic "StressHead" that lasts permanently on or in the strengthened structure.

Preparation of the surface of the concrete and application of Epoxy adhesive to CFRP-laminatesand concrete is done in the same way as for unstresed CFRP-laminates.

One part of the "StressHead" is anchored to theconcrete while the other is connected to the CFRP-laminates under controlled conditions. Thecompact dimensions of "StressHead" (length width height = 99mm 72mm 56mm) allow awide range of applications. Prestressing the CFRPis done by a hydraulic press. Then "StressHead" isfixed in its final position and the prestressingequipment can be removed.

All parts of "StressHead" that remain permanentlyon the strengthened structure are made of CFRP, sothe system has the same durability as the CFRP-laminates. A prototype of a anchor can be seen inFig. 1. On the right, the prestressed CFRP-plate

leaves the "StressHead". The straps on the left arejust used to fix the StressHead in the testingmachine. The CFRP-laminates are prestressed to

about P0 = 1500 N/mm2

with a CFRP-plate of 60mm 2.4mm. It is planned to increase thedimension of the prestressed CFRP plate and to achieve a prestressing force of up to 500 kN.

4. Laboratory testing

The development a anchor for prestressing CFRP-laminates started in 1995. First prototypes weremade of steel and aluminium because these materials, in contrary to CFRP, allow a easy variation ofthe geometry. A series of eight StressHeads of four different types was tested in 1999. It was

possible to anchor CFRP-laminates of 80mm 2.4mm up to their tensile strength. The next stepwas achieved by the end of 1999 with the first tests on anchors made completely of CFRP.

5. On site application

As a first test application a transverse girder of a highway bridge was strengthened with aprestressed CFRP-plate in 1999. The CFRP-plate served as a temporary external prestressing of aexisting bridge deck during construction time. This offered the opportunity to test the prestressingsystem "StressHead" under on site conditions and to inspect the complete in the course of thedemoliton.

A StressHead made of steel and aluminium as it was tested at the beginning of 1999 was used forthis application. There was no danger of corrosion because the prestressing system was only used

for a few months. Prestressing was controlled with two strain gauges on each CFRP-plate and bythe pressure of the hydraulic press. The strain of the CFRP-laminates was controlled about sevenmonths after prestressing. There was no significant loss of strain in the CFRP.

Fig. 1 Prototype of StressHead


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The use of prestressed CFRP-Laminates as post-strengthening

Gregor SCHWEGLER

Dr. sc. techn.

dipl. Ing. ETHPlssMeyerPartner AGLuzern, Switzerland

Gregor Schwegler, born 1963,received his civil engineeringdegree from the ETH Zurich in1990. His PhD was conferred onhim in 1994 by the sameuniversity and the EMPA

Thierry BERSET

dipl. Ing. ETH

Institute of StructuralEngineering, ETH ZurichZurich, Switzerland

Thierry Berset, born 1973,received his civil engineeringdegree from the ETH Zurichin1998

Summary

This article describes the use of prestressed externally glued Carbon Fibre Reinforced Plastic(CFRP) - laminates as post-strengthening of existing structures. The advantages of CFRP-laminatescompared to steel are discussed. Prestressing the CFRP-laminates results in varying positive effectson the strengthened section and allows to increase the utilisation factor of the high tensile strengthof CFRP. A system for prestressing and anchoring CFRP-laminates under on-site conditions ispresented. A series of laboratory tests was carried out. As a first test-application a transverse girderof a highway bridge was strengthened by a prestressed CFRP-plate.

Keywords: Carbon Fibre Reinforced Plastics, CFRP-laminates, Prestressing, post-strengthening

1. Introduction

There exist many structures such as bridges, storage-buildings or parking-decks with a insufficient

bending resistance that have to be replaced or reinforced. The reason can be a change in use, newstandards, changes in the structure itself due to conversion, ageing (corrosion and fatigue), damage(impact or fire) or mistake in design or realisation. In many of these cases strengthening allows thereuse of major parts of the existing structure instead of a complete replacement. Prestressed CarbonFibre Reinforced Plastic (CFRP) - laminates offer new possibilities for reinforcing existingstructures.

Externally bonded steel plates are used successfully for reinforcing since more than 30 years. Longterm studies on weather exposed beams with external steel-plate reinforcement showed thatcorrosion below the steel plates appears even without the influence of thawing salt [1 and 2]. Underharder environmental conditions, such as the strengthening of highway bridges with extensive useof thawing salt, corrosion of the steel plates is a serious problem. Mainly because of this reason thetechnology of strengthening concrete beams by externally bonded CFRP-laminates was developed

at the EMPA since 1985 [3]. CFRP-laminates are non-corroding in most environmental conditions,especially in the alkaline environment on a concrete surface [4].

In contrast to steel plates, where fatigue due to friction in the zone of cracks in the concrete can be aproblem, fatigue is normally no problem for CFRP-laminates. A CFRP-plate is a composite materialwhich consists of millions of carbon fibres that are only connected by the epoxy resin. Crackgrowing as in steel is therefore impossible, even if a single carbon fibre is extremely brittle.

Another disadvantage of steel plates compared to CFRP-plates is their heavy weight whichcomplicates the handling on site. The need to carry steel plates by hand restricts the length to about6-8 m. Joints are realised with glued single shear lap joints. The steel plates have to be hold inposition by dowels or another fixing system until the adhesive is hardened. Crossings of plates arevery expensive to realise. Normally, the thickness of one plate is doubled except in the zone of thecrossing, where a recess is arranged to offer place for the crossing plate. By contrast, CFRP-laminates are extremely light weight and can be delivered on coils in lengths up to 300 m or more.Thanks to the light weight, CFRP-laminates can be glued without the necessity of a temporary


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fixation. They are just pressed on the concrete surface. Crossings can be realised without anyspecial measures due to the thin CFRP-laminates that result from the very high strength of thematerial.

A major disadvantage of CFRP-laminates was until recently the high cost of the material. The costper kg of CFRP-laminates is about 100 times higher than that of ordinary steel. But they have a very

high tensile strength and a low specific gravity. This is why the cost of CFRP is only about twotimes higher than that of steel if the tensile strength is compared (Table 1). If the very simpleapplication of CFRP-laminates is also considered, the overall cost of a CFRP-reinforcing is alreadynow cheaper than strengthening with steel plates. The excellent durability of CFRP-laminatesresults in very low long term cost compared to steel.

Table 1 Comparison steel-plates - CFRP-laminates

steel plates CFRP-laminates

developed since 1967

in use since 1970

30 years experience

developed since 1985

in use since 1994

5 years experience

Mechanical properties and costPrice k = 2 CHF/kg k = 220 CHF/kg

Tensile strength f y = 235 MPa f t = 3000 MPa

Modulus of elasticity E = 210'000 MPa E = 170'000 MPa

specific gravity = 8000 kg/m3

= 1600 kg/m3

specific tensile strength fy / = 30 kNm/kg ft / = 1900 kNm/kg

specific modulus of

elasticityE / = 26 MNm/kg E / = 120 MNm/kg

strength / cost1

fy / (k)

=

15 kNm/CHF ft / (k) = 8.5 kNm/CHF

stiffness / cost2

E / (k) = 13 MNm/CHF E / (k) = 0.5 MNm/CHF

Application

Application expensive

temporary fixation, sometimes crane

simple and cheap

no fixation, just roll to the surface

Length of a unit 6...8m (weight, handling)joints necessary

coils up to 300m length

Crossings expensive very simple

Aesthetics steel plates are thick CFRP laminates are very thinpractically invisible

Durability

Corrosion between steel and gluede-icing salts cause problems

no corrosion under practically allenvironmental conditions

Fatigue fatigue due to friction at cracks no fatigue

1The unit 1 kNm/CHF means, that for the cost of 1 CHF a force of 1 kN can be transferred overa length of 1 m

2The unit 1 MNm/CHF means, that for the cost of 1 CHF at a strain of 1

0/00 a force of 1 kN is

transferred over a length of 1 m.


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The cost of CFRP-laminates that is given in Table 1 is rather too high. It is to be expected that thecost of CFRP will decrease to a level that is not higher than that of steel, if the tensile strength iscompared. The cost of CFRP to provide a certain stiffness however is very high compared to steel.This disadvantage can be partially compensated by prestressing the CFRP in many cases.

The use of externally bonded CFRP-laminates for strengthening structures in concrete, masonry or

wood is nowadays a established and reasonable technology.

2. The effect of prestressing

As shown above, the cost of CFRP-laminates is not higher than that of steel if the tensile strength iscompared. The cost of CFRP-laminates to provide a certain stiffness however is about 25 timeshigher than that of steel. There are many applications where the high tensile strength of CFRP cannot be used due to a lack of stiffness. CFRP-laminates with a higher stiffness (so called high-modulus CFRP) are available, but they are very expensive and have a lower tensile strength. Onepossible solution of this problem is to prestress the CFRP. The behaviour of RC-beams with astrengthening consisting of prestressed CFRP-laminates is described in [5]. Prestressing the CFRPresults in a later yielding of the internal steel reinforcement and a higher bending resistance. Inmany cases there is not enough place for a strengthening with unstressed CFRP-laminates. As

prestressing allows to make use of the high tensile strength of CFRP, the number of laminates canbe reduced. The cost of prestressing are partially compensated by the reduction of the number oflaminates.

Prestressed CFRP-laminates decrease the stress of the internal steel reinforcement even under deadloads. This results in a reduced deflection and overall crack width. A good crack distribution resultsfrom the stiff bond between CFRP and concrete. The overall behaviour of the strengthened structureunder service loads is therefore improved.

The reduction of tensile stress of the internal steel reinforcement together with a better crackdistribution results in a improved fatigue resistance of the strengthened structure.

In most cases the utilisation factor of the high tensile strength of CFRP-laminates is limited becausebond failure between CFRP and concrete at the rim of shear cracks can appear before tension failure

of the CFRP. Prestressing the concrete results in a reduction of the displacement of the rims of shearcracks and therefore allows to increase the admissible tensile stress in the CFRP.

The positive effect of externally bonded prestressed CFRP-laminates on the behaviour of thestrengthened structure is proven by calculation and testing. The CFRP-laminates should beprestressed as high as possible. The main problem is how to anchor the high tensile force at the endof the CFRP-plate. The ultimate shear stress between CFRP and concrete has a wide range and isinfluenced mainly by the material properties of the concrete and the crack distribution. As alreadymentioned, bond failure between CFRP and concrete limits the possible tensile stress in the CFRPin most cases. Continuous bond failure ends in failure of the CFRP-plate. The necessary highprestressing ratio can hardly be reached without a solution to anchor the end of the CFRP-plate.

3. The Prestressing System

The presented prestressing system that was used in laboratory tests and in test applications is basedon the idea to anchor the end of the CFRP-plate in a non-metallic "StressHead" that lastspermanently on or in the strengthened structure. The connection between CFRP-laminates and"StressHead" is assembled under controlled conditions. The range of the tensile resistance of thisconnection can be reduced to a acceptable extent. The reliability is much better than that of a gluedconnection between CFRP-plate and concrete, where the material properties of the concrete can notbe controlled.


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The application of a prestressed CFRP-plate using this System is realised in the following steps:

1. Preparation of the concrete surface in the same way as for conventionally glued CFRP-laminates

2. Connection of the CFRP-plate to the "StressHead". Mistakes in manipulation are prevented bythe design of the "StressHead". The connection will always reach the necessary resistance.

3. The "StressHead" is connected to the strengthened structure. The compact dimensions of the"StressHead" (length width height = 99mm 72mm 56mm) allow to adapt thisconnection to many situations.

4. Application of the epoxy adhesive to the CFRP-plate and the concrete surface in the same wayas for conventional strengthening with CFRP-laminates.

5. Pre-stressing the CFRP-plate by a hydraulic press. Force and displacement are controlled duringpre-stressing action. The "StressHead" is fixed in its final position and the prestressingequipment can be removed.

6. The CFRP laminates are pressed to the concrete in the same way as unstressed CFRP-laminates.

All parts of the "StressHead" that remain permanently on the strengthened structure are made of

CFRP. The whole system has the same durability as the CFRP-laminates. A prototype of thisStressHead as it was tested at the end of 1999 can be seen in Fig. 1. On the right, the prestressed

CFRP-plate leaves the StressHead. Thestraps on the left are just used to fix theStressHead in the testing machine andare not part of the StressHead used in aapplication. The CFRP-laminates areprestressed to about P0 = 1500 N/mm

2

which is equivalent to a pre-stressingratio of about 50 - 60% of the tensilestrength. The first prototypes are ableto prestress a CFRP-plate of 60mm

2.4mm. The prestressing force istherefore about 220 kN. It is planned toincrease the dimension of the pre-stressed CFRP plate and to anchor aprestressing force of up to 500 kN.

Fig. 1 Prototype of a "StressHead"


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4. Laboratory testing

The development of a anchor for prestressing CFRP-laminates started in 1995. The need for areliable and compact end anchor for CFRP-laminates came up during the research work on CFRP-

strengthened masonry [6]. Laboratory tests withprototypes made of steel were carried out from

1996 to 1999. Steel and aluminium were chosenbecause these materials, in contrary to CFRP,allow a easy variation of the geometry ofprototypes. A series of eight StressHeads of fourdifferent types was tested in 1999. It was possibleto anchor CFRP-laminates of 80mm 2.4mm upto their tensile strength. One example of a tension- strain - diagram is given in Fig. 2. The nominaltensile strength of the CFRP-plate used in this testwas 2800 N/mm

2. Rupture was observed outside

the anchor at a stress of 2900 N/mm2. The next

step was achieved by the end of 1999 with the first

tests on anchors made completely of CFRP. Theseanchors have the same durability as the CFRP-laminates (Fig. 1).

5. On site application

As a first test application a cross girder of a highway bridge was strengthened with a prestressedCFRP-plate in 1999 (Fig. 3). The bridge crosses the river Reuss in the canton Uri with three spans.

0

500

1000

1500

2000

2500

3000

0 5 10 15 20

Typ 3

[N/mm

2]

[0/00]

Fig. 3 Reussbrcke N4

Fig. 2 Example of a tension - strain diagram of a StressHead made of steel


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The cross section consists of two traffic lanes and a sidewalk. The existing bridge was built of fiveprestressed precast concrete beams and a precast deck with a in situ concrete covering. The mainbeams are connected by nine transverse girders. Shearing resistance was insufficient at the joints ofthe main beams in the mid-span. The thin precast concrete elements of the deck are also insufficientfor actual load cases, especially for wheel pressures. Concrete cover of reinforcement and tendonswas insufficient and corrosion appeared. A first study showed that strengthening and repair of the

existing bridge deck would cost about as much as a replacement by a new steel-concrete compositedeck.

One traffic lane had to be provided during construction time without interruption. As the bridgeaccess is on a dam on both sides of the bridge, a temporary bridge was too expensive to be realised.As first step, one side of the existing bridge was demolished. The new bridge deck was constructedin a displaced position (on the left in Fig. 4) while the rest of the old bridge served as traffic lane(on the right in Fig. 4). In a next phase, the rest of the existing bridge was demolished and the newbridge deck was pulled to its final position.

Fig. 4 Cross section during the construction of the new bridge deck

In the course of the demolition of the first half of the existing bridge, the prestressed concrete crossgirders had to be cut. There was no continuous reinforcement in the cross girders except of thetendons. The tendons are placed in flat-surfaced plastic ducts and therefore loose a significant

amount of prestressing when cut. It was necessary to replace these internal tendons by a externalprestressed steel ties before cutting the cross girders in order to provide load distribution and thestabilisation of the main beams. Eight of the nine cross girders were strengthened by external pre-stressed steel bars. One cross girder was prestressed by a CFRP-plate. This offered the possibility totest the prestressing system "StressHead" under on site conditions and to inspect the completesystem before and after demolition of the second half of the bridge.

A StressHead made of steel and aluminium as it was tested at the beginning of 1999 was used forthis application. There was no danger of corrosion because the prestressing system was only usedfor a few months. The CFRP-laminates are arranged in the form of a loop around the transversebeam as shown in Fig. 5. Both StressHeads in the middle of the bridge are supported by the samesteel plate. This guaranties a equivalent distribution of the prestressing force to both CFRP-laminates. The loop on the outer side of the bridge is formed by a steel saddle.

Prestressing was controlled with two strain gauges on each CFRP-plate and by the pressure of thehydraulic press.


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Fig. 5 Ground plan and sectional view of the arrangement of the CFRP-laminates

The strain of the CFRP-laminates was controlled just before demolition of the second part of theexisting bridge about seven months after prestressing. There was no significant loss of stress in theCFRP.

6. Conclusion

The use of CFRP-laminates is a efficient method for post-strengthening existing structures.Prestressed CFRP-laminates are in many cases superior to other strengthening methods in economy,

quality and durability. The main advantage of the presented prestressing system is that it is based ona simple anchor for CFRP-plates that is made completely of CFRP. This anchor could also be usedfor other applications such as no-bond prestressing.


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7. Literature

[1] LADNER, M., PRALONG, J. and WEDER, CH., Geklebte Bewehrung: Bemessung undErfahrungen, EMPA Bericht Nr. 116/5, Eidgenssische Materialprfungs- undForschungsanstalt, EMPA, April 1990

[2] EGGER, G. and CANTIENI, R., Fnfzehnjhrige Stahlbetonbalken mit aufgeklebterBewehrung, EMPA Bericht Nr. 150'582, Eidgenssische Materialprfungs- undForschungsanstalt, EMPA, September 1994

[3] KAISER, HP., Bewehren von Stahlbeton mit Kohlenstoffaserverstrkten Epoxidharzen, ETHDissertation Nr. 8918, Eidgenssische Materialprfungs- und Forschungsanstalt EMPA, 1989

[4] HANCOX, N.L. and MAYER, R.M., Design Data for Reinforced Plastics, A Guide forEngineers and Designers, Chapman & Hall, London, 1994

[5] DEURING, M., Verstrken von Stahlbeton mit gespannten Faserverbundwerkstoffen, EMPABericht Nr. 224, Eidgenssische Materialprfungs- und Forschungsanstalt, EMPA, 1993

[6] SCHWEGLER, G., Verstrken von Mauerwerk mit Hochleistungsfaserverbund-Werkstoffen,ETH Dissertation Nr. 10672, EMPA-Bericht Nr. 229, Eidgenssiche Materialprfungs- undForschungsanstalt EMPA, 1994

[7] SCHWEGLER, G., Verstrkung von Mauerwerk mit CFK-Lamellen sowie CFK-Verstrkungen im Mauerwerks- und Holzbau. Nachtrgliche Verstrkung von Bauwerken mitCFK-Lamellen, EMPA/SIA-Studientagung. Schweizerischer Ingenieur- und Architekten-Verein, Dokumentation D 0128, S.71-83 sowie S.61-65, 21. September 1995

[8] MEIER U., DEURING M., MEIER H. and SCHWEGLER G., Strengthening of Structureswith CFRP Laminates: Research and Applications in Switzerland, Advanced CompositeMaterials in Bridges and Structures, Canadian Society for Civil Engineering, 1992, pp 243-251

[9] MEIER U., Carbon Fiber-Reinforced Polymers: Modern Materials in Bridge Engineering.Structural Engineering International No 1, 1992

[10] Nachtrgliche Verstrkung von Bauwerken mit CFK-Lamellen, EMPA/SIA Studientagung,Schweizerischer Ingenieur- und Architekten- Verein, Dokumentation D 0128, 21. September1995

[11] Verstrken von Tragwerken mit CFK-Lamellen, TFB Fachveranstaltung, TechnischeForschung und Beratung fr Zement und Beton, 12. Mai 1998

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