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Post-strengthening ofRC Structures withExternally Bonded

Prestressed CFRP Strips

Hans-Peter ANDR

Leonhardt, Andr & PartnerConsulting EngineersStuttgart, Germany

Markus MAIER

Leonhardt, Andr & PartnerConsulting EngineersStuttgart, Germany

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


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2000 by Leonhardt, Andr & Partners, Consulting Engineers, Germany Taiwan

page 1 of 8

Post-strengthening of RC Structures with

Externally Bonded Prestressed CFRP Strips

Dr.-Ing.

Hans-Peter ANDRManaging DirectorLeonhardt, Andr & PartnerConsulting Engineers VBIStuttgart, Germany

Dipl.-Ing.Markus MAIER

Project Engineer

Leonhardt, Andr & Partner

1972 Diplom Ingenieur

University of Stuttgart, Germany1977 Master of ScienceUniversity of Calgary, Canada

1979 Project EngineerLeonhardt, Andr und Partner

1982 Dr.-IngUniversity of Stuttgart, Germany

Since 1988 Managing Director and PartnerLeonhardt, Andr & Partner

Since 1989 Control Engineer

Ministry of the Interior, Germany

SummaryA new prestressing and anchorage system for post-tensioned, externally bonded CFRP stripswhich is suitable for on-site application has been developed by Leonhardt, Andr & Partner.This system provides a maximum prestress of about 1000 N/mm (at a strain of 5.5 0/00 ) toeach laminate and can be directly anchored on the surface of the member to be strengthened.The system was tested at the Swiss Federal Laboratories for Material Testing and Research(EMPA) in Zrich.

This paper describes the first on-site application anywhere in the world, carried outsuccessfully at a prestressed concrete bridge in southern Germany in October 1998, as well asa second application carried out to post-strengthen an existing r/c shear wall in July 1999.

In addition, the features of the stressing system, the results of the large-scale EMPA-test, thefinal on-site application and the problems which had to be faced as well as new experiencesgained with prestressed CFRP strips will be presented and illustrated in this paper.

Keywords :Prestressed Carbon Fibre Reinforced Polymer (CFRP) Laminates, ExternallyBonded CFRP Surface Tendon, Post-tensioning System, On-Site Application,Anchorage Shear Stress

1. IntroductionExternally bonded carbon fibre reinforced polymer laminates are particularly suitable forin-situ rehabilitation, post-strengthening and structural repair of reinforced/prestressedconcrete structures.

The advantages of CFRP strips for external adhesive reinforcement can be summarised asfollows:

Available in any length Corrosion resistant Very high tensile strength Extremely low weight Low installation costs Easy to join Flexible (the minimum radius at which the strips can be transported in coiled form is

0.90m)


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page 2 of 8

The flexural strengthening of reinforced concrete using externally bonded Sika CarboDurCFRP strips has been authorised by the German Institute of Construction Technology since1997. Design regulations and guidelines for strengthening with Sika CarboDur have beendefined and the use of CFRP strips which are unstressed at the time of bonding is alreadycommon in practical structural design.

As the CFRP possesses no yielding deformation reserves (figure 1), the maximum bendingstrength of a strengthened section is obtained if the CFRP plate ruptures once the internal steelreinforcement has reached its yield limit, but prior to concrete fracture. Various tests showedthat the bending post-strengthened cross-section (with full composite action) cannot follow anunlimited laminate strain, that means Bernoullis hypothesis can no longer be assumed forstrains in the upper range. Delamination and debonding effects occur.

Figure 1 shows the linear elastic stress-strain relationship of a Sika CarboDur CFRP strip

For this reason, the strain which can be used in the design of non-prestressed CFRP strips islimited to 0.8%, i.e. only 50% of the ultimate strain capacity. In order to also make use of theupper range of the CFRP strain capacity, the strip has to be prestrained or prestressed right

before the bonding action. In that way it is possible to make maximum use of the full loadbearing capacity of this high-tensile material.

For this reason, a post-tensioning system suitable for on-site application was developed byLeonhardt, Andr & Partner and the first practical on-site application anywhere in the worldwas carried out successfully.

2. Prestressed Externally Bonded CFRP Laminates2.1 Advantages of Prestressed CFRP Strips

The advantages of using prestressed CFRP strips as externally bonded surface tendons can besummarised as follows:

The upper range of the strain and stress capacity can be used

Strains on the internal reinforcement can be relieved

The strips participate in carrying dead load and permanent loads at the time of bonding

The strips participate in bearing the load of additional loadings without any slip-delay

At low temperatures (member-shrinkage) the CFRP strip still remains a tension element

Cracked sections can be stressed/compressed back to the uncracked state, thus gaining in

stiffness

Crack widths and bending deflection can be reduced, thus improving serviceability

0,5 % 1,0 % 1,5% 1,6%

3000

f strip/mm

2000

1000

Linear elastic stress strain relationship

=170.

000/

mm

2800

Posttensioning Loading


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page 3 of 8

2.2 Post-tensioning system for on-site applications

Already in 1993, Deuring [1] succeeded in prestressing CFRP strips at EMPA laboratories inZrich, Switzerland. However, this tensioning system is not suitable for on-site application.

The system is based on the principle of a stationary violin bow, the tensioned string being

the CFRP strip and the bow the necessary complementary compression element. This methodprovides for the member to be strengthened to be placed onto the ready-stressed CFRP strip.

However, once the violin bows ends were cut off, the laminate started peeling off themembers surface, starting at the end of the strips and resulting in the concrete cover beingripped off, as the concretes tensile strength was exceeded due to an excessive anchorage

bonding shear stress.

Figure 2 in principle shows the difference between tensioning in laboratories and thetensioning system developed by Leonhardt, Andr & Partner for on-site application

For practical on-site application, a new post-tensioning and anchorage system was thereforedeveloped by Leonhardt, Andr & Partner. This system can be directly anchored on thesurface of the member to be strengthened. The system can be easily handled on-site and issuitable for both vertical and overhead application. It provides a maximum prestress of about1000 N/mm (at a strain of 0.55%) to each laminate during the pot life of the adhesive.

Figure 3: Miniature-jack, the systems core Figure 4: Stressing operationperformance being 70 kN at 1450 bar

compression in violin bow

lt2 lt2

peeling-off effect

Tensioning in laboratories

s

s

non-

prestressed

non-

prestressed

prestressed

compression in member

anchorage smoothens shear peaks

Tensioning on site


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page 4 of 8

A plate embedded and bonded in a concrete cover recess is used as jacking platform and tosmoothen the residual shear peaks of the bonded anchorage. The peeling and debondingtendencies affecting the ends of a prestressed laminate could thereby be counteracted.

The bonded unstressed dead strip ends serve as an additional anchorage reinforcement.The stressing operation is carried out using a hydraulic tandem cylinder miniature jack

(figure 3) during the pot life of the adhesive.The single stages of the prestressing and anchorage operation at the live end of the CFRPtendon are shown in figures 4 and 5.

Figure 5: Stages (1) to (3) of the stressing/anchorage operation(1) Stressing operation (2) Curing of the adhesive (3) Final (permanent) stage

The sequence of operations is shown in figure 6

Figure 6: Sequence of operations during application of prestressed CFRP surface-tendon

8. Installation of permanent anchor

plates (wet)

9. Removal of stressing system

10. Filling of recesses

11. Bonding of laminates non-prestressed

ends

12. Final surface treatment,

covering with mortar

1. Injection of resin into crack2. Preparation of concrete surface

3. Installation of anchor plates in concrete

cover recess

4. Application of adhesive to strip

5. Application of strip to concrete

6. Installation of sleigh plates and

preliminary dead anchor plate (dry)

7 Tensioning operation during pot life

of adhesive

Live Dead

s


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page 5 of 8

2.3 Large Scale Test at EMPA Laboratories, Zrich

The prestressing system has been tested successfully in a large scale test at EMPA, SwissFederal Laboratories for Materials Testing and Research. The testing set and the recordings ofthe strain gauges during the stressing operation are shown in figures 7 and 8.

As the prestressed externally bonded CFRP tendon is meant to be applied directly to thecracked member, the test beam was loaded before the application until cracks showed upclearly. Additional strain gauges were installed to monitor the laminates behaviour in thevicinity of those cracks while cyclic loadings close to the ultimate load range were applied

It was suspected that in-between the crack faces the strain on the laminate will rise rapidlyclose to the ultimate strain capacity and brittle fracture will occur.

However, the recordings showed some kind of ductile behaviour effect. The strainsimmediately adjacent to the monitored cracks were running behind the in-between cracks.

Figure 7: Final test of the LEOBA Figure 8: Strain gauges recordings

CarboDur post-tensioning system during stressing operation(EMPA test report [4])

3. Practical On-site Application of Prestressed CFRP Strips

3.1 Rehabilitation of the Lauterbridgein October 1998

3.1.1Damage

The bridge, designed as a 4-span continuous beam for each individual lane with internallybonded tendons, showed wide cracks at the bottom of the section right next to the first

columns supports. It was built in 1970 and is located near Gomadingen, Southern Germany.These cracks, which were found in the supposed compression area of the section, can beexplained as follows:

- The internal tendons were prestressed excessively.

- The tendons parabolic line was not adapted correctly to the bending moment of asupported section.

- The dead load of the concrete structure itself is presumed to be too high.

- The combined structural effect of the two beams due to the obliquely angled support ofthe columns was not considered in design.

LamellendehnungVersuchsbalkenV1,SpannenderLamelleam7.7.1998

-1,0

0,0

1,0

2,0

3,0

4,0

5,0

6,0

0 20 40 60 80 100 120

[min]

Dehnung[

o/oo

]

DMS K4

DMS K5

DMS K6


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page 6 of 8

Figure 9: Bottomview and sideview of the Lauterbridge in Gomadingen. Structure showed wide

cracks right next to the columns support in the supposed compression area

3.1.2Rehabilitation

The first practical on-site application of bonded prestressed CFRP strips anywhere in theworld was carried out successfully in October 1998.

By applying prestressed externally bonded CFRP strips, combined with a prior injection ofresin, a local compression zone was created. The cracked section could be forced back into anuncracked state.

This gain in stiffness improved the serviceability and relieved the strains on the internalreinforcement. The crack faces were taped and fixed together. They will no longer breathe

while bearing cyclic and reversed loading.

A non-prestressed application could not have achieved the same effect in this sensitivecompression-tension reversal zone.

Figure 10: Sideview of the Lauterbridge in Gomadingen

Figure 11: Four LEOBA-CarboDUR CFRP tendons at each cracked section with a maximumprestress of 1000 N/mm at a pre-strain of 0.55 % for each strip; loose ends not yetbonded

Underside

cracks


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page 7 of 8

3.2 Post-strengthening of Existing RC Shear Wall at Bank of Langen in July 1999

Structural alterations required two new openings to be cut into an existing reinforced concreteshear wall structure under full loading at a bank building in Langen, Germany.

As can be gathered from the simplified truss model shown in figure 12, the horizontal tensionforces, resulting from the change in direction of the compression stress on top of and below

the new openings, tend to form cracks in the existing structure.The internal reinforcement below the new openings turned out to be sufficient, as thereinforcement of the slab could be taken into account.

However, the section above the openings lacked a horizontal internal reinforcement and, ascalculations showed, could not withstand the developing tension stresses without formingwide cracks, reaching probably up to the slab above.

An additional external reinforcement, which would participate in bearing the tension stresswithout any slip delay, was required.

The method used to prevent any cracks from forming was to concentrically pre-compressthe concrete section above the new openings by applying externally bonded prestressed CFRP

strips before cutting out the openings. The degree of concrete pre-compression was designedaccording to the expected tension stress.

Therefore seven CFRP strips, with a total of 420 kN prestressing force, were applied asexternally bonded surface tendons before cutting work began.

The application was carried out successfully in July 1999.

Figure 12: Simplified truss model and the arrangement of the post-tensioned CFRP stripsapplied in order to pre-compress the concrete section before cutting the newopenings.

Figure 13: Rear view of the strengthened shear wall to which seven LEOBA-CarboDURprestressed CFRP strips and an additional four unstressed CFRP strips wereapplied; the front is already covered with fire protection plates


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page 8 of 8

4. Conclusion

In addition to the well known advantages of using unstressed CFRP composites as externallybonded reinforcement in structural repair and rehabilitation work, the application of externallybonded prestressed CFRP strips will be an even more effective and economic method of

improving structural performance.

With this method, the load bearing capacity of a strengthened section can be activelyinfluenced by the external stressing force before the time of bonding.

Combined with a prior injection of resin, cracked sections can be compressed back to theuncracked state, thus gaining back flexural rigidity with a minimum of additional crosssectional area. This gain in stiffness will improve the serviceability and relieve the strains onthe internal reinforcement. The crack faces are taped and tightened up and will not breatheanymore while bearing cyclic loading. This might be a convenient and low-budget method tosolve the increasing coupler-joint cracking problems encountered at a huge number of

prestressed concrete bridges equipped with internally bonded tendons.

The likelihood of cracks forming in existing reinforced concrete structures due to structuralalterations (e.g. new openings) and changes to the statical system can be completely avoided

by applying post-tensioned CFRP strips. On account of the residual truss-model tensionforces, the concrete can be precompressed before alteration work begins. In addition, the

prestressed external CFRP reinforcement can be activated immediately for additional loadingswithout any slip-delay.

5. References

[1] Martin Deuring, EMPA, Swiss Federal Laboratories for Materials Testing,

Report No. 224, 1993[2] Rostasy, Holzenkmpfer, Hankers, Geklebte Bewehrung fr die Verstrkung von

Betonbauteilen, Betonkalender 1996, Berlin Ernst& Sohn

[3] Sika CarboDur Bonded Carbon Fibre Plate, German Institute of ConstructionTechnology, Berlin, authorisation No. Z-36.12-29

[4] EMPA, Swiss Federal Laboratories for Materials Testing, ReportNo172745/21998

[5] H P Andr, M. Maier, Zukunftsweisende Entwicklung fr Bauteilverstrkung undErtchtigung, LEOBA-CarboDur als Oberflchenspannglied, IBK-Fachtagung 241,Darmstadt 1999

[6] H P Andr, Verstrkung von Brcken durch aufgeklebte, vorgespannte

Kohlefaserlamellen, Brckenbautagung der Bayerischen Straenbauverwaltung, 28./29.Sept. in Kulmbach

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