Carbon Fibre Sheet Polymer

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2nd Canadian Conference onEffective Design of Struc turesMcMaster UniversityHamil ton, Ontario, CanadaMay 20 – 23, 2008

Development of a Carbon Fiber Reinforced Polymer System forStrengthening and Repair of Steel Bridges and Structures

M. Dawood 1 and S. Rizkalla 2 1Ph.D. Candidate, Constructed Facilities Laboratory, Department of Civil, Construction and Environmental

Engineering, North Carolina State University, Raleigh, NC, 27695-7533, USA, [email protected] Professor of Civil Engineering and Construction, Constructed Facilities Laboratory, Department of

Civil, Construction and Environmental Engineering, North Carolina State University, [email protected]

Abstract

This paper summarizes the results of a comprehensive research program conducted to develop astrengthening and repair system for steel bridges and structures using carbon fiber reinforced

polymers (CFRP). The research program was completed in five phases. In the first phase asuitable adhesive was selected to bond CFRP materials to steel surfaces. In the second phasethree large-scale beams were tested to study the effectiveness of various strengthening

configurations. The third phase of the research program examined the overloading and fatigue behavior of the system. A detailed study of the bond characteristics was conducted in the fourth phase. The fifth phase consists of an accelerated environmental durability program to evaluatethe durability of the system. Based on the research findings flexural design guidelines are

presented. This paper demonstrates that the proposed CFRP strengthening system could be usedeffectively to enhance the serviceability and ultimate load-carrying capacity of typical steelstructures and bridges.

Keywords: CFRP, fatigue, overloading, bond, environmental durability, design guidelines

Introduction

A number of researchers have demonstrated that carbon fiber reinforced polymer (CFRP)materials can be used to strengthen steel flexural members 1,2 . In one early study a conventionalmodulus CFRP system was developed by considering a number of factors including materialselection, system behavior under static and fatigue loads, environmental durability and basic

bond characteristics 3. However, the research did not provide guidelines to facilitate the designand implementation of the system for field application. Most early research focused on the use



of conventional CFRP materials with a modulus of elasticity typically equal to or less than thatof steel. Thus, although the materials could effectively enhance the strength of the structure,large amounts of strengthening materials were required to improve the serviceability.

Recent research has focused on the bond characteristics of CFRP materials bonded to steelsurfaces. A number of analytical \models have been developed to describe the bond behavior 4,5 .

Other research, primarily in the mechanical, aerospace and marine industries, have demonstratedthat bond strength is highly affected by the shape of the end of the strengthening plate 6,7,8 .Research on the environmental durability of the CFRP-to-steel bond is limited, particularly in thecivil engineering and infrastructure disciplines.

This paper summarizes the various aspects which have been considered to develop a highmodulus (HM) CFRP strengthening system for steel flexural members. The system developmentwas based on a comprehensive research program, which has been on-going for the last six years.The experimental and analytical research program has been designed to take into fullconsideration the significant research advances which have been made throughout this timeframe. This paper describes the adhesive selection, large-scale verification, fatigue andoverloading behavior, bond characteristics and environmental durability of a proposed HM

CFRP system for strengthening steel flexural members. Flexural design guidelines are also presented. This paper demonstrates that the proposed CFRP system can be effectively used toenhance the serviceability and ultimate strength of steel bridges and structures.

CFRP strengthening system

The proposed strengthening system consists of HM carbon fiber materials and a two-part roomtemperature cure epoxy. The carbon fiber materials included dry fiber tow sheets and precuredCFRP plates. The fiber sheets are suitable for applications with complex geometricalconfigurations such as curved girders or monopole towers. These are typically impregnated withan epoxy resin in-situ which also acts to bond the fibers to the structure. The CFRP plates aresuitable for applications requiring a higher level of strengthening. The relatively high modulusof the CFRP materials makes them well suited to enhance the serviceability of steel structures.

Several different types of fibers, CFRP plates and adhesives were investigated in the first two phases of the research. Based on the initial trials, the materials presented in Table 1 wereselected for further investigation in the remaining four phases of the research program.

Table 1: HM carbon fiber material properties

Carbon Fiber 9 CFRP Plate * Adhesive **

Elastic modulus, E 640 GPa 418,000 MPa 2980 MPaUltimate strength, f u 2600 MPa 1540 MPa 38 MPaTensile rupture strain, εu 0.004 0.0037 0.0148

*measured according to ASTM D3039, **measured according to ASTM D638

Phase I: Adhesive and resin selection

The first phase of the system development focused on the selection of appropriate adhesives to bond the HM carbon fiber materials to steel surfaces. This included selection of a saturatingresin to bond the dry fiber tow sheets and selection of an appropriate structural adhesive for

bonding the pultruded CFRP plates 10.



Ten different saturating resins were compared through a series of double-lap shear coupontests to select a suitable resin for bonding carbon fiber tow sheets to steel surfaces. For the best

performing resins, failure of the coupons was predominantly by rupture of the carbon fibersindicating complete utilization of the HM CFRP materials. An average shear stress of 12 MPawas measured for these resins prior to rupture of the fibers. A second group of resins failed due

to pull-out of the fibers from the resin which indicated possible incomplete wetting of the fibers.The use of a wetting agent was investigated to improve the saturation of the fibers and increasethe capacity of the bond. However, no significant improvement of the performance wasobserved. The poor performance of some of the resins was due to debonding of the CFRP fromthe steel surface. To enhance the performance of the resins an elevated temperature cure cyclewas also investigated, however, no significant improvement of the performance was observed.

Small-scale flexural tests were conducted to select an effective adhesive for bonding CFRP plates to steel beams. These tests were selected to represent typical bond stress distributionwithin the adhesive layer for beam applications. The test specimens consisted of an 813 mmlong steel super light beam (SLB) with a steel plate welded to the compression flange to simulatethe presence of a composite concrete deck slab. The beams were strengthened by bonding

36 mm wide x 1.4 mm thick CFRP plates to the bottom of the tension flange. Various platelengths were considered to determine the minimum bond length required to develop the fulltension strength of the CFRP materials. The beams were subsequently loaded to failure in thefour point bending configuration shown in the figure. A total of six different structural adhesiveswere evaluated using this test configuration. The two part epoxy adhesive that was selected forfurther development as a part of the HM CFRP strengthening system was capable of developingthe full rupture strength of the 1.4 mm thick HM CFRP strips within a development length of102 mm.

Phase II: Large-scale verification

Three large-scale steel-concrete composite beams were tested to investigate the effectiveness ofdifferent configurations of CFRP plates to increase the strength and stiffness of typical steel-concrete composite bridge girders 10. Details of the testing program are presented in Table 2.Both intermediate (IM) and high modulus CFRP plates were considered. The elastic modulus ofthe HM CFRP was twice that of the IM CFRP. However, the high modulus fibers were alsoconsiderably more brittle, with an ultimate strain of only 0.004 compared to 0.006 for the IMfibers 9. The appropriate material should be selected for the particular strengthening application

being considered. The possibility of prestressing the HM CFRP strips to a stress levelcorresponding to 18 percent of the ultimate strength of the CFRP was also investigated.

Table 2: Experimental program for large-scale verification phase

CFRP Strip PropertiesBeamIdentifier

CFRP Ratio,*

TensionModulus

TensionStrength

Thick. FVF InstallationTechnique

IM-4.5-AB 4.5% 229 GPa 1220 MPa 3.2 mm 55% BondedHM-7.6-AB 7.6% 460 GPa 1530 MPa 4.0 mm 70% BondedHM-3.8-PS 3.8% 460 GPa 1530 MPa 2.9 mm 70% Prestressed

*Ratio of the CFRP cross-sectional area, accounting for the fiber volume fraction (FVF), to thetotal cross-sectional area of the steel section



The typical test beams consisted of a W310x45 steel section acting in composite action withan 840x100 mm reinforced concrete deck slab. Prior to installation of the CFRP materials, theunstrengthened beams were loaded to 60 percent of the yield load and unloaded to determinetheir initial stiffness. After strengthening all of the beams were loaded monotonically to failureusing a four-point bending configuration with a total span of 6400 mm and a 1000 mm constant

moment region.The load-deflection behavior of the three beams, given in Figure 1, was essentially linear upto rupture of the CFRP with a slight deviation from linearity occurring after yielding of the steel

beam. The behavior of the beams after rupture of the CFRP was similar to that of anunstrengthened beam. Failure occurred due to crushing of the concrete.

Figure 1 – Load-deflection behavior of large-scale steel-concrete composite beams 11

The percentage increase of the flexural strength and elastic stiffness due to the presence of theCFRP is given in the table in Figure 2(a) for the three tested beams. The findings demonstratethat the different strengthening systems increased the elastic stiffness and the ultimate capacityof the strengthened beams by up to 36% and 45% respectively. The reported stiffness increasewas determined as compared to the measured initial stiffness of the unstrengthened beams. Thestrength increase was determined by comparing the load at rupture of the CFRP to the load atcrushing of the concrete. The latter represents the typical failure mode of the unstrengthened

beams. While the HM and IM systems were designed to increase the ultimate capacity of the beams, the prestressed beam was designed primarily to increase the stiffness, without increasingthe flexural capacity of the section. This may be advantageous in cases where it is desired toimprove the serviceability of the member while maintaining the full ductility of the original

section. By comparing the results for beams HM-7.6-AB and HM-3.8-PS that are tabulated inFigure 2(a), it is clear that the use of the prestressed strips helped to improve the efficiency of thestrengthening system by reducing the amount of strengthening required to obtain a comparableincrease of the elastic stiffness. A typical failed beam is shown in Figure 2(b).



(a)

Figure 2 – Large-scale beam behavior (a) Increase of flexural strength and stiffness(b) Typical failed beam

Phase III: Overloading and fatigue performance

Six small-scale steel-concrete composite beams were tested under fatigue and overloadingconditions to study the behavior of the strengthening system under severe loading conditions 11 .The beams were tested in four-point bending with a span of 3050 mm and a 610 mm longconstant moment region.

Two of the test beams were strengthened with different reinforcement ratios of high modulusCFRP materials. Several loading and unloading cycles were conducted to simulate severeoverloading of the beams. A third unstrengthened beam was tested as a control beam forcomparison purposes. In addition to increasing the ultimate capacity and elastic stiffness of the

beams, installation of the high modulus CFRP materials also helped to increase the yield load.The presence of the CFRP materials also reduced the residual deflection of the strengthened

beams after unloading compared to the unstrengthened beam. This suggests that due to an

overloading event an unstrengthened beam would likely exhibit a significant residual deflectionwhich may necessitate replacement of the member while a similar strengthened beam couldremain in excellent serviceable condition.

The remaining three test beams were tested under fatigue loading conditions. Two of the beams were strengthened using the same reinforcement ratio of CFRP materials however, usingdifferent bonding techniques to investigate the effect of the bond on the fatigue performance ofthe system. The third beam remained unstrengthened to serve as a control beam for the fatiguestudy. All three of the test beams were subjected to three million fatigue loading cycles at afrequency of 3 Hz. The minimum applied load used for the cyclic loading for all three beamswas selected to be equivalent to 30 percent of the calculated yield load of the unstrengthened

beams to simulate the effect of the sustained dead-load for a typical bridge structure. For the

unstrengthened beam, the maximum load in the loading cycle was selected to be equivalent to60 percent of the calculated yield load to simulate the combined effect of dead-load and live-load. The maximum load for the strengthened beams simulated an increase of 20 percent of theallowable live-load level in comparison to the unstrengthened beam. The maximum applied loadin a fatigue cycle was thus equal to 60 percent of the calculated increased yield load of thestrengthened beams. Both strengthened beams sustained the three million load cycles at thesimulated increased live load level without showing any indications of degradation or failure. All

SpecimenIdentifier

Strengthincrease

Stiffnessincrease

IM-4.5-AB 16% 10%HM-7.6-AB 45% 36%HM-3.8-PS - 31%

(b)Failed CFRP plate



three beams were tested to failure at the completion of the fatigue study. The behavior wassimilar to the behavior of unfatigued beams.

Phase IV: Bond behavior

In the fourth phase of the research, a detailed experimental program was conducted to study the bond behavior of the proposed strengthening system 12. A thorough understanding of the bond behavior is necessary for application of the strengthening system to longer span structures whichrequire bonded splice joints to develop total continuity and complete composite action of thestrengthening system with the steel beam. The experimental program consisted of a total of eightdouble lap shear coupons and ten steel beams with a CFRP splice joint at the midspan locationwithin the constant moment region. The different parameters studied included the shape of the

plate end, the presence of additional mechanical anchorage by either a transverse CFRP wrap orsteel clamp and the total length of the splice plate.

All of the tested coupons and beams failed by sudden debonding of the splice plate prior torupture of the CFRP. The maximum measured strain in the main CFRP plate ranged from 23%

to 67% of the measured rupture strain of the CFRP. The findings indicate that the shape of the plate end had the most significant effect on the joint capacity. While a square plate end iscurrently the most commonly used configuration, the experimental results indicate thatimplementation of a reverse tapered joint detail as shown in Figure 3, can approximately doublethe strength of the spliced connection. The double-lap shear coupon tests suggest that for some

plate end configurations the presence of a steel clamp near the plate ends could further increasethe joint capacity by up to 80%. However, the beam tests indicated that both types ofmechanical anchorage near the plate end, the transverse CFRP wrap and the steel clamp, did notincrease the joint strength. Inspection of the failed specimens suggested that careful detailing ofthe anchorage system is essential if any increase of joint strength is to be achieved. The findingsindicate that, using proper detailing of the reverse tapered plate end, the proposed strengtheningsystem can be used to enhance the serviceability of a long-span steel beam even if a splice jointis located at a location of relatively high moment.

Figure 3 – Details of reverse tapered plate end configuration

A suitable failure criterion is currently being developed based on the fundamental material properties of the adhesive and the characteristics of the bond interface. The adhesive used in thestrengthening system is essentially linear, elastic and brittle in nature. Thus, the maximum

principal stress in the adhesive can be calculated using a linear-elastic finite element analysis.This stress can be used to predict the ultimate strength of the spliced connections. The ultimatestrength of the spliced connections could possibly be governed by the shear strength or thetension strength of the adhesive or of the bond interface. To determine the pure tension strengthof the bond interface, pull-off tests were conducted as shown in Figure 4a. The average

Steel tension flange

End of CFRPsplice plate

MainCFRPplate

Center of spliceNot to scale



measured bond strength of the interface was 18 MPa. This corresponds to approximately 50% ofthe measured tension strength of the adhesive. To determine the pure shear strength of theadhesive and of the bond interface, a specially designed torsion test device, shown in Figure 4bwas fabricated. Load was applied to the steel shaft using a torque wrench and the applied torquewas measured using a strain gauge based torque transducer. The average measured shear

strength of the adhesive and the bonded interface were 48 MPa and 23 MPa respectively.

Figure 4 – Failure criterion test setup (a) pure tension (b) pure shear

A 2-D linear finite element analysis was conducted for the double-lap shear coupons withsquare plate ends. The results were used to determine the magnitude of the principal stress nearthe end of the splice plate for the tested coupons immediately prior to failure. Since thecalculated stresses directly at the end of the splice plate were highly affected by the presence ofthe singularity at this location, the maximum principal stress near, but not at, the plate end wasconsidered to asses the failure of the joints. The maximum principal stress determined from theFE analysis 0.4 mm away from the plate end was between 32 MPa and 35 MPa for all of thetested coupons. These values are within the range of the measured tension strength of theadhesive and the bond interface. Thus, the average tension strength of the bond interface asmeasured by pull-off tests can possibly be used as a conservative failure criterion for the designof bonded joints. The analysis further indicates that implementation of the reverse tapered plateend detail can reduce the magnitude of the shear stress concentration by approximately 20% andcan practically eliminate the peeling stress concentration at that location.

Phase V: Environmental durability

A comprehensive experimental program is currently ongoing to study the environmentaldurability of the proposed strengthening system. The experimental program includes a total of52 CFRP-to-steel double lap shear coupons. Two methods of enhancing the durability of thesystem are considered. An additional glass fiber insulating layer was included between the steeland the carbon fiber for some of the test specimens to prevent galvanic corrosion. For somespecimens the steel was pretreated with a silane coupling agent to enhance the resistance of the

Pull-offspecimen

Pull-offtester

Steelshaft

Torquewrench

Fixedbase

Supportbearing

Adhesive

(b)(a)



steel-adhesive bond line to ingress of moisture. Additional specimens implemented bothmethods of protection while the final series of specimens did not include any additional

protection. The effect of sustained load was also considered. A load of 14 kN was sustained on32 of the specimens using specially fabricated screw-based fixtures. The load level wasmonitored using a pair of electrical resistance strain gauges on each specimen. The degradation

of the bond was accelerated by exposing the specimens to wet/dry cycles in a 100o

F, 5% saltwater solution. The severe environment accelerated the deterioration of the bond which allowedcomparison of the performance of the various parameters in a relatively short time. To simulatethe performance of the system under typical environmental conditions, another series ofspecimens was subjected to typical outdoor environmental conditions. The test setup is shown inFigure 5(a).

To date a total of 34 coupons have been tested. These include 14 unconditioned controlcoupons, 10 coupons which were subjected to accelerated corrosion for one month and 10coupons exposed to four months of accelerated corrosion. The remaining tests are currently in

progress. The average measured tension strength and the range of measured strengths for all ofthe tested coupons are given in Figure 5(b). The figure indicates that, after one month of

exposure to accelerated environmental conditions, the bond strength in all cases remainedessentially unchanged. The variability in the results for this exposure duration was likely due primarily to the inherent variability of the bond strength of the joints. The figure furtherindicates that after four months of exposure significant deterioration was observed. The averagetension strength of the specimens that did not include any form of additional environmental

protection decreased by 45% from 44 kN to 24 kN. While the use of a glass fiber layer helped toincrease the initial strength of the coupons to 76 kN, after four months the average tensionstrength decreased by 50% to 38 kN. The average strength of the specimens which included asilane coupling agent remained essentially constant for the four month duration. The averagestrength of the coupons which included both glass and silane dropped from 70 kN to 62 kN. Itcan be further observed that the inclusion of a glass fiber layer in the bond line significantlyincreased the scatter of the measured strengths which should be accounted for in the design.

Figure 5 – Environmental durability phase (a) test setup (b) test results

Submergedwater heater

Test couponsduring dry cycle

Load fixtures

0

20

40

60

80

100

120

Adhesive (A) Adhesive +Silane (AS)

Adhesive +Glass (AG)

Adhesive +Glass +

Silane (AGS)

M e a s

u r e

d T e n s

i o n

S t r e n g

t h ( k N )

Control1 Month4 Months

(a) (b)



Proposed flexural design guidelines

For a given beam, the CFRP strengthening system can be designed to achieve a specifiedincrease of the allowable live load level. The design is based on a non-linear moment-curvature

analysis and accounts for the non-linear behavior of the concrete and the steel materials. Theanalysis procedure and a worked example of the proposed design guidelines are presented indetail elsewhere 13. The proposed design methodology is based on three criteria as shown inFigure 6 relative to the moment-curvature relationship of a typical strengthened steel-concretecomposite beam. The combined effect of the dead load, M D, and the increased live load, M L,should not exceed 60% of the increased yield load of the strengthened member, M y,S to ensureelastic behavior of the member at service load levels. To satisfy the strength limit state, the totalfactored load, with the appropriate dead and live load factors, α D and α L respectively, should notexceed the ultimate capacity of the strengthened member, M U,S , with an appropriate strengthreduction factor, φ. To maintain the safety of the structure in case of an unexpected loss of thestrengthening system, the total effect of the applied dead load, M D, and the increased live load,

ML, should not exceed the residual nominal capacity of the unstrengthened beam, M n,US . Theremaining limit states which are applicable to unstrengthened steel-concrete composite beamsshould also be checked at the increased load level.

Figure 6 – Proposed design guidelines

Conclusions

This paper presents the development of a high modulus CFRP system for strengthening steel bridges and structures. The experimental and analytical results demonstrate that the proposedsystem can be effectively used to enhance the serviceability and ultimate strength of steel beams.The findings demonstrate that the proposed system is effective and robust. The modulus of theCFRP, the level of prestressing, the bond detailing and the degree of environmental protectionapplied can all be varied to suit the needs of the specific application. The experimental resultswere confirmed by analytical and finite element techniques. Flexural design guidelines are

presented which ensure the safety and serviceability of the strengthened member. This paper

Moment

C u r v a

t u r e

Strengthened

MD + M L ≤ 0.6 M YS

α D MD + α L ML ≤ MU,S = φMn,S MD + M L ≤ Mn,US

Live Load, M L

Dead Load, M D

Factored Moment(α D MD + α L ML)

Nominal Capacity (strengthened), M n,S

MY,SMY,U

Unstrengthened

Ultimate Capacity (strengthened), M U,S = φMn,S

Nominal Capacity(unstrengthened),Mn,US



demonstrates that carbon fiber materials can be effectively used for strengthening of steel bridgesand structures.

Acknowledgements

The authors would like to acknowledge the support of the National Science Foundation (NSF)Industry/University Cooperative Research Center (I/UCRC) on Repair of Buildings and Bridgeswith Composites (RB 2C). The generous support and contributions of Mitsubishi Chemical FPAmerica Inc. and Fyfe Co. LLC are also greatly appreciated.

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