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Engineering Structures

journal homepage: www.elsevier.com/locate/engstruct

CFRP shear strengthening system for steel bridge girders

Hamid Kazema,⁎, Ye Zhanga, Sami Rizkallaa, Rudolf Seracinoa, Akira Kobayashib

a Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC, USAbNippon Steel Sumikin Material Co., Ltd., Japan

A R T I C L E I N F O

Keywords:Small-diameter CFRPSteel GirderShearStrengthening

A B S T R A C T

This paper presents an investigation undertaken to study the effectiveness of using small-diameter CFRP strandsfor shear strengthening of steel bridge girders. The study includes a comprehensive experimental program tostudy effects of the CFRP reinforcement ratio and orientation of the strands. An analytical model, calibrated bythe experimental tests, was used to provide design recommendation. Results of the study showed that theproposed strengthening system is effective in increasing the shear capacity of steel bridge girders and there wasno sign of CFRP debonding or rupture failure commonly observed by CFRP laminates up to approximately 80%of the steel yield stress.

1. Introduction

Use of FRP materials has gained wide acceptance worldwide forstrengthening of concrete structures and bridges due to their interestingbenefits [1,2,3]. There is also a growing demand for strengthening steelstructures and bridges due to increasing of the load demand or reduc-tion of the load-carrying capacity due to corrosion of steel [3]. Use ofexternally bonded FRP materials for strengthening steel structures didnot progress as the use of FRP for strengthening concrete structures dueto the lower elastic modulus of typical FRP material relative to steel [3].However, the recent production of high-modulus Carbon FRP (CFRP)laminates with an elastic modulus similar to or higher than that of steeloffers a promising alternative for externally bonded strengthening ofsteel structures [4,5].

FRP is a composite material consisting of fibers commonly carbon,glass, basalt or aramid embedded in a resin matrix [6]. A critical issuein strengthening of structures using externally bonded FRP compositesystems is failure due to debonding of the strengthening materials [7].Due to the uncertainties and the sudden nature of the typical de-bonding failure experienced between the FRP composite and steelsubstrate, since the laminates are only bonded to the substrate fromone side, numerous studies were conducted experimentally and ana-lytically to investigate the bond behavior [8,9,10]. Flexuralstrengthening of steel beams is typically used as initial step to un-derstand the bond mechanism between the CFRP and steel substrate.Many researches have also explored substantial enhancement inflexural capacity and stiffness of strengthened steel members usingexternally bonded high-modulus CFRP materials [3,11,12], and [13].

Also, effectiveness of using the recent innovative pre-stressed CFRPplates is proven for strengthening steel structures [14,15], and [16].However, to increase the total load-carrying capacity of a strength-ened steel member, it is often necessary to also increase the shearcapacity along with the increased flexural capacity. Despite this, thereis very little research published on shear strengthening of steelstructures using externally bonded CFRP systems.

The typical failure mode of the steel web of steel bridge girders isbuckling in the direction of the principal compressive stress of the ap-plied shear. Thus, the shear capacity of a steel bridge girder can beincreased by enhancing the buckling resistance of the web panel. Todate, the published research on the FRP shear strengthening of steelbridge girders is very limited. Patnaik et al. [17] reported that the shearcapacity of a steel I-beam could increase by up to 26% when the web isstrengthened with high tensile unidirectional CFRP laminate. However,the failure mode was governed by debonding of the CFRP laminatefrom the beam web. Okeil et al. [18,19], and [20] conducted servalstudies to investigate the shear strengthening using T-shaped pultrudedGFRP sections used as a stiffener. The results of these tests showed thatthe GFRP could increase the shear capacity up to 56% and the observedfailure mode was debonding of the T-shaped pultruded GFRP [3].Narmashiri et al. [21] published a research report on the use of CFRPstrips to strengthen the web of steel beams using different reinforce-ment ratio. The research results showed that the shear capacity of thesteel I-beam increased by up to 51% and the failure mode governed bydebonding of the CFRP strips.

This paper introduces a relatively new material which is in the formof small-diameter CFRP strands, shown in Fig. 1, for shear

https://doi.org/10.1016/j.engstruct.2018.08.038

⁎ Corresponding author.E-mail address: hkazem@ncsu.edu (H. Kazem).

Engineering Structures 175 (2018) 415–424

0141-0296/ © 2018 Elsevier Ltd. All rights reserved.

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strengthening of steel plates. Previous research using the same CFRPstrands have already demonstrated excellent behavior and very effec-tive for strengthening effect in flexural and compressive strengtheningapplications of steel members [12,22,3], and [23]. The CFRP strandsare stitched together with a gap between each strand allowing the ad-hesive to completely encapsulate each strand resulting in a superiorbond behavior [3].

This paper presents the research undertaken to study the efficiencyof these small-diameter high-modulus CFRP strands to enhance theshear strength of steel bridge girders and the development of a pro-posed guideline for field application.

2. Experimental program

2.1. Material properties

2.1.1. SteelDog-bone steel coupon specimens, cut from built-up steel beam

specimen, were prepared and tested in tension according to ASTM A370[24] using a universal testing machine (UTM). Elongation of the cou-pons was measured by an extensometer and the applied load wasmeasured directly by the UTM load cell. A total of four coupons weretested. The average of the measured tensile properties is summarized in

Table 1.

2.1.2. CFRPSelection of High Modulus (HM) small-diameter CFRP strands used

in this program was based on the performance of these strands inprevious research undertaken using different CFRP material to increasethe compressive strength of steel plate [22,3], and [23]. Three testspecimens were prepared using an individual strand in accordance withASTM D3039 [25] and Test Method L2 of ACI 440.3R-04 [26]. Thecompressive characteristics of the high modulus CFRP strands weretested and are reported in previously published papers by authors[22,3], and [23]. Dog-bone coupon specimens, cut from CFRP witnesspanels, were tested to determine the properties of the CFRP compositematerial. Compressive specimens were prepared in accordance withASTM D6641 [27] and tested in compression. The measured tensileproperties of small-diameter CFRP strands and the measured compres-sive properties of the CFRP composite coupons are summarized inTable 2 and Table 3, respectively. The measured tensile load-strain HMCFRP strands and the measured compressive load-strain of the CFRPcomposite coupons are shown in Fig. 2 and Fig. 3, respectively. Materialproperties of the putty and epoxy adhesive are reported by Okuyamaet al. [28]. Further details of composite system including chemicalproperties and the compatibility between the putty layer, epoxy ad-hesive layer, and CFRP strands can be found in Komori et al. [29,3].

2.2. Test setup

The experimental program included testing a large scale 6.4m(21 ft) built-up steel beam. The 4.8 mm (3/16 in.) thick web of the beamwas stiffened by 9.5mm (3/8 in.) thick plate to create914mm×914mm (36 in.× 36 in.) panels. The flanges were both381mm (15 in.) wide and 12.7 mm (½ in.) thick. The beam was simplysupported with a span of 3.7 m (12 ft) and the beam was laterallybraced using pre-fabricated A-frames. The steel girder was designed tofail in shear prior to flexure. The applied load was induced by two andfour 445 kN (100 kip) capacity jacks for the control and strengthenedpanels, respectively, the same beam was used multiple times to testeach panel. Hence, the applied load point and supports were moved foreach test. Details of the test setup are shown in Fig. 4 (a) and (b).

2.3. Instrumentation

The strains at the mid-point of each tested panel was measured byelectrical resistance strain gauge rosettes. Two strain gauges were at-tached to the un-strengthened control panel, with one strain gauge oneach face [3]. Total of four strain gauges were attached at the mid-pointof each strengthened panel; one strain gauge on each face of the sub-strate steel web, and one on each outer face of the CFRP composite. Theout of plane deformation of each test panel was measured by an Op-totrackCertus motion capturing system and linear string potenti-ometers. The OptotrackCertus motion capturing system uses InfraRedEmitting Diodes (IRED) to measure the deformation. A total of 23 IREDswere placed along the vertical and horizontal center line at one side ofthe test panel with a spacing of 75mm (3 in.). Five linear string po-tentiometers were attached to the four mid-edges and mid-point of thetest panel on the other face of IRED targets. The layout of the

Fig. 1. Small-diameter CFRP strand sheet [3].

Table 1Measured average mechanical tensile properties of steel.

Nominal thickness Yield strength Modulus of elasticity Yield strainmm (in.) MPa (ksi) MPa (ksi) mm/mm (in./in.)

5 (3/16) 380 (55) 186,000 (27,000) 0.00204

Table 2Measured tensile properties of small-diameter high-modulus CFRP strands [3].

Strand area Rupture strain Rupture stress Elastic modulusmm2 (in2) mm/mm (in./in.) MPa (ksi) MPa (ksi)

1.14193 0.0032 807 255,000(0.00177) (1 1 7) (37,000)

Table 3Measured compressive properties of CFRP coupons [22].

Area Compressive strength Elastic modulusmm2 (in2) MPa (ksi) MPa (ksi)

70.518 69.89 47,358(0.1093) (10.14) (6,869)

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instrumentation was used to measure the behavior of the test panes isshown in Fig. 5.

2.4. Strengthening system Application [3]

Application of the CFRP strands started by sandblasting the steeland cleaning the surface with acetone. The installation process startedby covering the surface with primer resin which was applied and left tocure at least two hours. The strain gauge rosettes were attached to the

center of each side of the test panel after the primer was applied and thepanel was cleaned using pressurized air [3]. After the primer resin wasfully cured, polyurea putty was applied and left to cure for at least sixhours. The epoxy and CFRP strands were finally applied while the steelgirder was in the vertical orientation to simulate field condition. Afterapplying the epoxy layer, the CFRP strands were bonded to the testpanel according to the prescribed orientation. The lead wires for thestrain gauge rosette attached to the steel face of the panel were passedthrough the gap between the strands. The CFRP strands were firmly

Fig. 2. Tensile stress-strain behavior of the HM CFRP strands [3].

Fig. 3. Compressive stress-strain behavior of the HM CFRP witness panels [3].

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pressed against the steel panel using hand rollers until the excess epoxywas squeezed out to remove any air pockets. A second layer of epoxywas applied on the top of the CFRP strands. Finally, the strengthenedpanel remained in a control environment for at least seven days to curebefore testing. The application process is illustrated in Fig. 6 [3].

2.5. Test scheme

A total of 12 tests were carried on four steel web panels are sum-marized in Table 4. The factors included in this study were fiber or-ientation and CFRP reinforcement ratio. Each panel was tested threetimes. First, a control test without strengthening. Second, a strength-ened test with one layer of CFRP material on each face. Third, astrengthened test after installation of the second layer of the CFRP oneach face. The applied concentrated load point and supports weremoved to create the test configuration for each test. To prevent steelyielding, loading was stopped when the maximum measured steel strainin the test steel panel reached 1600 με (micro-strain). The orientation ofthe HM CFRP strands used in each test panel is illustrated schematicallyin Fig. 7.

3. Test results

3.1. Shear buckling

Fig. 8 shows a flat plate simulating typical web of steel plate beamof large span bridge considering shear stresses distributed uniformlyalong the four edges. The stresses induce equivalent principal stresses intension and compression as shown in Fig. 8. Orientation of the tensionand compression are at 45 degrees with respect to the shear stresses.These type of loading conditions induces buckling in the form of wavesor wrinkles inclined at about 45 degrees [22,30].

When a plate is initially imperfect and is not flat, it deflects beforereaching the theoretical buckling load (Pb). Due to the plate imperfec-tion the behavior shows a gradual increase of load with increasing in-plane or lateral deformation. The transition point between pre-bucklingand post-buckling behavior vanishes as shown in Fig. 9 and experiencedin plate tests. Moreover, at loads far beyond buckling load (Pb) thecurve may approach the curve for flat plate as shown in Fig. 9 [22,31].

3.2. Measured principle strain

The total applied shear load and the measured principle strain at

Fig. 4. Test setup for shear loading, (a) Control test, (b) Strengthened test.

Fig. 5. Test instrumentation, (a) IREDs and strain gauge rosette attached on the panel front face, (b) string potentiometers and strain gauge rosette attached on thepanel back face.

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mid-point of the tested panel strengthened with CFRP strands areshown in Fig. 10 (a) through (d). The graphs shown for each test panelpresent the results of the panel before strengthening, the strengthenedpanel with one layer of CFRP strands on each face, and the panelstrengthened with two layers of CFRP strands on each face [3]. Theshear resisted by the strengthened panel for one and two layers iscompared to the control test result before yielding of the steel. Theresults demonstrated the effectiveness of the externally bonded small-diameter CFRP strands in increasing the shear strength before yieldingof the steel web. The measured shear strength can be measured by theincrease of the load resistance at any selected value of strain in thefigures. The CFRP strands reduced the principle compression strain inthe web at the same magnitude of applied shear, which ultimately leads

to increased shear capacity. Further, it can be seen in all Fig. 10 thatregardless of the CFRP strand orientation, increasing the number ofCFRP layers increases the shear capacity of the web.

The percent increase in shear strength for the different CFRP strandorientations considered relative to the control test for the same panel isshown in Fig. 11. The results show that one and two layers of the CFRPstrands can increase the shear resistance by up to 46% and 91%, re-spectively. It is also confirmed that regardless to the reinforcementratio, aligning the CFRP strands in the principal compressive stressdirection improves the efficiency of the strengthening system by redu-cing out of plane deformation and delaying elastic/inelastic buckling.However, since CFRP strands are currently manufactured in standardsheets of 500mm (20 in.) width [22], applying strands with an angle of45° could be ineffective for web plate more than 0.7×0.7m(28×28 in.) due to wastes of the material. Therefore, using verticalscheme for one layer and then attaching additional layers orthogonal tothe bottom layer is recommended.

3.3. Load-Lateral deformation

The mid-point net lateral deformation at the of the web panels re-lative to the edge points under the applied shear is shown in Fig. 12 (a)through (d). These results reflect the effectiveness of the different or-ientation in increasing the overall stiffness of the steel web. The per-centage increase of lateral stiffness for all four sets of tests are shown inFig. 13. The behavior clearly indicates that increase of the lateralstiffness is related to increase of the reinforcement ratio for all CFRPstrand orientations. Results also showed that regardless to the re-inforcement ratio, CFRP strands oriented parallel to the compressivestress direction have the highest increase in lateral stiffness followed byCFRP strands oriented vertically. Discrepancy in the shear capacityincrease and the lateral stiffness increase of the similar test configura-tions are due to the differences in the plate imperfection observed forthe test plates [22].

Fig. 6. Strengthening system application, (a) Application primer resin on sandblasted steel panel, (b) Application of polyurea putty, (c) Application of CFRPcomposite [3].

Table 4Shear strengthening test matrix.

Test name Panel no. No. of CFRP layers perface

Fiber orientation

I-Control I 0 –I-0-1HM 1 0 (Horizontal)I-0–90-2HM 2 0–90 (Orthogonal Grid)

II-Control II 0 –II-90-1HM 1 90 (Vertical)II-90-2HM 2 90 (Vertical)

III-Control III 0 –III-45-1HM 1 45 (Compressive

Direction)III-45-2HM 2 45 (Compressive

Direction)

IV-Control IV 0 –IV-45-1HM 1 45 (Compressive

Direction)IV-± 45-2HM 2 ±45 (Orthogonal Grid)

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3.4. Deformed shape

Typical measured net lateral deformed shape along the vertical andhorizontal center lines for the tested panels strengthened in direction ofthe principal compressive stresses is shown in Fig. 14. The lateral de-formed shapes are shown for the control, one layer and two layers ofHM CFRP prior to steel yielding using the Optotrak system [3]. Theresults are similar for all the panels tested. The deformed shapes in-dicate that the lateral deformation decreased by increasing the re-inforcement ratio of the CFRP strengthening system at the same loadlevel. Furthermore, adding more layers of the CFRP strands induces lessdistortion of deformed shape. It should be noted that despite the lateraldeformation, there was no sign of CFRP debonding or rupture up toapproximately 80% of the steel yield stress.

4. Analytical model

Reported research related to the use of small-diameter CFRP strandsto increase the compressive and shear strength of steel plates indicatedthat finite element models and simple hand calculations are capable to

predict the behavior and increase of strength using this system [22].Use of the transformed section method to calculate the shear strength ofthe panel strengthened by CFRP strands is presented in this section. Thefollowing well-known elastic relationship is used to calculate the shearstress in the panel.

=τ VQI tt t (1)

where,

∑= +=

I IEE

It si

nfi

sfi

1 (2)

∑= +=

t ttt

tt si

nfi

sfi

1 (3)

τ is the shear stress for the strengthened specimen with one or twolayers of CFRP strands, V is shear force, Q is first moment of area, Es andEfi are elastic modulus of steel and the compressive elastic modulus ofeach CFRP layer, respectively. It, Is, and Ifi are moment of inertia of thetransformed cross-section, moment of inertia of steel, and moment ofinertia of each CFRP layer. tt, ts, and tfi are thickness of the transformedcross-section, thickness of steel, and thickness of each CFRP layer [3].CFRP compressive elastic modulus of the witness panels was used incalculations. Also, Tresca criteria was used to calculate shear stressfrom steel normal stress and later calculate the theoretical shearstrength [32]. It is worth noting that the proposed analytical approachis not considering the effects of the CFRP strands orientation and plateimperfections. This approach is proposed to conservatively use simpleequation for design purposes without considering complexities of thecomposite system. The predicted shear resistance is compared to themeasured values for all tested specimen, including the control andstrengthened plates, as shown in Fig. 15 [3]. Each point higher thandashed line clearly indicate that design value is lower that experimentalvalue supporting conservative design. The comparison indicates that forthe design purposes, the shear load can be sufficiently estimated usingcomposite transformed section properties of the strengthened panel [3].

Fig. 7. Schematic of Shear Strengthening Test Matrix.

Fig. 8. State of principle and shear stresses on the plate subjected to pure shear[22,30].

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5. Conclusions

This paper examined the effectiveness of small-diameter CFRP ma-terial for shear strengthening of steel bridge girder. Based on the test

results, it can be concluded that the HM CFRP is effective in increasingthe shear capacity of steel girders with no de-bonding failure [3]. De-tailed conclusions can be summarized as follows,

Fig. 9. Typical behavior of steel plate under pure shear with and without imperfection [22,31].

Fig. 10. Applied shear vs. principle steel strain for web panels strengthened with HM CFRP at an angle of (a) 0–90°, (b) 90°, (c) 45°, and (d)± 45°.

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(1) The HM small-diameter CFRP strand is an effective material forincreasing the shear capacity of steel bridge girders.

(2) The strengthening system offers excellent bonding behavior. No de-bonding failure was observed up to approximately 80% of the steelyielding.

(3) The effectiveness of the strengthening system increased by in-creasing the CFRP reinforcement ratio.

(4) Use of the proposed strengthening system increases the initial

lateral stiffness of the strengthened plate.(5) Applying the CFRP strands in direction of principle compressive

stresses gives the most effective strengthening orientation.However, to avoid waste of material and labor cost, it is re-commended to apply first layer of CFRP material in vertical direc-tion and then attach additional layers orthogonal to the bottomlayer.

(6) The simple analytical method is accurate enough to estimate theshear capacity of the test results of the composite cross section.

Fig. 11. Increase in shear capacity percentage of strengthened panels using oneand two layers of HM CFRP strands.

Fig. 12. Applied shear load vs. net lateral deformation for web panel strengthened with HM CFRP at an angle of (a) 0–90°, (b) 90°, (c) 45°, and (d)± 45°.

Fig. 13. Increase of the lateral stiffness for strengthened web panel using oneand two layers of HM CFRP.

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Acknowledgments

The authors would like to acknowledge the contribution of thefollowing parties for their support of this research: The National ScienceFoundation (NSF) Center of Integration of Composites intoInfrastructure (CICI) at North Carolina State University, the staff of theConstructed Facilities Laboratory (CFL) at North Carolina StateUniversity for their technical assistance during the experimentaltesting, and Nippon Steel & Sumikin Material Co., Ltd, CompositesCompany for providing the CFRP strand materials used in this testprogram.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, in theonline version, at https://doi.org/10.1016/j.engstruct.2018.08.038.

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