Retrofitting of Severely Shear-Damaged Concrete T-Beams Using Externally Bonded Composites and Mechanical End Anchorage

Retrofitting of Severely Shear-Damaged ConcreteT-Beams Using Externally Bonded Composites

and Mechanical End AnchorageTamer El-Maaddawy1 and Yousef Chekfeh2

Abstract: The effectiveness of using an externally bonded carbon-fiber-reinforced polymer (EB-CFRP) system with mechanical endanchorage to retrofit severely shear-damaged reinforced concrete (RC) beams is examined in this paper. A total of 14 tests were performedon eight RC beams with a T-shaped section and low compressive strength. To represent a severe shear-damage condition, five beamswere tested to failure, retrofitted, then retested to failure for a second time. Test parameters included the number of EB-CFRP layersand type of end anchorage system. The results demonstrated that retrofitting of severely shear-damaged RC T-beams with EB-CFRP com-posites and proper mechanical end anchorage can fully restore the original shear capacity of the beams. The use of a sandwich compositepanel in combination with a threaded anchor rod inserted through the entire web width (thru-bolt) as an end anchorage system was moreeffective than using the panel with side powder-actuated fasteners. Increasing the number of EB-CFRP layers did not result in additionalgain in shear capacity. The accuracy and validity of four different international guidelines/standards were examined by comparing theirpredictions with the experimental results. DOI: 10.1061/(ASCE)CC.1943-5614.0000299. © 2012 American Society of Civil Engineers.

CE Database subject headings: Concrete beams; Fiber reinforced polymer; Anchorage; Fastening; Rehabilitation; Damage.

Author keywords: Concrete; Shear; Externally bonded carbon-fiber-reinforced polymer (EB-CFRP); Retrofitting; Mechanical anchorage;Fasteners.

Introduction

Accidental shear damage by overheight vehicles to reinforcedconcrete (RC) bridge girders is a common problem facing bridgemaintenance departments worldwide. Shear damage may alsooccur in RC bridge girders due to overloading by overweight trucksor under extreme events such as a sudden earthquake. For severelyshear-damaged RC girders, the common practice is to saw-cut thedeck, remove the girder, and replace it with a new one (Miller et al.2007). Complete replacement is a very costly and time-consumingsolution. The long traffic delays during replacement would result insafety hazards and large financial burdens with negative economi-cal and psychological impacts. Hence, there is a need for a fast,cost-effective, and easy-to-install retrofitting solution for severelyshear-damaged RC bridge girders. An externally bonded carbon-fiber-reinforced polymer (EB-CFRP) retrofitting system has a greatpotential to replace the complete replacement option. Typically,RC bridge girders have a T-shaped section with integrated deckslab. This makes the full EB-CFRP wrapping scheme for shearstrengthening an impractical solution due to the presence of aflange. More attention should then be given to study the viabilityof using a U-wrap and side-bonding EB-CFRP configuration with

mechanical end anchorage to retrofit RC bridge girders deficient inshear strength (Brancaccio et al. 2009; Ortega et al. 2009; Kimet al. 2011). Although retrofitting is generally used to repair andstrengthen damaged structures, most of the previous studies onEB-CFRP shear strengthening have focused on undamaged beams(Triantafillou and Antonopoulos 2000; Chaallal et al. 2002; Khalifaand Nanni 2002; Chen and Teng 2003; Carolin and Taljsten 2005;Bousselham and Chaallal 2006; Pellegrino and Modena 2006;Grande et al. 2009; Godat et al. 2010; Chaallal et al. 2011). Theeffectiveness of the EB-CFRP system to restore and upgradethe shear capacity of severely shear-damaged RC T-beams hasreceived little attention in the literature.

This research was initiated to evaluate the effectiveness of anEB-CFRP composite system with mechanical end anchorage asa retrofitting solution for severely shear-damaged RC bridge gird-ers. Findings of the present research will expand the experimentaldatabase with results from tests on retrofitting of predamagedRC beams with a T-shaped section. The accuracy and validityof four different international guidelines/standards to predict thecontribution of the EB-CFRP system to shear capacity were exam-ined by comparing their predictions to the experimental results.

Experimental Investigation

Test Matrix

The test matrix of the experimental program is summarized inTable 1. A total of eight beams were constructed and tested.The specimens were divided into two main groups, [A] and [B],according to their initial damage state. Specimens of group [A] con-sisted of three specimens that were not damaged prior to strength-ening and/or testing. One beam was not strengthened to serve as acontrol sample. The other two beams were strengthened with one

1Associate Professor, UAE-Univ., Al-Ain, Abu Dhabi, P.O. Box 17555,United Arab Emirates (corresponding author). E-mail: [email protected]

2Academic Assistant, UAE-Univ., Al-Ain, Abu Dhabi, P.O. Box 17555,United Arab Emirates.

Note. This manuscript was submitted on November 28, 2011; approvedon April 3, 2012; published online on April 10, 2012. Discussion periodopen until May 1, 2013; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Composites for Con-struction, Vol. 16, No. 6, December 1, 2012. © ASCE, ISSN 1090-0268/2012/6-693-704/$25.00.

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and two layers of an EB-CFRP system without end anchorage.Specimens of group [B], five beams, were tested to failure, retro-fitted, and then tested to failure for a second time. In the first sheartest, failure was considered when the beam reached its peak load.This was identified when a sudden drop in applied load took place.Beams of this group will be called predamaged throughout thispaper. On the occurrence of the first shear failure, the load was re-moved and then the deteriorated side and bottom concrete coverswere chipped. Epoxy patch repair was then applied on the beams’lateral and bottom faces. Following the epoxy patch repair, onebeam was retested to failure without being strengthened withEB-CFRP. The remaining four beams were strengthened, afterbeing patch repaired with epoxy, with one and two layers ofEB-CFRP in combination with two different mechanical endanchorage systems, and then retested again to failure.

Test Specimens

The test specimen was a 3,200-mm-long RC beam with a T-shapedcross section as shown in Fig. 1. The cross section had a webwidth of bw ¼ 120 mm, flange width of bf ¼ 300 mm, total depthof h ¼ 240 mm, and effective depth of tensile steel of d ¼200 mm. The specimens were tested under three-point bendingwith an effective span of 3,000 mm and a short shear span (testregion) of 600 mm, rendering a shear span to effective depth ratioof a=d ¼ 3. The beam was heavily reinforced in flexure to ensure

that shear failure will be dominated. The tensile steel reinforcingbars had a 90° hook at each end to provide sufficient anchorage.The internal shear reinforcement in the test region was No. 6 plainstirrups with a clear cover of 15 mm placed at a spacing ofs ¼ 120 mm. The beam was designed in a way to ensure that itwill fail in shear at a load value less than the capacity of the existingtest facility (200 kN). The spacing between stirrups in the testregion corresponded to 0.6 d, which is slightly greater than themaximum limit of smax ¼ 0.5 d specified by the American Con-crete Institue (ACI) 318M-05 (2005). This was done in an effortto represent a shear-deficient RC beam with a poor steel detailing.Adequate shear reinforcement was provided in the long shear spanto avoid shear failure outside the test region.

Materials

Condition assessment investigations of several old buildings inthe United Arab Emirates undertaken by the first author revealedthat their concrete compressive strength was typically in the rangeof 15 to 25 MPa. Hence, a concrete with a low compressivestrength of f 0

c ¼ 20� 0.4 MPa was used in this study to re-present an old concrete that might be encountered in a retrofitting/repair condition. The No. 6 stirrups provided in the test regionhad a measured yield strength of 344 MPa and a diameter of5.5 mm. The longitudinal steel reinforcement had a nominal yieldstrength of 520 MPa. Unidirectional carbon fiber fabric having

Table 1. Test Matrix

GroupDamage state prior

to retrofitting

Retrofitting regime

Specimendesignationb

Epoxy patchrepair

Number ofU-shaped

CFRP layers

HorizontalCFRP sheeteach side

End anchoragesystema

[A] No damage — — — — U-NS— 1 — — U-EB1— 2 — — U-EB2

[B] Damaged to failure Yes — — — D-PRYes 1 Yes SCP+PAF D-EB1-PAFYes 2 Yes SCP+PAF D-EB2-PAFYes 1 Yes SCP+TB D-EB1-TBYes 2 Yes SCP+TB D-EB2-TB

aSCP+PAF and SCP+TB = sandwich composite panel anchored with powder-actuated fasteners and thru-bolts, respectively.bU and D = undamaged and damaged beams, respectively; NS = no strengthening; EB1 and EB2 = strengthening with one and two layers of EB-CFRP,respectively; PR = patch repair with epoxy with no CFRP strengthening; PAF and TB = powder-actuated fasteners and thru-bolts, respectively.

Fig. 1. Test specimen

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a tensile modulus of 230 GPa, tensile strength of 3.45 GPa, andultimate elongation of 1.5% was used in the EB-CFRP system(data was obtained from the manufacturer). The fabric was im-pregnated and bonded to the specimen with a compatible epoxyresin having a tensile strength of 30 MPa and ultimate elongationof 1.5% (data was obtained from the manufacturer). A curedCFRP composite laminate (fibers+resin) typically has a thicknessof 0.381 mm, tensile modulus of 65.4 GPa, tensile strength of894 MPa, and ultimate elongation of 1.33% (data was obtainedfrom the manufacturer).

Retrofitting/Strengthening Methodology

Strengthening of the undamaged beams (group [A]) included sur-face preparation and wet lay-up application of vertical U-shapedCFRP sheets without mechanical end anchorage. The U-shapedCFRP sheets had a width of 70 mm and a center-to-center spacingof sf ¼ 120 mm (Fig. 2). Retrofitting of the predamaged beams(group [B]), except specimen D-PR, comprised removal of deterio-rated concrete, application of epoxy-based surface repair, surfacepreparation, and wet lay-up application of CFRP composites.

Retrofitting of specimen D-PR did not include CFRP application.For other CFRP-retrofitted specimens of group [B], one horizontalCFRP sheet with a width of 190 mm and fibers oriented in longi-tudinal direction was first installed at each side of the beam web.The vertical U-shaped CFRP sheets were then applied followedby the installation of a mechanical end anchorage. Despite thenegligible contribution of the horizontal CFRP sheets to shearcapacity (Teng et al. 2002; Zhang et al 2004), they were installedin an effort to minimize the opening of existing internal shearcracks developed during the first shear test to failure. This wouldallow a transfer of load through the entire shear span during thesecond test after retrofitting. The web corners were chamfered toa radius of approximately rw ¼ 10 mm prior to CFRP application.

Mechanical End Anchorage System

The CFRP sheets within the test region in specimens of group [B]had a mechanical end anchorage as shown in Fig. 3. The CFRPsheets above the support and below the load point did not havemechanical end anchorage. This would not have an effect on thebeam failure mode or shear capacity because the CFRP sheets out-side the test region were not intercepted by any shear cracks.Pultruded precured composite plates were used for the mechanicalend anchorage system rather than steel plates to avoid corrosionproblems [Fig. 4(a)]. This type of composite plates is a hybridcarbon and glass fiber composite with vinylester matrix (Lamannaet al. 2001; Lee et al. 2009). A typical composite plate has a thick-ness of 3.2 mm, elastic modulus of 68.3 GPa, and tensile strengthof 848 MPa (Lamanna et al. 2001; Lee et al. 2009). On the com-pletion of the wet lay-up process, two 50 mm × 100 mm precuredcomposite plates were bonded longitudinally on top of each im-pregnated CFRP sheet, one at each side, directly below the flange.The ends of the wet-impregnated CFRP sheet were rolled over theprecured composite plates and then secured tightly by overlapp-ing them with additional precured composite plates, forming a

Fig. 2. Layout of EB-CFRP strengthening system without endanchorage

(a)

(b)

Horizontal CFRP sheet with fibers

oriented in longitudinal direction

Thru-bolt

Internal composite plate

U-shaped vertical CFRP sheets with SCP+TBmechanical end anchorage system

External composite plate

Ends of CFRP sheet rolled over

the internal composite plate

PAFs

Internal composite plate

Horizontal CFRP sheet with fibers

oriented in longitudinal direction

U-shaped vertical CFRP sheets with SCP+PAF mechanical end anchorage system

External composite plate

Ends of CFRP sheet rolled over

the internal composite plate

Fig. 3. EB-CFRP retrofitting regime with mechanical end anchorage: (a) SCP+PAF; (b) SCP+TB

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sandwich composite panel (SCP) (Ortega et al. 2009). The SCPswere mechanically fastened to concrete using two different meth-ods. In one method, two galvanized steel powder-actuated fasteners(PAFs), 50 mm apart, were used to fix each SCP at each side ofthe beam as shown in Fig. 3(a). The fasteners were 52-mm longand 4 mm in diameter. The PAFs were shot into the concrete usinga special gun and power cartridge [Fig. 4(b)]. To prevent crackingof concrete cover from fastener penetration, holes of 2-mm diam-eter were predrilled into the concrete to a depth of approximately15 mm. Steel washers were used to prevent damage of thecomposite plates by heads of the fasteners. In the second method,each SCP was fixed on the concrete surface by using a 10-mmdiameter galvanized steel threaded anchor rod, i.e., thru-bolt(TB), as shown in Fig. 3(b). The TBs were embedded into holespredrilled through the entire width of the beam web in the middle ofeach SCP. A photo of the thru-bolts is given in Fig. 4(c). Steelwashers and nuts were then installed. The nuts were tightenedon the threaded rod until they reached a snug position.

Test Setup and Instrumentation

The specimens were tested under three-point bending. The loadwas applied incrementally at a distance a ¼ 600 mm from the near-est support by means of a hydraulic jack until failure (see Fig. 1).The deflection under the load point was monitored using a linearvariable displacement transducer (LVDT). Electrical resistancestrain gauges were bonded to steel stirrups and CFRP sheets tomeasure their strains. Concrete clip gauges, with a gauge lengthof 100 mm, were mounted on the concrete surface at the midpointof the shear span at an angle of 45° to measure the diagonalcompressive and tensile displacements (refer to Fig. 1).

Experimental Results and Discussion

Failure Mode

Failure modes of specimens U-NS and D-PR that were notstrengthened with CFRP are shown in Figs. 5(a and b),

respectively. A photo of a typical specimen from group [B] atthe end of the first shear test to failure prior to retrofitting isshown in Fig. 5(c). These specimens exhibited a pure shear modeof failure in which diagonal shear cracks developed in the webprior to initiation of any flexural cracks. The shear cracks wereinitiated at the middle of the shear span at an average angle inthe range of 40° to 45°. The cracks then propagated each wayand became horizontal as they reached the support and the com-pression table. As the load progressed, more shear cracks devel-oped, rendering a more widespread cracking pattern at the onsetof failure.

Failure modes of the CFRP-strengthened specimens of groups[A] and [B] are depicted in Figs. 6 and 7, respectively. SpecimensU-EB1 and U-EB2 from group [A] and D-EB1-PAF and D-EB2-PAF from group [B] failed by sudden debonding of CFRP sheetsaccompanied by separation of the concrete cover of the beam’s lat-eral faces. Inspection of the debonded CFRP sheets revealed thatthe concrete was adhered to the CFRP, which indicates that failureof the strengthening system was due to excessive shear deformationthat caused CFRP debonding involving peeling of the concretecover. This was more evident in specimens D-EB1-PAF andD-EB2-PAF with SCP+PAF mechanical end anchorage. SpecimensD-EB1-TB and D-EB2-TB failed in shear by web crushing(i.e., diagonal compression shear mode of failure). Longitudinalsplitting cracks running parallel to the compressive steel reinforce-ment were observed at the top of the compression table at peakload. The use of SCP+TB mechanical end anchorage preventeddebonding of CFRP/peeling of concrete cover, and hence allowedthe beam to develop its full shear capacity. The overstrengtheningfor shear combined with the low concrete strength may havecontributed to the development of the longitudinal splitting cracksat the top surface. A similar mode of failure was reported in theliterature by other researchers for beams with a rectangular crosssection overstrengthened for shear by CFRP wraps (Khalifa andNanni 2002). Rupture of the CFRP sheets was observed at thebottom corners of the web in specimen D-EB2-TB at the onsetof failure (i.e., at the end of postpeak stage) due to excessive sheardeformation.

Fig. 4. Equipment and materials used for installation of end anchorage systems: (a) pultruded precured composite plates; (b) PAFs, washers, gun, andpower cartridge; (c) TBs

Fig. 5. Typical failure mode of specimens with no CFRP strengthening: (a) specimen U-NS; (b) specimen D-PR; (c) typical specimen from group [B]at the end of the first shear test

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Shear Capacity

The main test results are summarized in Table 2. Results of speci-men U-NS from group [A] indicate that a slender RC beam, afterthe first shear failure, can still maintain approximately 71% of itsoriginal shear capacity when reloaded back to failure. Results ofthe same group indicate that strengthening of undamaged RCbeams with one layer of an EB-CFRP system without end anchor-age resulted in an approximately 12% increase in shear capacity.Increasing the number of CFRP layers did not result in a furtherincrease in shear capacity. This is because of the prematuredebonding of CFRP attributable to the propagation of shear cracksthat became horizontal as they reached the compression table. Thiscaused the beams with the greater amount of CFRP to fail at aneffective CFRP strain lower than that recorded for the beams withthe lower amount of CFRP.

Results of group [B] indicate that the use of epoxy-patch repairsolely can restore approximately 91% of the original beam shearcapacity. Retrofitting of severely shear-damaged RC beams withEB-CFRP in combination with a mechanical end anchorage fullyrestored the original shear capacity. The shear capacity of thebeams after CFRP-retrofitting was even higher than the originalshear capacity recorded in the first shear test to failure. The gainin shear resistance depended on the fastening method used to fix theSCP. The use of TB to fix the SCP was more effective than the useof the PAFs. The shear capacity of the beams retrofitted with anEB-CFRP system mechanically anchored with SCP+TB wasapproximately 30 to 45% higher than the original shear capacityof the beam recorded in the first shear failure. For the beamsretrofitted with an EB-CFRP system mechanically anchored withSCP+PAF, the shear resistance increased by up to 17%, relativeto the original shear capacity recorded in the first shear test tofailure. This is because the use of SCP+PAF delayed debonding ofCFRP sheets and peeling of the concrete cover whereas the useof SCP+TB prevented debonding of CFRP and allowed the beamto develop its full shear capacity by failure of concrete struts.

Increasing the number of CFRP layers had no effect on the shearresistance of the retrofitted beams. This is because for the beamsretrofitted with an EB-CFRP system and SCP+PAF mechanicalend anchorage, the effective CFRP strain reduced with increased

number of CFRP layers. The beams retrofitted with an EB-CFRPsystem mechanically anchored with SCP+TB were overstrength-ened for shear and thus failed by crushing of diagonal concretestruts. This indicates that the shear capacity of the beams withSCP+TB end anchorage system was dominated by the concretecompressive strength and the beam cross section dimensions ratherthan the amount of the EB-CFRP. Therefore, increasing the amountof EB-CFRP had no further effect on the gain in shear capacity.

Deflection Response

The load-deflection response of specimens of group [A] and that ofspecimen D-PR from group [B] are depicted in Fig. 8. Shearstrengthening of undamaged beams with EB-CFRP had no effecton the beam stiffness. This is expected because CFRP sheets didnot increase the beam’s moment of inertia. The load-deflectioncurves of specimens of group [B] retrofitted with EB-CFRP andmechanical end anchorage are depicted in Fig. 9. The use of theSCP+TB mechanical end anchorage system significantly reduced

Fig. 6. Failure mode of CFRP-strengthened specimens of group [A]

Fig. 7. Failure mode of CFRP-retrofitted specimens of group [B]

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the rate of stiffness degradation in the postpeak stage, and henceremarkably improved the beam ductility relative to those of theircounterparts retrofitted with an EB-CFRP and SCP+PAF endanchorage system. Because of the proper end anchorage providedby the SCP+TB system, and hence the effective confinement pro-vided by the CFRP wraps, the beams could undergo significantdeformation after the peak load without collapse, debonding ofCFRP, and/or peeling of the concrete cover. Increasing the numberof CFRP layers had no effect on the deflection response of thebeams having SCP+PAF mechanical end anchorage system. Forthe beams having SCP+TB, increasing the number of CFRP layersslightly improved the beam ductility as manifested by the increasedarea under the load-deflection curve and increased deflection at theonset of failure.

Diagonal Deformation across Cracks

The load versus diagonal tensile displacement curves for specimensof group [A] and that of specimen D-PR from group [B] are de-picted Figs. 10(a and b), respectively. Shear strengthening ofundamaged beams with EB-CFRP slightly increased the crackingload. After crack initiation, the EB-CFRP system reduced the rateof increase of diagonal deformation across cracks relative to that ofthe control unstrengthened beam. The rate of increase of diagonaldeformation across cracks reduced with an increased number ofCFRP layers. Specimen U-NS showed a plastic response priorto reaching its peak load. The trend of the diagonal tensile displace-ment curve of specimen D-PR from group [B] during the secondshear test to failure (i.e., after being patch repaired with epoxy) wassimilar to that recorded in the first shear test prior to strengthening.However, the specimen exhibited higher diagonal displacementacross cracks in the second shear test relative to that of the firstshear test. This is because of the reopening of existing internalcracks during the second test at a load value of approximately20 kN, which was approximately 50% lower than the initial crack-ing load recorded in the first shear test.

The load versus diagonal tensile displacement curves for spec-imens of group [B] are depicted Fig. 11. The response of specimenD-EB2-TB in the first shear test to failure was not recorded due tomalfunction of the clip gauge. For other specimens of group [B],the diagonal tensile displacement response in the first test was dif-ferent than that recorded in the second test. In the first test to failure,prior to retrofitting, the diagonal tensile displacement featuredthree phases during loading similar to those of the control specimenU-NS. The first phase started from zero load and ended at thecracking load. No diagonal tensile displacement was recorded inthe precracking phase. In the second stage, after crack initiation,the diagonal tensile displacement increased at almost a constantrate up to a load value corresponding to approximately 95% ofthe peak load. In the third stage, the diagonal tensile displacementshowed almost a plastic response. The diagonal tensile displace-ment in the second shear test, after retrofitting, showed almost alinear response up to the peak load. The number of CFRP layersand type of end anchorage had no effect on the diagonal tensiledisplacement response recorded in the second test to failure. How-ever, it is evident that the rate of increase of diagonal deformationacross cracks after retrofitting was significantly lower than the rateof the postcracking stage recorded in the first shear test prior toretrofitting.

Diagonal Compressive Strain Response

The load-diagonal concrete compressive strain curves for speci-mens of group [A] are shown in Fig. 12. The unstrengthened

Table 2. Test Results

Group Specimen

Original loads (first shear failure) New loads (second shear failure)

RatioVnew=Vorg

Load capacityPorg (kN)

Shear resistanceVorg (kN)a

Load capacityPnew (kN)

Shear resistanceVnew (kN)a

[A] U-NS 116 93 83b 66 0.71U-EB1 130 104 — — —U-EB2 128 102 — — —

[B] D-PR 107 86 97 78 0.91D-EB1-PAF 111 89 130 104 1.17D-EB2-PAF 112 90 130 104 1.16D-EB1-TB 105 84 152 122 1.45D-EB2-TB 115 92 150 120 1.30

aV ¼ 0.8P.bThis beam has been reloaded to failure without being strengthened.

0

(a)

(b)

20

40

60

80

100

120

140

160

0 15 30 45 60 75 90 105 120 135 150

Tota

l loa

d (k

N)

Deflection (mm)

U-NS

U-EB1

U-EB2

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135 150

Tota

l loa

d (k

N)

Deflection (mm)

D-PR (first test)

D-PR (second test)

Fig. 8. Load-deflection curves: (a) specimens of group [A]; (b) speci-men D-PR

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specimen U-NS exhibited almost a trilinear diagonal compressivestrain response. In the first stage, the diagonal concrete struts werenot strained until initiation of diagonal cracks. After crack initia-tion, the diagonal compressive strains increased rapidly until thebeam reached a compressive strain of approximately 0.002, beyondwhich no further increase in the strain was recorded due to malfunc-tion of the clip gauge caused by development of a shear crack cross-ing the fixation points of the clip. The diagonal compressive strainresponse for the strengthened specimens U-EB1 and U-EB2 fea-tured three phases. No strains were recorded in the precrackingstage. Following crack initiation, the concrete struts strained at arate lower than that recorded for the unstrengthened specimenU-NS. For the CFRP-strengthened specimens, the diagonal com-pressive strains continued to increase in the postcracking stageup to a threshold limit beyond which the curves exhibited some-what a plastic response. This threshold was in the range of 0.0025to 0.0035. At peak load, the CFRP-strengthened specimensexhibited higher diagonal compressive strain than that exhibitedby specimen U-NS.

The diagonal concrete compressive strain curves for the CFRP-retrofitted specimens of group [B] recorded in the second test tofailure are plotted in Fig. 13. The response of specimen D-EB1-PAF was not recorded due to malfunction of the clip gauge. Fromthis figure, it can be seen that the concrete struts strained from thevery beginning of the second shear test because of the presence ofinternal cracks initiated earlier during the first shear test. The spec-imens exhibited a quasi-linear response up to the peak load. Therate of increase of diagonal compressive strain in specimensD-EB1-TB and D-EB2-TB with SCP+TB mechanical end anchor-age system was insignificantly different. However, it is evident thatthe use of SCP+TB end anchorage system remarkably reduced therate of increase of the diagonal compressive strain relative to that ofspecimen D-EB2-PAF with SCP+PAF end anchorage system.

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135 150

Tota

l loa

d (k

N)

Deflection (mm)

D-EB1-PAF (first test)

D-EB1-PAF (second test)

Specimen D-EB1-PAF

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135 150

Tota

l loa

d (k

N)

Deflection (mm)

D-EB2-PAF (first test)

D-EB2-PAF (second test)

Specimen D-EB2-PAF

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135 150

Tota

l loa

d (k

N)

Deflection (mm)

D-EB1-TB (first test)

D-EB1-TB (second test)

Specimen D-EB1-TB

0

20

40

60

80

100

120

140

160

180

0 15 30 45 60 75 90 105 120 135 150To

tal l

oad

(kN

)Deflection (mm)

D-EB2-TB (first test)

D-EB2-TB (second test)

Specimen D-EB2-TB

Fig. 9. Load-deflection curves of specimens of group [B] retrofitted with EB-CFRP and end anchorage

0

(a)

(b)

20

40

60

80

100

120

140

160

180

0 1 2 3 4

Load

(kN

)

Diagonal tensile displacement (mm)

U-NSU-EB2U-EB1

Failure of clip gauge

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4

Load

(kN

)

Diagonal tensile displacement (mm)

D-PR (first test) (before epoxy-patch repair)

D-PR (second test) (after epoxy-patch repair)

Fig. 10. Load-diagonal tensile displacement: (a) specimens of group[A]; (b) specimen D-PR

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CFRP Strain Response

Fig. 14 shows the load versus maximum measured CFRP strainresponse of test specimens. The maximum measured CFRP strainspresented in Fig. 14 are not necessarily the absolute maximumCFRP strains. This is because the CFRP strains are typicallyaffected by the location of the shear crack with respect to thelocation of the strain gauge. For specimens U-EB1 and U-EB2,which were not damaged prior to CFRP strengthening, the CFRPstrain response featured two phases during loading as shown inFig. 14(a). In the initial precracking stage, the CFRP did not con-tribute to shear resistance. In the second stage, the CFRP began tostrain at an applied load in the range of 45 to 50 kN. The rate of

increase of CFRP strains in the second stage reduced with increasednumber of CFRP layers. The CFRP strains continued to increaseas load progressed up to a certain threshold, i.e., effective CFRPstrain, beyond which no further increase in CFRP strains was re-corded. The effective CFRP strain measured in specimen U-EB2with two CFRP layers was approximately 40% of that of specimenU-EB1 with one layer of CFRP. This explains why increasingthe number of CFRP layers had no effect on the gain in shear capac-ity attributable to CFRP strengthening. Specimens of group [B]exhibited a quasi-linear CFRP strain response up to failure withalmost no precracking stage as shown in Figs. 14(b and c).Specimens D-EB2-PAF and D-EB2-TB with the higher amountof CFRP exhibited lower CFRP strains than those exhibited by

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4

Load

(kN

)

Diagonal tensile displacement (mm)

D-EB1-PAF (first test) (before retrofitting)

D-EB1-PAF (second test) (after retrofitting)

Specimen D-EB1-PAF

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4

Load

(kN

)

Diagonal tensile displacement (mm)

D-EB2-PAF (first test) (before retrofitting)

D-EB2-PAF (second test) (after retrofitting)

Specimen D-EB2-PAF

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4

Load

(kN

)

Diagonal tensile displacement (mm)

D-EB1-TB (first test) (before retrofitting)

D-EB1-TB (second test) (after retrofitting)

Specimen D-EB1-TB

0

20

40

60

80

100

120

140

160

180

0 1 2 3 4Lo

ad (

kN)

Diagonal tensile displacement (mm)

D-EB2-TB (second test) (after retrofitting)

Specimen D-EB2-TB

Fig. 11. Load-diagonal tensile displacement curves for CFRP-retrofitted specimens of group [B]

0

20

40

60

80

100

120

140

160

180

0 0.002 0.004 0.006 0.008 0.01

Load

(kN

)

Diagonal compressive strain

U-EB1U-EB2

U-NS

Failure of clip gauge

Fig. 12. Load-diagonal compressive strain curves for specimens ofgroup [A]

0

20

40

60

80

100

120

140

160

180

0 0.002 0.004 0.006 0.008 0.01

Load

(kN

)

Diagonal compressive strain

D-EB2-TB (second test)

D-EB1-TB (second test)

D-EB2-PAF (second test)

Failure of clip gauge

Fig. 13. Load-diagonal compressive strain curves for CFRP-retrofittedspecimens of group [B]

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their counterparts D-EB1-PAF and D-EB1-TB, respectively, withthe lower amount of CFRP. The maximum CFRP strain measuredin the specimens with the higher amount of CFRP was approxi-mately 50% of that measured in the specimens with the loweramount of CFRP. This further explains why no additional shearstrength gain was recorded as a result of increasing the number ofCFRP layers. From Figs. 14(b and c), it can be seen that the rateof increase of CFRP strains in the CFRP-retrofitted specimens ofgroup [B] was not affected by the type of mechanical endanchorage.

Analytical Investigation

The shear resistance attributable to EB-CFRP obtained by testing,Vf;exp, was compared with the nominal CFRP shear resistance,

Vfn, predicted by four different international guidelines/standards,namely, the American ACI 440.2R (2008), European InternationalFederation for Structural Concrete (fib) Task Group (TG) 9.3(2001), Italian Consiglio Nazionale delle Ricerche CNR-DT200(2004), and Australian HB 305 (Standards Australia 2008). Thefib TG 9.3 and HB 305 analytical formulations are based onanalytical models introduced previously by Triantafillou andAntonopoulos (2000) and Chen and Teng (2003), respectively.

Basis of Comparison

The experimental value of the CFRP contribution to shear capacityis the difference between the measured shear capacity before andafter CFRP strengthening/retrofitting. The shear capacity of spec-imens U-EB1 and U-EB2 from group [A] prior to CFRP strength-ening was assumed to be equal to the average shear capacity ofall unstrengthened beams recorded in the first shear test to failure.As mentioned previously, the predamaged specimens of group [B]were patch repaired with epoxy prior to CFRP retrofitting. Resultsof specimen D-PR indicated that the shear capacity of a predam-aged beam that was patch repaired with epoxy without CFRP wasapproximately 91% of the original shear capacity recorded in thefirst shear test to failure. Hence, for CFRP-retrofitted specimensof group [B], Vf;exp was taken to be equal to Vnew − 0.91Vorg.In an effort to calculate the nominal CFRP shear resistance,Vfn, no partial safety factors were adopted in all analytical formu-las. It is evident from previous studies that the contribution of hori-zontal (longitudinal) CFRP sheets to shear capacity is negligible(Teng et al. 2002; Zhang et. al 2004). The is why internationalguidelines/standards do not typically take into consideration thecontribution of horizontal CFRP sheets to shear capacity (the for-mulas are not valid for horizontal CFRP sheets where α ¼ 180°).Including the horizontal CFRP sheets in the analysis would resultin an unrealistic increase in the calculated CFRP shear resistance,which would be misleading while examining the accuracy ofinternational guidelines/standards. Hence, for specimens of group[B], only the vertical U-shaped CFRP sheets with α ¼ 90° wereconsidered in the analysis. The angle of inclination of shear crackswas assumed as θ ¼ 45°. Specimens D-EB1-TB and D-EB2-TB,with the SCP+TB anchorage system, did not exhibit CFRP debond-ing/peeling of the concrete cover mode of failure. The U-shapedanalytical formulas developed for a CFRP debonding mode offailure are not applicable for these two beams. The full-wraps ana-lytical formulas were then used to estimate the CFRP contributionfor the beams with SCP+TB anchorage system. The U-shapedanalytical formulas were used to predict the CFRP shear contribu-tion for all other beams because they had a CFRP debonding/peeling of concrete cover mode of failure.

Maximum CFRP Shear Contribution (Vf ;max )

In RC beams overstrengthened for shear, the crushing of web strutscan be the dominating mode of failure. Code provisions of themaximum shear resistance for RC beams strengthened in shear withCFRP are duplicates of those used in conventional concrete codesand standards. These provisions are overly conservative and in-clude hidden safety factors, and hence cannot be used to determinea realistic value for the maximum CFRP shear resistance, Vf;max .Accordingly, a unified rational approach based on stress fieldanalysis has been adopted in the present analytical study forprediction of the maximum CFRP shear resistance. A schematicdiagram showing a web crossed by shear cracks is given inFig. 15. From equilibrium, the maximum shear resistance of anRC beam can be calculated by

0

(a)

(b)

(c)

20

40

60

80

100

120

140

160

180

0 2000 4000 6000 8000

Load

(kN

)

CFRP strain (microstrain)

U-EB1

U-EB2

Failure of strain gauge

0

20

40

60

80

100

120

140

160

180

0 2000 4000 6000 8000

Load

(kN

)

CFRP strain (microstrain)

D-EB1-PAFD-EB2-PAF

Failure of strain gauge

0

20

40

60

80

100

120

140

160

180

0 2000 4000 6000 8000

Load

(kN

)

CFRP strain (microstrain)

D-EB2-TB D-EB1-TB

Failure of strain gauge

Fig. 14. Load-CFRP strain response: (a) CFRP-strengthened speci-mens with no end anchorage; (b) CFRP-retrofitted specimens withSCP+PAF end anchorage; (c) CFRP-retrofitted specimens with SCP+TB end anchorage

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Vnmax ¼ νf 0cbwjd cos θ sin θ (1)

According to the European code EN 1992-1-1 (EuropeanCommittee for Standardization 2004), the effectiveness factor,ν, for inclined concrete struts where the compression band isintercepted by reinforcement running obliquely to the directionof compression is given by

ν ¼ 0.6

�1 − f 0

c

250

�(2)

Combining Eqs. (1) and (2) and assuming θ ¼ 45° andjd ¼ 0.9d, the maximum nominal shear resistance of an RC beamis given by

Vnmax ¼ 0.27

�1 − f 0

c

250

�f 0cbwd (3)

The maximum CFRP shear resistance, Vf;max, is then given by

Vf;max ¼ Vnmax − ðVc þ VsÞexp (4)

The term ðVc þ VsÞexp represents the shear capacity prior tostrengthening or retrofitting. For the CFRP-strengthened specimensof group [A], the term ðVc þ VsÞexp was taken to be equal to theaverage shear resistance of all unstrengthened beams. For theCFRP-retrofitted specimens of group [B], the term ðVc þ VsÞexpwas taken to be equal to 0.91Vorg (refer to shear capacity ofspecimen D-PR, which was patch repaired with epoxy withoutCFRP).

Analytical Results and Discussion

A comparison between the experimental and analytical results ispresented in Table 3. The analytical predictions are also plotted ver-sus the experimental values in Fig. 16. All guidelines/standards

provided a nonconservative prediction for CFRP shear resistanceof specimen U-EB2, which was strengthened with two layers ofEB-CFRP without end anchorage. This is because the analyticalformulas do not take into consideration the interaction betweenthe amounts of internal stirrups and external CFRP shear reinforce-ment. This drawback seems to be more pronounced when a highnumber of CFRP sheets have been used without end anchorage inthe presence of internal steel stirrups. The ACI 440.2R and theCNR-DT200 analytical formulas provided a conservative predic-tion for the CFRP contribution for all other specimens. Similarly,the HB 305 Australian standards provided a conservative predictionfor the CFRP shear resistance for all other test specimens exceptspecimen D-EB2-TB, which was retrofitted with the higher amountof CFRP and SCP+TB end anchorage. However, if the maximumCFRP shear resistance predicted by Eq. (4) had been considered,the HB 305 Australian standards would have provided a conser-vative prediction for the CFRP shear resistance of specimenD-EB2-TB.

The fib TG 9.3 guidelines overestimated the CFRP shear resis-tance for all test specimens except specimens D-EB1-PAF andD-EB1-TB with the lower amount of CFRP and mechanical endanchorage. This is because the fib TG 9.3 assumes that the effectivedepth of CFRP, df , is always equal to 0.9d. For a T-shaped section,the actual value of df is typically less than 0.9d. This in turn wouldoverestimate the number of CFRP sheets intersected by a shearcrack, and hence the CFRP contribution to shear capacity. Thefib TG 9.3 guidelines would have provided a conservative predic-tion for the CFRP shear resistance of specimen D-EB2-TB, whichwas overstrengthened for shear, if the maximum CFRP shearresistance predicted by Eq. (4) had been considered.

θ

θT

fc

'νC

V

Fig. 15. Schematic diagram showing a web crossed by shear cracks

Table 3. Comparison of Test Results with International Guidelines/Standards

Specimen

Experimental Analytical results (Vfn) Ratio (Vfn=Vf;exp)

Vf;exp(kN)

ACI 440.2R(kN)

fib TG9.3 (kN)

CNR-DT200(kN)

HB 305(kN) Vf;max

ACI440.2R

fibTG 9.3

CNR-DT200

HB305

U-EB1 15 11.1 23.0 15.1 13.3 30.2 0.74 1.53 1.0 0.89U-EB2 13 18.6 31.4 19.9 16.7 30.2 1.43 2.42 1.53 1.28D-EB1-PAF 23 11.1 23.0 15.4 13.3 38.2 0.48 1.00 0.67 0.58D-EB2-PAF 22 18.6 31.2 19.9 16.7 37.3 0.85 1.42 0.90 0.76D-EB1-TB 46 17.4 31.4 18.0 25.7 42.8 0.38 0.68 0.39 0.56D-EB2-TB 36 34.9 53.5a 26.8 51.4a 35.5 0.97 1.49 0.74 1.43aValue exceeds the maximum calculated CFRP shear resistance Vf;max.

0

5

10

15

20

25

30

35

40

45

50

55

60

0 5 10 15 20 25 30 35 40 45 50 55 60

Ana

lytic

al V

fn(k

N)

Experimental Vf,exp (kN)

ACI 440.2R

fib TG 9.3

CNR-DT200

HB 305

Safe side

Fig. 16. Analytical versus experimental CFRP shear resistance

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Conclusions

Based on the results of this study, the following conclusionsare drawn:• The experimental study demonstrated that retrofitting of se-

verely shear-damaged RC girders with EB-CFRP compositesand proper mechanical end anchorage can fully restore the ori-ginal shear capacity of the beams. The shear capacity of the pre-damaged beams retrofitted with EB-CFRP and mechanical endanchorage was even higher than the original shear capacityrecorded in the first shear test to failure. The use of an SCPin combination with a TB as an end anchorage system was moreeffective than using the panel with PAFs. The use of an SCP+PAF end anchorage system delayed debonding of the CFRPand peeling of the concrete cover, whereas the use of SCP+TB prevented debonding of CFRP and allowed the beam todevelop its full shear capacity by failure of concrete struts. In-creasing the number of CFRP layers did not result in additionalgain in shear capacity regardless of the presence of the mechan-ical end anchorage. Premature debonding of CFRP can preventthe beams with a high amount of CFRP from reaching anadditional gain in shear resistance. Crushing of low-strengthconcrete struts can also limit the gain in shear resistance pro-vided by additional CFRP sheets despite using a proper andeffective end anchorage system.

• The analytical investigation demonstrated that all internationalguidelines/standards examined in this study provided a noncon-servative prediction for the nominal CFRP shear resistancewhen a high amount of CFRP (two layers) was used withoutend anchorage. When a low amount of CFRP (one layer)was used without end anchorage, all guidelines/standards pro-vided a conservative prediction for the CFRP shear resistanceexcept the fib TG 9.3, which overestimated the CFRP shearresistance by approximately 53%. All guidelines/standards pro-vided a conservative prediction for the CFRP shear resistancewhen the lower amount of CFRP was used in combination withmechanical end anchorage. For the beams retrofitted with thehigher amount of CFRP in combination with SCP+PAF endanchorage system, all guidelines/standards, except the fib TG9.3, provided a conservative prediction for the nominal CFRPshear resistance. For the beams with the higher amount of CFRPin conjunction with an SCP+TB end anchorage system, the ACI440.2R and the CNR-DT200 guidelines provided a conservativeprediction for the nominal CFRP shear resistance, whereas thefib TG 9.3 and HB 305 overestimated the CFRP shear contribu-tion. If the maximum CFRP shear resistance, Vf;max, had beenconsidered, the fib TG 9.3 and HB 305 would have also pro-vided a conservative prediction for the CFRP shear resistanceof the beams retrofitted with the higher amount of CFRP in com-bination with an SCP+ TB mechanical end anchorage system.

Acknowledgments

The authors wish to express their gratitude to the UAE Universityand Emirates Foundation grant no. 2009/052 for financing this re-search work. The authors would like to thank Mr. Abdelrahman Al-Sallamin, Eng. Tarek Salah, and Mr. Faisel Abdel-Wahab for theirassistance throughout testing.

Notation

The following symbols are used in this paper:a = shear span;

bf = width of beam flange;bw = width of beam web;C = compression force caused by flexure;d = effective depth of tensile steel reinforcement;df = effective depth of EB-CFRP;f 0c = concrete compressive strength;h = beam depth;jd = flexural level arm;

Pnew = maximum applied load recorded in the second shear test;Porg = maximum applied load recorded in the first shear test;rw = radius of web corner;s = spacing between steel stirrups;sf = center-to-center spacing between EB-CFRP shear

reinforcement;smax = maximum spacing between stirrups specified by ACI

318-05 (2005);T = tension force caused by flexure;V = shear resistance;Vc = concrete contribution to shear resistance;

Vf;exp = experimental CFRP shear resistance;Vf;max = maximum calculated CFRP shear resistance;

Vfn = calculated (nominal) CFRP shear resistance;Vnew = shear resistance recorded in the second shear test;

Vnmax = maximum calculated shear resistance of an RC beam;Vorg = shear resistance recorded in the first shear test;Vs = steel stirrups’ contribution to shear resistance;α = angle of inclination of CFRP shear reinforcement with

respect to the longitudinal axis of the beam(90° ≤ α < 180°);

θ = angle of inclination of shear cracks; andν = effectiveness factor that accounts for the lower strength of

inclined concrete struts.

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