flexural strengthening of concrete beams using externally bonded compositematerials

Flexural strengthening of concrete beamsusing externally bonded compositematerials

Michael J. Chajes*, Theodore A. Thomson Jr, Ted F. Januszka and WilliamW. Finch Jr

Department of Civil Engineering, University of Delaware, 137 Dupont Hall, Newark, DE19716, USA

Received 22 March 1994; revised and accepted 3 May 1994

In this study, a series of reinforced concrete beams were tested in four-point bending to determine theability of externally bonded composite fabrics to improve the beams' flexural capacity. The fabrics usedwere made of aramid, E-glass and graphite fibres, and were bonded to the beams using a two-partepoxy. The different fabrics were chosen to allow a variety of fabric stiffnesses and strengths to bestudied. The external composite fabric reinforcement led to a 36 to 57% increase in flexural capacity anda 45 to 53% increase in flexural stiffness. For the beams reinforced with E-glass and graphite fibrefabrics, failures were a result of fabric tensile failure in the maximum moment region. The beamsreinforced with aramid fabric failed due to the crushing of the compression concrete. In addition to thetest results, an analytical model based on the stress-strain relationships of the concrete, steel andcomposite fabrics is presented. Using the model, beam response is computed and compared with theexperimental results. The comparisons indicate that the flexural behaviour of composite-fabricreinforced concrete beams can be accurately predicted using the described method.

Keywords: composite materials; flexural strength; reinforced concrete beams

Many modern cities are faced with a rapidly deteriorating infrastructure. Because of the prohibitive cost ofreplacing large numbers of deteriorated structures,research efforts have focused on methods of strengthening existing structures. The rehabilitation of concretestructures represents one of the more challengingproblems faced by engineers today. The research presented here focuses on flexural strengthening of reinforced concrete beams through the use of externallyapplied composite materials.

Research and field application of steel plates bondedto concrete has been the subject of ongoing research formany years. Steel plates bonded to the tension face ofconcrete beams have been shown to be effective in (i)increasing flexural capacity and (ii) enhancing flexuralstiffness thereby reducing deflections and controllingcrackingt". Included in these works are studies regardingthe anchorage of steel plates bonded to concrete beams.Stress concentrations at the ends of the plates can lead topremature debonding of the plates, and several differentend-anchorage schemes were employed to help alleviatethis problem. This work provides useful insight into themethods of bonding plates to concrete, the types of adhesives that can be used, and the various modes of failure.

While steel plates have the advantage of possessing ahigh strength-to-weight ratio (i.e. fairly thin plates can be

*Correspondence to Dr Michael J. Chajes

used), they have the disadvantages of being susceptible tocorrosion and debonding.

As an alternative to steel, the use of advanced composite materials for structural rehabilitation shows greatpromise. Along with having a high strength-to-weightratio, composite materials have the beneficial characteristics of being non-corrosive and generally resistant tochemicals. Several survey papers have discussed promising applications of composite materials for a variety ofcivil structures'"!", In terms of strengthening concretestructures, research efforts have included (i) the bondingof fibre-rein forced-plastic (FRP) composite plates to reinforced concrete and prestressed concrete beams toimprove flexural stiffness and strength's-"; (ii) the wrapping of concrete columns with fibreglass/epoxy jackets toprovide the additional flexural and shear strength neededin seismic regions>; (iii) the confinement of concretecolumns using composite materials, thereby increasingthe columns' axial capacityv"; and (iv) the wrapping ofconcrete beams with composite fabric/epoxy jackets toprovide additional shear strength>?'.

Work involving the bonding ofcomposite plates to thetension face of concrete beams can be divided into twocategories: (i) the bonding of non-prestressed compositeplates, and (ii) the bonding of prestressed compositeplates. In the studies involving non-prestressed composite plates bonded to reinforced concrete beams, various types of glass, carbon and aramid fibre composite

0950-0618/94/03/0191-11© 1994 Butterworth-Heinemann Ltd Construction and Building Materials 1994 Volume 8 Number 3 191

Flexural strengthening of concrete beams: M. J. Chajes et al.

Figure 1 Test beam geometry, strain gauge locations and appliedloading

loads do not have a detrimental effect on the fabricreinforced beams. Both of these tests should help toidentify the most appropriate fabrics for this type ofapplication.

I 127mm I

Q2 # 3 Bars

Test Set-Up

I 127mm I

~

Beam Cross-Sections

1#3Bar

I 127 mm I

D

~ I 406.4mm31.75 mm

Experimental study

Test programme

This programme consisted of testing 14 under-reinforcedrectangular concrete beams. The tests were designed toevaluate the effect of externally bonded, composite fabricreinforcement on the beams' flexural behaviour. Thebeams were loaded monotonically to failure in fourpoint bending using an 890 kN capacity Tinius Olsenuniversal testing machine (see Figure 1).

Beam design

Fourteen rectangular beams with span lengths of 1.12 mand cross-sectional dimensions of 76.2 mm x 127 mmwere used (see Figure 1). The set of beams consisted ofthree control beams having only traditional steel reinforcement (beams CI, C2, C3); three sets of three beams,each having the same steel reinforcement as the controlbeams but with additional, externally applied aramid, Eglass and graphite fibre fabric reinforcement (beams A I,A2, A3, EI, E2, E3, GI, G2, G3); and two beams havingtwice the steel reinforcement of the control beams but noexternal composite fabric reinforcement (beams Sl, S2).The beams with additional steel reinforcement were usedto allow a comparison of beams having additional external fabric reinforcement with beams having additionalinternal steel reinforcement. Cross-sections of the various types of beams are shown in Figure 1.

All beams were designed to fail in flexure. The controlbeams and the beams with additional steel reinforcementwere designed according to the specifications of the ACIBuilding Code". In satisfying the ductility requirement(200/h~Pb~0.75Pb), a single Grade 60#3 bar (9.525mm diameter with a yield strength of 413 MPa) was used.Two # 3 bars were used in beams having extra steel

plates have been tested 1s- 25• These studies have shownthat the externally reinforced concrete beams exhibitedsubstantial increases in flexural capacity and stiffness. Inthe earliest work, many of the beams failed due to thedebonding of the composite plate. In later studies, adhesives with sufficient strength to transfer the shear forcesbetween the composite and concrete were found. Whenthese adhesives were used, the failure mode with regardto bond was a shear failure of the concrete. Finally,analytical models were developed for predicting theflexural behaviour of the beams.

The technique of prestressing composite plates andthen bonding them to reinforced concrete beams has alsobeen studied>:". These studies found that prestressed,bonded reinforcement produced a moderate gain overnon-prestressed reinforcement. The most significantimprovement found was the control of cracking. In onestudy>, a theory for the optimal design of externallybonded composite plates was presented.

While FRP composite plates can be used as an effectivemeans of providing additional reinforcement, they dopossess some drawbacks including (i) the need for a flatsurface for bonding, (ii) the cost associated with manufacturing large plates, and (iii) the difficulty in achievinga bond between the concrete and the composite platesufficient enough to prevent debonding from governingthe failure mode.

As an alternative to the use of composite plates, thispaper investigates the strengthening of reinforced concrete beams using epoxy-bonded composite fabrics madeof aramid (Kevlar*), E-glass, and graphite fibres. Likethe composite plates, these fabrics are non-corrosive andpossess high strength-to-weight ratios. They also havethe beneficial qualities of (i) being able to conform toirregular surface geometries, (ii) being manufactured inlong lengths, and (iii) being able to be bonded to beamsin such a way as to develop full tensile capacity prior todebonding. Jackets made from glass-fibre fabrics expoxied to concrete columns have been studied as a possibleseismic retrofitting procedure and have been found toperform quite well>.

This research can be directly applied to the upgradingand rehabilitation of concrete beams. In situations wherebridges will be required to handle increased traffic, composite fabrics can be adhered to the beams, therebyincreasing allowable loads. Likewise, damaged beamscan be reinforced and restored to their original capacity.Because of their ability to conform to the shape of thebeam, composite fabrics can be used in a wide variety ofsituations, and by wrapping the exposed cross-section,strong bonds can be achieved and a simultaneousincrease in shear capacity can be gainedv-". It should benoted that additional environmental and fatigue testingmust be run before actual field application is attempted.The environmental testing should investigate the abilityof the various fabrics to withstand aggressive environments, while the fatigue testing should verify that cyclic

*Kev!ar is the registered tradename of a family of aramid fibres produced by the E.I. duPont de Nemours and Co. Inc., Wilmington, DE,USA

192 Construction and Building Materials 1994 Volume 8 Number 3


0.025

-Aramid (1 layer)......... E-glass (3 layers)

- - - - -Graphite (2 layers)

0.010 0.Q15 0.020

Strain (mm1mm)

.....'

,,,

.. '

0.005

,,,,,,,,

, .', .', .:

r .', .', .'0...jL..-'-............+--'-~'-'-f--'---"---'-~+-'-~-'-1--'---'-'---'--+

0.000Ultimate strength,};' (MPa)

Table I Beams tested250

Beams Internal steel External fabricreinforcement reinforcement

200

CI, C2, C3 I #3 bar

ISI, S2 2#3 bars 150AI, A2, A3 I #3 bar Aramid (one layer)El, E2, E3 I #3 bar E-glass (three layers) '"Gl, G2, G3 I #3 bar Graphite (two layers) '"4) 100J:l

tf.l

50

Concrete batch

Table 2 Concrete strengths

12345

34.833.636.042.542.1

Figure 2 Stress-strain behaviour of impregnated composite fabrics

reinforcement (beams Sl and S2). The beams with external composite fabric reinforcement were designed so thatthe tensile capacity of the fabric was close to the yieldload of the additional # 3 bar (29.4 kN). To accomplishthis, different numbers of fabric layers were used according to the ultimate strength of the various types of fabric.According to the tensile capacity of the aramid, E-glassand graphite fabrics (see section on composite fabrics),one layer of aramid fabric (29.5 kN), three layers of Eglass (24.9 kN) and two layers of graphite fabric (29.4kN) were used. Shear reinforcement was not needed inany of the beams. Table 1 provides a summary of thevarious beams tested.

Beam fabrication

A concrete mix having a water-to-cement ratio of 0.50 byweight was used to cast the 14 beams. Type I Portlandcement was used, and the maximum aggregate size was9.5 mm. Five batches of concrete were needed to fabricate the 14 beams and associated standard test cylinders(152.4 mm diameter by 304.8 mm high). The beams werecast in the University of Delaware Structures Laboratoryand were allowed to cure in a water bath for 28 days.

The concrete strengths for the five batches are shownin Table 2. The average strength for the five batches was37.8 MPa, and none differed by more than 12.5% fromthis average. The Grade 60 steel reinforcing bars usedwere tested and found to have an actual yield strength of493.0 MPa.

Composite fabrics

The three types of composite fabrics used in this studywere (i) plain-weave aramid (Kevlar") fabric, (ii) crowfoot satin-weave E-glass fabric, and (iii) plain-weavegraphite fabric. All of these woven composite fabrics aremade up of fibres oriented at 0° and 90° with an equaldistribution of fibres in each direction. The use of thethree different fabrics allowed the effect of varying thestiffness of the external reinforcement to be studied. Itshould be noted, however, that the use of some of these

fabrics may prove to be undesirable if they are found tolack acceptable environmental durability.

For each type of fabric, three tensile test specimensimpregnated with the same adhesive used to bond thefabrics to the concrete (see next section) were prepared.The number of layers of fabric used for the specimenswas the same as used for the beams (one layer of aramidfabric, three layers of E-glass fabric and two layers ofgraphite fabric). All test specimens were cured under avacuum. After curing, the tensile specimens were loadedto failure using an Instron universal testing machine.From the test results, their respective elastic moduli,failure strains and ultimate strengths were computed.Stress-strain plots for the three impregnated compositefabrics are shown in Figure 2, and a summary of theaverage test values is presented in Table 3.

Adhesive selection

Based on ongoing research by the authors dealing withthe bonding of composite plates to concrete surfaces,Sikadur 32 was selected as the adhesive for this application. Sikadur 32 is a two-component, high-modulus,high-strength, construction epoxy. The two parts aremixed in a ratio of 1:1and can be cured at room temperature.

Since the previous bond tests involved compositeplates bonded to concrete, additional bond tests usingSikadur 32 and the three types of composite fabrics wereconducted. A series of pull-off tests were run using theset-up shown in Figure 3. The tests involved the bondingof a 1 inch wide piece of epoxy-impregnated compositefabric to a concrete block. Nine specimens having bondlengths of 25.4, 50.8 and 76.2 mm were tested (three ateach length). The tests indicate that a single layer ofaramid, a triple layer of E-glass, and a double layer ofgraphite fabric can be expected to develop full tensilecapacity in approximately 50.8 mm for both the E-glassand graphite fabric, and in approximately 76.2 mm forthe aramid fabric. Based on these results, Sikadur 32 wasdeemed to be a suitable adhesive choice. A summary ofthe bond test results is given in Table 4.

Construction and Building Materials 1994 Volume 8 Number 3 193


Table 3 Properties of resin-impregnated composite fabrics

Composite fabric Specimen thickness Modulus of elasticity Failure strain, 4u Failure stress, a,u(mm) E,(MPa) (mm mm- I ) (MPa)

Aramid (one layer) 1.04 11020 0.0225 223E-glass (three layers) 1.42 13090 0.0122 138Graphite (two layers) 1.22 22050 0.00748 190

p p

t t.- Composite Fabric---. Composite Fabric (no end tabs)

(b)

FrontView SideView Figure 4 Externally bonded fabric: (a) beams A I, EI, E2, E3, G I, G2,G3; (b) beams A2, A3

Figure 3 Experimental set-up for bond tests

Table 4 Bond test results

Bond length (mm) Aramid fabric E-glass fabric Graphite fabric

25.4 B B BB B FB F F

50.8 B F FB F FB F F

76.2 F F FF F FF F F

B - Bond failureF - Fabric tensile failure

Bonding ofcomposite fabric

Prior to the bonding of the fabric to the beams, theconcrete surface was mechanically abraded using agrinding wheel, creating a somewhat porous surface. Thefabric was coated with adhesive on both sides and placedonto the tension face of the beam, which itself had beencoated with adhesive. The fabric was then smoothed toensure a uniform distribution of adhesive and placed in avacuum bag. The beam was allowed to cure undervacuum for I day and for an additional 2 days once itwas removed from the vacuum.

The two bonding schemes used are shown in Figure 4.Originally, all beams were bonded without end tabs asshown in Figure 4(a). During the initial round of testing,however, the first aramid-reinforced beam (beam AI)experienced some fabric debonding. As a result, end tabswere added (see Figure 4b) as suggested in a study byRitchie et a/.21•

Instrumentation and test procedure

The test procedure consisted of loading all 14 beamsmonotonically to failure according to the configuration

Figure 5 Typical beam during testing

shown in Figure I. Failure was defined as (i) the crushingof the concrete in the compression region, (ii) tensilefailure of the composite fabric, or (iii) the debonding ofthe composite fabric. During the testing, the deflectionsof all beams were measured at the end points, at thelocation of the applied loads, and at midspan. A photograph of a typical beam taken during testing is shown inFigure 5.

To provide additional information about the behaviour of the externally reinforced beams, electrical resistance strain gauges were used to measure strains in boththe concrete and composite on one of each type of fabricreinforced beam (beams A2, E2 and G2). Four gaugeswere mounted through the depth at midspan with anadditional two gauges mounted on the fabric on eitherside of the midspan gauge. The location of all straingauges is shown in Figure I.



Figure 6 (a) Stress-strain curve for concrete, (b) internal strain, stressand force distributions for beam

Theoretical analysis

General methodology

Beams properly designed according to the ACI Code>'will fail in flexure due to the crushing of the compressionconcrete. The ultimate strength analysis of such underreinforced beams is relatively straightforward. This ispartially due to the fact that the design procedure forcesthe longitudinal reinforcing steel to be at yield when thebeam fails. Because composite materials loaded in tension behave linear elastically to failure (see Figure 2), theanalysis of concrete beams reinforced with compositematerials must be handled differently than the ACIapproach. An iterative analytical method for predictingthe flexural behaviour of reinforced concrete beams withexternally bonded composite plates has been presentedby An et al.». This method, which involves assuming astrain distribution for the cross-section and then utilizingthe material constitutive relationships to check equilibrium, will form the basis for the analysis used herein.

Cross-Section Strain

(a)

(b)

Stress Force

Assumptions

Before proceeding to the details of the analytical procedure, the following assumptions are noted: (i) planesections remain plane; (ii) no slip betwen the longitudinalreinforcing steel and the concrete; (iii) no slip betweenthe impregnated composite fabric and the concrete; (iv)no shear failure of the concrete adjacent to the bondedfabric reinforcement; (v) concrete carries no tension; and(vi) maximum strain in the concrete in compression is0.003.

Material stress-strain relationships

Use of the analytical model assumes that the dimensionsof the beam are known and that complete stress-strainrelationships are known for the steel rebar and impregnated composite fabric in tension, and the concrete incompression. In our case, the steel rebar is assumed tobehave elastic-perfectly plastic, and the steel tensilestressfs is given by

Values for the modulus" ultimate strain and ultimatestrength for the three composite fabrics are given inTable 3.

For the concrete, Hognestad's idealized stress-straincurve for concrete in compression, described by Park andPaulay>, is used. This relationship, shown in Figure6(a), is given by

(4)

and

where

So = 2fc"/s; (6)

fs = E,6, for 0 ~ 6, < 6y (1)fc"';:::,0.92fc' for 32.8 MPa ~ fc' ~44.8 MPa (7)

where E, is the modulus of the epoxy-impregnated fabric,Gr is the fabric strain, and Gru is the ultimate fabric strain.

where E, is the elastic modulus of the steel, 6, is the strainin the steel, and hand 6y are the yield stress and associated yield strain. From tension tests, it was found thatthe # 3 bars used have a yield strength of 493 MPa and amodulus of 200000 MPa. It is assumed that all beamsfail before the steel reaches its ultimate strain.

The impregnated composite fabrics are modelled asbehaving linear elastically to failure, and the fabrictensile stressfr is given by

and

I, = h for 6y~ 6,

fr = ErGr for 0~ 6r < Gru

(2)

(3)

E; = 4730.jfc'MPa for normal weight concrete (8)

in which fc is the stress in the concrete, fc' is the cylinderstrength, fc" is the maximum compressive stress reachedin the concrete, e; is the strain in the concrete, So is thestrain in the concrete at maximum stress, Seu is the maximum strain in the concrete, and E, is the initial tangentmodulus of the concrete in compression.

Analytical method

The method presented is iterative in nature and is bestsuited for computer solution. It begins by selecting avalue of concrete strain at the outer compression fibreand guessing the associated location for the neutral axis.Based on the values chosen, the assumptions discussedabove, and the stress-strain relationships for the concrete, steel and composite fabric, a state of strain, stress



Table 5 Test results for each beam

Beam Concrete strength j, Failure mode Experimental beam strength(MPa) (N)

CI 34.8 Concrete crushing 10 182C2 34.8 Concrete crushing II 236C3 36.0 Concrete crushing II 125SI 33.6 Concrete crushing 18330S2 33.6 Concrete crushing 1698 6AI " 42.1 Fabric debonding 16269A2 42.5 Concrete crushing 147 56A3 34.8 Concrete crushing 16 888EI 33.6 Fabric tensile failure 15 280E2 42.1 Fabri c tensile failure 152 82E3 42.5 Fabric tensile failure 153 79GI 42.5 Fab ric tensile failure 15059G2 42.1 Fabri c tensile failure 17035G3 36.0 Fabric tensile failure 144 89

Cont rol beam strength(N)

10849b

to 849b

to 849'10849'10849'10 849b

10 849b

10849'10 849'10 849h

to 849h

Percentage increase

+ 69.0+56.6+ 50.0+ 36.0+55.7+ 40.8+ 40.9+ 41.8+ 38.8+57.0+ 33.6

"This was the only aramid-reinforced beam without end tabs'Average strength of control beams CI, C2 and C3

Figure 8 Load versus midspan deflection behaviour of fabricreinforced beams after fabric failure

- ControlBeam (C2)

.........E-glassBeam (E3)

--ControlBeam (C2)-Steel Beam (SI)- - - - -Aramid Beam (A3).........E-glassBeam(E3)- -Graphite Beam (G3)

10.0 20.0 30.0 40.0 50.0 60.0MidspanDisplacement(nun)

10.0 20.0 30.0 40.0 50.0 60.0MidspanDisplacement(nun)

Load versus midspan deflection plot for typical beamsFigure 7

20000

15000

~-g 10000.3

5000

16000

14000 .'.'

12000

~10000 :

-g 80000

...J6000

4000

2000

and 52) also failed in flexure owing to concrete crushing.Like the control beams, these beams also developedinitial flexural tension cracks at loads near 2000 N. How"ever, because of the additional tensile reinforcement, thesteel in these beams yielded at loads ranging from 15500to 16500 N and finally failed at loads between 17000 and

and internal resultant forces can be established (seeFigure 6b). By summing the resultant forces, the equilibrium of the section can be checked. If the resultantforces are not in equilibrium, a different location for theneutral axis is guessed while the selected value for thestrain at the outer fibre of the concrete is maintained.Once a neutral axis location that satisfies equilibrium isfound, the internal moment, curvature and strain anywhere in the cross-section can be computed. By followingthis procedure, and by allowing the value of the strain atthe outer fibre of the concrete to begin at a small valueand incrementally increa se, the flexural behaviour of thesection can be establi shed . Failure of the section is said tooccur either when the strain in the concrete reaches itsultimate compression strain E:cu (taken to be 0.003 in thisstud y), or when the composite fabric reaches its ultimatetensile strain Cfu'

All of the beams tested were analysed using acomputer program that followed the procedure described above . The analyses provide predictions regardingthe ultimate capacity of the beams, their mode of failureand the load-deformation behaviour.

Experimental and analytical results

The results of the 14 beam tests are summarized in Table5 and Figures 7-9. These results, along with the corresponding analytical predictions, are discussed in thesections that follow.

Beam behaviour and mode of failure

Upon being loaded , each of the three control beams (CI ,C2 and C3) began developing flexural tensile cracks inthe const ant moment region at loads around 2000 N. Atloads around 9000 N, the reinforcing steel in these beamsyielded. Finally, each of the three control beams failed inflexure at loads of around II 000 N as a result of thecompression concrete crushing. The load-deflectionbehaviour of a typical control beam is shown in Figures 7and 8, while a picture showing beam C3 after failure canbe seen in Figure 9.

The two beams with add itional steel reinforcement (51

196 Construction and Bu ild ing Mater ials 1994 Volume 8 Number 3


18500 N. Figure 7 includes a typical load-deflection plotfor these beams.

The general flexural behaviour to failure of all thecomposite-fabric-rein forced beams was similar, althoughthe flexural stiffness and final mode of failure var ieddepending upon the fabric used. Like the standard steelreinforced beams, the fabric-reinforced beams also developed flexural cracks at loads around 2000 N. Aftercracking, the beams continued to deform at stiffnessesbetween that of the control beams and the beams havingadditional steel reinforcement. At loads around 12 000N, the steel in these beams began to yield as evidenced bythe no ticeable change in flexural stiffness seen in Figur es7 and 8. Because the impregnated fabric remains elastic,

Figure 9 Photographs of typical beams afte r failure: (a) control beamwith I # 3 bar (C3); (b) aramid fabric reinforced without end tabs (AI);(c) aramid fabric reinforced with end tabs (A2); (d) E-glass fabricreinforced (E3); (e) graphite fabric reinforced (G3)

these beams exhibit an appreciable amount of flexuralstiffness until failure occurs at loads ranging fromapproximately 15 000 to 17000 N. Figur e 7 shows typicalload-deflection behaviour for the beams reinforced witheach of the three types of fabric.

For all of the beams reinforced with E-glass and graphite fabrics , beam failure resulted following the tensilefailure of the fabric. For the beams instrumented withstrain gauges, the strains recorded just prior to failure aresimilar to the values obtained from tensile tests. Following the tensile failure of the fabric , the beams' resistancedropped to that of the control beams , and the beamscontinued to resist load until the compression concretecrushed. This beha viour is shown for one of the E-glass



Table 6 Average ultimate strength increase for beams Table 7 Average stiffness increase for beams in service load region

Beams Experimental Control beam Percentage Beams Experimental Control beam Percentagebeam strength strength increase beam stiffness stiffness increase

(N) (N) (N mm- I) (N mm")

CI, C2, C3 10 848a CI, C2, C3 1203a

81,82 17658b 10 848a +62.8 81,82 1805b 1203a +50.0A2,A3 15822b 10 848a +45.9 AI, A2,A3 I 843b 1203a +53.2EI, E2, E3 153J4b 10 848a +41.2 EI, E2, E3 1752b 1203a +45.6GI, G2, G3 15528b 10 848a +43.1 GI,G2, G3 1744b 1203a +45.0

-Average value for control beamshAverage values

-Average value for control beamsbAverage values

Table 8 Measured versus theoretical ultimate strength of beams

beams, as compared with the control beams, are evidentin the load-deflection plots of Figures 7 and 8. To quantify and compare the flexural stiffness of the beams in theservice load region, stiffnesses of the various beams werecomputed by finding the slope of the line connecting apoint at the origin of the load-deflection plot and a pointon the displacement curve corresponding to a deflectionof L/360 (where L is the beam's span length). The average of these flexural stiffnesses for each of the differenttypes of beams is given in Table 7. From the results inTable 7, we can see that both the fabric-reinforced beamsand the beams with additional steel reinforcement exhibited similar increases in stiffness in the working loadregion ranging from 45.0 to 53.2%.

Comparison ofanalytical and experimental results

Using the analytical method discussed earlier, the flexural behaviour and ultimate flexural capacity of the threetypes offabric-reinforced beams were computed. Table 8contains a comparison of the average measured beamstrengths and the values obtained using the analyticalmodel. From these results, we see that the model is quiteaccurate in predicting the ultimate capacity of the various beams. The computed values differed by 17.0, 2.6and 4.5% respectively for the aramid-, E-glass- and graphite-reinforced beams. These differences are quite smallwhen one considers the expected variation associatedwith the behaviour of reinforced concrete structures. Infact, computed values of ultimate capacity for the beamshaving only steel reinforcement differed by approximately 20% from the experimental results.

Another useful comparison involves looking at thestrain behaviour of the composite fabric. In Figure 10,plots of the applied load versus fabric strain for the threebeams with strain gauges (A2, E2 and G2) are comparedwith values computed analytically. Like the computation

"Based on an average concrete strengthhAverage values

+17.0-2.6-4.5

Percentagedifference

I3 523a

15728"16266"

Theoreticalbeam strength

(N)

15822b153J4b15528b

Experimentalbeam strength

(N)

Beams

A2,A3EI, E2, E3GI,G2,G3

beams in Figure 8. Figure 9 shows typical E-glass- andgraphite-reinforced beams after failure.

The first aramid-reinforced beam tested (A 1) failedwhen a sudden debonding of the aramid fabric occurred.The other two beams with aramid fabric (A2 and A3)utilized end tabs and did not experience debonding.These beams, unlike the beams with E-glass and graphitereinforcement, reached the ultimate compression strainof the concrete before reaching the fabric's tensile capacity. As a result, these two beams failed owing to crushing of the compression concrete (see Figure 9). Thisdifference in failure mode from the other fabric-reinforced beams is understandable since the ultimate strainof the impregnated aramid fabric is twice that of theimpregnated E-glass fabric and three times that of theimpregnated graphite fabric.

It is important to note that while the fabric-reinforcedbeams do not display as much ductility as the beamsreinforced with steel alone, they do exhibit some measureof ductility prior to failure. If ductility is taken to be theultimate displacement divided by the displacementoccurring when the steel yields (Llu/Lly) , the steel-reinforced beams have ductilities on the order of four to fivewhile the fabric-reinforced beams have ductilities on theorder of two or three. It should be noted that the ductilities of the E-glass and graphite fabric-reinforced beamsare similar to steel-reinforced beams if the behaviourafter fabric failure is considered (see Figure 8).

Changes in strength and stiffness

All of the externally reinforced beams showed significantincreases in ultimate flexural capacity as compared withthe control beams. As shown in Table 5, the increasesranged from 33.6 to 57%. When the experimental valuesare averaged, we find that the E-glass-, graphite- andaramid-reinforced beams showed similar increases instrength over the control beams of 41.2,43.1 and 45.9%respectively (see Table 6). By comparison, the averageincrease in strength resulting from the additional steelreinforcement was 62.8%. The effect of the externallyapplied reinforcement on ultimate flexural capacitybecomes very similar to that of beams having additionalinternal steel reinforcement when one recalls that theactual (tested) strength of the additional steel was 15.7%higher than the strength of the aramid fabric, 16.0%higher than the strength of the E-glass fabric, and 28.9%higher than the strength of the graphite fabric.

Differences in flexural stiffness of the fabric-reinforced



Finally, based on the analytical computations, thepercentage of the fabric-reinforced beams' flexural resistance contributed by the fabric reinforcement at bothinitial yield of the steel reinforcement and at the ultimateload is shown in Table 9. From these numbers we see thatthe composite reinforcement contributes between 23 and42% of the moment resistance at initial yielding of thesteel reinforcement, and between 43 and 54% of themoment resistance at failure .......... E-glass Beam (Theoretical)

-E-glass Beam (Measured)

4000

2000

6000

sooo

16000 +-.~..,.......j~~-+-~~+-'-~'-+~~+--->~-'--+

14000

12000

gl0000

]

Conclusion

The results of this study indicate that externally appliedcomposite fabrics can be effectively used to rehabilitateor strengthen concrete beams, and analytical methodsneeded to describe their behaviour are available. Foreach of the three types of fabrics tested, increases inflexural strength similar to those found in beams havingadditional steel reinforcement were achieved. Specifically, the beams reinforced with epoxy-impregnated aramid, E-glass and graphite fabrics displayed increases inflexural capacities of 53.2, 45.6 and 45.0%, respectively,over control beams having only internal steel reinforcement. The fabric-reinforced beams also displayed anapproximately 40% increase in flexural stiffness. Finally,the bond between the fabric and the concrete, combinedwith the additional anchorage provided by easily appliedend tabs, ensured failure of either the concrete in compression or the fabric in tension. Both of these failuremodes displayed a reasonable amount of ductility priorto failure.

In this paper, an analytical method was also presented.By comparing the measured and computed behaviour ofthe three types of fabric-reinforced beams, the methodwas found to predict the beams' ultimate flexural capacities accurately. The method was also accurate in predicting the mode of failure. For the beams reinforced withfabrics having smaller ultimate strain capacities (E-glassand graphite), predicted and observed failure resultedfrom the tensile fracture of the fabric. For the beamsreinforced with aramid fabric, which has a higher ultimate strain capacity, the predicted and observed mode offailure was crushing of the compression concrete.

Before this type of rehabilitation procedure can besafely applied, further studies involving the durability ofthese externally reinforced beams are needed. These testsshould provide information regarding the ability of thefabrics to withstand aggressive environments and cyclicloads.

0.010O.OOS0.004 0.006

Strain (mm1mm)

......... Aramid Beam (Theoretical)

- Aramid Beam (Measured)

........Graphite Beam (Theoretical)

- Graphite Beam (Measured)

0.002

o+-~~-I-~--+-~~-+~~-+~~'--I-

0.000

5000

g'i ioooo.3

5000

g'0 10000

.3

(b)

15000

o-J.S~-'--+~~-+-~~\---'-~'-+~~+-"~-'---+

0.000 0.002 0.004 0.006 0.008 0.010 0.012

Strain (mmlmm)

15000

(c)

(a)

o+--'-~-'-+~~-I-~~f---'-~-+~~t--'-~-'-t

0.000 0.002 0.004 0.006 O.OOS 0.010 0.012

Strain (mm1mm)

Figure 10 Load versus fabric strain diagrams: (a) aramid fabric;(b) E-glass fabric; (c) graphite fabric

of ultimate strengths, the computed load versus fabricstrain behaviour agrees reasonably well with the measured behaviour. Even for the results of the aramid-reinforced beam, in which the computed and measuredvalues differ most notably, the general stiffness exhibitedbefore and after the steel yielded are quite similar. Infact, the assumption that the concrete carries no tensionis responsible for a lot of the discrepancy between thetwo curves in this case.

Acknowledgements

The authors of this study would like to thank MarkCourtney and Hercules Incorporated for the graphitefabric used in these tests. We would also like to extendthanks to the Center for Composite Materials, with special thanks to Anthony Thiravong, for giving us access totheir equipment and supplies. Finally, we would like tothank Doug Baker and Michael Davidson for their useful contributions in preparing for the tests.


Flexural strengthening of concrete beams: M. J. Chajes at al.

Table 9 Contribution of composite fabric to flexural resistance

Fabric-reinforced beams Moment resisted bysteel-concrete couple

(N m)

Moment resisted byfa bric-concrete couple

(N m)

Percentage of momentresisted by fabric-concrete couple

156015351523

Nomenclature

nal reinforcement of concrete beams using fiber reinforced plastics. ACI Struct. J. 1991,88,490-500

22 Rostasy, F.S., Hankers, e. and Ranisch, E.B. Strengthening ofR/C- and PIC-structures with bonded FRP plates. Advanced Composite Materials in Bridges and Structures, CSCE, Sherbrooke,Canada, 1992,pp. 253-263

23 Saadatmanesh, H. and Ehsani , M.R. Flexural strength of externally reinforced concrete beams. Proc. First Materials EngineeringCongr., ASCE. 1990, pp. 1152-1161

24 Saadatmanesh, H. and Ehsani, M.R . Fiber composite plates canstrengthen beams. Concr. Int., ACI 1990, 12(3),65-71

25 Saadatrnanesh, H. and Ehsani , M.R . RC beams strengthened withGFRP plates. I: Experimental stud y. J. Struct . Eng. 1991, 117(11),3417-3433

26 Karam, G.N. Optimal design for prestressing with FRP sheets instructural members . Advanced Composite Materials in Bridges andStructures, CSCE, Sherbrooke, Canada, 1992, pp . 277-285

27 Triantafillou, T.e. and Deskovic, N . Innovative prestressing withFRP sheets: Mechanics of short-term behaviour. J. Eng. Mech.'1991, 117(7), 1652- 1672

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30 Katsumata, H. , Yoshirou, K. and Toshikaza, T. A study withcarbon fiber for earthquake-resistant capacity of existing reinforced concrete columns. Proc. Ninth World Conf on EarthquakeEngineering, Tokyo, Japan, 1988, Vol. 7, pp. 517-522

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32 Chajes, M.J., Januszka, T.F ., Thompson, TA., Finch, W.W. andMertz, D.R. Shear strengthening of reinforced concrete beamsusing externally applied composite fabrics . ACI Struct. J. In press

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34 ACI Committee 318. Building code requirements for reinforcedconcrete (ACI 318-89) Revised 1992. American Concrete Institute, Detroit, 1992

35 Park, R. and Paulay, T. Reinforced Concrete Structures, JohnWiley, New York , 1975

(a) At load corresponding to init ial yielding of steel reinforcementAramid 1541E-glass 1529Graphite 1519(b) At ultimate loadAramidE-glassGraphite

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AbCde

ds

458.9756.11095

118816611782

23.033.041.9

43.352.053.9

aramid-reinforced beamwidth of beamcontrol beamdistance from extreme compression fibre tocentroid of tension fabricdistance from extreme compression fibre tocentroid of tension reinforcementE-glass-reinforced beammodulus of elasticity of concretemodulus of elasticity of resin-impregnatedfabric



!e"ifishGhp

resultant force in compression concreteforce in fabric reinforcementforce in tensile steel reinforcementconcrete compression stresscompressive strength of concrete fromcylinder testmaximum compressive stress of concretefabric tensile stresssteel tensile stressspecified yield strength of non-prestressedsteel reinforcementgraphite-reinforced beamdepth of beamapplied load

CTr

CTru

CTs

beam reinforced with additional steelstrain in the concreteultimate compressive strain of the concretestrain in fabric reinforcementultimate tensile strain of the fabricstrain in concrete at maximum stressstrain in tensile steel reinforcementratio of non-prestressed steel tension reinforcement to cross-sectional area of beamreinforcement ratio producing balancedstrain conditionstress in fabric reinforcementultimate tensile stress of the fabricstress in steel tensile reinforcement


flexural strengthening of concrete beams using externally bonded compositematerials

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