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STRENGTHENING OF REINFORCED CONCRETEBEAMS UNDER TORSION USING CFRP SHEETS

El Mostafa Higazy*, Ain Shams University, EgyptMahmoud El-Kateb, Ain Shams University, Egypt

36thConference on OUR WORLD IN CONCRETE & STRUCTURES: 14 - 16 August 2011,Singapore

Article Online Id: 100036033

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36th

Conference on Our World in Concrete & StructuresSingapore, August 14-16, 2011

STRENGTHENING OF REINFORCED CONCRETE

BEAMS UNDER TORSION USING CFRP SHEETS

El Mostafa Higazy* and Mahmoud El-Kateb

*Department of Structural EngineeeringAin Shams University

Cairo, Egypte-mail: [email protected]

Keywords: Strengthening, Beams, Torsion, FRP

Abstract. Repair and strengthening of bridges, buildings, and other civilengineering structures have become necessary due to aging, environmentallyinduced degradation, increases in service loads, changes in use of the structure,design and/or construction errors, changes in design code regulations, andseismic retrofits. Many buildings and bridge elements are subjected to significanttorsional moments that affect the design, and may require strengthening.Reinforced concrete (RC) beams may be deficient in torsional shear capacityand in need of strengthening. These deficiencies may occur for several reasons,such as insufficient stirrups resulting from construction errors or inadequate

design, reduction in the effective steel area due to corrosion, or increaseddemand due to a change in occupancy. FRP composites have shown greatpromise as a state-of-the-art material in flexural and shear strengthening asexternal reinforcement. However, little attention has been paid to its applicabilityin torsional strengthening of RC beams in terms of both experimental andnumerical research.The main objectives of the current study are to investigate the torsional behaviorof RC beams strengthened with externally bonded carbon fiber reinforcedpolymer (CFRP) sheets, and to identify the influence of the investigatedparameters affecting the torsional behavior on the effectiveness and feasibility ofstrengthening.

The experimental program recounts an overall investigation of torsionalstrengthening of five rectangular reinforced concrete beams with externallybonded carbon fiber-reinforced polymer (CFRP) sheets under the effect ofcombined flexure, shear and torsion. One specimen is kept withoutstrengthening as control specimen, while four specimens are strengthened withCFRP composite wraps. The main parameters investigated are the CFRP warpsspacing and number of plies. The concrete grade and steel reinforcement ratioare kept constant for all specimens.In the experimental findings, the increase in the ultimate torsional moment fordifferent strengthening configurations, performance improvement and crackpatterns are presented.

Ain Shams University

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El Mostafa Higazy and Mahmoud El-Kateb

1. INTRODUCTION

In recent decades, due to aging of most structures and for sustainability of satisfactoryperformance of the existing structural system, repair and strengthening of bridges, buildings, andother civil engineering structures have become necessary. Generally, most structures require repair

and strengthening for several reasons such as; environmentally induced degradation, increases inservice loads, changes in use of the structure, design and/or construction errors, changes in designcode regulations, and seismic retrofits. Particularly, some reinforced concrete (RC) elements such asbeams may be deficient in torsional shear capacity and in need of strengthening. These deficienciesmay occur for several reasons, such as insufficient stirrups resulting from construction errors orinadequate design, reduction in the effective steel area due to corrosion, or increased demand due toa change in occupancy.In current practice, torsional strengthening of concrete members is achieved by one of the followingmethods; (1) increasing the member cross-sectional area combined with adding of transversereinforcement, (2) using externally bonded steel plates and pressure grouting the gap between plateand concrete element, and (3) applying an axial load to the member by post-tensioning.

Although these methods will continue to be used in many more instances, fiber reinforced polymer(FRP) composites provide another option for strengthening. FRP composites have shown great

promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement.Studies of flexural and shear strengthening of RC beams using FRP have been formed since the earlynineties of the last century

1. However, little attention has been paid to its applicability in torsional

strengthening of RC beams in terms of both experimental and numerical research due to thespecialized nature of the problem and the difficulties in conducting realistic tests and representativeanalyses.This paper attempts to fill that gap in the experimental area by investigating the torsional behavior ofRC beams strengthened with externally bonded CFRP sheets (wraps) under the effect of combinedflexure, shear and torsion. The main objective of the current study is to identify the influence of theinvestigated parameters affecting the torsional behavior on the effectiveness and feasibility ofstrengthening.

2. EXPERIMENTAL PROGRAM

2.1 Specimens details

The experimental program recounts an overall investigation of five rectangular reinforced concretebeams of 120x300 mm cross section and 3100 mm long. The dimensions of the test specimens andthe reinforcement details are shown in figure 1.

3100

3#16

1#16

2#10

2#10

5#6/m

5#6/m

5#6/m

3#16

1#16

120

300

Figure 1: Dimensions and reinforcement details of test specimens

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The main parameters investigated are the CFRP composite wrap spacing and the number of plies.One specimen was kept without strengthening as control specimen, while four specimens werestrengthened with 100 mm width CFRP composite all around wraps which were placed at spacing of100 mm and 150 mm. Two beams were wrapped using one layer and the other two beams werewrapped using two layers of the CFRP composites. The compressive strength of concrete (fc

`) was 20

MPa and was kept constant for all specimens. Test specimen matrix is listed in table 1.

Specime

nWrap spacing

No. of

layers

BR1 Control specimen

BR2

3100

100150100150100150100150100 100 400400

1 ply

BR3

3100

100150100150100150100150100 100 400400

2 plies

BR4

100100

3100

300 100 100 100 100 100 100 100 100 100 100 100 100 300

1 ply

BR5100100

3100

300 100 100 100 100 100 100 100 100 100 100 100 100 300

2 plies

Table 1: Test specimen matrix

In this research, Sikawrap Hex 230C CFRP sheets and Sikadur 330 impregnation resin were used. Allmaterials are supplied by Sika. The mechanical properties of the CFRP sheets are given in table 2. Inpreparation of the reinforced concrete beam for fiber wrapping, the edges were rounded so that therewere no sharp corners that would contribute to the reduction of the fiber strength.

Thickness(mm)

Tensile StrengthMPa

Tensile ModulusMPa

Ultimate Strain%

0.12 4,100 231,000 1.70Table 2: Mechanical properties of CFRP sheets

2.2 Test Setup

Specimens were eccentrically loaded with two concentrated loads at both thirds of the beam span.Eccentric loads were transferred to the beam using two steel jackets specially designed toaccommodate and transfer the applied load without being failed or distorted. Beams were fixed fromboth ends to the laboratory steel frame against vertical and transverse displacements but were free tohave other displacements. A load cell is placed in the middle of the two steel jackets to divide theapplied load on both loading points equally using a rigid steel beam. A diagrammatic sketch for thetest setup is shown in figure 2.

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Fixation bolts

Fixation bolts

Specimen

Steel jacketSteel jacket

Lab. steel frame

Load cell Lab. steel frame

Steel jacket

Rigid beam

Rigid beam

Load cell

Sec. (2-2)

2

2

1

1

Sec. (1-1)

Elevation

Figure 2: Diagrammatic sketch for the test setup

2.3 Instrumentation

Displacement transducers were the main devices used for measuring the imposed and resulting

displacements in the test. Linear variable displacement transducers (LVDTs) of maximum gaugelength of 100mm and precision of 1/100 of mm were used to measure the various types ofdisplacement that make up the total displacements of the specimens. The combination of LVDTs wasinstalled at the loading location, and at support to measure the components of displacement in twodirections, in order to evaluate the angle of rotation of the beam.Electrical strain gauges were the main devices used for measuring the strain in the test. Threeelectrical resistance strain gauges with 10mm gauge length were bounded to the longitudinal andtransverse reinforcement bars in appropriate locations. In addition, two strain gauges were bounded toeach CFRP wrap on both its sides. Also, two strain rosettes were set on the specimen side in theloaded part to evaluate the principal strains in concrete. All the LVDTs and strain gauges wereconnected to a channel box. Then, all the data were recorded by a computer controlled dataacquisition system on load intervals of 5.0 kN.

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3. EXPERIMENTAL RESULTS

3.1 Deformational Behavior

The deformational behavior studied in this section emphasis relationship between the torsionalmoment carried by the test specimen versus the maximum twist angle of the specimen cross sectionwhich has found to be at loading location. The ultimate twist angle, the corresponding torsionalmoment and the maximum strain in FRP wraps for all specimens are listed in table 3. Also, thetorsional moment twist angle relationships for all specimens are plotted in figure 3.

SpecimenUltimate twist angle

(Degree)Ultimate torsional moment

(kN.m)

Max. strain in FRP wraps

()

BR1 3.082 10.80 NA

BR2 3.135 11.25 7041

BR3 2.788 11.70 4044

BR4 2.855 11.70 6293

BR5 3.026 12.60 3971

Table 3: Summary of results

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

Twist angle (Deg.)

Torsionalmoment(kN.m

)

BR1

BR2

BR3

BR4

BR5

Figure 3: Torsional moment versus twist angle relationships

The deformational behavior of all specimens followed the same trend and it was noticeable that theFRP wraps had no effect on beam stiffness in early loading stages.Specimen BR1, which is the control specimen in this research, followed almost a linear behavior forthe first four loading steps up to a twist angle of 0.217

o(about 7.0% of the ultimate twist angle) then

followed a parabolic trend up to failure at ultimate twist angle of 3.082o and a corresponding torsionalmoment of 10.8 kN.m.

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Specimen BR2, which had the lowest amount of FRP wraps, followed almost a linear behavior up to atwist angle of 0.217

o(about 6.9% of the ultimate twist angle) then followed a parabolic trend up to

failure at ultimate twist angle of 3.135o

and a corresponding torsional moment of 11.25 kN.m. Thetorsional capacity of BR2 was improved to 104.2% of that of the control specimen while the twist anglewas reduced to 78.5% of the ultimate twist angle in control specimen at same loading level.

Specimens BR3 and BR4 almost showed an identical behavior and deformation values. SpecimenBR3 failed at ultimate twist angle of 2.788

oand a corresponding torsional moment of 11.7 kN.m. The

torsional capacity of BR3 was improved to 108.3% of that of the control specimen while the twist anglewas reduced to 72.5% of the ultimate twist angle in control specimen at same loading level. BR4 failedat ultimate twist angle of 2.855

oand a corresponding torsional moment of 11.7 kN.m. The torsional

capacity of BR4 was improved to 108.3% of that of the control specimen while the twist angle wasreduced to 73.2% of the ultimate twist angle in control specimen at same loading level.Specimen BR5, which had the largest amount of FRP wraps, showed as expected the bestbehavior. It was deformed linearly up to a twist angle of 0.217

o(about 7.2% of the ultimate twist angle)

then followed a parabolic trend up to failure at ultimate twist angle of 3.026o

and a correspondingtorsional moment of 12.6 kN.m. The torsional capacity of BR5 was improved to 116.7% of that of thecontrol specimen while the twist angle was reduced to 66.4% of the ultimate twist angle in controlspecimen at same loading level.

Figure 4: Specimen BR2 prior to failure

3.2 Crack Patterns

The control specimen BR1 showed a wider range of crack propagation and a faster rate of crack

progression than other specimens due to the absence of any FRP wrapping along the beam. All thestrengthened four specimens showed almost the same trend of crack progression, this could beattributed to the relatively similar wrapping scheme and the identical type of reinforcement details.

Although, final stages of loading prior to failure showed slightly different cracking behavior.Diagonal cracks initiated and propagated through the loaded third of the beam from the loading pointtowards the support. At twist of nearly 75% of the ultimate twist, cracks were completely developed. Insubsequent loading steps, there was a gradual built-up of cracks preceded by widening of existingcracks and loss of stiffness. Also, delamination of FRP wraps was noticed at failure.

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Figure 5: Crack patterns and modes of failure

4. ANALYSIS

fib Bulletin 142

states that an externally bonded FRP wraps will provide contribution to thetorsional capacity only if full wrapping around the elements cross section is applied, so that the tensileforces carried by the FRP on each side of the cross section may form a continuous loop. Ultimatetorsional moment calculations are based on the fiber orientation and the mode of failure. In case werethe FRP wraps are in vertical (transverse) direction of the beam, and diagonal cracks are assumed tobe inclined 45

oto the longitudinal beam axis, the contribution of FRP wraps to ultimate torsional

moment is determined by using the effective strain in the fibers and can be predicted by the followingexpression:

Mt frp = 2 frp Afrp Efrp Ac / S (1)

Where frp is the strain in fiber; AFRP is the area of FRP wrap; EFRP is the modulus of elasticity of theFRP material; AC is the gross area of the concrete cross section; S is the center to center spacing ofFRP wraps.

Also, the ultimate torsional moment of the reinforced concrete beam cross section (Mt conc) can bedetermined by the ACI 318-08

3, using equation (11-18) as follows:

)66.0()7.1

()( `22

2

c

w

c

oh

hu

w

u fdb

V

A

pT

db

V++ (2)

Then the total ultimate torsional moment capacity of a strengthened beam cross section can bedetermined using equations (1) and (2) as follows:

Mt calc = Mt frp + Mt conc(3)

Table 4 lists a comparison between the calculated value of the ultimate torsional moment and theexperimental value, all in kN.m. Calculated values showed relatively a good agreement with themeasured values.

Specimen Mt frp Mt conc Mt Calc Mt Experimental Variation

BR2 5.42 4.81 10.23 11.25 -9.1%

BR3 6.18 4.81 10.99 11.70 -6.1%

BR4 6.06 4.81 10.87 11.70 -7.1%

BR5 7.92 4.81 12.73 12.60 1.1%

Table 4: Comparison between calculated and test results

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5. CONCLUSIONS

An experimental program comprised testing five rectangular cross section reinforced concretebeams was conducted to evaluate the efficiency and feasibility of strengthening beams subjected totorsion using CFRP wraps. Based on the obtained results, the following conclusions can be furnished:

Strengthening reinforced concrete rectangular beams subjected to torional loads using CFRPwraps helped in improving the overall performance of strengthened beams.

Strengthening beams with CFRP wraps helped in improving the torsional capacity of beamsup to 116.7% of its non-strengthened value.

Strengthening beams with CFRP wraps helped in increasing the torsional stiffness bydecreasing the twist angle up to 66.4% of its non-strengthened value.

There is no effect of CFRP wraps on beam torsional stiffness in loading stages beforecracking and in early loading stages after cracking.

Even with lower amount of FRP , using single ply of wraps with small wrap spacing helped toachieve the same improvement in performance of that when using two plies of wraps withlarger wrap spacing. Hence, it is more economic and feasible.

REFERENCES

[1] A. Ghobarah, M. N. Ghorbel, and S. E. Chidiac, "Upgrading Torsional Resistance of ReinforcedConcrete Beams Using Fiber-Reinforced Polymer", Journal of Composites for Construction

ASCE, Vol. 6, No. 4, November 1, 2002. pp. 257-263[2] fib Bulletin 14. Externally bonded FRP reinforcement for RC structures, fib - International

Federation for Structural Concrete, Lausanne, Switzerland. 2001[3] American Concrete Institute (ACI) Committee 318. (2008). Building,code requirements for

structural concrete. ACI 318-08, American Concrete Institute, Detroit.[4] Adrian K. Y. Hii and Riadh Al-Mahaidi, "Torsional Capacity of CFRP Strengthened Reinforced

Concrete Beams", Journal of Composites for Construction ASCE, Vol. 11, No. 1, February 1,2007. pp. 71-80

[5] Saravanan Panchacharam and Abdeldjelil Belarbi, "Torsional Behavior of Reinforced Concrete

Beams Strengthened With FRP Composites", First FIB Congress, Osaka, Japan, October 13-19,2002

[6] Mehran Ameli, Hamid R. Ronagh, and Peter F. Dux, "Behavior of FRP Strengthened ReinforcedConcrete Beams Under Torsion", Journal of Composites for Construction ASCE, Vol. 11, No. 2,

April 1, 2007. pp. 192-200

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