STRENGTHENING OF REINFORCED CONCRETE BEAMS UNDER TORSION USING CFRP SHEETS

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

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

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36th Conference on Our World in Concrete & Structures

Singapore, August 14-16, 2011

STRENGTHENING OF REINFORCED CONCRETE

BEAMS UNDER TORSION USING CFRP SHEETS

El Mostafa Higazy* and Mahmoud El-Kateb†

*Department of Structural Engineeering

Ain Shams University Cairo, Egypt

e-mail: melkateb@gmail.com

Keywords: Strengthening, Beams, Torsion, FRP

Abstract. Repair and strengthening of bridges, buildings, and other civil engineering structures have become necessary due to aging, environmentally induced degradation, increases in service loads, changes in use of the structure, design and/or construction errors, changes in design code regulations, and seismic retrofits. Many buildings and bridge elements are subjected to significant torsional moments that affect the design, and may require strengthening. Reinforced concrete (RC) beams may be deficient in torsional shear capacity and 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 increased demand due to a change in occupancy. FRP composites have shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement. However, little attention has been paid to its applicability in torsional strengthening of RC beams in terms of both experimental and numerical research. The main objectives of the current study are to investigate the torsional behavior of RC beams strengthened with externally bonded carbon fiber reinforced polymer (CFRP) sheets, and to identify the influence of the investigated parameters affecting the torsional behavior on the effectiveness and feasibility of strengthening. The experimental program recounts an overall investigation of torsional strengthening of five rectangular reinforced concrete beams with externally bonded carbon fiber-reinforced polymer (CFRP) sheets under the effect of combined flexure, shear and torsion. One specimen is kept without strengthening as control specimen, while four specimens are strengthened with CFRP composite wraps. The main parameters investigated are the CFRP warps spacing and number of plies. The concrete grade and steel reinforcement ratio are kept constant for all specimens. In the experimental findings, the increase in the ultimate torsional moment for different strengthening configurations, performance improvement and crack patterns are presented.

† Ain Shams University

El Mostafa Higazy and Mahmoud El-Kateb

1. INTRODUCTION

In recent decades, due to aging of most structures and for sustainability of satisfactory performance of the existing structural system, repair and strengthening of bridges, buildings, and other civil engineering structures have become necessary. Generally, most structures require repair and strengthening for several reasons such as; environmentally induced degradation, increases in service loads, changes in use of the structure, design and/or construction errors, changes in design code regulations, and seismic retrofits. Particularly, some reinforced concrete (RC) elements such as beams may be deficient in torsional shear capacity and 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 increased demand due to a change in occupancy. In current practice, torsional strengthening of concrete members is achieved by one of the following methods; (1) increasing the member cross-sectional area combined with adding of transverse reinforcement, (2) using externally bonded steel plates and pressure grouting the gap between plate and 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 early nineties 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 the specialized nature of the problem and the difficulties in conducting realistic tests and representative analyses. This paper attempts to fill that gap in the experimental area by investigating the torsional behavior of RC beams strengthened with externally bonded CFRP sheets (wraps) under the effect of combined flexure, shear and torsion. The main objective of the current study is to identify the influence of the investigated parameters affecting the torsional behavior on the effectiveness and feasibility of strengthening.

2. EXPERIMENTAL PROGRAM

2.1 Specimen’s details

The experimental program recounts an overall investigation of five rectangular reinforced concrete beams of 120x300 mm cross section and 3100 mm long. The dimensions of the test specimens and the 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

El Mostafa Higazy and Mahmoud El-Kateb

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 were strengthened with 100 mm width CFRP composite all around wraps which were placed at spacing of 100 mm and 150 mm. Two beams were wrapped using one layer and the other two beams were wrapped 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

n Wrap 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

BR5

100100

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. All materials are supplied by Sika. The mechanical properties of the CFRP sheets are given in table 2. In preparation of the reinforced concrete beam for fiber wrapping, the edges were rounded so that there were no sharp corners that would contribute to the reduction of the fiber strength.

Thickness (mm)

Tensile Strength MPa

Tensile Modulus MPa

Ultimate Strain %

0.12 4,100 231,000 1.70

Table 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 to accommodate and transfer the applied load without being failed or distorted. Beams were fixed from both ends to the laboratory steel frame against vertical and transverse displacements but were free to have other displacements. A load cell is placed in the middle of the two steel jackets to divide the applied load on both loading points equally using a rigid steel beam. A diagrammatic sketch for the test setup is shown in figure 2.

El Mostafa Higazy and Mahmoud El-Kateb

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 gauge length of 100mm and precision of 1/100 of mm were used to measure the various types of displacement that make up the total displacements of the specimens. The combination of LVDTs was installed at the loading location, and at support to measure the components of displacement in two directions, 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. Three electrical resistance strain gauges with 10mm gauge length were bounded to the longitudinal and transverse reinforcement bars in appropriate locations. In addition, two strain gauges were bounded to each CFRP wrap on both its sides. Also, two strain rosettes were set on the specimen side in the loaded part to evaluate the principal strains in concrete. All the LVDTs and strain gauges were connected to a channel box. Then, all the data were recorded by a computer controlled data acquisition system on load intervals of 5.0 kN.

El Mostafa Higazy and Mahmoud El-Kateb

3. EXPERIMENTAL RESULTS

3.1 Deformational Behavior

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

Specimen Ultimate twist angle

(Degree) Ultimate torsional moment

(kN.m)

Max. strain in FRP wraps

(µε)

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.)

Torsional moment (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 the FRP 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 for the 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 torsional

moment of 10.8 kN.m.

El Mostafa Higazy and Mahmoud El-Kateb

Specimen BR2, which had the lowest amount of FRP wraps, followed almost a linear behavior up to a twist 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. The

torsional capacity of BR2 was improved to 104.2% of that of the control specimen while the twist angle was 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. Specimen BR3 failed at ultimate twist angle of 2.788

o and 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 angle was reduced to 72.5% of the ultimate twist angle in control specimen at same loading level. BR4 failed at ultimate twist angle of 2.855

o and 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 was reduced 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 best behavior. 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 corresponding

torsional moment of 12.6 kN.m. The torsional capacity of BR5 was improved to 116.7% of that of the control specimen while the twist angle was reduced to 66.4% of the ultimate twist angle in control specimen 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 the strengthened four specimens showed almost the same trend of crack progression, this could be attributed 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 point towards the support. At twist of nearly 75% of the ultimate twist, cracks were completely developed. In subsequent loading steps, there was a gradual built-up of cracks preceded by widening of existing cracks and loss of stiffness. Also, delamination of FRP wraps was noticed at failure.

El Mostafa Higazy and Mahmoud El-Kateb

Figure 5: Crack patterns and modes of failure

4. ANALYSIS

fib Bulletin 142 states that an externally bonded FRP wraps will provide contribution to the

torsional capacity only if full wrapping around the element’s cross section is applied, so that the tensile forces carried by the FRP on each side of the cross section may form a continuous loop. Ultimate torsional moment calculations are based on the fiber orientation and the mode of failure. In case were the FRP wraps are in vertical (transverse) direction of the beam, and diagonal cracks are assumed to be inclined 45

o to 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 following expression:

Mt frp = 2 εfrp Afrp Efrp Ac / S (1)

Where εfrp is the strain in fiber; AFRP is the area of FRP wrap; EFRP is the modulus of elasticity of the

FRP material; AC is the gross area of the concrete cross section; S is the center to center spacing of FRP wraps. Also, the ultimate torsional moment of the reinforced concrete beam cross section (Mt conc) can be determined by the ACI 318-08

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

)66.0()7.1

()( `2

2

2

c

w

c

oh

hu

w

u fdb

V

A

pT

db

V+≤+ φ (2)

Then the total ultimate torsional moment capacity of a strengthened beam cross section can be determined 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 the experimental value, all in kN.m. Calculated values showed relatively a good agreement with the measured 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

El Mostafa Higazy and Mahmoud El-Kateb

5. CONCLUSIONS

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

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

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

• Strengthening beams with CFRP wraps helped in increasing the torsional stiffness by decreasing 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 before cracking and in early loading stages after cracking.

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

REFERENCES

[1] A. Ghobarah, M. N. Ghorbel, and S. E. Chidiac, "Upgrading Torsional Resistance of Reinforced Concrete 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 Reinforced Concrete Beams Under Torsion", Journal of Composites for Construction ASCE, Vol. 11, No. 2, April 1, 2007. pp. 192-200