STRENGTHENING OF REINFORCED CONCRETE COLUMNS USING

FRP FABRIC

SHUJAATULLAH SHEIKH

A project report submitted in partial fulfillment of the

requirements for the award of the degree of Master of Engineering

(Civil – Structure)

Faculty of Civil Engineering

Universiti Teknologi Malaysia

JANUARY 2012

iii

To my beloved family…

iv

ACKNOWLEDGEMENTS

First and foremost the author wishes to express his profound gratitude to his

research supervisor Associate Professor Dr. Abdul Rahman Mohd. Sam for his

guidance and valuable advice throughout the research period. His insight into the

behaviour of CFRP-concrete substrate was of immense help in the preparation of this

thesis.

The author would also like to thank his brother Dr. Usman Ullah Sheikh in

proofreading the final draft and for his many technical comments.

Last but not least, the author remembers his family, especially his mother, for

her countless blessings which have always been a source of inspiration in achieving

success up to this level.

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ABSTRACT

In a number of cases, the compressive strength of reinforced concrete

members have been found to be less than the design strength. Therefore, forcing the

structural engineer to strengthen these members in order to cater for the dead, super-

dead and superimposed loads to which these structural members are subjected to

throughout their service life. In the case of under-strength columns, an efficient

method of increasing the strength and ductility of these columns is by wrapping them

with fibre-reinforced polymer (FRP) fabric. However, most previous studies on

concrete short columns confined with FRP sheets were based on small-scale testing

which did not take into consideration the size effect of these columns. In this study,

40 MPa concrete cylinders having height-to-width ratios (λ) of 4 and 8 are wrapped

with CFRP sheets impregnated with epoxy and tested under uniaxial compression.

Their ultimate failure loads are recorded and the performance of the wrapped

cylinders is compared to the unwrapped control specimens in terms of the strength

gain produced by the confining effects of the wraps.

vi

ABSTRAK

Dalam beberapa kes, kekuatan mampatan struktur konkrit bertetulang

didapati tidak mencapai kekuatan rekabentuk. Justeru itu, jurutera struktur terpaksa

mencadangkan agar langkah-langkah penambahbaikan dilakukan terhadap struktur-

struktur yang lemah ini untuk membolehkan struktur-struktur berkenaan terus dapat

menanggung beban hidup dan beban mati yang dikenakan dengan selamat. Salah

satu kaedah yang efisien untuk menguatkan struktur tiang konkrit yang lemah adalah

dengan membalut tiang berkenaan dengan jaket FRP (serat polimer yang diperkuat).

Dalam kajian ini, silinder konkrit gred 40 MPa dengan nisbah kelangsingan (λ)

bersamaan 4 dan 8 dibalut dengan CFRP (serat karbon polimer yang diperkuat) dan

diuji di bawah aplikasi daya mampatan. Beban puncak direkodkan serta kelakunan

sampel silinder konkrit yang berbalut dibandingkan dengan sampel kawalan yang

tidak dibalut daripada segi kesan penambahbaikan yang mungkin dihasilkan oleh

balutan CFRP berkenaan.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES ix

LIST OF FIGURES x

LIST OF SYMBOLS xii

LIST OF ABBREVIATIONS xiv

LIST OF APPENDICES xv

CHAPTER 1 INTRODUCTION 1

1.0 Introduction 1

1.1 Objectives of the study 7

1.2 Scope of the study 7

1.3 Case study 8

CHAPTER 2 LITERATURE REVIEW 9

2.1 Introduction 9

CHAPTER 3 RESEARCH METHODOLOGY 15

3.1 Introduction 15

3.2 Concrete 18

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3.3 Reinforcement 19

3.4 Compression Testing of Samples 23

CHAPTER 4 RESULTS AND DISCUSSION 24

4.1 Introduction 24

4.2 Concrete cube results 24

4.3 Ultimate loads at failure 25

4.4 Stiffness (k) 29

4.5 Modes of failure 30

4.6 Discussion 33

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 41

5.1 Conclusions 41

5.2 Recommendations for future research 43

REFERENCES 44

APPENDIX A 46

ix

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Various expressions for kl [7]. 12

3.1 40 MPa concrete design mix. 19

3.2 Cylindrical concrete sample properties. 20

4.1 Test results on 40 MPa concrete. 25

4.2 Table depicts peak loads (kN) and axial compressions

(mm) at failure and failure modes.

33

4.3 Recommended values for parameters kl, kg and ks [7]. 35

4.4 The actual and calculated load capacities of wrapped

samples compared.

36

x

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Schmidt Rebound Hammer Testing being carried out

on the 1000 mm diameter bored pile.

2

1.2 Concrete cores taken from the 1000 mm diameter

“under-strength” bored pile being tested.

2

1.3 Strengthening of an RC column by enlarging its cross

sectional area.

3

1.4 Two new columns are introduced in order to reduce

the stresses experienced by the under-strength column

C/2.

4

1.5 Retrofitting of under-strength bridge column with a

steel jacket.

5

1.6 Under-strength concrete column in a building wrapped

with carbon fibre reinforced polymer (CFRP) sheets.

6

1.7 Retrofitting of a concrete bridge pier with glass fibre

reinforced polymer (GFRP) sheets.

6

1.8 Axial load enhancement for a rectangular RC column

achieved by CFRP wrapping.

8

2.1 Unconfined circular column experiences lateral stress

fl when vertical load is applied.

9

2.2 FRP confined circular column. 10

2.3 Free-body diagram of circular column confined by an

FRP jacket.

10

3.1 Dimensions of sample type A and sample type B. 15

3.2 The direction of fabric wrapping on the cylindrical

sample in order to enable confinement of concrete.

16

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3.3 The CFRP sheet is lapped at a length of 100 mm in

order to prevent debonding.

16

3.4 Sample preparation and wrapping with CFRP sheet. 17

3.5 Flow chart for the experimental procedure. 18

3.6 Compressive testing of 150 mm concrete cubes. 19

3.7 Cross-sectional dimensions and reinforcement details

of circular concrete samples.

21

3.8 Reinforcement cage for the concrete samples used in

this study.

21

3.9 Plan view of the reinforcement cage used in this study. 22

3.10 SikaWrap-200C woven carbon fibre fabric (width 600

mm) for structural strengthening.

22

3.11 A wrapped sample being tested under the application

of uniaxial compressive loading.

23

4.1 Load-compression curve for samples A1 and A2. 26

4.2 Load-compression curve for samples B1 and B2. 27

4.3 Load increase for all samples tested. 28

4.4 Load-compression curve for samples A2 and B2. 28

4.5 Load-deflection behaviour of wrapped samples A2 and

B2.

29

4.6 Load-deflection behaviour of unwrapped samples A1

and B1.

30

4.7 Compressive failure of the unwrapped concrete

sample.

31

4.8 Mode of failure of (a) wrapped and (b) unwrapped

samples.

32

4.9 Peak failure loads for wrapped samples A2 and B2. 37

4.10 Bits of concrete stuck to the CFRP fabric. 38

4.11 Typical stress-strain behaviour of steel and FRP

materials [12].

40

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LIST OF SYMBOLS

Ac - cross sectional area of concrete

Ag - gross cross-sectional area of concrete

Asc - cross-sectional area of longitudinal steel

Ast - cross-sectional area of longitudinal steel

D or d - diameter of circular concrete column

Ec - modulus of elasticity of concrete

EFRP - modulus of elasticity of FRP

f’c - compressive strength of the unconfined concrete, measured by

testing 150 mm x 300 mm cylinders according to ASTM C39

f’cc - strength of concrete due to the confinement effect of the FRP

f’cc2:1 - confined concrete strength of a concrete specimen with L/D = 2

f’co - unconfined strength of concrete

fcu - characteristic strength of concrete

fFRP - ultimate tensile strength of the FRP

fl - confinement pressure exerted by FRP

fy - characteristic strength of reinforcement

k - Stiffness = P/Δ

kg - gap factor

k1 - confinement effectiveness coefficient

ks - shape factor

L - length of column

le - effective length of column

M - bending moment

Nu - ultimate axial load capacity of column

P - axial compressive load

Pn - axial load capacity of column

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t or tFRP - thickness of FRP or total thickness for multiple plies

γc - partial safety factor for concrete

γfrp - partial factor for FRP tensile strength

γs - partial safety factor for steel reinforcement

Δ - axial deformation (shortening)

λ - column slenderness ratio (L/D)

vc - Poisson ratio of concrete

ψf - reduction factor for concrete

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LIST OF ABBREVIATIONS

AFRP Aramid Fibre Reinforced Polymer.

CFRP Carbon Fibre Reinforced Polymer.

EBR Externally Bonded Reinforcement.

FRP Fibre Reinforced Polymer.

GFRP Glass Fibre Reinforced Polymer.

HPC High Performance Concrete

HSC High Strength Concrete

RC Reinforced Concrete.

xv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Axial Load Estimation for CFRP-wrapped Samples 46

CHAPTER 1

INTRODUCTION

1.0 Introduction

In a number of cases, the structural strength of reinforced concrete members

have been found to be less than the design strength, therefore, forcing the structural

engineer to strengthen these members in order to cater the dead, super-dead and

superimposed loads to which these structural members are subjected throughout their

service life.

In the case of under-strength reinforced concrete columns, a few options are

available to the structural engineer, among them:

(a) complete demolition and re-construction – this is the most radical

alternative whereby strengthening of the member in question is either not

feasible structurally or economically, or the structural engineer is unwilling to

take any risks pertaining to the dubious strength of the said member.

For example, a 1000 mm diameter bored pile for a bridge foundation as

shown in Figure 1.1 was found to be of under-strength concrete. The

consulting engineer stipulated that the actual strength of the concrete has to

2

be established by Schmidt Rebound Hammer Testing as shown in Figure 1.1.

Core samples were also taken and tested as shown in Figure 1.2.

Figure 1.1: Schimdt Rebound Hammer Testing being carried out on the

1000 mm diameter bored pile.

Figure 1.2: Concrete cores taken from the 1000 mm diameter “under-strength” bored pile being tested.

3

The consulting engineer stated that if the in-situ concrete strength determined

from rebound hammer testing and core tests was satisfactory, then the bored pile

in question can be accepted as being of sound quality. On the contrary, if the

concrete strength is determined to be lower than the design strength, then the

bored pile should be abandoned and a replacement pile had to be constructed.

(b) enlargement – structurally this is the simplest and most obvious solution

to strengthen under-strength columns as shown in Figure 1.3.

Figure 1.3: Strengthening of an RC column by enlarging its cross

sectional area.

However, this method has a few drawbacks. First of all, expensive rentable

floor area is lost when the dimensions of columns are enlarged. For example,

when a 450 mm-square column is enlarged to 600 mm x 600 mm,

approximately 2 square feet of floor space is lost. This leads to a loss in

revenue from floor rental.

From project budgeting point of view, enlargement will incur additional

material and labour costs.

New concrete cast against

under-strength column

4

(c) stress reduction – this can be achieved by the addition of extra columns,

resulting in load distribution away from the under-strength column to the

newly added columns as shown in Figure 1.4. Once again, this solution is

both expensive and ruins the architectural functionality of the floor space.

Figure 1.4: Two new columns are introduced in order to reduce the

stresses experienced by the under-strength column C/2.

(d) installation of steel plates – this traditional method of strengthening

reinforced concrete structural elements as shown in Figure 1.5 is now deemed

both expensive and inefficient as the strength enhancement achieved per

kilogramme of steel used is rather low.

5

Figure 1.5: Retrofitting of under-strength bridge column with a steel

jacket.

This method also involves the lugging of thick steel plates to the construction

site. Installation is difficult due to the weight of these plates which would

have to be mechanically attached to the under-strength columns using bolts

and nuts. Surface treatment of these plates is absolutely necessary for

corrosion and fire protection, thus incurring further costs to the contractor.

(e) wrapping with FRP sheets – currently this method as shown in Figures 1.6

and 1.7 is gaining popularity as it is a clean and hassle-free method of

strengthening structural concrete members. The confining effect of the FRP

sheets enhances the compressive strength of the concrete.

6

Figure 1.6: Under-strength concrete column in a building wrapped with

carbon fibre reinforced polymer (CFRP) sheets.

Figure 1.7: Retrofitting of a concrete bridge pier with glass fibre

reinforced polymer (GFRP) sheets.

7

This study seeks to investigate the behaviour of circular concrete columns

wrapped with CFRP fabric under the application of monotonic axial loading.

1.1 Objectives of the study

The objectives of the study are as follows:

i. To study the mode of failure of circular reinforced concrete columns

wrapped with carbon fibre reinforced polymer (CFRP) sheet.

ii. To study the increase in axial load capacity of CFRP-wrapped

columns as compared to unwrapped columns.

iii. To study the effects of column slenderness ratio, λ, on the axial

strength gain of CFRP-wrapped columns.

1.2 Scope of the study

This study is restricted to circular column specimens wrapped with a single

ply of CFRP sheet consisting of mid-strength carbon fibres under the application of

purely compressive loading. As no eccentricity is considered, no bending moment is

applied to the column specimens tested in the study. Normal concrete having a

characteristic 28-day cube strength of 40 MPa is used. Neither high-strength (HSC)

nor high-performance concrete (HPC) is considered. The slenderness ratios of the

concrete specimens are restricted to values of λ = 4 and λ = 8.

8

1.3 Case study

It was required to increase the load capacity of a 300 mm by 450 mm

rectangular RC column as shown in Figure 1.8 reinforced with 6 numbers of 20 mm

diameter steel bars with the characteristic yield strength of steel is 400 N/mm2. The

compressive 28-day cube strength of the concrete [1] was 22.4 MPa giving an

ultimate axial load carrying capacity of 1983 kN. By wrapping the column with four

plies of BBR carbon fibre sheet CFS 240 – 300 gm/m2, the ultimate load bearing

capacity of the column was increased by 21.5% to 2409 kN.

Figure 1.8: Axial load enhancement for a rectangular RC column achieved by CFRP wrapping.

REFERENCES

[1] “A Guide to the Design and Application of BBR FRP Strengthening Systems (2002)”, BBR Systems Ltd. Schwerzenbach, Switzerland.

[2] Karbhari, V.M. and Gao, Y. (1997) “Composite jacketed concrete under uniaxial compression – verification of simple design equations.” ASCE Journal of Materials in Civil Engineering, Vol. 9, No. 4.

[3] Toutanji, H.A. (1999) “Stress – strain characteristics of concrete columns externally confined with advanced fiber composite sheets.” ACI Materials Journal, Vol. 96, No. 3.

[4] Saafi, M., Toutanji, H.A. and Li, Z. (1999) “Behavior of concrete columns confined with fiber reinforced polymer tubes.” ACI Materials Journal, Vol. 96, No. 4.

[5] Miyuachi, K., Inoue, S., Kuroda, T. and Kobayashi, A. (1999) “Strengthening effects of concrete columns with carbon fiber sheet.” Transactions of The Japan Concrete Institute, Vol. 21.

[6] Samaan, M., Mirmiran, A. and Shahawy, M. (1998) “Model of concrete confined by fiber composite.” ASCE Journal of Structural Engineering, Vol. 124, No. 9.

[7] Teng, J.G., Chen, J.F., and Lam, L. (2002) “FRP-strengthened RC structures.” John Wiley & Sons, Ltd., West Essex, England.

[8] Mirmiran, A., Shahawy, M., Samaan, M. and El Echary, H. (1998) “Effect of column parameters on FRP-confined concrete.” ASCE Journal of Composites for Construction, Vol. 2, No. 4.

[9] Mander, J.B., Priestley, M.J.N. and Park, R. (1988) “Theoretical stress – strain model for confined concrete.” ASCE Journal of Structural Engineering, Vol. 114, No. 8.

45

[10] Saadatmanesh, H., Ehsani, M. R. and Li, M. W. (1994) “Strength and ductility of concrete columns externally reinforced with fiber composite straps.” ACI Structural Journal, V. 91, No. 4 July-August 1994.

[11] American Concrete Institute (ACI) (2002) “Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures.” ACI-440, ACI-440 Committee, Detroit.

[12] FIB Bulletin 14 Technical Report (2001) “Externally Bonded FRP Reinforcement for RC Structures.” International Federation of Structural Concrete, Lausanne.

[13] Spoelstra, M.R. and Monti, G. (1999) “FRP-confined concrete

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practice for design and construction (Incorporating Amendments Nos. 1, 2, 3 and 4).” British Standards Institution, London.

[15] “Product Information Edition 02/2008.” Sika Kimia Sdn. Bhd., Lot 689, Nilai Industrial Estate, 71800 Nilai, Negeri Sembilan, Malaysia.

[16] Razavi, M. (2009) “Strengthening of under-strength concrete using FRP fabric.” Master’s Thesis, Universiti Teknologi Malaysia, Johor Bahru, Malaysia.

[17] Aire, C., Gettu, R. and Casas, J.R. (2001) “Study of the compressive behavior of concrete confined by fiber reinforced composites - Composites in Construction (ISBN 90 2561 858 7).” Swets & Zeitlinger, Lisse, The Netherlands.

[18] Balaguru, P., Nanni, A. and Giancaspro, J. (2009) “FRP Composites for Reinforced and Prestressed Concrete Structures – A guide to fundamentals and design for repair and retrofit.” Taylor & Francis, New York.

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