BORANG PENGESAHAN STATUS TESIS€¦ · 2.5.4 Joint Geometry Effect on Joint Strength 45 2.5.5 Elastic Properties and Deformation 47 2.6 Double Lap Joint 48 2.7 Surface Treatments

PSZ 19 : 16 (Pind 1/97)

UNIVERSITI TEKNOLOGI MALAYSIA

BORANG PENGESAHAN STATUS TESIS

JUDUL: THE DEVELOPMENT OF MATHEMATICAL MODEL FOR

PREDICTING BONDING BEHAVIOUR OF CFRP PLATE-EPOXY-CONCRETE BONDED SYSTEM

SESI PENGAJIAN : 2006 / 2007 Saya, KHUSYAIRI BIN MOHAMED

( HURUF BESAR )

mengaku membenarkan tesis ( PSM / Sarjana / Doktor Falsafah )* ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan syarat-syarat kegunaan seperti berikut : 1. Tesis adalah hakmilik Universiti Teknologi Malaysia. 2. Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian

sahaja. 3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi

pengajian tinggi. 4. ** Sila tandakan ( √ ). SULIT (Mengandungi maklumat yang berdarjah keselamatan atau

kepentingan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972)

TERHAD (Mengandungi maklumat TERHAD yang telah ditentukan oleh

organisasi/badan di mana penyelidikan dijalankan)

TIDAK TERHAD Disahkan oleh ( TANDATANGAN PENULIS ) ( TANDATANGAN PENYELIA )

Tarikh : 6 MAY 2007 Tarikh : 6 MAY 2007

CATATAN * Potong yang tidak berkenaan.

** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai

SULIT atau TERHAD. *** Tesis dimaksudkan sebagai tesis bagi Ijazah Doktor Falsafah dan Sarjana secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana Muda (PSM).

Alamat Tetap : KG. BUKIT TANAH, 16810 SELISING, PASIR PUTEH, KELANTAN

MR SHUKUR HAJI ABU HASSAN Nama Penyelia

id84526781 pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com

UTM(FKM)-1/02

Fakulti Kejuruteraan Mekanikal

Universiti Teknologi Malaysia

PENGESAHAN PENYEDIAAN SALINAN E-THESIS

Judul Tesis : THE DEVELOPMENT OF MATHEMATICAL MODEL FOR PREDICTING BONDING BEHAVIOUR OF CFRP PLATE-EPOXY-CONCRETE BONDED SYSTEMPENYERAP KEBISINGAN UNTUK SISTEM EKZOS

Ijazah : MECHANICAL ENGINEERING (PURE)

Fakulti : FACULTY OF MECHANICAL ENGINEERING

Sesi Pengajian : 2006 / 2007

Saya, KHUSYAIRI BIN MOHAMED

(HURUF BESAR)

No. Kad Pengenalan 830602-13-5547 mengaku telah menyediakan salinan e-thesis sama

seperti tesis asal yang telah diluluskan oleh panel pemeriksa dan mengikut panduan

Penyedian Tesis dan Disertasi Elektronik (TDE), Sekolah Pengajian Siswazah, Universiti

Teknologi Malaysia, Disember 2006.

(Tandatangan pelajar) (Tandatangan penyelia sebagai saksi)

Nota: Borang ini yang telah dilengkapi hendaklah dikemukakan kepada FKM bersama penyerahan CD.

Alamat Tetap :

KG. BUKIT TANAH, 16810 SELISING, PASIR PUTEH, KELANTAN. Tarikh: 6 MAY 2007

Nama : MR. SHUKUR HAJI ABU HASSAN Penyelia Fakulti : FACULTY OF MECHANICAL ENGINEERING Tarikh : 6 MAY 2007


�I hereby declare that I have read the content of the thesis and in my opinion, the

content of the thesis have fulfilled the scope and quality for the purpose of achieving

the Degree of Bachelor of Engineering (Mechanical-Pure).�

Signature : ��.

Supervisor : MR. SHUKUR HAJI ABU HASSAN

Date : 6 MAY 2007


THE DEVELOPMENT OF MATHEMATICAL MODEL FOR PREDICTING

BONDING BEHAVIOUR OF CFRP PLATE-EPOXY-CONCRETE BONDED

SYSTEM

KHUSYAIRI BIN MOHAMED

A Dissertation Submitted to the Faculty of Mechanical Engineering in Partial

Fulfillment of the Requirement for The Award of the Degree of Bachelor of

Mechanical Engineering (Pure)

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

MAY 2007


ii

�I hereby declare that this writing is my original work except for quotations and summaries,

each one of which I have clearly stated its source.�

Signature : ..................................................

Author�s Name : KHUSYAIRI BIN MOHAMED

Date : 6 MAY 2007


iii

First of all, all the praises and thanks be to Allah S.W.T for His Love,

This thesis is dedicated to my family,

To my beloved parent, Mohamed b. Jusoh & Che Nab Bt. Awang,

Maslina Bt. Abu Bakar

And all my friends,

Thank you very much for your unstinting help and encouragement.

iv

ACKNOWLEDGEMENTS

In the name of Allah, the most Gracious and most Compassionate

Firstly, i want to to thank Allah Almighty for blessing me and giving me

strength to accomplish this thesis. I would like also to take this opportunity to gather

all my most sincere gratitude towards my gracious and benevolent supervisor, Mr.

Shukur Haji Abu Hassan for his proper guidance and generous assistance throughout

the accomplishment of the work contain in the thesis. I have benefitted through his

kind help, valueable suggestion and encouragement in completing my work and

compiling this thesis.

I would like to express my earnest appreciation and thanks to Mr. Ismail

Kamis for providing helpful and significant information when i needed him most.

Many thank to all of the technicians especially Mr. Rizal and Mr. Fadli for their

cooperation and assisting me in the various laboratory tasks.

Last but not least, to those who contribute towards the accomplishment of this

thesis especially Maslina, Bob, Matzul, Pokceq, Chuban, Shah, Adib and all my

family members, i offer my deepest and genuine gratefulness for their support,

opinion and suggestion.


v

ABSTRACT

The double-lap joint CFRP plate-epoxy-concrete bonded system is

considered in this investigation. A simplified one dimensional model based on the

classical elasticity theory by Volkersen/ de Bruyne is presented. The shear

deformation in the adhesive, thickness of adherend and adhesive are assumed

constant along bond length. Adherends shear deformation and peeling effect are

neglected. The analytical solutions of shear stress in the adhesive is obtained and

almost agreed with experimental results studied by Shukur A.H. The influential

parameters on bond stress distribution are determined by the equation


vi

ABSTRAK

Double-lap joint bagi CFRP-epoksi-konkrit dipertimbangkan di dalam kajian

ini. Model satu dimensi yang dipermudahkan berdasarkan teori elastik klasik oleh

Volkersen/ de Bruyne dipersembahkan. Perubahan bentuk daya ricih pada adhesive,

ketebalan adhesive dan juga adherend dianggap malar sepanjang bond length. Selain

itu, ubah bentuk ricih pada adherend dan juga efek kupasan adalah diabaikan.

Penyelesaian tegasan ricih secara analitikal yang diperolehi sangat hampir dengan

keputusan dari eksperimen yang dijalankan oleh Shukur A.H. Seterusnya, pengaruh

parameter-parameter terhadap taburan tegasan ricih ditentukan berdasarkan

penyelesaian analitikal tersebut.


vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE PAGE i

DECLARATION OF ORIGINALITY ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xvi

LIST OF APPENDICES xvii

1 INTRODUCTION

1.1 Project background 1

1.2 Objective 2

1.3 Scope 2

2 LITERATURE REVIEW

2.1 The Technology and Application of

FRP or Steel Plate Bonded System in

Construction Industry 4

2.1.1 Definition of Durability 9


viii

2.1.2 The Bond Durability of Steel Plate as

Externally Bonded System 9

2.1.3 The Bond Durability of FRP Plate as

Externally Bonded System 10

2.1.4 Factors Affecting Bond Strength 15

2.1.5 Factors Affecting Bond Durability 19

2.1.6 Failure Modes of FRP-Concrete Bonded

System 21

2.1.6.1 Failure at Interface 22

2.1.6.2 Adhesive Failure 23

2.1.6.3 Adherend Failure 23

2.2 Fibre Reinforced Polymer (FRP) Composites 24

2.3 Advanced FRP Composites Applied for Load

Bearing Structures 24

2.3.1 Carbon Fibre 26

2.3.2 The Pultrusion Process 27

2.3.3 FRP Pultruded Composites Plates 28

2.3.4 Durability of FRP Composites 29

2.4 Adhesive Bonding Technology 37

2.4.1 Adhesive Selection 38

2.4.2 Adhesive Mechanical Properties 39

2.4.3 Effects of Loading Configuration on

Adhesive Joint 40

2.4.4 Advantages and Limitations of

Adhesive Bonding 41

2.5 The Principles of Adhesive Bonding Technology

for Structural Applications 42

2.5.1 Factors Considered in Adhesive

Joint Design 43

2.5.2 Bond Mechanism 43

2.5.3 Joining Technique 44

ix

2.5.4 Joint Geometry Effect on Joint Strength 45

2.5.5 Elastic Properties and Deformation 47

2.6 Double Lap Joint 48

2.7 Surface Treatments 50

2.8 Adhesive Joint Design Principles 50

2.9 Failure Modes 52

2.10 Mathematical Model for Predicting Bond Stress

Behaviour 54

3 METHODOLOGY

3.1 The Parameters Effect 67

3.2 Bond Test for CFRP Plate-Epoxy-Concrete

Specimen 68

3.2.1 Experimental Details 68

3.2.2 Details of Test Materials 70

3.2.3 Determination of Bond Stress

Characteristics 71

3.3 Conclusion of Research Methodolgy 74

4 DEVELOPMENT OF MATHEMATICAL MODEL

4.1 Theoretical Analysis on Tension-Compression

CFRP Plate-Concrete Prism Bonded System 76

5 ANALYSIS AND DISCUSSION

5.1 Equation validation 85

5.2 Discussion of Equation Validation 89

5.3 Parametric study 90

5.3.1 The Effect of CFRP Plate Young�s

x

Modulus 92

5.3.2 The Effect of CFRP Plate Thickness 93

5.3.3 The Effect of Concrete Young�s Modulus 94

5.3.4 The Effect of Adhesive Shear Modulus 96

5.3.5 The Effect of Adhesive Thickness 97

5.3.6 The Effect of Bond Length 98

5.4 Conclusion of Parametric Study 99

6 CONCLUSION AND RECOMMENDATION

6.1 Conclusion 101

6.2 Suggestion for Future Study 103

REFERENCES 104

APPENDICES 110

xi

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 List of projects using CFRP for structures

rehabilitation in Malaysia since 2001 5

2.2 Experimental results of control and exposed

Beams 12

2.3 Properties of Selfix Carbofibe Pultruded CFRP Plates

System 25

2.4 Typical reinforcing unidirectional fibre properties 26

5.1 Material and physical properties of testing specimens 86

5.2 Three different parameter�s value 91

5.3 Influential parameters matrix 91


xii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Steel plate member used for strengthening RC

Beam 7

2.2 FRP laminate system used for strengthening RC

Beam 8

2.3 Muar Bridge Beams strengthened with CFRP Sheet 9

2.4 Typical behaviour of load vs deflection for control

Beams 12

2.5 Typical behaviour of load vs defelection for exposure

beams under wet/dry cycles 13

2.6 Typical Load-Strain Bi-linear Curve for FRP

Sheet-Concrete Prism Bonded Specimen 17

2.7 Comparison of bond strength due to different

concrete surface preparation methods 18

2.8 Typical force transfer distributions for concrete mix

A: (a) control; (b) wet�dry; (c) freeze�thaw and

(d) dual. 20

2.9 Typical force transfer distributions for concrete

mix B: (a) control; (b) wet�dry; (c) freeze�thaw;and

(d) dual 20


xiii

2.10 Progressive failure of CFRP plate externally bonded

to concrete due to vertical and horizontal concrete

crack openings near to loading point 21

2.11 (a)Cohesive failure 22

(b)Adhesive failure 23

(c)Adherend failure i.e. concrete shearing 24

2.12 The Pultrusion process in manufacturing FRP plate 27

2.13 Products produced by Pultrusion Process 28

2.14 CFRP Plate (black strip) externally bonded to tension

face of reinforced concrete beam 29

2.15 (a)The changes of width and thickness respectively of

T300/934 graphite/epoxy immersed in distilled water

at different temperatures 31

(b)The weight change of T300/934 graphite/epoxy

immersed in distilled water at different temperatures 31

2.15 Stress-life (S-N) data for pultruded composite coupons

tested in fatigue at room temperature using

environmental conditions A through to F 33

2.17 Flexural strength and modulus for 0 specimens of

pultruded composite coupons before and after

environmental aging 34

2.18 Flexural modulus for 90° specimens of pultruded

composite coupons before and after environmental

aging 34

2.19 Sorption behaviour of pultruded composite

coupons under various aging conditions. 35

2.20 Properties of as-delivered (control), fresh-water-aged

and salt-water-aged materials 36

2.21 S�N curves of the as-delivered (control), water-aged

and 3.5% salt-solution-aged materials 36

2.22 Typical brittle and ductile adhesive behaviour 39

xiv

2.23 Loading modes or type of stresses 40

2.24 Good wetting (A) Poor wetting (B) 44

2.25 Type of adhesive joints techniques for flat

Adherends 45

2.26 Areas of failure initiation and critical strength 46

2.27 A typical adhesive shear stress distribution in a lap

joint according to elastic-plastic model 46

2.28 (a) Deformation of rigid members and

(b) Deformation of elastic members 47

2.29 Double lap joint configuration specimen under

pull-push loads 48

2.30 Relative joint strength of various joint

configurations 51

2.31 The development of the damage process for out of

plane and cohesive failure modes 54

2.32 a): Specimen geometry and material parameters of

double-lap joint under pull-push loads 55

(b)Force analysis on elementary bar model 55

2.33 Two models of stress relationship 57

2.34 Bond slip model for pull � push joint 60

2.35 Equilibrium of the CFRP 61

2.36 Strain of the intervening materials of the bond

region 61

2.37 Loaded end slip vs. pullout force relationship 63

2.38 The free-body diagram of a single-lap joint. 64

2.39 The free-body diagram of a double-lap joint. 64

2.40 The shear stress obtained by Eqs. (11) and FEM

along the adhesive region for a single-lap joint 65

3.1 Instrumentation set-up onto CFRP Plate-epoxy-

concrete prism specimen for this study 69

3.2 Standard test rig used by Swamy et. al.

xv

for GFRP Plate-epoxy-concrete pull-out test 69

3.3 Arrangement of pull-out test rig onto CFRP Plate-

epoxy-concrete prism specimen 71

3.4 Elementary force analysis 72

4.1 Geometry and material parameters of the CFRP

plate-epoxy-concrete prism 76

4.2 Force analysis on elementary bar model 77

4. 3 Linear shear stress and strain distribution through the

thickness of adherends 78

5.1 Theoretical and experimental local bond stress for

BOSTUS specimens at 10 kN 86





5.4 Roughness and irregularities of concrete surface

profile 90

5.5 Local bond stress with different CFRP Young 93

5.6 Local bond stress with different CFRP thickness 94

5.7 Local bond stress with different concrete Young�s

modulus 95

5.8 Local bond stress with different adhesive shear

modulus 97

5.9 Local bond stress with different adhesive thickness 98

5.10 Local bond stress with different bond length 99

6.1 Exponential line of theoretical and experimental local

bond stress 102

xvi

LIST OF SYMBOLS

L - Length of overlap

E - Elastic modulus

T - Thickness

o - Outer adherend

i - Inner adherend

a - Adhesive

G - Shear modulus

T - Applied force per unit width (kN/m)

x - Distance from loaded end point

y - Local coordinates system with the origin at the top surface

- Shear strain

u - Displacement

- Longitudinal normal stress

cf - Concrete compressive strength

F - Force

A - Area


xvii

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Data of theoretical and experimental local bond stress 110

B Data of parametric study 113

C Determine local bond stress from governing equation 116


CHAPTER 1

INTRODUCTION

1.0 Project background

Joint which connects two components together is a common technology for

assembling structures, and is increasingly being used in aerospace and automotive

industries. Statistics shows that approximately 70% of the failure of structures is

initiated from joints [39].

Most of structures are formed by connecting different components through

the joints. In adhesive bonding, the load is transmitted from one adherend to another

adherend smoothly through the adhesive layer in the overlap region, which means the

adhesive serves as a medium for load transmission. In this study, the adherends are

the FRP plate and concrete, while the adhesive is epoxy.

Fiber reinforced polymer (FRP) arises as a strong alternative to replace steel

material. The advantages of this new material over traditional construction materials

are its low weight, high strength and greatly improved resistance against corrosion


2

and durability. FRP is usually used to strengthen deteriorated reinforced concrete

structures.

Thus, to ensure the safety of that joint in structures, it is necessary to analyze

the stress distribution on the joint. The double�lap joint with the characteristics of

simplicity and efficiency has been used widely in many applications and becomes a

standard test specimen for determining the mechanical properties of adhesives beside

the single lap joint.

The major difference between adhesive bonding and mechanical fastener is

the bonded area. The area of adhesive bonding is larger than that of mechanical

fastener. The stress concentration is minimized due to the larger bonded area, and the

stress distributions become more uniformly in the overlap region

1.1 Objective

The objective of this study is to develop and produce a mathematical

governing equation that can be used to predict the local bond stress characteristics for

FRP plate-epoxy-concrete bonded system.

3

1.2 Scope

The scope of this study is to cover the literature review that relates to bonding

technology and existing bonding formulation. Then, a mathematical model of

bonding behaviour for CFRP plate-epoxy-concrete bonded system under pull-push

loading configuration will be developed. The results from the equation will be

analyze and discussed. Conclusion and suggestion for the future study also will be

included in the report writing.

CHAPTER 2

LITERATURE REVIEW

2.0 Introduction

In this chapter, the literature reviews were focused into technical aspects that

started from the structure strengthening applications and durability problems

identification, exposure site weathering characteristics, FRP composites technology

and applications, adhesive bonding technology, application and formulation and

finally focused into the development of test rigs for experimentation purposes that

have been developed and applied by past researchers. The important of focusing into

those aspects are the key elements to answer the study programme objectives.

2.1 The Technology and Application of FRP or Steel Plate Bonded System in

Construction Industry

The technique for strengthening structures such as reinforced concrete beam,

column and slab by method of bonding of FRP Plate or laminating of FRP sheet was


5

slowly accepted by Malaysian authority for the past few years and the momentum

was kept moving positively. Table 2.1 shows list of selected projects that were

undertaken by FOSROC Sdn. Bhd. i.e. one of the leading company that deal with

strengthening works in Malaysia.

The technique was proven to be successful applied in most European

countries and in United States of America and has been referred as a technological

benchmark to be used in Malaysia. The main concept of applying this technique for

damaged or deteriorated structures is to strengthen and stiffen the stress-critical

areas. The advantages of using FRP composites for that kind of application compared

to the existing conventional technique that used of steel bars are FRP far less light

than steel and also extremely resistant to corrosion. Besides that FRP composite also

offers flexibility in site handling and flexibility in applying onto irregular structure

shapes.

Table 2.1: List of projects using CFRP for structures rehabilitation in Malaysia since

2001 (Source: FOSROC Sdn. Bhd.)

NO

PROJECT APPLICATION YEAR

1 Rehabilitation/Strengthening Works to Muar

Bridge At Muar, Johor.

(Fosroc SK-N200)

R.C.Beams 2001

2

Rehabilitation/Strengthening Works to Kuala

Besut Bridge At Terengganu.

(Fosroc SK-N200)

R.C.Beams &

Slabs

2001

3 Rehabilitation/Strengthening Works to Muar

Bridge, Johor. (Fosroc SK-N200)

R.C.Columns 2002

4 Strengthening Works to AIA Building At Jalan

Ampang, Kuala Lumpur.

(Fosroc SK-N300)

R.C.Beams 2002

6

5 Strengthening Works to Beams At Cyberia

Homes, Cyberjaya.

(Fosroc SK-N300)

R.C.Beams 2002

6

Rehabilitation/Strengthening Works to Kuala

Besut Bridge At Terengganu.

(Fosroc SK-N200)

R.C.Beams &

Slabs

2003

7 Rehabilitation/Strengthening Works to Chukai

Bridge, Terengganu.

(Fosroc SK-N200)

R.C.Beams 2003

8 Strengthening Works to R.C.Beams at Cyberjaya

For Kenwin Engineering Sdn Bhd, Cyberjaya.

(Fosroc SK-N300)

R.C.Beams 2003

9 Strengthening Works to Beams at Palace of

Justice, Putrajaya.

(Fosroc SK-N300)

R.C.Beams 2003

10

Rehabilitation/Strengthening Works to Dungun

Bridge At Terengganu.

(Fosroc SK-N200)

R.C.Beams 2003

11 Strengthening Works to Muar Bridge at Muar,

Johor.

(Fosroc SK-N200)

R.C.Columns 2005

12 Strengthening Works to �Projek Perumahan

Rakyat, Lembah Pantai, Kerinchi, Wilayah

Persekutuan.

(Fosroc SK-N300)

R.C. Slab and

Shearwall

2006

13 Strengthening Works to Tan Chong Showroom,

Kuala Lumpur.

(Fosroc SK-N200 & N-200)

R.C. Slab &

Beam

2006

The long-term durability of any construction materials is a key element in

order to ensure that the structure able to maintains its integrity and provides the

service according to its design throughout its service life. The deteriorated concrete

7

structures require repair and maintenance or sometimes need strengthening to extend

their service life. The development of epoxy material as an adhesive system since

1960s has shown a great opportunity for strengthening of existing reinforced

concrete structures by externally bonded steel plate technique. In the area of

strengthening deteriorated reinforced concrete members the steel plate-bonding

system has been widely used and proven to be the most successful externally repair

technique and Fig. 2.1 shows the application of steel plate bonded to the tension face

of reinforced concrete beam at site.

Fig. 2.1: Steel plate member used for strengthening RC beam [5]

From a survey conducted by McKenna and Erki [2], its shows that steel plate-

bonded system has been used since 1964, when malleable steel plate with an

adhesive bonded were applied to load bearing structures of apartment building, in

Durbain, South Africa. The same system was also applied for upgrading several

buildings in Switzerland in early 1970s, Tee-beam bridges in France in 1972 and

1974. The survey also reported that in Japan over 200 highway bridges have been

strengthened with steel plate bonded with epoxy together with anchorage bolted

system. Most of problems of the existing structures member are due to poor design,

inadequate of reinforcement, corrosion and creep. The survey also indicates that the

steel plates epoxy bonded system was quite successfully used to rehabilitate wide

range of structural problems for the last 30 over years.

8

However due to possible corrosion problem and handling aspect during

installation other techniques are being investigated. Nowadays, with the

advancement in the material technology an advanced composite materials or

technically known as Fibre Reinforced Polymer (FRP) shows a great opportunity to

be used for renewal programme in the construction industry in the rehabilitation

work throughout the world [3]. Fig. 2.2 shows the laminating technique of CFRP to

the reinforced concrete beam and Fig. 2.3 show the application of CFRP sheet for

rehabilitation to the damage bridge beams.

Fig. 2.2: FRP laminate system used for strengthening RC beam [6]

9

Fig. 2.3: Muar Bridge Beams strengthened with CFRP Sheet (Source: FOSROC

Sdn. Bhd.)

2.1.1 Definition of Durability

The durability of a material or a structure is defined as the ability to resist

cracking, oxidation, chemical degradation, delamination, wear, and/or the effects of

foreign object damage for a specified period of time, under the appropriate load

conditions, under specified environmental conditions [3,4].

2.1.2 The Bond Durability of Steel Plate as Externally Bonded System

In the area of steel plate bonding system numerous researches have been

carried out and the knowledge on the long-term performance of the system is well

10

established [1]. Results showed that maximum composite action could be achieved

by the adhesive bonded and together with significant improvement in performance in

terms of ultimate load, stiffness and crack control [7]. However, in the exposure test

that was carried out indicates that significant amount of corrosion of steel may take

place during exposure at site. A localized bond failure due to corrosion that resulting

loss in bond strength at the steel-epoxy interface was observed and the reduction of

the overall strength of the exposed beams was attributed to the corrosion [8]. Finally

it could be concluded that the use of externally bonded steel plate to rehabilitate

reinforced concrete structures have shown some disadvantages in handling at site and

also sensitive to an aggressive environment.

2.1.3 The Bond Durability of FRP Plate as Externally Bonded System

The FRP plate bonding system whether using CFRP or GFRP plate is seen to be

applicable as strengthening technique but several aspects of structural implications

and long-term behaviour and durability of the system need to be understood and

designed for before such techniques can be widely applied. In addition, the long-term

durability of the plate-adhesive and concrete-adhesive interface exposed to tropical

environment with heavy rain and sunshine throughout the year is the important factor

to determine the suitability of the FRP plate-bonded system to be used in this region.

Long-term durability is one of the most important properties of most polymeric

based adhesive bonds. Although it can be difficult to achieve in aggressive

environments, there are some methods to slow the degradation process. Material

selection, proper surface preparation and right joint design able to increased and

maintain the durability of joints. Such study is essential for any modification or

recommendation, if necessary, pertaining to the use of the FRP plate bonding system

in particular using the CFRP plate.

11

In contrast to steel, CFRP composite is seen to be more durable to most

aggressive environment which corrosion problem that facing by steel plate system

can be eliminated. However the CFRP plate-bonded system is relatively new

technology in the construction industry although the design concept is quite similar

to the steel plate bonding system. Therefore, there are still many areas of material

and structural implication arising from the use of CFRP plates-bonded system that

are not yet clear and need further research especially in durability aspects [21].

Furthermore, most of the studies have been conducted in Japan, Europe, Canada and

United States of America in which the weather pattern is different from tropical

environment. Research on the short-term structural performance of reinforced

concrete beams strengthened with CFRP plate bonded system that have been

conducted showed a significant improvement in the ultimate flexural capacity of the

beams [9]. Another testing programme on strengthening undamaged beams with FRP

plates demonstrated that bonded FRP plates improved the strength and stiffness of

reinforced concrete beams [10].

Many researches have been conducted on the flexural behaviour of reinforced

concrete beams strengthened on the tension face with either GFRP or CFRP plates

and fabric wet lay-up system. The findings showed that due to higher tensile strength

and higher modulus of elasticity of CFRP plate compared to GFRP plate, the overall

structural performance of reinforced concrete beams strengthened with CFRP plate is

better than GFRP plate. The typical failure mechanisms of the strengthened beams

are plate peeling or debonding close to the plate ends, flexural tensile cracks in

concrete with rupture of FRP plate and shear cracks in concrete starting from the

plate ends [11]. Due to the abrupt curtailment of the plate-adhesive system adjacent

to the support a high concentration of interface shear stress in the vicinity of the plate

occurred [12]. This may lead to abrupt and non-ductile failure of the member, which

is undesirable in the design. It can be seen that the bond between adhesive-FRP plate

and adhesive-concrete interface is of particular important for the member to develop

maximum flexural capacity and affect the long-term performance of the strengthened

member. Toutanji and Gomez [13] on their durability study of FRP composites

bonded to concrete beams had shows that exposure to salt water and dry condition at

35 °C under 90 % humidity under wet/dry cycles was exhibited less improvement in

12

term of ratio of ultimate load for both exposed and control specimens (Table 2.2).

The behaviour of beams strengthened with CFRP and GFRP sheet were shows by

Figs. 2.4 and 2.5. Their results show that debonding of FRP sheets from concrete

interface shows by all the FRP bonded beams.

Table 2.2: Experimental results of control and exposed beams [13]

Fig. 2.4: Typical behaviour of load vs deflection for control beams [13]

13

Fig. 2.5: Typical behaviour of load vs defelection for exposure beams under wet/dry

cycles [13]

A similar study also was conducted by Chajes [14] on durability of concrete

beams externally bonded with aramid, glass and carbon composites. The externally

bonded beam specimens were exposed to freeze thaw and calcium chloride solution

under wet/dry conditions. The results shows that the effects of aggressive

environments were degraded the FRP externally bonded beam strength

performances. The beams bonded with aramid and glass fibre system exhibited about

50 % reduction of strength due to both exposure conditions.

Experimental study conducted by Karbhari and Zhao [15] had shows that the

effects of exposure conditions onto composite and composite-concrete interfaces was

degraded due to moisture uptake. Their study involved the application of GFRP and

CFRP composites system that were externally bonded to the tension face of concrete

beam specimens. The specimens were exposed to fresh water, salt water, freeze-thaw

and under freezing temperature. The flexural load test results were indicates that the

14

degradation was occurred primarily at the interface level of FRP-concrete and FRP

itself due to changes composite stiffness caused by resin plasticization. They also

discovered that the moisture absorption rate was high in fresh water compared to

exposure to sea water.

On the other hand, not many studies have been conducted on the long-term

performance of reinforced concrete beams strengthen using FRP plate-bonded

system that exposed to natural weather. Thus, this area needs further investigation

especially with different exposure conditions. In relation to that, the long-term

durability of the FRP-plate bonded system exposed to different aggressive

environments need to be addressed especially exposure to tropical climate in which

at present the data are very limited or non-existence. In the plate bonding system

penetration of moisture may also occur through the resin via micro cracks, which can

leads to local debonding of the FRP plate [16]. Furthermore, most of the FRP

reinforcement that had been developed in temperate countries was tested for

durability under conditions simulating those countries. Since the tropical climate

experience abundant rain and sunshine throughout the year, therefore, it would be

essential to assess the long-term durability of the FRP plate-bonded system in this

region [17].

The environmental resistance of any bonded assembly FRP system depends

on the durability of the individual components materials, as well as on the bond

between them [22]. For example, in the use of FRP Plate as a material for external

strengthening of reinforced concrete structures, the individual components are the

reinforced concrete, the FRP and the adhesive. The long-tem integrity of bonded

joints implies both chemical and mechanical durability under varying temperature,

moisture and other environmental factors. Adhesive bonded joints with equivalent

bond strength values in short-term static tests may differ markedly with respect to the

durability.

15

The measured residual joint strength after environmental exposure is a

function of change in the cohesive properties of the adherend and in the adhesion

between the adhesives and the adherend. Therefore, joint durability demands a three-

fold consideration of the structural integrity of the cured adhesive, the adherends and

the environmental stability of the interface.

Adhesive bonded joints are generally attacked by exposure to moisture and

elevated temperature. In a well mode joint where a sound bond has been achieved,

the main effect will be on the adhesive layer. A small amount of moisture will

induces plasticization of the adhesive in a highly stressed regions may actually be

beneficial in reducing stress concentrations. However, a small reduction in joint

strength should normally be anticipated in relation to the effects of environmental

conditions on the adhesive itself.

2.1.4 Factors Affecting Bond Strength

The strength of the joint depends on the tensile yield strength (i.e. for ductile

materials), its modulus and thickness of the adherend as well as shear modulus and

the thickness of an adhesive. The adhesive layer must be as thin as possible to avoid

joint starvation and the shear modulus should be higher in order to provide joint

toughness (i.e. able to absorb or to resist stresses at the bond interface). The analysis

of bond durability strength can be related to the following parameters;

Type of adhesive

Different adhesive provides different bond strength, toughness and

durability level. The selection of the adhesive should be done

carefully based on the type of joint, strength needed, and the

materials to be bonded and working environments.

Bonded materials

16

The bonded materials (i.e. adherends) should be compatible as

possible to the adhesive. Each adherend has it own properties that

will provide different strength and durability level especially at the

bond adherend-adhesive interface.

Adherend preparation

Adherend preparation should follow the strict procedure to produce

good adhesion and absorption by chemical contact or by mechanical

interlocking mechanism between adherend and the adhesive.

Curing parameters (i.e. temperature and pressure)

The adhesive only will provide high bond strength if completely

cured. To reach this level, the bonding needs enough time for curing,

dry and a clean environment during preparation, suitable curing

temperature and pressure. Uncompleted curing process can weaken

the bond strength which finally caused slipping problems at

adhesive-adherend interfaces.

Adhesive thickness

The thickness of the adhesive should be controlled; not too thick or

less. Thick bond layer will create an unexpected force and moment.

Besides, it will risk a peel failure. The less thickness could cause

lower strength of bonding and easily fractured.

The study conducted by Horiguchi and Saeki [23] had shows that the shear

test method exhibit the lowest bond strength relative to compressive strength of the

concrete. The shear test method showed relatively low bond strength and less effect

toward the concrete compressive strength. The type of failure mode was dominated

by debonding between CFRP and concrete interface. However, they are not focusing

into depth the factors such as concrete surface preparation, local force distribution,

local strains and the durability aspects (long-term effect) in their study.

17

Toutanji and Ortiz [24], in their research finding has shown that the cracks

have occurred on the bonded FRP-concrete prism for all test specimens i.e. around

the specimen centre. The measured strains i.e. at the center of test specimen on FRP

sheets was gradually increased from the centre toward the outside i.e. due to

formation or development of cracks at concrete surface where the separation

occurred and slowly widen finally leads to the final concrete fracture. It shows that

final concrete fracture was occurred at FRP/Concrete interface. The concrete surface

preparation by water jet treatment on concrete surface has shown 50 % higher of load

up to failure compared to sanding method. They are also found that the high modulus

CFRP composite produced bond strength about 25 % higher than low modulus type.

Their finding also shows that the fibre stiffness and concrete surface treatment were

the main factors that contributing to specimen stiffness after first cracking as shown

in Figs. 2.6 and 2.7 respectively.

Fig. 2.6: Typical load-strain bi-linear curve for FRP sheet-concrete prism bonded

specimen [24]

18

Fig. 2.7: Comparison of bond strength due to different concrete surface preparation

methods [24]

Arden and Nanny [25] in their research finding have showed that the

reinforced concrete beam surface preparation by sandblasting method is slightly

improved their ultimate failure load and the beam stiffness compared to the tested

beam treated by sanding method. Its could be estimated that an increased of failure

load and deflection is about 20 kN (i.e. about 15%) and 5.5 mm (i.e. about 78.60%)

compared to both beams surface preparation methods. Depending failure at the

adhesive-concrete interface has occurred on each tested beam. The failure initially

started within the constant moment region which started by cracks development that

produced high stress level at bond interface at the higher load. This implies failure at

bond interface that finally propagated towards the sheet end. Their finding has also

includes that pre-cracked concrete beams surface treatments produced negligible

effects in the increment of the ultimate failure loads and the deflections.

19

2.1.5 Factors Affecting Bond Durability

One of the most important factors in bond durability is the environmental

stability factor occurs at adhesive-adherend interfaces. The changes in the adhesive

and the adherend mechanical or chemical properties can be the factors that allowed

for changes in adhesion properties. Therefore, bond surface conditions and pre-

treatments often represent the key to enhancing the bond durability. In FRP-concrete

bonded system for example, if the bond procedure is well followed, the surfaces of

both concrete and FRP materials are relatively stable; finally the durable bonds with

epoxy adhesives can be achieved. The substitution of FRP materials for steel in

strengthening reinforced concrete members is motivated by the assurance of superior

bond integrity.

The most outstanding durability study was conducted by Mukhopadhyaya

[21] onto GFRP-epoxy-concrete that were exposed to various aggressive conditions.

They used two different concrete mixed with compressive strength of 35 MPa and 50

MPa for mix A and B respectively. They were discovered that aggressive

environmental conditions, i.e. wet and dry cycles and freeze-thaw do create further

damage to the plate-concrete-adhesive interfaces. All the exposure specimens that

exposed to aggressive regimes showed higher dimensional changes and differential

movement between the plate and concrete compared to the control specimens. They

also had found that the exposure regime has a distinct and strong influence on the

nature of the bond transfer length (refer to Figs. 2.8 (a) to (d) and Figs. 2.9 (a) to (d).

The exposure regime not only increases the length over which the force is transferred

from the plate to the concrete, but it is also progressively increases the process of

debonding at the stressed end.

20

Fig. 2.8: Typical force transfer distributions for concrete mix A: (a) control; (b) wet�

dry; (c) freeze�thaw and (d) dual. [21]

Fig. 2.9: Typical force transfer distributions for concrete mix B: (a) control; (b)

wet�dry; (c) freeze�thaw;and (d) dual [21]

21

2.1.6 Failure Modes of FRP-Concrete Bonded System

Externally FRP bonded to concrete beams could fail in several ways when

loaded in bending. If both reinforcing steel and FRP cross sectional area fractions are

small, reinforcing steel yielding may be followed by rupture of FRP composite sheet

or plate. If the FRP cross sectional area fraction is high, failure is due to concrete

crushing while the steel may have yielded or not, depending in its cross sectional

area fraction. Debonding of FRP from concrete cover may occurs due to the

following phenomenon [18,19,20,26];

i. The sudden propagation of cracks in the adhesive-concrete interface

(i.e. due to brittleness of both materials).

ii. Peeling-off of the FRP sheet/plate due to opening caused by shear

cracks in the concrete (Fig. 2.10).

iii. Shear failure between concrete cover and FRP sheet layer and the

longitudinal reinforcement.

Fig. 2.10: Progressive failure of CFRP plate externally bonded to concrete due to

vertical and horizontal concrete crack openings near to loading point [26]

22

By referring to Mukhopadhyaya [21], the failure of FRP plate-adhesive-

concrete bonded that subjected to tension-compression loads could occur in three

different ways, namely; (a) cohesive failure in the adhesive layer, (b) adhesion

failure and (c) concrete shearing failure. Those types of failures are shown in Figs.

2.11 (a) to (c).

2.1.6.1 Failure at Interface

This may arise through failure of an interlayer between the substrate material

and adhesive (i.e. an oxide coating or primer layer) or through failure of the adhesive

bond surface. In practice, the interface is not perfectly flat and the surface

topography acts to create a layer where there is both adhesive and substrate present.

Fig. 2.11 (a): Cohesive failure

23

2.1.6.2 Adhesive Failure

Cohesive failure occurs through excessive strain with the adhesive material

and may occur anywhere within the adhesive layer. Stresses and strains peak at the

ends of the overlap and generally close to substrate.

Fig. 2.11 (b): Adhesive failure

2.1.6.2 Adherend Failure

This type of failure will arise through development of excessive strain within

the adherend material and it is more common to occur for brittle type materials. In

particular, joints made with adherends of FRP composite with concrete bonded with

toughened adhesive failed by adherend failure, usually by concrete shearing.

24

Fig. 2.11 (c): Adherend failure i.e. concrete shearing

2.2 Fibre Reinforced Polymer (FRP) Composites

Composite material can be defined as a material that consists of multiphase

material that exhibits a proportion of the properties of both constituent phases such

that a better combination of properties can be produced. Traditionally, a composite

material can be modeled as a material that consists of a matrix phase and a

reinforcement phase, with the overall quality and efficiency of the material being

primarily determined by the efficiency of the load transfer mechanisms. Advanced

composites materials is classified as a material that possess high strength, high

modulus to weight ratio and high fracture toughness whilst not exhibiting an increase

in weight [27].

2.3 Advanced FRP Composites Applied for Load Bearing Structures

Advanced Fibre Reinforced Polymer (FRP) composites have been

successfully used as an engineering structures or members for many years in the

25

aerospace, automotive, marine, chemical industries, etc. FRP composite is more

preferable than steel for such specific applications due to their durability aspects

when being subjected to extreme environment conditions such as exposure to

highly polluted or coastal area, high temperature fluctuation and high moisture

condition. FRP materials have characteristics that are different from most

conventional engineering material. For example, the characteristic of carbon

fibres are low bulk density, high tensile strength and modulus to weight ratios

and excellent fatigue behaviour, however, the variation in type of carbon and the

sheets or plates forming process have resulted different characteristic of

mechanical and physical performances.

The mechanical properties of three different types of CFRP plates that

produced through Pultrusion process which classified as an advanced composite

is shown in Table 2.4. The CFRP plates listed in Table 2.4 can be categorized in

three types; Type S referred as high tensile strength, Type M referred as high

strength with intermediate modulus and Type H known as high modulus

composite. In the application of strengthening steel structure, type M is most

preferred due to compatibility of elastic properties with mild steel material. The

three most popular reinforcing fibres system that has been classified as advanced

material is shown in Table 2.5.

Table 2.4: Properties of Selfix Carbofibe Pultruded CFRP Plates System (Source:

Exchem EPC Group Ltd., United Kingdom)

Plate

type

Ult. tensile strength

(average)

(MPa)

Tensile

modulus

(GPa)

Plate width

(mm)

Plate thickness

(mm)

S 2800 150 50/80/120 1.2/1.4

M 3200 200 50/80/120 1.2/1.4

H 1600 280 50/80/120 1.2/1.4

26

Table 2.5: Typical reinforcing unidirectional fibre properties [28]

Fibre Tensile

strength

(MPa)

Modulus of

elasticity

(GPa)

Elongation

(%)

Specific

density

Carbon: high

strength*

Carbon: high

modulus*

Carbon: ultra high

modulus**

4300-4900

2740-5490

2600-4020

230-240

294-329

540-640

1.9-2.1

0.7-1.9

0.4-0.8

1.8

1.78-

1.81

1.91-

2.12

Aramid: high

strength and high

modulus

3200-3600 124-130 2.4 1.44

Glass 2400-3500 70-85 3.5-4.7 2.6

2.3.1 Carbon Fibre

Carbon fibres are currently the predominant high strength to high modulus

fibres used in the manufacture of advanced polymer composites load bearing

structures such as for automotive driveshaft, bridge beam, wings skin for jet fighter

etc. The genesis of carbon fibre technology was the need to produce lightweight, stiff

and strong materials for the rapidly growing aerospace industry. Typical sizes of

carbon fibres are in between 6 ìm and 8 ìm in diameter and consist of small

crystallites of turbo static graphite i.e. one of the allotropic forms of carbon. The

carbon fibres are formed by treating organic fibres (precursors) with heat and tension

to form a highly ordered carbon structure. In standard graphite single crystals, the

carbon atoms exist as hexagonal arrays stacked in a regular ABAB sequence. The

atoms in each layer are held together by very strong covalent bonds, whereas the

27

layers are only connected by weak van der Waals forces, and hence graphite is

anisotropic. In turbo static graphite, the stacking sequence is highly irregular.

2.3.2 The Pultrusion Process

Pultrusion is a process that enables hybrid composite components in the

forms of rod, profile sections, and tubular sections to be manufactured in continuous

lengths [29]. The basic technique employed is to impregnate the reinforcing fibers, in

continuous form with resin matrix such as polyester or epoxy prior pulling the

impregnated fibres through a curing and post curing die zones which imparts the

desired shape to the composite. A diagrammatic representation of the process is

shown in Fig.2.12.

Fig. 2.12: The Pultrusion process in manufacturing FRP plate [29]

Pultrusion machine is capable in producing hundreds of metres of profile

section per hour under single operative control. A wide range of component shapes

can be manufactured by this process at a very competitive cost due to its highly

automated nature (Fig.2.12). Polyesters, vinylesters and epoxy resins are among the

principal matrix systems that have been used for the process coupled with carbon,

glass, aramid or hybrid of those reinforcement materials respectively.

28

Fig. 2.13: Products produced by Pultrusion Process (Source: Strongwell

Corporation, USA)

2.3.3 FRP Pultruded Composites Plates

One of the most important properties of FRP composites is the tensile

behaviour whereby the stiffness and strength of FRP composites can be varied in

magnitude and direction to meet the structural design requirements. From

experienced in the testing fields, it can be seen that the mechanical properties of FRP

under tensile load are greatly influences by fibre properties, fibre forms, fibre

volume fraction, fibre orientation, matrix properties and processing methods [30]. In

design practice, the reinforcing fibres are specifically oriented parallel to the applied

load in order to gain the maximum strength capacity of the fibres. The best example

is orthotropic pultruded CFRP plate that being used for upgrading the flexural

performances of reinforced concrete (Fig. 2.14) or steel structures.

29

Fig. 2.14: CFRP Plate (black strip) externally bonded to tension face of reinforced

concrete beam (Source: Behaviour of Beams Strengthened with CFRP Composite,

IRPA 72272, 1999-2001)

2.3.4 Durability of FRP Composites

More recently, the use of FRP composite materials was extended to be used

as primary structures in aircrafts, automotive applications and infrastructure such as

for rehabilitation or strengthening of steel or reinforced concrete bridges and

buildings. This fact brings the issue of durability which the long-term

experimentation results can be used to predict the long-term properties and residual

life, as a determinant factor in the success of the referred applications. FRP

composite materials find to be increased in infrastructure applications, where design

lives cycles are about ten times longer than those in aerospace, the issue of durability

becomes more critical and must seriously focused. The tolerance of composites to

damage induced by mechanical loading and moisture ingress is the most importance

factors should be considered in real life applications.

30

The studies conducted by previous known researchers that related to the

durability performances of FRP composites are the main focused in this following

literatures review. The review is to gain the knowledge that related to the study FRP

composite degradation effects from exposure to moisture environment conditions

(i.e. outdoor, fresh water and salt water). Referring to the study conducted by Zhou

and Lucas [31], onto the effects of unidirectional graphite/epoxy composite under

water environment at temperature of 45°C, 60°C, 75°C and 90°C that exposed for

more 8000 hours were revealed two important factors that are summarised as

follows;

i. Water sorption in graphite/epoxy (T300/934) material exhibited both

Fickian and non-Fickian diffusion behaviour. The materials obey the

Fickian diffusion behaviour at lower temperatures and non-Fickian

behaviour at higher temperatures. The non-Fickian behaviour was

resulted from chemical modification and physical damage to the epoxy

resin. Cracks, voids and surface peeling were observed clearly through

SEM and optical microscopy.

ii. Moisture-induced expansion of T300/934 composite was measured in

length (fibre direction), width and thickness directions. There was no

expansion due to water absorption was detected in the fibre direction

dimension. Significant dimensional changes resulting from moisture-

induced expansion were observed in the width and thickness directions

of the laminate. The thickness decreased of the specimen at high

temperature was associated with surface resin dissolution and peeling.

Those characteristics are shown in Figs. 2.23 (a), (b) and (c).

31

Fig. 2.15 (a) and (b): The changes of width and thickness respectively of T300/934

graphite/epoxy immersed in distilled water at different temperatures [31]

Fig. 2.15 (c): The weight change of T300/934 graphite/epoxy immersed in distilled

water at different temperatures. The solid lines represent theoretical Fickian diffusion

[31]

Amer [32], on their studies related to hydrothermal effect on single fibre

composite have revealed that the interfacial degradation mechanism due to

environmental exposure of graphite/epoxy was mechanical in nature. Matrix swelling

was the main factor that degraded the interfacial stresses which finally produced a

complex state of stress within the fibre-matrix interface. Their study also revealed

that the analysis done on the single fibre system able to predict the bulk composites

behaviour. This was confirmed by experimental and FEA on bulk composites with

volume fraction ranging from 63%� 71%.

(a (b)

32

The long term durability study conducted by Liaoa [33] onto pultruded glass-

fiber-reinforced vinyl ester composite coupons subjected to various environment

conditions to study the long-term durability for infrastructure applications. Several

groups of specimens were aged in water or in salt solutions containing mass fractions

of either 5% NaCl or 10% NaCl for up to 6570 hours. The control (as-received) and

aged specimens were cyclically tested in air or while immersed in water or in salt

solution. For specimens cyclically loaded at or above 45% of the average flexural

strength of the dry coupons, no substantial difference in fatigue life was observed

among all the specimen groups. For samples cyclically loaded at 30% of the dry

flexural strength, however, all specimens tested in air survived beyond 107 cycles

while all those tested in water environments did not. It is found that long-term

environmental fatigue behaviour is not controlled by the quantity of water absorbed;

rather, it is governed by a combination of both load and fluid environment. No

difference in fatigue life was found for specimens aged in different fluid

environments at room temperature prior to fatigue testing. Relative to these samples,

however, a significant difference was seen for specimens aged in water at 75°C for

2400 hours prior to cyclic test at load levels above 30% of the dry flexural strength

(Fig. 2.25). When tested at 30% of the dry flexural strength the differences were

within the experimental uncertainty. Microscopic examination of the fatigue

specimens revealed evidence of a degraded fiber/matrix interphase region in those

specimens where environmental exposure caused premature failure so this is

believed to be a controlling factor in the environmental performance of the glass

composite.

33

Fig. 2.16: Stress-life (S-N) data for pultruded composite coupons tested in fatigue at

room temperature using environmental conditions A through to F [33]

Liaoa [34], on their durability study of pultruded glass�fiber-reinforced vinyl

ester matrix composite coupons subjected to environmental aging in water or salt

solutions at room temperature (25°C) or in water at 75°C for various times. The

flexural properties (strength and modulus) were determined for bending

perpendicular to the 0 degree orientations for all aging conditions. In addition,

flexural properties in the 90 degree orientation and tensile properties in the 0 degree

orientation were also tested for the as-received specimens and the specimens exposed

to selected aging conditions. Both strengths and moduli were generally found to

decrease with environmental aging. A group of specimens were also aged in room

temperature water for 9120 h before being tested for failure in tension. The mean

tensile modulus after aging (14.4 GPa) is 23% lower than that before aging (18.6

GPa). The mean tensile strength after aging (227 MPa) dropped by 29% compared to

those without aging (160 MPa). The failure strain for the control and the aged

specimens are 2.1 and 1.4%, respectively.

In addition, examination of the failure surfaces and comparisons between the

strength of the 90 degree specimens suggested that degradation of the fiber/matrix

34

interphase region also occurred during the aging process. Their durability study

results were presented in the following respective graphs shows in Fig. 2.17 (a) and

(b), Fig. 2.18 and Fig. 2.19.

Fig. 2.17 (a) and (b): Flexural strength and modulus for 0 specimens of pultruded

composite coupons before and after environmental aging [34]

Fig. 2.18: Flexural modulus for 90° specimens of pultruded composite coupons

before and after environmental aging [34]

(a) (b)

35

Fig. 2.19: Sorption behaviour of pultruded composite coupons under various aging

conditions [34]

The study conducted by McBagonluri [35] on the effects of short-term cyclic

moisture aging on the strength and fatigue performance of a glass/vinyl ester

pultruded composite system exposed to fresh and salt water. They have found that

the quasi-static tensile strength was seen to reduce by 24% at a moisture

concentration of 1% by weight. This reduction in strength was not recoverable even

when the material was dried, suggesting that the exposure to moisture caused

permanent damage in the material system. Even though the fatigue damage process

of the control, fresh-water- and salt-water-saturated material was similar, the cyclic

moisture absorption�desorption experiments altered the fatigue performance of the

composite system tested. Their elastic properties and fatigue strength results are

shown in Fig. 2.20 and Fig. 2.21 respectively.

36

Fig. 2.20: Properties of as-delivered (control), fresh-water-aged and salt-water-aged

materials [35]

Fig. 2.21: S�N curves of the as-delivered (control), water-aged and 3.5% salt-

solution-aged materials [35]

Experimental study conducted by Gautier [36] has shown that glass fibre

reinforced polyester pultruded composites immersed in water at different

temperatures (ranging from 30°C to 100°C) were produced three types of damage:

osmotic cracking in the matrix, at the interphase and interfacial debonding. The

result shows that the matrix osmotic cracking is the factor for specimen weight loss.

The result also shows that the decreased of interlaminar shear strength (ILSS) is

caused mainly by interfacial debonding, induced by differential swelling, and by

osmotic cracking at the interphase but the matrix also contributes to the decrease.

They finally concluded that the composite life time is greatly dependent on the

ability of the matrix to microcrack under the service conditions.

37

2.4 Adhesive Bonding Technology

Adhesives have been successful used for a number of decades in joining most

of aircraft and automotive components. Nowadays, the adhesives had also successful

used in textile industry, medicine and construction industry. The adhesive bonded

joint offers or form a major proportion of modern aircraft and automotive

construction to reduce weight, mechanical stresses and production time. Adhesives

have significant advantages over other mechanical joints such as rivets, bolts and

screws which adhesive bonding technology has a great potential to avoid excessive

stresses concentration by spreading stresses over a larger area. This finally able to

permit thinner joining surfaces that very important in low-weight applications.

By referring to Eurocomp Design Code and Handbook [31], bonded joint can

be defined as where the materials (similar or dissimilar) bond surfaces are held

together (i.e. by mechanical or chemical mechanism) by means of structural

adhesive. In order to achieve their functions, the following conditions must be

reached;

i. The adhesive should not exceed an allowable shear stress. The

performances of the joint depend to the adjustment of the maximum

shear stresses to be less than the joint shear strength.

ii. The adhesive also not exceed an allowable tensile (peel stress).

iii. The adherend is not exceed the through thickness tensile stress allowable

iv. The adherend must not exceed the allowable in-plane shear stress.

In design practice, the in-plane shear stress will be by testing before put in

design analysis. Typically, one or more of the three conditions above will become

critical before in-plane shear stress limit in the adherends exceed.

38

2.4.1 Adhesive Selection

The selection for suitable adhesive depends on several important factors as

mentioned by Budinski [40]. Among the factors that need to be considered are as

follows;

i. Service temperature

ii. Chemical level

iii. Duration of application

iv. Adherend materials

There are a few groups of adhesives system can be considered for bonding

most of the structural parts, and they are as follows:

Epoxy: Two-part adhesive system that cured at room temperature based on

epoxy-polyamide which has shear strength as high as 13.8 MPa at 38°C and

0.68 MPa at 149°C.

Anaerobic adhesives: Polyester-acrylic resins which cured with absence of

air. Suitable for metal to metal joints. Shear strength in excess of 13.8 MPa

can be obtained on metal bond strength test.

Cyanoacrylates: Suitable for metal bonding process which cured by

moisture absorption from adherends. Shear strength can be developed as high

as 20.6 MPa.

39

2.4.2 Adhesive Mechanical Properties

In structural bonding applications, there are important mechanical properties

that must be given full attention and understood in the design process, and they are as

follows;

i. shear modulus

ii. shear strength

iii. maximum shear strain

iv. tensile modulus

v. tensile (peel) strength

All the important related properties should be obtained from the manufacturer

or by established testing methods. This is important due to the effects from durability

factors such as moisture ingression, temperature fluctuation etc. Referring at creep

property, adhesives will creep under constant load even at the room temperature

especially at elevated temperature. Usually, thermoset based adhesives have better

creep resistance than thermoplastic adhesives. Fig. 2.22 shows two different

behaviour of adhesives system that has characteristics of ductile and brittle

respectively. Brittle type adhesive normally failed at high stress level with low strain

value compared to ductile adhesive system which shows high strain to failure.

Fig. 2.22: Typical brittle and ductile adhesive behaviour [31]

40

2.4.3 Effects of Loading Configuration on Adhesive Joint

The joint structure typically loaded by several of loading systems as shown in

Fig. 2.23. The tensile, cleavage and peel loads for example should be avoided

because it will weaken the joint strength. In principal, the adhesive layers of the joint

should primarily be stressed in shear or compression, the excessive strains (due to

deformation) should also be considered at the area where non-linear behaviour of

adherends or adhesive is expected.

In bonded joint, there is four main loading modes may be subjected to most

bonded structures;

i. Out-plane loads acting on a thick adherends produce peel stresses.

ii. Tensile, torsion or pure shear loads imposed on adherends produce shear

stresses.

iii. Out-of-plane tensile loads produce tensile and bending stresses.

iv. Out-of-plane tensile loads acting on stiff and thick adherends at the end of

the joint produce cleavage.

Fig. 2.23: Loading modes or type of stresses [31]

41

2.4.4 Advantages and Limitations of Adhesive Bonding

Adhesive may be the logical choice as a fastening method for bonding any

structural materials for a variety of reasons [41]. Each type of joint has it own

advantages and disadvantages that make differences between them which are listed

as follows;

Advantages:

i. The ability to joint similar and dissimilar materials.

ii. The ability to minimize stress concentration usually associated with

mechanical joints such as bolts, rivets and spot welds.

iii. Adhesive is not a electrical conductor, therefore no formation of

electrolytic corrosion in joining dissimilar materials.

iv. Ductile adhesive system able to absorb shock and vibration which

could increase fatigue life. Normally adhesive-bonded metal have

ten times more fatigue life than mechanical joints.

v. Dissimilar materials thickness can be joined; for example, concrete

beam can be joined to very thin FRP plate in strengthening

application.

vi. Adhesives acting as a sealant in addition to bonding.

vii. The elimination of fastener holes allows lighter materials to be

used and able to maintain equal or better mechanical properties.

Limitations

i. Adhesives are more subject to deterioration due to environmental

influences especially in adhesive-metal joints.

ii. Difficult to inspect the bond quality once assembled.

iii. Poor resistance to peeling type loading and may require additional

fastener to support extra stresses (bonded-bolted joints).

iv. Polymeric based adhesives properties tend to degrade over time

especially that expose to aggressive environment conditions.

v. Proper jigs and fixtures is needed for bonding process in order to

apply heat and pressure which depend on bonding cycle.

42

vi. Most adhesive have limited shelf life.

vii. Less reliable when expose to extreme temperature above 300 °C.

2.5 The Principles of Adhesive Bonding Technology for Structural

Applications

The basic principles of adhesive bonded joints shall be follows for the

production of strong and durable adhesive bonds as well to minimize the bond

defects. There are types of adhesive joint configurations and each type offer different

criteria to be considered while applying the connection. Davis and Bond [42] have

listed few important joint principles that related to the application of adhesive

bonding in practices. Among the important preferred principles are listed as follows;

i. The basic principle for design of adhesive bonds is to design the joint

such that the adhesive is always stronger than the unnotched strength of

the adherends.

ii. The basic principle for adhesive fatigue design is therefore to ensure

that the overlap length is sufficient to enable the adhesive shear stress to

decay to near zero to make the joint resistant to creep and load effects.

iii. The basic principles of surface preparation are that the surface must be

free of contamination, sufficiently chemically active to enable

formation of chemical bonds between the adhesive and the adherends,

and resistant to environmental deterioration in service, especially by

hydration.

43

iv. The basic principle for integrity of an adhesive bond is that the

inspection will not assure quality, it must be obtained by management

of all aspects of the bonding process during production.

2.5.1 Factors Considered in Adhesive Joint Design

It is really need to give an attention to a few factors to make an appropriate

and effective adhesive bond joint. Among the important factors are need to be

seriously considered are as follows;

i. Adherend mechanical and physical characteristics to be joined.

ii. Adherend surface hardness conditions.

iii. Adherend thickness.

iv. The temperature of environment for most of service life and period

time.

v. Contamination in contact to the bond (solvents, oil and other fluids).

vi. Required joint strength.

vii. Stresses due to type of loading configurations (tensile, shear, peel,

compression, impact, vibration and etc.).

2.5.2 Bond Mechanism

The theory considers adhesion to be the result of the mechanical interlocking

of polymer adhesive into the pores and other superficial asperities of adherend. The

roughness and porosity of adherend are generally the factors as wet ability by the

adhesive is sufficient as shown in Fig. 2.24. Otherwise, the non-wetted parts

44

originate failures. However, mechanical interlocking is not a mechanism at the

molecular level. It is merely a technical means to increase the adsorption of the

adhesive to the adherends at macro level [43].

Fig. 2.24: Good wetting (A) Poor wetting (B) [43]

2.5.3 Joining Technique

Referring to EUROCOMP Design Code and Handbook [31], the joint design

process should start with recognizing the joint requirements such as for supporting

and distributing the internal forces and moments. The following stage is selecting the

joint category normally determined by loading configuration or by the required joint

efficiency as a fraction of the strength. The geometry of the adherends, suitability of

the fabrication, component dimensions, manufacturing environment and number of

components to be produced must also be considered. Another factors are includes

service environment and the lifetime of the structure, requirements set for the

reliability of the joint, disassembly or not, need or fluid and weather tightness,

aesthetics and cost.

The adhesive joint must be carefully designed and prepared. The aim of the

joint is to obtain maximum strength for a given bond area. In designing adhesive

45

joint the basic characteristics of adhesives must dictate the design. The type of joints

used in adhesive-bonding flat adherends is shown in the Fig. 2.25.

Fig. 2.25: Type of adhesive joints techniques for flat adherends [43]

2.5.4 Joint Geometry Effect on Joint Strength

The joint strength also affected by the joint geometry with certain

configuration. The most basic problems of bonded joint are the unavoidable shear

stress concentrations and inherent eccentricity of the forces. The two problems

causing peel stresses in both, adhesive and adherends. From the Fig. 2.26 it can be

seen that the shear stresses are at the maximum at the end of the overlap.

The effects of the eccentricity are the greatest in lap and strap joints. It should

be known that the static load-bearing capacity of a bonded lap or strap joint cannot

be increased significantly by increasing the lap length beyond the minimum needs.

But, the bond length must long enough to provide a moderate loaded adhesive area in

the middle to resist creep deformations of the adhesive.

46

The peel stresses can be reduced by increasing the adherend stiffness without

increasing its thickness, increasing the lap length, tapering the ends of the adherend

and using adhesive fillets. Adhesive fillets used and adherend ends tapered will

reducing stress concentrations at the end of the overlap.

Fig. 2.26 shows the typical locations of possible failure initiation and critical

strength. It can be seen that when the joint (i.e. single lap) loaded with in-plane loads,

the concentration of stress failure exist at the ends of the over lap. Fig. 2.27 shows

the shear stress distribution along the bonded length and the location where higher

shear stresses occurred. The higher shear stresses location can be said as the region

of failure initiation.

Fig. 2.26: Areas of failure initiation and critical strength [31]

Fig. 2.27: A typical adhesive shear stress distribution in a lap joint according to

elastic-plastic model [31]

47

2.5.5 Elastic Properties and Deformation

Fig. 2.28 shows the schematic diagram of a single-lap joint with uniform lap

thickness loaded in tension. By assuming the deformation of double-lap joint follows

the deformation shown in Fig. 2.28. Theoretically the deformation and initiation of

bond failure occurred at the loaded end (i.e. the most stressed region). The upper and

lower part represent as an adherends while adhesive in the middle. The members

deform concentrically and the adhesive in shear when load applied. There are two

types that the specimen can be categorized; as a rigid members and as an elastic

members. If the members were rigid, equal amount of load would transfer along the

adhesive, and the shear deformation would be equal in all part of adhesive. In reality,

the members are always have elasticity and will deform continuously through their

lengths. The greater amount of load were transferred at the center of overlap mean by

the higher displacement between the members occurs there.

Fig. 2.28: (a) Deformation of rigid members (b) Deformation of elastic members

[31]

(a) (b)

48

2.6 Double Lap Joint

The double lap joint is a balanced joint construction configuration because it

consists of two outer substrates that are bonded on both sides of centre (inner)

adherend. The joint configuration that shows in Fig. 2.29 will experience internal

bending if the outer adherends are thick. In a well symmetrical double lap joint, the

centre of the adherend experiences no net bending moment, but the outer adherend

will (if it is thick), which could increased tensile and compressive stresses at loaded

ends.

Fig. 2.29: Double lap joint configuration specimen under pull-push loads

The joint is designed based on standard bond area, maximum proportion of

bond area that contribute to strength, the direction of maximum stress applied to high

strength area and the minimum stress in direction of the weakest joint. When the

adherends are subjected to the tension load, the loading effect can divided to normal

force, shear force and internal bending moment. It is reasonable to ignore axial stress

in the bond layer as it regarded to thin layer and the adhesive is assumed more

flexible than the adherends. Another assumption consequence to the strain in vertical

direction in the adhesive is zero, makes the shear stress and shear strain were

assumed to be constant over the adhesive layers.

From the bond stress distribution shows in Fig. 2.29, it is clear that a uniform

shear stress was distributed symmetrically along the bond length. Shear stress is

directly proportional to the width of the joint, but increasing the bonded area beyond

49

certain limits is not significantly affected. The strength of the joint is depending on

the yield strength (or ultimate strength for brittle materials) of the adherend, its

modulus and thickness. The thickness of the adhesive bond is important and must be

as thin as possible to avoid joint starvation. The analysis of the bond strength and

durability are related to the following parameters;

Type of adhesive: Different adhesive provides different bond strength and

characteristic. The selection of the adhesive should be done carefully based

on the type of joint, strength needed, materials to be connected and working

environments.

Adherend materials: The adherends be used should be suitable to the

adhesive. Each adherend has it own mechanical and physical properties that

will provide different strength and durability.

Adherend preparations: Adherend should be prepared follows the correct

procedure to provide a good adhesion and absorption by the contact between

adherend and the adhesive.

Curing process; temperature and pressure: The adhesive only provide

high bond strength if it is completely cured. To reach this level, the bonding

needs enough time, dry and clean environment and standard curing

temperature. Incomplete curing process can cause bond slippage.

Adhesive thickness: The thickness of the adhesive should be controlled; not

to thick or less. Thick bond layer will create an unexpected force and

moment, finally will produce peel failure and less bond thickness could cause

lower bond strength.

50

2.7 Surface Treatments

Adherend surface treatment is really an important parameter that will affect

an adequate joint strength if not properly prepared. Therefore, all the bond surfaces

shall be properly treated prior to bonding. In order to produce a good bonding

performance, the entire adherends bond surface shall be treated by the following

procedure:

i. Solvent degreasing using a clean absorbent material which does not

itself contaminate the surface.

ii. Abraded using medium grit abrasive paper (i.e. for FRP composite),

sandblasting (i.e. for metal or concrete), etc.

iii. Degreasing.

During the surface roughening process, the pressure applied (i.e. by any type

of tool) shall be adjusted to suit with the condition as not to damage the adherend

material structure (i.e. not to produce permanent stresses within material structure).

Commonly, the prepared surface must be pretreated immediately after surface

treatment to avoid contamination or voids that can cause poor bonding.

2.8 Adhesive Joint Design Principles

In general, the loads imposed on the bonded joint structure must be obtained

from the whole structure analysis. Besides, the bond line must ensure capable to

transfer the applied loads between the joints members. While the adherends are

capable of with-standing, the joint induced internal loadings. The evaluation of the

components basic strength which to be joined under the applied external loads is a

part of the component design process.

51

The experimental specimen that to be tested is designed based on analytical

models for plate-to-plate connection and supplemented by testing. The assumption

made that the joint is a perfect bonding between the adhesive and the adherends. This

means, there are no slip occurred along the bond area and the force applied were

transferred uniformly to each part of the adherends. It shown from the failure of

cohesive in the adhesive or adherend always occur before the adhesive failure at the

interface. If the matter as follows occurred, the assumption may become invalid; so

must be considered properly:

i. Non-suitable chemical of the adhesive and adherends. The adhesive

cannot provide a good bonding and high strength needed. Besides, the

adhesive will give a chemical reaction between the adhesive matrix and

the adherends matrix.

ii. In adequate surface treatment. For examples, the surface is not roughen

perfectly, the surface of bonding area is contaminated and not fully

degrease by the solvent, the pressure applied while bonding is also not

enough.

iii. Environment factors such as temperature and pressure during bonding.

The bonding process should not been done during high humidity where

the water will dissolved between the adhesive pore and will effect the

bond strength. There must be enough time for the adhesive to cure and

should be applied on suitable dry environment.

iv. Other bonding defects.

Referring from most material testing, it is a perfect bonding if the failure

mode is not an adhesive failure. If the slip occurred, the surface treatment should be

improved or the adhesive or a joint configuration shall be changed. The design of

bonded joints shall be based on practically and tolerances of the manufacturer.

52

Referring to the Fig. 2.30, it can be seen that the different type of joint has it

own mode of failure. For double lap joint (i.e. same characteristic as double strap

joint), the major problem occurs is peel failure if compared to the others joint

technique likely to have shear failure.

Fig. 2.30: Relative joint strength of various joint configurations [31]

2.9 Failure Modes

Sheppard [43] in their study on developing damage zone model for adhesive

bonded joint were listed four types of failure modes by which an adhesive bonded

joint can fail. The four primary failure modes are summarised as follows;

i. Adhesive failure that means a rupture of an adhesive bond, such that the

separation is at the adhesive - adherend interface. This failure is mainly

due to a material mismatch or in adequate surface treatment.

ii. Cohesive failure of adhesive means that when the adhesive fails due to

loads exceeding the adhesive strength.

53

iii. Cohesive failure of adherend means that when the adherend fails due to

loads in excess of the adherend strength, for example due to bending,

tension or compression.

iv. Out-of plane adherend failure (this failure mode only occurs for

composite adherends and is in form of intra-laminar and/or inter-

laminar failure in adherends.

They also proposed three damage zone models that could be developed at

three important load levels; low load, medium load and ultimate load. The following

are the description for those three models.

i. Low load level: localised damage will occur at the end of the joint

(loaded end). This damage occurs due to the adherend is locally

subjected to excessive strain which is greater than ultimate material

strain. If the joint consist of concrete and FRP, a damage will develop

on both adherend either by fibre/matrix interface failure (inter-laminar)

or concrete shearing for FRP and concrete respectively.

ii. Medium load level: the damage zones will grow in size and the

concentration of points of specific damage will increase.

iii. Failure load level: the damage zone will grow to a critical size when

the individual components of damage will coalesce and form a crack.

54

Fig. 2.31 (a) and (b) shows the development of the damage process for out of plane

and cohesive failure modes [43]

2.10 Mathematical Model for Predicting Bond Stress Behaviour

The mathematical model of FRP Plate-concrete bonded system under pull-

push loads can be referred to elementary force diagram in Figure 2.32 (a) and (b).

The figure shows a double-lap joint under pull- push loads of a FRP plate bonded to

concrete by adhesive joint. FRP plate, adhesive layer and concrete prism are assumed

constant along the bond length. In such a joint, the adhesive layer is mainly subjected

to shear deformations. A simple mechanical model for this joint can be thus

established by treating the plate and the concrete prism as being subject to axial

deformations while the adhesive layer was assumed to be subjected to shear

deformations only. That is, both adherends are assumed to be subject to uniformly

distributed axial stresses, with any bending effect was neglected, while the adhesive

layer is assumed to be subject to shear stresses which are also constant across the

(b) (a)

(b)

55

thickness of the adhesive layer. It should be noted that in such a model, the adhesive

layer represents not only the deformation of the actual adhesive layer but also that of

materials adjacent to the adhesive layer and is thus also referred to in the paper as the

joint interfaces.

Fig. 2.32(a): Specimen geometry and material parameters of double-lap joint under

pull-push loads

Fig. 2.32 (b): Force analysis on elementary bar model

A series of pull-push shear tests on single-lap bonded joints were carried out

by Yao [44] to determine the effect of various parameters on the bond behaviour of

To To + dTo

dxc

dx

Ti Ti + dTi dxc

dxc

dxc To + dTo To

56

FRP plate-to-concrete joints. The thin FRP plates were instrumented with strain

gauges at 10 mm intervals along the bond length, L to monitor the variation of plate

strains with load. The results showed that the plate strains in the debonded zone were

substantially affected by plate bending due to the thinness of the plate and the

roughness of the cracked interface. Their mathematical model was developed

through the following assumptions for simplicity of the problems [45];

i. Adherents are homogeneous and linear elastic

ii. Adhesive is exposed only to shear forces

iii. Bending effects are neglected

iv. Normal stresses are uniformly distributed over the cross section

v. Thickness and width of the adherents are constant throughout the bond

line.

The two models of stress-slip relationship shown in Fig. 2.33 are

considered to be possible in representing the nonlinear interfacial behaviour are

introduced here. It is noted that the area below the curve represents the

interfacial fracture energy of mode II, fG , which is defined as the energy required to

bring a local bond element to shear fracture (debonding). f is the local bond

strength, and f maximum slip.

57

Fig. 2.33: Two models of stress relationship [44]

For model I, the stress-slip relation is linearly ascending before the

occurrence of interfacial fracture and the value of shear stress suddenly drops to zero

when the value of slip exceeds f without consideration of the softening behavior.

While for model II the stress-slip relation is linearly ascending when the value of the

slip is smaller than 1 . After the occurrence of an interfacial microcrack the stress

slip relation is linearly descending with a range of f 1 . The value of shear stress

is reduced to zero and an interfacial macrocrack (debonding) occurs when the value

of slip exceeds f .

For the case of the pull-push joint, the theoretical solutions can be obtained as

follows:

Model I: Linear Shear Stress-Slip Relationship with a Sudden Stress Drop

The shear stress along the FRP-concrete interface can be written in the form

L

x

b

P

sinh

cosh.

1

58

Model II: t-d Relationship with Linearly Ascending and Descending Branches

The shear stress along the FRP-concrete interface can be written in the form

aL

xf

1

1

cosh

cosh

if 0 1

and

aLxaLxaLf 2211

2 cossintanh

if f 1

where:

P=

aaaLbf

2211

2

2

1 sincostanh

222

1

11

22 1

2 tEb

b

tEG f

f

f

fG

1

221

2

ff

fG

1

222

2

21

1

2

97.0arcsin1

f

fa

Analytical solutions for adhesively bonded balanced composite and metallic

joints are conducted to reveal the adhesive peel and shear stresses [46].The coupling

between the adherents is established through the constitutive relations for the

59

interface ��resin rich�� layer, which is assumed homogeneous, isotropic and linear

elastic. The constitutive equations are suggested as follows:

dx

dwhu

dx

dwhu

h

Gaa

1101

2202

0 22

where aG is the shear modulus, aE is the elastic modulus of the interface ��resin

rich�� layer, and ra, sa are the interface ��resin rich�� layer transverse normal and

shear stress components.

Cho [37] assumed a bond-slip model type for a coarse sand coated interface

in the form of their suggested model types in order to use Yuan�s analytic solutions,

and finds appropriate model parameters for an assumed bond-slip model type

applicable to the coarse sand coated interface.

In Figure 2.34, the bond-slip model is expressed by the bond stress function,

where xuxu 21 represents the slip between adherent 1 and 2. f and f

are the bond strength and maximum slip, respectively. And, fG expresses the fracture

energy, which corresponds to the area under the curve. P is the axial force, E, t

and b are the axial stiffness, thickness and width of adherent, respectively. Subscripts

1 and 2 indicate adherent 1 and 2.

Different stress distributions are considered for the ascending and descending

parts of the bond-slip curve. When the stress in the whole bonding area occurs in the

ascending part of the curve in the bond-slip model, the solution is as follows:

xG

xf

f

2

2

60

On the other hand, when the stress in the adhesion area develops also in the

descending part of the bond-slip curve, the solution should be subdivided and

expressed separately for the ascending part aLx 0 and descending part

LxaL of the bond-slip curve, as follows:

xx ff

f

1

Fig. 2.34 : Bond slip model for pull � push joint [37]

A formularization method to obtain the bond-slip model and optimized bond-

slip model have been presented based on experiments and analyses of the bond

behavior between FRP plate and concrete. A multi-objective optimization problem

has been formulated by means of physical programming technique, which minimizes

the difference between shear bond test results and analytic solutions of the bond-slip

model derived from fracture mechanics. The optimization has been performed

through a genetic algorithm. .

Cruz and Barros [38] assumed that CFRP has a linear-elastic behavior and

neglecting the thickness of this composite material, the equilibrium of a CFRP of

length xd bonded to concrete can be given by the following expression (see Figure

2.35 and Figure 2.36)

61

dx

dtEx fff

2

where x is the bond stress acting on the contact surface between CFRP and epoxy

adhesive, and fE , ft and f are the Young�s modulus, the thickness and the strain

of the CFRP, respectively.

Fig. 2.35: Equilibrium of the CFRP [38]

Fig.2.36: Strain of the intervening materials of the bond region [38]

62

The quality of the local bond stress�slip relationship, s , has decisive

importance on the accuracy of this simulation. In the present work the local bond

stress�slip relationship is composed by the following two equations:

mm s

ss if mss

and

,

mm s

ss if mss

where m and ms are the bond strength and its corresponding slip, and and , are

parameters defining the shape of the curves.

To assure that peak pullout force and its corresponding slip obtained

numerically are similar (less than a tolerance of 1%) to the values registered

experimentally, the following method was used:

Step 1: fixing the parameters a and a0, the values of sm and sm of the best

fitting

and , , the values of ms and m of the best fitting were found

Step 2: using the values of ms and m obtained in the previous step, the values

of and , giving the best fitting were determined.

Figure 2.37 shows that the loaded end slip vs. pullout force

relationship obtained analytically (thick line) fits quite well the corresponding

experimental envelop (hatch).

63

Fig. 2.37: Loaded end slip versus pullout force relationship [38]

The error is the difference, in absolute value, between the areas

corresponding to the experimental and analytical curves. From these study, the

following observations can be pointed out:

The error of each series is quite acceptable.

A reasonable coefficient of variation was obtained in the bond strength.

Large scatter in the values of ms , and , was obtained.

Her [39] make the basic assumptions when she study about adhesively-

bonded lap joints as follows:

The shear stress in the adhesive layer do not vary through the thickness.

The longitudinal stresses in the adherends do not vary through the thickness.

The adherends and adhesive layer are linear elastic, and joint edge moment is

neglected.

64

Fig. 2.38: The free-body diagram of a single-lap joint [39]

Fig. 2.39: The free-body diagram of a double-lap joint [39]

For single-lap joint, the shear deformation in the adhesive layer as follows:

l

l

tEtE

tEtE

l

xP

ooii

ooii

cosh

sinh

sinh

cosh

2

While for double-lap joint :

2cosh

sinh

2

2

2sinh

cosh

4 l

l

tEtE

tEtE

l

xP

ooii

ooii

65

Where:

ooii

a

tEtE

G 12

oi TTP 2

and the constants iE and oE are the equivalent longitudinal moduli in upper and

lower adherends, respectively. oE and ot are the longitudinal displacement, strain,

Young's modulus, and thickness of the outer adherend, iE respectively and it are the

respective components relative to the inner adherend, aG is the shear modulus of

adhesive, and are the shear strain and thickness of the adhesive layer. l is the

length of the bonded region.

Fig. 2.40: The shear stress obtained by Eqs. (11) and FEM along the adhesive region

for a single-lap joint [39]

From these results one can observe:

The stress obtained by finite element method is higher than the analytical

solution. It may be due to the bending effect caused by the eccentric load

which has been ignored in the analytical solution.

66

The result of maximum shear stress in the analytical solution is occurred on

both the ends of overlap region. However, the maximum shear stress occurs

at a short distance away from the ends in the finite element result.

A simplified one-dimensional approach has been developed to model the

adhesive bonding for single-lap joint and double-lap joint. A simply analytical

solution is obtained, and compared with the two-dimensional finite element results.

Good agreement between these two results demonstrates that present approach can

provide a simple but accurate solution which is very useful in joint design.

The effects of varying parameters such as thickness of adhesive and

adherend, modulus of adhesive and adherend can be concluded as follows:

High stress concentration occurs on the free ends of adhesively bonding

region. The shear stress and transverse normal stress in the adhesive layer are

responsible for the initiation of the failure of the adhesively bonding joints.

Increase of the thickness of the adhesive layer, leads to the lower shear stress

in the adhesively bonding region. Thus, thicker adhesive layer is able to

improve

the strength of the adhesively bonding joint.

In the case of two different adherends connected by the adhesive joint, the

maximum shear stress occurs at the free end of adhesive region near to the

adherend with higher stiffness.

As the thickness of adherents is di�erent, the maximum shear stress occurs at

the free end of adhesive region near to the thinner adherend.

CHAPTER 3

RESEARCH METHODOLOGY

3.0 Introduction

In order to answer the research questions an equation is derived basic from

classical theories of Volkersen/ de Bruyne�s solution for double-lap. Then, from the

theory, the parameters will be varied to find out its influence to local bond stress.

The results of experimental works by Shukur [30] were compared to the theoretical

result in order to prove the validity of the equation.

3.1 The Parameters Effect

From the equation that will be derived, each parameter that is suspected effects

the local bond stress of the joint, will be verified to three different values at Other

parameters will be fixed to a constant value as shown in to make sure the effect to the

joint were caused by that parameter. Effect means the changes in maximum value of

the local bond stress and the effective bond length.


68

3.2 Bond Test for CFRP Plate-Epoxy-Concrete Specimen

Data and results studied by Shukur [30] will be compared to the theoretical

results in order to validate the equation.

The test was conducted onto three specimens of CFRP plate-epoxy-concrete

under pull-push loading configuration and all of the specimens were loaded starts

from 0 kN till it failed and the strain readings were taken each 5 kN of incremental.

but only the bond stress at load 40 kN will be compared with the theory at that

loading the load transfer from the CFRP plate to the concrete are most fairly linear

and occurs at almost uniform rate [30].

3.2.1 Experimental Details

The experimental set-up for the test referred to Swamy [15,22] as a standard

guidelines. These include the geometry of the specimen as shown in (Fig. 3.1) and

the test rig shown in (Fig. 3.2)

69

Fig 3.1: Instrumentation set-up onto CFRP Plate-epoxy-concrete prism specimen for

this study

Fig. 3.2: Standard test rig used by Swamy [15,22] for GFRP Plate-epoxy-concrete

pull-out test

70

3.2.2 Details of Test Materials

The Selfix Carbofibre Pultruded CFRP Plate type S, 50 mm wide, 1.4 mm

thick and 555 mm in length was bonded onto concrete prism surface using Selfix

Carbofibe two parts epoxy adhesive system with 2 GPa shear stress. The epoxy

thickness was 1mm for each part. The concrete prism was ad to have design

compressive strength of 40 MPa with 100 mm thickness. The surface of concrete

prisms was roughened using air tool hammer.

71

Fig. 3.3: Arrangement of pull-out test rig onto CFRP Plate-epoxy-concrete prism specimen

3.2.3 Determination of Bond Stress Characteristics

The local longitudinal CFRP Plate force that acted along the bond length was

made from the assumption that the bonding between CFRP Plate-adhesive-concrete

was perfect, therefore the following local force, Fi and local bond stress, ô equations

were developed through local strain, åi testing data [15,22,27]. Based on statics

72

equilibrium force analysis on specimen joint element, dx shows in Fig. 3.4, the

analysis was done as follows;

Ó Fx = 0

[(ó + dó) (A) � (ó) (A)] � ô (dA) = 0

dó (w x t) = ô (dx x w)

where,

A = (t x w) is CFRP Plate cross sectional area (mm2)

ó is normal stress onto CFRP Plate cross sectional area (MPa)

therefore,

CFRP Plate local force at i location, Fcfrp,i = dó (w x t)

= Ecfrp.åi (w x t) [3.7]

and, finally the local bond stress at location i - j,

ôi-j = (Fcfrp,i � Fcfrp,j)/(dx x w) [3.8]

where,

w = CFRP Plate width (mm)

t = CFRP Plate thickness (mm)

ÄLi-j = dx = the distance between two consecutive strain gauges

(Fcfrp,i � Fcfrp,j) = ÄF i-j = the variation of the local longitudinal force

dx

Fcfrp

Fcon

dx

(ó + dó)A óA

ô (dA) adhesive

CFRP Plate

Fig. 3.4: Elementary force analysis

73

Ecfrp = CFRP Plate Young�s Modulus (GPa)

åi/cfrp = CFRP Plate local strain at location i (µå)

i. Local Force Transfer, Fi

Local CFRP Plate force at any location along the bond length can be

determined by an equation (3.7).

ii. Local Bond Stress, ôL

The local bond stress distributions along the bonded length can be

determined by assuming a linear variation of the longitudinal force along the

CFRP Plate between two consecutive strain gauge locations. By referring to two

strain readings åi/cfrp and åj/cfrp at position i and j, the plate thickness tp, its elastic

Young modulus Ecfrp, and the distance ÄLi-j between two consecutive strain

gauges positions, the local average bond stress between two consecutive gauges

position can be determined by an equation (3.8).

iii. Average Bond Stress, ôavg

An average bond stress for double lap joint system can be established and

determined by the following equation;

Average bond stress, av = 2

P

A

= 2( )B

P

b L [3.9]

where,

Average bond strength, av ,s = max

2

P

A

= max

2( )B

P

b L [3.10]

where,

P = nominal load (N)

Pmax = Ultimate load (N)

74

A = Bond area (b x LB) (mm²)

b = Width of CFRP plate (mm)

LB = Bond length (mm)

3.3 Conclusion of Research Methodology

For the comparison between theoretical and experimental, the joint will be

assumed to experience perfect bonding which the bond stress at the CFRP is equals

to adhesive shear stress. The derived equation wills valid if the result shows a good

agreement with experimental results. If validated, the parameters effect on local bond

stress distribution can be determined from the equation.

CHAPTER 4

DEVELOPMENT OF BOND GOVERNING EQUATION FOR

FRP-CONCRETE BONDED SYSTEM

4.0 Introduction

This mathematical governing equation is derived from the classical theories of

Volkersen/ de Bruyne�s solution for double-lap joint system [41]. The assumptions

made for the equation are stated as follows:

i. adherend shear deformations are neglected

ii. linear shear stress distributions through the thickness of the adherends

iii. thickness of adherend and adhesive were assumed constant along the bond

length


76

4.1 Theoretical Analysis on Tension-Compression CFRP Plate-Concrete

Prism Bonded System

The geometry of the CFRP plate-epoxy- concrete under pull push loading

configuration is shown in Fig 4.1. The length of the overlap is L. The thicknesses of

the outer and inner adherends are ot and it respectively while at is the thickness of

adhesive. The elementary force diagram is shown in Fig 2.32 (b).

Fig. 4.1: Geometry and material parameters of the CFRP plate-epoxy-concrete prism

ti

dx

ot

0

ta

2

T

2

T

T

L

x

77

The assumption of linear shear and strain distribution through out the

thickness of the adherends is shown in Fig 4.2.

From the elementary force diagram in Fig 2.32 (b), the average adhesive shear stress

over the bond line is:

L

Tdx

L

L

aave 2

10

[1]

where a is adhesive shear stress. Following the notation in Fig.4.2, the equilibrium

equations for the basic elements of the outer and inner adherends can be written as

follows:

ve F = 0

0 oaoo TdxdTT

00 adx

dT [2]

02 dxTdTT aiii dxc

02 ai

dx

dT [3]

dxa

To +dTo To 0o

ao

to U Uo

aoU y�

Ti +dTi

Ti

dxa

2

t i

0i

ai aiU

iaU

y��

Fig. 4.2: Linear shear stress, and strain distribution through the thickness of adherends

78

Therefore, the adherend shear o for the outer adherend and i for the inner adherend

can be expressed as:

'yto

ao

[4a]

and

iai t

y ''21 [4b]

where y� and y�� are the local coordinates system with the origin at the top surface of

the outer and inner adherend. Eqns. [4a] and [4b] are based on zero shear stresses at

the top surface of the outer adherend (i.e. at y� = 0) and at the centre of the inner

adherend (i.e. at y�� = ti/2), and o = a at y�= to and ai at y��= 0. Then with a

linear material constitutive relationship the adherend shear strain o for the outer

adherend and i for the inner adherend are written as:

'ytG oo

aO

[5a]

and

ii

ai t

y

G

''21

[5b]

The longitudinal displacement functions uo for the outer adherend and ui for the inner

adherend are given by:

2'

0'

2'�'�' y

tGuydyuyu

o

aos

y

ooso

[6a]

and

i

y

i

aaiiaii t

yy

Guydyuyu

2

0

''''''�''�''

'' [6b]

79

where osu represents the displacement at the top surface of the outer adherend and

aiu is the adhesive displacements at the interface between the adhesive and inner

adherend. Note that, due to the perfect bonding of the joints, the displacements are

continuous at the interfaces between the adhesive and adherends. As a result, the aiu

should be equivalent to the inner adherend displacement at the interface and aou (the

adhesive displacement at the interface between the adhesive and outer adherend)

should be the same as the outer adherend displacement at the interface. Based on eqs.

[6a], the aou can be expressed as:

o

oaosooao G

tutyuu

2'

[7]

Using eqs. [7] eqs. [6a] can be rewritten as:

o

oa

oo

aaoo G

ty

tGuyu

2'

2' 2

[6c]

The longitudinal resultant forces To and Ti for the outer and inner adherend,

respectively, are:

ot

oo dyyT0

'' [8a]

and

2

0''''2

t

ii dyyT [8b]

where o and i are longitudinal normal stress for the outer and inner adherends,

respectively. By transforming these stresses into functions of displacement and

substituting eqs. [6b] and eqs. [6c] into the displacements, eqs. [8a] and eqs. [8b] can

be rewritten as:

80

dxG

dt

dx

dutEdy

dx

duET

o

aoaooo

to

oo

o

3'

0

[9a]

and

dxG

dt

dx

dutEdy

dx

duET

i

aiaiii

ti

ii

i

6''2

2/

0

[9b]

The adhesive shear strain a is simply defined as:

aoaia

a uut

1 [10]

The adhesive shear stress can be written as:

aoaia

aa uu

t

G [11]

By differentiating eqs. [11] with respect to x, the equation becomes:

dx

du

dx

du

t

G

dx

d aoai

a

aa [12]

Substituting eqs. [9a] and [9b] into eqs. [12] leads to:

dx

d

G

t

G

t

tE

T

tE

T

t

G

dx

d a

o

o

i

i

oo

o

ii

i

a

aa 36

[13]

By differentiating eqn. [13] with respect to x and substituting eqn. [2] and [3] into the

differential equation, the equation becomes:

2

2

2

2

36

2

dx

d

G

t

G

t

tEtEt

G

dx

d a

o

o

i

i

oo

a

ii

a

a

aa [14]

81

By rearranging eqn. [14], one obtains:

aooiia

aa

o

o

i

i

a

a

tEtEt

G

dx

d

G

t

G

t

t

G

12

361

2

2

[15]

which governs the adhesive shear stress. It can be rewritten as:

022

2

aa

dx

d

or 0'' 2 aa [16]

with

o

o

i

i

a

a

ooiia

a

G

t

G

t

t

G

tEtEt

G

361

12

2 [17]

The parameter of is redefined by and and is given as follows;

222 [18]

where

ooiia

a

tEtEt

G 122 [19]

1

2

361

o

o

i

i

a

a

G

t

G

t

t

G [20]

82

Assume the exact solution is y = emx

Therefore y� = memx

y�� = m2emx

so, m2- 02 [21]

m = [22]

The general solution for the governing eqn. [16] is:

xxa BeAe [23]

From Fig. 2.32 (b), CFRP plate-concrete prism will experience 2 times of a value

for each layer for bond length of L, the eqn. [23] becomes:

22

xx

a BeAe

[24]

The appropriate boundary conditions are stated as follows;

avgiavgo LTTLT

T 2,2

at x= 0 [25a]

and

LxatTTT io 0,0 [25b]

Based on eqn. [24] and eqn. [25a],

BAavg [26]

83

Based on eqn. [24] and eqn. [25b],

220LL

BeAe

LBeA [27]

From eqn. [26] and [27] BBe Lavg

1 L

avg

eB

[28]

From eqn. [27] and [28] 1

L

Lavg

e

eA

[29]

Therefore, the final mathematical governing equation�s:

22

21

1 xL

x

La eeeL

T

e

[30]

with condition Lx 0

This equation is not limited to any value of local bond stress. But, the fact is

if the local bond stress exceed the bond strength (shear, compressive and tensile

strength of those bonded materials), the bonding will experience micro cracking and

finally fail when the direct tensile strength exceeding the value of pull-off strength.

The bond failure is normally occurs at adhesive-concrete interface as concrete

tensile shear strength is very low compared to tensile strength of CFRP and shear

strength of epoxy [23, 30]. Then, a limitation can be estimated for the a as follows:

a Concrete tensile shear strength

84

The equation that relates the concrete Young�s modulus and concrete�s

compressive strength as follows [47]:

cE = 5.061073.4 cf

CHAPTER 5

ANALYSIS AND DISCUSSION

5.0 Introduction

In order to validate the equation, the theoretical local bond stress value was

compared to the experimental value. Then, the parametric study was done to study

the effects of material properties to local bond stress distribution.

5.1 Equation validation

For the validation of the equation, local bond stress of three specimens

(BOSTUS 1, BOSTUS 2, BOSTUS 3) at load 10 kN, 40 kN and 60 kN which

studied by Shukur [30] were compared to the results from theoretical value. The

material and physical properties of the specimens is shown in Table 5.1


86

Table 5.1: Material and physical properties of testing specimens

Parameter Value

Epoxy shear modulus, aG (GPa) 2.7

CFRP Young�s modulus, CFRPE (GPa) 135

Concrete Young�s modulus, concreteE

(GPa)

30

Thickness of epoxy, at (mm) 1.5

Thickness of CFRP CFRPt (mm) 1.4

Thickness of concrete, concretet (mm) 100

Bond length, L (mm) 200

Local Bond Stress vs Bond Length (Theoretical and experimental value)

-1

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

Theory

BOSTUS 1

BOSTUS 2

BOSTUS 3

Load = 10 kN

Fig. 5.1: Theoretical and experimental local bond stress for BOSTUS specimens at

load 10 kN

Fig 5.1 shows the comparison between theoretical and experimental values

for BOSTUS specimens. From the graph, it can be observed that the maximum

theoretical local bond stress at loaded end for theoretical is 5.1 MPa, which is 21%

higher than BOSTUS 1, 27% higher than BOSTUS 2 and 31% higher than BOSTUS

87

3. At 15 mm bond length, the theoretical local bond stress shows the value of 1.6

MPa, while the local bond stress of BOSTUS 1 is 1.99 MPa. The difference is about

20%.The value of local bond stress for BOSTUS 2 and BOSTUS 3 at 15 mm bond

length are 18% and 16% higher respectively than the theoretical value.

For 35 mm bond length, the theoretical value is 0.2 MPa, while BOSTUS 1 is

33% higher; 0.3 MPa, and BOSTUS 3 is 20% higher; 0.25 MPa. The value for

BOSTUS 2 is the same with the theoretical value; 0.2 MPa. Both of theoretical and

experimental effective bond length is about 105 mm from the loaded end, which

means that the tensile force from CFRP plate was transferred to the concrete.


-5

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

Theory

BOSTUS 1

BOSTUS 2

BOSTUS 3

Load = 40 kN


load 40 kN

From Fig. 5.2, it can be observed that in the experiment, debonding has

occurred at the loaded end and the maximum local bond stress has shifted to 15 mm

bond length at 40 kN load level.

At 15 mm bond length, the theoretical local bond stress shows the value of

10.71 MPa, while the local bond stress of BOSTUS 1 is 12.4 MPa. The difference is

88

about 16%.The value of local bond stress for BOSTUS 2 and BOSTUS 3 at 15 mm

bond length are 10% and 12% higher respectively than the theoretical value.

For 35 mm bond length, the theoretical value is 3.97 MPa, while BOSTUS 1

is 17% lower; 3.31 MPa, and BOSTUS 2 is 18% lower; 3.27 MPa. The value for

BOSTUS 3 is 4.01 MPa, 1% higher than theoretical value. Both of theoretical and

experimental effective bond length is about 105 mm from the loaded end.

Local Bond Stress vs. Bond Length(Theoretical and experimental)

-5

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140 160 180

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

Theory

BOSTUS 1

BOSTUS 2

BOSTUS 3

Load = 60 kN


load 60 kN

Due to the growth of the crack, the maximum local bond stress has shifted to

35 mm bond length for 60 kN load level in the experiment. From Fig. 5.3, it can be

observed that experimental local bond stress values show very big differences with

the theoretical.

89

5.2 Discussion of Equation Validation

From Figs. 5.1 to 5.3, it can be observed that the equation only valid for low

and medium load level. The acceptable range of error between the theory and

experiment actually happened due to assumption that were made when the derivation

of the equation. The assumption of the thickness of adherends and adhesive were

assumed constant along the bond length which practically impossible in specimen�s

preparation also caused the error in determined the local bond stress. The error also

exists due to the assumption that all of the materials are fully linear elastic even the

concrete has a plastic region.

The higher value of maximum local bond stress for theory at loaded end

possibly because the adherends shear deformation and peeling effect was neglected

in the calculation. As we can observe from the graph, the load was transferred further

in experiment than in theoretical, which caused the experimental local bond stress at

point 15 mm and 35 mm higher than the theoretical. This happened due to a little

bond slip that happened between the concrete and CFRP plate.

The strains behaviour for BOSTUS specimens may also be affected by

roughness and irregularities of concrete surface profiles as shown in Fig. 5.4 that

reflects the strains distribution characteristics along the bond length. The mechanical

inter-locking between adhesive and adherends contributed a big effect in strain

distribution [30].

90

Fig. 5.4: Roughness and irregularities of concrete bond surface

5.3 Parametric study

From the equation that has been developed from previous chapter, the

parameters that influence the stress distributions in the adhesively bonding region

(interface) can be classified into two categories.

One is called mechanical properties which include the Modulus Young of

adherends and the shear modulus of the adhesive. The other is called physical

properties which include the thickness of the adhesive layer, the thickness of the

adherends and the bond length.

To find out the effect of those parameters, the local bond stress at load 20 kN

was calculated using the equation. Those parameters were varied with three different

values as shown in Table 5.2 while other parameters were fixed to be constant .Table

5.3 shows the parameters matrix.

91

Table 5.2: Three different parameter�s value

Parameter Value 1 Value 2 Value 3

adhesiveG (GPa) 1.5 2.0 2.5

outerE (GPa) 100 150 200

innerE (GPa) 20 30 40

adhersivet (mm) 0.5 1 1.5

outert (mm) 1 1.5 2

Bond length, L

(mm)

100 200 250

Table 5.3: Influential parameters matrix

Parameter adhesiveG

(GPa)

outerE

(GPa)

innerE

(GPa)

adhersivet

(mm)

outert

(mm)

innert

(mm)

Bond

length

(mm)

adhesiveG (GPa) varies 150 30 1 1.5 100 200

outerE (GPa) 2.0 varies 30 1 1.5 100 200

innerE (GPa) 2.0 150 varies 1 1.5 100 200

adhersivet (mm) 2.0 150 30 varies 1.5 100 200

outert (mm) 2.0 150 30 1 varies 100 200

Bond length,

L (mm)

2.0 150 30 1 1.5 100 varies

92

5.3.1 The Effect of CFRP Plate Young�s Modulus

From the graph shows in Fig. 5.5, it can be seen that local bond stress at 20

kN load level increased when the CFRP Young�s modulus increased. The maximum

local bond stress for 140 GPa CFRP Young�s modulus is 7.07 MPa, 19.8% higher

than local bond stress for 100 GPa CFRP Young�s modulus, 5.9 MPa. While for 180

GPa CFRP Young�s modulus, the local bond stress is 7.88 MPa, 11.5% higher than

140 GPa CFRP Young�s modulus.

At 20 mm bond length, local bond stress for 140 GPa and 180 GPa CFRP

Young�s modulus are 2.5 MPa and 3.11 MPa, 43.7% and 78.7% higher, respectively

to the 100 GPa CFRP Young�s modulus

For 40 mm bond length, local bond stress for 140 GPa CFRP Young�s

modulus is 0.88 MPa, 72.5% higher than local bond stress for 100 GPa CFRP

Young�s modulus, 0.51 MPa while for 180 GPa CFRP Young�s modulus, the local

bond stress was 1.22 MPa, 38.6% higher than 140 GPa CFRP Young�s modulus.

The effective bond length of local bond stress for 100 GPa CFRP Young�s

modulus was only about 80 mm, while 100 mm and 120 mm for 150 GPa and 200

GPa CFRP Young�s modulus. It can be seen that the higher CFRP Young�s modulus,

the longer bond length required to transfer the force from concrete to CFRP.

93

Local Bond Stress vs Bond Length(Effect of CFRP Young's modulus )

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

E=100GPa

E=150GPa

E=200GPa

Ga =2.0GPaEc =30GPata =1mmta =1.5mmtconcrete =100mmLoad =20kN

Fig 5.5: Local bond stress with different CFRP Young�s modulus

5.3.2 The Effect of CFRP Plate Thickness

Fig. 5.6 shows that the local bond stress at 20 kN load level is higher for the

thicker CFRP plate thickness. At loaded end, the maximum local bond stress for 1.5

mm CFRP plate thickness was 7.07 MPa, 19.8% higher than local bond stress for 1

mm thickness, 5.9 MPa. For 2 mm thickness, the local bond stress was 7.85 MPa,

11% higher than 1.5 mm thickness.

The local bond stress for 1.5 mm and 2 mm thickness were 2.5 MPa and 3.08

MPa for 20 mm bond length, 43.7% and 77% higher, respectively to the 1 mm

thickness. For 40 mm bond length, local bond stress for 1.5 mm thickness was 0.88

MPa, 72.5% higher than 1 mm thickness, 0.51 MPa while for 2 mm thickness; the

local bond stress was 1.21 MPa, 37.5% higher than 1.5 mm thickness.

94

The effective bond length for was about 80 mm for 1 mm CFRP plate

thickness and it increased to 100 mm and 120 mm for 1.5 mm and 2.0 mm CFRP

plate thickness.

Local Bond Stress vs Bond Length(Effect of CFRP thickness )

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

t=1mm

t=1.5mm

t=2mm

Ga =2.0GPaEc =30GPaEcFRP =150GPata =1mmtconcrete =100mmLoad =20kN

Fig. 5.6: Local bond stress with different CFRP thickness

5.3.3 The Effect of Concrete Young�s Modulus

Fig. 5.7 shows the variation of concrete Young�s modulus on local bond

stress at 20 kN load level. The graph indicates that the concrete Young�s modulus

does not significantly affect the interface shear stress values. However, it can be seen

that there is a marginal tendency of developing a higher interface shear stress value

when the concrete Young�s modulus is increased.

95

At loaded end, maximum local bond stress for 20 GPa concrete Young�s

modulus was 6.86 MPa, and it increased 3% to 7.07 MPa, 4% to 7.14% when the

concrete Young modulus increased to 30 GPa and 40 GPa.

At bond length 20 mm, local bond stress for 40 GPa concrete Young�s

modulus was 2.55 MPa, 8.5% higher than 20 GPa concrete Young�s modulus and

2% higher than 30 GPa concrete Young�s modulus.

For 40 mm bond length, local bond stress for 30 GPa concrete Young�s

modulus was 0.88 MPa, 8.6% higher than the local bond stress for 20 GPa concrete

Young�s modulus, 0.81 MPa while for 40 GPa concrete Young�s modulus, the local

bond stress was 0.91 MPa, 3.4% higher than 30 GPa concrete Young�s modulus.

It also can be observed from the graph that concrete Young�s modulus does

not significantly affect the effective bond length.

Fig. 5.7: Local bond stress with different concrete Young�s modulus

Local Bond Stress vs Bond Length(Effect of Concrete Young's Modulus)

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

E=20GPa(fc=18MPa)

E=30GPa(fc=40MPa)

E=40GPa(fc=70MPa)

Ga =2.0GPaECFRP =150GPata =1mmtCFRP =1.5mmtconcrete =100mmLoad =20kN

96

5.3.4 The Effect of Adhesive Shear Modulus

As can be seen from Fig. 5.8, the local bond stress at 20 kN load level

increased when the adhesive shear modulus increased. The maximum local bond

stress for 1.5 GPa adhesive shear modulus was 6.08 MPa. When the adhesive shear

modulus increased to 2.0 GPa, the maximum local bond stress increased 16.2 % to

7.07 GPa. For 2.5 GPa adhesive shear modulus, the maximum local bond stress was

8.36 MPa, 18.2 % higher than 2.0 GPa adhesive shear modulus.

Local bond stress at 20 mm bond length for 2.0 Pa and 2.5 GPa adhesive

shear modulus were 2.5 MPa and 3.50 MPa, 35.1% and 89.2% higher; respectively

to the 1.5 GPa adhesive shear modulus.

For 40 mm bond length, local bond stress for 2.0 GPa adhesive shear

modulus was 0.88 MPa, 57.1% higher than 1.5 GPa adhesive shear modulus, 0.56

MPa while for 2.5 GPa adhesive shear modulus; the local bond stress was 1.46 MPa,

65.9% higher than 2.0 GPa shear modulus. The effective bond length increased about

25% from 80 mm to 100 mm when the adhesive shear modulus increased from 1.5

GPa to 2.0 GPa and 20 % to 120 mm when the adhesive shear modulus increased to

2.5 GPa from 2.0 GPa.

97

Fig. 5.8: Local bond stress with different adhesive shear modulus

5.3.5 The Effect of Adhesive Thickness

Fig. 5.9 shows that the local bond stress at 20 kN load level is lower for the

thicker adhesive thickness. Maximum local bond stress for 1 mm adhesive thickness

was 7.07 MPa, 11.8% lower than local bond stress for 0.5 mm thickness, 8.02 MPa.

For 1.5 mm adhesive thickness, the local bond stress was 5.76 MPa, 18.5% lower

than 1 mm thickness. At 20 mm bond length, local bond stress for 1 mm and 1.5 mm

adhesive thickness were 2.5 MPa and 1.6 MPa, 22.4% and 48.4% lower, respectively

to the 0.5 mm thickness.

For 40 mm bond length, local bond stress for 1 mm thickness was 0.88 MPa,

31.8% lower than 0.5 mm thickness, 1.29 MPa while for 1.5 mm thickness, the local

Local Bond Stress vs Bond Length(Effect of Adhesive Shear Modulus )

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

G=1.5GPa

G= 2.0GPa

G= 2.5GPa

EC = 30GPaECFRP =150GPata =1mmtCFRP =1.5mmtconcrete =100mmLoad =20kN

98

bond stress was 0.48 MPa, and 45.5 % lower than 1 mm thickness. The curves show

that the effective bond length increased when the thickness of adhesive is increased.

Fig. 5.9: Local bond stress with different adhesive thickness

5.3.6 The Effect of Bond Length

Fig. 5.10 shows that the bond length has no effect to the local bond stress as

long as it is exceeding the effective bond length that is required. When the bond

length is 100 mm, the maximum local bond stress is about 8.01 MPa, 13% higher

than local bond stress with 200 mm and 250 mm bond length. It occurs due to the

effective bond length required at 20 kN load level is about 105 mm to transfer the

force from concrete to CFRP.

Local Bond Stress vs Bond Length(Effect of Adhesive Thickness )

0

1

2

3

4

5

6

7

8

9

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

t=0.5mm

t= 1mm

t= 1.5mm

Ga =2.0GPaEC = 30GPaECFRP =150GPatCFRP =1.5mmtconcrete =100mmLoad =20kN

99

Local Bond Stress vs Bond Length(Effect of Bond Length )

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

L=100 mm

L=200 mm

L=250 mm

Ga =2.0GPaECFRP =150GPata =1mmta =1.5mmtconcrete =100mmLoad =20kN

Fig. 5.10: Local bond stress with different bond length

5.4 Conclusion of Parametric Study

The local bond stress increased when the thickness of CFRP increased. This

is due to the assumption made that the linear shear stress distribution through the

thickness of adherend. The shear stress was assumed as zero at the top surface and

linearly increased with the increment of thickness untill it is equals to adhesive shear

stress at the adherend-adhesive interface.

However, the local bond stress decreased when the thickness of adhesive

increased. It is because, the adhesive shear strain, is a function of adhesive

displacement, u and thickness, at .

100

When the thickness of adhesive increased while the adhesive displacement

valu is constant, the adhesive shear strain will decreased and cause the reduction of

local bond stress.

The less-stiffness of adhesive and adherends will reduce the local bond stress

along the bond length compared to stiffer one. Assumes a point at the loaded end

(named as point i) and another point with certain distance from the loaded end

(named as point j ).

For the less stiff of materials, the jiF value is smaller than the stiffer one

for the constant of jiA because the force at point j almost equal with the force at

point i.

The concrete Young�s modulus does not significantly effect the local bond

stress due to small value compared to CFRP plate Young�s modulus.

CHAPTER 6

CONCLUSION AND SUGGESTION

6.1 Conclusion

The local bond stress calculated from the equation has show good agreement

with the experiment�s for low and medium load level and the error still in acceptable

range. Fig 6.1 shows the exponential line between theoretical and experimental

values at 10 kN load level.

The value of maximum local bond stress from the theory can be used as a

guideline for a safe design of the joint. It�s because, the displacement experienced

were not same from one joint with another joint. So, the safest way is to assume that

no bond slip occur which will produce the joint experienced the highest possible

bond stress.


102


y = 4.7242e-0.0707x

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140 160 180 200

Bond length, x (mm)

Bo

nd

str

ess,

Tau

(M

Pa)

Theory

BOSTUS 1

BOSTUS 2

BOSTUS 3

Expon.(BOSTUS 3)Expon.(BOSTUS 2)Expon.(Theory)Expon.(BOSTUS 1)

Load = 10 kN

Fig. 6.1: Exponential line of theoretical and experimental local bond stress at 10 kN

load level

From the parametric study, we can conclude that:

Maximum local bond stress and effective bond length increase when

the CFRP Young modulus and thickness increase.

Maximum local bond stress and effective bond length increase when

the adhesive shear modulus increases.

Maximum local bond stress and effective bond length decrease when

the adhesive thickness increases.

Concrete Young�s modulus and compressive strength doesn�t

significantly effect the local bond stress distribution.

The bond length must exceed the required effective bond length to

transfer the load from concrete to CFRP.

For the safety of joint design, a few things must be considered:

Less-stiffness of adherends and adhesive

Low adherends thickness

High adhesive thickness

103

6.2 Suggestion For Future Study

The equation that has been derived should have a boundary condition based

on all materials strength. The CFRP plate-epoxy-concrete bonded system usually

experienced failure at concrete interface due to its lowest shear strength compared to

CFRP plate and epoxy. The boundary condition should able to predict the limitation

of maximum local bond stress for the joint before it failed. The equation also must be

developed to meet the need for cyclic loading

The effects of long term environmental exposure to the joint also can be

determined by this equation as long as all of the material properties of the joint are

known.

104

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110

APPENDIX A

( Data of theoretical and experimental local bond stress )


111

Data for Fig.5.1

Bond length,

x (mm)

BOSTUS 1

Local Bond

Stress, a (MPa)

BOSTUS 2

Local Bond

Stress, a (MPa)

BOSTUS 3

Local Bond

Stress, a (MPa)

Theoritical

Local Bond

Stress,

a (MPa)

0-15 4 3.7 3.6 5.1

15-35 1.99 1.97 1.9 1.6

35-65 0.3 0.2 0.25 0.2

65-105 0.1 0.1 0.2 0.1

105-155 0.001 0.001 0.001 0.001

155-200 0.0001 0.0001 0.0001 0.0001

Data for Fig.5.2

Bond length,

x (mm)

BOSTUS 1

Local Bond

Stress, a (MPa)

BOSTUS 2

Local Bond

Stress, a (MPa)

BOSTUS 3

Local Bond

Stress, a (MPa)

Theoritical

Local Bond

Stress,

a (MPa)

0-15 27

15-35 12.4 11.8 12 10.7

35-65 3.31 3.27 4.01 3.97

65-105 0.1 0.1 0.2 0.1

105-155 0.001 0.001 0.001 0.001

155-200 0.0001 0.0001 0.0001 0.0001

Data for Fig.5.3

Bond length,

x (mm)

BOSTUS 1

Local Bond

Stress, a (MPa)

BOSTUS 2

Local Bond

Stress, a (MPa)

BOSTUS 3

Local Bond

Stress, a (MPa)

Theoritical

Local Bond

Stress,

a (MPa)

0-15 36

15-35 9.8

35-65 7.05 7.01 5.01 2.1

65-105 6.99 6.98 4.98 0.1

105-155 2.01 2.0 2.01 0.001


112

155-200 0.7 0.7 0.71 0.0001

113

APPENDIX B

( Data of parametric study)


114

Data for Fig.5.5 Bond length,

x(mm)

Local bond stress

(MPa)

[ CFRPE =100 GPa]

Local bond stress

(MPa)

[ CFRPE =150 GPa]

Local bond stress

(MPa)

[ CFRPE =200 GPa]

0-20 5.90 7.07 7.88

20-40 1.74 2.50 3.11

40-60 0.51 0.88 1.22

60-80 0.15 0.31 0.48

80-100 0.04 0.11 0.19

Data for Fig.5.6

Bond length,

x(mm)

Local bond stress

(MPa)

[ CFRPt =1mm]

Local bond stress

(MPa)

[ CFRPt =1.5mm]

Local bond stress

(MPa)

[ CFRPt =2.0mm]

0-20 5.90 7.07 7.85

20-40 1.74 2.50 3.08

40-60 0.51 0.88 1.21

60-80 0.15 0.31 0.48

80-100 0.04 0.11 0.19

Data for Fig.5.7

Bond length,

x(mm)

Local bond stress

(MPa)

[ concreteE =20 GPa]

Local bond stress

(MPa)


Local bond stress

(MPa)


0-20 6.86 7.07 7.14

20-40 2.35 2.50 2.55

40-60 0.81 0.88 0. 91

60-80 0.28 0.31 0.32

80-100 0.09 0.11 0.12


115

Data for Fig.5.8

Bond length,

x(mm)

Local bond stress

(MPa)

[ adhesiveG =1.5GPa]

Local bond stress

(MPa)

[ adhesiveG =2.0GPa]

Local bond stress

(MPa)

[ adhesiveG =2.5 GPa]

0-20 6.08 7.07 8.36

20-40 1.85 2.50 3.50

40-60 0.56 0.88 1.46

60-80 0.17 0.31 0.61

80-100 0.05 0.11 0.26

Data for Fig.5.9

Bond length,

x(mm)

Local bond stress

(MPa)

[ adhersivet =0.5mm]

Local bond stress

(MPa)

[ adhersivet =1mm]

Local bond stress

(MPa)

[ adhersivet =1.5mm]

0-20 8.02 7.07 5.76

20-40 3.22 2.50 1.66

40-60 1.29 0.88 0.48

60-80 0.52 0.31 0.14

80-100 0.21 0.11 0.04

Data for Fig.5.10

Bond length,

x(mm)

Local bond stress

(MPa)

[L=100mm]

Local bond stress

(MPa)

[L=200mm]

Local bond stress

(MPa)

[L=250mm]

0-20 8.01 7.07 7.07

20-40 3.01 2.50 2.50

40-60 1.29 0.88 0.88

60-80 0.52 0.31 0.31

80-100 0.21 0.11 0.11

116

APPENDIX C

( Determine local bond stress from governing equation )

117

Material properties:

t,inner 0.1 t,outer 0.0014

t,adhesive 0.0015 E,inner 3.00E+10 E,outer 1.35E+11

G,adhesive 2.70E+09

Determine value from material properties:

m n l

Eo*to 1/(Eo*to) Ei*ti 2/(Ei*ti) m+n Ga/ta l(m-n)

1.89E+08 5.29E-09 3.00E+09 6.67E-

10 5.96E-

09 3.86E+12 2.30E+04 151.5902

Determine the local bond stress for 20 kN load level:

x L' T/2L' exp- L x/2 Exp (- x/2) exp( x/2) m a

0 0.02 20000000 6.13252E-13 0 1 1 1 20000000

0.02 0.02 20000000 6.13252E-13 1.52 0.218712 4.572225 0.218712 4374238

0.04 0.02 20000000 6.13252E-13 3.04 0.047835 20.90524 0.047835 956697.8

0.06 0.02 20000000 6.13252E-13 4.56 0.010462 95.58348 0.010462 209241.2

0.08 0.02 20000000 6.13252E-13 6.08 0.002288 437.0292 0.002288 45763.53

0.1 0.02 20000000 6.13252E-13 7.6 0.0005 1998.196 0.0005 10009

0.14 0.02 20000000 6.13252E-13 10.64 2.39E-05 41772.77 2.39E-05 478.2684

0.16 0.02 20000000 6.13252E-13 12.16 5.24E-06 190994.5 5.12E-06 102.3725


Documents

BORANG PENGESAHAN STATUS TESIS€¦ · 2.5.4 Joint Geometry Effect on Joint Strength 45 2.5.5 Elastic Properties and Deformation 47 2.6 Double Lap Joint 48 2.7 Surface Treatments