PSZ 19:16 (Pind. 1/97)

Universiti Teknologi Malaysia

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).

BORANG PENGESAHAN STATUS TESIS♦ JUDUL: BOND TRANSFER LENGTH AND BOND STRESS ANALYSIS OF CFRP-

EPOXY-CONCRETE DOUBLE LAP JOINT

SESI PENGAJIAN: 2005/2006 Saya ANANDA RAJ A/L SADACHARAM_____________

(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

__________________________________

(TANDATANGAN PENULIS)

Disahkan oleh

_____________________________________

(TANDATANGAN PENYELIA)

DR. YOB SAED BIN ISMAIL

Alamat Tetap:

G-2-4, TAMAN BUKIT JAMBUL,

11900, BAYAN LEPAS,

PULAU PINANG

____________________________________

(TANDATANGAN PENYELIA)

EN. SHUKUR ABU HASSAN

Tarikh: DISEMBER 2005 Tarikh: DISEMBER 2005

UTM(PS)-1/02

Fakulti Kejuruteraan Mekanikal

Universiti Teknologi Malaysia

PENGESAHAN PENYEDIAAN SALINAN E-THESIS

Judul tesis : BOND TRANSFER LENGTH AND BOND STRESS ANALYSIS OF

CFRP-EPOXY-CONCRETE DOUBLE LAP JOINT

Ijazah: SARJANA MUDA KEJURUTERAAN MEKANIKAL

Fakulti: KEJURUTERAAN MEKANIKAL

Sesi Pengajian: 2005/2006

Saya ANANDA RAJ A/L SADACHARAM__________________________

(HURUF BESAR)

No Kad Pengenalan 820909-07-5631 __ mengaku telah menyediakan salinan e-thesis sama seperti tesis

asal yang telah diluluskan oleh panel pemeriksa dan mengikut panduan Tesis dan Disertasi Elektronik

(TDE), Sekolah Pengajian Siswazah, Universiti Teknologi Malaysia, November 2002.

……………………….. ……………………………..

( Tandatangan Pelajar ) (Tandatangan penyelia sebagai saksi)

Nama Penyelia: DR.YOB SAE ISMAIL

Alamat tetap

G-2-4, Taman Bukit Jambul, ……………………………..

11900, BayanLepas, PulauPinang (Tandatangan penyelia sebagai saksi)

Nama Penyelia: EN.SHUKUR ABU HASSAN

Fakulti: Kejuruteraan Mekanikal Tarikh: DISEMBER 2005 Tarikh: DISEMBER 2005

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

CD.

i

“ We hereby declare that we have read this thesis and in our

opinion this thesis is sufficient in terms of scope and quality for the

award of the Bachelor of Mechanical Engineering”

Signature : ____________________

Name Of Supervisor : Dr. Yob Saed bin Ismail

Date : December

Signature : ______________________

Name of Supervisor : En. Shukur bin Abu Hassan

Date : December

ii

BOND TRANSFER LENGTH AND BOND STRESS ANALYSIS OF CFRP-

EPOXY-CONCRETE DOUBLE LAP JOINT

ANANDA RAJ A/L SADACHARAM

A thesis is submitted in fulfillment of the requirements for the award of

Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

Universiti Teknologi Malaysia

DECEMBER 2005

iii

“ I declare that this thesis entitled ‘ Bond Transfer Length and Bond Stress Analysis of

CFRP-epoxy-concrete double lap joint’ is the result of my own research except

as cited in the references. This thesis has not been accepted for any degree and is not

concurrently in candidature of any other degree”

Signature :

Name : Ananda Raj A/L Sadacharam

Date :

iv

Specially dedicated to my beloved father, mother, my lovely sister, my

loving grandmother as well as my relatives . Thank you for your love,

support, advice, all the positive encouragement that was given and the

most important thing is the understanding and patience that they showed

to me all this while,

You will always be my highest inspiration……

v

ACKNOWLEDGMENT

I would like to take this time to thank and praise the ALMIGHTY GOD for

giving me the opportunity to study for a degree program under UTM and HIS steadfast

love, guidance and care throughout the whole study period.

Sincere gratitude and appreciation is dedicated to my supervisors, Dr. Yob Saed

Ismail and En. Shukur Abu Hassan for their valued guidance, advice, encouragement

and helpful discussions throughout the study.

Thanks to En. Shukur Abu Hassan and lab technician En. Rizal, who both

rendered considerable assistance in the setup and experimental testing at the Strength

Lab, UTM. I would also like to thank my friends particularly Mohd. Supian Abu Bakar

for helping and supporting me throughout my work pursue.

vi

ABSTRACT

Carbon Fibre Reinforced Polymer (CFRP) composites have been used

successfully as strengthening materials for reinforced concrete structures by external

plate-bonded technique. The CFRP materials have excellent characteristics such as

lightweight, high tensile strength, non-magnetic and highly corrosion resistant. Many

studies have shown that by using plate-bonded technique, the performance of the

strengthened concrete member can be enhanced. In this technique, the surface

preparation of concrete substrate and CFRP material along with the type of adhesive

used are the critical factors affecting the bonding performance of the system. This

project discusses the experimental testing and results discussions on the bonding

behavior between CFRP plates and concrete through ultimate failure load of the CFRP-

epoxy-concrete specimen, bond transfer length and bond stress distribution along the

bonded joint. Two concrete prism samples were choosen, which are BOLTALS50-

OD03-1 yr and BOLTUS OD01-1 yr. Both specimens were exposed to outdoor

environment to further look into the durability of the joint. A 50 mm width by 1.2 mm

thick CFRP plate was used and bonded to 100 x 100x 295 mm concrete prism. The two

specimens was given the final loading test using INSTRON 100kN Universal Testing

Machine. The results of local bond stress for BOLTALS50-OD03-1 yr and BOLTUS

OD01-1 yr specimens shows that exposure condition and sustainable load had

influenced the performance of bonding characteristics between CFRP and concrete

prism. Finally, it can be concluded that though the final loading test shows significant

results, the extension of the 12 months exposure period would better improve the overall

CFRP-epoxy-concrete bond performance.

vii

ABSTRAK

Gentian Carbon Fibre Reinforced Polymer (CFRP) komposit telah digunakan

dengan meluasnya sebagai bahan penguat untuk struktur konkrit bertetulang dengan

menggunakan teknik pengikatan plat luaran. Gentian komposit CFRP terutamanya

merupakan suatu ciptaan yang sungguh bermaanfat dalam aplikasi industri kerana

mempunyai kriteria yang tinggi seperti ringan, mempunyai kekuatan tegangan yang

tinggi, bukan bahan magnetic dan tahan karat. Pelbagai kajian yang dilakukan terhadap

bidang ini mendapati dengan penggunaan teknik pengikatan plat luaran, keupayaan

untuk konkrit menunjukkan prestasi yang tinggi boleh dipertingkatkan. Dalam teknik

ini, penyediaan permukaan bagi substrat konkrit bertetulang dan gentian CFRP berserta

dengan bahan pelekat adalah faktor kritikal yang mempengaruhi prestasi sistem

pengikatan. Dua jenis spesimen berlainan disediakan, iaitu BOLTAL S50-OD03 1 yr

dan BOLTUS OD01-1 yr. Kedua-dua specimen ini didedahkan kepada persekitaran

untuk jangka masa 12 bulan. Tujuannya untuk melihat kebolehharapan substrat konkrit

tersebut. Tegasan keseragaman di sepanjang pengikatan plat CFRP diperhatikan. Plat

CFRP yang berukuran kira-kira 50mm lebar dan 1.2mm tebal digunakan dalam kajian

ini. Plat ini diikat pada konkrit yang berukuran 100 x 100 x 295mm. Kedua-dua

specimen kemudiannya diuji pada ujian bebanan terakhir menggunakan mesin

INSTRON 100kN Universal Testing Machine. Keputusan terhadap tegasan ikatan

setempat menunjukkan pendedahan kepada persekitaran mempengaruhi kekuatan ikatan

antara plat CFRP dan konkrit. Kesimpulannya, walaupun ujian bebanan terakhir

menunjukkan keputusan yang baik, tapi jangka masa pendedahan lebih daripada 12

bulan boleh menunjukkan perubahan yang lebih baik ke atas ikatan.

viii

CONTENTS

CHAPTER TITLE PAGE

TITLE ii

DECLARATION iii

DEDICATION iv

ACKNOWLEDGMENT v

ABSTRACT vi

ABSTRAK vii

CONTENT viii

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS xviii

I INTRODUCTION

1.1 Project Background 1

1.2 Objectives 2

1.3 Scope 3

1.4 Methodology 3

II LITERATURE STUDY ON CFRP, EPOXY AND

CONCRETE

ix

2.0 Introduction 5

2.1 Definition of FRP 5

2.2 General properties of Fibre Reinforced Polymer 6

2.2.1 Reinforcing Fibre System 7

2.2.2 Carbon Fibre Reinforced Polymer (CFRP) 8

2.3 Epoxies 10

2.3.1 Advantages of Epoxy Resins

2.4 Concrete 15

2.4.1 Portland cement 15

2.4.2 Types of Portland cement 16

2.5 Hardened concrete 17

2.5.1 Water: Cement Ratio Law 17

2.5.2 Gain in strength with ages 18

2.6 Other factors affecting concrete strength 19

2.6.1 Compaction 19

2.6.2 Temperature 20

2.6.3 Heat of hydration 20

2.7 Aggregates 21

2.7.1 Soundness 23

2.7.2 Grading 23

2.8 Particle shape 24

2.9 Water Demand 24

2.10 Modulus of Elasticity 25

2.11 Tensile Strength 26

2.12 Resin Adhesive for Concrete Bonding 27

2.13 Summary 27

III LITERATURE STUDY ON BONDING TECHNOLOGY

3.0 Introduction 28

3.1 Adhesive Bonding Definition 29

x

3.1.1 Advantages of Adhesive Bonding 30

3.1.2 Disadvantages of Adhesive Bonding 30

3.2 Mechanism and Models of Adhesive Bonding 32

3.2.1 Mechanical Interlocking Model 32

3.2.2 Electrical Model 32

3.3 Joining Technique 33

3.3.1 Joint design 34

3.4 Bonded Joints 36

3.5 Stresses and Strains 37

3.6 Failure Modes 37

3.7 Joint Geometry Effect on Joint Strength 39

3.8 Design Principles 40

3.9 Adhesive 42

3.10 Adhesive mechanical properties 42

3.11 Lap Joints 43

3.11.1 Single lap joint 44

3.11.2 Double lap joint 46

3.12 Factors considered in designing Adhesive Bonding 48

3.13 Mechanism of Bond Failure 49

3.13.1 Failure at Interface 49

3.13.2 Failure in the Adhesive 49

3.13.3 Failure in the Adherend 50

3.14 Summary 50

IV FORMULATION OF FRP-EPOXY-CONCRETE

BONDED SYSTEM

4.1 Definition of average bond strength 52

4.1.1 Definition of Local Shear Stress Distribution 53

xi

4.2 Bond Transfer Lengths 54

4.3 Behavior of FRP to Concrete Bonded Joint 55

4.4 Failure Patterns 56

4.5 Summary 57

V EXPERIMENTAL TESTING SET-UP

5.1 Experimental Evaluation 58

5.1.1 Details of material 59

5.1.2 Adhesive 60

5.2 Significant of pull-out test 60

5.2.1 Specimen preparation 61

5.2.1.1 Sustainable load on BOLTALS50-OD03-1 yr 61

5.2.1.2 Summary on pre-stressed process 65

5.2.2 Surface preparation 65

5.2.3 Curing time and pressure 66

5.2.4 Instrumentation and Measurement 67

5.2.4.1 Strain gauges 67

5.2.4.2 Loading Machine 68

5.2.4.3 Data Acquisition System 69

5.3 Experimental Procedure 70

5.3.1 Strain Gauge Installation 70

5.3.2 Soldering Technique 71

5.3.3 Testing Set-up 72

5.3.3.1 Specimen (BOLTUS OD01-1 yr) 72

5.3.3.2 Specimen (BOLTALS50-OD03-1 yr) 76

VI DATA ANALYSIS AND RESULTS DISCUSSION

6.0 Introduction 77

xii

6.1 Force transfer between CFRP plate and concrete prism 77

6.2 Bond Stress Distribution 79

6.2.1 Local load, average bond stress and 79

local bond stress

6.2.2 Local Bond Stress Concentration Factor, λ 82

6.3 Modes of failure 83

VII CONCLUSION AND RECOMMENDATION

FOR FUTURE STUDY

7.1 Force transfer between CFRP plates and concrete 86

7.2 Local Bond Stress Distribution 87

7.3 Bond Strength 87

7.4 Failure mode analysis 88

7.5 Recommendation for future study 88

REFERENCES 89

APPENDIX

APPENDIX (A) 91

APPENDIX (B) 92

APPENDIX (C) 93

APPENDIX (D) 94

APPENDIX (E) 95

APPENDIX (F) 100

APPENDIX (G) 101

APPENDIX (H) 102

APPENDIX (I) 103

APPENDIX (J) 104

xiii

LIST OF TABLES

NO. TITLE PAGE

1.1 The exposure condition on both specimens 4

2.1 Mechanical properties of three different grades of carbon fibres 9

2.2 Typical mechanical and physical properties of epoxy 12

2.3 Water requirements for two slumps and three sizes of stone for 26

average materials

5.1 Technical data of Selfix Carbofibe Pultruded CFRP Plate type-S 59

5.2 Mechanical properties of Resifix 31 epoxy adhesive 60

5.3 Strain Gauge Technical Data 68

6.1 Bond Stress Experimental Result for BOLTALS50-OD03-1 yr 82

6.2 Bond Stress Experimental Result for BOLTUS OD01-1 yr 83

xiv

LIST OF FIGURES

NO. TITLE PAGE

2.1 Stress-strain curves for typical fiber, resin and FRP composite 7

2.2 Tensile stress versus strain of various type of reinforcing fibers 8

2.3 Epoxy resin synthesis 12

2.4 Reaction of epichlorhydrin and diphenylolpropane to form an 13

epoxy resin

2.5 Relation between cement: water ratio and minimum 28-day 17

compressive strength of concrete

2.6 Effect of cement type on the strength of concrete 18

( at equal water: cement ratios )

2.7 Suitable grading limits for concrete sands 24

3.1 Basic steps of adhesive bonding process 31

3.2 Good Wetting(A) and Poor Wetting(B) 32

3.3 Electrical double layer at polymer-metal interfaces 33

3.4 Type of joints used in adhesive-bonding flat adherends 33

3.5 Design Procedure of Adhesive Joint 34

3.6 Areas of failure initiation and critical strength 38

3.7 Stress distribution of lap joint 38

3.8 A typical adhesive shear stress distribution in a lap joint 39

according to elastic-plastic model

xv

3.9 Relative joint strength of various joint configurations 41

3.10 Typical brittle and ductile adhesive behaviour 43

3.11 Tensile force on lap joint showing (a) unloaded joint, 44

(b) joint under stress, and (c) stress distribution in adhesive

3.12 Exaggerated deformations in loaded single lap joint 45

3.13 Bending moments induced in the outer adherends of a double 46

lap joint

3.17 Cohesive and adhesive bond failure 50

4.1 Double lap joint of CFRP and concrete bonded system 52

4.2 Pull push test of a single lap plate to concrete bonded joint 55

(a) elevation (b) plan

4.3 Different potential failure modes in a plate bonded joint (a) 56

cohesive failure, (b)adhesive failure (c)concrete shearing failure

5.1 Selfix Carbofibe Pultruded CFRP plate 59

5.2 CFRP Local Plate Load versus Bond Length BOLTALS50-OD03-1 yr 61

5.3 Local Bond Stress versus Bond Length BOLTALS50-OD03-1 yr 62

5.4 Applied Hydraulic Load versus CFRP Local Plate Force 63

BOLTALS50-OD03-1 yr

5.5 Applied Hydraulic Load versus Local Plate Strain along 63

Bonded Length BOLTAL S50-OD03-1 yr

5.6 BOLTALS50-OD03-1 yr Time Stress 64

5.7 The schematic diagram positioning of strain gauges and LVDT’s 68

5.8 Universal Testing Machine ( Model Instron 100kN ) 69

5.9 Data Logger Type TDS-302 69

5.10 Final instrumentation set-up for pull-out test 70

5.11 Placement of specimen (BOLTUS OD01-1 yr) on the Instron machine 73

5.12 Placement of upper shear connector to the test rig specimen 73

5.13 Lead wires connection to Data Logger TDS-302 74

5.14 Application of roller bearing to concrete prism top surface 74

5.15 Placement of LVDT on the concrete specimen 74

5.16 Test rig set-up 75

xvi

5.17 Final inspection before testing 75

5.18 & Monitoring the progress of the test through strains output and 75

5.19 machine control panel

5.20 Set-up of BOLTALS50-OD03-1 yr on Instron machine 76

5.21 Data acquisition set-up 76

5.22 Monitoring testing progress 76

5.23 Removing the LVDT’s at 40kN load level 76

length BOLTAL S50-OD03-1 yr

6.1 CFRP Local Load versus Bond Length BOLTALS50-OD03-1 yr 78

6.2 CFRP Local Load versus Bond Length BOLTUS OD01-1 yr 78

6.3 Local Bond Stress versus Bond Length BOLTAL S50-OD03-1 yr 81

6.4 Local Bond Stress versus Bond Length BOLTUS OD01-1 yr 81

6.5 Remains of concrete prism (specimen BOLTAL S50-OD03-1 yr) 85

6.6 Remains of concrete prism (specimen BOLTUS OD01-1 yr) 85

6.7 Failure mode for specimen BOLTALS50-OD03-1 yr 85

6.8 Failure mode for specimen BOLTUS OD01-1 yr 85

xvii

LIST OF SYMBOLS

ρ - density ( kg/m3 )

Ec - modulus of elasticity of concrete ( GPa )

τ - shear stress ( MPa )

P - load ( kN )

b - width of the joint ( mm )

l - length ( mm )

Pmax - ultimate applied load ( kN )

Fult - ultimate applied load ( kN )

A - bonded area ( = WL ) ( mm2 )

∆Fi-j - Difference in longitudinal force between two consecutive

gauge locations ( kN )

∆L - Spacing between two consecutive gauge locations ( mm )

Lo - transfer length

τξo - maximum value of shear stress

bp - laminate width ( mm )

ξo - relative load level at which cracking initiates

ECFRP - Elastic modulus of plate ( MPa )

εL - plate strain measured by strain gauge

τave - average bond stress ( MPa )

LB - length of bonded surface ( mm )

τlocal - local bond stress ( MPa )

b - width of the CFRP plate ( mm )

λ - local bond stress concentration factor

c - half of length of overlap

xviii

o - outer adherend

i - inner adherend

η - adhesive thickness

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

xix

1

CHAPTER 1

INTRODUCTION

1.1 Project Background

Infrastructure facilities for transportation and housing such as bridges and

buildings which were developed and rapidly expanded in the middle of last century is

aging and in due to various factors. The security of bridges and buildings which

represents a huge amount of investment is being questioned. Deficiencies in the existing

inventory range from those related to wear, environmental deterioration and aging of

structural components, to increased traffic demands and changing traffic patterns. The

long term durability of this construction material is very much important in order to

ensure the structure able to maintain its integrity and provides the service according to its

design throughout its service life. Thus the deteriorated concrete structures due to

environmental attack require repair and maintenance or sometimes need further

strengthening to extend their service life.

The introduction of steel in the early 1960’s as external strengthening member by

method of bonding able to increase both strength and stiffness of reinforced concrete

2

structures such as beam, slab and wall. This technique said to be an economical choice

and can be rapidly applied in the field with little or no disturbance to structure operation,

and that it does not alter the configuration of the structure. However, the corrosion of

steel plates can be a big problem since corrosion can damage the bond and integrity

resulting in the failure of the structure. Also, shear and flexural de-bonding could develop

after the formation of shear diagonal cracks or when the curvature in the beam increased.

In order to avoid the problem of using steel plates is the use of a relatively high

durability composite material known as fibre reinforced polymer (FRP). The use of FRP

materials such as carbon fibre reinforced polymer (CFRP) is now receiving widespread

attention for applications in plate bonding technology and structural elements ranging

from the retrofit and rehabilitation of buildings and bridges to the construction of new

structural system in most developed country such as USA, Canada, Japan and most

European countries. The advantages of using CFRP plate compare to steel plate were as

follows; low stress concentration, no bearing stress, lighter weight, high-stiffness-to-

weight ratios and strength-to-weight, stiffer joints beside higher durability to environment

degradation.

1.2 Objective

The main aim of this study is to study the bond stress distribution characteristics

along the bonded length of CFRP plate-epoxy-concrete bond interfaces.

3

1.3 Scope

The study will focus on the mechanical properties and behavior of the CFRP

plate, epoxy and concrete material. The activities of the project contains literature

reviews of carbon fibre reinforced plastic/ polymer and concrete, literature reviews that

focuses on the concept of bonding technology (adhesive) which involves two kinds of

joint configuration, single lap shear and double lap shear and the literature reviews on

analysis and behavior of CFRP- epoxy- concrete bond characteristics.

Finally, the investigation of bond characteristics was studied through final load

test of the unstressed and stress specimens exposed to outdoor condition for duration of

12 months. Part of the study will cover on test rig design and fabrication for pull-out test,

specimen preparation, experimental and data analysis and finally the conclusion based on

the project.

1.4 Methodology

In this project, two specimens were prepared and exposed to outdoor condition.

The first specimen is known as BOLTAL S50-OD03 1yr and the second specimen is

known as BOLTUS OD01-1yr.

These specimens were prepared and exposed to the outdoor environment for a

period of 12 months. During the exposure period, CFRP plate and adhesive are

4

monitored to detect any durability problem. After completion of exposure duration, final

load test (pull-out test) was carried out.

Exposure Condition Duration

(i) Outdoors

BOLTAL S501yr-OD03

BOLTUS OD01-1 yr

12 months

12 months

Table 1.1: The specimen experimentation exposure programme

5

CHAPTER II

LITERATURE STUDY ON CFRP, EPOXY AND CONCRETE

2.0 Introduction

In this chapter, the literature study will focus on the properties and characteristics

of Carbon Fibre Reinforced Polymer (CFRP), the adhesive system (epoxy) and the

concrete materials.

2.1 Definition of FRP

Referring to J.J Bikerman [4], FRP composite material is a material composed of

a mixture or combination of two or more constituents or phases that differ in form and

chemical composition and which are essentially insoluble in each other. In general, FRP

composite consist of reinforcing material such as glass or carbon fibre and a matrix such

as polyester or epoxy.

6

The constituents retain their identities that is, they do not dissolve or merge

completely into one another although and normally, the components can be physically

identified and exhibit an interface between one another.

2.2 General properties of Fibre Reinforced Polymer

Fibre reinforced composites are blends of a high strength, high modulus

fibre with a hard enable liquid matrix. The typical FRP composite material comprises

reinforcing fibres embedded in a polymer matrix that protects them against mechanical

damage, transfer stresses between them and maintains the shape of the composite. In this

form, both fibres and matrix retain their physical and chemical identities yet they produce

a combination of properties that cannot be achieved with either of the constituents acting

alone. The bonding of these aligned fibres into the softer matrix material results in a fibre

direction. Since the fibres are highly directional, the resultant composite will exhibit

anisotropic behavior much like steel reinforced concrete. This anisotropic behavior gives

the designer freedom to tailor the strengthening system to reinforce specific high stresses.

Typical composite material properties include low specific gravity modulus to

weight ratio. Most FRP materials are resistant to corrosion. Matrix materials deform

plastically, whereas the fibres in general do not. The stress and strain relationship

between resins, fibres and composites is shown in Figure 2.1. Since the FRP composite

behavior is generally dominated by the reinforcement, this plastic deformation or yield is

seldom exhibited by the composite for structural purposes. Brittle failure is the typical

failure mode for FRP composite under excessive stresses.

7

Figure 2.1: Stress-strain curves for typical fibre, resin and FRP composite [1]

2.2.1 Reinforcing Fibres System

Fibres ideally comprise not more than 60% ( by volume ) of the composite and

are the principal load carrying members. Hand lay-up methods may produce laminates

with lower fibre volumes, which may range between 30-50% of fibre content. Fibres

primarily act in tension tend to have low transverse strength. For handling purposes in

some forms of composite, the individual fibres are brought together in ‘bundles’ called

tows and roving. The fibres can be used in this form or further processed into tow sheets,

fabrics or mats. The three most common types of fibres used in forming polymeric

composites are carbon, glass and aramid.

Figure 2.2 shows the strengths and maximum strains of the different types of

fibres at failure. The gradient from the graph also indicates the stiffness (modulus) of the

composite, the steeper gradient and higher stiffness.

8

Figure 2.2: Tensile stress versus strain of various type of reinforcing fibres [1]

2.2.2 Carbon Fibre Reinforced Polymer (CFRP)

Carbon fibres consist of tows, each of them made up from numerous filaments

(~10000). The filaments are from 7 to 15µm in diameter, and they consist of small

crystallites of ‘turbo static ‘graphite which is one of the allotropic forms of carbon. In the

graphite crystal, carbon atoms are arranged in a hexagonal array in a plane and the planes

are stacked together, with covalent bonds acting within the planes and weaker Van der

Waals forces holding the planes together.

To obtain high modulus and high strength of fibre, the layered planes of graphite

must be with weaker Van der Waals forces holding the planes together. To obtain a high

modulus and high strength, the layered planes of graphite must be aligned parallel to the

fibre axis. However, the structure of the stacked planes is not ideally regular. So

therefore, the properties of carbon fibres can be vary over a wide range, depending upon

the degree of perfection, which is function of production process as mentioned by H.

Derek. [3]

9

Three precursor materials are used at present to produce carbon fibres: rayon,

polyacrylonitrile (PAN) and isotropic and liquid crystalline pitches. Rayon and isotropic

pitch precursors are used to produce low modulus carbon fibres (≤ 50 GPa). Higher

modulus carbon fibres (≥ 200 GPa) are made from PAN or liquid crystalline

(mesophase) pitch precursor. It gives poor bonding to the matrix. Carbon fibres are not

wet by molten metals and are difficult to wet with resins, especially for the higher

modulus fibres. [4]

Carbon fibres are not affected by moisture, atmosphere, solvents, bases and weak

acids at room temperature. However, oxidation becomes a problem at elevated

temperature. The carbon fiber is anisotropic material, and its transverse modulus is an

order of magnitude less than its longitudinal modulus. [4]

Table 2.1: Mechanical properties of three different grades of carbon fibres [4]

Typical Properties High Strength High Modulus Ultra-High Modulus

Density ( g/cm3 ) 1.8 1.9 2.0-2.1

Young’s Modulus ( GPa ) 230 370 520-620

Tensile Strength ( MPa ) 2.48 1.79 1.03-1.31

Tensile Elongation ( % ) 1.1 0.5 0.2

The application of CFRP started in the 1980’s, particularly in Japan and Europe as

a replacement/alternative to the old conventional steel plate bonding. The use of carbon

fibre composite for structural applications was first studied at the Swiss Federal Testing

Laboratories (EMPA). It has emerged to be an efficient and versatile strengthening

method for construction and repairing of concrete structures.

10

The CFRP composites are directional, in which the maximum mechanical

properties and strength is offered in fibre direction. These material has fantastic

characteristics: they are very strong light weight material, have high strength to weight

ratio, excellent fatigue properties, outstanding corrosion and weather resistance, low

maintenance, long term durability, parts consolidation and formable as they can be

shaped to any desired form and surface texture. The CFRP laminates provides an

engineered system that potentially offers to civil construction industry with improved

performance for a longer period of time than conventional construction material.

CFRP are more expansive than glass and aramid composites, however CFRP offer

an excellent combination of strength, low weight and high modulus. The tensile strength

of CFRP is equal to glass fibres composite while its modulus is about three to four times

higher than glass or aramid. CFRP are supplied in a number of different forms, from

continuous filament tows chopped fibres and mat. The highest strength and modulus are

obtained by using unidirectional continuous reinforcement. Twist free tows of continuous

filament carbon contain 1,000 to 7,500 individual filaments, which can be woven or

knitted into woven roving and hybrid fabrics with glass fibre and aramid fibre.

2.3 Epoxies

Referring to thesis literature, Tan Sek Lin [5], epoxy resins are used extensively

in composite materials for a variety of demanding structural applications. They are the

most versatile of the commercially available matrices. Depending on the chemical

structures of the resin and the curing agent, the availability of the numerous modifying

reactants, and the conditions of cure, it is possible to obtain toughness, chemical and

solvent resistance, mechanical response, ranging from extreme flexibility to high strength

11

and hardness, resistance to creep and fatigue, excellent adhesion to most fibers, heat

resistance and excellent electrical properties.

All epoxy resins contain the epoxide, oxirane, or ethoxylene group:

O R

C C

Where, R represents the point of attachment to the remainder of the resin molecule. The

epoxide function is usually a 1, 2- or α-epoxide that appears in the form:

O

CH2 CH CH2

Which called the glycidyl group, which is attached to the remainder of the molecule by

oxygen, nitrogen or carboxyl linkage, hence the terms glycidyl ether, glycidyl amine or

glycidyl ester. Curing of the resin results from the reaction of the oxirane OH of the

compounds that contain active hydrogen atoms:

O

R

R’ H + C C R C C R

Where R’ H is:

12

O

R’NH2 R’R’’NH R’ C OH

Primary Secondary Carboxylic Acid

Amine Amine

R’SH O OH

Mercaptan Oxygen Phenol

Figure 2.3: Epoxy resin synthesis [4]

Three chemical reactions are of major importance to the curing of epoxy

composite matrices: the amine/epoxide reaction, the anhydride/epoxide reaction and the

Lewis acid-catalyzed epoxide homoplymerization. Epoxies tend to have higher

viscosities than polyesters and vinylester systems. Typical properties of epoxies are given

in Table 2.2.

Properties Value

Tensile Strength ( MPa ) 55-130

Tensile Modulus ( GPa ) 2.8-4.1

Elongation ( % ) 3.0-10.0

Density ( g/cm3 ) 1.2-1.3

Shrinkage ( % ) 1-5

Table 2.2: Typical mechanical and physical properties of epoxy [4]

13

The epoxy resins constitute an important and rapidly growing class of resins for

use in reinforced plastic and adhesives. The exact structure of the epoxy resins is not

known and their reactions are not fully understood. However a fairly good model of the

structure can be built and knowledge is increasing rapidly on the molecular structure and

its various reactions.

Figure 2.4: Reaction of epichlorhydrin and diphenylolpropane to form an epoxy

resin [4]

The reaction product shown in Figure 2.4 is the base resin for a majority of

commercial resins. The low-molecular-weight resins are quite liquid. The higher-

molecular-weight resins with large values for n are fusible solids. The first are curing

reactions in which the epoxy groups play the most important role. The second are those

curing reactions in which the epoxy groups are important, but insufficient to effect

complete curing. Both types of reactions are important in laminating and in the curing of

adhesives.

14

2.3.1 Advantages of Epoxy Resins

The advantages of epoxy resins over other polymers as adhesive agents for civil

engineering use can be summarized as follows (Mays and follows Hutchinson, 1992);

• High surface activity and good wetting properties for a variety of substrates.

• May be formulated to have a long open time (the lime between mixing and closing of

the joint).

• High cured cohesive strength, so the joint failure may be dictated by the adherend

strength, particularly with concrete substrates.

• May be toughened by the inclusion of a dispersed rubber phase.

• Minimal shrinkage on curing, reducing bond line, strain and allowing the bonding of

large areas with only contact pressure.

• Low creep and superior strength retention under sustained load.

• Able to accommodate irregular or thick bond lines.

• Blending with a variety of materials to achieve desirable properties can be readily

modified formulation.

These various modifications make epoxy adhesives relatively expensive in

comparison to other adhesives. However, the toughness, range of viscosity and curing

conditions, good handling characteristics, high adhesive strength, inertness, low

shrinkage and resistance to chemicals have meant that epoxy adhesives have found many

applications in constructions, for example, repair materials, coatings and as structural and

non-structural adhesives.

15

2.4 Concrete

Concrete is composed of crushed stone, sand, Portland cement and water. When

these materials are mixed together, a plastic mass is formed which may be placed in a

box and compacted in it. After a period, the box may be stripped to reveal a block of

hardened concrete having the same shape as the interior of the box into which it was

placed. The fundamental characteristic that gives concrete its unique usefulness is its

capacity to change in a short space of time from a plastic, mouldable mass to hard stone

like material of considerable durability.

2.4.1 Portland cement

R. N. Swamy, P. S. Mangat, and C. V. S. K. Rao [6] in their research provide the

fundamental property of Portland cements is that when it’s mixed with water,

chemical reactions (called hydration) take place, which change the structure of the plastic

mass whereby it becomes hard and rigid. This chemical reaction does not rely on the

absorption of carbon dioxide will take place under water.

16

2.4.2 Types of Portland cement

a) Ordinary Portland cements (OPC): used for all normal concrete work.

b) Rapid hardening Portland cement: more finely ground than OPC; develops a higher

strength at early ages; allows early stripping of shutters.

c) Portland blast furnace cement ( PBFC ): manufactured by inter grinding Portland

cement clinker with a proportion of granulated blast furnace slag; slower rate of

strength development than OPC and more affected by low temperature and poor

curing; low heat of hydration and so useful in mass concrete.

d) Portland cement: Portland cement clinker ground with not more than 15% granulated

blast furnace slag; may be considered to be the same as OPC for all practical

purposes.

e) Portland cement/slag blends: ordinary Portland cement and milled granulated blast

furnace slag are mixed together or mixer, the milled slag (available as “Slagment”)

replaces OPC in the mix; a 50/50 blend is normally used and may be regarded as

similar to PBFC. The hydraulic properties of milled slag on its own are very weak but

the OPC appears to activate the slag so that the two act together as hydraulic cement.

f) Sulphate resisting Portland cement: this practically the same as OPC but gives better

resistance to sulphate attack.

g) Masonry cement: used making mortars and plasters that are fatty and water holding.

h) Marine cement: special cement for use in marine environments; a specially

formulated clinker inter ground with a pozzolan and air-entraining agent. [6]

17

2.5 Hardened concrete

2.5.1 Water: Cement Ratio Law

Duff Abrams in the USA stated the first fundamental law, which is the basis of all

concrete technology. This is the famous water: cement ratio law, which may be stated as

follows:

“For the same materials and conditions of the test, the strength of fully compacted

concrete depends only on the ratio of water to cement used in the mix.”

The relation between 28 day compressive strength and the proportion of cement to

water in the mix is shown in Figure 2.5, for weight batching and for volume batching.

This applies to ordinary Portland cement, average aggregates, reasonable site control and

standard test procedure. In practice, the test result shows considerable scatter and the

curves on Figure 2.5 give the “minimum strengths” which may be expected at various

cement : water ratio by mass.

Figure 2.5: Relation between cement: water ratio and minimum 28-day compressive

strength of concrete [6]

18

2.5.2 Gain in strength with ages

As the chemical reactions of hydration proceeds, the strength of the concrete

increases. Figure 2.6 shows the typical age/strength relationships for three types of

cement. The strengths at the various ages are referred to the strength developed at 28 days

as 100%. The criterion of 28 days age was chosen arbitrarily and was adopted by early

designers on the basis that it was desirable to load the structure about four weeks after the

concrete had been placed. Working stresses used by the design engineer are based on the

strength at 28 days. At this age, the concrete has reached a high proportion of its potential

strength. The diagram is based on the compressive strength of standard concrete test

cubes. So long as water is present, the concrete will continue to gain strength and may be

20% higher than the 28 day strength at 3 months and 30% higher at 6 months.

Figure 2.6: Effect of cement type on the strength of concrete ( at equal water:

cement ratios ) [6]

19

2.6 Other factors affecting concrete strength

2.6.1 Compaction

Concrete and Aggregates, Annual Book of ASTM Standards [7] shows the

relation between strength and water: cement ratio is applicable only to concrete, which

can be fully compacted with the available equipment. In badly compacted concrete, there

are air voids and spaces, which obviously reduce its strength.

For an average structural concrete, an increase in the water content of 25% would

result in an equivalent reduction. It is therefore obvious that the elimination of voids by

proper compaction is as important in securing good quality concrete as close control of

the water: cement ratio. This was recognized by Duff Abrams and mentioned in his

original paper on water : cement ratio.

Dry mixes of low workability cannot be consolidated by hand methods

(damping, ramming, roding and spading) and vibrators must be used. Vibration makes

the concrete fluid so that it flows into the corners and round the reinforcement, and

allows air to be expelled. Mixes of high workability should not be vibrated because

vibration is liable to cause segregation. When vibrators are used, it is imperative that

adequate standby equipment is ready for immediate use in case of breakdowns.

20

2.6.2 Temperature

Most chemical reactions proceed more rapidly at higher temperatures. The

hydration of cement follows the rule and consequently concrete gains strength more

rapidly when the curing temperature is increase. On the other hand, the gain in strength is

slower when the curing temperature is reduced. During cold spells, it is advisable to place

concrete on a rising temperature, not when the temperature is falling. [7]

2.6.3 Heat of hydration

As with many other chemical reactions, heat is generated when cement hydrates.

This is useful in cold weather concreting because once the hydration has started, the heat

generated will compensate for the heat lost from exposed surfaces and through the

shutters. In large mass concrete dams, the heat of hydration causes a rise in the

temperature of the concrete because the rate of heat generation is greater than the rate at

which it is dissipated to the atmosphere on the surface. Internal stresses may be created in

the concrete which might cause cracking in the structure. For such conditions, low heat

cement is used. The concrete mix is designed to have low cement content. [7]

21

2.7 Aggregates

In concrete work, the term “aggregate” refers to material that is mixed with

cement and water to provide bulk in the concrete. Material which is retained on a 4.75

mm (3/16) in sieve is defined as “coarse aggregate” and that which passes through this

sieve as “fine aggregate”. The stone and sand mentioned previously would more correctly

classify as coarse and fine aggregate respectively.

Coarse aggregate is defined as that material of which not less than 90% by mass is

retained on 4.75 mm sieve. Increasing the proportion of large particles (say 20 mm and

larger) in the coarse aggregate fraction of a mix results in a reduction in water demand,

but if an excessive proportion of large material is used, a harsh non-cohesive mix will

result.

For fine aggregate, natural sands are major source of fine aggregate and they are

usually of alluvial or marine origin. If suitable natural sands are unavailable, fine

aggregates can be produced by the crushing or grinding of rock. Fine aggregate should be

well graded and should not have excessive fine material passing the 150µm and 75µm

sieves. Sand with excessive fines will increase water demand with detrimental effects on

the strength and shrinkage of the concrete. Concrete aggregates are usually divided into

two main classes-dense and lightweight. [7]

(i) Dense Aggregates

Dense aggregates are composed of particles that have a bulk density when dry of

not less than 2300kg/m3. They are mainly derived from naturally occurring sands, gravels

and rocks of igneous, sedimentary or metamorphic origin. Dense aggregates are also

22

produced from slag, a by-product of metallurgical furnaces. These aggregates are derived

mainly from blast furnaces that produce iron, but they can also be produced from slag

obtained from the smelting of non-ferrous ores.

(ii) Lightweight Aggregates

Lightweight aggregates include those materials that have a bulk density when dry

of less than 2000kg/m3. Aggregates that are used in the manufacture of structural

lightweight concrete include scoria, expanded shale and clays, foamed blast-furnace slag,

sintered fly-ash and furnace clinker. Low-density aggregates that are mainly used for

insulation purposes in non-structural components such as screeds and blocks include

pumice and the processed materials perlite, vermiculite, polystyrene beads. Organic

materials like sawdust, wood shavings and rice husks-have also been used to make non-

structural lightweight concrete.

(iii) Special Aggregates

In addition to two types of aggregate described above, there are also aggregates

with high bulk density that are used in concrete for shielding against nuclear radiation.

These aggregates include barites, steel punching, scrap iron and the iron ores hematite,

magnetite and limonite.

23

2.7.1 Soundness

The aggregate will constitute up to 85% of the overall mass of the concrete and

their properties have a considerable influence on the characteristics of the concrete in

both the plastic and hardened states. In general, aggregates should consist of hard, strong

durable particles, free from clay or organic materials. Some materials are known to

expand slowly when used as an aggregate while others shrink. Both tend to cause

disruption of the concrete and should be avoided. If it is not possible to obtain aggregates

from a source which has records of satisfactory performance, it is wise to have aggregates

from unknown sources tested. [7]

2.7.2 Grading

An aggregate is made up of particles of various sizes and the quantity present in

various sizes is referred to as “grading” of the aggregate. The grading of the fine

aggregate is particularly important. Grading is determined by passing sample of the

aggregate through a series of standard sieves starting with the largest size and weighing

the mass retained on each sieve. The percentage passing each sieve is then calculated and

a grading curve drawn. It has been found that grading curve of sand should fall within the

envelope shown on Figure 2.7. [7]

24

Figure 2.7: Suitable grading limits for concrete sands [7]

2.8 Particle shape

The particle shape of an aggregate is an important characteristic in concrete work.

It is easy to visualize that an aggregate with elongated, flat key or needle shaped particles

will produce a concrete, which will be more difficult to consolidate than one with

rounded particles, the “place ability” will be lower. The badly shaped particles do not

slide over one another as easily as the better shaped rounded particles. The badly shaped

particles do not pack together as well and leave larger spaces (voids) between the

particles. Such sand requires more cement paste to fill the voids and badly shaped stone

will require more mortar (cement paste and fine aggregate) to fill the voids in the stone.

2.9 Water Demand

Many physical properties of the aggregates, including its grading, particle shape,

surface area and void cement, influence its concrete making qualities. The collective

25

effect of these and other characteristics can be evaluated and expressed in an index

known as the “water demand”. Water demand is a characteristic of the aggregates and is

independent of the cement: water ratio or the amount of aggregate in the mix. If the

coarse aggregate is kept unchanged throughout a series of tests, we can see that the use of

different sands results in different water requirements. Water demand is measured in

kilograms (liters) of water per cubic meter of compacted concrete.

The following values are approximate but give an indication of the water

requirements of different sands with 20 mm nominal size stone and 35 mm slump:

Very good sand 165kg/m3

Average pit sand 195kg/m3

Average crusher sand 205kg/m3

Poor sand 230kg/m3

Compensates for the higher water requirement, the cement content must be

increased to maintain the correct cement: water ratio. Thus it will be seen that poor sand

at a cheaper price may take the concrete more expensive because of extra cement

required. The size of the stone affects the water requirements of the mix; a small stone

requires more water for the same workability (slump) than a larger sized stone.

26

Table 2.3: Water requirements for two slumps and three sizes of stone for average

materials [6]

Slump Nominal stone size (mm) Water demand (kg/m3)

For compaction

By vibration-

35 mm slump

40

20

10

180

195

210

For hand

Compaction-

85 mm slump

40

20

10

190

205

220

2.10 Modulus of Elasticity

The stress strain relationship for concrete is not linear, but the portion of the curve

at low stresses may be regarded as being linear and the modulus of elasticity determined

from it. The modulus of elasticity of concrete, Ec, may be taken as 20 GPa for most

design purpose.

2.11 Tensile Strength

P. N. Balaguru and V. Ramakrishnan [8] in their research says that the tensile

strength of concrete is very much lower than the compressive strength and although there

is no direct relationship, the strength in tension is often assumed to be one tenth ( 1/10 )

that in compression. Concrete has a brittle type of failure and consequently the use of

27

plain (un-reinforced) sections could be dangerous if high tensile stresses occur. The

compression strength and tensile strength of concrete are as follows:

σc/c = 40 – 60 MPa

σc/t = 1/10 ( 40 – 60 )MPa ≈ 4 – 6 MPa

2.12 Resin Adhesive for Concrete Bonding

Wet, fresh or plastic concrete has little tendency to adhere to hardened concrete.

Good sub-surfaces preparation usually aimed at increasing the mechanical bond is a pre-

requisite. It is desirable to interpose at the bonding interface a layer of material that

adheres well to the hardened concrete and also to the fresh concrete being placed on it.

Concrete-concrete bonds using epoxy adhesives have mechanical properties greater than

those of concrete itself.

Typical average properties of such adhesives are

( i ) compressive strength = 77 MN/m2

(ii ) shear strength = 35 MN/m2

(iii) tensile strength = 20 – 28 MN/m2

When cured, epoxide resin bonding agents resist very well with the alkali from the

concrete as well as the moisture. The adhesive transmits compressive, tensile and shear

forces excellently, but situation causing peel stress should be avoided. Where these

forces cannot be avoided, special flexible compositions may be needed.

28

2.13 Summary

The FRP composite materials have excellent mechanical properties and suits any

application works. Advantages and disadvantages should be taken into consideration

while using FRP composite materials to avoid technical difficulties and reach the

required objective of usage.

The stress-strain curves for typical fibre, epoxy resin and FRP are different. The

fibre has higher stiffness properties compared to the FRP composite and epoxy resin.

The fibre content is 60% to 70% from its volume and the principal load carrying

elements for FRP made by pultrusion process. Epoxy is a solid organic material, usually

of high molecular weight, that exhibits a tendency to flow when subjected to stress. The

advantages of epoxy resins are such as high surface activity, minimal shrinkage, low

creep and superior strength retention under sustained load. The development of the basic

theory of reinforced concrete particularly as an efficient strengthening method should be

carried on.

29

CHAPTER III

LITERATURE STUDY ON BONDING TECHNOLOGY

3.0 Introduction

This chapter focuses on adhesive bonding, joint design and mechanisms of bond

failures, bonding characteristics and bonding formulation.

3.1 Adhesive Bonding Definition

Adhesive bonding is the process of uniting materials with the aid of an adhesive, a

substance capable of holding such materials together by surface attachment as mentioned by

H. Landrock. [9]

30

3.1.1 Advantages of adhesive bonding

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

structures for a variety of reasons as follows;

• Provide uniform distribution of stress and larger stress bearing area.

• Join thick or thin materials of any shape.

• Join similar or dissimilar materials.

• Minimize or prevent electrochemical corrosion between dissimilar materials.

• Resist fatigue and cyclic loads.

• Provide joints with smooth contours.

• Seal joints with smooth contours.

• Insulate against heat transfer and electrical conductance.

• Damp vibration and absorb shock.

• Provide and attractive strength/weight ratio.

3.1.2 Disadvantages of adhesive bonding

Adhesive bonding presents certain limitations as a fastening method for a

number of reasons as follows;

• The bond does not permit examination of the bond area (unless the adherends are

transparent).

31

• Careful surface preparation is required to obtain durable bonds, often with

corrosive chemicals.

• Holding fixtures, presses, ovens and autoclaves which are not required for other

fastening method are necessities for adhesive bonding.

• Rigid process control, including emphasis on cleanliness is required for most

adhesives.

• The useful life of the adhesives joint depends on the environment to which it is

exposed.

• Exposure to solvents used in cleaning or solvent cementing may cause health

problems.

Figure 3.1: Basic steps of adhesive bonding process [11]

32

3.2 Mechanism and Models of Adhesive Bonding

3.2.1 Mechanical Interlocking Model

This oldest adhesion theory considers adhesion to be result of the mechanical

interlocking of polymer adhesive into pores and other superficial asperities of substrates.

The roughness and porosity of substrates are generally suitable factors only in so far as

the wet ability by the adhesive is sufficient as shown in Figure 3.2. Otherwise, the non

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

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

adhesive on the substrates, referring to Shuo Yang. [10]

Figure 3.2: Good Wetting (A) and Poor Wetting (B) [10]

3.2.2 Electrical Model

This model treats the adhesive-substrates system as a plate capacitor whose

plates consist of electrical layer that occurs when two materials of different electrical

properties in nature are bought in contact. This model is only applicable in the case of

incompatible materials, like polymer and metallic substrates shown in Figure 3.3.

33

Figure 3.3: Electrical double layer at polymer-metal interfaces [10]

3.3 Joining Technique

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

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

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

adhesive-bonding flat adherends is shown in Figure 3.4.

Figure 3.4: Type of joints used in adhesive-bonding flat adherends [9]

34

3.3.1 Joint Design

Select joining technique

Select joint configuration

Select adhesive

Define lap length

Calculate τmax

continue

τmax ≤ τ allowable

Calculate σmax

σmax ≤ σallowable

Select new • Adhesive yes• Joint configuration • Joining technique

Does increase lap length reduce τmax or σmax No

No

yes

No

yes

Figure 3.5: Design Procedure of Adhesive Joint [12]

35

Referring to Maxwell Davis and David Bond [12], joint for adhesive bonding

must be designed particularly for the use of adhesives. The aim of joint design is to

obtain maximum strength for a given bond area. In designing joints especially for

adhesive bonding the basic characteristics of adhesives must dictate design of joints.

The flow system shown in Figure 3.5 is on the bonding design procedure which

starts with the recognizing the joint requirements such as to support and distributing the

internal forces and moments. Then it is followed by selecting the joint category. This

step is usually determined by loads that need to be transferred, or by the required joint

efficiency as a fraction of the strength. The geometry of the member to be joint,

suitability of the fabrication, component dimensions, manufacturing environments and

number of components in a production run also must be considered. Another factor

effecting includes service environment and the lifetime of the structure, requirements for

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

aesthetics and the cost.

The third step is selecting the joining technique. From the three types of

categories, there are a few type of joining technique such as:

i) Mechanical connections: bolted and riveted joints (shear loaded fastener), bolted

and riveted joints (axially loaded fasteners), clamped joints, threaded, contact

joints, strap joints and embedded fasteners

ii) Bonded connections: adhesively bonded joints, laminated joints, molded joints,

bonded insert joints and cast-in-joints

iii) Combined connections: bonded-bolted joints and bonded-riveted joints.

36

Then, the selection of joint configuration must be made. Typical of joint

configurations and loading within each joining technique are illustrated in Figure 3.5.

3.4 Bonded Joints

Bonded joint is where the surfaces are held together by means of structural

adhesive. This type of joint must satisfy all these conditions to reach its objective

correctly.

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 should not exceed an allowable tensile stress.

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

iv) The adherend must not exceed the allowable in-plane shear stress.[12]

Practically, in case of in-plane shear stress is not included with the design

conditions and may only be verified by testing. Typically, one or more of the three

conditions above will become critical before in-plane shear stress limit in the adherends

exceed.

37

3.5 Stresses and Strains

In bonded joint connection, there are four main loading modes may occur under

such loads;

i) Out-plane loads acting on a thick adherend produce peel loads.

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

stresses.

iii) Out-of-plane tensile loads produce tensile stresses.

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

joint produce cleavage.

Simultaneously, the joint typically loaded by several of these load components.

In bonded joint, the tensile, cleavage and peel loads must be avoided because it will

effect the joint connection. While the adhesive layers of bonded joint should primarily

be stressed in shear or compression, the strains (deformation) should also be considered

at the area where non-linear behavior of adherends or adhesive is expected.

3.6 Failure Modes

There are three primary types of failure modes that can be seen on joint failure,

namely;

38

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, so should be avoided.

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

exceeding the adhesive strength.

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

in excess of the adherend strength.

Figure 3.6 shows the typical locations of possible failure initiation and critical

strength.

Figure 3.6: Areas of failure initiation and critical strength [12]

When a (single/double-lap) connection loaded with in-plane loads, the stress

concentration failure exists at the end of the over lap. Figure 3.7 shows the shear stress

distribution. The location where high shear stresses occurred can be said as the failure

initiation.

Figure 3.7: Stress distribution of lap joint [12]

39

3.7 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 Figure 3.8, it can be seen that the maximum shear stresses

occurs at the end of the overlap.

Figure 3.8: A typical adhesive shear stress distribution in a lap joint according to

elastic-plastic model [12]

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 be long enough to provide a moderate loaded adhesive area in the

middle to resist creep deformations of the adhesive.

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

increasing its thickness, increasing the lap length, tapering the ends of the adherends and

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

concentrations at the end of the overlap.

40

3.8 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. [12]

The experimental specimen which will 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 is 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 adherend

matrix.

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

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

by the solvent, the pressure applied while bonding is also not enough.

41

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.

Referring from the testing of the specimen, 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. Referring

to the Figure 3.9 from Boyajian, D.V Davalos [13], it can be seen that the different type

of joint has it own mode of failure. For double strap joint, major problem that occurs are

peel failure if compared to the others joint technique likely to have shear failure.

Figure 3.9: Relative joint strength of various joint configurations [13]

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3.9 Adhesive

There are a few ways in which adhesives can be categorized;

i) Adhesive type: this factor divide the adhesives based on the polymer type

whether it has thermo set (infusibility and insoluble after curing) or

thermoplastic (fusible and soluble and soften when heated) base.

ii) Curing process activation; whether it chemical, solvent, heat or other

activation.

iii) Curing process requirements; it looks at curing temperature and cycle, curing

pressure and cycle or post-curing.

iv) Form of adhesive; the form of adhesive whether paste, liquid or film.

3.10 Adhesive mechanical properties

In applications, there are few important mechanical properties to look into and

understood;

i) shear modulus

ii) shear strength

iii) maximum shear strain

iv) tensile modulus

v) tensile (peel) strength.

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All the properties should be obtained from the manufacturer or by testing.

Consideration of environmental factors, such as temperature, moisture and chemical

among the factors will affect the mechanical properties of the bond characteristics.

From the Figure 3.10, we can see than the adhesives have either ductile or brittle

behavior and should be also considered when the joint is applied.

Figure 3.10: Typical brittle and ductile adhesive behavior [13]

Referring at creep property, adhesives will creep under constant load even at the

room temperature especially at elevated temperature. Usually, thermo set adhesives has

better creep resistance than thermoplastic adhesives.

3.11 Lap Joints

These are the most commonly used adhesive joints. They are simple to make, can

be used with thin adherends and stress the adhesive in its strongest direction. The most

effective way to increase bond strength is to increase the joint width. In addition to

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overlap width and length, the strength of the lap joint is dependent on the yield strength

of the adherend. Modulus and the thickness of the adherend determine its yield strength,

which should not exceed the joint strength. Figure 3.11 shows the tensile force on lap

joint at various condition.

Figure 3.11: Tensile force on lap joint showing (a) unloaded joint, (b) joint under

stress, and (c) stress distribution in adhesive [5]

3.11.1 Single lap joint

The lap joint, in which two straps are joined together with an overlay, is one of

the most common joints encountered in practice. The joint is easy to make and the

results are sensitive to both adhesive quality and adherend surface preparation.

The simplest analysis considers the adherends to be rigid and the adhesive to

deform only in shear. This is shown in Figure 3.14(a). If the width of the joint is b, the

length l, and the load P, then the shear stress τ is given by:

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τ = P [1]

bl

While adherend tensile stress will decrease linearly to zero over the joint length. In

Figure 3.14(b) is shown a similar joint but in which the adherends are now elastic. For

the upper adherend, the tensile stress is a maximum at A and falls to zero at B. Thus, the

tensile strain at A is larger than that at B and this strain must progressively reduce over

the length l. The converse is true for the lower adherend. Thus, assuming continuity of

the adhesive/adherend interface, the uniformly sheared parallelograms of adhesive

shown in Figure 3.14(a) become distorted to the shapes given in Figure 3.14(b). This

phenomenon is called differential shear. Essentially, this is what Volkersen analysed. It

is instructive to study Volkersen’s analysis first and then to add to it the subsequent,

more refined analyses and to see what effect they have.

(a)

(b)

Figure 3.12: Exaggerated deformations in loaded single lap joint [13]

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3.11.2 Double lap joint

The lap joint is a balanced construction because it is bonded on both sides of

concrete. The double lap joint experiences internal bending, as shown diagrammatically

in Figure 3.15. In a symmetrical double lap, the centre adherent experiences no net

bending moment, but the outer adherents bend, giving rise to tensile stresses across the

adhesive layer at the end of the overlap where they are not loaded and compressive

stresses at the end where they are loaded as shown in Figure 3.15.

Figure 3.13: Bending moments induced in the outer adherents of a double

lap joint [13]

The strength of the joint depends on the tensile yield strength of the adherent, its

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

be as thin as possible to avoid joint starvation.

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The analysis of the durability of joint related to a few matters:

i) 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, and the materials to be connected and

considering the application.

ii) Adherent used: The adherents be used should be suitable and to the adhesive.

Each adherend has it own properties that will provide different durability.

iii) Adherent preparations: Adherent prepared should follow the correct

procedure to give good adhesion and absorption by the contact between

adherent and the adhesive.

iv) Curing process; temperature and pressure: The adhesive only will give high

bond strength if completely cured. To reach this situation, the bonding needs

enough time, dry and clean environment and suitable curing temperature.

Uncompleted curing process can cause slipping problems of the adherents.

v) 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.

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3.12 Factors considered in designing Adhesive Bond Joint

Attention should be given to a few factors to make an appropriate and effective

adhesive bond joint. Basically, it involves four main points, the material involved in the

job requirement, the job design and cost.

The sub-factors are referring to:

• Type of material to be joined

• Hardness conditions and surface finishes

• Adherents thickness

• Part function

• The temperature range that the joint or assemble must be able to withstand

• The temperature of environment for most of service life and period time

• Humid environment; service time and the temperature

• Test referred

• Contamination in contact to the bond (solvents, oil and other fluids); the

temperature and exposure type

• Electrical continuity

• Required joint strength

• Stress withstands (tensile, shear, peel, compression, impact, vibration and

etc.)

• Tolerance of temperature and pressure of bonded part. [13]

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3.13 Mechanism of Bond Failure

Failure of adhesive can occur in three locations, that is failure at interface,

failure in the adhesive and failure in the adherent, referring from P. Mukhopadhyaya,

R.N Swamy (1998). [14]

3.13.1 Failure at Interface

This may arise through failure of an interlayer between the parent adherend

material and adhesive (an oxide coating or primer layer) or through failure of the

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

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

complexity of the layer militates against modeling at this level in any detail. However,

the purpose of the layer is to transfer normal and shear loads between the adherend and

the adhesive and its effect on joint performance (other than failure) is negligible. [14]

3.13.2 Failure in the Adhesive

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 an adherend. The various mode of failure are

illustrated in Figure 3.16.

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Figure 3.14: Cohesive and adhesive bond failure [14]

3.13.3 Failure in the Adherend

This will arise through excessive strain within the adherend material and is more

common for brittle materials. In particular, joints made with adherends of fibre

reinforced composite material and toughened adhesive initially fail by adherend failure,

usually de-lamination of ply closest to the adhesive. [14]

3.14 Summary

There are a few types of joint configurations and each type has different criteria

to be considered while applying the connection. Different types of joint has different

51

stress distribution and failure. The preparation process and suitable adhesive type will

determine the strength of the joint and should be considered carefully.

The mechanism and models of adhesive bonding may act on three ways;

Mechanical Interlocking Model, Electrical Model and Diffusion Model. Mode of joint

failure can be categorized as adhesive, cohesive or combination of both, and all these

depend on bond integration and loading imposed on the joint system.

Long term durability is one of the most important properties of many adhesive

bonds. Although it can be difficult to achieve in different environments, there are some

methods to slow the degradation process. Material selection and preparation, proper

surface preparation and proper design of the joint may maximize the durability of the

joints.

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CHAPTER IV

FORMULATION OF FRP-EPOXY-CONCRETE BONDED SYSTEM

In this chapter, the theoretical calculations on bond strength, distribution between

the stress along the joint and analysis on the adherend is shown. Furthermore, the aspect

on the behavior of bonding materials and the modes of failure that develops on the

bonded properties is discussed.

4.1 Definition of average bond strength

Figure 4.1: Double lap joint of CFRP and concrete system

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From Figure 4.1, when a structure is subjected to tensile forces, the bond strength

Bond Strength, τcfrp = Pmax 2A

= Pmax

2wL

τcfrp = average bond strength of CFRP plate ( MPa )

Pmax = ultimate load (N)

A = bonded area (= wL) (mm2)

w = width of CFRP bond plate (mm)

L = bond length (mm)

4.1.1 Definition of Local Shear Stress Distribution

Shear stress distribution, τ = ∆Fi-j

w∆L

Where, ∆Fi-j = Difference in longitudinal force between two

consecutive gauge locations

∆L = Spacing between two consecutive gauge locations

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4.2 Bond Transfer Lengths

L. Bizindavyi and K.W Neale [15] in their research shows the initial transfer

length is the distance from the loaded end of the joint to the point where the exponential

strain profiles reach zero strain. This value is constant for all load levels lower than the

initial cracking load, but varies with the number of plies. Beyond the cracking load, the

transfer length increases with crack propagation.

Assuming uniform shear stresses along the composite laminate or plate, the

values of the initial transfer length Lo for the CFRP to concrete joints can be estimated

as:

Lo = ξo . Fmax bp . τξo

τξo= maximum value of shear stress

bp = CFRP laminate/ plate width

ξo = relative load level at which cracking initiates

The estimated evolution of the transfer lengths, Lt for loads below and above

service load levels can be obtained by:

Lt = Lo for ξ = F/Fmax ≤ ξo

Lt = Lo + [ ] ]. (Lj – Lo) for ξo ≤ ξ ≤ ξmax where Lj is the joint length. ξ – ξo___ξmax – ξo

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4.3 Behavior of FRP to Concrete Bonded Joint

Figure 4.2: Pull push test of a single lap plate to concrete bonded joint. (a)

Elevation (b) Plan [16]

Figure 4.2 shows a single lap pull push test of a plate to concrete bonded joint, in

which the width and the thickness of each of the three components ( plate, adhesive

layer and concrete prism are constant along the length. The width and the thickness of

the plate are represented by bp and tp respectively, those of the concrete prism by bc and

tc respectively and the bonded length of the plate is denoted by L. The Young’s modulus

of the plate and concrete are Ep and Ec respectively. 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 ( the two adherends ) as

being subject to axial deformations while the adhesive layer can be assumed to be

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

uniformly distributed axial stresses only, with any bending effects neglected, while the

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

the 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.

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4.4 Failure Patterns

The failure of an adhesive bonded plate-concrete joint subjected to a uniaxial

tensile can occur in three ways:

(a) Cohesive failure in the adhesive layer

(b) Adhesion failure

(c) Concrete shearing failure