FIBER GLASS REINFORCED PLASTIC REBARS FOR CONCRETE …prr.hec.gov.pk/jspui/bitstream/123456789/1570/1/2120S.pdf · 2018-07-23 · FIBER GLASS REINFORCED PLASTIC REBARS FOR CONCRETE

FIBER GLASS REINFORCED PLASTIC

REBARS FOR CONCRETE STRUCTURES

RIAZ AHMAD GORAYA

2005-Ph.D-Civil-03

SUPERVISOR

PROF. DR. MUHAMMAD AKRAM TAHIR

_______________________________________________________________________

DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF ENGINEERING AND TECHNOLOGY

LAHORE, PAKISTAN

2013

FIBER GLASS REINFORCED PLASTIC

REBARS FOR CONCRETE STRUCTURES

by

RIAZ AHMAD GORAYA

2005-Ph.D-Civil-03

INTERNAL EXAMINER EXTERNAL EXAMINER

(Prof. Dr. Muhammad Akram Tahir) (Prof. Dr. Abdullah Saand)

CHAIRMAN DEAN

Civil Engineering Department Faculty of Civil Engineering

Dissertation submitted in partial fulfillment of the requirements for the Degree of Doctor of Philosophy in Civil Engineering

_______________________________________________________________________

DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF ENGINEERING AND TECHNOLOGY

LAHORE, PAKISTAN

2013

This dissertation is dedicated to

My Parents, Family and Friends

ACKNOWLEDGEMENTS

i

ACKNOWLEDGEMENTS

The author expresses his profound gratitude to his advisor, Professor Dr.

Muhammad Akram Tahir, for the highly valuable advices, extra-ordinary guidance and

consistent encouragement provided by him throughout this research work. The author

falls short of words while paying gratitude to him for the patience he observed during

improving the written work as well as for his kind and helping attitude throughout the

research work. This work could never has been completed without his outstanding

guidance and extended cooperation. The author feels extremely proud to have worked

under his supervision and consider it a great privilege.

The author is highly thankful to Professor Dr. Tamon Ueda of Hokkaido

University at Sappporo in Japan, Dr. Waseem Uddin Khalifa of University of Akron OH,

USA and Dr. Salman Azhar of Auburn University at Auburn in USA, for devoting their

precious time to serve as External Examiners. Author is also thankful to Professor Dr.

Abdullah Saand of Quaid-e-Azam University at Nawabshah in Pakistan for allocating his

valueable time to serve as inland External Examiner.

The author is highly thankful to Engr. Mehmood Khalid, CEO M/s Fiber Craft

Industries Lahore, Pakistan, for his tremendous assistance and cooperation during the

experimentation for development of GFRP rebars, which was done at his premises for the

first time in Pakistan. The author remain indebted to him for spending his resources

without any commercial interest, including the provision of pultrusion setup and making

necessary improvements in it, at his own cost, required to develop the GFRP rebars,

devoting his precious time for technical discussions as well as reviewing the experimental

schemes prepared by the author for the experimentation related to the development of

GFRP rebars.

The author owes his deep gratitude to late Prof. Dr. Muhammad Ashraf for his

encouraging attitude towards the research scholars. His positive and helping attitude will

always be memorized.

ACKNOWLEDGEMENTS

ii

Thank are also due to Chairman Civil Engineering Department, Dean Faculty of

Civil Engineering and staff of concrete, strength of materials as well as test floor

laboratories for their cooperation in successful completion of this research work.

Author is thankful to his colleagues for their sincere support in reducing the

problems encountered during the research work.

Finally the author is very thankful to his family members, who always prayed for

his success and have been a constant source of encouragement, including his elder son,

Mr. Shaoib Ahmad Goraya.

Riaz Ahmad Goraya

ABSTRACT

iii

ABSTRACT

An experimental program was conducted to develop Glass Fiber Reinforced

Plastic (GFRP) reinforcing bars (rebars) for the first time in Pakistan using available local

resources, with tensile and bond strengths closely conforming to the international

standards. The average bond strength of locally developed GFRP rebars was evaluated

using normal strength concrete through direct pullout and beam bond tests by varying the

bonded length, rebar diameter, concrete cover, surface texture as well as the concrete

strength. Sequence and methodology of research work was divided into three distinct

phases, in which first two were related with the development of GFRP rebars.

The optimum composition of resin mixture was determined first of all basing on barcol

hardness criterion through hit and trial approach using standard pultrusion process. Fifty

trial productions of GFRP rebars with barcol hardness tests were executed for this

purpose, and the optimum composition of resin mixture was finalized.

The next stage of experimental program was to achieve the optimized combination of

three process parameters namely, fiber content, pull speed and heating die temperature for

9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm diameter rebars. It was achieved initially

through hit and trial approach using the optimum composition of resin mixture for 9.5mm

and 25mm diameter rebars and production models were developed for these two rebar

diameters relating the tensile strength of rebar with fiber content, pull speed and heating

die temperature. These production models helped to reduce the trials for two comparable

diameter rebars of 13mm and 22mm respectively. Similarly optimum combinations of

process parameters were determined for remaining diameter rebars based on their

production models developed on same analogy thus reducing the time and cost of GFRP

rebars. Total 165 trial productions along with simple tension tests were executed for this

purpose. Finally a single and comprehensive model named as ‘unified production model’

was developed in which fiber content, pull speed, heating die temperature, rebar diameter

and its square were the main parameters. The experimental tensile strength results were

validated using the unified production model. The unified model is recommended as a

comprehensive guideline for the development of GFRP rebars in future where patent

details are not available.

ABSTRACT

iv

GFRP rebar surface texture was finalized through preliminary bond study with plain

GFRP rebars by conducting 16 direct pullout tests using four diameter, 9.5mm, 13mm,

19mm and 25mm rebars, two bonded lengths, 5.0 db & 7.0 db with concrete strength of

41.4 MPa, to check for comparable bond strength as per American reference GFRP

rebars, Aslan-100TM, developed by Hughes Brothers Inc. USA. The bond stress of plain

rebars was found quite low, therefore, deformed uncoated rebars were next developed and

subjected to simple direct pullout tests. A set of 24 simple direct pullout tests (without

recording the stroke or slip values) was conducted using 27.0 MPa concrete by combining

four diameter rebars of 9.5mm, 13mm, 19mm, & 25mm and three bonded lengths of 3.0

db, 5.0 db and 7.0 db. Two pullout specimen sizes, Ø150mm x 300mm and Ø100mm x

200mm, were used. These deformed rebars exhibited the bond stress well comparable

with the above reference GFRP rebars.

The final production of deformed uncoated and sand coated GFRP rebars was made in six

diameter rebars of 9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm using optimum

composition of resin mixture and optimum combinations of process parameters. Each lot

of final production was tested for quality assurance tests including barcol hardness,

tensile strength, tensile modulus of elasticity and the average bond strength.

The average bond stress of locally developed deformed GFRP rebars was evaluated

through 48 direct pullout tests using 41.4 MPa concrete, four diameter rebars of 9.5mm,

13mm, 19mm, & 25mm and three bonded lengths of 3.5 db, 5.0 db & 7.0 db. Two pullout

specimen sizes, Ø150mm x 300mm and Ø100mm x 200mm, were used for this purpose.

The bond study was carried out by varying the bonded length, rebar diameter, concrete

cover/confinement and surface texture of GFRP rebars. Average bond stress of locally

developed deformed GFRP rebars in flexure was evaluated through six beams using 41.4

MPa concrete, two diameter rebars of 13mm and 19mm with above three bonded lengths

by varying the bonded length as well as rebar diameter. The effect of joint action on

average bond stress of primary beams of junctions was also studied using the same

parameters as of individual beams. The bond evaluation studies were carried out to ensure

the bond performance of locally developed GFRP rebars for their effective composite

action in RC members.

A model for predicting the average bond stress was developed basing on the direct pullout

experimental results; half of which were used to calibrate the model and remaining half to

validate. The proposed pullout bond model was further validated using the published data

ABSTRACT

v

of direct pullout results by several researchers for the rebars whose surface textures were

comparable to the developed rebars. The model prediction agreed closely with the

experimental results.

The beam bond experimental results were also in close agreement with the published

beam bond results by several other researchers.

Basing on the barcol hardness, tensile strength, tensile modulus of elasticity, bond

strength comparisons with the ACI/ASTM requirements, reference GFRP rebars as well

as experimental results of several researchers, it may safely be claimed that the successful

development of GFRP rebars in Pakistan has been achieved, which is a major

breakthrough considering the poor to moderate technological facilities available in

Pakistan. The indigenous development process will help the country to economically

develop and use the GFRP rebars in RC flexure members for special applications as well

as to maintain the safety and durability of theses members.

CONTENTS

vi

TABLE OF CONTENTS Page

Acknowledgements

Abstract

Table of Contents

Chapter One: Introduction

i

iii

vi

1.1 Background and Problem Identification 1

1.2 Problem Statement and Research Need 2

1.3 Research Objectives 3

1.4 Research Scope and Methodology 5

1.5 Research Constraints and Limitations 13

Chapter Two: Literature Review

2.1 General 14

2.2 Materials 15

2.2.1 Plain Concrete 15

2.2.2 Steel Reinforcement 16

2.2.3 Corrosion of Steel Reinforcement 17

2.2.4 Fiber Reinforced Plastic Reinforcement and Glass Fibers 18

2.2.5 Matrix/ Resin 23

2.2.6 Filler, Accelerator and Catalysts for Resin Mixture 26

2.3 Processes 28

2.3.1 Fiberizing and Sizing 28

2.3.2 Process Parameters and Pultrusion Process 30

2.4 Bond Anchorages – GFRP Rebars 32

2.4.1 Average Bond Stress 32

2.4.2 Flexure Bond Stress 34

2.4.3 Development Length 35

2.4.4 Factors Influencing the Bond Stress of GFRP Rebars 36

CONTENTS

vii

2.4.5 Bond Failure Types 39

2.4.6 State of Bond Stress in Surrounding Concrete 41

2.5 Summary 42 Chapter Three: Experimentation for Development of GFRP Rebars

3.1 General 43

3.2 Collaboration with Local Industry 44

3.3 Selection and Procurement of Raw Materials 45

3.3.1 Glass Fibers 45

3.3.2 Resin 47

3.3.3 Other Raw Materials 48

3.4 Experimentation for Development of GFRP Rebars 48

3.4.1 Determination of Optimum Composition of Resin Mixture

51

3.4.2 Determination of Optimum Combination of Process Parameters 59

3.5 Final Production of GFRP Rebars and Quality Assurance Tests 92

3.5.1 Quality Assurance Tests 93

3.5.2 Discussion on Results

3.5.3 Geometry of Deformed GFRP Rebars

97

98

3.6 Comparison of GFRP Rebar Properties with Steel Rebars 99

3.7 Summary 99

Chapter Four: Production Models for GFRP Rebars

4.1 Hardness and Tensile Strength Experimental Results 101

4.2 Production Models for 9.5 mm and 25 mm Diameter Rebars 102

4.3 Production Models for Intermediate Diameter Rebars 106

4.4 Unified Production Model 111

4.5 Summary 114

CONTENTS

viii

Chapter Five: Experimentation for Bond Stress Evaluation

5.1 General 116

5.2 Experimental Program 116

5.3 Materials 117

5.3.1 Cement 117

5.3.2 Fine Aggregates 118

5.3.3 Coarse Aggregates 119

5.3.4 GFRP Rebars 119

5.3.5 Concrete Mix Proportions 119

5.4 Direct Pullout Testing 121

5.4.1 Test Specimens 121

5.4.2 Testing Setup and Procedure 121

5.5 Direct Pullout Test Results 122

5.6 Discussion on Direct Pullout Results 131

5.6.1 Effect of Bonded Length and Rebar Diameter Variation on Average Bond Stress 131

5.6.2 Effect of Cover Variation on Average Bond Stress

5.6.3 Effect of Surface Texture Variation on Average Bond Stress

5.6.4 Effect of Concrete Strength Variation on Average Bond Stress

132

137

141

5.7 Beam Bond Tests 144

5.7.1 Test Specimens and Testing of Beams 144

5.8 Results and Discussion on Beam Bond Tests 146

5.9 Evaluation of Reduction in Bond Stress of Junctions 153

5.9.1 Test Specimens and Testing of Junctions 153

5.10 Results and Discussion on Testing of Junctions 155

5.11 Summary 161

Chapter Six: Comparison and Verification of Bond Stress Experimental Results

6.1 General 163

CONTENTS

ix

6.2 Comparison of Pullout and Beam Bond Test Results 163

6.3 Comparison of Beam and Junction Test Results 166

6.4 Comparison of Experimental Bond Stress Results with Other Researchers 168

6.5 ACI Beam Bond Equation 176

6.6 Proposed Model for Direct Pullout Tests and Validation of Experimental Results

177

6.7 Summary

Chapter Seven: Conclusions and Recommendations

183

7.1 Conclusions 185

7.2 Recommendations for Use of Local GFRP Rebars in Concrete Flexural Members

189

7.3 Recommendations for Future Research Work 191

References 192

Appendices 197

CHAPTER-1 INTRODUCTION

1

INTRODUCTION

1.1 BACKGROUND AND PROBLEM IDENTIFICATION

The rapid increase in the use of Fiber Reinforced Plastic (FRP) reinforcements for

civil/structural engineering applications that has occurred over last two decades can be

attributed to continuing reductions in life cycle cost (LCC), and to the numerous

advantages of FRPs as compared to conventional materials such as concrete and steel.

According to John and Politécnica (2003), concrete is the largest man made item

consumed by human after food, hence, any improvement related to concrete structures

would have a significant impact on the human civilization.

During the preliminary stage of this research work, efforts were made to identify

the laggings of reinforced concrete (RC) members/structures. During the identification

stage, it was found that majority of structures in the world as well as in Pakistan are

comprised of reinforced concrete members, therefore, an enhancement in the structural

performance and overall durability of reinforced concrete would have a direct influence

not only on construction industry but the society as a whole.

Based on extensive site investigations carried out during the preliminary stage of

this research work, it was revealed that reinforced concrete members/structures were

inherently prone to cracking especially in tension zone leading to failure. The major cause

of cracking has been categorized into several classes but the most usual is corrosion of

steel reinforcing bars due to number of reasons including chemical attack, carbonation

effect, access of chlorides, moisture and air to the steel rebars, poor concrete quality etc.

Plain concrete is strong in compression but weak in tension and embedded steel

reinforcing bars (rebars) play an important role in carrying the tensile load through an

effective bond between these two materials. In RC members, steel rebars corrode due to

above stated reasons and volumetric expansion takes place resulting into cracking,

spalling of concrete and subsequently reduction in cross-sectional area of rebars, which

has detrimental effect on safety and durability of the RC members. It has also been


2

noticed that production of steel rebars is an energy expansive process and in energy

deprived country like Pakistan, an alternative of steel rebars would be welcomed and a

need of time.

In view of afore mentioned problem, civil engineering community in Pakistan was

striving for a change in the construction industry where an alternative to traditional steel

rebars may be used in RC flexural members, which would overcome the above stated

inherent problems of steel rebars. Keeping in view the above discussion, the research

work presented here focuses on the development of Glass Fiber Reinforced Plastic

(GFRP) rebars, with quality assurance tests, for their use in RC flexural members for

special applications in Pakistan. It is pertinent to note that these rebars have neither been

developed and used in Pakistan nor in any other developing country before this research

was undertaken; and even till to date GFRP rebars are being produced only by a limited

number of manufacturers in a very few technological advanced countries.

GFRP reinforcing bars possess number of potential advantages over conventional

steel rebars including non-corrosiveness, high resistivity against environmental effects,

high tensile strength to weight ratio etc.

1.2 PROBLEM STATEMENT AND RESEARCH NEED

As mentioned earlier, reinforced concrete’s tendency to develop cracks is well

known. Air and water content seep down to steel reinforcement through these cracks,

causing steel to rust and expand. The expansion forces spall the concrete and gradually

push it away from the steel rebars. The concrete spalling process accelerates in humid,

saline or chemically aggressive environments. It has been widely observed during the

field investigations that substantial amount of cross-sectional area of embedded steel

rebars in RC bridge decks was lost due to corrosion in just 20-25 years of their

construction. The reduction in cross-sectional area was as high as upto 35-40% in the

deck slab in certain cases.

Conventional rehabilitation techniques are not feasible and use of advance

imported synthetic materials is quite expansive in such large scale rehabilitation works.

Hence there was a dire need of development of economical alternative solution to address


3

such problems faced by the construction industry in general and from the civil

engineering point of view in particular.

GFRP rebars have presented a viable solution to the above stated problems and

acted as an effective alternate of conventional steel rebars in some technological

advanced countries including USA, Canada, Japan, China etc. as well as in some

European countries. FRP reinforcements including GFRP rebars are being manufactured

and used quite frequently in these countries in RC members of buildings, bridges and

other structures subjected to corrosive environments as well as in electromagnetic

applications etc.

However, due to technological limitations, presently GFRP rebars are not in

production and use anywhere in Pakistan as well as in any of the developing countries.

Being a patent and proprietary product, technical details related to the development of

GFRP rebars are not available publically. Moreover, it is not economically feasible to

purchase the patent technology, therefore, this study was undertaken to discover the

process details for the development of this new construction material in the form of GFRP

rebars, closely comparable with the international standards, to make it an open source

technology.

1.3 RESEARCH OBJECTIVES

The research objectives have been categorized into two phases. The first and

major was the development of GFRP rebars, closely conforming to the international

standards, for the first time in Pakistan using available local resources. This phase

required extensive knowledge of locally available resources and detailed analysis of

prevailing conditions for the successful development of these rebars. The sequence of this

important research objective/phase has been summarized as below:

1) Identification and collaboration with appropriate local industry, associated with

production of general fiber reinforced plastic (FRP) products, for the assistance

in development of GFRP rebars, where the following tasks could be

accomplished.


4

a) Search, selection and procurement of appropriate type, grade and form of

glass fibers, resin and other raw materials.

b) Implementation of standard pultrusion process.

c) Determination of optimum composition of resin mixture as well as

combination of process parameters through the trial productions and testing

of GFRP rebars.

2) Development of production models for cost reduction of production process

through reducing the number of trial production as well as for validation of

experimental tensile strength results of GFRP rebars.

3) Quality assurance testing of locally produced GFRP rebars.

The second objective of research work deals with the evaluation of average bond

stress of locally developed GFPR rebars. Since the projected ribs were not present on the

rebar surface as in case of steel rebars, therefore, the fundamental purpose of this phase

was to evaluate the average bond stress of these rebars with normal strength concrete to

ensure effective transfer of tensile forces from concrete to rebar for proper composite

action. The following tasks were performed in this phase.

Direct Pullout Tests

Beam Bond Tests

Testing of Junctions/ Intersecting Beams, to determine the reduction in average

bond stress of primary beams of junctions due to joint action.

Development of pullout bond model for validation of experimental pullout bond

stress results as well as comparison of beam bond results with the published

results of beam bond stress by several researchers.


5

1.4 RESEARCH SCOPE AND METHODOLOGY

The research scope was categorized into two parts. First and major was the

development of GFRP rebars with tensile and bond strengths closely conforming to the

international standards. It also included identifying and procuring the raw materials

required for the development of GFRP rebars. It is pertinent to note that no guideline

related to development of these rebars was available in the literature, that is why hit and

trial approach was adopted.

Second part comprised of evaluation of average bond strength of locally

developed GFRP rebars with normal strength concrete through direct pullout and beam

tests by varying the various bond affecting parameters. Sequence and methodology of

research scope was divided into following three distinct phases, in which first two were

related with the development of GFRP rebars.

1- Determination of optimum composition of resin mixture through fifty (50) trial

productions of GFRP rebars based on barcol hardness criterion using hit and trial

approach and standard pultrusion process.

2- Determination of optimum combination of three prime process parameters

namely, fiber content, pull speed and heating die temperature of the pultrusion

machine for each, 9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm diameter

deformed rebars. Selection criterion for optimum combinations of process

parameters was the tensile strength. Optimum combination was determined

initially through hit and trial approach, using the optimum composition of resin

mixture, for 9.5mm (the smallest) and 25mm (the largest) diameter rebars and

production models were developed for these two rebar diameters relating the

tensile strength of rebar with fiber content, pull speed and heating die temperature.

These production models helped to reduce the trials for two comparable diameter

rebars of 13mm and 22mm respectively. Similarly optimum combinations of

process parameters were determined for remaining diameter rebars based on their

production models developed on same analogy thus reducing the time and cost of

GFRP rebars. Total 165 trial productions along with simple tension tests were


6

executed for this purpose against the initially/originally planned two hundred

eighty (280) trial productions based on hit & trial approach.

Finally a single and comprehensive model named as ‘unified production

model’ was developed in which fiber content, pull speed, heating die temperature,

rebar diameter and its square were the main parameters. The experimental tensile

strength results were validated using the unified production model, and which will

also serve as a comprehensive guideline for the development of GFRP rebars in

future where patent details are not available.

It is pertinent to note that in order to finalize the surface texture of locally

developed GFRP rebars, which may result the comparable bond stress with the

American reference GFRP rebars, Aslan-100TM, developed by Hughes Brothers

Inc. USA; sixteen (16) plain GFRP rebars with and without sand coating treatment

were developed in four diameter rebars (db) of 9.5, 13, 19 and 22mm. The effect

of surface texture of GFRP rebar on average bond stress was studied by direct

pullout tests using 41.4 MPa concrete and two bonded lengths of 5.0 db as well as

7.0 db. The results of this bond study have been published (Goraya et al, 2010) and

experimental scheme as well as results have been given in Appendix-A. The

average bond stress of plain GFRP rebars was quite low, therefore deformed

rebars were next developed and subjected to simple direct pullout tests for

determining their average bond stresses.

A set of twenty four (24) simple direct pullout tests (without recording the

stroke of slip) was conducted using 27.0 MPa concrete by combining four

diameter rebars of 9.5, 13, 19 and 25mm and three bonded lengths of 3.0 db, 5.0 db

and 7.0 db. Two pullout specimen sizes, Ø150mm x 300mm and Ø100mm x

200mm, were used. The experimental schemes as well as results have been

provided in Appendix-B. The deformed GFRP rebars exhibited the average bond

stress well comparable with the reference rebars. Thus the deformed surface

texture for GFRP rebars was finalized due to its better bond performance.

The final production of deformed uncoated as well as sand coated GFRP

rebars in six diameters of 9.5mm, 13mm, 16mm, 19mm, 22mm and 25mm was


7

made using the optimum composition of resin mixture as well as combinations of

process parameters along with quality assurance tests. Barcol hardness, tensile

strength, tensile modulus of elasticity, water absorption, specific gravity, bond

strength etc. were determined and compared with ACI/ASTM standards as well as

with the reported properties of reference GFRP rebars, Aslan-100TM.

3- In the last phase of research work, bond study of locally developed GFRP rebars

was carried out to determine their average bond stresses at maximum pullout load

to ensure their effective composite action in RC members. The evaluation of

average bond stress of these rebars was made through direct pullout tests by

varying bonded length, rebar diameter, concrete cover and surface texture. Forty

eight (48) direct pullout tests were performed using 41.4 MPa compressive

strength concrete, two pullout specimen sizes of Ø150mm x 300mm and Ø100mm

x 200mm, two surface textures of deformed & sand coated, four diameter rebars

of 9.5mm, 13mm, 19mm and 25mm with three bonded lengths of 3.5 db, 5.0 db

and 7.0 db.

Average bond stress of locally developed GFRP deformed rebars in

flexure was evaluated through six beam bond tests by varying bonded length and

rebar diameter. Two diameters rebars of 13mm and 19mm, three bonded lengths

of 3.5 db, 5.0 db and 7.0 db and 41.4 MPa concrete were used in beam bond tests.

The effect of joint action on average bond stress of primary beams of junctions

was also studied through six junctions with same parameters as of

individual/reference beams.

A model for predicting the average bond stress was developed basing on

the direct pullout experimental results; half of which were used to calibrate the

model and remaining half to validate. The proposed pullout bond model was

further validated using the published data of direct pullout results by several

researchers for the rebars whose surface texture was well comparable to the

developed rebars.

The published pullout data used for the validation of proposed pullout bond model

has been given in Appendix-D. The experimental schemes of above three phases

have been presented in tables 1.1 to 1.5:


8

Table 1.1: Experimental Scheme for determination of optimum composition of resin

mixture based on hit and trial approach.

Preliminary Trial Production

Set ID

Quantities of Resin Mixture Ingredients (Phr) Planned Trials CO BPO TBPB

PTPS-1 0.20

1.00,1.33,1.67 & 2.00

1.00,1.33,1.67 & 2.00

16

PTPS-2 0.24 16

PTPS-3 0.28 1.00, 1.33 & 1.67 10

PTPS-4 0.26, 0.30 1.00 8

Total Planned Trials 50

The term “Phr” is an abbreviation of “Parts per hundred resin”, which is a standard term

used for resin mixture ingredients. The resin mixture ingredients CO, BPO and TBPB are the

abbreviations of Cobalt Octoate, Benzoyl Per Oxide and Tertiary Butyl Peroxy Benzoate

respectively. The optimum composition of resin mixture ingredients was same for all rebar

diameters; only the quantity of resin mixture has to vary for different diameter rebars.

The maximum desired hardness of GFRP rebars as per ASTM D-2583 was 50. A

closer value to 50 was required for finalizing the optimum composition of resin mixture.

After finalization of deformed surface texture, the next phase experimental work

was the determination of optimum combination of process parameters for each diameter

rebar.

The criterion for the selection of optimum combination of process parameters for

each rebar diameter was to have the tensile strength of trial production of GFRP rebar

close to the tensile strength of American reference GFRP rebars.

The experimental scheme for the determination of optimum combinations of

process parameters has been given in table 1.2.


9

Table 1.2: Original experimental scheme for determination of optimum combinations of

process parameters based on hit and trial approach.

Trial Production

Set ID

Rebar Diameter

(mm)

Process Parameters Planned

Trial Productions

Fiber Content

(%)

Pull Speed (mm/minute)

Heating Die Temperature (oC)

TPS-1

9.5

71

110,120,130,140

185,190,195,200,205

60 TPS-2 72

TPS-3 73

TPS-4

13 73

100,110,120,130

190,195,200,205,210

40

TPS-5 74

TPS-6

16 74

90,100,110,120

195,200,205,210,215

40

TPS-7 75

TPS-8

19 75

80,90,100,110

200,205,210,215,220

40

TPS-9 76

TPS-10

22 76

70,80,90,100

205,210,215,220,225

40

TPS-11 77

TPS-12

25

77

60,70,80,90

210,215,220,225,230

60 TPS-13 78

TPS-14 79

Total Planned Trials based on hit & trial approach 280


10

Using the optimum composition of resin mixture and combination of process

parameters, final product of deformed uncoated and sand coated GFRP rebars was

developed. Determination of average bond stress of this final production was carried out

through the direct pullout and beam bond tests as detailed below.

Table 1.3: Experimental Scheme of Pullout Tests for Uncoated Deformed GFRP rebars

using Ø150mm x 300mm Test Specimens and 41.4 MPa Compressive Strength Concrete.

Rebar ID

Rebar Diameter db (mm)

Clear Cover C (mm)

C/db

Ratio db/Lb Ratio

GFRD9-Lb3.5C70 9.5 70.25

7.39

1/3.5

GFRD9-Lb5.0C70 1/5.0

GFRD9-Lb7.0C70 1/7.0

GFRD13-Lb3.5C68 13 68.50

5.27

1/3.5

GFRD13-Lb5.0C68 1/5.0

GFRD13-Lb7.0C68 1/7.0

GFRD19-Lb3.5C65 19 65.50

3.45

1/3.5

GFRD19-Lb5.0C65 1/5.0

GFRD19-Lb7.0C65 1/7.0

GFRD25-Lb3.5C62 25 62.50

2.50

1/3.5

GFRD25-Lb5.0C62 1/5.0

GFRD25-Lb7.0C62 1/7.0

Note: GFRDxx-LbyyCzz stands for GFRP uncoated Deformed rebar, with xx diameter, Bonded Length

(Lb) of yy times the rebar diameter (db), and concrete clear cover (C) zz to GFRP rebars,

respectively.

The effect of bonded length and rebar diameter variation on average bond stress

was also studied using the deformed sand coated GFRP rebars with rebar ID, GFRSxx-

LbyyCzz and above same scheme.


11

Table 1.4: Experimental Scheme of Pullout Tests for Deformed Uncoated GFRP rebars


Rebar ID


Clear Cover C (mm)

C/db

Ratio db/Lb Ratio

GFRD9-Lb3.5C45 9.5

45.25

4.76

1/3.5

GFRD9-Lb5.0C45 1/5.0

GFRD9-Lb7.0C45 1/7.0

GFRD13-Lb3.5C43 13

43.50

3.35

1/3.5

GFRD13-Lb5.0C43 1/5.0

GFRD13-Lb7.0C43 1/7.0

GFRD19-Lb3.5C40 19

40.50

2.13

1/3.5

GFRD19-Lb5.0C40 1/5.0

GFRD19-Lb7.0C40 1/7.0

GFRD25-Lb3.5C37 25

37.50

1.50

1/3.5

GFRD25-Lb5.0C37 1/5.0

GFRD25-Lb7.0C37 1/7.0

The effect of bonded length and rebar diameter variation on average bond stress

was also studied using the deformed sand coated GFRP rebars with rebar ID, GFRSxx-

LbyyCzz and above same scheme.

After conducting the above stated direct pullout tests, beam bond tests were

performed to study the average bond stress of locally developed uncoated deformed

GFRP rebars in flexure. The beam size was 150mm x 225mm x 1165mm and remained

same for all the beam as well as junction tests.

Finally the effect of joint action on average bond stress of primary beams of

junctions was studied. Both intersecting beams were of the same size as of individual


12

beams. The experimental schemes for beam and junction tests have been given in table

1.5.

Table 1.5: Experimental scheme for beam and junction tests to study the effect of bonded

length and rebar diameter variation on average bond stress of deformed GFRP rebars.

Beam/Junction

ID

Main Rebar Diameter db (mm)

Supporting and Hanger Rebars Diameter

dbs (mm)

Lb/db Ratio

i1GFR19-Lb3.5

19

13 and 9.5

3.5

i2GFR19-Lb5.0 5.0

i3GFR19-Lb7.0 7.0

i4GFR13-Lb3.5

13

9.5

3.5

i5GFR13-Lb5.0 5.0

i6GFR13-Lb7.0 7.0

Note: inGFRxx-Lbyy stands for Beam or Junction No. ‘n’ with deformed uncoated GFRP main rebar

of Diameter xx and Bonded Length (Lb) of yy times the rebar diameter (db), respectively.

The successful development of GFRP rebars in Pakistan closely conforming, in

tensile strength, tensile modulus of elasticity, barcol hardness, water absorption as well

average bond stress properties, to the international standards is a major breakthrough

considering the poor to moderate technological facilities available in Pakistan and will

save a substantial amount of foreign currency which has to be paid either to import these

rebars or to purchase the patent technology. The indigenous development process will

help the country to economically develop and use the GFRP rebars in RC flexural

members for special applications like in highly corrosive environments, electromagnetic

fields etc., which otherwise substantially reduce the safety and durability of the structures

with the passage of time.


13

1.5 RESEARCH CONSTRAINTS AND LIMITATIONS

Identification and collaboration with appropriate local industry associated with the

production of general glass fiber items and having necessary basic infrastructure as well

as willingness to sacrifice time/resources for assistance in a non-commercial research

based assignment of development of GFRP rebars, was a huge challenge. Most of the

industries refused due to prevailing energy crises in Pakistan. Only one industry, M/s

Fiber Craft Industries, showed its consent for the needful assistance.

The available old pultrusion setup with the local industry was being used for the

production of general fiber reinforced plastic products like pipes, sheets etc. and was not

suitable, in the existing condition, for the development of GFRP rebars. Some necessary

improvements were made in the pultrusion setup prior to start the trial production process.

The production process for the development of GFRP rebars was started without

any previous experience as well as any specific guideline due to its non-availability in the

literature. There was no definite starting point for selecting the composition of resin

mixture ingredients except the recommended dose limits of accelerator and catalysts

provided by their manufacturers through data sheets. That is why hit and trial approach

was adopted for the production of GFRP rebars. Guidelines for selecting the

combinations of process parameters were derived from ASTM standard and general

experience of the industry individuals with the existing pultrusion machine. Due to lack

of experience of development of GFRP rebars, anchorage grips at the ends of each GFRP

rebar for tension test were not hardened enough to be gripped in the testing machine jaws

and crushed frequently in the start of development process. Various trials were run with

different hardeners and this process consumed a lot of time and resources.

Limited time allocation of technical personnel of the industry for assistance due to

heavy engagements in their own commercial assignments as well as prevailing energy

crises in Pakistan in the form of electric load shedding caused the delays, heavy

disturbance in the production process as well as wastage of raw materials etc.

CHAPTER-2 LITERATURE REVIEW

14

LITERATURE REVIEW

This chapter is a compilation of relevant knowledge that was useful for this

research work and has been presented here with the view that some of the engineers,

researchers and readers might not be very much familiar to this knowledge.

Extensive research is being carried out around the world on the issue of Fiber

Reinforce Plastic (FRP) reinforcement including the GFRP rebars along with their

performance evaluation. This research work has two folds namely, development of GFRP

rebars and evaluation of their properties including the tensile strength and average bond

strength with normal strength concrete. The effective bond between GFRP rebar and the

concrete can only ensure the proper composite action between these two materials.

This chapter describes the inherent corrosion problems of steel rebars along with

its effects on the performance of reinforced concrete members/structures, possible

ingredients of FRP/GFRP reinforcement with their properties and the pultrusion process

with various process parameters. This chapter also includes the necessary details related

to average bond strength of GFRP rebars and factors affecting the bond strength along

with different failure modes as well as the relevant research work done so far by several

researchers around the globe.

2.1 GENERAL

Concrete is comprised of cement, fine and coarse aggregates along with water.

Fresh concrete hardens after placement due to a chemical process known as hydration.

During the hydration process, water reacts with cement, which binds the other

components together, eventually creating a composite material.

Concrete construction dates back to the early ages of human civilizations.

Concrete is among one of the first man made construction materials which were produced

and used on large scale by human being. In essence, little more has been changed in the

evolution of concrete since its original inception, in the sense that newer materials have


15

been added to achieve better strength and performance. Evidences of early age concrete

have been found around the globe.

The history of modern concrete effectively begins, however with Joseph

Aspdin’s patent for the manufacture of Portland cement concrete in 1824 as early as the

1830’s, of reinforcing Portland cement with iron rods and bars. During the later half of

nineteenth century, practical techniques for reinforcing the concrete began to emerge and

the structural possibilities for this new material explored, cumulating in the erection of

Weaver’s flour Mill at Swansea in 1898, the first multi-storey concrete building in

Britain.

During the twentieth century, growth in the use of structural concrete was rapid as

engineers and architects began to realize its potential. This produced innovative and

ground breaking designs for civil engineering and building structures, both in terms of

their engineering technology as well as architectural design.

In Pakistan, most of the construction is comprised of reinforced concrete

members. Concrete industry provides immense support to the socio-economic setup of the

construction industry by employing manpower and consuming locally available materials

which gives a boost to the local industry.

The main problem faced by civil/concrete engineers today is just how to deal

with reinforced concrete and its architectural & engineering heritage. Whilst there has

been developments in the treatments to halt the damaging effects of corroding

reinforcement, which are detrimental to overall performance and esthetics of structures.

2.2 MATERIALS

2.2.1 Plain Concrete

Plain concrete contains specific proportions of Portland cement, sand, crushed

stone and water. Sometimes, chemical admixtures are also added in it to improve/modify

its properties. Fresh concrete hardens after wet mixing, placement and compaction by the


16

hydration process. Furthermore, additional binder and micro fillers may be added to

develop advanced form of concrete with enhanced performance and strength capabilities.

Concrete powers a US $35 billion industry which employs more than two million

workers in the United States alone. More than 55,000 miles of freeways and highways in

America have been made of this material. The People's Republic of China currently

consumes 40% of the world's cement (concrete) production (Lomborg, 2001 and

Wikipedia).

Plain concrete is strong in compression but weak in tension and shear, therefore to

overcome this potential weakness, reinforcing bars (rebars) are used in tension and shear

zones of the plain concrete and resulting concrete is called reinforced concrete (RC). The

concrete compressive strength has an important role in the structural performance of the

concrete. Better strength concrete performs better than the low strength concrete. The

concrete having 28 days compressive cylinder strength in the range of 20 to 45 MPa is

usually called the normal strength concrete. Concrete tensile strength is generally

believed to be around 1/10th of its compressive strength.

2.2.2 Steel Reinforcement

The aggregates in the hardened plain concrete efficiently carry the compression

load. However, it is weak in carrying tensile stress as cement binding the aggregates in

position can break, allowing the concrete member to crack and fail. Reinforced concrete

resolves this problem by introducing the steel reinforcing bars or glass fiber reinforced

plastic reinforcing bar to carry the tensile loads in tension zone of a RC member

(Macdonal, 2003).

To overcome the deficiency of plain concrete and to combat early shrinkage,

control the thermal expansion and contraction, steel reinforcement is included in locations

where tension occurs to form reinforced concrete. Since steel and concrete have almost

same coefficients of thermal expansion, therefore, they form an effective composite

material. The performance and appearance of RC depends on the individual materials as

well as on the environmental conditions and its maintenance in its service life. Improved

quality control and mix designs greatly reduce the problems associated with poor quality

construction, premature decay and high maintenance cost.


17

The major potential problem of steel reinforcing bars is the corrosion due to

which cracking in concrete members/structures takes place. Cracks initiate from the

interface of concrete and steel rebars which lead to spalling of concrete as well as

reduction in cross-sectional area of steel rebars resulting into safety threats, structural

instability and poor appearance of reinforced concrete members/structures.

2.2.3 Corrosion of Steel Reinforcement

Reinforcing steel bar present in concrete is protected against corrosion by high

alkalinity of the surrounding concrete, which creates a passivating layer at the surface of a

steel rebar. This passivating layer is composed of ferric oxide as well as of stable

compounds, which are more reactive. When ferrous oxide compounds come into contact

with aggressive agents like chloride ions, they react chemically with oxygen to form

solid, iron oxide corrosion products, which result in volumetric increase of steel rebar and

create an expansion force greater than the concrete tensile strength. The end result of

expansion of corroded steel rebar is the deterioration of concrete (Rixom and

Mailvaganam, 1999).

For occurrence of corrosion, chloride in the range of 0.59 to 0.89 kg/m3 is

required to be present. The reinforced concrete may be protected against corrosion by

lowering the water cement ratio or adding entraining air in concrete or increasing the

concrete cover over steel rebar or use of calcium-nitrate admixture or catholic protection

or adding an internal-barrier admixture or a combination of these techniques, incase

whenever there is possibility of chloride attack on concrete from external source like de-

icing salts (Rixom and Mailvaganam, 1999).

In the corrosion process of steel rebar in a moist environment, moisture on or near

the rebar surface behaves like electrolyte of corrosion cell, and the anode and cathode are

close together, e.g. across a single crystal or grain. The oxide is formed and deposited

away from rebar surface allowing the corrosion to continue. In concrete, electrolyte is the

pore water which is in contact with rebar surface, usually highly alkaline (pH = 12-13)

due to Ca (OH)2 from hydration of cement. It is also due to presence of Na2O and K2O in

the cement. The primary anodic product is Fe3O4, not the Fe++, deposited a thin film at

the rebar surface, which prevents any further corrosion. Steel reinforcing bar is said to be


18

protective and under such conditions, concrete provides an effective protective covering.

This protective layer can be ineffective in the following two situations:

i) Occurrence of carbonation of concrete, which results in loss of alkalinity.

ii) Due to chloride ions resulting from sea water.

In order to prevent corrosion and avoid the above indicated problems, calcium

nitrate admixtures may be added to concrete at the time of batch mixing. These

admixtures do not create a physical barrier to the ingress of chloride ion but modify the

concrete chemistry near steel rebar surface. The nitrite ions oxidize the present ferrous

oxide and convert it to the ferric oxide. The rebar surface absorbs the nitrite and fortifies

the ferric oxide passivating layer. The dose of calcium nitrite admixture is adjusted

according to exposure condition of concrete to the corrosive agents, for its maximum

effectiveness. Larger dosage should be used for greater exposures. The correct dosage of

a specific admixture for a particular situation can be determined based on the nature of

project as well as exposure data. Internal barrier chemicals/admixtures come in two

groups. One group comprises of damp proofing and water proofing chemicals. The

second group comprises of compounds that create an organic film around the steel rebar,

supplementing the passivating layer. The second type of admixture is usually promoted

for use at a fixed rate irrespective of expected chloride exposure.

It is pertinent to note that these corrosion protection measures are quite expensive

and economically not feasible. Corrosion resistance GFRP rebars have presented a viable

solution and acted as an effective alternative of conventional steel rebars in number of

technological advance countries.

2.2.4 Fiber Reinforced Plastic Reinforcement and Glass Fibers

Interest in the use of fiber reinforced plastic (FRP) reinforcement for civil

engineering structures has been increased steadily especially since the early 1990’s and

there are various field applications of FRPs in reinforced concrete structures. Japan has

been a leading country in terms of practical applications and the amount of FRP

reinforcement used for concrete (Ueda T, 2005), which has been steadily increasing.


19

Figure 2.1 and 2.2 show the number of practical applications for FRP reinforcement in

Japan.

Fig. 2.1: Number of practical applications for FRP reinforcing bars in Japan (Ueda T,

2005)

Fig. 2.2: Number of practical applications in Japan (Ueda T, 2005)


20

Some of the common FRP reinforcement applications for concrete structures include:

FRP reinforcing bars and pre-stressing tendons for the flexural members of concrete

structures.

Externally bonded FRP sheets, plates as well as wraps for rehabilitation and

strengthening of reinforced concrete, steel, and aluminium structural members.

Hybrid structures made of FRP materials.

Fiber reinforced plastic reinforcements/products are composite materials mainly

comprised of a resin matrix and the reinforcing fibers. Fibers are much stronger than the

matrix. The mechanical properties of FRP reinforcement primarily depend on the type of

fiber, quality, shape, orientation, adhesion properties of fibers with the matrix, volumetric

proportions of fibers as well as on the type of manufacturing process (ISIS, EC Module 3,

2003). The stress-strain relationship for fibrous reinforcement and resin matrix has been

given in figure 2.3.

Fig. 2.3: Stress-Strain relationship for fibers and matrix (ISIS, EC Module 3, 2003)

Figure 2.3 shows typical stress strain curves for fibers, matrices, and the FRP

materials that result from combination of fibers and matrix. Fibers used for the production

of composite materials have high tensile strength, toughness and stiffness, where as resin

matrix has low strength as well as stiffness. FRP reinforcement/product is a composite

Matrices

Strain [%]

Fibers

FRP

0.4 – 4.8 > 10

Stress [MPa]

1800-4900

600-3000

34-130


21

material combining the properties of fibers and matrix. The efficiency of fibers is affected

by their length, chemical composition and the cross-sectional shape.

There are various types of fibers usually used in civil/structural engineering

applications, out of which the most commonly used are glass, carbon (graphite), and to

the lesser extent aramid. The suitability of a particular fiber type for specific applications

depends on the required length, the stiffness, durability considerations, cost constraints

and the availability of component materials (ISIS, EC Module 2, 2003). The stress-strain

relationship for various pure fibers, excluding the effect of matrix, has been given in

figure 2.3.

Fig. 2.4: Stress-Strain relationship for various fibers and steel (ISIS, EC Module 2, 2003)

GFRP rebars are composed of glass fibers and matrix. Matrix results from resin

mixture, which consists of thermoset resin, filler, catalyst, accelerator etc. Glass fibers are

manufactured by a process known as direct melt with rapid and continuous drawing from

a glass melt. The diameters of glass fibers vary from 3 to 25 microns. It is an established

fact that glass fibers are cost efficient and consequently the most commonly used fibers in

civil engineering applications.

Two types of glass fibers, E-glass (Electrical) and S-glass (Strength) are usually

used. E-glass has the lowest cost of all commercially available fibers, and is used for

general purposes where strength, durability, acid resistance, electrical resistance and cost

efficiency are the main considerations. S-glass has relatively high strength, stiffness and

ultimate strain than the E-glass but with higher cost as well as more susceptible to

degradation in alkaline environments than the E-glass (Aslanova, 1985). Other types of

Strain (%)

0 1 2 3 4 5

Str

ess

(MP

a)

0

1000

2000

3000

4000

5000

6000E-GlassAramid-49Standard CarbonHigh-Modulus CarbonUltra High-ModulusReinforcing Steel


22

glass fibers include C (chemical) and AR (Alkali Resistant). The C-glass is usually used

where high chemical stability in an acidic environment is required. According to

Aslanova, 1985, AR-glass fibers are most commonly used to reduce the loss in strength as

well as in weight, when subjected to alkaline environment.

Typical physical and mechanical properties of various commercial glass fibers

have been presented in table 2.1.

Table 2.1: Typical Physical and Mechanical Properties of Glass Fibers.

Parameters E-glass S-glass C-glass AR-glass

Tensile Strength (GPa) 3.45 4.30 3.03 2.50

Tensile Modulus (GPa) 72.40 86.90 69.00 70.00

Ultimate Strain (%) 4.80 5.00 4.80 3.60

Poisson’s Ratio 0.20 0.22 - -

Density (g/cm3) 2.54 2.49 2.49 2.78

Diameter (um) 10.00 10.00 4.50 -

Longitudinal CTE (10-6/ oC) 5.00 2.90 7.20 -

Dielectric Constant 6.30 5.10 - -

Source: Benmokrane et al. (1995). CTE: Coefficient of Thermal Expansion

Another type of glass fibers is the E-CR glass (Electrical Corrosion Resistant glass

exhibiting corrosion resistant properties) which is an improved form of E-glass and

without Boron and Fluorine making it environment friendly. E-CR glass has better

resistance to acids and alkalis, which is obtained through the application of special

treatment and sizing to the E-glass. Sizing is a process to improve the bond between

filament (individual fiber) surface and resin in a composite material by applying the

treatment to filament. Furthermore, sizing usually consists of ingredients which provide

lubricity to the filament surface, safeguard against abrasive damage during handling and a


23

binder which enhances strand integrity and facilitates the packing of filaments. Typical

properties of ECR glass fibers have been given in table 2.2.

Table 2.2: Typical Properties of E-CR Glass Fibers.

Coefficient of linear

expansion

(10-6/oC)

Reference

index

(bulk)

Weight loss in

24 hours in 10%

H2SO4 (%)

Tensile strength

at 23 oC

(MPa)

Young’s

modulus

(GPa)

Filament

elongation

at break (%)

5.9 1.576 5 3100 - 3800 80 - 81 4.5 – 4.9

Source: Frederick T. et al. (2001)

2.2.5 Matrix/ Resin

Matrix/resin is a binder for glass fibers in the production of various composites as

well as GFRP rebars (ISIS, EC Module 2, 2003). It is used for the following functions:

binding the fibers/reinforcement together;

protection of fibers from abrasion as well as environmental degradation;

separation and dispersion of fibers within the composite;

transfer of force among the individual fibers; and

development of compatibility with the fibers (chemically as well as thermally)

Fibers/reinforcement provides strength and stiffness to the composite/GFRP rebar

and matrix is necessary to transfer forces among the individual fibers. This force transfer

phenomena is achieved through shear stresses that develop in matrix among the

individual fibers. The quality of bond between fibers and matrix is a prime factor in

getting the good mechanical properties of composites. Matrix selection has the single

major impact on long term performance of the composite/GFRP rebar and have a

significant impact on the cost as well (ISIS EC Module 3, 2003).

Thermoset resins are used in the production of composites as well as GFRP rebars

instead of thermoplastic resins due to low molecular weight and low viscosity. Their


24

molecules are joined together by chemical cross links; hence, these resins form a rigid

three dimensional structure that once set, cannot be changed or reshaped by applying heat

or pressure. Commonly used thermosetting resins are polyesters, vinyl esters and epoxies.

These resins have good chemical resistance as well as thermal stability and undergo low

creep & stress relaxation. However, these resins have comparatively low strain to failure

resulting in low impact strength along with less shelf life. The most commonly used resin

for the production of GFRP rebars is the corrosion resistant vinyl ester. Typical physical

and mechanical properties of commercially available thermoset resin materials have been

given in table 2.3.

Table 2.3: Typical Physical and Mechanical Properties of Thermoset Resin Materials.

Parameters Polyester Epoxy Vinyl ester

Tensile Strength (MPa) 20.00 - 100.00 55.00 - 130.00 70.00 – 80.00

Tensile Modulus (GPa) 2.10 - 4.10 2.50 - 4.10 3.00 - 3.50

Ultimate Strain (%) l.00 - 6.00 1.00 - 9.00 3.50 - 5.50

Poisson’s Ratio - 0.20 - 0.33 -

Density (g/cm3) 1.00 - l .45 1.10 - 1.30 1.10 - 1.30

CTE (10-6/oC) 55.00 - 100.00 45.00 - 90.00 21.00 - 73.00

Cure Shrinkage 5.00 - 12.00 l.00 - 5.00 5.40 - 10.30

Source: Bakis, (1993).

Another important selection criterion for matrix/resin materials is that it should

have low density, usually much less than the fibers, such that overall weight of composite

is minimized. One of the most widely used corrosion resistant vinyl ester resin for the

production of FRP/GFPR rebars is with the brand name of Hetron 922, prepared by M/s

Ashland USA. The important reported properties of this vinyl ester resin based on

laminates have been given below:


25

Thermal Conductivity (K-Value): When glass content of a glass reinforced laminate

increases, the thermal conductivity increases. The resin has low thermal conductivity than

the glass fibers. Table 2.4 indicates the thermal conductivity values.

Table 2.4 - Thermal Conductivity (Typical K Values: W/(m °C)

Composite Composite

Resin Casting M/M M/Wr/M/Wr

Glass Content (%) 0 25 40

HETRON-922 0.18 0.20 0.22

M = Chopped Mat 0.50 kg/m2 Wr = Woven Roving 0.80 kg/m2

Source: M/s Ashland USA, Manufacturer’s data sheet, 2001.

Thermal Expansion/Contraction: When fiber content increases, the thermal expansion

of composite decreases. The thermal expansion depends on fiber content, type and

orientation of the fibers. Table 2.5 gives the thermal expansion values.

Table 2.5: Coefficient of Linear Thermal Expansion1 (Typical Values: x 105mm/mm/°C)

Laminate Laminate

Resin Casting M/M M/Wr/M/Wr

Glass Content (%) 0 25 40

HETRON 922 5.68 2.83 2.19 1 Harrop Thermodilatometric Analyzer from –30 to 30° C. The CLTE is linear from –30 to 100 °C for

the glass reinforced laminates.



Volumetric Cure Shrinkage: During the curing process of a composite, the liquid resin

decreases in volume. The linear shrinkage of a glass reinforced laminate depends on

content, type and orientation of the fiber. Table 2.6 indicates the typical volumetric

shrinkage values.


26

Table 2.6: Volumetric Cure Shrinkage of Castings (Typical Values)

Resin

Density of Liquid

(g/cm3)

Density of Solid

(g/cm3)

Shrinkage (%)

HETRON-922

1.04 1.14 9.6


2.2.6 Filler, Accelerator and Catalysts for Resin Mixture

A resin mixture consists of resin, filler, accelerator, and catalysts etc., used for the

production of FRP/GFRP reinforcement including rebars through the standard pultrusion

process. The function of filler is to reduce the surface voids. The filler selection is based

on required degree of saturation (wet out) of the fibers. Lesser filler quantities are used

for the composites based on rovings whereas larger quantities are used for mat

composites. The viscosity of resin mixture depends on the size of filler particle and small

particles increase the viscosity due to more resin absorption. Calcium carbonate as filler is

usually considered the most economical one.

A highly active oxidizing accelerator (promoter) is used to accelerate the chemical

reaction between resin and the catalyst. Examples may include diethyl aniline, cobalt

octoate and cobalt naphthanate. Cobalt octoate is a cost efficient as well as compatible

with the tropical weather conditions. Its structural formula is

[CH3(CH2)3CH(C2H5)COO]2Co.

Catalyst is a liquid chemical which changes the rate of chemical reaction without

itself undergoing permanent change in its composition. It initiates and accelerates the

polymerization reaction of resin for curing of a composite and improves the corrosion

resistance, when added in small quantity. There are numerous catalyst systems available.

A resin catalyst system that provides high resistance against corrosion in chemical

environments is MEKP/CoNap catalyst system, which is called as Benzoyl

peroxide/Dimethylaniline (BPO/DMA).


27

Suitability of a catalyst also depends on the temperature requirements as well as

the cost efficiency. Keeping in view the tropical weather conditions of Pakistan as well as

the cost efficiency, the suitability of combination of following two catalysts is quite high.

a) Benzoyl Peroxide (BPO)

Benzoyl Peroxide (BPO) is an organic compound in peroxide family. It consists of

two benzoyl groups bridged by a peroxide link. Its structural formula is

[C6H5C(O)]2O2. It is one of the most important organic peroxides in the form of

applications and scale of its commercial production. The usual recommended dosage

of BPO by the manufacturers is 1% to 2%.

b) Tertiary Butyl Peroxybenzoate (TBPB)

Organic Peroxides are quite powerful oxidizing agents which release the oxygen.

These are commonly used as catalysts, initiators and cross linking agents for

polymerization process in the manufacturing of plastics, chemical intermediates,

cleaning and bleaching agents etc. Tertiary Butyl Peroxybenzoate (TBPB) is a strong

free radical source consisting of more than 8.1% of active oxygen which is used as a

polymerization initiator, catalyst and vulcanizing agent, cross linking agent as well

as chemical intermediate with structural formula of C6H5CO2OC(CH3)3. The usual

recommended dosage of TBPB by the manufacturers is 1% to 2%.

The combination of resin, filler, accelerator and catalysts results into formation of

a resin mixture which is then combined with the fibers to produce FRP/GFRP

reinforcement including rebars. The criterion of hardness is used for the determination of

optimum composition of resin mixture ingredients. Norwood H, (1983) has reported that

the standard industrial method for determining the effects arising from cure conditions

and corrosive media is to measure the barcol hardness. The value of barcol hardness is an

indication of surface cure and determined as per ASTM D-2583 standard. For each resin,

the fiber reinforced plastic laminates shall have minimum 90% hardness value of the

manufacturer’s specified value as per ASTM D-2583.

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30

2.3.2 Process Parameters and Pultrusion Process

Number of researchers has studied the impact of process parameters on

mechanical properties of general composites/laminates produced by pultrusion process.

The process parameters include fiber volume fraction, pull speed and heating die

temperature of the pultrusion machine, resin viscosity, impregnation duration etc. Out of

these parameters, first three are usually considered the most influential on mechanical

properties of the final product.

Cowen et al. (1986) investigated the flexural strength and tensile modulus of

elasticity for different glass fiber material combinations and concluded that the flexural

strength decreased with increasing pulling speed while initial modulus was affected much

less. Ma et al. (1990) determined the mechanical properties for different glass material

combinations as function of change in fiber volume content as well as pulling speed. The

flexural strength was found to increase with increasing fiber volume fraction up to certain

limit and decrease with the higher pull speeds. Vaughan et al. (1990) found that pre-

heater temperature, pulling speed and cooling rate were the influential process variables

in their overall effect on the mechanical properties of pultruded composites. Astrom et al.

(1994) investigated flexural properties of pultruded shapes/sections as a function of

pulling speed and temperatures of preheater, heating die as well as cooling die. It was

found that only the pulling speed had significant influence on the flexural properties,

where an increased pulling speed lead to decrease in the strength.

Moschiar SM et al. (1996) and Joshi Sunil C, (2006) have studied that temperature

profile along the heating die is important because if the temperature is low, resin mixture

will not completely cure due to less heat transfer and if the temperature is too high, it will

degrade the composite surface with over curing effects which creates manufacturing

problems like the most critical one is, when produced profile breaks inside the heating

die. Sarrionandia M et al. (2002) investigated that pulling speed depends on various

conditions including the bar size, heating die length, die temperature and resin

formulation. They also concluded that in order to incorporate all these conditions,

experience is the fundamental to achieve the optimum speed with high quality standards.


31

Wilcox et al. (1998) analyzed the optimization of pultrusion process. When they

examined the process in detail, number of parameters affecting the quality and process

efficiency of final product was quite large. Process parameters may range from the basic

such as process speed to the more complex such as cross-linking reaction of resin mixture

in the heating die. Relationships between different parameters are more important than

the individual ones. The usual compact mathematical description of the whole process is

not feasible in commercial production environments due to complexity of the problem.

Wilcox et al. described the effect of design on pultrusion material requirements and on

inter-process relationship. They explained the requirements of design, which initially

dictate the fiber/reinforcement type and lay-up as well as the resin formulation. The raw

materials specified as a result of design specifications will affect the pultrusion process.

The parameters affected are not only those set by the operator, but also important are the

inter-process parameters that occur as a result of materials and process settings. These

include the pull force, position of peak exotherm as well as the impregnation time.

Figure 2.7 demonstrates the process parameters breakdown for pultrusion process.

Each parameter has a dominant effect on the overall properties of final product of

composite. It may be observed that heat transfer to the composite in heating die through

the control of die temperature and pull speed plays a vital role for mechanical properties

of composites.

Fig. 2.7: Schematic Pultrusion Process (Wilcox et al, 1998)


32

Despite of having general understanding of inter-relation and impact of process

parameters on mechanical properties of general composites, no guideline was found for

the combination of these parameters for the development of FRP/GFRP rebars. Therefore,

trial and error approach has to be employed to determine the required combination of

process parameters. Fiber volume fraction, pull speed and heating die temperature have

major impact on the mechanical properties of composites. Thus these three prime

parameters have to be considered for the production of FRP/GFRP rebars. Range of

heating die temperature as well as of pull speed varies from machine to machine and

required combination of them, at which the composite have maximum strength, can be

determined only through the trial productions of rebars.

The criterion for finalizing the process parameters is the tensile strength

requirement. An optimum combination of process parameters gives the fully cured profile

with maximum tensile strength. Excess to the optimum value of any parameter results

into over curing effects on composite/rebar surface including the distortion of surface,

undulations, twisting, cracks as well as the worst situation of stucking of composite/rebar

in heating die, which may cause damage to the die.

Hunay Y et al. (2001) found after experimental study that under cure of the resin

mixture/matrix will not generally produce optimum tensile properties of the composite

and in applications where corrosive environments are faced by the composite, further

degradation of the mechanical properties can occur.

2.4 BOND AND ANCHORAGE – GFRP REBARS

2.4.1 Average Bond Stress

Average bond stress is the shear stress present at the interface of rebar and

concrete. This shear stress is main cause of load transfer from concrete to the rebar.

“Bond stress is usually defined as the shear force per unit area of rebar surface”.

When this bond is effectively developed, it enables the two individual materials to form a

composite material. Considering the uniform bond stress distribution along the bonded

length in a direct pullout test, the average bond stress ‘u’ can be determined as


33

u ∑

= ∆ / ∑

= ∆fs db2

4 db

u = ∆ .

Where;

u = average bond stress

q = change of force in rebar (fs) per unit length

Ab = cross-sectional area of rebar

∑o = surface area of rebar per unit length

∆fs = change of rebar force

db = nominal diameter of rebar; all in consistent units

Bond stress develops in a reinforced concrete member due to change in rebar force along

its length with the change in loading and/or from anchorage of rebar.

Alternatively, the average bond stress can be determined from the pullout force

‘F’, using the following basic equation:

Where;

Lb = bonded length of rebar in concrete

The bond stress distribution for a specific bonded length (length of rebar surface

in contact with concrete) is usually assumed uniform. The actual bond stress distribution

is not possible to determine as the bond force changes at cracks and magnitude of tensile

force also changes (Nilson & Winter, 1991).


34

2.4.2 Flexure Bond Stress

For an ideal beam action, the tension force ‘T’ should vary at similar rate of the

bending moment.

∆T = u∑o∆x

∆M = ∆T. jd

Rate of change of bending moment (∆M) = Shear force (V)

V = ∆M/∆x

V∆x = ∆M

V∆x = ∆T. jd

V∆x/jd = ∆T

Comparing ∆T

V∆x/Jd = u∑o∆x

u = V/jd. ∑o ; all in consistent units

Where;

∆T = change in internal resultant tension force

∆x = change in distance

jd = internal lever arm between resultant compression and tension forces

Above equation shows that when shear force is high, average bond stress ‘u’ will

have high value. However, actual bond stress is higher than the above because of

presence of cracks in the concrete at discrete intervals along concrete member which

results into additional bond stress due to tension carried by the concrete between cracks.

Alternatively, the average bond stress can be determined in flexure from beam

bond tests using the following equation:


35

Where;

T = resultant tension force = M/jd

M = bending moment

The average bond stress can also be determined in a beam bond tests by directly

measuring the strain in the GFRP rebar and using the following equation.

4

Where;

E = tensile modulus of elasticity

The average bond stress determined from the beam bond tests is always lower

than the bond stress obtained from the direct pullout tests due to difference in their

structural behaviors. Experimental results of bond performance evaluation obtained by

Benmokrane et al. (1996) concluded that the bond stress values of beam bond tests were

more than 50% smaller than that of direct pullout tests.

2.4.3 Development Length

The development length ‘Ld’ is defined as the length of embedded rebar necessary

to develop the full tensile strength of the rebar, controlled by either pullout or splitting

(Nilson et al, 2010). For the safety against bond failure, the rebar should be extended by a

distance ‘Ld’ beyond any section at which it is required to develop a given force hence

distance ‘Ld’ is required to transmit the rebar tension force ‘T’ to the concrete through the

bond.

T = Ab fs = db². fs ….……………….……1

Bond shear force = u∑o Ld

= u. db. Ld ………..…………..…….….2

Equating 1 and 2

db². fs = u db Ld

. = Ld …………………………………...…3


36

The ultimate stress of a rebar can only be established at a particular section if that

rebar is embedded in concrete up to a sufficient distance beyond a certain point. This

length of rebar beyond that certain point is known as development length. The certain

point may be the section of maximum bending moment. It is to note that development

length is proportional to the tensile strength of concrete.

2.4.4 Factors Influencing the Bond Stress of GFRP Rebars

Bond is an important phenomenon for effective composite action between GFRP

rebar and the concrete. There are number of factors which affect the average bond stress

of GFRP rebars with concrete including the bonded length, rebar size/diameter, concrete

cover/confinement, rebar surface texture, concrete compressive strength etc. Bond stress

studies have been carried out by several researchers around the globe to determine the

impact of these factors on the average bond stress. Most of these studies are based on

pullout tests and a few on beam tests. Pullout test method consists of embedding the

GFRP rebar for a specific length into a concrete cylinder or concrete block.

The experimental work carried out by several researchers states that average bond

stress decreases with increasing bonded length. The decrease in bond stress is not

proportional to increase in the bonded length. The distribution pattern of bond stress

along bonded length is usually determined through the ratio of maximum to average bond

stress. Bond stress is said to be uniform if this value is minimum whereas large values

indicate the non-uniform bond stress distribution.

Johnston and Zia (1982) performed a series of beam bond tests on epoxy coated

rebars to compare their performance with normal mild steel rebars. They found that the

epoxy coated rebars exhibited higher slips than mild steel rebars at lower loads.

Moreover, cracks and pullouts in the concrete developed earlier for epoxy coated rebars.

They also concluded that epoxy coated rebars have less strength & slip resistance and

form cracks earlier than normal mild steel rebars. In both epoxy coated and mild steel

rebars, bond stress decreased with the increase in bonded length and rebar size, therefore,

epoxy coating has no effect on these parameters. Johnston and Zia also recommended a

15% increase in the development length to compensate for the reduced performance of

the epoxy coated rebars.


37

Clark and Johnston (1983) performed a study using beam bond tests. They

explained the variation (increase then decrease of bond stress with bonded length) of

results by their belief that high localized stresses are encountered at a certain point and

after that point, decrease in bond stress will begin. Other results showed that earlier

loaded rebars had higher values of slip than those loaded at 28 days. They also concluded

that controlled early loading resulted in no harmful effect on the ultimate bond stress.

Larralde and Silva-Rodriguez (1993) did a study with 9.5 mm and 15.9 mm

diameter GFRP rebars and compared their results with steel rebars of same sizes. They

found that reduction in bond stress due to an increased bonded length was due to non-

linear bond stress distribution along the bonded length. They compared their GFRP

experimental data with steel rebar data and concluded that GFRP has a lower bond stress

and higher slip at failure than the typical steel reinforcing bars.

Brown and Bartholomew (1993) tested No. 3 (9.5mm diameter) rebars, from two

manufacturers, to characterize the rebars bond behavior. They determined that bond stress

decreases with longer bonded lengths. They also compared their bond stress values with

that of steel and found that the GFRP reinforcing bars had approximately two third the

bond stress of steel rebars.

Larralde, et al. (1994) performed a series of tests with pullout test method. They

concluded that GFRP rebars exhibited a decrease in bond stress for an increase in the

bonded length. Upon analysis of results, they concluded that a non-linear bond stress

distribution exists between concrete and the rebar. According to their results, majority of

the stress was taken by concrete surface near the loaded end of concrete cylinder.

Chaallal and Benmokrane (1995) carried out a study using the direct pullout test

method. They found that development length required to develop full capacity of GFRP

rebar is 20 times the rebar diameter.

Ehsani, et al. (1996) conducted a study that compared the bond behavior of GFRP

rebars using pullout test method as well as beam bond tests. Pullout specimens were

tested with No. 3, 6 and 9 rebar sizes with varying bonded lengths, and compared with

beam tests results. Upon comparison of the results, they found that pullout test method


38

gave non-conservative bonded lengths. Analyzing of experimental data, they found the

ultimate bond stresses increased by an average of 13% when the pullout test method was

adopted. From this comparison, they concluded that beam tests should be performed to

determine the bond behavior of GFRP reinforcing bars in concrete.

Al Zahrani, et al. (1999) conducted an experimental study that investigated the

modes of bond failure of lugged GFRP rebars. They observed that all rebars failed due to

shearing off the machined lugs. It was found that bonded lengths equal to five times rebar

diameter had 25% more bond stress than the ten times rebar diameter bonded lengths.

They believed that a non-linear bond stress distribution exists along the longer bonded

lengths. They also studied whether height of lugs affect the bond stress or not, and found

that it did not. Finally, they concluded that type of fibers also affects the bond stress of

GFRP rebars.

Beam bond test is a method to determine the bond stress response in flexure. In a

beam test, concrete around the rebar is in tension and considered as more realistic than the

pullout test method. Beam test may have two pieces of rebar cast in opposite sides of a

rectangular block and rebars run parallel to the longer side of rectangle having sufficient

concrete cover on all sides, so that splitting of concrete does not occur. Loads are applied

to one of the rebar specimens and vertical & horizontal reactions are applied at bottom

corner of the loaded sides. Another vertical reaction is also applied at the top of free end

to ensure the level of block. Loads are measured and deformation readings are taken at

the loaded as well as free ends of rebars.

The bond stress increases with the increase in concrete cover to rebar due to more

confining pressure on the rebar. Because of high confining pressure due to large concrete

covers or confining reinforcement, the size effect vanishes, according to Ichinose et al.

(2004).

Okelo et al. (2005) tested 151 pullout specimens and proposed an equation

showing the average bond stress as a function of concrete compressive strength and the

rebar diameter, which is as follow:


39

14.7

Whereas;

u = average bond stress (in MPa)

db = rebar diameter (in mm)

= specified concrete compressive strength (in MPa)

According to S.P Tastani et al. (2006), average bond stress is equivalent to

consider a uniform bond stress distribution along the bonded length. This assumes a linear

variation of normal stresses and strains, as the GFRP rebars are linear elastic up to failure

without de-bonding. The validity of this approximation is for shorter bonded lengths, few

times the rebar diameter so as to preclude development of considerable shear lag in the

concrete cover. For longer bonded lengths, actual bond behavior changes from the

assumption of uniform bond stress distribution.

Qingduo et al. (2009) tested ninety direct pullout specimens using GFRP ribbed

rebars with normal strength concrete. They recommended that the optimum rib spacing

should be equal to the rebar diameter and rib height equal to 6% of the rebar diameter.

Marta Baena et al. (2009) studied the interfacial bond behavior between various

types of glass and carbon FRP rebars using two types of concrete and 5.0 db bonded

length with eighty eight direct pullout tests. Their experimental results confirmed the

rebars tendency to have lower bond strength with larger diameter rebars. The effect of

rebar surface treatment on bond strength was also found less important in low strength

concretes as compared to higher strength concretes.

2.4.5 Bond Failure Types

Primarily there are following two types of bond failure.

a) Splitting

b) Pullout


40

Sometimes a mixed mode failure occurs containing both of above two patterns.

The mode of bond failure is dependent on concrete cover/confinement, surface texture of

rebar, concrete compressive strength and the bonded length (Okelo et al, 2005).

Incase of small concrete cover/confinement, splitting failure occurs. Resistance to

splitting failure is primarily offered by tensile strength of concrete which is a function of

its compressive strength. A better strength concrete as well as surface roughness of the

rebar resists better against the splitting of concrete.

Splitting failure occurs when concrete around the rebar develops transverse

splitting cracks along planes that are parallel and perpendicular to the rebar. As the rebars

are loaded, they exert radial pressure on the surrounding concrete, which results into

splitting crack at the interface and propagates towards the outer surface of concrete

leading to failure.

Pullout failure occurs when the radial forces from the loaded rebars are smaller

than the resistance of surrounding concrete. Shear strength of concrete plays an important

role in the pullout failure. In case of large concrete covers, there is more confining

pressure on the rebar and splitting of concrete is difficult, resulting into pullout failure.

Okelo et al. (2005) observed the pullout failure in his experimental work once the shear

strength of bond between concrete and GFRP rebar was exceeded. The ultimate bond

stress of pullout specimens was controlled by the shear strength of concrete adjacent to

GFRP rebar as well as its surface texture.

Okelo et al. (2005), after doing the experimental work on evaluation of bond

performance of GFRP rebars, concluded that for shorter bonded lengths with low

compressive strength concretes and small rebar diameters, pullout of rebar occurs. For

longer bonded lengths with high compressive strength concretes, either rebar fracture in

flexure or splitting or concrete shear compression failure occurs.

Qingduo et al. (2008) found that the concrete cover has significant impact on the

failure mode of GFRP rebars. For concrete covers ranging from one to three times the

rebar diameter, splitting failure took place. For larger concrete covers, pullout failure or

fracture of rebar occurred.


41

2.4.6 State of bond stress in surrounding concrete

The state of bond stress in concrete surrounding of a reinforcing bar may be

summarized as follow.

1. Stress condition in adjacent concrete to the rebar varies along embedded rebar and

also affects the bond performance.

2. Stress in concrete surrounding of a rebar leads to cracks and deformations of that

concrete.

3. Bond stress in surrounding concrete tends to pull the concrete away from rebar

surface in the vicinity of major cracks.

4. Some of the tension in concrete is usually lost when crack opens near the surface

of a rebar.

When concrete separates itself from the rebar surface at a crack, circumference of

concrete around the rebar (previously in contact with rebar) increases. Thus the

circumferential tensile stresses are introduced. These stresses may lead to longitudinal

splitting cracks in concrete which run parallel to the rebar.

It is pertinent to note that in case of intersecting beams at 90o in a building frame,

compression and tension is developed transversely in the rebars. The transverse tensile

stresses may lead to early cracking along the main rebars and also affect their bond

performance. The transverse compression can improve the bond performance by

developing confining pressure on the rebar. The beams that support slab, top rebars of

beams are subjected to transverse tension.

The effect of joint action on bond stress of primary beam of junction/intersecting

beams at 90o, has been studied by Kafeel. A (2009) using deformed steel rebars in normal

as well as high strength concretes and found that bond stress reduced up to 32% in

primary beams of junctions due to joint action.


42

2.5 SUMMARY

Literature review helped in identifying and understanding the inherent problems

and limitations of steel rebars related to corrosion along with its mechanism as well as

effects on the performance of RC members.

No specific guideline was available in the literature related to the development of

GFRP rebars. Various characteristics and properties of FRP laminates were studied.

General properties of possible ingredients of FRP/GFRP reinforcement, like glass fibers,

resin, filler, accelerator as well as catalysts were studied along with their behavior with

varying conditions. This study helped in establishing the selection basis for the

development of GFRP rebars. Detailed technical discussions with industry individuals

also helped in understanding the development process. The dose limits of accelerator and

catalysts recommended by their manufacturers through data sheets were adopted.

General understanding of prime process parameters, fiber volume fraction, pull

speed as well as heating die temperature and their impact on mechanical properties of

laminates/composites also helped in establishing the guidelines for the local development

of GFRP rebars. As these GFRP rebars have to be used in concrete, therefore their

average bond stress response was essential to study for proper composite action with the

concrete.

Literature review also helped in understanding the key findings of several

researchers related to their studies on bond performance of FRP/GFRP rebars as well as

factors affecting the bond performance. Based on review of these bond studies, it was

possible to select the most influential factors like bonded length, rebar size, concrete

cover, concrete compressive strength and surface texture for studying their effects on

bond stress of locally developed GFRP rebars for this research work.

CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS

43

EXPERIMENTATION FOR DEVELOPMENT OF GFPR REBARS

3.1 GENERAL

The fundamental aspects of experimental program for the development of glass

fiber reinforced plastic (GFRP) rebars have been presented in this chapter. Literature

review revealed that although numbers of researchers have studied the mechanical

properties of fiber reinforced plastic laminates/composites as well as the effect of process

parameters on their mechanical properties but no specific detail had been made available

on the development of GFRP rebars. It may be noted that there are only a few

manufacturers of GFRP rebars in some technological advanced countries and there was

no access to any data related to the development of GFRP rebars. Thus a rigorous

experimental program was devised for the development of these reinforcing bars based on

hit and trial approach.

After detailed survey and assessment of various industries in Pakistan associated

with the production of general glass fiber products like pipes, sheets etc. and preliminary

technical discussion with the agreed one, working collaboration was made for the

assistance in developing the GFRP rebars. Appropriate raw materials were identified,

selected and procured. Experimental details of development process have been discussed

in this chapter for various trial productions of GFRP rebars. The main objective of trial

productions of these rebars was to determine the optimum composition of resin mixture

as well as optimum combination of process parameters. The trial productions for

determining the optimum process parameters were reduced with the help of proposed

production models thus reducing the cost of GFRP rebars.

In order to finalize the surface texture, number of plain GFRP rebars were

developed for preliminary bond study for the effect of surface texture on bond stress

through direct pullout tests using 41.4 MPa strength concrete and subsequently deformed

GFRP rebars were also developed to study their bond stress through simple direct pullout

tests using 27.0 MPa concrete.


44

The comparison of properties including the barcol hardness and tensile strength

was made between final production of local deformed GFRP rebars and the reference

GFRP rebars, Aslan-100TM, developed by Hughes Brothers Inc. USA. The results of

various trial productions as well as finally developed GFRP rebars have also been

included in this chapter.

3.2 COLLABORATION WITH LOCAL INDUSTRY

A number challenges were faced during the development phase of GFRP rebars.

The prime one was the identification and affiliation with appropriate local industry

associated with production of general glass fiber products, and which would have the

necessary infrastructure and willingness to participate in the non-commercial research

based assignment of development of GFRP rebars for the first time, ever in the history of

any developing country like Pakistan. A detailed survey for the availability of basic

facilities in various relevant industries across the country was carried out in order to

assess the level of willingness and required resources, which could be used for the

development of GFRP rebars. Most of such industries were not willing due to number of

reasons. Only one industry, Messers Fiber Craft Industries Lahore, Pakistan gave its

consent for the needful assistance.

After having the assistance of appropriate local industry which had necessary pre-

requisites facilities and willingness, the next step was to have elaborated technical

discussions on the available existing infrastructure and required up-gradation in the

pultrusion setup, necessary to achieve the goal of development of GFRP rebars. It is

pertinent to note that identifying the industry/individual that is willing to sacrifice time

and resources for a non-profitable activity was a huge challenge. Furthermore, convincing

the local industry for up-gradation in the existing old pultrusion setup at their-own cost

also required some level of persuasion which was a mile stone.

After deliberation and up-gradation of pultrusion setup, the next step was to apply

‘trial and error’ approach for the use of procured raw materials to discover the optimum

raw materials (resin mixture) composition, whose barcol hardness after curing may be

well close to 50, as per ASTM requirement as well as the maximum reported hardness of

the reference GFRP rebars. First preliminary trial production was run with the raw


45

materials composition based on general guidelines derived from material data sheets,

literature review and general experience of the industry individuals related to the

production of FRP pipes, sheets etc. This stage required immense amalgam from the

general technical know-how of the local industry and detailed data analysis carried out by

the author.

The final outcome of whole exercise was to have a setup/ infrastructure with the

ability to produce quality assured GFRP rebars of international standard on demand. The

details of this development process have been presented in the subsequent sections.

3.3 SELECTION AND PROCUREMENT OF RAW MATERIALS

Glass fibers and thermoset resin were selected to use for the development of

GFRP rebars being cost efficient and widely used in world for this purpose. For the

development of GFRP rebars, other raw materials including filler, accelerator and

catalysts were selected based on the guidelines derived from literature review and

materials data sheets along with author’s data analysis as well as general experience of

the industry individuals. The brief description of each raw material has been presented

below.

3.3.1 Glass Fibers

Corrosion resistant E-glass (E-CR) fibers with brand mark of “ECR-469L-2400”,

were selected, being continuous & single end roving and in straight alignment produced

according to ASTM D-578, due to following advantages over the E-glass fibers:

1. Environment friendly E-CR glass is better than E-glass and did not contain harmful

constituents like Boron and Fluorine.

2. Due to elimination of Boron and Fluorine in the glass formulation, chemical

resistance to water, acids and alkalis is greatly improved.

3. E-CR glass possesses higher temperature resistance because its softening point is

much higher than the E-glass.


46

4. It has better dielectric strength, lower electrical leakage, and higher surface

resistance. It has low resin demand with easily openable strands during the

production process.

5. The products of E-CR glass show better mechanical properties as compared to E-

glass products. It has stable roving density and excellent abrasion resistance with

lower fuzz.

The comparison of chemical compositions of E-CR and E-glass fibers has been

presented in table 3.1.

Table 3.1: Comparison of Chemical Compositions of E-CR and E-glass Fibers.

Chemical Ingredient (%) E-CR glass E-glass

SiO2 57 - 62 52 - 56

Al2O3 9 - 12 12 - 16

CaO 20 - 23 16 - 25

MgO 2 - 4 0 - 5

B2O3 0 6 - 8

ZnO 2 - 4 0

Na2O+K2O 0.8 Max. 0.8 Max.

TiO2 1 - 3 0.2 Max.

Fe2O3 0.5 Max. 0.5 Max.

F2 0 0.6 Max.

Source: CPIC China, Manufacturer’s data sheet, 2006.

The reported properties of ECR-469L rovings have been given in table 3.2.

Table 3.2: ECR Direct Roving Product.

Source: CPIC China, Manufacturer’s data sheet, 2006.

Product Code Tex Yield Filament Diameter

LOI (%) Moisture

Absorption

ECR-469L-2400 2400 207 24um 0.40 0.10 Max.


47

The term Tex is used for expressing the linear density of fibers and equal to the

mass or weight of fibers in grams per 1000 meters length of fibers, whereas term Yield is

used to define the linear density of a roving, measured by number of meters per gram.

Loss on ignition (LOI) is the weight loss after burning off an organic sizing from the glass

fibers, or an organic resin from a glass fiber laminate, expressed as percentage of total.

The E-CR glass fibers, as shown in figure 3.1, were imported from CPIC

(Chongqing Polymer International Corporation Ltd.) China, being a highly specialized

manufacturer of E and E-CR glass rovings.

Fig. 3.1: ECR-469L-2400 glass fibers Fig. 3.2: Hetron-922 vinyl ester resin

3.3.2 Resin

Corrosion resistant vinyl ester resin of M/s Ashland USA with brand name of

HETRON-922TM was selected and used after importing from Saudi Arabia as shown in

figure 3.2 and with reported properties by the manufacturer given in table 3.3.

Table 3.3: Composite Properties of Hetron-922 versus Glass Content (Typical Values)

Resin M/M M/Wr/M/Wr/M

Glass Content (%) 25 40

Tensile Strength (MPa) 91 125

Tensile Modulus (GPa) 6 11

Flexural Strength (MPa) 185 258

Flexural Modulus (GPa) 7 10




48

Barcol hardness value is an indication of surface cure and determined as per

ASTM D-2583 standard. For each resin, fiber reinforced plastic laminates shall have

minimum 90% hardness value of the manufacturer’s specified value as per ASTM

standard. The manufacturer’s value of hardness for Hetron 922TM was 30 for laminates

cured at room temperature and 50 for the GFRP rebars.

3.3.3 Other Raw Materials

The raw materials selected and used for the development of GFRP rebars other

than the glass fibers and vinyl ester resin were as follow, which were also the ingredients

of resin mixture.

Filler is a relatively inert material added to a resin mixture to improve the cost

efficiency and surface texture as well to provide thixotropy. Filler acts as a resin extender

and reduce the porosity of the composite surface. It also keeps the die surface clean.

Locally available calcium carbonate being cost efficient filler was used in the

development of GFRP rebars.

Accelerator speeds up the chemical reaction between resin and catalyst. Among

the various available accelerators, Cobalt Octoate (CO) was the cost efficient accelerator

compatible with tropical conditions of Pakistan, hence selected.

Catalyst initiates and accelerates the polymerization reaction of resin for curing

of a composite as well as improves the corrosion resistance of composite in some reactive

chemical environments. Based on manufacturer’s data sheets, cost efficiency and

experience of the industry individuals with our local tropical conditions, combination of

following two catalysts was used:

a) Benzoyl Per Oxide (BPO)

b) Tertiary Butyl Peroxy Benzoate (TBPB)

3.4 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS

After having collaboration with the local industry along with detailed technical

discussions and up-gradation of pultrusion setup including the fiber alignment/tensioner


49

system, the next step was to run the trial productions of GFRP rebars using the most

commonly employed technique for the production of composites including FRP/GFRP

rebars, called Pultrusion, which is automated and economical. It is similar to extrusion

process by which many metal sections are fabricated, but in this case the composite is

produced by pulling action hence called pultrusion.

The experimental program for the development of GFRP rebars was implemented

through the Pultrusion machine, consisting of a creel, resin impregnation tank, pre-

forming plates, heated metal die, and the pulling mechanism. The schematic pultrusion

process has been presented in figure 3.3.

The pultrusion process starts with creels holding doffs of fiber roving and is

accomplished by pulling raw fibers through a resin bath and then through a heated die.

Fig. 3.3: Schematic Pultrusion Process for GFRP rebars (Source: Tighiouart, 1998).

Fibers were pulled from creel system into a resin bath for thorough wetting. The

wetting of glass fibers with resin mixture performed the function of glue, which joined

different parts of composite. It may be noted that fibers passed through the transverse

breaker bars, not directly through the resin bath in a straight line. The breaker bars

spreaded the fibers for better wetting with resin mixture.

The fiber resin system was then pulled through pre-form plates to squeeze out

excess resin mixture and properly form the fiber resin cross section before it entered the

curing/heating die.

The change of state from wet saturated to solid state is called the “curing” of

GFRP rebar, which occurred in heating die. As the GFRP rebar passed through the

heating die, constant heat transfer initiated the cure reaction and pulling speed was


50

adjusted such that the resin mixture had fully cured by the time it left the die. Although

the pulling speed range vary from machine to machine but in present study the pulling

speed could be varied from 20 to 250 mm/minute. The heating die temperature of

pultrusion machine could be varied from room temperature to 285 oC. Optimum

combination of pull speed and heating die temperature can alone ensure the proper curing

of a GFRP rebar.

As the GFRP rebar passed out of heating die, resin mixture was cured and a

solidified product of cross section equal to that of diameter of heating die straight portion

was obtained. By having a suitable gap between die exit and the pulling device, GFRP

rebar cooled to a stage where it was sufficiently hard to be gripped by the pulling device.

Finally, the GFRP rebar was cut to desired length with a cutting device, which in present

case was done manually. The statistics of available existing pultrusion setup were as

follow:

Overall length of pultrusion machine = 8 meters

Length of heating die = 1 meter

Minimum distance of die exit and moveable pulling device = 2.5 meters

Cutting of GFRP rebars = manual/automatic

It is pertinent to note that although the existing old pultrusion machine was

imported by the industry a few years ago for the production of general composite

products but it was never used for the development of GFRP rebars nor it was fit for this

purpose. That is why certain improvements in the pultrusion machine were made prior to

start the development process of GFRP rebars.

As the development of GFRP rebars was done for the first time, number of

difficulties and problems were encountered during this development phase. These

difficulties included but not limited to crushing and slippage of anchorage ends/grips

during tensile strength testing of preliminary trial productions, wastage of number of trial

productions, limited time allocation of technical personnel of the industry due to heavy

engagements in their own commercial assignments etc. and worst of all the prevailing

energy crises in Pakistan in the form of excessive electric load shedding resulted into

heavy disturbance in the development process as well as wastage of raw materials leading


51

to considerable financial loss. Learning from mistakes also helped the introduction of

further improvements in pultrusion setup, including the fixing of resin injection system

for improvement in orientation of fibers and equalizing the tension to fibers.

Two types of discoveries/findings were made through various trial productions of

GFRP rebars. Firstly, determination of optimum composition of resin mixture and

secondly, the determination of optimum combination of process parameters for each

diameter rebar. These optimum settings were then used to produce the final production of

GFRP rebars.

3.4.1 Determination of Optimum Composition of Resin Mixture

In the first part of experimentation for development of GFRP rebars, optimum

composition of resin mixture ingredients including accelerator, Cobalt Octoate (CO), two

catalysts, Benzoyl Per Oxide (BPO) and Tertiary Butyl Peroxybenzoate (TBPB) was

determined through preliminary trial productions of GFRP rebars. There was no definite

starting point for selecting the proportions of these ingredients except the recommended

dose limits provided by their manufacturers through data sheets and the general

experience of industry individuals. The quantity of filler (Calcium Carbonate) was kept

constant as 5% based on general experience of industry individuals. Four sets of

preliminary trial productions were planned and implemented, comprising of fifty trials, to

determine the effect of variation of CO, BPO and TBPB on the hardness of GFRP rebar.

The barcol hardness of each preliminary trial production was determined with barcol

impressor as per ASTM D-2583. The recommended dose limits by the manufacturers for

CO, BPO and TBPB were 0.10% to 0.50%, 1% to 2% and 1% to 2% respectively.

For the preliminary trial productions, initial setting of process parameters was kept

as fiber content = 72%, heating die temperature as 195 oC and pull speed as 120 mm/

minute. These values were adopted based on guidelines derived from ASTM standard and

the general experience of industry individuals as well as based on initial un-recorded trial

productions of GFRP rebar. As these values were not optimum, therefore, barcol hardness

of preliminary trial productions was somewhat lower than the maximum required value of

50. Therefore, target value of hardness was set in the range of 45-50.


52

The criterion for selection of optimum composition of resin mixture ingredients

was to have barcol hardness of GFRP rebars within the target range, which was also close

to the maximum reported hardness value of 50 of reference GFRP rebars. The

experimental scheme for preliminary trial production sets based on hit and trial approach

has been given in table 3.4.

Table 3.4: Experimental Scheme for Preliminary Trial Production Sets for determination

of optimum composition of Resin mixture.

Preliminary Trial Production

Set ID

Quantities of Resin Mixture Ingredients (Phr) Planned Trials CO BPO TBPB

PTPS-1 0.20

1.00,1.33,1.67 & 2.00

1.00,1.33,1.67 & 2.00

16

PTPS-2 0.24 16

PTPS-3 0.28 1.00, 1.33 & 1.67 10

PTPS-4

(Confirmatory)

0.26, 0.30 1.00 8

Total Trial Productions 50

The term “Phr” is an abbreviation of “Parts per hundred resin”, which is a standard term

used for composition of resin mixture. The optimum composition of resin mixture was kept same

for all rebar diameters, only the quantity of resin mixture was varied for different diameter rebars.

In preliminary trial production set-1 (PTPS-1), comprised of 16 trials, the CO was

fixed at 0.20 Phr. Four proportions of BPO as 1.00, 1.33, 1.67 and 2.00 Phr were

combined with four values of TBPB as 1.00, 1.33, 1.67 and 2.00 Phr. The composition of

resin mixture ingredients and hardness results of preliminary trial production set-1

(PTPS-1) have been presented in table 3.5.


53

Table 3.5: Composition of resin mixture and Experimental results of hardness of PTPS-1

Rebar ID

CO

(Phr)

BPO

(Phr)

TBPB

(Phr)

Barcol Hardness

Remarks

GFR-C20B100TB100

0.20

1.00

1.00

*

-

GFR-C20B100TB133 1.33

* -

GFR-C20B100TB167 1.67 13 UCP

GFR-C20B100TB200 2.00 18 UCP

GFR-C20B133 TB100

1.33

1.00

*

-

GFR-C20B133 TB133 1.33

* -

GFR-C20B133 TB167 1.67 15 UCP

GFR-C20B133 TB200 2.00 21 UCP

GFR-C20B167 TB100

1.67

1.00

* -

GFR-C20B167 TB133 1.33 11 UCP

GFR-C20B167 TB167 1.67 18 UCP

GFR-C20B167 TB200 2.00 25 UCP

GFR-C20B200 TB100

2.00

1.00 10 UCP

GFR-C20B200 TB133 1.33 16 UCP

GFR-C20B200 TB167 1.67 22 UCP

GFR-C20B200 TB200 2.00 29 UCP

* Not Measurable.

Note: Three process parameters, i.e, fiber content (72%), heating die temperature (195 oC) and

pull speed (120 mm/min.) were kept constant for all PTPS. The abbreviations ‘UCP’, ‘FCP’ and

‘OCE’ stand for Under Cured Profile, Fully Cured Profile and Over Curing Effects respectively.

The rebar identification, for example, GFR-C20B100TB100 represents the GFRP deformed rebar

with CO = 0.20 Phr (C20), BPO = 1.00 Phr (B100) and TBPB = 1.00 Phr (TB100).

Results of preliminary trial production set-1 have also been shown graphically in figure

3.4.


54

Fig. 3.4: Effect of variation in BPO and TBPB on the hardness of GFRP rebars.

The results of PTPS-1 revealed that at low dosage (1.00 and 1.33) of BPO and

TBPB, barcol hardness of GFRP rebars was quite low even not measureable in some

cases due to insufficient curing and termed as under cured profile. As the proportions of

BPO and TBPB were increased to 1.67 and 2.00, value of hardness also increased,

maximum up to 29 due to improvement in curing condition at the maximum

recommended dose value of 2.0 for BPO as well as TBPB. As the maximum barcol

hardness value was much lesser than the target value, therefore, second preliminary trial

production set (PTPS-2), comprising of 16 trials, was implemented.

In PTPS-2, value of CO was given an increment of 0.04 and fixed at 0.24 Phr.

Again four proportions of BPO, 1.00, 1.33, 1.67 and 2.00 Phr were combined with four

values of TBPB, 1.00, 1.33, 1.67 and 2.00 Phr, to study the effect of variation of BPO and

TBPB on the hardness of GFRP rebars.

The composition of resin mixture and hardness results of PTPS-2 have been

shown in table 3.6, and graphically in figure 3.5 as well.

The results of PTPS-2 exhibited the similar trend as of PTPS-1 but with higher

values of hardness due to higher proportion of CO which improved the curing of GFRP

rebars. The maximum obtained value of hardness was 38, again at maximum

recommended value of 2.00 for each, BPO as well as TBPB. As the hardness value was

still lower than the target value hence third preliminary trial production set (PTPS-3),

0

10

20

30

40

50

1.00 1.33 1.67 2.00

Bar

col H

ardn

ess

TBPB (Phr)

At CO = 0.20 Phr

BPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 PhrBPO = 2.00 Phr

0

10

20

30

40

50

0.67 1.00 1.33 1.67 2.00 2.33

Bar

col H

ardn

ess

TBPB (Phr)

At CO = 0.20 Phr

BPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 PhrBPO = 2.00 Phr


55

comprising of 10 trials was judiciously planned and implemented. The experience gained

from first two trial sets helped in reducing the trials in PTPS-3.


Rebar ID

CO

(Phr)

BPO

(Phr)

TBPB

(Phr)

Barcol Hardness

Remarks

GFR-C24B100TB100

0.24

1.00

1.00

* -

GFR-C24B100TB133 1.33 10 UCP

GFR-C24B100TB167 1.67 16 UCP

GFR-C24B100TB200 2.00 23 UCP

GFR-C24B133 TB100

1.33

1.00

* -

GFR-C24B133 TB133 1.33 13 UCP

GFR-C24B133 TB167 1.67 19 UCP

GFR-C24B133 TB200 2.00 26 UCP

GFR-C24B167 TB100

1.67

1.00 10 UCP

GFR-C24B167 TB133 1.33 17 UCP

GFR-C24B167 TB167 1.67 25 UCP

GFR-C24B167 TB200 2.00 32 UCP

GFR-C24B200 TB100

2.00

1.00 14 UCP

GFR-C24B200 TB133 1.33 22 UCP

GFR-C24B200 TB167 1.67 31 UCP

GFR-C24B200 TB200 2.00 38 UCP

* Not Measurable.

Note: The rebar identification, for example, GFR-C24B100TB100 represents GFRP deformed rebar

with CO = 0.24 Phr, BPO = 1.00 Phr and TBPB = 1.00 Phr.


56


In PTPS-3, value of CO was given further uniform increment of 0.04 Phr in its

previous value of 0.24 resulting into 0.28 Phr. Three values of BPO, 1.00, 1.33 and 1.67

Phr were combined with four values, 1.00, 1.33, 1.67 and 2.00 Phr of TBPB. The

composition of resin mixture and hardness results of PTPS-3 have been shown in table

3.7 and graphically in figure 3.6 as well.


Rebar ID

CO

(Phr)

BPO

(Phr)

TBPB

(Phr)

Barcol Hardness

Remarks

GFR-C28B100TB100

0.28

1.00

1.00 11 UCP

GFR-C28B100TB133 1.33 23 UCP

GFR-C28B100TB167 1.67 37 UCP

GFR-C28B100TB200 2.00 46 FCP

GFR-C28B133 TB100

1.33

1.00 22 UCP

GFR-C28B133 TB133 1.33 33 UCP

GFR-C28B133 TB167 1.67 43 FCP

GFR-C28B133 TB200 2.00 46 OCE

GFR-C28B167 TB100 1.67

1.00 39 UCP

GFR-C28B167 TB133 1.33 46 OCE

NO FURTHER TRIAL PRODUCTION WAS NECESSARY DUE TO APPEARANCE OF OVER CURING EFFECTS ON THE REBAR SURFACE

0

10

20

30

40

50

1.00 1.33 1.67 2.00

Bar

col H

ardn

ess

TBPB (Phr)

At CO = 0.24 PhrBPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 PhrBPO = 2.00 Phr

0

10

20

30

40

0.67 1.00 1.33 1.67 2.00 2.33

Bar

col H

ardn

ess

TBPB (Phr)

At CO = 0.24 PhrBPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 PhrBPO = 2.00 Phr


57

Note: The rebar identification GFR-C28B100TB100, for example, represents the GFRP deformed

rebar with CO = 0.28 Phr, BPO = 1.00 Phr and TBPB = 1.00 Phr.


The results of PTPS-3 revealed that at CO = 0.28, BPO = 1.00 and TBPB = 2.00

Phr, value of barcol hardness was the maximum i.e. 46 (within the target range) without

appearance of any over curing effect on the rebar surface. Higher value of hardness than

46 was not achieved in any other trial production; therefore, this composition of resin

mixture ingredients was considered as the optimum. When the value of BPO was further

increased from 1.00 to 1.33 Phr and combined with TBPB value of 1.67 Phr, hardness

was not increased from the previous value of 46 but over curing effects appeared on the

rebar surface. As this value of hardness was within the target range and close to the

maximum required value of 50, therefore confirmatory PTPS-4, comprises of 8 trials was

also planned and implemented to observe any further possible improvement in the

hardness of GRFP rebar.

Two confirmatory values of CO, 0.26 and 0.30 Phr, were combined with optimum

value of BPO = 1.00 (selected as the optimum) and four values, 1.00, 1.33, 1.67 and 2.00

of TBPB. The composition of resin mixture and hardness results of confirmatory PTPS-4

has been shown in table 3.8 as well as graphically in figure 3.7.

0

10

20

30

40

50

1.00 1.33 1.67 2.00

Bar

col H

ardn

ess

(TBPB) Phr

At CO = 0.28 Phr

BPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 Phr

0

10

20

30

40

50

0.67 1.00 1.33 1.67 2.00 2.33B

arco

l Har

dnes

s(TBPB) Phr

At CO = 0.28 Phr

BPO = 1.00 PhrBPO = 1.33 PhrBPO = 1.67 Phr


58


Rebar ID

CO

(Phr)

BPO

(Phr)

TBPB

(Phr)

Barcol Hardness

Remarks

GFR-C26B100TB100

0.26

1.00

1.00 13 UCP

GFR-C26B100TB133 1.33 21 UCP

GFR-C26B100TB167 1.67 32 UCP

GFR-C26B100TB200 2.00 42 UCP

GFR-C30B100 TB100

0.30 1.00

1.00 18 UCP

GFR-C30B100 TB133 1.33 28 UCP

GFR-C30B100 TB167 1.67 38 UCP

GFR-C30B100TB200 2.00 46 OCE

Fig. 3.7: Effect of variation in CO and TBPB on the hardness of GFRP rebars.

It may be noted from table 3.8 and figure 3.7 that none of the hardness values has

gone beyond 46. The value of 46 was achieved at GFR-C30B100TB200 only and that too

with over curing effects. The four trial production sets have confirmed that resin mixture

composition for the rebar ID, GFR-C28B100TB200 given in table 3.7 was the desired

optimum composition determined using the available resources.

0

10

20

30

40

50

1.00 1.33 1.67 2.00

Bar

col H

ardn

ess

TBPB (Phr)

At BPO = 1.00 Phr

Cobalt Octoate = 0.26 Phr

Cobalt Octoate = 0.30 Phr

0

10

20

30

40

50

0.67 1.00 1.33 1.67 2.00 2.33

Bar

col H

ardn

ess

TBPB (Phr)

At BPO = 1.00 Phr

Cobalt Octoate = 0.26 PhrCobalt Octoate = 0.30 Phr


59

After conducting experimental work in the form of four preliminary trial

production sets comprising of fifty productions of GFRP rebars along with hardness tests,

the following optimum composition of resin mixture was concluded against the hardness

value of 46 and adopted for onward experimental work.

Table 3.9: Optimum Composition of Resin Mixture.

Resin Mixture

Ingredients

Vinyl Ester Resin

(Phr)

Filler (CaCO3)

(Phr)

CO

(Phr)

BPO

(Phr)

TBPB

(Phr)

Achieved Barcol

Hardness

Target Barcol

Hardness

Optimum Composition

100

5

0.28

1.00

2.00

46

45 - 50

Note: The above optimum composition was independent of rebar diameter. In subsequent trial

productions, only the quantity of resin mixture was increased with increase in rebar diameter.

It is evident from the above experimental results that as the Cobalt Octoate (CO)

proportion was increased, hardness also increased with optimum value of 0.28 Phr. With

further increase in CO, there was no improvement in the hardness of GFRP rebars but

over curing effects appeared including surface distortions, undulations, surface cracks etc.

It is also concluded from above results that with the increase in proportions of

BPO and TBPB, hardness increased at optimum values of BPO = 1.00 Phr and TBPB =

2.00 Phr. Further increase in their values has exhibited the over curing effects.

3.4.2 Determination of Optimum Combination of Process Parameters

After finalization of optimum composition of resin mixture, the next phase

experimental work was to determine and finalize the optimum combination of process

parameters for each rebar diameter. Literature review and the development process both

were indicative that for a specific type of glass fiber and vinyl ester resin; fiber volume

fraction, heating die temperature and pull speed were the major and important process

parameters in a pultrusion process.


60

The study of effect of these three process parameters on the tensile strength of six

deformed rebar diameters, 9.5, 13, 16, 19, 22 and 25 mm was planned by fourteen trial

production sets initially based on hit and trial approach, comprising of 280 productions of

GFRP rebars. The optimum composition of resin mixture, as discussed in the earlier

section 3.4.1, was adopted for these trials. The original experimental scheme of these trial

production sets has been given in table 3.10.

It may be noted that trial production set numbers TPS-n tabulated in table 3.10

have been designated in increasing order from the smallest to the largest diameter rebars,

whereas the actual trial production order was little different. First of all, 9.5mm diameter

rebars were developed and then 25mm diameter rebars. Subsequently 13mm and 22mm

diameter rebars were developed and finally 16mm and 19mm diameter rebars were

produced.

The criterion for selection of optimum combination of process parameters for each

rebar diameter was to have tensile strength close to the maximum reported tensile

strength of reference GFRP rebars. The tensile strength of each trial production of GFRP

rebar was determined by simple tension test according to ACI 440.3R-04 Method B.2,

and compared with the maximum reported tensile strengths of reference rebars. The

maximum reported tensile strengths of reference rebars were 760, 690, 655, 620, 586, and

550 MPa for 9.5, 13, 16, 19, 22 and 25mm diameter rebars respectively. These tensile

strengths were called the maximum target tensile strengths.

As stated above, first of all optimum combination of process parameters for the

smallest, 9.5mm, rebar diameter was determined by hit & trial approach. A production

model was proposed for this rebar diameter, refer chapter-4 for details. Subsequently

same approach was adopted to get the optimum combination of process parameters for

the largest, 25mm, diameter rebar as well as development of its production model.

Production models for other rebar diameters were also developed. These production

models helped to reduce the number of trial productions, which were initially planned on

hit & trial approach, for the intermediate rebar diameters.


61

Table 3.10: Original Experimental Scheme for trial production sets based on hit & trial

approach for the determination of optimum combination of process parameters.

Trial Production

Set ID

Rebar Diameter

(mm)

Process Parameters Planned

Trial Productions

Fiber Content

(%)


Heating Die Temperature (oC)

TPS-1

9.5

71

110,120,130,140

185,190,195,200,205

60 TPS-2 72

TPS-3 73

TPS-4

13 73

100,110,120,130

190,195,200,205,210

40

TPS-5 74

TPS-6

16 74

90,100,110,120

195,200,205,210,215

40

TPS-7 75

TPS-8

19 75

80,90,100,110

200,205,210,215,220

40

TPS-9 76

TPS-10

22 76

70,80,90,100

205,210,215,220,225

40

TPS-11 77

TPS-12

25

77

60,70,80,90

210,215,220,225,230

60 TPS-13 78

TPS-14 79

Total Planned Trial Productions 280

As per ASTM D-2584, fiber content should not be taken less than 70 percent to

provide proper reinforcing action in a composite, thus the trial production set-1 (TPS-1)


62

for 9.5mm diameter rebar was started with fiber content equal to 71%. Four values, 110,

120, 130 and 130 mm/minute, of pull speed were combined with five values, 185, 190,

195, 200 and 205 oC, of heating die temperature. The results of TPS-1 have been shown

in table 3.11 as well as in figures 3.8 and 3.9 graphically.

Table 3.11: Combination of Process Parameters and Experimental Results of Tensile

Strengths of TPS-1 for 9.5 mm diameter rebars.

Rebar ID

Fiber Volume Fraction

(%)

Pull Speed

(mm/min.)

Heating Die Temperature

(oC)

Tensile Strength

(MPa)

Remarks

GFR9-F71P110T185

71

110

185 653 UCP

GFR9-F71P110T190 190 698 FCP

GFR9-F71P110T195 195 729 OCE

GFR9-F71P110T200 200 - NR

GFR9-F71P110T205 205 - NR

GFR9-F71P120T185

120

185 640 UCP

GFR9-F71P120T190 190 681 UCP

GFR9-F71P120T195 195 711 FCP

GFR9-F71P120T200 200 732 OCE

GFR9-F71P120T205 205 - NR

GFR9-F71P130T185

130

185 610 UCP

GFR9-F71P130T190 190 651 UCP

GFR9-F71P130T195 195 682 FCP

GFR9-F71P130T200 200 704 OCE

GFR9-F71P130T205 205 - NR

GFR9-F71P140T185

140

185 587 UCP

GFR9-F71P140T190 190 629 UCP

GFR9-F71P140T195 195 658 FCP

GFR9-F71P140T200 200 681 OCE

GFR9-F71P140T205 205 - NR

Note: The abbreviations ‘UCP’, ‘OCE’ and ‘NR’ stand for Under Cured Profile, Over Curing

Effects and Not Required, respectively and kept constant for all TPS. The rebar identification, for

CHA

exam

71%

Fig.diam

Fig.

tem

incr

thus

subs

tem

over

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h(M

Pa)

APTER-3

mple, GFR9-

%, Pull speed

. 3.8: Effecmeter rebars

. 3.9: Effect

The ten

mperature fo

reased. It in

s resulted in

sequently te

mperature re

r curing eff

550

600

650

700

750

185H

At

550

600

650

700

750

110

Ten

sile

Str

engt

h (M

Pa)

A

F71P110T185 d

= 110 mm/m

t of variatios.

t of variatio

nsion test re

r a particul

ndicates tha

nto low ten

ensile streng

sulted into

ffects on the

190 19eating Die Tem

t Fiber Conten

120Pull Speed (m

At Fiber Conte

EXPER

denotes GFRP

minute and h

on in Heati

n in Pull Sp

esults of TP

ar fiber con

at at low die

nsile strengt

gth of rebar

small incre

e rebar surf

5 200mperature (oC)

nt = 71%

Pull(mm

130 14mm/min.)

ent = 71%

185

190

195

200

Heating DieTemperature

RIMENTATIO

63

P deformed r

heating die T

ng Die Tem

peed on Ten

PS-1 revea

ntent and pu

e temperatu

th. With the

r was consi

ease in the t

face. For ex

205

110

120

130

140

l Speed m/min.)

Ten

sile

Str

engt

h (M

Pa)

40

e e (oC)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

rebar of 9.5m

emperature =

mperature o

nsile Strengt

aled that wi

ull speed, t

ures, GFRP

e increase i

derably imp

tensile stren

xample, at p

550

600

650

700

750

180 18

g(

)H

At

550

600

650

700

750

100 1

g(

)

At

VELOPMENT

mm diameter

= 185 oC.

on Tensile S

th of 9.5 mm

ith the incre

ensile stren

rebar was

in die temp

proved. Fur

ngth along

pull speed

85 190 19Heating Die Tem

t Fiber Conten

10 120Pull Speed

Fiber Conten

NT OF GFRP R

r with Fiber c

Strength of

m diameter

ease in hea

ngth of GFR

not properl

perature, cur

rther increas

with appear

of 110 mm

5 200 205mperature (oC)

nt = 71%

Pul(mm

130 140(mm/min.)

nt = 71%


REBARS

content =

9.5 mm

rebars.

ating die

RP rebar

ly cured

ring and

se in die

rance of

m/minute

5 210

110120130140

l Speed m/min.)

150

185190195200

(oC)

CHA

with

of 5

over

trial

com

incr

tem

be e

can

heat

die.

resu

with

tem

hav

The

Fig.reba

Ten

sile

Str

engt

h (M

Pa)

APTER-3

h die tempe

5oC in 190o

r curing eff

l production

mbination of

It is int

rease in d

mperature, in

explained w

be said tha

t energy wa

On the oth

ulted into sim

In TPS

h same com

mperatures (

e been pres

e similar tren

. 3.10: Effears.

550

600

650

700

750

800

185He

At

erature of 1

C increased

ffects and th

ns was war

f fiber conte

teresting to

ie tempera

ncrease in p

with the help

at when, at

as provided

her hand, a

milar effect

-2 and TPS

mbinations

185, 190, 1

ented in tab

nd of result

ct of variati


Fiber Content

EXPER

90 oC, the a

d the tensile

his conditio

rranted with

ent and pull

note from

ature impro

pull speed d

p of heat en

a specific d

for curing o

at a given p

t of more he

S-3, fiber co

of pull spe

95, 200 an

bles 3.12 an

ts, as of TPS

ion in Die T


t = 72%

Pull(mm

RIMENTATIO

64

achieved te

e strength b

on of rebar

h any increm

l speed.

figures 3.8

oved the t

decreased th

nergy requir

die tempera

of rebar due

pull speed,

eat energy.

ontent were

eeds (110,

nd 205 oC).

nd 3.13 as w

S-1, has bee

Temperatur

205

110120130140

Speed m/min.)

5

6

6

7

7

8

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

nsile streng

by 31 MPa,

was not de

ment of die

and 3.9 tha

tensile stre

he tensile str

red for prop

ature, the pu

e to longer d

when die

e increased

120, 130 a

The results

well as in fig

en observed

re on Tensil

550

600

650

700

750

800

180 185H

At F

VELOPMENT

gth was 698

, however, w

esirable. Th

e temperatur

at at any pa

ength and

rength. This

per curing o

ull speed w

duration of

temperature

to 72% and

and 140 m

s of these tr

gures 3.10 to

d in TPS-2 a

le Strength


Fiber Content

NT OF GFRP R

8 MPa. An

with appear

herefore, no

re at this p

articular pul

at a spec

s phenomen

of a GFRP

was decrease

rebar in the

e was incre

d 73% resp

mm/minute)

rial product

o 3.13 respe

and TPS-3.

of 9.5mm d

200 205mperature (oC)

= 72%

Pull Sp(mm/m

REBARS

increase

rance of

o further

articular

ll speed,

cific die

non may

rebar. It

ed, more

e heating

eased; it

pectively

and die

tion sets

ectively.

diameter

210

110120130140

peed min.)


65



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR9-F72P110T185

72

110

185 694 FCP

GFR9-F72P110T190 190 725 OCE

GFR9-F72P110T195 195 - NR

GFR9-F72P110T200 200 - NR

GFR9-F72P110T205 205 - NR

GFR9-F72P120T185

120

185 672 UCP

GFR9-F72P120T190 190 710 FCP

GFR9-F72P120T195 195 735 OCE

GFR9-F72P120T200 200 - NR

GFR9-F72P120T205 205 - NR

GFR9-F72P130T185

130

185 639 UCP

GFR9-F72P130T190 190 694 UCP

GFR9-F72P130T195 195 747 FCP

GFR9-F72P130T200 200 748 OCE

GFR9-F72P130T205 205 - NR

GFR9-F72P140T185

140

185 603 UCP

GFR9-F72P140T190 190 654 UCP

GFR9-F72P140T195 195 698 FCP

GFR9-F72P140T200 200 731 OCE

GFR9-F72P140T205 205 - NR

Note: The designation of rebar identification is same as in table 3.11 and has been kept uniform

throughout from TPS-1 to TPS-14.

CHA

Fig.

Fig.reba

Fig.

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.11: Effec

. 3.12: Effears.

. 3.13: Effec

550

600

650

700

750

800

110

A

550

600

650

700

750

800

110

A

550

600

650

700

750

800

110

A

ct of variati

ct of variati

ct of variati

120Pull Speed (m

At Fiber Conte

120Pull Speed (m

At Fiber Conte

120Pull Speed (m

At Fiber Conte

EXPER

ion in Pull S

ion in Die T

ion in Pull S

130 1mm/min.)

ent = 72%

185

190

195

200

Heating Die Temperature (

130 1mm/min.)

ent = 73%

185

190

195

200


130 1mm/min.)

ent = 73%

185190195200


RIMENTATIO

66

Speed on Te

Temperatur

Speed on Te

40

(oC)

Ten

sile

Str

engt

h (M

Pa)

40

e e (oC)

Ten

sile

Str

engt

h (M

Pa)

140

(oC)

Ten

sile

Str

engt

h(M

Pa)

ON FOR DEV

ensile Stren

re on Tensil

ensile Stren

550

600

650

700

750

800

100 1

550

600

650

700

750

800

100 1

g(

)A

550

600

650

700

750

800

100 1

Ten

sile

Str

engt

h (M

Pa)

A

VELOPMENT

gth of 9.5m

le Strength

gth of 9.5m

10 120Pull Speed (m

At Fiber Cont

10 120Pull Speed (

At Fiber Conten

110 120Pull Speed (

At Fiber Cont

NT OF GFRP R

mm diameter

of 9.5mm d

mm diameter

130 140mm/min.)

tent = 72%

Heating DiTemperatu

130 140(mm/min.)

nt = 73%

Heating Di

Temperatur

130 140(mm/min.)

ent = 73%


REBARS

r rebars.

diameter

r rebars.

150

185

190

195

200

ie ure (oC)

150

185

190

195

e

re (oC)

150

185190195

e e (oC)


67



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR9-F73P110T185

73

110

185 676 FCP

GFR9-F73P110T190 190 718 OCE

GFR9-F73P110T195 195 - NR

GFR9-F73P110T200 200 - NR

GFR9-F73P110T205 205 - NR

GFR9-F73P120T185

120

185 661 UCP

GFR9-F73P120T190 190 703 FCP

GFR9-F73P120T195 195 734 OCE

GFR9-F73P120T200 200 - NR

GFR9-F73P120T205 205 - NR

GFR9-F73P130T185

130

185 626 UCP

GFR9-F73P130T190 190 681 FCP

GFR9-F73P130T195 195 716 OCE

GFR9-F73P130T200 200 - NR

GFR9-F73P130T205 205 - NR

GFR9-F73P140T185

140

185 594 UCP

GFR9-F73P140T190 190 638 UCP

GFR9-F73P140T195 195 678 FCP

GFR9-F73P140T200 200 704 OCE

GFR9-F73P140T205 205 - NR

The reported tensile strength of same diameter reference rebar was 760 MPa. It

may be noted from table 3.12 as well as from figures 3.10 and 3.11, that at pull speed of


68

130 mm/minute with die temperature of 195 oC, tensile strength of locally developed

GFRP rebar was 747 MPa. When the die temperature was further increased at same fiber

content of 72%, and pull speed of 130 mm/minute, there was no appreciable gain in the

tensile strength, rather over curing effects appeared on the rebar surface. No other

combination of fiber content, pull speed and die temperature gave the tensile strength

greater than 747 MPa.

In TPS-3, all trials were conducted with fiber content = 73%. Same pull speeds,

110, 120, 130, 140 mm/minute were combined with same die temperatures, 185 to 210 oC

with an interval of 05 oC. It may be noted from table 3.13 as well as from figures 3.12 and

3.13 that all tensile strength values were lower at same combinations of pull speed and die

temperature as compared to TPS-2. The possible reason in reduction of tensile strength

values was the disturbance in desired balance between fiber content and the resin mixture

quantity. Due to higher fiber content, lesser resin mixture affected the proper wetting and

binding of fibers which caused decrease in the tensile strengths.

Thus the optimum combination of process parameters for 9.5 mm diameter rebars

was concluded as fiber content = 72%, pull speed = 130 mm/minute and heating die

temperature = 195 oC, with achieved tensile strength of 747 MPa against the maximum

target tensile strength of 760 MPa.

After finalizing the optimum combination of process parameters for 9.5mm

diameter rebar, a similar study was executed to determine the optimum combination of

process parameters for the largest, 25mm, rebar diameter based on hit and trial approach

as per experimental scheme shown in table 3.10.

TPS-12 was started with fiber content of 77%. Four values, 60, 70, 80 and 90

mm/minute, of pull speed were combined with five values of heating die temperature,

210, 215, 220, 225 and 230 oC. As the cross-sectional area of 25mm diameter rebars was

almost seven times the area of 9.5mm diameter rebars, therefore higher fiber content and

more heat energy (higher die temperature and lower pull speed) was necessary for 25mm

diameter rebars. The results of TPS-12 have been shown in table 3.14 as well as in figures

3.14 and 3.15 graphically.


69


Strengths of TPS-12 for 25 mm diameter rebars.

Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR25-F77P60T210

77

60

210 485 FCP

GFR25-F77P60T215 215 512 OCE

GFR25-F77P60T220 220 - NR

GFR25-F77P60T225 225 - NR

GFR25-F77P60T230 230 - NR

GFR25-F77P70T210

70

210 472 UCP

GFR25-F77P70T215 215 497 FCP

GFR25-F77P70T220 220 518 OCE

GFR25-F77P70T225 225 - NR

GFR25-F77P70T230 230 - NR

GFR25-F77P80T210

80

210 453 UCP

GFR25-F77P80T215 215 481 FCP

GFR25-F77P80T220 220 502 OCE

GFR25-F77P80T225 225 - NR

GFR25-F77P80T230 230 - NR

GFR25-F77P90T210

90

210 440 UCP

GFR25-F77P90T215 215 467 UCP

GFR25-F77P90T220 220 488 FCP

GFR25-F77P90T225 225 503 OCE

GFR25-F77P90T230 230 - NR

CHA

Fig.

diam

Fig.

incr

tens

For

tens

incr

next

stre

effe

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.14: Effe

meter rebars

. 3.15: Effec

The res

rease in hea

sile strength

example,

sile strength

reased the te

t trial run

ngth of reb

ects; which

400

450

500

550

600

210H

At

400

450

500

550

600

60

At

ect of variat

s.

ct of variati

sults of TPS

ating die te

h of GFRP r

at pull spe

h of 25mm

ensile streng

with anoth

bar was furt

h were not

215 220Heating Die Tem

t Fiber Conten

70Pull Speed (m

t Fiber Conten

EXPER

tion in Heat

ion in Pull S

S-12 exhibit

emperature

rebars incre

ed of 80 m

diameter re

gth by 28 M

her increme

ther increas

desirable.

0 225mperature (oC)

nt = 77%

Pull(mm

80 9mm/min.)

nt = 77%

210

215

220

225


RIMENTATIO

70

ting Die Te

Speed on Te

ted similar t

at 77% fib

eased due to

mm/minute,

ebars was 4

MPa with no

ent of 5o C

sed by 21 M

Therefore,

230

60708090

l Speed m/min.)

Ten

sile

Str

engt

h (M

Pa)

90

(oC)

Ten

sile

Str

engt

h(M

Pa)

ON FOR DEV

emperature

ensile Stren

trends as of

ber content

o improvem

when die

453 MPa. A

o over curin

. At die te

MPa but w

, no furthe

400

450

500

550

600

205 2

g(

)

H

A

400

450

500

550

600

50 6

Ten

sile

Str

engt

h (M

Pa)

A

VELOPMENT

on Tensile

gth of 25mm

f TPS-1. It w

and at a s

ment in curin

temperatur

An increase

ng effects, w

emperature

with appeara

er trial run

210 215

Heating Die Te

At Fiber Cont

60 70Pull Speed

At Fiber Cont

NT OF GFRP R

Strength o

m diameter

was found t

specific pul

ng of GFRP

re was 210

e of 5o C in

which deman

of 220 oC

ance of ove

n was exec

220 225

emperature (oC)

tent = 77%

Pull(mm

80 90(mm/min)

tent 77%

Heating DiTemperatu

REBARS

of 25mm

rebars.

that with

ll speed,

P rebars. oC, the

n 210 oC

nded the

, tensile

r curing

uted by

230

60708090

Speed m/min.)

100

210215220

ie ure (oC)

CHA

incr

othe

tabl

pull

show

tens

die

Fig.diam

Fig.

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

reasing the

er pull spee

le 3.14.

The TP

l speed and

wn in table

sile strength

temperature

. 3.16: Effemeter rebars

. 3.17: Effec

400

450

500

550

600

210

H

A

400

450

500

550

600

60

A

HeaTem

die tempera

eds were ex

PS-13 was e

d heating d

3.15 as we

h value in T

e of 220 oC

ect of variats.

ct of variati

215 220

Heating Die Tem

At Fiber Cont

70Pull Speed (m

At Fiber Conte

210215220225

ating Die mperature (oC)

EXPER

ature at this

ecuted in th

executed wi

die tempera

ell as graphi

TPS-13 was

against the

tion in Heat

ion in Pull S

0 225

mperature (oC)

tent = 78%

Pul(mm

80 9mm/min.)

ent = 78%

RIMENTATIO

71

s particular p

he similar m

ith fiber con

ature, as of

ically in fig

527 MPa a

maximum

ting Die Te

Speed on Te

230

60

70

80

90

ll Speed m/min.)

Ten

sile

Str

engt

h (M

Pa)

90

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

pull speed.

manner, wh

ntent of 78

f TPS-12. R

gures 3.16 a

at pull speed

target tensi

emperature

ensile Stren

400

450

500

550

600

205 2

g(

)

At

400

450

500

550

600

50 6

A

VELOPMENT

However, t

ose details

% with sam

Results of

and 3.17. Th

d of 80mm/

le strength o

on Tensile

gth of 25mm

10 215Heating Die Te

t Fiber Conten

60 70Pull Speed

At Fiber Conte

NT OF GFRP R

trial produc

have been

me combina

TPS-13 ha

he highest a

/minute and

of 550 MPa

Strength o

m diameter

220 225emperature (oC)

nt = 78%

Pull(mm

80 90(mm/min.)

ent = 78%

Heating DTemperatu

REBARS

ctions on

given in

ations of

ve been

achieved

d heating

a.

of 25mm

rebars.

230)

60708090

l Speed m/min.)

100

210215220225

Die ure (oC)


72


Strengths of TPS-13 for 25mm diameter rebars.

Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR25-F78P60T210

78

60

210 496 FCP

GFR25-F78P60T215 215 521 OCE

GFR25-F78P60T220 220 - NR

GFR25-F78P60T225 225 - NR

GFR25-F78P60T230 230 - NR

GFR25-F78P70T210

70

210 485 UCP

GFR25-F78P70T215 215 507 FCP

GFR25-F78P70T220 220 519 OCE

GFR25-F78P70T225 225 - NR

GFR25-F78P70T230 230 - NR

GFR25-F78P80T210

80

210 468 UCP

GFR25-F78P80T215 215 497 UCP

GFR25-F78P80T220 220 527 FCP

GFR25-F78P80T225 225 529 OCE

GFR25-F78P80T230 230 - NR

GFR25-F78P90T210

78

90

210 452 UCP

GFR25-F78P90T215 215 475 UCP

GFR25-F78P90T220 220 498 FCP

GFR25-F78P90T225 225 517 OCE

GFR25-F78P90T230 230 - NR


73

The last TPS-14 was executed with fiber content of 79% with same combinations

of pull speed and heating die temperature as of TPS-12. Results of TPS-14 have been

shown in table 3.16 as well as graphically in figures 3.18 and 3.19.


Strengths of TPS-14 for 25mm diameter rebars.

Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR25-F79P60T210

79

60

210 481 FCP

GFR25-F79P60T215 215 508 OCE

GFR25-F79P60T220 220 - NR

GFR25-F79P60T225 225 - NR

GFR25-F79P60T230 230 - NR

GFR25-F79P70T210

70

210 466 UCP

GFR25-F79P70T215 215 487 FCP

GFR25-F79P70T220 220 506 OCE

GFR25-F79P70T225 225 - NR

GFR25-F79P70T230 230 - NR

GFR25-F79P80T210

80

210 449 UCP

GFR25-F79P80T215 215 472 FCP

GFR25-F79P80T220 220 493 OCE

GFR25-F79P80T225 225 - NR

GFR25-F79P80T230 230 - NR

GFR25-F79P90T210

90

210 - NR

GFR25-F79P90T215 215 - NR

GFR25-F79P90T220 220 - NR

GFR25-F79P90T225 225 - NR

GFR25-F79P90T230 230 - NR

CHA

Fig.diam

Fig.

cont

valu

opti

fibe

achi

MP

diam

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3


. 3.19: Effec

It may b

tent, for all

ues were low

imum comb

er content =

ieved tensil

a of 25mm

Product

meter rebar

400

450

500

550

600

210H

At

400

450

500

550

600

60

A

ect of variats.

ct of variati

be observed

l combinati

wer than th

bination of p

= 78%, pull

le strength o

diameter re

tion models

rs, as detai


Fiber Conten

70Pull Speed (m

At Fiber Cont

EXPER

tion in Heat

ion in Pull S

d from table

ons of pull

he tensile str

process par

l speed = 8

of 527 MPa

eference GF

s were dev

iled in cha

0 225mperature (oC)

t = 79%

Pull Sp(mm/m

80mm/min.)

tent = 79%

212122


RIMENTATIO

74

ting Die Te

Speed on Te

e 3.16 and f

speed and

rength valu

ameters for

80 mm/min

a against th

FRP rebar.

veloped for

apter-4, whi

230

607080

peed min.)

Ten

sile

Str

engt

h (M

Pa)

90

101520

e e (oC)

Ten

sile

Str

engt

h(M

Pa)

ON FOR DEV

emperature

ensile Stren

figures 3.18

heating die

es obtained

r 25mm diam

nute and die

e maximum

9.5mm an

ich, subseq

400

450

500

550

600

205 2

H

At

400

450

500

550

600

50

Ten

sile

Str

engt

h (M

Pa)

A

VELOPMENT

on Tensile

gth of 25mm

8 and 3.19 t

e temperatu

d with 78%

meter rebar

e temperatu

m target tens

d 25mm as

quently, hel

210 215

Heating Die Tem

Fiber Content

60 70Pull Speed

At Fiber Conten

NT OF GFRP R

Strength o

m diameter

that with 79

ure, tensile

fiber conten

rs was concl

ure = 220 o

sile strength

s well as f

lped to red

220 225

mperature (oC)

t = 79%

Pull S(mm/

80 90(mm/min.)

nt = 79%

Heating DieTemperatur

REBARS

of 25mm

rebars.

9% fiber

strength

nt. Thus

luded as oC, with

h of 550

for other

duce the

230

607080

Speed /min.)

100

210215220

e re (oC)


75

number of trial productions for intermediate diameter rebars thus reducing the time and

cost of GFRP rebars. Furthermore, lesson learnt from executed six trial production sets

have also helped in reducing the onward trial productions of GFRP rebars. Some

additional confirmatory trial productions were also executed over and above the

minimum required based on model prediction, as detailed in chapter-4.

For 13mm diameter rebars, minimum fiber content of 73% was considered

because optimum combination of process parameters for 9.5 mm diameter rebar had been

achieved with fiber content = 72% and at the same fiber content, optimum combination of

higher diameter rebar did not look possible. With the increase in rebar diameter,

requirement of fiber/reinforcement as well as heat energy for proper curing increased to

have the desired tensile strength of GFRP rebar. To increase the amount of heat energy

for 13mm diameter rebars, as compared to 9.5mm rebars, an increment of 5 oC in die

temperature and reduction of 10 mm/minute in pull speed (for longer duration of rebar in

heating die) was opted.

TPS-4 for 13mm diameter rebars was started with fiber content = 73% and four

values of pull speed, 100, 110, 120 and 130 mm/minute, were combined with five values

of die temperature, 190, 195, 200, 205 and 205 oC.

Experimental results of TPS-4, showing the effect of variation in pull speed and

heating die temperature on tensile strength of 13 mm diameter rebars, have been

presented in table 3.17 as well as in figures 3.20 and 3.21 graphically. It may be noted

that trial productions, which were not implemented due to appearance of over curing

effects on rebar surface at lower temperatures have been reported with term ‘NR’

representing ‘ Not Required’.


76



Rebar ID


(%)

Pull Speed

(mm/min)


(oC)

Tensile Strength

(MPa)

Remarks

GFR13-F73P100T190

73

100

190 628 FCP

GFR13-F73P100T195 195 659 OCE

GFR13-F73P100T200 200 - NR

GFR13-F73P100T205 205 - NR

GFR13-F73P100T210 210 - NR

GFR13-F73P110T190

110

190 617 UCP

GFR13-F73P110T195 195 647 FCP

GFR13-F73P110T200 200 665 OCE

GFR13-F73P110T205 205 - NR

GFR13-F73P110T210 210 - NR

GFR13-F73P120T190

120

190 598 UCP

GFR13-F73P120T195 195 635 UCP

GFR13-F73P120T200 200 674 FCP

GFR13-F73P120T205 205 674 OCE

GFR13-F73P120T210 210 - NR

GFR13-F73P130T190

130

190 577 UCP

GFR13-F73P130T195 195 610 UCP

GFR13-F73P130T200 200 635 FCP

GFR13-F73P130T205 205 656 OCE

GFR13-F73P130T210 210 - NR

CHA

Fig.

Fig.

incr

tens

cont

achi

to th

Wh

app

com

than

GFR

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.20: Effec

. 3.21: Effec

TPS-4 r

rease in hea

sile strength

tent of 73%

ieved tensil

he maximu

en die temp

eared on r

mbination of

n 674 MPa.

RP rebars,

550

600

650

700

750

190Hea

A

550

600

650

700

750

100

A

HeatTem

ct of variati

ct of variati

revealed the

at energy thr

h of 13mm d

%, pull spe

le strength f

um target te

perature wa

rebar surfa

f pull speed

. Another t

with fiber

195 200ating Die Temp

At Fiber Conten

110Pull Speed (m

At Fiber Conte

190195200205

ting Die mperature (oC)

EXPER

ion in Die T

ion in Pull S

e similar tre

rough incre

diameter reb

eed of 120

for 13mm d

nsile streng

as further in

ace with n

d and die te

rial produc

content of

0 205perature (oC)

nt = 73%

Pull S(mm/

120 1mm/min.)

ent = 73%

RIMENTATIO

77

Temperature

Speed on Te

end of resul

ement in die

bars was im

0 mm/minu

diameter reb

gth of 690 M

ncreased at

no useful g

emperature

tion set, TP

74% and s

210

100

110

120

130

Speed /min.)

Ten

sile

Str

engt

h (M

Pa)

130

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

e on Tensile

ensile Stren

lts, as of 9.5

e temperatur

mproved. Ta

ute and die

bars was 67

MPa of 13m

the same p

gain in the

gave the te

PS-5, was c

ame combi

550

600

650

700

750

185 190

g(

)

H

A

550

600

650

700

750

90

g(

)A

VELOPMENT

e Strength o

gth of 13mm

5mm diamet

re and redu

able 3.17 dep

e temperatu

4 MPa, whi

mm diamete

pull speed,

e tensile s

ensile streng

conducted f

inations of


At Fiber Conten

110Pull Speed

At Fiber Conte

NT OF GFRP R

f 13mm reb

m diameter

ter rebars. W

uction in pul

picts that w

ure of 200

ich was fair

er reference

over curing

strength. N

gth equal or

for 13mm d

pull speed


nt = 73%

Pull(mm

130d (mm/min.)

ent = 73%


REBARS

bars.

rebars.

With the

ll speed,

with fiber oC, the

rly close

e rebars.

g effects

No other

r greater

diameter

and die

0 215

100

110

120

130

Speed m/min.)

150

190195200205

e re (oC)


78

temperature to study their effects on the tensile strength. The experimental results of TPS-

5 have been shown in table 3.18 as well as in figures 3.22 and 3.23 graphically.



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR13-F74P100T190

74

100

190 614 FCP

GFR13-F74P100T195 195 647 OCE

GFR13-F74P100T200 200 - NR

GFR13-F74P100T205 205 - NR

GFR13-F74P100T210 210 - NR

GFR13-F74P110T190

110

190 598 UCP

GFR13-F74P110T195 195 632 FCP

GFR13-F74P110T200 200 656 OCE

GFR13-F74P110T205 205 - NR

GFR13-F74P110T210 210 - NR

GFR13-F74P120T190

120

190 581 UCP

GFR13-F74P120T195 195 617 FCP

GFR13-F74P120T200 200 640 OCE

GFR13-F74P120T205 205 - NR

GFR13-F74P120T210 210 - NR

GFR13-F74P130T190

130

190 562 UCP

GFR13-F74P130T195 195 594 UCP

GFR13-F74P130T200 200 624 FCP

GFR13-F74P130T205 205 641 OCE

GFR13-F74P130T210 210 - NR

CHA

Fig.reba

Fig.

of 7

appr

max

para

120

as o

four

205

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.22: Effears.

. 3.23: Effec

It is evi

74% were

roached the

ximum targ

ameters for

0mm/minute

TPS-6 a

of TPS-4 an

r pull speed

, 210 and 2

550

600

650

700

750

190H

At

550

600

650

700

750

100

At

ct of variat

ct of variati

ident from a

lower than

e tensile str

get strength

13mm diam

e and die tem

and 7 for 16

d 5. TPS-6

ds, 90, 100,

215 oC. Sim


Fiber Conten

110Pull Speed (m

Fiber Conten

EXPER

tion in Die

ion in Pull S

above result

n the value

rength of 6

h of 690 M

meter rebars

mperature =

6mm diame

was started

, 110 and 1

milarly TPS

0 205mperature (oC)

nt = 74%

Pull S(mm/

120 1mm/min.)

nt = 74%

190

195

200

205


RIMENTATIO

79

Temperatur

Speed on Te

ts that all va

es with fib

674 MPa, w

MPa. Thus

s was conclu

= 200 oC.

eter rebars w

d with fiber

20 mm/min

S-7 was sta

210

100110120130

Speed /min.)

Ten

sile

Str

engt

h (M

Pa)

130

(oC)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

re on Tensi

ensile Stren

alues of ten

er content

which was a

s the optim

uded as fibe

were implem

content = 7

nute and fiv

arted with f

550

600

650

700

750

185 19

g(

)

H

At

550

600

650

700

750

90 1

g(

)A

VELOPMENT

le Strength

gth of 13mm

sile strength

of 73%. N

achieved in

mum comb

er content =

mented on

74% along w

ve die temp

fiber conten

0 195 200

Heating Die Te

Fiber Conten

00 110Pull Speed (

At Fiber Conte

NT OF GFRP R

of 13mm d

m diameter

h with fiber

None of an

n TPS-4 aga

ination of

= 73%, pull

the similar

with combin

peratures, 19

nt of 75% a

0 205 210

emperature (oC)

nt = 74%

Pull Sp(mm/m

120 130(mm/min.)

ent = 74%


REBARS

diameter

rebars.

r content

ny trials

ainst the

process

speed =

analogy

nation of

95, 200,

and with

0 215

)

100110120130

peed min.)

140

190195200

e e (oC)


80

same combinations of pull speed and die temperature as of TPS-6. The results of TPS-6

and 7 have been presented in table 3.19, figures 3.24 & 3.25 and table 3.20, figures 3.26

& 3.27 respectively.

Table 3.19: Combination of Process Parameters and Experimental Results of Tensile Strengths of TPS-6 for 16 mm diameter rebars.

Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR16-F74P90T195

74

90

195 571 FCP

GFR16-F74P90T200 200 613 OCE

GFR16-F74P90T205 205 - NR

GFR16-F74P90T210 210 - NR

GFR16-F74P90T215 215 - NR

GFR16-F74P100T195

100

195 558 UCP

GFR16-F74P100T200 200 599 FCP

GFR16-F74P100T205 205 624 OCE

GFR16-F74P100T210 210 - NR

GFR16-F74P100T215 215 - NR

GFR16-F74P110T195

110

195 547 UCP

GFR16-F74P110T200 200 591 UCP

GFR16-F74P110T205 205 629 FCP

GFR16-F74P110T210 210 630 OCE

GFR16-F74P110T215 215 - NR

GFR16-F74P120T195

120

195 539 UCP

GFR16-F74P120T200 200 574 UCP

GFR16-F74P120T205 205 605 FCP

GFR16-F74P120T210 210 621 OCE

GFR16-F74P120T215 215 - NR

CHA

Fig.reba

Fig.

Fig.reba

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.24: Effears.

. 3.25: Effec

. 3.26: Effears.

500

550

600

650

700

195H

At

500

550

600

650

700

90

g(

)

At

HeatinTemp

500

550

600

650

700

195H

ct of variat

ct of variati

ct of variat


Fiber Conten

100Pull Speed (m

t Fiber Conten

195200205210

ng Die perature (oC)


At Fiber Co

EXPER

tion in Die

ion in Pull S

tion in Die


nt = 74%

Pull(mm

110 1mm/min.)

nt = 74%

5 210mperature (oC)

ntent = 75%

Pull S(mm/

RIMENTATIO

81

Temperatur

Speed on Te

Temperatur

215)

90100110120

Speed m/min.)

Ten

sile

Str

engt

h (M

Pa)

120

Ten

sile

Str

engt

h(M

Pa)

215

90

100

110

120

Speed /min.)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

re on Tensi

ensile Stren

re on Tensi

500

550

600

650

700

190 195

g(

)

H

At

500

550

600

650

700

80 9

Ten

sile

Str

engt

h (M

Pa)

A

500

550

600

650

700

190 19

g(

)

At

VELOPMENT

le Strength

gth of 16mm

le Strength


t Fiber Conten

90 100Pull Speed

At Fiber Conten

5 200 205Heating Die Te

t Fiber Conten

NT OF GFRP R

of 16mm d

m diameter

of 16mm d


nt = 74%

Pull S(mm/m

110 120(mm/min.)

nt = 74%


5 210 215emperature (oC)

nt = 75%

Pull(mm

REBARS

diameter

rebars.

diameter

5 220

90100110120

Speed min.)

130

195200205210

(oC)

5 220)

90100110120

Speed m/min.)


82



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR16-F75P90T195

75

90

195 552 FCP

GFR16-F75P90T200 200 586 OCE

GFR16-F75P90T205 205 - NR

GFR16-F75P90T210 210 - NR

GFR16-F75P90T215 215 - NR

GFR16-F75P100T195

100

195 539 UCP

GFR16-F75P100T200 200 568 FCP

GFR16-F75P100T205 205 591 OCE

GFR16-F75P100T210 210 - NR

GFR16-F75P100T215 215 - NR

GFR16-F75P110T195

110

195 521 UCP

GFR16-F75P110T200 200 548 FCP

GFR16-F75P110T205 205 567 OCE

GFR16-F75P110T210 210 - NR

GFR16-F75P110T215 215 - NR

GFR16-F75P120T195

120

195 506 UCP

GFR16-F75P120T200 200 531 UCP

GFR16-F75P120T205 205 549 FCP

GFR16-F75P120T210 210 563 OCE

GFR16-F75P120T215 215 - NR

CHA

Fig.

conoC,

tens

to T

sim

had

sam

Tab

TProd

Se

TP

TP

TP

TP

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.27: Effec

Optimu

cluded as fi

at which t

sile strength

Based o

TPS-11, for

ilar manner

also guided

me combinat

ble 3.21: Sch

Trial duction et ID

RD

m(m

PS-8

PS-9

PS-10

PS-11

500

550

600

650

700

90

A

ct of variati

um combina

iber content

the achieve

h of 655 MP

on similar t

r 19mm and

r and as per

d for execut

tion of fiber

heme for on

Rebar Dia-

meter mm)

FibVoluFrac

(%

19 7

7

22 7

7

100Pull Speed (m

At Fiber Conten

EXPER

ion in Pull S

ation of pr

t = 74%, pu

ed tensile st

Pa of 16mm

trends of res

d 22mm dia

r table 3.21

ting no furth

r content an

nward Trial

ber ume ction

%)

Pul(mm

75 80,9

76

76 70,8

77

110 12mm/min.)

nt = 75%

195200205210


RIMENTATIO

83

Speed on Te

rocess para

ull speed = 1

trength was

m diameter re

sults of pre

ameter reba

. Appearanc

her trial wit

nd pull speed

Production

ll Speeds m/minute)

90,100,110

80,90,100

20

e (oC)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

ensile Stren

ameters for

110mm/min

s 629 MPa

eference reb

vious trial p

ars respectiv

ce of over c

th any incre

d.

n Sets for 19

HeatiTempera

200,205,2

205,210,2

500

550

600

650

700

80

A

VELOPMENT

gth of 16mm

r 16mm di

nute and die

a against th

bars.

production

vely, were i

curing effec

ement in die

9 and 22 mm

ing Die atures (oC)

210,215,220

215,220,225

100Pull Speed (m

At Fiber Conten

NT OF GFRP R

m diameter

ameter reb

e temperatur

he maximum

sets onward

implemente

cts on rebar

e temperatu

m diameter r

Results Table No.

0 3.22

3.23

5 3.24

3.25

120mm/min.)

nt = 75%

192020


REBARS

rebars.

bars was

re = 205

m target

d TPS-8

ed in the

r surface

ure at the

rebars.

GraphicaResults

Figure N

3.28 - 3.2

3.30 - 3.3

3.32 - 3.3

3.34 - 3.3

140

950005

e e (oC)

al s

No.

29

31

33

35


84



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR19-F75P80T200

75

80

200 580 FCP

GFR19-F75P80T205 205 605 OCE

GFR19-F75P80T210 210 - NR

GFR19-F75P80T215 215 - NR

GFR19-F75P80T220 220 - NR

GFR19-F75P90T200

90

200 567 UCP

GFR19-F75P90T205 205 589 FCP

GFR19-F75P90T210 210 601 OCE

GFR19-F75P90T215 215 - NR

GFR19-F75P90T220 220 - NR

GFR19-F75P100T200

100

200 554 UCP

GFR19-F75P100T205 205 579 UCP

GFR19-F75P100T210 210 606 FCP

GFR19-F75P100T215 215 607 OCE

GFR19-F75P100T220 220 - NR

GFR19-F75P110T200

110

200 542 UCP

GFR19-F75P110T205 205 563 UCP

GFR19-F75P110T210 210 581 FCP

GFR19-F75P110T215 215 599 OCE

GFR19-F75P110T220 220 - NR


85



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR19-F76P80T200

76

80

200 547 FCP

GFR19-F76P80T205 205 568 OCE

GFR19-F76P80T210 210 - NR

GFR19-F76P80T215 215 - NR

GFR19-F76P80T220 220 - NR

GFR19-F76P90T200

90

200 530 UCP

GFR19-F76P90T205 205 551 FCP

GFR19-F76P90T210 210 574 OCE

GFR19-F76P90T215 215 - NR

GFR19-F76P90T220 220 - NR

GFR19-F76P100T200

100

200 511 UCP

GFR19-F76P100T205 205 532 FCP

GFR19-F76P100T210 210 561 OCE

GFR19-F76P100T215 215 - NR

GFR19-F76P100T220 220 - NR

GFR19-F76P110T200

110

200 - NR

GFR19-F76P110T205 205 - NR

GFR19-F76P110T210 210 - NR

GFR19-F76P110T215 215 - NR

GFR19-F76P110T220 220 - NR


86



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR22-F76P70T205

76

70

205 542 FCP

GFR22-F76P70T210 210 559 OCE

GFR22-F76P70T215 215 - NR

GFR22-F76P70T220 220 - NR

GFR22-F76P70T225 225 - NR

GFR22-F76P80T205

80

205 529 UCP

GFR22-F76P80T210 210 548 FCP

GFR22-F76P80T215 215 558 OCE

GFR22-F76P80T220 220 - NR

GFR22-F76P80T225 225 - NR

GFR22-F76P90T205

90

205 508 UCP

GFR22-F76P90T210 210 538 UCP

GFR22-F76P90T215 215 566 FCP

GFR22-F76P90T220 220 567 OCE

GFR22-F76P90T225 225 - NR

GFR22-F76P100T205

100

205 486 UCP

GFR22-F76P100T210 210 513 UCP

GFR22-F76P100T215 215 535 FCP

GFR22-F76P100T220 220 554 OCE

GFR22-F76P100T225 225 - NR

CHA

Fig.reba

Fig.

Fig.reba

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h(M

Pa)

APTER-3

. 3.28: Effears.

. 3.29: Effec

. 3.30: Effears.

450

500

550

600

650

200H

At

450

500

550

600

650

80

A

HeatinTemp

450

500

550

600

650

200

Ten

sile

Str

engt

h (M

Pa)

He

At

ct of variat

ct of variati

ct of variat


Fiber Conten

90Pull Speed (m

At Fiber Conte

200205210215

ng Die perature (oC)


Fiber Content

EXPER

tion in Die

ion in Pull S

tion in Die

0 215mperature (oC)

nt = 75%

Pull(mm

100 1mm/min.)

ent = 75%

0 215mperature (oC)

t = 76%

9

Pull S(mm/m

RIMENTATIO

87

Temperatur

Speed on Te

Temperatur

220

8090100110

l Speed m/min.)

Ten

sile

Str

engt

h (M

Pa)

110

Ten

sile

Str

engt

h (M

Pa)

220

80

90

100

Speed min.)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

re on Tensi

ensile Stren

re on Tensi

450

500

550

600

650

195 20

g(

)

A

450

500

550

600

650

70 8

g(

)A

450

500

550

600

650

195H

At

VELOPMENT

le Strength

gth of 19mm

le Strength

00 205 210Heating Die T

At Fiber Conte

80 90Pull Speed

At Fiber Conte


Fiber Conten

NT OF GFRP R

of 19mm d

m diameter

of 19mm d

0 215 220Temperature (oC

ent = 75%

Pull(mm

100 110(mm/min.)

ent = 75%

Heating DTemperatu


nt = 76%

Pull(mm

REBARS

diameter

rebars.

diameter

0 225C)

8090100110

l Speed m/min.)

120

200205210215

Die ure (oC)

215)

8090100

Speed m/min.)

CHA

Fig.

Fig.diam

Figureba

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3

. 3.31: Effec


ure 3.33: Ears.

450

500

550

600

650

80

g(

)

A

450

500

550

600

650

205H

At

450

500

550

600

650

70

A

ct of variati

ect of variats.

Effect of va

90Pull Speed (m

At Fiber Conten


Fiber Conten

80Pull Speed (m

At Fiber Conte

EXPER

ion in Pull S

tion in Heat

ariation in P

100 1mm/min.)

nt = 76%

222



nt = 76%

Pull(mm/

90 1mm/min.)

ent = 76%

205210215220


RIMENTATIO

88

Speed on Te

ting Die Te

Pull Speed

110

200205210

e e (oC)

Ten

sile

Str

engt

h (M

Pa)

225)

708090100

Speed /min.)

Ten

sile

Str

engt

h (M

Pa)

100

e e (oC)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

ensile Stren

emperature

on Tensile

450

500

550

600

650

70 8

g(

)

450

500

550

600

650

200 205

g(

)

H

At

450

500

550

600

650

60 7

g(

)

A

VELOPMENT

gth of 19mm

on Tensile

e Strength

80 90Pull Speed (m

At Fiber Con

5 210 215Heating Die Te

t Fiber Conten

70 80

Pull Speed (m

At Fiber Conte

NT OF GFRP R

m diameter

Strength o

of 22mm d

100 110mm/min.)

tent = 76%

222


5 220 225emperature (oC)

nt = 76%

Pull(mm

90 100

mm/min.)

ent = 76%


REBARS

rebars.

of 22mm

diameter

120

200205210

e e (oC)

5 230)

708090100

Speed m/min.)

110

205210215220

e re (oC)


89



Rebar ID


(%)

Pull Speed

(mm/min.)


(oC)

Tensile Strength

(MPa)

Remarks

GFR22-F77P70T205

77

70

205 532 FCP

GFR22-F77P70T210 210 545 OCE

GFR22-F77P70T215 215 - NR

GFR22-F77P70T220 220 - NR

GFR22-F77P70T225 225 - NR

GFR22-F77P80T205

80

205 516 UCP

GFR22-F77P80T210 210 531 FCP

GFR22-F77P80T215 215 547 OCE

GFR22-F77P80T220 220 - NR

GFR22-F77P80T225 225 - NR

GFR22-F77P90T205

90

205 491 UCP

GFR22-F77P90T210 210 513 FCP

GFR22-F77P90T215 215 534 OCE

GFR22-F77P90T220 220 - NR

GFR22-F77P90T225 225 - NR

GFR22-F77P100T205

100

205 - NR

GFR22-F77P100T210 210 - NR

GFR22-F77P100T215 215 - NR

GFR22-F77P100T220 220 - NR

GFR22-F77P100T225 225 - NR

CHA

Fig.diam

Fig.

prod

proc

tabl

Ten

sile

Str

engt

h (M

Pa)

Ten

sile

Str

engt

h (M

Pa)

APTER-3


. 3.35: Effec

After c

ductions of

cess param

le 3.26, whi

450

500

550

600

650

205H

At

450

500

550

600

650

70

A

ect of variats.

ct of variati

onducting t

f GFRP reb

eters were

ch were sub


Fiber Conten

80Pull Speed (m

At Fiber Cont

EXPER

tion in Heat

ion in Pull S

the above e

bars along w

finalized fo

bsequently u

5 220mperature (oC)

nt = 77%

Pull(mm/

90 1mm/min.)

ent = 77%


RIMENTATIO

90

ting Die Te

Speed on Te

exhaustive

with simple

for each reb

used for the

225

708090

Speed /min.)

Ten

sile

Str

engt

h (M

Pa)

100

205210215

e e (oC)

Ten

sile

Str

engt

h (M

Pa)

ON FOR DEV

emperature

ensile Stren

experiment

e tension te

bar diamete

e final produ

450

500

550

600

650

200 20

g(

)

A

450

500

550

600

650

60

g(

)

A

VELOPMENT

on Tensile

gth of 22mm

tal work of

ests, optimu

er and have

uction of GF

05 210 21Heating Die T

At Fiber Conte

70 80Pull Speed (

At Fiber Conte

NT OF GFRP R

Strength o

m diameter

f 165 record

um combin

e been prese

FRP rebars.

5 220 225emperature (oC

ent = 77%

Pull S(mm/m

90 100(mm/min.)

ent = 77%

Heating DiTemperatu

REBARS

of 22mm

rebars.

ded trial

nation of

ented in

.

5 230C)

708090

Speed min.)

110

205210215

ie ure (oC)


91

Table 3.26: Optimum Combinations of Process Parameters for each diameter GFRP rebar.

Rebar Diameter

(mm)

Optimum Combination of Process Parameters Achieved Tensile Strength

(MPa)

Max. Target Tensile Strength

(MPa)

Fiber Content (%)


(oC)


9.5 72 195 130 747 760

13 73 200 120 674 690

16 74 205 110 629 655

19 75 210 100 606 620

22 76 215 90 566 586

25 78 220 80 527 550

Note: Maximum target tensile strengths of reference GFRP rebars have been taken from the data

sheet of Aslan-100TM GFRP rebars (2007) produced by Hughes Brothers Inc. USA.

It may be noted from above results that for each rebar diameter, there was a

unique combination of process parameters, which was used for the development of GFRP

rebars in order to achieve the desired tensile strength closely comparable with the

maximum reported tensile strength of reference GFRP rebars.

It is pertinent to note that in order to finalize the surface texture of locally

developed GFRP rebars, which may result the comparable bond stress with the reference

GFRP rebars, sixteen (16) plain GFRP rebars with and without sand coating treatment

were developed in four diameter rebars (db) of 9.5, 13, 19 and 22mm. The effect of

surface texture of GFRP rebar on average bond stress was studied by direct pullout tests

using 41.4 MPa concrete and two bonded lengths of 5.0 db as well as 7.0 db. The results of

this bond study have been published (Goraya et al, 2010) and experimental scheme as

well as results have been given in Appendix-A. The average bond stress of plain GFRP

rebars was quite lower than the reference rebars.


92

In order to improve the bond stress of local rebars, deformations were produced

on rebar surface by wrapping the surface helically with a resin wetted strand/yarn of

fibers along the length of rebar before its entrance in heating die. These rebars were

named as deformed or deformed uncoated GFRP rebars. Fixing of sand particles on rebar

surface with the same resin mixture just after its development made the rebars sand

coated and named as sand coated or deformed sand coated GFRP rebars.

The deformed rebars were next developed and subjected to simple direct pullout

tests for determining their average bond stresses. A set of twenty four (24) simple direct

pullout tests (without recording the stroke of slip) was conducted using 27.0 MPa

concrete by combining four diameter rebars of 9.5, 13, 19 and 25mm and three bonded

lengths of 3.0 db, 5.0 db and 7.0 db. Two pullout specimen sizes, Ø150mm x 300mm and

Ø100mm x 200mm, were used. The experimental details have been provided in chapter-5

and Appendix-B. The deformed GFRP rebars exhibited the average bond stress well

comparable with the reference rebars. Thus the deformed surface texture for GFRP rebars

was finalized due to its better bond performance.

3.5 FINAL PRODUCTION OF GFRP REBARS AND QUALITY ASSURANCE

TESTS

Using the experimentally determined optimum composition of resin mixture as

well as optimum combination of process parameters through exhaustive trial productions

along with barcol hardness and simple tension tests, final production of deformed

uncoated and sand coated GFRP rebars was made in six diameter rebars of 9.5, 13, 16,

19, 22 and 25mm.

Various properties of finally developed GFRP rebars were determined by

conducting the tests according to relevant ASTM/ ACI standards and compared with the

reported properties of reference GFRP rebars. The comparisons of properties have been

presented in tables 3.27, 3.28 and 3.29.


93

Fig. 3.36: Final production of GFRP rebars, deformed and sand coated.

3.5.1 Quality Assurance Tests

Three samples of each developed GFRP rebar were tested for the following

properties and results were compared with the reference rebars.

a) Barcol Hardness.

b) Specific Gravity.

c) Moisture Absorption.

d) Tensile Modulus of Elasticity.

The test results have been shown in table 3.27.

Table 3.27: Results of quality assurance tests of finally developed GFRP rebars.

Properties Relevant Standard

Average Results of Local Rebars

Reported Results of Reference Rebars

Barcol Hardness ASTM D-2583 48 50

Specific Gravity ASTM D-792 1.90 2.0

24 Hours Moisture Absorption at 50 oC

ASTM D-570

0.24%

0.22%

Tensile Modulus of Elasticity (GPa)

ACI 440.3R-04

39.4

40.8

Note: The statistical data of local GFRP rebars has been given in table E.1 of Appendix-E.


94

The above tests results indicate that GFRP rebars developed with available local

resources were closely conforming to ACI/ASTM standards as well as comparable with

the reference GFRP rebars.

The tensile strength and tensile modulus of elasticity of each diameter rebar was

determined, based on three samples, as per ACI 440.3R-04 Method B.2 and compared

with the reported values of reference rebars. The results of tensile strength and tensile

modulus of elasticity have been shown in tables 3.28 and 3.29 for deformed uncoated and

sand coated GFRP rebars respectively.

Table 3.28: Properties of Finally Developed GFRP uncoated deformed rebars.

Rebar ID

RebarDia-meter

(mm)

Rebar X-Sec. Area

(mm2)

Unit Wt.

(gm/m)

Tensile Strength of Local Rebars

(MPa)

Tensile Strength of Reference

Rebars

(MPa)

Elastic Modulus of Local Rebars

(GPa)

Elastic Modulus of Reference

Rebars

(GPa)

GFR9-D 9.5 70.88 133.10 747 760 39.10 40.80

GFR13-D 13 126.40 251.13 674 690 39.70 40.80

GFR16-D 16 197.70 412.52 629 655 39.30 40.80

GFR19-D 19 285.30 588.15 606 620 39.50 40.80

GFR22-D 22 387.60 783.78 566 586 39.60 40.80

GFR25-D 25 506.50 1012.30 527 550 39.40 40.80

Reference rebar values have been taken from the data sheet of Aslan-100TM (2007) GFRP

deformed rebars manufactured by Hughes Brothers Inc. USA. The statistical data of local GFRP

rebars has been given in table E.2 of Appendix-E.

The tensile modulus of elasticity was determined from the slope of stress-strain curve

being linear up to failure.


95

Table 3.29: Properties of Finally Developed GFRP Sand coated rebars.

Rebar ID

RebarDia-meter

(mm)

Rebar X-Sec. Area

(mm2)

Unit Wt.

(gm/m)

Tensile Strength of Local Rebars

(MPa)

Tensile Strength of Reference

Rebars

(MPa)

Elastic Modulus of Local Rebars

(GPa)

Elastic Modulus of Reference

Rebars

(GPa)

GFR9-S 9.5 70.88 134.42 761 760 38.96 40.80

GFR13-S 13 126.40 252.94 689 690 39.20 40.80

GFR16-S 16 197.70 414.35 644 655 38.90 40.80

GFR19-S 19 285.30 590.16 616 620 39.40 40.80

GFR22-S 22 387.60 785.58 577 586 39.70 40.80

GFR25-S 25 506.50 1014.00 540 550 39.50 40.80

Note: The statistical data of local GFRP rebars has been given in table E.3 of Appendix-E.

It is evident from the above results that tensile strength of locally developed

GFRP rebars improved in the range of 1.62% to 2.40% with sand coating treatment.

The tensile stress-stain graphs of GFRP deformed uncoated rebars have been

shown in figures 3.37 to 3.39 and sand coated rebars in figures 3.40 to 3.42.

Fig. 3.37: Tension test graphs for 9.5 and 13mm diameter deformed rebars respectively.

747

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

674

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

E = 39.70 GPa E = 39.10 GPa


96

Fig. 3.38: Tension test graphs for 16 and 19mm diameter deformed rebars respectively.

Fig. 3.39: Tension test graphs for 22 and 25mm diameter deformed rebars respectively.

Fig. 3.40: Tension test graphs for 9.5 and 13mm diameter sand coated rebars respectively.

629

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

606

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

566

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

527

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

761

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

689

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

E = 39.30 GPa E = 39.50 GPa

E = 39.60 GPa E = 39.40 GPa

E = 39.96 GPa E = 39.20 GPa


97

Fig. 3.41: Tension test graphs for 16 and 19mm diameter sand coated rebars respectively.

Fig. 3.42: Tension test graphs for 22 and 25mm diameter sand coated rebars respectively.

3.5.2 Discussion on Results

The literature review revealed that the tensile strength is a function of rebar

diameter. Tensile strength of a GFRP rebar increases with the decrease in its diameter.

Thus larger diameter GFRP rebars have less tensile strength as well as efficiency as

compared to smaller diameter rebars. Fibers located near the centre of rebar cross section

are not subjected to as much stress as those fibers which are situated near the outer

surface of rebar. The locally developed GFRP rebars followed the same trend.

Tensile stress-strain graphs of all the GFRP rebars indicated a linear elastic

behavior up to failure without any yield point. Failure of rebars was abrupt and quite

violent.

644

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

616

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

577

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

540

0

200

400

600

800

0.000 0.005 0.010 0.015 0.020 0.025

Ten

sile

Str

engt

h (M

Pa)

Strain

E = 39.90 GPa E = 39.40 GPa

E = 39.70 GPa E = 39.50 GPa

CHA

3.5.

wett

reba

wra

vary

is d

spac

Geo

Tab

Reb

Note

APTER-3

.3 Geometr

The ext

ted strand/

ars was pr

apped surfac

y. Sometim

efined as th

cing, as sho

ometric prop

ble 3.30: Ge

bar Diamete

db (mm)

9.5

13

16

19

22

25

e: Rib height

ry of Deform

ternal surfa

yarn of fib

imarily def

ce presented

es geometri

he ratio of th

own in figur

Fig.

perties of lo

eometric Pro

er Avg. R

hr

0

0

0

0

0

0

t and spacing

EXPER

med GFRP

ce of the G

ers along th

fined with

d a surface

ical charact

he projected

re 3.43.

. 3.43: Geom

ocal GFRP d

operties of F

Rib Height

(mm)

0.18

0.24

0.24

0.24

0.24

0.24

g may vary f

RIMENTATIO

98

P Rebars

GFRP rebar

he length o

two terms

with almos

teristic is de

d rib area no

metry of de

deformed re

Finally Dev

Avg. Rib

Sr (m

17.1

18.7

18.7

18.7

18.7

18.7

from product

ON FOR DEV

was made

f rebar and

s; rib spaci

st constant r

efined with

ormal to the

formed GFR

ebars have b

veloped GFR

Spacing

mm)

10

70

70

70

70

70

tion to produ

VELOPMENT

deformed b

geometry o

ing and rib

rib height b

the geomet

e axis to the

RP rebars

been presen

RP deforme

Avg. Rib Area

Ar (mm2)

5.27

9.62

11.88

14.14

16.61

18.67

uction.

NT OF GFRP R

by wrapping

of deformed

b height. H

but rib spac

tric ratio ‘as

centre to ce

nted in table

ed rebars.

Rib AreSpacing

As = A

0.30

0.51

0.63

0.75

0.88

0.99

REBARS

g a resin

d GFRP

Helically

ing may

’, which

entre rib

e 3.30.

ea to Ratio

Ar /Sr

08

4

5

6

8

98


99

3.6 COMPARISON OF GFRP REBAR PROPERTIES WITH STEEL REBARS

A comparison of local GFRP rebars properties was also made with the reported

properties of steel rebars as presented in table 3.31.

Table 3.31: Comparison of Properties of local GFRP rebars with Steel rebars.

Properties Local GFRP Rebars Steel Rebars

Tensile Strength (MPa) 527 - 761 483 - 620

Tensile Modulus of Elasticity (GPa) 39.4 200

Yield Strength (MPa) N/A 276 - 414

Hardness Barcol; 48 Rockwell; 43-54

Specific Gravity 1.90 7.9

Density (gm/cm3) 1.25 – 2.10 7.9

Ultimate Strain at Failure (%) 1.30 – 1.90 6 - 12

Moisture Absorption (%) 0.24 -

Unit Weight (gm/m) of 9.5mm to 25mm diameter rebars

133 to 1012 785 to 3925

Above comparison revealed that GFRP rebars have high tensile strength to weight

ratio with low tensile modulus of elasticity and ultimate strain. Based on this comparison

as well as average bond stress values (as detailed in chapter-5), the recommendations

have been made in the chapter-7.

3.7 SUMMARY

After having collaboration with the relevant local industry for assistance in the

development of GFRP rebars, necessary improvements were made in the existing old

pultrusion setup. Raw materials were finalized and 50 trial productions of GFRP rebars

were executed with barcol hardness tests, using trial and error approach, to determine the

optimum composition of resin mixture based on hardness criterion.


100

Optimum combination of three prime process parameters, fiber volume fraction,

pull speed and heating die temperature, were determined for each rebar diameter, through

165 executed trial productions of GFRP rebars with simple tension tests, initially through

hit and trial approach and based on tensile strength criterion. Subsequently production

models were developed, which helped to reduce the number of trial productions and

hence the cost of GFRP rebars. It was found that die temperature had more impact on the

tensile strength characteristics of GFRP rebar.

Surface texture of GFRP rebars was finalized through comparison of plain and

deformed GFRP rebar bond stresses, determined with direct pullout tests, with the

average bond stress of reference GFRP rebars.

Final production of GFRP deformed rebars was made using optimum composition

of resin mixture and combination of process parameters. Quality assurance tests were also

performed on the final production of GFRP rebars and comparison of properties of these

rebars was made with the reported properties of reference GFRP rebar as well as with

steel rebars. It was found that locally developed GFRP rebars were closely conforming to

the ACI/ASTM standards as well as comparable with the reference GFRP rebars,

developed in USA with more advance technology and resources.

It is pertinent to note that no detail related to the development of GFRP rebars

exists in the literature. Optimum composition of resin mixture as well as combination of

process parameters are always a trade secret due to proprietary issue, which have been

now made available through this research as an open source technology. Moreover, it

may be considered as the addition in the existing body of knowledge. None of any

developing countries as well as many of developed countries has started the development

and use of GFRP rebars.

CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS

101

PRODUCTION MODELS FOR GFRP REBARS

The optimum composition of resin mixture has been determined based on the

maximum barcol hardness of 50 as per ASTM standard as well as hardness of the

reference GFRP rebars. Optimum combination of three prime process parameters have

also been identified experimentally in the preceding chapter-3 based on the tensile

strength criterion for each GFRP rebar diameter.

This chapter describes the comparisons of results obtained in the preceding

chapter, as well as the development of proposed production models. In the absence of any

detail in the literature related to the development of GFRP rebars, trial and error approach

was adopted, which consumed a lot of time and resources. In order to reduce the number

of trial productions and optimize the cost of production process, individual production

models were developed for each diameter rebar. Finally a single and comprehensive

production model was also developed for the validation of experimental results as well as

to serve as fundamental guideline for the development of GFRP rebars in future.

4.1 HARDNESS AND TENSILE STRENGTH EXPERIMENTAL RESULTS

After conducting the trial productions of GFRP rebars by varying the composition

of resin mixture and subsequent hardness tests on each developed rebar with barcol

impressor as per ASTM D-2583, a value of 46 against the target range of 45-50 was

achieved. The optimum composition of resin mixture ingredients so achieved was then

used to determine the optimum combination of process parameters. Experimental versus

the maximum required hardness value was as follow:

Optimum Composition of Resin Mixture Ingredients Experimental Hardness

Value

Max. Required Hardness as per ASTM D-2583

Vinyl Ester Resin (Phr)

Filler (CaCO3) Phr

CO (Phr)

BPO (Phr)

TBPB (Phr)

100 5 0.28 1.00 2.00 46 50


102

It is evident that at optimum composition of resin mixture the experimental

hardness value was 92% of the maximum required value without optimization of the

process parameters.

In the subsequent part of experimental work for the development of GFPR rebars,

optimum combinations of process parameters for each rebar diameter were determined

based on tensile strength requirements through large number of trial productions. For

each trial production of GFRP rebar, simple tensile strength test was performed to

determine its maximum value. Upon achieving a tensile strength value closer to the target

tensile strength of reference GFRP rebars, the corresponding combination of process

parameters was selected as the optimum.

4.2 PRODUCTION MODELS FOR 9.5mm AND 25mm DIAMETER REBARS

First of all, optimum combination of process parameters was determined for 9.5mm

(the smallest) diameter rebars through 40 trial productions by hit and trial approach. After

having optimum combination for 9.5mm diameter rebars, production model relating the

tensile strength of GFRP rebar with the three process parameters was proposed. The detail

of this individual production model has been given below:

For a particular GFRP rebar diameter, the tensile strength ft (in MPa) is associated

with fiber content, F (in %), pull speed, P (in mm/minute) and heating die temperature,

Thd (in oC), the following relationship was considered for the first order approximations:

ft = α F + β P + γ Thd (4.1)

Where α, β and γ are the coefficients determined by regression analysis, ‘α’

represents the change in tensile strength per unit increase in fiber content (in %), ‘β’

denotes change in strength per unit change in pull speed (in mm/minute) and similarly ‘γ’

is the change in tensile strength per oC change in heating die temperature.

Seventeen trial production results were used to calibrate the above equation for

9.5mm diameter rebars. This data pertained to the trials which were either resulted in

under cured profile or fully cured profile. The over cured profile data was not considered

in this calibration process. Table 4.1 shows the statistics of this calibration.


103

Table 4.1: Statistics of Calibration of Proposed Model for 9.5mm diameter GFRP rebars:

Parameters

Calibrated Values -6.1972 -2.4089 +7.3805

t-Values -5.88 -16.23 18.45

Coefficient of Correlation 0.99

The coefficient of correlation is very close to 1.0, which indicates the perfection

of proposed production model.

It was noted that increase in fiber content and pull speed decreased the tensile

strength of 9.5 mm diameter rebars whereas increase in heating die temperature

contributed to increase in the tensile strength. The quality of calibration and validation

may be seen from the plot shown in figure 4.1. It may be noted that almost all the data

lies around the zero error line and well within 10 % error lines which indicates that

proposed production model represents the data very well.

Fig. 4.1: Experimental and Predicted Tensile Strengths of 9.5mm diameter GFRP rebars.

0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)

Experimental Tensile Strength (MPa)

+ 10%

- 10%


104

It is pertinent to note that proposed production model for 9.5mm diameter rebars

may not be truly applicable for larger diameter rebars, therefore at second stage, another

production model for 25mm diameter rebars was proposed after implementation of

various trial productions by hit and trial approach. Second set of data was taken from the

trial productions of 25mm diameter GFRP rebars for the calibration of proposed model.

This production model was calibrated and validated in the same way as was done for 9.5

mm diameter rebars. The statistics of this calibration have been shown in table 4.2.

Table 4.2: Statistics of Calibration of Proposed Model for 25mm diameter GFRP rebars:

Parameters


t-Values -3.84 -6.28 8.46


The quality of calibration and validation has been demonstrated by the plot shown in figure 4.2.

Fig. 4.2: Experimental and Predicted Tensile Strengths for 25mm diameter GFRP rebars.

0

200

400

600

0 200 400 600

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


105

The comparison of coefficients obtained for above two proposed production models has

been made in table 4.3.

Table 4.3: Comparison of Coefficients of Proposed Production Models for 9.5mm and 25mm diameter GFRP rebars.

Parameters

Calibrated Values

9.5 mm -6.1972 -2.4089 +7.3805

25 mm -5.6094 -1.4931 +4.8381

The scatter of data for 25 mm diameter rebars was quite well within the limits of

10%, and much better than the scatter of 9.5 mm diameter rebars. Moreover, 25mm

diameter rebars were less sensitive to the three process parameters as compared to 9.5mm

diameter rebars; yet the change in fiber content, pull speed and heating die temperature

affected the tensile strength more or less in the similar way.

It was also noted that the linear production model was more sensitive in case of

smaller diameter rebars and this variation of sensitivity, from 9.5 mm to 25 mm diameters

rebars for three process parameters, may be used to reduce the number of trial

productions for intermediate diameter rebars. When hit and trial approach was adopted

for the trial production of 9.5mm and 25mm diameter rebars, the statistics of production

efforts, which also affected the cost of production, has been shown in table 4.4.

Table 4.4: Statistics of Efforts for Trial production of 9.5mm and 25mm diameter GFRP

rebars based on hit and trial approach.

GFRP Rebar Diameter

(mm)

Planned No. of Trial

Productions

Executed No. of Trial

Productions

Non-Executed No. of Trials due to

appearance of OCE

%age of Executed Trial

Productions

9.5 60 40 20 66.67

25 60 33 27 55.00

OCE = Over Curing Effects


106

4.3 PRODUCTION MODELS FOR INTERMEDIATE DIAMETER REBARS

The proposed production models for 9.5mm and 25mm diameter rebars were used

to reduce the number of trial productions of comparable and closer intermediate diameter

rebars i.e. 13mm and 22mm respectively. From the trial productions data of 13mm and

22mm diameter rebars, their production models were also proposed, in the similar

manner, to help in reducing the trial productions of 16mm and 19mm diameter rebars

respectively. The statistics of reductions in trial productions with the use of proposed

production models have been presented in table 4.5.

Table 4.5: Statistics of Reduction in Trial Production of 13, 22, 16 and 19mm diameter

GFRP rebars based on Model Prediction.

Rebar Diameter

(mm)

Planned Trial Productions initially based on Hit & Trial

Approach

No. of Trial Productions based on Model Prediction

%age Reduction in No. of Trial

Productions due to Model Prediction Min.

RequiredAdditional

Confirmatory

Total

13 40 17 8 25 37.5

22 40 14 7 21 47.5

16 40 17 8 25 37.5

19 40 14 7 21 47.5

Although the minimum required trial productions based on model prediction were

quite few i.e. in the range of 35.0% to 42.5% of the initially planned based on hit and trial

approach, however keeping in view the limitations of proposed production models, some

additional confirmatory trial productions for each rebar diameter were also executed. The

additional confirmatory trial productions were also required to build the database for the

development of proposed production models for other diameter rebars. The reduction in

number of trial productions increased the time and cost efficiency of production process.


107

The proposed production models for intermediate diameter rebars were calibrated

using the fewer data sets of 13, 22, 16 and 19mm diameter rebars accordingly. The

statistics of calibrations of proposed model for 13mm diameter rebars have been shown in

table 4.6.

Table 4.6: Statistics of Calibration of Production Model for 13mm diameter GFRP rebars:

Parameters


t-Values -6.54 -9.87 15.05


Calibration and validation of quality for 13mm diameter rebars has been shown through

the plot given in figure 4.3.


The scatter of data for 13mm diameter rebars is much better than the scatter of

data for 25mm diameter rebars, almost on the zero error line. Similarly the statistics of

0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


108

calibration of production models for 22mm, 16mm and 19mm diameter rebars have been

given in tables 4.7, 4.8 and 4.9 respectively.

The quality of calibration and validation for 22mm, 16mm and 19mm diameter

rebars has been presented graphically in figures 4.4, 4.5 and 4.6 respectively.


Parameters


t-Values -4.77 -8.16 10.41



0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


109


Parameters


t-Values -4.41 -3.58 7.49



Table 4.9: Statistics of Calibration of proposed Model for 19mm diameter GFRP rebars

Parameters


t-Values -2.56 -2.07 4.80


It may be note that in statistics, a coefficient of correlation equal to or more than 0.80 has

been described as strong.

0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


110


The statistics of calibration of all the six individual production models for six

rebar diameters have been combined and presented in table 4.10.

Table 4.10: Calibrated Values of Coefficients for Proposed individual Production Models.

Parameters COC

Calibrated Values

25mm -5.6094 -1.4931 +4.8381 0.90

22 mm -6.6031 -1.8596 +5.7141 0.96

19 mm -8.5752 -1.2483 +6.5204 0.80

16 mm -11.8959 -1.5533 +8.1303 0.88

13 mm -7.6934 -1.8984 +7.2606 0.97

9.5 mm -6.1972 -2.4089 +7.3805 0.99

COC = Coefficient of Correlation

0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


111

4.4 UNIFIED PRODUCTION MODEL

At this stage, when optimum combinations of process parameters for all the six

diameters of GFRP rebars have been determined through exhaustive experimental work

and using individual production models, a better and final single comprehensive model,

called the unified production model, was proposed. When data of all six rebar diameters

was combined, the tensile strength was not only depending upon F, P and Thd but also on

the size of GFRP rebar. The heat energy consumed to cure GFRP rebar during pultrusion

process was the function of π db 2 as well as of its circumference, π db. The unified model

equation in terms of five coefficients has been given below, in which ‘db’ is nominal

diameter of rebar (in mm), ‘db 2’ is square of rebar diameter (in mm2), whereas ‘F’ Fiber

content (in %), ‘P’ pull speed (in mm/minute) and ‘Thd’ the Heating Die Temperature (in oC) as defined earlier.

ft = α F + β P + γ Thd + λ db + μ db 2 (4.2)

In the above equation, α, β, γ, λ and μ are the coefficients determined by

regression analysis, α, β, γ have already been defines in section 4.2 whereas ‘λ’ indicates

the variation in tensile strength due to unit change in the diameter of rebar and finally ‘μ’

represents the change in tensile strength of rebar due to unit change in db2 of the GFRP

rebar.

The above unified production model was developed for GFRP rebar diameters

ranging from 9.5mm to 25mm and fiber content ‘F’ in the range of 71% to 79% by

weight.

Pultrusion machine settings of heating die temperature and pull speed vary from

machine to machine, manufactured by different manufacturers. However, ranges of pull

speed and heating die temperature of pultrusion machine used for the development of

GFRP rebars were as follow:

Pull Speed ‘P’ 140 to 60 mm/minute with the interval of 10mm/minute and

Heating Die Temperature ‘Thd’ in the range of 185 oC to 230 oC with interval of 5 oC.


112

Half of the data was used to calibrate the unified production model and remaining

half was used for the validation of the model. The values of above five coefficients

determined by regression analysis have been given in table 4.11 and quality of fit has

been shown in figure 4.7.

Table 4.11: Calibrated Values of Coefficients for Unified Production Model for GFRP rebars.

Parameters µ

Calibrated Values

-3.2398 -1.7526 6.9530 -20.7686 -0.1521

t-Values -3.02 -9.79 16.02 -8.17 -2.30


Fig. 4.7: Quality of fit of Unified Production Model for GFRP rebars.

0

200

400

600

800

0 200 400 600 800

Pre

dict

ed T

ensi

le S

tren

gth

(MP

a)


+ 10%

- 10%


113

The scatter of data for unified production model for GFRP rebars is quite well

within the limits of 10%, thus the model given in the equation 4.2 fits the data quite

accurately.

It is pertinent to note that unified production model would also serve as

fundamental guideline for the production of GFRP rebars in future where patent details

are not available. The final step in this part of study was to validate the experimentally

achieved maximum tensile strengths of local GFRP rebars at the optimum combination of

process parameters for each rebar diameter. Comparison of experimental tensile strengths

of final production of deformed GFRP rebars was made with the predicted tensile

strengths, using the unified production model, as well as with the reported tensile

strengths of reference GFRP rebars. This comparison has been presented in table 4.12 as

well as in figure 4.8 graphically.

Table 4.12: Comparison of experimental tensile strengths of local GFRP deformed rebars

with Predicted and Reference rebars tensile strengths.

Rebar Dia-meter (mm)

Optimum Process Parameters

Tensile Strength (MPa)

%age Diff. of 1 & 2

%age Diff. of 1 & 3

Fiber Content

(%)

Pull Speed

(mm/min)


(oC)

1 2 3

Experi-mental

Predicted by Unified

Model

Reference Rebars

9.5 72 130 195 747 684 760 8.43 1.71

13 73 120 200 674 648 690 3.86 2.31

16 74 110 205 629 622 655 1.11 3.96

19 75 100 210 606 592 620 2.31 2.25

22 76 90 215 566 560 586 1.06 3.41

25 78 80 220 527 522 550 0.95 4.18

Standard Deviations 2.906 1.017

Tensile strengths of reference rebars have been taken from the data sheet of Aslan-100TM (2007)

GFRP rebars.


114

Fig. 4.8: Comparison of experimental and predicted tensile strengths with the tensile

strengths of reference GFRP rebars.

4.5 SUMMARY

The comparison of experimental tensile strengths with the predicted tensile

strengths based on unified production model revealed the variation in the range of 1% to

8% whereas this variation was in the range of 2% to 4% for reference GFRP rebars,

which indicates the quality of fit of unified production model. It may be noted that barcol

hardness of final production of GFRP rebars was 48 as compared to the maximum

reported hardness of 50 for the reference rebars. In the absence of any data related to

development of GFRP rebars in the literature, the local development of GFRP rebars,

closely comparable to the international standard as well as to the GFRP rebars

manufactured by a technological advanced country, was a major breakthrough/

achievement.

The proposed individual production models helped to reduce the number of trial

productions in the range of 37.5% to 47.5% of the initially planned trials based on hit and

0

200

400

600

800

9.5 13 16 19 22 25

Ten

sile

Str

engt

h (M

Pa)

GFRP Rebar Diameters (mm)

Experimental Tensile Strength

Reference Tensile Strength

Predicted Tensile Strength


115

trial approach, thus reducing the cost of GFRP rebars. The unified production model

including the effects related to rebar diameter would also serve as comprehensive

guideline for the production of GFRP rebars in future.

It is pertinent to note that no such production models were available in the existing

body of knowledge, which have now been added through this research work.

CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION

116

EXPERIMENTATION FOR BOND STRESS EVALUATION

5.1 GENERAL

After successful development of GFRP rebars as well as production models, the

last phase experimental work was the evaluation of average bond strength of these rebars.

In reinforced concrete members, transfer of tensile stress from concrete to rebar is

through the bond between the two materials. Therefore, it was necessary to evaluate the

average bond strength of locally developed GFRP rebars to ensure their effective

performance/composite action with concrete. GFRP rebar bond with concrete is

controlled by internal mechanisms including the chemical adhesion between rebar and

concrete at interface, frictional resistance caused by surface roughness against the rebar

slip and thirdly mechanical interlock between rebar and concrete due to irregularities of

the interface. Friction and mechanical interlocking are usually considered to be the most

effective means of stress transfer.

Literature review revealed that bonded length, rebars diameter, concrete cover,

surface texture, concrete compressive strength are the main factors which affect the

average bond stress of GFRP rebars.

In chapter-5, details of experimental schemes for direct pullout tests, beam bond

tests and junction tests along with results of effect of variation in bonded length, rebar

diameter, concrete cover, surface texture and concrete strength on the average bond stress

of locally developed GFRP rebars as well as comments on these results have been

presented.

5.2 EXPERIMENTAL PROGRAM

The experimental program for determination of average bond stress of GFRP

rebars with normal strength concrete had three phases. Phase-1 experimental program,

comprised of direct pullout tests, was conducted as per ACI 440.3R-04 method B3. Four

GFRP deformed uncoated and sand coated rebar sizes of 9.5mm, 13mm, 19mm and

25mm diameter were used with three bonded lengths of 3.5 db, 5.0 db and 7.0 db. Forty


117

eight direct pullout tests were performed using 41.4 MPa compressive strength concrete.

It may be noted that another set of twenty four simple direct pullout tests were conducted

using 27.0 MPa concrete at the time of development of GFRP rebars in connection with

finalization of rebar surface texture, refer Appendix-B for experimental schemes and

results. Five aspects were studied, firstly the effect of variation of bonded length, secondly

the effect of rebar diameter, thirdly the effect of concrete cover, fourthly the effect of

surface texture and lastly the effect of concrete compressive strength variation on the

average bond stress at maximum pullout force.

In phase-2 of this experimental program, determination of average bond stress was

carried out through beam tests as per RILEM 1994a specifications. Three GFRP

deformed uncoated rebar sizes of 9.5mm, 13mm and 19mm diameter were used with

three bonded lengths of 3.5 db, 5.0 db and 7.0 db. Six beams were tested to determine the

average bond stress response in terms of effect of bonded length and rebar diameter

variation on the average bond stress of locally developed GFRP rebars.

Third phase of experimentation was to evaluate the average bond stress of an

assembly of two intersecting beams at right angles, also called a junction. The purpose of

this testing was to determine the effect of joint action on the average bond stress of

primary beam of junction using local GFRP rebars. The assembly of primary and

secondary beams had the same dimensions as that of individual beams tested in phase-2

of this experimental program for comparison purposes. The beam in junction with more

effective depth of main rebar i.e. with bottom rebar below the other intersecting beam

rebar, was named as primary beam and other as secondary. Six junctions were tested to

study the average bond stress response in the form of effect of bonded length and rebar

diameter variation. Three uncoated deformed GFRP rebar sizes of 9.5, 13 and 19mm

diameter were used with three bonded lengths of 3.5 db, 5.0 db and 7.0 db.

5.3 MATERIALS

5.3.1 Cement:

Cement acts as primary binder in a mortar or concrete mixture. Locally available

Ordinary Portland (ASTM type-I) Cement was used with the brand name of ‘Bestway’.


118

Various laboratory tests as per ASTM C-150 were performed on the cement for

determining its physical and mechanical properties and the results have been shown in

table 5.1.

Table 5.1: Physical and Mechanical Properties of ASTM type-I Cement.

Sr. No. Properties Test Results

1 Standard Consistency 29%

2 Initial Setting Time 1 hour 50 minutes

3 Final Setting Time 3 hours 25 minutes

4 Fineness Value 6%

5 Soundness Value 7mm

6 Compressive Strength of 68mm mortar cubes

(1:3 c/s mortar with w/c ratio of 0.40)

3 days strength = 16 MPa


7 Compressive Strength of 100mm concrete

cubes (PCC 1:2:4 with w/c ratio of 0.52)



5.3.2 Fine Aggregates:

Fine aggregates (sand) fill the voids/pores of coarse aggregates in a concrete

mixture. Locally available coarse river sand i.e. lawrencepur sand having fineness

modulus of 2.65 was used after conducting necessary tests as per ASTM C-33. The test

results have been shown in table 5.2.

Table 5.2: Properties of Fine Aggregates.

Source of Sand Specific

Gravity

Bulk Density

(Kg/m3)

Water

Absorption (%)

Fineness Modulus

Lawrencepur 2.70 1445 1.20 2.65


119

5.3.3 Coarse Aggregates:

Well graded crushed stone was used as coarse aggregates. The maximum size of

coarse aggregate was 13mm. Various laboratory tests were performed as per ASTM C-33

on locally available margalla crushed stone and the results have been shown in table 5.3:

Table 5.3: Properties of Coarse Aggregates.

Source of Coarse

Aggregates

Specific Gravity

Bulk Density

(Kg/m3)

Water

Absorption (%)

Fineness Modulus

Margalla

Crushed Stone 2.65 1380 1.00 7.64

Clean tab water was used for the preparation of concrete mix.

5.3.4 GFRP Rebars:

The deformed GFRP uncoated and sand coated rebars, after their successful

development and quality assurance tests, were used in the experimentation to determine

their average bond stress with normal strength concrete. The properties of deformed

uncoated rebars and sand coated rebars have already been given in the tables 3.28 and

3.29 of chapter-3 respectively.

5.3.5 Concrete Mix Proportions:

For the preparation of normal strength concrete in laboratory, weight batching of

concrete ingredients (cement, sand and crushed stone) was carried out as per ASTM C-

192 with the following mix proportions:

a) For 41.4 MPa compressive strength concrete: 1:1.35: 2.75 by weight with water

cement ratio (w/c) of 0.40 and slump value of 50-75 mm.


120

b) For 27.0 MPa concrete: 1:1.80: 3.75 by weight with water cement ratio (w/c) of 0.40

and slump value of 50-75 mm. It is pertinent to note that this concrete was used for

determining the average bond stress with simple direct pullout tests related to

finalization of surface texture of the GFRP rebars, as discussed in the chapter-3.

The concrete ingredients were mixed at room temperature and in a rotating concrete

mixer. The fine and coarse aggregates were mixed together first, then cement was added

and allowed to mix thoroughly. Finally clean water was added and allowed to mix until

concrete became uniform in appearance. Cylinders were casted for confirmation of concrete

strength (also called control cylinders) as well as for the pullout test specimens. Soon after

pouring of concrete, exposed concrete surfaces were covered with polythene sheet to avoid

evaporation of moisture from the fresh concrete. After 24 hours of casting of concrete

cylinders, curing was started as per ASTM C-192. During the curing process of concrete, it

was made sure that GFRP rebar outside the concrete area of a pullout specimen is not

submerged in curing water.

Compressive strength of control cylinders (Ø150mm x 300mm) was determined

as per ASTM C-39 using universal testing machine at maturity period of 3, 7, 14 and 28

days. The test results of 41.4 MPa concrete have been shown in figure 5.1.

Fig. 5.1: Rate of Gain of Compressive Strength of Concrete at 3, 7, 14 and 28 Days.

0

10

20

30

40

50

0 5 10 15 20 25 30

Com

pres

sive

Str

engt

h (M

Pa)

Maturity Period of Concrete (Days)


121

5.4 DIRECT PULLOUT TESTING

GFRP rebar bond with concrete influences the behavior of concrete in number of

ways. It affects the anchorage of rebars, strength of lap splices and serviceability etc. The

pullout test method is usually adopted to study these parameters. Furthermore, in order to

study average bond stress response of local GFRP rebars with concrete in case of beams,

it was felt appropriate to determine bond stress through direct pullout tests for

comparison/reference purposes. Pullout test is comparatively simple and gives an

assessment of parameters that affect the average bond stress.

Direct pullout tests were conducted to study the effect of variation of bonded length,

rebar diameter, cover, surface texture and concrete strength variation on average bond stress

between GFRP rebar and concrete.

5.4.1 Test Specimens

GFRP rebars were cut into 1200mm lengths with an anchor on one end. Deformed

GFRP uncoated as well as sand coated rebar test specimens were prepared in four diameters

of 9.5, 13, 19 and 25mm and used in the pullout testing. Three bonded lengths of 3.5 db, 5.0

db and 7.0 db, were selected for the experimentation. Two cylinder specimen sizes of

Ø 150mm x 300mm and Ø 100mm x 200mm, were used.

The bond between GFRP rebar and concrete was broken by covering the rebar with

polyvinyl chloride (PVC) pipe, where it was not required.

5.4.2 Testing Setup and Procedure

The testing setup was comprised of universal testing machine (UTM) as shown in

figure 5.2, specially designed pullout assembly and the data acquisition system. A hinge has

been provided on one side of pullout assembly for the purpose of eliminating eccentricity,

which may be developed during fixing of rebar in the UTM. Pullout specimen rebar was

gripped from anchor side in the upper jaw of testing machine and hinged rod of pullout

assembly in the lower jaw of machine. Assuming that bond stress is uniformly distributed


122

along the bonded length ‘Lb’, the average bond stress ‘u’ can be determined by dividing the

maximum measured pullout force ‘F’ with the bonded surface area (db x Lb) of the GFRP

rebar, u =

, where db is the nominal diameter of GFRP rebar, which is an average

diameter assuming the shape of rebar as a circle. This bond stress was an average value

over the bonded length.

5.5 DIRECT PULLOUT TEST RESULTS

GFRP rebar embedded in concrete develops bond initially by chemical adhesion

between concrete and rebar surface. With the increase in pullout force, adhesion vanishes

and bond stress is made up of friction resistance of interface against slip as well as of

mechanical interlock between GFRP rebar and surrounding concrete due to irregularities

of interface. Experimental results revealed that the bond failure occurred at the interface

of concrete and rebar surface and in some cases of sand coated rebars, failure occurred at

the interface of rebar original surface and sand coating due to relative high shear strength

of concrete. Sand coating increased the frictional component of bond stress.

Different types of failure mode were observed during the pullout experimentation.

Firstly, the pullout failure, secondly splitting failure of concrete and thirdly, the mixed

mode failure having both the effects, as shown in figures 5.3 to 5.5. Pullout failure

occurred when shear strength of bond between rebar surface and concrete was exceeded

and there was relative movement of the rebar. Pullout failure was observed for smaller

(9.5 and 13mm) diameter rebars as well as for shorter bonded lengths. Pullout failure

occurred when concrete cover was large enough resulting into higher confining pressure

on the rebar and restraining the splitting of concrete. With the increase in rebar diameter

from 13mm to 19mm and bonded length from 3.5 db to 5.0 db, pullout failure shifted into

mixed mode failure. For larger diameter rebars, 25mm with 5.0 db & 7.0 db bonded

lengths, and 19mm with 7.0 db bonded length, the failure was splitting type, which was

abrupt and highlighted by formation of flexural cracks as well as concrete splitting.

According to Okelo et al. (2005), type of bond failure mainly depends upon bonded

length, rebar diameter, surface texture, concrete cover to the rebar and the concrete

compressive strength.


123

Fig. 5.2: Pullout Testing Setup Fig 5.3: Pullout Failure

Fig 5.4: Mixed Mode Failure Fig 5.5: Splitting Failure

The experimental schemes and results of direct pullout bond study have been

presented in tables 5.4 to 5.7 as well as in figures 5.6 to 5.15 graphically.


124

Table 5.4: Scheme and Results of Pullout Tests for Uncoated Deformed GFRP rebars


Rebar ID


Clear Cover

C (mm)

C/db

Ratio

db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

Stroke s

(mm)

Mode of

Failure

GFRD9-Lb3.5C70 9.5 70.25

7.39

1/3.5 18.13 18.27 17.91 P

GFRD9-Lb5.0C70 1/5.0 22.33 15.75 14.35 P

GFRD9-Lb7.0C70 1/7.0 25.03 12.61 11.72 P

GFRD13Lb3.5C68 13 68.50

5.27

1/3.5 31.81 17.12 16.68 P

GFRD13Lb5.0C68 1/5.0 37.85 14.26 12.21 P

GFRD13Lb7.0C68 1/7.0 43.52 11.71 9.27 M

GFRD19Lb3.5C65 19 65.50

3.45

1/3.5 62.36 15.71 13.81 P

GFRD19Lb5.0C65 1/5.0 74.28 13.10 10.34 M

GFRD19Lb7.0C65 1/7.0 83.99 10.58 7.13 S

GFRD25Lb3.5C62 25 62.50

2.50

1/3.5 78.76 11.46 10.12 M

GFRD25Lb5.0C62 1/5.0 93.36 9.51 8.77 S

GFRD25Lb7.0C62 1/7.0 110.51 8.04 5.53 S

Note: GFRDxx-LbyyCzz stands for GFRP uncoated Deformed rebar, with xx diameter, Bonded Length (Lb) of yy times the rebar diameter (db), and zz concrete clear cover (C) to GFRP rebars, respectively. The letters P, M and S represent the Pullout, Mixed mode and Splitting failure respectively.

It is pertinent to note that stroke values have been used as the pullout tests were

conducted to determine the average bond stress at maximum pullout force. Stroke, in this

case, is only a qualitative indicator of slip, although no relation exists between the two.

Higher stroke values are the indicative of higher slips and vice versa. The graphical

representation of above results has been given in figures 5.6 to 5.10, the plot data has

been smoothened using the bezier curve techniques. For reference purposes, the original

pullout test graphs (bond stress versus stroke), containing large number of data points,

have also been given in Appendix-C.


125

Fig. 5.6: Effect of bonded length variation on bond stress of 9.5mm and 13mm diameter rebars respectively.

Fig. 5.7: Effect of bonded length variation on bond stress of 19mm and 25mm diameter rebars respectively.

Fig. 5.8: Effect of diameter variation on bond stress of GFRP rebars for 3.5 db and 5.0 db

bonded lengths respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db 0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm13mm19mm25 mm

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm13mm19mm25mm


126

Fig. 5.9: Effect of diameter variation on bond stress of GFRP rebars for 7.0 db bonded

length.

Fig. 5.10: Effect of bonded length variation on maximum pullout force and average bond

stress of deformed uncoated GFRP rebars respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm13mm19mm25mm

0

25

50

75

100

125

0 2 4 6 8

Pul

lout

For

ce (

KN

)

Bonded Length (x db)

9.5 mm13 mm19 mm25 mm

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


9.5 mm13 mm19 mm25 mm


127

Table 5.5: Scheme and Results of Pullout Tests for deformed Sand Coated GFRP rebars


Rebar ID

Rebar Dia-meter

db (mm)

Clear Cover

C (mm)

C/db

Ratio

db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

Stroke s

(mm)

Mode of

Failure

GFRS9Lb3.5C70 9.5 70.25

7.39

1/3.5 19.73 19.88 15.20 P

GFRS9Lb5.0C70 1/5.0 25.06 17.68 11.13 P

GFRS9Lb7.0C70 1/7.0 28.80 14.51 9.47 P

GFRS13Lb3.5C68 13 68.50

5.27

1/3.5 33.69 18.13 14.20 P

GFRS13Lb5.0C68 1/5.0 42.13 15.87 9.68 M

GFRS13Lb7.0C68 1/7.0 47.05 12.66 7.12 M

GFRS19Lb3.5C65 19 65.50

3.45

1/3.5 65.77 16.57 11.92 P

GFRS19Lb5.0C65 1/5.0 80.52 14.20 8.79 M

GFRS19Lb7.0C65 1/7.0 88.12 11.10 5.54 S

GFRS25Lb3.5C62 25 62.50

2.50

1/3.5 88.10 12.82 12.27 M

GFRS25Lb5.0C62 1/5.0 101.41 10.33 8.89 S

GFRS25Lb7.0C62 1/7.0 125.07 9.10 7.52 S

Note: GFRSxx-LbyyCzz stands for GFRP sand coated rebar, with xx diameter, Bonded Length (Lb) of

yy times the rebar diameter (db), and zz concrete clear cover (C) to GFRP rebars, respectively.

The graphical representation of above results has been given in figures 5.11 to 5.13.


128

Fig. 5.11: Effect of bonded length variation on bond stress of 9.5mm and 13mm diameter deformed sand coated rebars respectively.

Fig. 5.12: Effect of bonded length variation on bond stress of 19mm and 25mm diameter deformed sand coated rebars respectively.

Fig. 5.13: Effect of bonded length variation on maximum pullout force and average bond stress of deformed Sand coated GFRP rebars respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

3.5 db5.0 db7.0 db

0

25

50

75

100

125

0 2 4 6 8

Pul

lout

For

ce (

KN

)


9.5 mm13 mm19 mm25 mm

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


9.5 mm13 mm19 mm25 mm


129

Table 5.6: Scheme and Results of Pullout Tests for Deformed Uncoated GFRP rebars


Rebar ID


Clear Cover

C (mm)

C/db

Ratio db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

Stroke s

(mm)

Mode of

Failure

GFRD9-Lb3.5C45 9.5

45.25

4.76

1/3.5 17.64 17.78 20.32 P

GFRD9-Lb5.0C45 1/5.0 21.95 15.48 16.47 P

GFRD9-Lb7.0C45 1/7.0 24.00 12.09 13.67 P

GFRD13-Lb3.5C43 13

43.50

3.35

1/3.5 26.42 14.22 18.54 P

GFRD13-Lb5.0C43 1/5.0 31.64 11.92 14.37 P

GFRD13-Lb7.0C43 1/7.0 36.27 9.76 11.13 P

GFRD19-Lb3.5C40 19

40.50

2.13

1/3.5 52.16 13.14 15.62 P

GFRD19-Lb5.0C40 1/5.0 61.81 10.90 11.94 P

GFRD19-Lb7.0C40 1/7.0 69.70 8.78 9.06 M

GFRD25-Lb3.5C37 25

37.50

1.50

1/3.5 65.84 9.58 12.04 P

GFRD25-Lb5.0C37 1/5.0 77.95 7.94 10.53 M

GFRD25-Lb7.0C37 1/7.0 92.91 6.76 7.19 S

The graphical representation of above results has been given in figure 5.14.

Fig. 5.14: Effect of bonded length variation on maximum pullout force and average bond stress of deformed uncoated GFRP rebars respectively.

0

25

50

75

100

125

0 2 4 6 8

Pul

lout

For

ce (

KN

)


9.5 mm13 mm19 mm25 mm

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


9.5 mm13 mm19 mm25 mm


130

Table 5.7: Scheme and Results of Pullout Tests for deformed Sand Coated GFRP rebars using Ø100mm x 200mm Test Specimens and 41.4 MPa Compressive Strength Concrete.

Rebar ID


Clear Cover

C (mm)

C/db

Ratio db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

Stroke s (mm)

Mode of

Failure

GFRS9-Lb3.5C45 9.5

45.25

4.76

1/3.5 19.57 19.72 17.39 P

GFRS9-Lb5.0C45 1/5.0 23.90 16.86 13.07 P

GFRS9-Lb7.0C45 1/7.0 24.13 12.16 11.18 P

GFRS13-Lb3.5C43 13

43.50

3.35

1/3.5 30.74 16.54 16.29 P

GFRS13-Lb5.0C43 1/5.0 37.38 14.08 12.01 P

GFRS13-Lb7.0C43 1/7.0 41.77 11.24 8.94 P

GFRS19-Lb3.5C40 19

40.50

2.13

1/3.5 55.73 14.04 14.06 P

GFRS19-Lb5.0C40 1/5.0 66.57 11.74 10.85 P

GFRS19-Lb7.0C40 1/7.0 75.26 9.48 7.92 M

GFRS25-Lb3.5C37 25

37.50

1.50

1/3.5 73.12 10.64 13.89 P

GFRS25-Lb5.0C37 1/5.0 83.84 8.54 11.14 M

GFRS25-Lb7.0C37 1/7.0 102.40 7.45 9.38 S

The graphical representation of above results has been shown in figure 5.15.

Fig. 5.15: Effect of bonded length variation on maximum pullout force and average bond stress of deformed Sand coated GFRP rebars respectively.

0

25

50

75

100

125

0 2 4 6 8

Pul

lout

For

ce (

KN

)


9.5 mm13 mm19 mm25 mm

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


9.5 mm13 mm19 mm25 mm


131

5.6 DISCUSSION ON DIRECT PULLOUT RESULTS

Following aspects have been studied in the direct pullout testing:

5.6.1 Effect of bonded length and rebar diameter variation on average bond stress

The results of direct pullout tests for study the effect of bonded length variation on

average bond stress of deformed uncoated GFRP rebars revealed that with the increase in

bonded length from 3.5 db to 5.0 db and then to 7.0 db, the pullout load increased whereas

average bond stress and corresponding stroke value, both decreased for all rebar

diameters and this reduction in bond stress generally increased with the increase in rebar

diameter as indicated from tables 5.4 to 5.7 as well as figures 5.6 to 5.15. Similar bond

stress response was observed in all the cases, uncoated as well as sand coated rebars in

both pullout specimen sizes. For example, 9.5mm diameter uncoated rebars, using

Ø150mm x 300mm test specimens, reduction in bond stress was within the range of 14%

to 20%. For 13mm diameter rebars, the range of reduction in bond stress was from 17%

to 18%. Similarly for 19mm and 25mm diameter rebars, bond stresses reduced within the

ranges of 17% to 19% and 15% to 17% respectively. The stroke values decreased in the

range of 7% to 21% for 9.5mm diameter rebars, 24% to 27% for 13mm, 25% to 31% for

19mm and 13% to 37% for 25mm diameter rebars. It is pertinent to note that effect of

bonded length variation on pullout force was increasing with the increase in bonded

length but not proportionately.

The effect of increase in rebar diameter on average bond stress for different

bonded lengths was also studied and found that when rebar diameter was increased,

average bond stress and stroke value, both decreased in all the cases. For example, when

rebar diameter was increased from 9.5mm to 13mm, for uncoated deformed GFRP rebars

using Ø150mm x 300mm test specimens, bond stress decreased within the range of 6% to

9%. Similarly, increase in rebar diameter from 13 mm to 19mm and then 19 mm to 25mm

caused the reduction in bond stress within the range of 8% to 10% and 24% to 27%

respectively for the three bonded lengths. The reduction in stroke values was 7% to 21%

when rebar diameter was increased from 9.5mm to 13mm and 15% to 23% for other

diameter rebars.


132

The sand coated GFRP rebars revealed the similar trend of increase in pullout

force and decrease in average bond stress with the increase in bonded lengths as well as in

rebar diameters but with higher values. Sand coating improved the overall bond stress

because of better surface roughness and friction. The increased surface roughness also

reduced the stroke values.

In sand coated rebars with Ø150mm x 300mm test specimens, the reduction in

average bond stress, for 9.5mm, 13mm, 19mm and 25mm diameter rebars, was in the

range of 11% to 22%, when the bonded length was increased from 3.5 db to 5.0 db and

then to 7.0 db.

The major reason of decrease in average bond stress with the increase in bonded

length and rebar diameter was that with the increase in bonded length, pullout load

increased but bond stress decreased due to the fact that pullout load increase was not

proportional to the increase in bonded length. Furthermore decrease in average bond

stress can be explained by considering the actual non-uniform bond stress distribution

instead of assuming it uniform over the bonded length.

The increase in bonded length as well as in rebar diameter caused the decrease in

stroke value due to increase in contact area of rebar surface with the concrete, which

increased the resistance against rebar slip.

5.6.2 Effect of cover variation on average bond stress

The effect of cover variation on average bond stress has been presented in table 5.8.

For each rebar diameter, there were three bonded lengths and for each bonded length, two

pullout specimen sizes were tested and compared.


133

Table 5.8: Comparison of Bond Stresses of Deformed Rebars using Ø150mm x 300mm, Ø100mm x 200mm Specimens and 41.4 MPa Compressive Strength Concrete.

Rebar ID


Clear Cover Ratio

C150/C100

db/Lb

Ratio

Bond Stress of Ø150mm

x 300mm Specimens u150 (MPa)

Bond Stress of Ø100mm

x 200mm Specimens u100 (MPa)

%age Diff. in Bond Stress (%)

GFR9-Lb3.5 9.5 1.55

1/3.5 18.27 17.78 2.68

GFR9-Lb5.0 1/5.0 15.75 15.48 1.71

GFR9-Lb7.0 1/7.0 12.61 12.09 4.12

GFR13-Lb3.5 13 1.57

1/3.5 17.12 14.22 16.94

GFR13-Lb5.0 1/5.0 14.26 11.92 16.41

GFR13-Lb7.0 1/7.0 11.71 9.76 16.65

GFR19-Lb3.5 19 1.62

1/3.5 15.71 13.14 16.36

GFR19-Lb5.0 1/5.0 13.10 10.90 16.79

GFR19-Lb7.0 1/7.0 10.58 8.78 17.01

GFR25-Lb3.5 25 1.67

1/3.5 11.46 9.58 16.40

GFR25-Lb5.0 1/5.0 9.51 7.94 16.51

GFR25-Lb7.0 1/7.0 8.04 6.76 15.92

The graphical representation of above results has been given in figures 5.16 and 5.22.

Fig. 5.15: Effect of cover variation on bond stress of 9.5mm diameter uncoated deformed rebars for 3.5 db and 5.0 db bonded lengths respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 4.76C/db = 7.39

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 4.76

C/db = 7.39


134

Fig. 5.16: Effect of cover variation on bond stress of 9.5mm (for 7.0 db bonded length) and 13mm diameter rebars with 3.5 db bonded lengths, respectively.

Fig. 5.17: Effect of cover variation on bond stress of 13mm diameter rebars for 5.0 db and 7.0 db bonded lengths respectively.

Fig. 5.18: Effect of cover variation on bond stress of 19mm diameter rebars for 3.5 db and 5.0 db bonded lengths, respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 4.76

C/db = 7.390

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 3.35

C/db = 5.27

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 3.35

C/db = 5.270

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 3.35

C/db = 5.27

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 2.13

C/db = 3.450

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 2.13

C/db = 3.45


135

Fig. 5.19: Effect of cover variation on bond stress of 19mm and 25mm diameter rebars for 7.0 db and 3.5 db bonded lengths respectively.

Fig. 5.20: Effect of cover variation of 25mm diameter deformed rebars for 5.0 db and 7.0 db bonded lengths respectively.

Fig. 5.21: Effect of bonded length variation on bond stress of 9.5mm and 13mm diameter rebars respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 2.13

C/db = 3.450

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 1.50

C/db = 2.50

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 1.50

C/db = 2.500

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

C/db = 1.50

C/db = 2.50

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


C/db = 4.76C/db = 7.39

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


C/db = 3.35C/db = 5.27


136

Fig. 5.22: Effect of bonded length variation on bond stress of 19mm and 25mm diameter rebars respectively.

The experimental results of effect of cover variation on average bond stress of

deformed uncoated GFRP rebars indicated that average bond stress increased with the

increase in C/db ratio for all rebar diameters and for all the three bonded lengths. For

9.5mm diameter rebars, increase in bond stress was in the range of 2% to 4%, when C/db

ratio was increased from 4.76 to 7.39 (clear cover ratio of 1.55).

The bond stress was increased by 16% to 17%, when C/db ratio was increased

from 3.35 to 5.27 (clear cover ratio of 1.57) for all bonded lengths of 13mm diameters

rebars. Similarly due to increase in C/db ratio from 2.13 to 3.45 (clear cover ratio of 1.62)

for 19mm diameters rebars, the same percentage of 16% to 17% increase in bond stress

was observed for all the three bonded lengths. The results of 25mm diameter rebars

revealed the increase of 16% in the average bond stress, when C/db ratio was increased

from 1.50 to 2.50 (clear cover ratio of 1.67).

Comparing the results of tables 5.5 and 5.7 for the effect of C/db ratio, deformed

sand coated rebars again performed better than the deformed uncoated rebars with higher

values of average bond stress. The bond stress values of sand coated rebars were 1% to

18% higher than those of uncoated rebars. While comparing the results of increase in

average bond stress with the increase in C/db ratio of sand coated rebars, it was found that

with the increase in C/db ratio, the increase in bond stress of 9.5mm, 13mm, 19mm and

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


C/db = 2.13C/db = 3.45

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


C/db = 1.50C/db = 2.50


137

25mm diameter rebars was in the range of 1% to 16%, 9% to 11%, 15% to 17% and 17%

to 18%, respectively.

With the increase in C/db ratio, bond stress increased as the higher confining effect

on rebar through more concrete cover resisted the circumferential tensile stresses, which

resulted into higher frictional force required to pullout the GFRP rebar. Higher C/db ratio

also caused the reduction in stroke values due to more resistance against rebar slip.

5.6.3: Effect of surface texture variation on average bond stress

As discussed earlier in chapter-3, sixteen number of plain GFRP rebars were

developed initially for preliminary average bond stress determination with direct pullout

tests for finalizing the surface texture of GFRP rebars. Four diameter of plain GFRP rebars

of 9.5, 13, 19 and 22mm were subject to direct pullout tests with two bonded lengths of 5.0

db and 7.0 db and using 41.4 MPa compressive strength concrete. The experimental scheme

and results of this preliminary bond stress study has been given in Appendix-A. It is

pertinent to note that average bond stress of local plain GFRP rebars was quite low than the

average bond stress of reference GFRP rebars, therefore, deformed GFRP rebars were

developed subsequently and subjected to simple direct pullout tests using 27.0 MPa

concrete.

The effect of surface texture variation on average bond stress of locally developed

GFRP rebars using four diameter of deformed uncoated as well as sand coated GFRP rebars

of 9.5, 13, 19 and 25mm, three bonded lengths of 3.5 db, 5.0 db and 7.0 db and 41.4 MPa

concrete, has been presented in table 5.9. For each rebar diameter, there were three bonded

lengths and for each bonded length, two surface textures have been tested and compared.

The graphical representation of results has been given in figures 5.23 and 5.30.


138

Table 5.9: Comparison of Bond Stress of Uncoated and Sand Coated deformed Rebars using Ø150mm x 300mm Specimens and 41.4 MPa Compressive Strength Concrete.

Rebar ID


Clear Cover

C (mm)

C/db

Ratio

db/Lb

Ratio

Bond Stress of Uncoated

Rebars uu (MPa)

Bond Stress of Sand Coated Rebars

uc (MPa)


GFR9Lb3.5C70 9.5 70.25

7.39

1/3.5 18.27 19.88 8.10

GFR9Lb5.0C70 1/5.0 15.75 17.68 10.92

GFR9Lb7.0C70 1/7.0 12.61 14.51 13.09

GFR13Lb3.5C68 13 68.50

5.27

1/3.5 17.12 18.13 5.57

GFR13Lb5.0C68 1/5.0 14.26 15.87 10.14

GFR13Lb7.0C68 1/7.0 11.71 12.66 7.50

GFR19Lb3.5C65 19 65.50

3.45

1/3.5 15.71 16.57 5.19

GFR19Lb5.0C65 1/5.0 13.10 14.20 7.75

GFR19Lb7.0C65 1/7.0 10.58 11.10 4.68

GFR25Lb3.5C62 25 62.50

2.50

1/3.5 11.46 12.82 10.61

GFR25Lb5.0C62 1/5.0 9.51 10.33 7.94

GFR25Lb7.0C62 1/7.0 8.04 9.10 11.65

Fig. 5.23: Effect of surface texture variation on bond stress of 9.5mm diameter rebar for 3.5 db and 5.0 db bonded lengths, respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

ded

Len

gth

(MP

a)

Stroke (mm)

Deformed

Sand Coated


139

Fig. 5.24: Effect of surface texture variation on bond stress of 9.5mm and 13mm diameter rebar for 7.0 db and 3.5 db bonded lengths, respectively.

Fig. 5.25: Effect of surface texture variation on bond stress of 13mm diameter rebar for 5.0 db and 7.0 db bonded lengths, respectively.

Fig. 5.26: Effect of surface texture variation on bond stress of 19mm diameter rebar for 3.5 db and 5.0 db bonded lengths, respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated


140

Fig. 5.27: Effect of surface texture variation on bond stress of 19mm and 25mm diameter rebars for 7.0 db and 3.5 db bonded lengths respectively.

Fig. 5.28: Effect of surface texture variation on bond stress of 25mm diameter rebar for 5.0 db and 7.0 db bonded lengths respectively.

Fig. 5.29: Effect of surface texture variation on bond stress of 9.5mm and 13mm diameter rebars respectively.

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated

0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated0

5

10

15

20

0 5 10 15 20 25

Bon

d S

tres

s (M

Pa)

Stroke (mm)

Deformed

Sand Coated

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


UncoatedSand Coated

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


UncoatedSand Coated


141

Fig. 5.30: Effect of surface texture variation on bond stress of 19mm and 25mm diameter rebars respectively.

Pullout test results for study the effect of variation of surface texture on average

bond stress revealed that, with the increase in rebar surface roughness through sand

coating was a mean for the enhancement of bond stress. For 9.5mm diameter rebars, bond

stress increased within the range of 8% to 13% due to sand coating.

13mm diameter rebars showed an increase in the average bond stress in the range

of 6% to 10%, whereas 19mm diameter rebars experienced the increase of about 5% to

8% in the bond stress. For 25mm diameter rebars, increase in bond stress was in the range

of 8% to 12% due to sand coating for all the three bonded lengths. Stroke values of sand

coated rebars were lesser than the uncoated rebars due to more surface friction. In few

cases like 25mm diameter rebars, the glued sand particles were detached from the rebar

surface resulting into more stroke values.

5.6.4: Effect of concrete strength variation on average bond stress

As discussed in chapter-3, twenty four deformed uncoated GFRP rebars were also

subjected to simple direct pullout tests for preliminary bond stress determination using

concrete compressive strength of 27.0 MPa to finalize the surface texture of GFRP rebars.

Four rebar diameters of 9.5, 13, 19 and 25mm were used in the study. Bonded lengths of

3.0 db, 5.0 db and 7.0 db, were used with Ø150mm x 300mm and Ø100mm x 200mm

cylinder specimens.

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


UncoatedSand Coated

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


UncoatedSand Coated


142

The experimental schemes and results for this preliminary bond stress study have

been given in Appendix-B. It is pertinent to note that the average bond stress of GFRP

rebars was determined at maximum pullout force and no stroke values were recorded as the

purpose was only to know the average bond stress at 27.0 MPa concrete strength for

comparison with the reference GFRP rebars.

The effect of variation of concrete compressive strength on average bond stress has

been presented in table 5.10. For each rebar diameter, there were three bonded lengths and

for each bonded length, two (41.4 and 27.0 MPa) concrete strengths have been compared.

The graphical representation of results has been given in figures 5.31 and 5.32.

Table 5.10: Comparison of Average Bond Stresses of 41.4 and 27.0 MPa Strength

Concretes using Ø150mm x 300mm Specimens and deformed uncoated GFRP rebars.

Rebar ID


Clear Cover Ratio

C150/C100

db/Lb

Ratio

Bond Stress of 41.4 MPa

Concrete Specimens u41.4 (MPa)

Bond Stress of 27.0 MPa

Concrete Specimens u27.0 (MPa)


GFR9Lb3.5C70 9.5 1.55

1/3.5 18.27 16.40 10.24

GFR9Lb5.0C70 1/5.0 15.75 12.65 19.68

GFR9Lb7.0C70 1/7.0 12.61 10.60 15.94

GFR13Lb3.5C68 13 1.57

1/3.5 17.12 14.97 12.56

GFR13Lb5.0C68 1/5.0 14.26 11.23 21.25

GFR13Lb7.0C68 1/7.0 11.71 8.95 23.57

GFR19Lb3.5C65 19 1.62

1/3.5 15.71 12.95 17.57

GFR19Lb5.0C65 1/5.0 13.10 10.33 21.15

GFR19Lb7.0C65 1/7.0 10.58 8.03 24.10

GFR25Lb3.5C62 25 1.67

1/3.5 11.46 9.50 17.10

GFR25Lb5.0C62 1/5.0 9.51 7.55 20.61

GFR25Lb7.0C62 1/7.0 8.04 6.35 21.02


143

Fig. 5.31: Effect of concrete strength variation on bond stress of 9.5mm and 13mm diameter rebars respectively.

Fig. 5.32: Effect of concrete strength variation on bond stress of 19mm and 25mm diameter rebars respectively.

Pullout test results for study the effect of variation of concrete strength on average

bond stress revealed that, with the increase in concrete compressive strength, the average

bond stress increased for all bonded lengths. It was due to the fact that bond stress

depends on the shear strength of concrete and with the increase in concrete compressive

strength, shear strength of concrete also increases. For 9.5mm diameter rebars, bond

stress increased within the range of 10% to 20% due to increase of concrete compressive

strength from 27.0 to 41.4 MPa.

13mm diameter rebars showed an increase in the range of 13% to 24% in average

bond stress, whereas 19mm diameter rebars experienced the increase of about 18% to

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


41.4 MPa Concrete

27 MPa Concrete0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


41.4 MPa Concrete

27 MPa Concrete

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


41.4 MPa Concrete

27 MPa Concrete0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


41.4 MPa Concrete

27 MPa Concrete


144

24% in the average bond stress. For 25mm diameter rebars, increase in bond stress was in

the range of 17% to 21% due to increase in concrete compressive strength.

Using the equation developed by Okelo et al. (2005) relating the average bond

stress of GFRP rebars with concrete compressive strength and rebar diameter,

. (in MPa) or ACI design code equation,

. (in MPa), difference

of average bond stresses for above four diameter rebars for 41.4 and 27.0 MPa concretes

was computed and found 19%. It is interesting to note that computed difference of

average bond stresses for the two concrete strengths was fairly close to the experimentally

determined difference of average bond stresses.

5.7 BEAM BOND TESTS

After conducting direct pullout tests, the next phase experimental work was

comprised of determination of average bond stress in flexure through beam tests.

Six beams, forming two sets, were casted and tested to determine the effect of

bonded length as well as rebar diameter variation on average bond stress response using

local GFRP deformed rebars. Casting of all beams was done with the same normal

strength concrete used for direct pullout tests with 28 days compressive cylinder strength

of 41.4 MPa. Pouring of concrete in beam moulds was done with care to avoid any

damage to the fixed strain gauge on main GFRP rebar surface. After pouring and

compaction of concrete, beam specimens were covered with polythene sheet to stop

moisture evaporation. De-moulding of beam specimens was done three days after

concrete pouring. Wrapping of exposed concrete surfaces was done with moist jute bags

followed by polythene sheets for curing purposes. Beam specimens were dried after

curing and painted with white paint for marking and then testing purposes.

5.7.1 Test Specimens and Testing of Beams

All beam specimens were 150mm x 225mm in section and 1165mm in length. The

typical elevation, plan and x-sections of beam specimen have been shown in figure 5.33.


145

The length of uncoated deformed GFRP rebars was kept same as that of beam. The

concrete clear cover to the GFRP rebars at bottom and top was kept 25mm.

Fig. 5.33: Details of Beam Specimens

The placing arrangement of GFRP deformed uncoated rebars has been shown in

figure 5.33 (b to d) for the first set comprising of three beams, B1GFR19-Lb3.5, B2GFR19-

Lb5.0 and B3GFR19-Lb7.0. At the bottom of these beams, one central main GFRP rebar of


146

19mm diameter with two adjacent supporting GFRP rebars of 13mm diameter lying on

either side of the central rebar along with two top hanger rebars of 9.5mm diameter were

used. PVC pipe was used to break the bond between rebar and the concrete. In second set

of three beams, B4GFR13-Lb3.5, B5GFR13-Lb5.0 and B6GFR13-Lb7.0, 13mm diameter rebar

instead of 19mm and 9.5mm diameter rebar instead of 13mm rebar were used keeping

other parameters same. The testing scheme and results for studying the effect of bonded

length variation, 3.5, 5.0 and 7.0 times the main rebar diameter, on bond stress response

has been shown in table 5.11. The equation used to compute the average bond stress in

case of beam tests was;

, where = measured strain in main GFRP rebar,

E = tensile modulus of elasticity of GFRP rebar, Ab = x-sectional area of rebar = db2,

db = nominal diameter of rebar and Lb = bonded length of rebar in contact with concrete.

Preparation of test specimens was done as shown in figure 5.34. Testing setup was

comprised of universal testing machine with two points loading arrangement through load

cell and data acquisition system. The strain in main rebar as well as slip of this rebar was

recorded with the help of strain gauge, data acquisition system and data logger along with

Linear Variable Differential Transformers (LVDTs). The LVDT-1 was used to measure

slip of main GFRP rebar and LVDT-2 for slip of concrete as shown in figure 5.35.

Fig. 5.34: Fixing of Strain Gauge Fig. 5.35: Testing Arrangement with LVDTs

5.8 RESULTS AND DISCUSSION ON BEAM BOND TESTS

The behavior of beams, subjected to two points loading, was observed carefully.

In all beams, at low magnitude of load, adhesion and friction played their role and


147

resisted the applied load resulting into zero relative movement between GRFP rebar and

the surrounding concrete with no cracking anywhere. With the increase in load level, role

of friction and adhesion vanished and slip of the rebar took place along with formation of

flexural cracks, which propagated upward with initiation of splitting failure. These cracks

were prominent in the shear zone on the strain gauge side of beam. Upon further increase

in load magnitude, crack width increased by failing the mechanical interaction between

GFRP rebar and the concrete.

It was observed that cracking of beams initiated near the location of PVC conduits

used for de-bonding of main rebars which demonstrated the start of bond failure before

flexural failure, as shown in figures 5.36 to 5.41. Width and depth of cracks were kept on

increasing with increase in load magnitude resulting into total bond failure. It is evident

from bond stress-slip graphs that after getting the peak values, bond stress showed sudden

drop as shown in figures 5.42 to 5.47 except the beam B-4. In beam B-4, 9.5mm

supporting GFRP rebar was broken as shown by the gradual decrease in the bond stress

after having its maximum value, as shown in figure 5.43. It is pertinent to note that slip of

beam B-4 was continued till the rupture of GFRP rebar. All other beams failed in bond

with development of small horizontal cracks associated with main diagonal cracks.

Bond failure resulting from splitting of concrete was observed in all beams. As the

rebars were loaded they exerted radial pressure on the surrounding concrete, which had

not adequate capacity to resist this pressure thus splitting cracks initiated at the interface

and propagated towards outer surface.

The relationships between bond stress and slip were plotted. The experimental

scheme and results of beam bond tests have been shown in table 5.11 as well as in figures

5.42 to 5.48, graphically.


148

Table 5.11: Experimental Scheme and Results of Beam Bond Tests for the Effect of

Bonded Length Variation on Average Bond Stress of uncoated Deformed GFRP rebars.

Beam ID

Main Rebar Diameter db (mm)

Supporting and Hanger Rebar

Diameter dbs (mm)

Lb/db Ratio

Avg. Bond Stress

u (MPa)

Slip s (mm)

B1GFR19-Lb3.5

19

13, 9.5

3.5 7.80 2.40

B2GFR19-Lb5.0 5.0 6.45 2.33

B3GFR19-Lb7.0 7.0 4.83 2.27

B4GFR13-Lb3.5

13

9.5

3.5 8.82 4.30

B5GFR13-Lb5.0 5.0 7.52 3.50

B6GFR13-Lb7.0 7.0 5.79 2.85

Note: BnGFRxx-Lbyy stands for Beam No. ‘n’ with uncoated deformed GFRP main rebar of Diameter

xx and Bonded Length (Lb) of yy times the rebar diameter (db), respectively.

Fig. 5.36: Diagonal Cracks in B1GFR19-Lb3.5 Fig. 5.37: Bond Failure in Beam B2GFR19-Lb5.0


149

Fig. 5.38: Bond Failure in Beam B3GFR19-Lb7.0 Fig. 5.39: Crack Pattern in Beam B4GFR13-Lb3.5

Fig. 5.40: Bond Failure in Beam B5GFR13-Lb5.0 Fig. 5.41: Bond Failure in Beam B6GFR13-Lb7.0

Fig. 5.42: Bond Stress and Slip Graphs for Beam B1GFR19-Lb3.5 and B2GFR19-Lb5.0

respectively.

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

5.0 db


150

Fig. 5.43: Bond Stress and Slip Graphs for Beam B3GFR19-Lb7.0 and B4GFR13-Lb3.5

respectively.

Fig. 5.44: Bond Stress and Slip Graphs for Beam B5GFR13-Lb5.0 and B6GFR13-Lb7.0 respectively.

Fig. 5.45: Effect of bonded length variation on bond stress for 19 mm and 13 mm GFRP

rebars respectively.

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

7.0 db0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

5.0 db

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

7.0 db

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db5.0 db7.0 db

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db5.0 db7.0 db


151

Fig. 5.46: Effect of diameter variation on bond stress for 3.5 db and 5.0 db bonded length beams respectively.

Fig. 5.47: Effect of diameter variation on bond stress for 7.0 db length beams.

Fig. 5.48: Effect of bonded length variation on bond stress of beams.

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

13 mm dia Rebar

19mm dia. Rebar0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

13 mm dia Rebar19 mm dia rebar

0

2

4

6

8

10

0.0 1.0 2.0 3.0 4.0 5.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

13 mm dia Rebar19mm dia. Rebar

0

2

4

6

8

10

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


19 mm Rebar13 mm Rebar


152

Comments on results:

The results of beam bond tests revealed that overall bond stress values were lower

than the corresponding bond stress values of direct pullout tests. The bond stresses

obtained from beam bond tests were lower in the range of 47% to 54% than the bond

stress obtained from direct pullout tests, as detailed in chapter-6. The concrete around the

GFRP rebars in beams was under tension leading to cracking under comparatively low

stresses, which resulted into smaller bond stresses. For 19mm diameter rebars, slip values

were lesser as compared to 13mm diameter rebars in beam tests due to more resistance

against slip as the larger surface area of 19mm rebar was in contact with concrete as

compared with 13mm rebars.

As shown in the bond stress-slip graphs, bond stress response in beams was

generally stiff and linear at initial stage especially for shorter bonded lengths. In most of

the cases, after having peak bond stress value there was a sudden drop indicating the bond

failure. In general, bond stress response may be categorized into two parts, first as linear

elastic and the second one as non-linear.

The effect of variation in bonded length on average bond stress as well as slip was

studied in beam bond tests. With the increase in bonded length, bond stress and slip both

decreased due to the same reasons as in direct pullout tests. For 13mm diameter main

rebars, 15% decrease in bond stress and 19% decrease in slip was observed when bonded

length increased from 3.5 db to 5.0 db. Whereas this decrease was 23% in bond stress and

19% in the slip when bonded length was increased from 5.0 db to 7.0 db. For 19mm

diameter rebars, reduction in bond stress was about 17% when bonded length was

increased from 3.5 db to 5.0 db. Similarly when the bonded length was increased from 5.0

db to 7.0 db, the reduction in bond stress was about 25% for 19mm diameter rebar. The

slip decreased in 19mm rebar by 3%. Comparing the bond stress results obtained from

beam tests with direct pullout test results, the average bond stress response was found

quite similar.

The effect of increase in rebar diameter for same bonded lengths was also

analyzed and found that with the increase in rebar diameter, bond stress decreased

similarly as in case of pullout tests. Slip was also decreased. When rebar diameter was


153

increased from 13mm to 19mm, decrease in average bond stress was in the range of 12%

to 17% whereas slip decreased in the range of 20% to 44%.

The decrease in bond stress with increase in rebar diameter can be understood

with the help of water bleeding in concrete. Larger the rebar diameter, higher the quantity

of bleeding water trapped below the rebar resulting into more void, which reduces the

contact area between rebar & concrete and hence reducing the bond stress.

5.9 EVALUATION OF REDUCTION IN BOND STRESS OF JUNCTIONS

This experimental work was comprised of determining the reduction in average

bond stress of primary beams of junction. A junction was an assembly of two, primary

and secondary beams intersecting at right angle. The purpose of this testing was to

determine the effect of joint action on average bond stress of primary beams using the

locally developed GFRP deformed rebars. The intersecting beam having bottom rebars

below the other beam rebars was called as primary and other as secondary beam. The

length and dimensions of intersecting beams as well as size and placement of GFRP

rebars of each intersecting beam was kept similar to that of individual beams, called the

reference beams, for comparison purposes.

Six intersecting beams/junctions, forming two sets, were casted and tested to

study the effect of bonded length as well as rebar diameter variation on average bond

stress response of primary beams. Casting of all the junctions was done with same 41.4

MPa concrete, which was used for reference beams in phase-2 experimental work. Same

precautions were observed for pouring, de-moulding and curing processes as were

observed in the reference beams. After curing of junctions, specimens were dried and

painted with white paint for marking and then testing purposes.

5.9.1 Test Specimens and Testing of Junctions

Each intersecting beam of the junction was 150mm x 225mm in section and

1165mm in length, same as of reference beams. Concrete cover was also kept same. The


154

typical elevation, plan and x-sections of each intersecting beam have been shown in

figure 5.33 of earlier section as well as in figure 5.49.

Fig. 5.49: Details of Intersecting Beams/Junction Specimens

Primary and secondary beams of first set consisting of three junctions, J1GFR19-

Lb3.5, J2GFR19-Lb5.0 and J3GFR19-Lb7.0 had one central main GFRP deformed uncoated


155

rebar of 19mm diameter with two adjacent supporting GFRP rebars of 13mm diameter

lying on either side of the central rebar along with two top hanger rebars of 9.5mm

diameter as shown in figure 5.49. PVC pipe was used to break the bond between rebar

surface and concrete, where it was required. In next set of three junctions, J4GFR13-Lb3.5,

J5GFR13-Lb5.0 and J6GFR13-Lb7.0, 13mm diameter rebar instead of 19mm and 9.5mm

diameter rebar instead of 13mm rebar were used in primary and secondary beams keeping

the other parameters same.

Preparation and marking of test specimens was done as shown in figures 5.50 and

5.51. Testing setup for intersecting beams was comprised of same universal testing

machine with load cell and data acquisition system used in the phase-2 experimental work

and as shown in figure 5.52. The strain in main GFRP rebar as well as its slip was

recorded with the help of strain gauge, data acquisition system, LVDT and data logger.

For primary beam, LVDT-1 and 2 were used to measure slip of main rebar and of

concrete respectively whereas LVDT-3 and 4 were used to measure the slip of main rebar

and concrete respectively for secondary beams.

Fig. 5.50: Junction Testing Specimen Fig. 5.51: Concrete Casting Arrangement

5.10 RESULTS AND DISCUSSION ON TESTING OF JUNCTIONS

The average bond stress response of intersecting beams subjected to two points

loading was observed and recorded carefully. Similar to the reference beams tested in

phase-2 experimental work, at smaller load level, adhesion and friction played their role

and resisted the applied load and there was no relative movement between GRFP rebar


156

and surrounding concrete of each intersecting beam. With the gradual increase in load

magnitude, role of friction & adhesion vanished and slip of rebar took place, which

caused the formation of flexural cracks, similar to the reference beams. In general,

chemical adhesion of secondary beams failed earlier, at lower load level, than the primary

beams as well as the bearing resistance of secondary beams exhausted earlier. The data of

bond stress determination and slip of intersecting beams was recorded and processed.

It was observed that two points loading on intersecting beam assembly increased

the tensile stress of primary beam due to flexural action of secondary beam causing the

reduction in bond stress of primary beam as compared to reference beam. The slip values

of intersecting beams were also lower than the corresponding reference beams due to

more stiffness of the junction.

It was also observed that cracking of both intersecting beams was initiated near

the location of de-bonded portion of main rebars adjacent to the joint of two beams.

Location and pattern of cracks was nearly similar in primary and secondary beams. Width

and depth of cracks were kept on increasing with the increase in applied load resulting

into complete bond failure. Both intersecting beams of all the junctions were failed after

development of cracks, as shown in figures 5.53 to 5.57.

In the first set, comprising of junctions J1, J2 and J3, 13mm diameter supporting

rebars of primary beams lying adjacent to 19mm diameter main rebar were broken, as

shown in figure 5.54. As a result of rebar fracture, bond stress of primary beam started

decreasing gradually till the complete breakage of rebar after having maximum values,

whereas the bond stress of secondary beams was in gradual ascending order till failure, as

shown in figures 5.58 and 5.59.

In the second set, comprising of junctions J4, J5 and J6, primary and secondary

beams both failed in bond as shown in figures 5.59 and 5.60. In all junctions, splitting

failure was observed. The experimental scheme and results of junction tests, average bond

stress and slip, have been shown in table 5.12.


157

Table 5.12: Experimental Scheme and Results of Junction Tests for study the Effect of

Bonded Length Variation on Average Bond Stress of Primary Beams using uncoated

Deformed GFRP rebars.

Junction ID

Main Rebar

Diameter

db (mm)

Suppor-ting

Rebar Diameter

dbs (mm)

Lb/db Ratio

Bond Stress of Primary Beam

u (MPa)

Slip of Primary Beam

s (mm)

Bond Stress of

Secondary Beam

u (MPa)

Slip of Secondary

Beam

s (mm)

J1GFR19-Lb3.5

19

13, 9.5

3.5 6.79 1.84 6.23 1.79

J2GFR19-Lb5.0 5.0 5.46 1.76 5.04 1.68

J3GFR19-Lb7.0 7.0 4.49 1.71 3.84 1.57

J4GFR13-Lb3.5

13

9.5

3.5 8.15 2.55 7.48 2.26

J5GFR13-Lb5.0 5.0 6.08 2.33 5.89 2.13

J6GFR13-Lb7.0 7.0 5.02 2.20 4.88 2.05

Note: JnGFRxx-Lbyy stands for Junction No. ‘n’ with uncoated deformed GFRP main rebar of

Diameter xx and Bonded Length (Lb) of yy times the rebar diameter (db), respectively.

The graphically presentation of above results has been made in figures 5.58 to 5.64.

Fig. 5.52: Testing Arrangement and Setup Fig. 5.53: Rebar Failure in J1GFR19-Lb3.5


158

Fig. 5.54: Rebar Failure in PB of J2GFR19-Lb5.0 Fig. 5.55: Cracks Pattern in J3GFR19-Lb7.0

Fig. 5.56: Cracks Pattern in J5GFR13-Lb5.0 Fig. 5.57: Failure Pattern in J6GFR13-Lb7.0

Fig. 5.58: Bond Stress and Slip of J1GFR19-Lb3.5 and J2GFR19-Lb5.0 respectively.

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

Primary BeamSecondary Beam

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)



159



Fig. 5.61: Effect of variation of Bonded Length on Bond Stress of Primary Beams of Junctions for 19 mm and 13 mm diameter rebars respectively.

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db5.0 db7.0 db

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

3.5 db5.0 db7.0 db


160

Fig. 5.62: Effect of variation of Rebar Diameter on Bond Stress of Primary Beams for 3.5 db and 5.0 db Bonded Length respectively.

Fig. 5.63: Effect of variation of Rebar Diameter on Bond Stress of Primary Beams for 7.0 db Bonded Length.

Fig. 5.64: Effect of bonded length variation on bond stress of primary and secondary beams of junctions.

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)

13 mm dia. Rebar19 mm dia Rebar

0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Bon

d S

tres

s (M

Pa)

Slip (mm)


0

2

4

6

8

10

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Primary Beam


0

2

4

6

8

10

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Secondary Beam



161

Comments on results:

The average bond stress experimental results of junctions revealed that the

primary beams performed better than the secondary beams. The bond stress of secondary

beams was lower than the bond stress of primary beams in the range of about 3% to 14%.

The effect of increase in bonded length of primary as well as in secondary beams

was observed and found the similar trend as of individual/reference beams. With the

increase in bonded length, bond stress and slip, both decreased for all rebar diameters.

The reduction in average bond stress and slip of primary beam for 19mm diameter rebar

was in the range of 18% to 20% and 3% to 4% respectively for three bonded lengths,

whereas this range of reduction was 17% to 25% in bond stress and 6% to 9% in slip for

13mm diameter rebars. Secondary beams had reduction of bond stress and slip in the

order of 19% to 24% and 6% to 7% respectively for 19mm diameter rebars whereas this

reduction order for 13mm diameter rebars was 17% to 21% and 4% to 6% respectively.

The effect of variation in rebar diameter on bond stress and slip was also analyzed

for primary beams and found that with the increase in rebar diameter, bond stress and slip

decreased similarly as in case of reference beams. When the rebar diameter was increased

from 13mm to 19mm, the decrease in bond stress was 17% for 3.5 db, 10% for 5.0 db and

11% for 7.0 db bonded length. The slip decreased in the order of 28%, 24% and 22% for

3.5 db, 5.0 db and 7.0 db bonded length respectively. The slip values were more for smaller

(13mm) rebars as compared to larger (19mm) diameter rebars due to their lesser

resistance against the slip. Thus the joint action reduced the average bond stress as well as

the slip of primary beams of junctions as compared to the reference beams.

5.11 SUMMARY

Average bond stress response of locally developed GFRP deformed rebars was

studied initially through direct pullout tests which gave a fair assessment of bond stress

with two concrete strengths of 41.4 and 27.0 MPa. Four diameter rebars of 9.5, 13, 19 and

25mm, three bonded lengths of 3.5 db, 5.0 db and 7.0 db and two pullout specimen of

Ø150mm x 300mm and Ø100mm x 200mm were used. Five aspects were studied in


162

pullout testing comprising of effect of variation in bonded length, rebar diameter,

cover/confinement, surface texture and concrete compressive strength on the average

bond stress response. It was found that an increase in the bonded length as well as in rebar

diameter decreased the average bond stress as well as stroke value. Large concrete

cover/confinement increased the resistance against pullout of rebar hence the bond stress

as well. Sand coating has increased the surface roughness which improved the overall

bond stress response of the GFRP rebars. Better strength concrete, 41.4 MPa, resulted

into higher bond stress due to improvement in the shear strength of concrete, as compared

to 27.0 MPa concrete.

Bond stress response of local GFRP rebars in flexure was studied through six

beams subjected to two points loading using 41.4 MPa concrete, two main rebar

diameters of 13mm and 19mm with the above three bonded lengths. It was concluded that

average bond stresses obtained from beam tests were lower in the range of 47% to 54%

than the corresponding bond stresses obtained from direct pullout tests due to difference

in structural behavior of two test methods. The bond stress of local GFRP rebars in

flexure conformed to the established experimental trend observed by several researchers.

Finally the effect of joint action on the average bond stress response of two

intersecting beams/junctions was also studied through six junctions using the same

parameters as of reference/individual beams. It was found that joint action reduced the

bond stress by 7% to 19% of the primary beams of junctions as compared to the

reference/individual beams.

Thus the average bond stress response of locally developed GFRP rebars was well

in line with international research carried out so far as well as closely comparable with

the reference GFRP rebars, Aslan-100TM, developed by Hughes Brothers Inc. USA.

CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS

163

COMPARISON AND VERIFICATION OF BOND STRESS

EXPERIMENTAL RESULTS

6.1 GENERAL

Factors affecting the average bond stress using local GFRP deformed rebars with

normal strength concretes have been identified and studied experimentally in the

preceding chapter-5. The effect of bonded length, rebar diameter, concrete cover, surface

texture and concrete compressive strength variation on average bond stress has been

studied in detail through direct pullout tests. The effect of bonded length as well as rebar

diameter variation on average bond stress in flexure has also been studied using beam

tests. The effect of joint action on the average bond stress of primary beams of junctions

was analyzed by testing the junctions/assembly of intersecting beams at right angles.

This chapter describes the analysis of results obtained in proceeding chapter, their

comparisons and verification of experimental bond stress results. A model for predicting

the average bond stress has been developed basing on pullout experimental results, half of

which were used to calibrate the model and remaining half for validation. Further

validation of proposed pullout bond model using the published data of direct pullout

results by several researchers has also been included in this chapter.

6.2 COMPARISON OF PULLOUT AND BEAM BOND TEST RESULTS

The structural behavior in direct pullout tests and beam bond tests subjected to

two points loading was different. In the direct pullout test, concrete around the GFRP

rebar was under compression which suppressed the tendency of cracking due to more

confining pressure thus increased the average bond stress. In case of beams in pure

bending, concrete around the GFRP rebar was under tension which was favorable to

produce cracking under comparatively low stresses thus decreased the bond stress.

The experimental results of beam bond tests revealed that average bond stress was

lower in beams as compared to bond stress obtained in the direct pullout tests. Table 6.1


164

and figure 6.1 demonstrates the comparison of average bond stresses obtained from direct

pullout tests and beam bond tests.

Table 6.1: Comparison of average bond stresses obtained from direct pullout and beam

bond tests.

Rebar Diameter

db(mm)

Lb/db

Ratio

Average Bond Stress from Pullout

Tests

u (MPa)

(1)

Reduction in Pullout Bond Stress with

Bonded Length (%)

Average Bond

Stress from Beam Tests

u (MPa)

(2)

Reduction in Beam

Bond Stress with

Bonded Length (%)

Reduction of Average Bond

Stress in Beam Bond Tests (%)

(Diff. of 1&2)

13

3.5 17.12

16.70

8.82

14.74 48.48

5.0 14.26 7.52 47.26

7.0 11.71

17.88 5.79 23.00 50.55

19

3.5 15.71

16.61 7.80

17.30 50.35

5.0 13.10 6.45 50.76

7.0 10.58

19.23 4.83 25.11 54.35

Fig. 6.1: Comparison of average bond stresses obtained from direct pullout and beam

bond tests for 13mm and 19mm diameter uncoated deformed GFRP rebars, respectively.

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Pullout ResultBeam Bond Result

0

5

10

15

20

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Pullout ResultBeam Bond Result


165

It is evident from table 6.1 that reduction in average bond stress in direct pullout

and beam bond tests was 17% and 15% to 17% respectively, when bonded length was

increased from 3.5 db to 5.0 db. The reduction in bond stress with increase in bonded

length from 5.0 db to 7.0 db in direct pullout tests was in the range of 18% to 19%,

whereas in beam tests, this reduction range was 23% to 25%. In case of beams, reduction

in average bond stress with the increase in bonded length was generally more than the

reduction in direct pullout tests.

The bond stresses obtained from beam bond tests were compared with the direct

pullout results and found that bond stresses obtained from beams were lower in the range

of 47% to 51% up to 5.0 db bonded length. When the bonded length was increased from

5.0 db to 7.0 db, average bond stress was decreased in the range of 51% to 54%.

Experimentally obtained bond stress from direct pullout tests for 5.0 db bonded

lengths of locally developed uncoated deformed GFRP rebars was also compared with the

corresponding reported bond stress of reference rebars (Aslan-100TM) and found in close

agreement. The reported bond stress of reference GFRP rebars has been determined by

the manufacturer using direct pullout test method and for 5.0 db bonded lengths. The

comparison of average experimental bond stress from direct pullout tests and reported

bond stress of the reference rebars has been presented below:

Average Experimental Bond Stress of Local

Deformed Rebars

(MPa)

Reported Bond Stress of Reference Rebars

(MPa)

Difference

(%)

13.15 11.60 11.78

The experimental bond stress was taken as the average of four diameters rebars for

5.0 db bonded lengths whereas bond stress of reference rebars has been taken as the

average of eight diameters rebars.


166

6.3 COMPARISON OF BEAM AND JUNCTION TEST RESULTS

Bond stress values of primary beams of junctions were also compared with the

corresponding reference/individual beams bond stresses to determine the effect of joint

action on the average bond stress of these primary beams. It has been noticed that average

bond stress values of all primary beams were lesser in the range of 7% to 19% as

compared to reference beams as shown in table 6.2 as well as in figures 6.2 and 6.3.

Table 6.2: Comparison of Average Bond Stresses of Reference Beams (RF) and Primary

Beams (PB) of Junctions.

Beam/ Junction

ID

Ji/Bi

Lb/db Ratio

Average Bond Stress

u (MPa)

Reduction of Bond Stress in

Primary Beams

(%)

(Diff. of 1&2)

Reference Beam (RF)

(1)

Reduction in Bond Stress of RF with

Bonded Length (%)

Primary Beam (PB)

(2)

Reduction in Bond Stress of PB with

Bonded Length (%)

1-GFR19-Lb3.5

3.5 7.80

17.30 to 25.11

6.79

17.76 to 19.58

12.95

2-GFR19-Lb5.0

5.0 6.45 5.46 15.34

3-GFR19-Lb7.0

7.0 4.83 4.49 7.04

4-GFR13-Lb3.5

3.5 8.82

14.74 to 23.00

8.15

17.43 to 25.40

7.60

5-GFR13-Lb5.0

5.0 7.52 6.08 19.14

6-GFR13-Lb7.0

7.0 5.79 5.02 13.30

Note: inGFRxx-Lbyy stands for Reference/Primary Beam No. ‘n’ with uncoated deformed GFRP main

rebar of Diameter xx and Bonded Length (Lb) of yy times the rebar diameter (db), respectively.


167

Fig. 6.2: Comparison of average bond stresses of reference beams and primary beams of

Junctions for 13mm and 19mm diameter deformed GFRP main rebars respectively.

Fig. 6.3: Comparison of average bond stresses of reference beams and primary beams of

Junctions.

The reduction in average bond stress of primary beams as compared to reference

beams was due to the flexural action of secondary beams on the primary beams. This

flexural action magnified the tensile stress of primary beam resulting into reduction in its

bond stress.

0

2

4

6

8

10

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Primary Beam

Reference Beam

0

2

4

6

8

10

0 2 4 6 8

Bon

d S

tres

s (M

Pa)


Primary Beam

Reference Beam

0

2

4

6

8

10

1 2 3 4 5 6

Bon

d S

tres

s (M

Pa)

Primary/ Reference Beam No.

Primary Beam

Reference Beam


168

It is also evident from table 6.2 as well as figures 6.2 and 6.3 that with the increase

in bonded length of primary beam from 3.5 db to 5.0 db and then to 7.0 db, average bond

stress decreased in the range of 18% to 20% for 13mm diameter rebars whereas for 19mm

diameter rebars, this decrease was in the range of 17% to 25%. Similarly in case of

reference beams, range of bond stress reduction was 17% to 25% for 13mm diameter

rebars and 15% to 23% for 19mm diameter rebars.

6.4 COMPARISON OF EXPERIMENTAL AVERAGE BOND STRESS RESULTS

WITH OTHER RESEARCHERS

Experimental results of average bond stress obtained from direct pullout tests as

well as from beam bond tests were compared with the published data of average bond

stresses by several researchers around the globe. The comparisons of trends as well as

numerical values of bond stresses, obtained from direct pullout and beam bond tests, have

been made and presented in tables 6.3 and 6.4 respectively. The graphical presentation of

theses comparisons has been made in figures 6.4 to 6.7 for direct pullout and figures 6.8

to 6.11 for beam bond tests.

It is revealed from these comparisons that experimental results of average bond

stresses were in close agreement to the published results of several international

researchers. Pattern and trends were exactly similar and variation in numerical values of

bond stresses was within the range of 4% to 19% for direct pullout and 5% to 29% for

beam bond tests. The possible reason for this variation was the difference in the testing

parameters and conditions.


169

Table 6.3: Comparison of direct pullout experimental results with published data of average bond stresses by several researchers.

Rebar ID

Experimental Bond Stress

u (MPa)

O. Chaallal & B. Benmokrane

(1995) u (MPa)

M.R Ehsani et al. (1996)

u (MPa)

Roman Okelo et al. (2005)

u (MPa)

Qingduo Hao et al. (2008)

u (MPa)

Yanlei Wang et al. (2008)

u (MPa)


u (MPa)

Marta Baena et al. (2009)

u (MPa)

% Diff.

Standard deviation

GFRD9- Lb 5.0

15.75

fc’= 41.4MPa fu = 747 MPa db= 9.50mm C/db= 7.39 Lb = 5.0 db

-

-

18.13 (10.09%)

fc’= 39.4 MPa fu = 772 MPa db= 10mm C/db = 9.65 Lb = 5.0 db

13.90 (11.74%)

fc’= 30.0 MPa fu = 1027MPa db= 10mm C/db = 4.50 Lb = 5 db

12.96 (17.71%)

fc’= 28.7 MPa fu = 710 MPa db= 10mm C/db = 3.67 Lb = 4 db

13.17 (4.25%)


17.45 (9.74%)

fc’=53.11MPa fu = 760 MPa db= 8mm C/db = 12.0 Lb = 5 db

4.25 to 17.71

2.23

GFRD9- Lb 7.0

12.61

fc’= 41.4MPa fu = 747 MPa db= 9.50mm C/db= 7.39 Lb = 7.0 db

-

-

15.33 (17.74%)


-

-

-

-

17.74

1.92

GFRD13

-Lb 5.0

14.26

fc’= 41.4MPa fu = 674 MPa db= 13mm C/db= 5.27 Lb = 5.0 db

15.00 (4.93 %)

fc’= 31.0 MPa fu = 689 MPa db = 12.7mm C/db = 9.56 Lb = 4.92 db

-

-

12.19 (14.51%)

fc’= 40.0 MPa fu = 761 MPa db= 14mm C/db= 3.0 Lb = 5 db

11.61 (18.58%)

fc’= 28.7 MPa fu = 710 MPa db= 12mm C/db= 3.67 Lb = 4 db

11.61 (18.58%)


16.77 (14.96%)

fc’=53.11MPa fu = 690 MPa db= 12mm C/db = 7.83 Lb = 5 db

11.50

to 18.58

2.11

GFRD13

-Lb 7.0

11.71 fc’= 41.4MPa fu = 674 MPa db= 13mm C/db= 5.27 Lb = 7.0 db

11.10 (5.21 %)

fc’= 31.0 MPa fu = 689 MPa db= 12.7mm Lb = 9.84 db

-

-

-

-

-

-

5.21

0.43


170

Table 6.3 (Cont’d): Comparison of direct pullout experimental results with published data of average bond stresses by several researchers.

Rebar ID


u (MPa)


(1995) u (MPa)


u (MPa)


u (MPa)


u (MPa)

Yanlei Wang et al. (2008)

u (MPa)


u (MPa)


u (MPa)

% Diff.

Standard deviation

GFRD19

-Lb 5.0

13.10


14.70

(10.88 %)

fc’= 31.0 MPa fu = 652 MPa db= 19.1mm C/db = 6.17 Lb = 4.71 db

-

13.80

(5.07 %)


-

-

-

15.08

(13.13 %)

fc’=53.54MPa fu = 620 MPa db= 19mm C/db= 4.76 Lb = 5.0 db

5.07 to 13.13

0.80

GFRD19

-Lb 7.0

10.58


11.90

(11.09 %)

fc’= 31.0 MPa fu = 652 MPa db= 19.1mm C/db= 6.17 Lb = 9.42db

9.20

(13.04 %)


12.07

(12.34 %)


-

-

-

-

11.09

to 12.34

1.34


171

The graphical presentation of comparisons of direct pullout experimental results

with published results of several researchers has been given in figures 6.4 to 6.7.

Fig. 6.4: Comparison of direct pullout experimental results with published results of

several researchers for 9.5mm and 13mm diameter rebars, respectively.


several researchers for 19mm diameter rebars.

0

5

10

15

20

Researchers

Bon

d S

tres

s (M

Pa)

9.5 mm dia. rebars with Lb = 5.0 db

Experimental

Roman Okelo (2005)



Yanlei et al. (2008)


0

5

10

15

20

ResearchersB

ond

Str

ess

(MP

a)

13 mm dia. rebars with Lb = 5.0 db

Experimental

O. Chaallal (1995)



Yanlei et al. (2008)


0

5

10

15

20

Researchers

Bon

d S

tres

s (M

Pa)

Lb = 5.0 db

Experimental

O. Chaallal (1995)

Roman Okelo (2005)


0

5

10

15

20

Researchers

Bon

d S

tres

s (M

Pa)

Lb = 7.0 db

Experimental

O. Chaallal (1995)


Tighiourat et al. (1998)

Roman Okelo (2005)


172


several researchers.



0

5

10

15

20

9.5 mm 13 mm 19 mm 9.5 mm 13 mm 19 mm 9.5 mm 13 mm 19 mm

Bon

d S

tres

s (M

Pa)

Experimental O. Chaallal (1995) Roman Okelo (2005)

Effect of Variation in Bonded Lengths on Average Bond Stress for Various Diameter Rebars

5.0 db7.0 db

0

5

10

15

20

5.0 db 7.0 db 5.0 db 10 db 5.0 db 7.0 db

Bon

d S

tres

s (M

Pa)

Experimental O. Chaallal (1995) Roman Okelo (2005)

Effect of Variation in Rebar Diameter on Average Bond Stress for Various Bonded Lengths

9.5 mm13 mm19 mm


173

Table 6.4: Comparison of beam bond experimental results with published data of average bond stresses by several researchers.

Beam ID


u (MPa)


(1995) u (MPa)

B. Tighiouart et al. (1998)

u (MPa)

Al Zahrani et al. (1999)

u (MPa)


u (MPa)

Bilal & Harajli (2005)

u (MPa)

% Diff.

Standard deviation

B2GFR19-Lb 5.0

6.45

fc’= 41.4MPa fu = 606 MPa db= 19mm Lb = 5.0 db

-

6.80

(5.14 %)

fc’=31.0 MPa fu = 640 MPa db = 19.1mm Lb = 6.0 db

-

-

7.70

(16.23 %)

fc’= 32.4 MPa fu = 620 MPa db=20.1mm Lb = 3.81 db

5.14 to 16.23

0.64

B3GFRD19-Lb 7.0

4.83


5.70 (15.26 %)


3.80 (21.32%)

fc’=31.0 MPa fu = 640 MPa db= 19.1mm Lb = 10.0 db

6.60 (26.82 %)


6.80 (28.97 %)


6.10 (20.82 %)

fc’= 34.9 MPa fu = 620 MPa db=20.1mm Lb = 5.71 db

15.26 to 28.97

1.14

B5GFRD13- Lb 5.0

7.52


-

8.80 (14.54 %)


-

-

-

14.54

0.90

B6GFRD13- Lb 7.0

5.79


7.50 (22.80 %)


7.30 (20.68 %)


-

-

-

20.68 to 22.80

0.93


174

The graphical presentation of comparison of beam bond experimental results with

published results of several researchers has been shown in figures 6.8 to 6.11.

Fig. 6.8: Comparison of beam bond experimental results with published results of several

researchers.

Fig. 6.9: Comparison of beam bond experimental results with published results of


0

2

4

6

8

10

Researchers

Bon

d S

tres

s (M

Pa)

Beam B2GFR19-Lb 5.0

Experimental

Tighiourat et al. (1998)Bilal & Hirajli (2005)

0

2

4

6

8

10

Researchers

Bon

d S

tres

s (M

Pa)

Beam B6GFR13-Lb 7.0

Experimental

O. Chaallal (1995)


0

2

4

6

8

10

Researchers

Bon

d S

tres

s (M

Pa)

Beam B3GFR19-Lb 7.0

Experimental

O. Chaallal (1995)

Al Zahrani et al. (1999)

Roman Okelo (2007)


Bilal & Hirajli (2005)


175





0

2

4

6

8

10

19 mm 13 mm 19 mm 13 mm 19 mm 13 mm

Bon

d S

tres

s (M

Pa)

Experimental O. Chaallal (1995) Tighourat et al. (1998)

Effect of Variation in Bonded Lengths on Average Bond Stress for Various Diameter Rebars

5.0 db7.0 db

0

2

4

6

8

10

5.0 db 7.0 db 5.0 db 7.0 db 5.0 db 7.0 db

Bon

d S

tres

s (M

Pa)

Experimental O. Chaallal (1995) Tighourat et al. (1998)

Effect of Variation in Rebar Diameters on Average Bond Stress for Various Bonded Lengths

13 mm19 mm


176

6.5 ACI BEAM BOND EQUATION

Based on Wambeke and Shield’s (2006) database of extensive experimentation on

bond performance evaluation of GFRP rebars in beams, a linear regression of normalized

average bond stress versus the normalized cover and bonded length resulted in the

following relationship after rounding the coefficients, which has also been published in

ACI committee report ACI 440.1R-06. This bond equation was suggested for beams with

concretes having compressive strength ranging from 28-45 MPa and C/db=3.50. The

tested beams by Wambeke and Shield were comprised of both types of failure; splitting as

well as the pullout failure.

0.083 ′4.0 0.30 100

Where u = Average bond stress, (in MPa).

′ = 28 days concrete compressive cylinder strength, (in MPa).

= Lesser of the cover to the center of rebar or one-half of the center-on-center spacing

of rebars, (in mm).

= Diameter of rebar, (in mm).

= Bonded length of rebar in concrete, (in mm).

The above equation demonstrates that the term is quite less sensitive as

compared to , similar to the direct pullout experimental results. Wambeke and Shield

created a consolidated data of 269 beam bond tests. The database was limited to beam-

end (also known as eccentric pullout) tests, notch-beam tests and splice beam tests. The

majority of the rebars represented in the database were composed of GFRP having both

spiral wrap and helical lug pattern rebars with and without confinement reinforcement.

As discussed earlier, the nature of structural behavior in beam subjected to two

points loading and direct pullout tests is little different. In beams, there is tension in

concrete at bottom whereas in direct pullout tests, there is compression. In this

experimental study, the bonded length portion of GFRP rebars was kept at the farthest


177

end of pullout specimen, where the effect of compression was reduced to quite extent.

Smaller bonded lengths had small compression forces in the bonded length portions.

Based on these facts, it was required to make necessary modifications in the above

equation to make it applicable for the direct (concentric) pullout test results.

It is pertinent to note that statically calibrated equation from the direct pullout data

would be slightly different from the ACI beam bond equation.

6.6 PROPOSED MODEL FOR DIRECT PULLOUT TESTS AND VALIDATION

OF EXPERIMENTAL RESULTS

The experimental results of direct pullout tests revealed that , as well as ′

were the major parameters which affected the average bond stress. It was evident from

the direct pullout experimental data that bond stress did not vary linearly with the ratio ,

hence a model for direct pullout test results was proposed. For a particular GFRP rebar

diameter, the average bond stress u (in MPa) was associated with the ratios , and √

,

and the following non-linear model for direct pullout tests was proposed.

0.083 ′

Where , , and are the coefficients determined by regression analysis;

‘ ’ represents the change in bond stress due to change in ratio , ‘ ’ denotes the

variation due to ratio and ‘ ’ represents the change in bond stress due to change in the

ratio .

It is pertinent to note that the term

was used in the proposed model due to its

non-linear relation with the average bond stress as evident from the direct pullout



178

The proposed pullout bond model was developed for the deformed GFRP rebar

diameters ranging from 9.5mm to 25mm, concrete compressive strength in the range of

27-41 MPa, and C/db ratio from 3.0 to 7.0 resulting into splitting as well as pullout failure.

Half of the experimental data was used to calibrate and remaining half for

validating the proposed model. Statistics of the calibration have been given in table 6.5.

Table 6.5: Statistics of Calibration of Proposed Pullout Bond Model:

Parameters

Calibrated Values +17.53 +0.36 +108.84 -9.82

t-Values 4.74 0.89 9.22 -4.46


Thus the proposed bond model for direct pullout tests was as follows:

0.083 ′ 17.34 0.36 108.84 9.82

The coefficient of correlation is close to 1.0, which indicates the perfection of

proposed model. The quality of fit of above model based on experimental data of this

research has been shown in figures 6.12 and 6.13.


179

Fig. 6.12: Quality of fit of proposed bond model for calibration data. (Refer tables 5.6

and B.1)

Fig. 6.13: Quality of fit of proposed bond model for validation data. (Refer tables 5.4

and B.2)

0

5

10

15

20

25

0 5 10 15 20

Pre

dict

ed B

ond

Str

ess,

MP

a

Experimental Bond Stress, MPa

+25%

-25%

0%

0

5

10

15

20

25

0 5 10 15 20

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%


180

The scatter of data for the proposed pullout bond model was quite well within the

limits of 25%.

It is pertinent to note that the proposed pullout bond model was further validated

using the published experimental data of direct pullout tests by several researchers.

Details of validation data have been provided in Appendix-D.

The quality of fit of proposed pullout bond model for validation data of various

researchers have been shown in figures 6.14 to 6.18.

Fig. 6.14: Quality of fit of proposed bond model for validation data of Okelo et al.

(2005) (Refer table D.3)

0

5

10

15

20

25

0 5 10 15 20

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%


181

Fig. 6.15: Quality of fit of proposed bond model for validation data of Hao et al. (2008)

(Refer table D.5a)

Fig. 6.16: Quality of fit of proposed bond model for validation data of Marta Baena et al.

(2009) (Refer table D.6a)

0

5

10

15

20

25

0 5 10 15 20

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%

0

5

10

15

20

25

30

0 5 10 15 20 25

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%


182

Fig. 6.17: Quality of fit of proposed bond model for validation data of Marta Baena et al.

(2009) (Refer table D.6b)

Combining the data of direct pullout test results of all the researchers, the quality of fit of

proposed pullout bond model has been shown in the figure 6.18.

Fig. 6.18: Quality of fit of proposed bond model for Validation Data of all researchers

0

5

10

15

20

25

30

0 5 10 15 20 25

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%

0

5

10

15

20

25

30

0 5 10 15 20 25

Pre

dict

ed B

ond

Str

ess,

MP

a


+25%

-25%

0%


183

The results of above comparisons and validations revealed that proposed pullout

bond model was applicable with good accuracy for the published experimental data of

direct pullout tests by several researchers despite of having differences in the testing

parameters and conditions. The variations in results using the proposed pullout model

were within the range 25%. Thus the accuracy and authenticity of proposed bond model

has been established.

It is an established experimental conclusion that in case of beams, average bond

stress is always less than the bond stress obtained from direct pullout tests due to

difference in structural behavior of concrete in two test methods. Benmokrane et al.

(1996) and other researchers concluded that average bond stress of GFRP rebars from

beam tests was lesser than that obtained from direct pullout tests by 50% and more.

Comparison of beam experimental bond stress with the direct pullout results as

discussed in section 6.2 revealed that average bond stress from beams was lower in the

range of 47% to 54% than that obtained from direct pullout tests, which also conformed

to the established experimental conclusions of various researchers. Hence the accuracy of

beam bond experimental results has also been established.

6.7 SUMMARY

The comparison of experimental results of direct pullout, beam bond and primary

beams of junctions revealed the similar trends as of published data of average bond

stresses by several researchers. The difference in numerical values of experimental bond

stresses and the published data was due to the difference in the testing parameters and

conditions. The difference for direct pullout test results was in the range of 4% to 19%

whereas in case of beam bond tests, it was in the range of 5% to 29%.

The comparison of direct pullout and beam bond test results revealed the bond

stress reduction in the range of 47% to 54% in case of beams and close to the limits

indicated in the literature despite of having variation in the testing conditions and

parameters.


184

The proposed pullout bond model was calibrated and validated using the research

experimental data and scatter of data was quite well within the limits of 25%. The

proposed model for direct pullout tests was also validated using the published

experimental data of direct pullout tests by several researchers and variation was found in

range of 25% despite of having quite difference in the testing parameters, thus the

accuracy and authenticity of proposed pullout bond model was established.

The beam bond experimental results also conformed to the published results of

beam tests by various researchers, which established the accuracy of beam bond


CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS

185

CONCLUSIONS AND RECOMMENDATIONS

7.1 CONCLUSIONS

The major objective of present research was to develop GFRP rebars for the first

time in Pakistan using available local resources, with tensile and average bond stress

properties closely conforming to the international standards. For this purpose, an

extensive experimental program was planned and executed through trial productions of

GFRP rebars for the determination of optimum composition of resin mixture ingredients

as well as combination of process parameters based on hardness and tensile strength

criteria respectively, which were subsequently used for the final production of GFRP

rebars. It is pertinent to note that no guideline related to the development of GFRP rebars

was available in the existing literature, that is why hit and trial approach was adopted.

Production models were developed to optimize/economize the production process as well

as for the validation of experimental results. The optimum composition of resin mixture

ingredients, optimum combination of process parameters and the development of

production models are the major findings of this research work as well as contribution to

the existing body of knowledge.

The research objectives also included the evaluation of average bond stress of these

local GFRP rebars with normal strength concretes. Another experimental program was

planned and executed to evaluate the effect of bonded length, rebar diameter, concrete

cover, surface texture of rebar and concrete compressive strength variations on the bond

stress through direct pullout tests. The effect of bonded length and rebar diameter

variation on the average bond stress in flexure was also studied through beam bond tests.

Finally, the effect of joint action on average bond stress of primary beams of

junctions/intersecting beams at right angles was investigated. A model for predicting the

average bond stress was developed basing on direct pullout experimental results; half of

which were used to calibrate the model and remaining half to validate. The proposed

pullout bond model was further validated using the published data of direct pullout results

by several researchers. The development of direct pullout bond model may be considered

as the addition in the existing body of knowledge. Based on the analysis of experimental

results and their comparisons, following conclusions can be drawn.


186

1- Optimum composition of resin mixture ingredients as well as combination of process

parameters are always a trade secret due to proprietary issue, which have been now

made available through this research as an open source technology. The locally

developed GFRP deformed rebars were closely conforming to ASTM & ACI

requirements and comparable with the GFRP rebars ‘Aslan-100TM‘ developed in USA

with more advanced technology and resources.

2- With the increase in heating die temperature, the tensile strength of GFRP rebars was

increased due to more heat energy. When the die temperature was low, the GFRP

rebars were not properly cured and hence exhibited low tensile strength.

3- At a specific die temperature, decrease in pull speed resulted into more heat energy to

the GFRP rebar, hence better curing and tensile strength was achieved.

4- The literature review revealed that the tensile strength is a function of rebar diameter.

The tensile strength increases with the decrease in rebar diameter. Thus larger

diameter rebars has less tensile strength as well as efficiency. The fibers located near

the centre of rebar cross section are not subjected to as much stress as those fibers

which are situated near the outer surface of rebar. The locally developed GFRP rebars

followed the same trend.

5- Proposed individual production models helped to reduce the number of trial

productions in the range of 37.5% to 47.5% thus reduced the cost of GFRP rebars.

The tensile strengths predicted with unified production model were in close

agreement (within 10%) with the experimental tensile strengths.

6- Average bond stress decreased in the range of 14% to 20% and stroke value by 13%

to 37% with the increase in bonded length from 3.5 db to 5.0 db and then to 7.0 db in

direct pullout tests, and this trend of decrease was similar in the beam bond as well as

junction tests. It was due to the fact that with increase in bonded length, distribution

of bond stress along the bonded length became non-uniform.


187

7- Bond stress decreased in the range of 6% to 27% and pullout load increased in the

range of 11% to 19%, when GFRP rebar diameter was increased in the range of

9.5mm to 25mm. The decrease in bond stress with the increase in rebar diameter as

well as bonded length was due to the fact that pullout load increase was not

proportional to increase in the bonded length.

8- The bond stress increased by 16% to 17% with the increase in C/db ratio for all GFRP

rebar diameters due to more confining pressure, which resisted the circumferential

tensile stresses and increased the frictional force required to pullout the rebar.

9- Sand coating caused the increase in average bond stress in the range of 5% to 13%

along with decrease in the corresponding stroke value due to better surface roughness.

10- The average bond stress increased in the range of 10% to 24% due to increase in

concrete compressive strength from 27.0 MPa to 41.4 MPa due to better shear

strength of higher strength concrete.

11- Smaller, 9.5 and 13mm, diameter rebars with all bonded lengths caused the pullout

failure due to more concrete cover/confining pressure on rebar and restraining the

splitting of concrete. Shorter, 3.5 db, bonded length for 19mm and 25mm diameter

rebars also resulted into pullout and mixed mode failure.

12- Larger diameter rebars, 19mm with 7.0 db and 25mm with 5.0 & 7.0 db bonded

lengths have shown splitting failure which was abrupt and highlighted by formation of

flexural cracks and concrete splitting.

13- The average bond stresses obtained from beam bond tests were lower in the range of

47% to 54% than the corresponding values of bond stresses obtained from direct

pullout tests due to difference in structural behavior of two test methods. The concrete

around the GFRP rebars in beams was in tension and caused cracking under

comparatively low stresses, hence exhibited smaller bond stresses.

14- With the increase in rebar diameter in beam bond tests, average bond stress decreased

in the same way as in the direct pullout tests. When the rebar diameter was increased


188

from 13mm to 19mm, the decrease in bond stress was in the range of 12% to 17% and

slip by 20% to 44%.

15- The increase in bonded length in beam bond tests caused the decrease in average bond

stress in the range of 15% to 25% and slip by 3% to 19% due to the same reasons as

for the direct pullout tests.

16- The joint action has reduced the average bond stress of primary beams of junctions by

7% to 19% as compared to individual/reference beams due to stress magnification in

primary beams by the secondary beams.

17- Comparison of experimental bond stress results with the published data of average

bond stress by several international researchers revealed the same trend of bond

stresses and difference in their numerical values was in the range of 4% to 19% for

direct pullout tests and 5% to 29% for beam bond tests due to difference in the testing

parameters and conditions.

18- The average bond stresses predicted with the proposed pullout bond model were in

close agreement with the experimental bond stresses. Authenticity of the proposed

model was also established by validating the proposed model with published pullout

experimental data by several researchers. Validation with more than 100 published

experimental pullout results by several researchers concluded the difference in a range

of 25% despite of having differences in the testing parameters and conditions.


189

7.2 RECOMMENDATIONS FOR USE OF LOCAL GFRP REBARS IN

CONCRETE FLEXURAL MEMBERS

Based on experimental results of locally developed GFRP deformed rebars, their

comparisons and results of quality assurance tests, following are some recommendations:

1- Locally developed GFRP rebars are suitable for use in reinforced concrete (RC)

flexural members in the following specific conditions:

a) in highly corrosive environments for bridge decks, approach slabs, parking

structures, railroad crossings, salt storage facilities, concrete manhole covers for

sewerage and other chemical effluents etc. This also includes the structural

concrete subjected to marine salts, seawalls, water fronts and the floating marine

docks.

Reinforced concrete used in the chemical plants, containers, pipeline and chemical

distribution facilities is also subjected to corrosive environment, therefore GFRP

rebars should be used for their more durability.

b) in magnetic resonance imaging (MRI) units like in hospitals or other equipment

sensitive to the electromagnetic fields as well as in the concretes near high voltage

cables/sub-stations/transformers etc. This also includes the concrete used in

manhole covers for high voltage ducts, compass calibration pads as well as in

radio frequency sensitive areas.

2- As compared to steel rebars, GFRP rebars have low tensile modulus of elasticity (about

one fifth) as well as low strain at failure (about 4 to 6 times lower), therefore shall not be

used in earth quake resistant RC flexural members. Earthquake resistant

structures/members have to be ductile for better performance in seismic risk zones.

3- Being a relatively new material with low tensile modulus of elasticity and strain,

additional material reduction factors for the use of GFRP rebars should be considered as

per relevant standards and structural design should be based on deflection as well as crack

width control. ACI 440.IR-06, “Guide for the design and construction of structural


190

concrete reinforced with FRP bars”, contains environmental reduction factors of 0.70 and

0.80 for GFRP rebars when concrete is exposed to and not exposed to earth and weather

respectively.

There is no need of developing/revising the existing design guidelines for the use of

GFRP rebars in public and commercial construction projects in Pakistan because of

availability of number of authentic design guidelines with sufficient details. These

authentic guidelines include ACI 440.1R-06, Canadian CSA S-806 Building Code,

Canadian Highway Bridge Design Code Section-16 and many others. However local and

environmental conditions of the project sites shall be considered appropriately while

designing the concrete flexural members.


191

7.3 RECOMMENDATIONS FOR FUTURE RESEARCH WORK

Based on experimental work performed in this research work, following are some

recommendations for the future research works.

1- Study the options for reduction in energy consumption of the pultrusion process

by developing more efficient heating system.

2- Study the effect of degree of cure and volume fraction of glass fibers on the

durability of GFRP rebars subjected to severe acidic environments.

3- Determination of bond behavior of locally developed GFRP rebars under cyclic

loading.

4- Determination of fatigue performance of locally developed GFRP rebars .

5- Conduct beam bond tests without auxiliary rebars to study the bond behavior of

GFRP rebars in unconfined concrete and compare the results with this research

work.

6- Conduct the similar bond evaluation study with high strength concrete and

compare the results with this study.

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192

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APPENDICES

197

APPENDIX-A

The experimental scheme and results of sixteen preliminary direct pullout tests

using 41.4 MPa concrete and Ø150mm x 300mm pullout specimens for the study of

effect of surface texture variation on the average bond stress of plain GFRP rebars with

and without sand coating have been presented in table A.1.

Table A.1: Experimental Scheme and Results of Preliminary direct Pullout tests to study

the effect of surface texture variation on bond stress of Plain GFRP rebars with and

without sand coating.


Plain Rebar Surface Texture

Lb/db Ratio Avg. Bond Stress u (MPa)

9.5

WSC

5.0

6.10

SC 9.00

WSC

7.0

4.90

SC 7.50

13

WSC

5.0 9.20

SC 14.50

WSC

7.0 10.00

SC 13.50

19

WSC

5.0 9.00

SC 10.50

WSC

7.0 7.30

SC 10.00

APPENDICES

198

22

WSC

5.0 5.25

SC 6.50

WSC

7.0 5.50

SC 7.50

WSC = Without Sand Coating. SC = With Sand Coating.

It may be noted from above bond stress results that average bond stress, of four

plain GFRP rebar diameters without sand coating and for 5.0 db bonded lengths, was 7.38

MPa. When this bond stress value was compared with bond stress value of 11.60 MPa for

5.0 db bonded length of reference GFRP rebars (Aslan-100TM) based on average of eight

rebar diameters, it was then decided to develop and test the deformed GFRP rebars for

evaluation of bond stress. Plain GFRP rebars had quite low average bond stress than the

reference rebars, whereas deformed GFRP rebars exhibited the comparable bond stress with

the reference GFRP rebars.

APPENDICES

199

APPENDIX-B

Table B.1: Experimental Scheme and Results of Simple Direct Pullout Tests for

Uncoated Deformed GFRP rebars using Ø150mm x 300mm Test Specimens and 27.0

MPa Compressive Strength Concrete.

Rebar ID


Clear Cover

C (mm)

C/db

Ratio

db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

GFR9-Lb3.0 9.5 70.25

7.39

1/3.0 13.95 16.40

GFR9-Lb5.0 1/5.0 17.93 12.65

GFR9-Lb7.0 1/7.0 21.04 10.60

GFR13-Lb3.0 13 68.50

5.27

1/3.0 23.84 14.97

GFR13-Lb5.0 1/5.0 29.81 11.23

GFR13-Lb7.0 1/7.0 33.26 8.95

GFR19-Lb3.0 19 65.50

3.45

1/3.0 44.06 12.95

GFR19-Lb5.0 1/5.0 58.58 10.33

GFR19-Lb7.0 1/7.0 63.75 8.03

GFR25-Lb3.0 25 62.50

2.50

1/3.0 67.74 11.50

GFR25-Lb5.0 1/5.0 74.12 7.55

GFR25-Lb7.0 1/7.0 87.28 6.35

Note: GFRxx-Lbyy stands for GFRP uncoated Deformed rebar, with xx diameter and Bonded Length (Lb) of yy times the rebar diameter (db), respectively. C, represents the clear cover of concrete to the GFRP rebar.

It is evident from above results that average bond stress, of four uncoated deformed

GFRP rebar diameters for 5.0 db bonded lengths, is 10.44 MPa, which was comparable with

bond stress value of 11.60 MPa for 5.0 db bonded length of reference GFRP rebars. Thus

deformed surface texture of GFRP rebars was finalized and used for all bond study

experimental works.

APPENDICES

200

Table B.2: Experimental Scheme and Results of Simple Direct Pullout Tests for

Deformed Uncoated GFRP rebars using Ø100mm x 200mm Test Specimens and 27.0

MPa Compressive Strength Concrete.

Rebar ID


Clear Cover

C (mm)

C/db

Ratio db/Lb

Ratio

Max. Pullout Force

F (KN)

Avg. Bond Stress

u (MPa)

GFR9-Lb3.0 9.5

45.25

4.76

1/3.0 12.09 14.21

GFR9-Lb5.0 1/5.0 17.04 12.02

GFR9-Lb7.0 1/7.0 17.35 8.74

GFR13-Lb3.0 13

43.50

3.35

1/3.0 21.38 13.42

GFR13-Lb5.0 1/5.0 29.49 11.11

GFR13-Lb7.0 1/7.0 30.36 8.17

GFR19-Lb3.0 19

40.50

2.13

1/3.0 42.36 12.45

GFR19-Lb5.0 1/5.0 58.97 10.40

GFR19-Lb7.0 1/7.0 62.95 7.93

GFR25-Lb3.0 25

37.50

1.50

1/3.0 48.77 8.28

GFR25-Lb5.0 1/5.0 71.37 7.27

GFR25-Lb7.0 1/7.0 85.22 6.20

It is also evident from above results that average bond stress, of four uncoated

deformed GFRP rebar diameters for 5.0 db bonded lengths, is 10.20 MPa, which was also

comparable with bond stress value of 11.60 MPa for 5.0 db bonded length of reference

GFRP rebars.

APPENDICES

201

APPENDIX-C

Representative Pullout Test Graphs for Deformed and Sand Coated GFRP Rebars

Fig. C.1: Average bond stress vs stroke graph for 5.0 db and 7.0 db bonded lengths of

9.5mm diameter deformed GFRP rebars respectively.

Fig. C.2: Average bond stress vs stroke graph for 3.5 db and 5.0 db bonded lengths of

13mm diameter deformed GFRP rebars respectively.

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm-5db0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm-7db

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

13mm-3.5db0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

13mm-5db

APPENDICES

202

Fig. C.3: Average bond stress vs stroke graph for 3.5 db and 5.0 db bonded length of 19mm and 25mm diameter deformed GFRP rebars respectively.

Fig. C.4: Average bond stress vs stroke graph for 7.0 db and 3.5 db bonded length of 25mm diameter deformed and 9.5mm diameter sand coated GFRP rebars respectively.

Fig. C.5: Average bond stress vs stroke graph for 5.0 db and 7.0 db bonded length of 9.5mm diameter sand coated GFRP rebars respectively.

0

5

10

15

20

0 5 10 15 20 25 30

Bon

d S

tres

s (M

Pa)

Stroke (mm)

19mm-3.5 db0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

25mm-5db

0

5

10

15

20

0 5 10 15 20

Bon

d S

tres

s (M

Pa)

Stroke (mm)

25mm-7db0

5

10

15

20

0 20 40 60 80

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm-3.5db

0

5

10

15

20

0 20 40 60 80 100

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm -5db0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

9.5mm-7db

APPENDICES

203

Fig. C.6: Average bond stress vs stroke graph for 3.5 db and 5.0 db bonded length of 13mm diameter sand coated GFRP rebars respectively.

Fig. C.7: Average bond stress vs stroke graph for 7.0 db and 3.5 db bonded length of 13mm and 19mm diameter sand coated GFRP rebars respectively.

Fig. C.8: Average bond stress vs stroke graph for 5.0 db and 3.5 db bonded length of 19mm and 25mm diameter sand coated GFRP rebars respectively.

0

5

10

15

20

0 20 40

Bon

d S

tres

s (M

Pa)

Stroke (mm)

13mm-3.5db0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

13mm-5db

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

13mm-7db

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

19mm-3.5db

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

19mm-5db

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

25mm-3.5db

APPENDICES

204

Fig. C.9: Average bond stress vs stroke graph for 5.0 db bonded length of 25mm diameter

sand coated GFRP rebars.

0

5

10

15

20

0 10 20 30 40 50

Bon

d S

tres

s (M

Pa)

Stroke (mm)

25mm-5db

APPENDICES

205

APPENDIX-D

Table D.1: Published Pullout Test Results by O. Chaallal and B. Benmokrane (1995) used

for the Validation of Proposed Pullout Bond Model.

Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)

Predicted Bond Stress

u (MPa)

% Diff.

1 12.7 121.15 62.5 31.0 15.0 12.62 15.88

2 12.7 121.15 125.0 31.0 11.1 9.64 13.12

3 15.9 119.55 75.0 31.0 12.5 11.68 6.53

4 15.9 119.55 150.0 31.0 11.4 8.79 22.88

5 19.1 117.95 90.0 31.0 14.4 10.67 25.93

Table D.2: Published Pullout Test Results by Ehsani et al. (1996) used for the Validation

of Proposed Pullout Bond Model.

Sr. No. Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 19.0 193.7 152.4 32.2 11.5 9.26 19.50

2 19.0 193.7 152.4 32.2 9.6 9.26 3.57

3 19.0 193.7 152.4 32.2 9.3 9.26 0.46

4 29.0 188.5 203.2 32.2 9.9 7.27 26.61

5 29.0 188.5 203.2 32.2 9.8 7.27 25.87

APPENDICES

206

Table D.3: Published Pullout Test Results by Okelo et al. (2005) used for the Validation


Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 6 98.5 30 44.3 16.57 19.03 -14.86

2 8 97.5 40 44.9 13.73 17.39 -26.64

3 10 96.5 50 44.8 18.93 16.05 15.23

4 10 96.5 50 44.6 12.83 16.01 -24.79

5 10 96.5 50 44.6 17.83 16.01 10.20

6 10 96.5 50 60.4 17.13 18.63 -8.77

7 10 96.5 50 41.9 19.50 15.52 20.42

8 10 96.5 70 41.9 15.77 13.33 15.46

9 10 96.5 90 41.9 12.83 12.22 4.75

10 10 96.5 50 39.4 18.13 15.05 17.00

11 10 96.5 70 39.4 15.33 12.93 15.67

12 10 96.5 90 39.4 13.43 11.85 11.76

13 10 96.5 50 30.8 15.80 13.31 15.79

14 10 96.5 90 30.8 13.9 10.48 24.62

15 10 96.5 50 46.4 18.83 16.33 13.27

16 16 93.5 80 41.8 16.67 12.79 23.27

17 16 93.5 80 45.5 17.50 13.34 23.75

18 19 92.5 171 48.3 11.06 9.87 10.77

19 19 92.5 171 44.1 8.30 9.43 -13.61

20 19 92.5 95 35.0 13.80 10.76 22.05

21 19 92.5 171 33.5 9.27 8.22 11.34

APPENDICES

207

Table D.3 (Con’d): Published Pullout Test Results by Okelo et al. (2005) used for the

Validation of Proposed Pullout Bond Model.

Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

22 19 92.5 133 40.0 12.43 9.79 21.23

23 19 92.5 95 49.0 16.70 12.73 23.78

24 19 92.5 133 49.0 12.27 10.84 11.68

Note: Each experimental bond stress value has been taken as average of three direct pullout tests.

Table D.4: Published Pullout Test Results by Tastani et al. (2006) used for the Validation


Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 19.05 69.9 95.05 29.0 7.40 9.60 22.91

2 19.05 69.9 95.05 29.0 7.40 9.60 22.91

3 19.05 69.9 95.05 29.0 8.10 9.60 15.62

4 19.05 69.9 95.05 29.0 8.00 9.60 16.66

5 19.05 69.9 95.05 29.0 7.30 9.60 23.95

6 19.05 69.9 95.05 29.0 7.20 9.60 25.00

APPENDICES

208

Table D.5a: Published Pullout Test Results by Hao et al. (2008) used for the Validation of

Proposed Pullout Bond Model.

Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 8 46 40 30 14.26 13.16 7.72

2 8 46 200 20 7.18 6.83 4.88

3 8 46 240 20 6.42 6.74 -4.98

4 10 45 50 20 11.81 10.03 15.04

5 10 45 200 20 7.54 6.55 13.12

6 10 45 250 20 6.01 6.42 -6.80

7 10 45 50 30 13.90 12.29 11.60

8 10 45 100 30 12.24 9.19 24.92

9 10 45 150 30 9.28 8.36 9.92

10 10 45 200 30 8.13 8.02 1.32

11 10 45 50 40 16.69 14.19 14.98

12 10 45 100 40 14.31 10.61 25.85

13 10 45 150 40 10.39 9.65 7.10

14 12 44 60 30 13.14 11.55 12.11

15 12 44 120 30 11.52 8.63 25.11

16 12 44 180 30 9.09 7.88 13.37

17 12 44 240 30 6.23 7.59 -21.76

18 14 43 70 40 12.19 12.58 -3.23

19 14 43 140 40 10.81 9.40 13.07

20 14 43 210 40 8.05 8.61 -6.97

21 8 46 40 20 13.14 10.74 18.23

APPENDICES

209

Table D.5a (Con’d): Published Pullout Test Results by Hao et al. (2008) used for the

Validation of Proposed Pullout Bond Model.

Sr. No.

Rebar Dia.

db (mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

22 10 45 50 20 10.77 10.03 6.84

23 10 45 50 20 11.41 10.03 12.07

24 10 45 50 30 11.31 12.29 -8.65

25 10 45 50 30 11.97 12.29 -2.66

26 10 45 50 30 13.10 12.29 6.20

27 10 45 50 40 14.01 14.19 -1.28

28 10 45 50 40 14.83 14.19 4.32

29 12 44 60 30 9.60 11.55 -20.31

30 12 44 60 30 12.41 11.55 6.94

31 14 43 70 40 10.94 12.58 -15.02

Table D.5b: Published Pullout Test Results by Hao et al. (2009) used for the Validation


Sr. No.

Rebar Dia.

db(mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 8 71 32 28.7 14.58 15.14 3.69

2 8 71 32 28.7 13.4 15.14 11.49

3 10 70 40 28.7 13.17 14.11 6.66

4 10 70 40 28.7 13.96 14.11 1.06

5 12 69 48 28.7 11.61 13.25 12.38

6 12 69 48 28.7 10.83 13.25 18.27

APPENDICES

210

Table D.6a: Published Pullout Test Results by Marta Baena et al. (2009) for concrete

strengths more than 48.0 MPa used for the Validation of Proposed Pullout Bond Model.

Sr. No.

Rebar Dia. db

(mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 8 96 40 54.93 24.33 19.19 21.11

2 12 94 60 54.93 19.51 16.55 15.16

3 8 96 40 53.54 15.47 18.95 -22.50

4 8 96 40 53.11 17.45 18.87 -8.11

5 12 94 60 53.11 15.44 16.27 -5.40

6 12 94 60 53.54 15.06 16.34 -8.50

7 19 90.5 95 53.54 15.08 13.28 11.93

8 19 90.5 95 53.11 14.73 13.23 10.22

9 8 96 40 49.55 22.99 18.23 20.71

10 8 96 40 53.65 20.78 18.97 8.72

11 12 94 60 49.55 15.34 15.72 -2.50

12 12 94 60 53.65 17.35 16.36 5.74

13 16 92 80 49.55 16.85 13.91 17.47

14 19 90.5 95 53.65 14.32 13.30 7.17

15 19 90.5 95 53.65 14.58 13.30 8.82

16 8 96 40 50.50 16.40 18.40 -12.23

17 8 96 40 56.30 17.70 19.43 -9.78

18 12 94 60 50.50 14.54 15.87 -9.13

19 12 94 60 56.30 15.75 16.76 -6.37

20 16 92 80 58.20 15.47 15.07 2.57

APPENDICES

211

Table D.6b: Published Pullout Test Results by Marta Baena et al. (2009) for concrete

strengths less than 32.0 MPa used for the Validation of Proposed Pullout Bond Model.

Sr. No.

Rebar Dia. db

(mm)

Clear Cover

C (mm)

Bonded Length

Lb (mm)

Concrete Strength

fc’ (MPa)


u (MPa)


u (MPa)

% Diff.

1 8 96 40 27.80 15.77 13.65 13.43

2 12 94 60 29.34 12.86 12.10 5.94

3 12 94 60 26.50 14.13 11.50 18.64

4 12 94 60 26.70 11.05 11.54 -4.42

5 16 92 80 28.30 12.17 10.51 13.65

6 16 92 80 26.70 12.03 10.21 15.15

7 8 96 40 31.30 16.97 14.49 14.64

8 12 94 60 30.00 9.89 12.23 -23.67

9 12 94 60 28.30 9.78 11.88 -21.47

10 16 92 80 30.00 10.47 10.82 -3.35

11 16 92 80 28.30 12.23 10.51 14.07

12 8 96 40 29.66 12.75 14.10 -10.60

13 16 92 80 26.67 11.70 10.20 12.80

14 16 92 80 27.16 9.84 10.30 -4.63

15 8 96 40 29.34 14.85 14.03 5.55

16 12 94 60 30.00 15.83 12.23 22.73

APPENDICES

212

APPENDIX-E

The statistical data of various quality assurance tests of final production of GFRP

rebars has been presented in the tables E.1, E.2 and E.3. Each test was performed on three

GFRP rebar samples. It is pertinent to note that no sample was rejected.

Table E.1: Statistical Data of quality assurance tests of finally developed deformed GFRP

rebars.

Properties

Relevant Standard

Results of Local GFRP Rebars

Min. Max. Median Avg. Standard

Deviation

Barcol Hardness ASTM D-2583

47

49

48

48

1.00

Specific Gravity ASTM D-792 1.88 1.92 1.90 1.90 0.02

24 Hours Moisture Absorption at 50 oC

(%)

ASTM D-570

0.24

0.24

0.24

0.24

0.00

Tensile Modulus of Elasticity (GPa)

ACI 440.3R-04

39.10

39.70

39.45

39.40

0.216

APPENDICES

213

Table E.2: Statistical Data of quality assurance tests of finally developed deformed GFRP

rebars.

Rebar ID

Rebar

Diameter

(mm)

Tensile Strength of Local GFRP Rebars

(MPa)

Elastic Modulus

of Local Rebars

(GPa) Minimum Maximum Median Standard Deviation

GFR9-D 9.5 740 753 747 6.506 39.10

GFR13-D 13 669 679 674 5.000 39.70

GFR16-D 16 624 635 629 5.507 39.30

GFR19-D 19 602 611 606 4.509 39.50

GFR22-D 22 560 571 566 5.507 39.60

GFR25-D 25 522 531 527 4.509 39.40

Note: Elastic modulus values were same.

APPENDICES

214

Table E.3: Statistical Data of quality assurance tests of finally developed sand coated

GFRP rebars.

Rebar ID

Rebar

Diameter

(mm)

Tensile Strength of Local GFRP Rebars

(MPa)

Elastic Modulus

of Local Rebars

(GPa) Minimum Maximum Median Standard Deviation

GFR9-S 9.5 756 765 761 4.509 38.96

GFR13-S 13 685 694 689 4.509 39.20

GFR16-S 16 638 651 644 6.506 38.90

GFR19-S 19 612 619 616 3.511 39.40

GFR22-S 22 570 583 577 6.506 39.70

GFR25-S 25 535 544 540 4.509 38.96

Note: Elastic modulus values were same.

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