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
CHAPTER-1 INTRODUCTION
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
CHAPTER-1 INTRODUCTION
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.
CHAPTER-1 INTRODUCTION
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.
CHAPTER-1 INTRODUCTION
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
CHAPTER-1 INTRODUCTION
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
CHAPTER-1 INTRODUCTION
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:
CHAPTER-1 INTRODUCTION
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.
CHAPTER-1 INTRODUCTION
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
CHAPTER-1 INTRODUCTION
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.
CHAPTER-1 INTRODUCTION
11
Table 1.4: Experimental Scheme of Pullout Tests for Deformed Uncoated GFRP rebars
using Ø100mm x 200mm 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-Lb3.5C45 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
CHAPTER-1 INTRODUCTION
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.
CHAPTER-1 INTRODUCTION
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
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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)
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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:
CHAPTER-2 LITERATURE REVIEW
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.
M = Chopped Mat 0.50 kg/m2 Wr = Woven Roving 0.80 kg/m2
Source: M/s Ashland USA, Manufacturer’s data sheet, 2001.
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.
CHAPTER-2 LITERATURE REVIEW
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
Source: M/s Ashland USA, Manufacturer’s data sheet, 2001.
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).
CHAPTER-2 LITERATURE REVIEW
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.
CHA
2.3
glas
rang
visc
stra
met
has
The
from
also
this
2.3.
atte
flow
APTER-2
PROCESS
All glas
ss is extrud
ging from 0
cous state a
nds are obt
ter/second a
been reprod
Fig. 2
The ma
e raw mater
m the melt.
o be down d
process.
.1 Fiberizin
The ch
nuation pro
ws through
SES
ss rovings a
ded through
0.793 to 3.17
are quickly
tained from
and wound
duced in fig
2.5: Typical
arble melt p
rials are me
These mar
drawn from
ng and Sizin
hange of m
ocess as sho
a platinum
are manufac
h a bushin
75 mm (Fre
drawn to a
individual
onto requir
gure 2.5.
Pultrusion
process is u
elted and 2
rbles are th
m the solid p
ng
molten glass
own in figur
m rhodium a
28
ctured with
ng having
ederick T. et
required d
fibers/filam
red package
Composite
used to deve
to 3 cm dia
hen remelted
performs su
s in forehe
re 2.6. As p
alloy bushin
the direct d
many indiv
t al., 2001).
iameter and
ments. Thes
es. Typical
Diagram (F
elop special
ameter solid
d and glass
urface. Opti
earth into c
er Frederick
ng with a h
LIT
draw metho
vidual orifi
. The result
d then solid
se are pulled
pultrusion
Francesco e
l purpose h
d glass mar
s fibers are
cal fibers a
continuous
k T. et al. (2
high numbe
TERATURE R
od, in which
ices with d
ting fibers i
dify. Multi-
d at speed u
composite
et al, 2004)
high strength
rbles are de
formed wh
are also mad
glass fiber
2001), molt
er of holes
REVIEW
h molten
diameter
n highly
filament
up to 61
diagram
h fibers.
eveloped
hich can
de using
rs is an
ten glass
(400 to
CHA
800
cont
draw
rota
glas
from
stra
filam
(sm
as s
APTER-2
00) during
trolled very
wn down an
The fib
ates continu
ss filaments
m another i
nd integrity
ments are c
mall bundles
hoes) are us
F
the typical
y accurately
nd rapidly c
ber surface
ously throu
s pass. It is
in addition
y, resin com
combined in
of fibers/fi
sed.
Fig. 2.6: Fib
l production
y to mainta
ooled as the
is treated
ugh the sizin
the step w
to original
mpatibility,
nto a strand
filaments) a
berglass For
29
n. The bus
ain a const
ey exit the b
with sizin
ng bath for m
which primar
l glass com
lubricity a
d before rea
re required
rming Proce
shing is el
tant glass v
bushing.
g, passing
maintaining
rily differen
mposition. T
as well as a
aching the t
, multiple g
ess (Frederic
LIT
ectrically h
viscosity. T
through an
g a thin film
ntiates one
The sizing c
adhesion. A
take up dev
gathering de
ck T. et al, 2
TERATURE R
heated and
The fibers a
n applicato
m through w
glass fiber
components
After this tre
vice. If split
evices (also
2001)
REVIEW
heat is
are then
r which
which the
product
s impart
eatment,
t strands
o known
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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)
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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).
CHAPTER-2 LITERATURE REVIEW
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:
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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:
CHAPTER-2 LITERATURE REVIEW
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
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-2 LITERATURE REVIEW
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
M = Chopped Mat 0.50 kg/m2 Wr = Woven Roving 0.80 kg/m2
Source: M/s Ashland USA, Manufacturer’s data sheet, 2001.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
Table 3.6: Composition of resin mixture and Experimental results of hardness of PTPS-2
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
56
Fig. 3.5: Effect of variation in BPO and TBPB on the hardness of GFRP rebars.
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.
Table 3.7: Composition of resin mixture and Experimental results of hardness of PTPS-3
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
Fig. 3.6: Effect of variation in BPO and TBPB on the hardness of GFRP rebars.
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
58
Table 3.8: Composition of resin mixture and Experimental results of hardness of PTPS-4
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
(%)
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 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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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%
Heating Die Temperature
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
190 195eating Die Tem
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
5 200 2mperature (oC)
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
190 195eating Die Tem
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.)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
65
Table 3.12: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-2 for 9.5 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
Heating DieTemperature
130 1mm/min.)
ent = 73%
185190195200
Heating Die Temperature
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%
Heating DieTemperature
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
67
Table 3.13: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-3 for 9.5 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
69
Table 3.14: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-12 for 25 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
Heating Die Temperature (
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
72
Table 3.15: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-13 for 25mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
Table 3.16: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-14 for 25mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
. 3.18: Effemeter rebars
. 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
215 220Heating Die Tem
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
Heating DieTemperature
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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’.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
76
Table 3.17: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-4 for 13 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min)
Heating Die Temperature
(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
0 195 200Heating Die Tem
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
0 205 210mperature (oC)
nt = 73%
Pull(mm
130d (mm/min.)
ent = 73%
Heating DieTemperatur
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
Table 3.18: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-5 for 13 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
195 200Heating Die Tem
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
Heating Die Temperature (
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%
Heating DieTemperature
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
200 205Heating Die Te
Fiber Conten
100Pull Speed (m
t Fiber Conten
195200205210
ng Die perature (oC)
200 205Heating Die Tem
At Fiber Co
EXPER
tion in Die
ion in Pull S
tion in Die
5 210emperature (oC)
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
5 200 205Heating Die Tem
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
5 210 215mperature (oC)
nt = 74%
Pull S(mm/m
110 120(mm/min.)
nt = 74%
Heating Die Temperature
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.)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
82
Table 3.20: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-7 for 16 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
Heating Die Temperature
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
Heating DieTemperature
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
84
Table 3.22: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-8 for 19 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
85
Table 3.23: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-9 for 19 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
86
Table 3.24: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-10 for 22 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
205 210Heating Die Tem
Fiber Conten
90Pull Speed (m
At Fiber Conte
200205210215
ng Die perature (oC)
205 210eating Die Tem
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
200 205Heating Die Te
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
5 210emperature (oC)
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
. 3.32: Effemeter rebars
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
210 215Heating Die Te
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
Heating DieTemperatur
5 220emperature (oC)
nt = 76%
Pull(mm/
90 1mm/min.)
ent = 76%
205210215220
Heating DieTemperatur
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
Heating DieTemperature
5 220 225emperature (oC)
nt = 76%
Pull(mm
90 100
mm/min.)
ent = 76%
Heating DieTemperatur
REBARS
rebars.
of 22mm
diameter
120
200205210
e e (oC)
5 230)
708090100
Speed m/min.)
110
205210215220
e re (oC)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
89
Table 3.25: Combination of Process Parameters and Experimental Results of Tensile
Strengths of TPS-11 for 22 mm diameter rebars.
Rebar ID
Fiber Volume Fraction
(%)
Pull Speed
(mm/min.)
Heating Die Temperature
(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
. 3.34: Effemeter rebars
. 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
210 215Heating Die Tem
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%
Heating DieTemperature
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)
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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 (%)
Heating Die Temperature
(oC)
Pull Speed (mm/minute)
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP 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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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.
CHAPTER-3 EXPERIMENTATION FOR DEVELOPMENT OF GFRP REBARS
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
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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.
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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
Calibrated Values -5.6094 -1.4931 +4.8381
t-Values -3.84 -6.28 8.46
Coefficient of Correlation 0.90
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)
Experimental Tensile Strength (MPa)
+ 10%
- 10%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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.
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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
Calibrated Values -7.6934 -1.8984 +7.2606
t-Values -6.54 -9.87 15.05
Coefficient of Correlation 0.97
Calibration and validation of quality for 13mm diameter rebars has been shown through
the plot given in figure 4.3.
Fig. 4.3: Experimental and Predicted Tensile Strengths for 13mm diameter GFRP rebars.
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)
Experimental Tensile Strength (MPa)
+ 10%
- 10%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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.
Table 4.7: Statistics of Calibration of Proposed Model for 22mm diameter GFRP rebars:
Parameters
Calibrated Values -6.6031 -1.8596 +5.7141
t-Values -4.77 -8.16 10.41
Coefficient of Correlation 0.96
Fig. 4.4: Experimental and Predicted Tensile Strengths for 22mm 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%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
109
Table 4.8: Statistics of Calibration of Proposed Model for 16mm diameter GFRP rebars:
Parameters
Calibrated Values -11.8959 -1.5533 +8.1303
t-Values -4.41 -3.58 7.49
Coefficient of Correlation 0.88
Fig. 4.5: Experimental and Predicted Tensile Strengths for 16mm diameter GFRP rebars.
Table 4.9: Statistics of Calibration of proposed Model for 19mm diameter GFRP rebars
Parameters
Calibrated Values -8.5752 -1.2483 +6.5204
t-Values -2.56 -2.07 4.80
Coefficient of Correlation 0.80
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)
Experimental Tensile Strength (MPa)
+ 10%
- 10%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
110
Fig. 4.6: Experimental and Predicted Tensile Strengths for 19mm diameter GFRP rebars.
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)
Experimental Tensile Strength (MPa)
+ 10%
- 10%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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.
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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
Coefficient of Correlation 0.97
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)
Experimental Tensile Strength (MPa)
+ 10%
- 10%
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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)
Heating Die Temperature
(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.
CHAPTER-4 PRODUCTION MODELS FOR 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
CHAPTER-4 PRODUCTION MODELS FOR GFRP REBARS
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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’.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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 days strength = 24 MPa
7 Compressive Strength of 100mm concrete
cubes (PCC 1:2:4 with w/c ratio of 0.52)
3 days strength = 11 MPa
7 days strength = 17 MPa
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
124
Table 5.4: Scheme and Results 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
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
9.5 mm13 mm19 mm25 mm
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
127
Table 5.5: Scheme and Results of Pullout Tests for deformed Sand Coated GFRP rebars
using Ø150mm x 300mm Test Specimens and 41.4 MPa Compressive Strength Concrete.
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
)
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)
Bonded Length (x db)
9.5 mm13 mm19 mm25 mm
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
129
Table 5.6: Scheme and Results of Pullout Tests for Deformed Uncoated GFRP rebars
using Ø100mm x 200mm 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
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
)
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)
Bonded Length (x db)
9.5 mm13 mm19 mm25 mm
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
Rebar Diameter 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
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
)
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)
Bonded Length (x db)
9.5 mm13 mm19 mm25 mm
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
Rebar Diameter db (mm)
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
C/db = 4.76C/db = 7.39
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
C/db = 3.35C/db = 5.27
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
C/db = 2.13C/db = 3.45
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
C/db = 1.50C/db = 2.50
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
Rebar Diameter db (mm)
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)
%age Diff. in Bond Stress (%)
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
UncoatedSand Coated
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
UncoatedSand Coated
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
UncoatedSand Coated
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
UncoatedSand Coated
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
Rebar Diameter db (mm)
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)
%age Diff. in Bond Stress (%)
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
41.4 MPa Concrete
27 MPa Concrete0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
41.4 MPa Concrete
27 MPa Concrete
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
41.4 MPa Concrete
27 MPa Concrete0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
41.4 MPa Concrete
27 MPa Concrete
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Bonded Length (x db)
19 mm Rebar13 mm Rebar
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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.
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
Primary BeamSecondary Beam
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
159
Fig. 5.59: Bond Stress and Slip of J3GFR19-Lb7.0 and J4GFR13-Lb3.5 respectively.
Fig. 5.60: Bond Stress and Slip of J5GFR13-Lb5.0 and J6GFR13-Lb7.0 respectively.
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)
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)
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)
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)
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)
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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)
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)
13 mm dia. Rebar19 mm dia Rebar
0
2
4
6
8
10
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
Primary Beam
19 mm Rebar13 mm Rebar
0
2
4
6
8
10
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
Secondary Beam
19 mm Rebar13 mm Rebar
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-5 EXPERIMENTATION FOR BOND STRESS EVALUATION
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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)
Bonded Length (x db)
Pullout ResultBeam Bond Result
0
5
10
15
20
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
Pullout ResultBeam Bond Result
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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)
Bonded Length (x db)
Primary Beam
Reference Beam
0
2
4
6
8
10
0 2 4 6 8
Bon
d S
tres
s (M
Pa)
Bonded Length (x db)
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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)
Qingduo Hao et al. (2009)
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%)
fc’= 28.7 MPa fu = 710 MPa db= 10mm C/db = 7.0 Lb = 4 db
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%)
fc’= 39.4 MPa fu = 772 MPa db= 10mm C/db = 9.65 Lb = 7.0 db
-
-
-
-
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%)
fc’= 28.7 MPa fu = 710 MPa db= 12mm C/db = 5.75 Lb = 4 db
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
170
Table 6.3 (Cont’d): 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)
Qingduo Hao et al. (2009)
u (MPa)
Marta Baena et al. (2009)
u (MPa)
% Diff.
Standard deviation
GFRD19
-Lb 5.0
13.10
fc’= 41.4MPa fu = 606 MPa db= 19mm C/db= 3.45 Lb = 5.0 db
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 %)
fc’= 35.0 MPa fu = 620 MPa db= 19mm C/db = 4.87 Lb = 5.0 db
-
-
-
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
fc’= 41.4MPa fu = 606 MPa db= 19mm C/db= 3.45 Lb = 7.0 db
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 %)
fc’= 32.2MPa fu = 641 MPa db= 19mm C/db= 10.19 Lb = 8.0 db
12.07
(12.34 %)
fc’= 38.2 MPa fu = 620 MPa db= 19mm C/db = 4.87 Lb = 7.0 db
-
-
-
-
11.09
to 12.34
1.34
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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.
Fig. 6.5: Comparison of direct pullout experimental results with published results of
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)
Qingduo Hao et al. (2008)
Qingduo Hao et al. (2009)
Yanlei et al. (2008)
Marta Baena et al. (2009)
0
5
10
15
20
ResearchersB
ond
Str
ess
(MP
a)
13 mm dia. rebars with Lb = 5.0 db
Experimental
O. Chaallal (1995)
Qingduo Hao et al. (2008)
Qingduo Hao et al. (2009)
Yanlei et al. (2008)
Marta Baena et al. (2009)
0
5
10
15
20
Researchers
Bon
d S
tres
s (M
Pa)
Lb = 5.0 db
Experimental
O. Chaallal (1995)
Roman Okelo (2005)
Marta Baena et al. (2009)
0
5
10
15
20
Researchers
Bon
d S
tres
s (M
Pa)
Lb = 7.0 db
Experimental
O. Chaallal (1995)
M.R Ehsani et al. (1996)
Tighiourat et al. (1998)
Roman Okelo (2005)
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
172
Fig. 6.6: Comparison of direct pullout experimental results with published results of
several researchers.
Fig. 6.7: Comparison of direct pullout experimental results with published results of
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
173
Table 6.4: Comparison of beam bond experimental results with published data of average bond stresses by several researchers.
Beam ID
Experimental Bond Stress
u (MPa)
O. Chaallal & B. Benmokrane
(1995) u (MPa)
B. Tighiouart et al. (1998)
u (MPa)
Al Zahrani et al. (1999)
u (MPa)
Roman Okelo et al. (2007)
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
fc’= 41.4MPa fu = 606 MPa db= 19mm Lb = 7.0 db
5.70 (15.26 %)
fc’= 31.0 MPa fu = 652 MPa db= 19.1mm Lb = 10.0 db
3.80 (21.32%)
fc’=31.0 MPa fu = 640 MPa db= 19.1mm Lb = 10.0 db
6.60 (26.82 %)
fc’=38.0 MPa fu = 624 MPa db= 19.1mm Lb = 6.65 db
6.80 (28.97 %)
fc’= 50.0 MPa fu = 620 MPa db= 19.1mm Lb = 10.0 db
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
fc’= 41.4MPa fu = 674 MPa db= 13mm Lb = 5.0 db
-
8.80 (14.54 %)
fc’=31.0 MPa fu = 690 MPa db= 12.7mm Lb = 6.0 db
-
-
-
14.54
0.90
B6GFRD13- Lb 7.0
5.79
fc’= 41.4MPa fu = 674 MPa db= 13mm Lb = 7.0 db
7.50 (22.80 %)
fc’= 31.0 MPa fu = 689 MPa db= 12.7mm Lb = 10.0 db
7.30 (20.68 %)
fc’=31.0 MPa fu = 690 MPa db= 12.7mm Lb = 10.0 db
-
-
-
20.68 to 22.80
0.93
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
several researchers.
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)
Tighiourat et al. (1998)
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)
Tighiourat et al. (1998)
Bilal & Hirajli (2005)
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
175
Fig. 6.10: Comparison of beam bond experimental results with published results of
several researchers.
Fig. 6.11: Comparison of beam bond experimental results with published results of
several researchers.
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
experimental results.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
Coefficient of Correlation 0.95
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.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
Experimental Bond Stress, MPa
+25%
-25%
0%
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
Experimental Bond Stress, MPa
+25%
-25%
0%
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
Experimental Bond Stress, MPa
+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
Experimental Bond Stress, MPa
+25%
-25%
0%
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
Experimental Bond Stress, MPa
+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
Experimental Bond Stress, MPa
+25%
-25%
0%
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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.
CHAPTER-6 COMPARISON & VERIFICATION OF BOND STRESS EXPERIMENTAL RESULTS
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
experimental results.
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.
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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.
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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.
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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.
CHAPTER-7 CONCLUSIONS AND RECOMMENDATIONS
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.
REFERENCES
192
REFERENCES
Aslanova M/S Glass fibers, “Handbook of Composites”, Vol. 4 I - Strong Fibers eds W
Watt and B.V. Perov, Elsevier, Amsterdam, 1985, pp. 3-60 I T I
Astrom B. T., Larsson P. H., Hepola P. J. and Pipes R. B., (1994), “Flexural properties of
pultruded carbon/PEEK composites as a function of processing history”, Composites, 2.5.
814.
B. Benmokrane, O. Chaallal and R. Masmoudi, (1995), “Glass fiber reinforced plastic
(GFRP) rebars for concrete structures”, Construction and Building Materials, Vol. 9, No.
6, pp. 353-364.
Bakis C.E., (1993), “FRP reinforcement: materials and manufacturing”, In Fiber-
Reinforced-Plastic (FRP) Reinforcement for Concrete Structures: Properties and
Applications, ed. A. Nanni, pp.13-58
Benmokrane B. et al., (1998), “Standard Test Methods for FRP Rod and Sheet”,
Intelligent Sensing for Innovative Structures (ISIS Canada), University of Manitoba,
Winnipeg, Canada, pg. 61
Benmokrane B., Tighiouart B, Chaallal O., (1996), “Bond strength and load distribution
of composite GFRP reinforcing bars in concrete”. ACI Mater J, 93:246-253.
Benmokrane B., Tighiouart B., D. Gao, (1998), “Investigation of bond in concrete
member with fiber reinforced polymer (FRP) bars”, Construction and Building Materials
12, pp. 453-462.
Concrete description on Wikipedia - http://en.wikipedia.org/wiki/Concrete
Cowen G., Measuria U. and Turner R. M., (1986), “Section Pultrusions of Continuous
Fiber Reinforced Thermoplastics”, C22/86, Institution for Mechanical Engineers,
London.
CPIC® glass fiber, technical data sheet, (2005), published by Chongqing Polymer
International Corporation Ltd., China.
REFERENCES
193
De Larrard F, Schaller I, Fuches J., (1993), "Effect of bar diameter on the bond strength
of passive reinforcement in high-performance concrete”. ACI Mater J, 333-339.
Ehasni M.R., (1993), “Glass-fiber reinforcing bars”, In Alternative Materials for the
Reinforcement and Pre-stressing of Concrete, ed. J.L. Clarke, Blackie Academic and
Professional, London, pp. 35-54
F.J.G. Silva, F. Ferreira a, C. Costa a, M.C.S. Ribeiro b, A.C. Meira Castro A.C., (2012),
“Comparative study about heating systems for pultrusion process” ISEP – Instituto
Superior de Engenharia do Porto do Instituto Politécnico do Porto, Rua Dr. António
Bernardino, de Almeida, 431, 4200-072 Porto, Portugal, (Article in Press).
Francesco Foccaci, Antonio Nanni, Charles E. Bakis, (2000), “Local Bond-Slip
Relationship for FRP Reinforcement in Concrete”, Journal of Composites for
Construction, Vol. 4, No. 1, pp. 24-31
Francesco Micelli, Antonio Nanni, (2004), “Durability of FRP rods for concrete
structures”, Construction and Building Materials, Vol. 18, pp. 491–503.
Frederick T. Wallenberger, James C. Watson, and Hong Li, (2001), “Glass Fibers”, ASM
Handbook, Vol. 21, Composites (#06781G), ASM International, PPG Industries, Inc.
Hetron® and AropolTM, Resin Selection Guide, International Edition 2000, For Corrosion
Resistant FRP Applications, published by Ashland® Composite Polymers Division USA.
http://en.wikipedia.org/wiki/GFRP (accessed on 15.12.2010)
Hunay Y, Less LJ., (2001), “A kinetic model for free-radical cross linking
copolymerization of styrene/vinyl ester resin”, Polymer Composites, 22(5):668–79.
Intelligent Sensing for Innovative Structures (ISIS) Canada, (2003), “An Introduction to
FRP Composites for Construction”, ISIS Educational Module 2, University of Manitoba,
Winnipeg, pg. 5
Intelligent Sensing for Innovative Structures (ISIS) Canada, (2003), “An Introduction to
FRP Composites for Construction”, ISIS Educational Module 3, University of Manitoba,
Winnipeg, pg. 3
REFERENCES
194
J.A.D. Wilcox & D.T. Wright, (1998), “Towards pultrusion process optimization using
artificial neural networks”, Journal of Materials Processing Technology, Vol. 83, pp.
131–141
Joshi Sunil C, Lam YC., (January 2006), “Integrated approach for modelling cure and
crystallization kinetics of different polymers in 3D pultrusion simulation”. J Mater
Process Technol, 174:178–82
Kafeel Ahmed, (2009), “Bond strength of ultra high strength concrete at intersection of
beams”, PhD dissertation, pp. 193-203
Lomborg B, (2001), “The Skeptical Environmentalist”: Measuring the Real State of the
World, pg.138
Luyckx S.B., Sannino A., (1988), “Crack branching and fracture mirrors in cemented
tungsten carbide, Journal of Material Science”, Vol. 23, pp. 1243-1247
M.R. Ehsani and Saadatmanesh H., (March 1996), “Design Recommendations for Bond
of GFRP Rebars to Concrete”, Journal of Structural Engineering, Vol. 122, No. 3, pp.
247-254
Marta Baena, Lluis Torres, Albert Turron, Cristina Barris, (2009), “Experimental study of
bond behavior between concrete and FRP rebars using pullout tests”, Composites: Part B
Elsevier, Vol. 40, pp. 784-797
Mathieu Robert, B. Benmokrane, (2010), “Effect of aging on bond of GFRP bars
embedded in concrete”, Cement and Concrete Composites Elsevier, Vol. 32, pp. 461-467
Mathieu Robert, Brahim Benmokrane, (2009), ”Effect of aging on bond of GFRP bars
embedded in concrete”, Department of Civil Engineering, University of Sherbrooke,
Sherbrooke, Quebec, Canada, Issue. 40, pp. 784-797
Mazaheripour H., Barros J.A.O, Sena-Cruz J.M, Pepe M., Martinelli E., (2012),
“Experimental study on bond performance of GFRP bars in self-compacting steel fiber
reinforced concrete”, Composite Structure Elsevier, Article in Press
(http://dx.doi.org/10.1016/j.compstruct.2012.07.009)
REFERENCES
195
Moschiar SM, Reboredo MM, Kenny JM, Vazquez A., (June 1996), “Analysis of
pultrusion processing of composites of unsaturated polyester resin with glass fibers”,
Polymer Composites, 17(3): 478–85
Nanni A, Al-Zahrani MM, Al-Dulaijan SU, Bakis CE, Boothby TE., (1999), “Bond of
FRP reinforcement to concrete. Experimental results”. In: Taerwe L, editor. Proceedings
of the 2nd International RILEM Symposium _FRPRCS-2. London: E and FN Spon.,
pp.135-145.
Norwood H., (1983), “In contact with acidic environments—a case study of Composite
Structures”, 2(1):1–22
Qingduo Hao, Yanlei Wang, Jinping Ou, (2008), “Design recommendations for bond
between GFRP/steel wire composite rebars and concrete”, Engineering Structures
Elsevier, Vol. 30, pp. 3239-3246
Qingduo Hao, Yanlei Wang, Jinping Ou, Zheng He. (2009), “Bond strength of glass fiber
reinforced polymer ribbed rebars in normal strength concrete”, Construction and Building
Materials Elsevier, Vol. 23, pp. 865-871
Riaz A Goraya, Ahmed K and M. Akram Tahir, (2010), “Effect of surface texture on
bond strength of GFRP rebars in concrete” Mehran University Research Journal of
Engineering and Technology, ISSN 0254-7821 Volume 30 (1), pp. 45-52.
Rixom MR, and NP Mailvaganam, (1984), “Chemical Admixtures for Concrete”, 2nd
edition, London, E. & FN Spon, pg. 50
Roman Okelo, (2007), “Realistic Bond Strength of FRP Rebars in NSC from Bond
Specimens”, Journal of Aerospace Engineering ASCE, Vol. 20, No. 3, pp. 133-140
Roman Okelo, Robert L. Yuan, P.E, (2005), “Bond Strength of Fiber Reinforced Polymer
Rebars in Normal Strength Concrete, Journal of Structural Engineering”, Vol. 9, No. 3,
pp. 203-213
Saleem M. and Tsubaki, T., (2010), “Multi-layer model for pull-out behavior of post-
installed anchors”, Proc. FRAMCOS-7, Fracture Mechanics of Concrete Structures,
AEDIFICATIO publishers, Germany, Vol. II, pp. 823-830
REFERENCES
196
Sarrionan diameter M, Mondrago I, Moschiar SM, Reboredo MM, Vazquez A., (February
2002), “Heat transfer for pultrusion of a modified acrylic/glass reinforced composite”,
Polymer Composites, 23(1): 21–7
Tastani S.P, Pantazopoulou S.J, (2006), “Bond of GFRP Bars in Concrete: Experimental
Study and Analytical Interpretation”, Journal of Composites for Construction ASCE, Vol.
10, No. 5, pp. 381-391
Ueda T, (2005), “FRP for construction in Japan”, JSCE-CICHE Joint Seminar on
Concrete Engineering in Mangolia, Ulan Batar, Mangolia, pp. 54-68.
Vanderley M. John, Escola Politécnica, (2003), “On the Sustainability of the Concrete”,
Paper commissioned by the UNEP Journal Industry and Environment, University of São
Paulo, Brazil, Ed. Engenharia Civil, Cidade Universitária São Paulo, 05508 900 Brazil.
Vaughan J. G. and Dillard. T. W., (1990), “A characterization of the important parameters
for graphite/PEEK pultrusion”, Journal of Thermoplastic Composite Materials., 3, I31.
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.
Rebar Diameter db (mm)
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
Rebar Diameter db (mm)
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
Rebar Diameter db (mm)
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)
Experimental Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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
of Proposed Pullout Bond Model.
Sr. No.
Rebar Dia.
db (mm)
Clear Cover
C (mm)
Bonded Length
Lb (mm)
Concrete Strength
fc’ (MPa)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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
of Proposed Pullout Bond Model.
Sr. No.
Rebar Dia.
db (mm)
Clear Cover
C (mm)
Bonded Length
Lb (mm)
Concrete Strength
fc’ (MPa)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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
of Proposed Pullout Bond Model.
Sr. No.
Rebar Dia.
db(mm)
Clear Cover
C (mm)
Bonded Length
Lb (mm)
Concrete Strength
fc’ (MPa)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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)
Experimental Bond Stress
u (MPa)
Predicted Bond Stress
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.
Top Related