a study on sustainable and eco-friendly fiber reinforced
TRANSCRIPT
THESIS
A STUDY ON SUSTAINABLE AND ECO-FRIENDLY FIBER
REINFORCED POLYMER COMPOSITE FOR
STRENGTHENING OF CONCRETE MEMBERS
NAUMAN WAHAB
GRADUATE SCHOOL, KASETSART UNIVERSITY
Academic Year 2019
THESIS APPROVAL
GRADUATE SCHOOL, KASETSART UNIVERSITY
DEGREE:
Master of Engineering (Sustainable Energy and Resources
Engineering)
MAJOR FIELD: Sustainable Energy and Resources Engineering
FACULTY: Engineering
TITLE: A Study on Sustainable and Eco-friendly Fiber Reinforced Polymer
Composite for Strengthening of Concrete Members
NAME: MR. NAUMAN WAHAB
THIS THESIS HAS BEEN ACCEPTED BY
(Associate Professor Penjit Srinophakun, Ph.D.)
THESIS ADVISOR
(Associate Professor Thongchai Rohitatisha Srinophakun,
Ph.D.)
GRADUATE COMMITTEE
CHAIRMAN
APPROVED BY THE GRADUATE SCHOOL ON
(Associate Professor Somwang Khantayanuwong, Ph.D.)
DEAN
THESIS
A STUDY ON SUSTAINABLE AND ECO-FRIENDLY FIBER REINFORCED
POLYMER COMPOSITE FOR STRENGTHENING OF CONCRETE MEMBERS
NAUMAN WAHAB
A Thesis Submitted in Partial Fulfillment of
the Requirements for the Degree of
Master of Engineering (Sustainable Energy and Resources Engineering)
Graduate School, Kasetsart University
Academic Year 2019
C
ABST RACT
NAUMAN WAHAB : A Study on Sustainable and Eco-friendly Fiber Reinforced
Polymer Composite for Strengthening of Concrete Members . Master of
Engineering (Sustainable Energy and Resources Engineering), Major Field:
Sustainable Energy and Resources Engineering, Faculty of Engineering. Thesis
Advisor: Associate Professor Penjit Srinophakun, Ph.D. Academic Year 2019
The strengthening and rehabilitation of concrete members is an
important issue which arises worldwide. Carbon, aramid and glass fiber reinforced
polymer (FRP) composites are mainly used for strengthening and rehabilitation.
However, its use is limited on a small scale because of its high price, lack of
availability and environmental impacts. The solution of this issue gives rise to the
use of locally available natural fibers and low-cost synthetic fibers. This paper
presents the experimental and analytical results of circular and square concrete
columns confined with juteβpolyester hybrid FRP composites. The main objective
of this study is to evaluate the viability and performance of concrete confined with
the hybridization of jute and polyester (FRP) composite sheets to utilize its superior
properties. A novel hybrid technique has been applied for the wrapping of fiber
sheets. The fiber sheets were applied in such a way that a uniform bond between the
inner and outer layer was achieved. A total of 48 plain, standard size circular and
square concrete specimens, externally wrapped with a juteβpolyester FRP (JPFRP)
composite, were tested under monotonic axial compressive loads. The result shows
that JPFRP confinement increased the strength, strain and ductility index ranged
between 1.24 and 2.61, 1.38 and 8.97, and 4.94 and 26.5 times the un-jacketed
specimen, respectively. Furthermore, the wrapping has a significant effect on the
low-strength specimens, having a circular cross-section. For high strength
specimens, the post-peak stress-strain behaviour was dominated by the outer
polyester jacket because of its large rupture strain. Additionally, the test results
were used to evaluate the existing strength-strain models derived for conventional
FRPs. The models predicted values either underestimated or overestimated the
compressive strength and strain of JPFRP-confined specimens. However, the
strength models performed better than the strain models. The JPFRP wrapping
significantly enhanced the strength, fracture energy, ductility index, and post-peak
response. Therefore, JPFRP confinement can be used for a small-scale application,
where little strength and high ductility is demanded. Moreover, it can be used to
prevent the peeling of the concrete cover and moisture penetration into the concrete.
_________________ _______________________________ ____ / ____ / ____
Student's signature Thesis Advisor's signature
D
ACKNOWLEDGEMENT S
ACKNOWLEDGEMENTS
First of all, I would like to express my heartiest gratitude to God Almighty,
who is the most merciful and kind, without whom it would be difficult for me to finish
this study.
I would like to express my profound and sincere gratitude to my advisor Assoc
Prof. Dr Penjit Srinophakun, for her encouraging guidance and valuable suggestion
throughout my study and related research. I am also grateful to Dr Qudeer Hussain for
his help in experimental work. I am equally indebted to Assoc. Prof Dr Tongchai
Srinophakun for his administrative support in providing scholarship and valuable
comments on my thesis whenever I approached him.
The author would like to thank his family members for their encouragement
and continuous support during the whole study period. The source of encouragement
and moral support for the study solely come from my respected father Prof. Dr Said
Wahab, whom I consider my source of motivation behind my success. Besides, I would
like to mention my brother M. Aamir Wahab for his support.
Last but not least, I would like to extend my gratitude to my seniors for their
support, guidance and motivation throughout my study as well as my stay in Thailand.
Thanks are also extended to TAIST-Tokyo Tech and Faculty of Engineering,
Kasetsart University for providing scholarship.
I dedicate this work to my loving parents, sisters and brother. Their endless
love, motivation and encouragement helped me to complete this study.
NAUMAN WAHAB
TABLE OF CONTENTS
Page
ABSTRACT .................................................................................................................. C
ACKNOWLEDGEMENTS .......................................................................................... D
TABLE OF CONTENTS .............................................................................................. E
List of Tables ................................................................................................................ G
List of Figures ............................................................................................................... H
INTRODUCTION ......................................................................................................... 1
OBJECTIVES ................................................................................................................ 6
LITERATURE REVIEW .............................................................................................. 7
2.1 General Strengthening Methods .......................................................................... 8
2.1.1 Reinforced Concrete Jacketing ................................................................... 9
2.1.2 Steel Jacketing .......................................................................................... 10
2.1.3 Fiber Reinforced Polymers ....................................................................... 12
MATERIALS AND METHODS ................................................................................. 22
3.1 Specimen Design ................................................................................................... 22
3.2. Material Properties ............................................................................................ 24
3.2.1. Concrete ................................................................................................... 24
3.2.2. Epoxy Resin ............................................................................................ 24
3.2.3. Jute Fiber ................................................................................................. 25
3.2.4. Polyester Fiber ......................................................................................... 26
3.2.5. Hybrid FRP Wrapping ............................................................................. 28
3.3. Instrumentation ................................................................................................. 31
3.4. Test Procedure .................................................................................................. 32
Results and Discussions ............................................................................................... 34
4.1 Failure Modes .................................................................................................... 34
4.1.1. Circular Columns .................................................................................... 34
F
4.1.2. Square Columns ...................................................................................... 36
4.2. Stress-Strain Response ...................................................................................... 37
4.2.1. Circular Concrete Specimens .................................................................. 41
4.2.2. Square Concrete Specimens .................................................................... 42
4.3. Ductility and Fracture Energy of the JPFRP-Confined Specimens .................. 44
4.4. Review of Strength and Strain Models ............................................................. 47
4.4.1. Existing Strength Models ........................................................................ 48
4.4.2. Existing Strain Models ............................................................................ 50
4.5. Evaluation of Strength and Strain Models ........................................................ 50
CONCLUSIONS.......................................................................................................... 64
LITERATURE CITED ................................................................................................ 66
CURRICULUM VITAE .............................................................................................. 71
List of Tables
Page
Table 1. Details of test specimens................................................................................ 23
Table 2. Concrete mix proportion (kg per cubic meter). ............................................. 24
Table 3. Mechanical properties of epoxy resin. ........................................................... 25
Table 4. Mechanical properties of the jute fiber reinforced polymer (JFRP). ............. 26
Table 5. Mechanical properties of the polyester FRP. ................................................. 27
Table 6. Average values of tested specimens. ............................................................. 39
Table 7. Average values of all tested specimens. ........................................................ 46
Table 8. Existing strength and strain models for FRP-confined circular specimens. .. 51
Table 9. Existing strength and strain models for FRP-confined square specimens. .... 52
Table 10. Comparison of model predictions with experimental results of circular
specimens. .................................................................................................................... 57
Table 11. Comparison of model predictions with experimental results of square
specimens. .................................................................................................................... 58
Table 12. Statistical performance of the strength model vs. experimental data of
circular specimens. ....................................................................................................... 62
Table 13. Statistical performance of the strength model vs. experimental data of
square specimens. ........................................................................................................ 62
List of Figures
Page
Figure 1. Strengthened RC columns after casting and jacketing ................................ 11
Figure 2. The typical mode of failure of tested SFRP confined circular specimen .... 16
Figure 3. Sheet of Glass Chopped Strand Mats Fiber. ................................................ 17
Figure 4. Beam confined by GCSM Sheet.................................................................. 17
Figure 5. Concrete specimen confined by vinylon rope with two steel collars at top
and bottom. .................................................................................................................. 20
Figure 6. Concrete cylinder confined by Polypropylene rope after applying axial
loading.......................................................................................................................... 21
Figure 7. Dimensions of circular and square specimens. ............................................ 23
Figure 8. Stress-strain curve of the jute FRP composite. ............................................ 26
Figure 9. Stress-strain behavior of the polyester FRP composite. .............................. 27
Figure 10. Specifications of the FRP flat coupon for tensile testing .......................... 28
Figure 11. The stress-strain curve of the juteβpolyester fiber reinforced polymer
(JPFRP) hybrid composite. .......................................................................................... 29
Figure 12. The M500-50-AT computer-controlled UTM. ......................................... 30
Figure 13. Hybrid Fiber wrapping configuration of circular specimens .................... 30
Figure 14. Hybrid fiber wrapping configuration of square specimens ...................... 31
Figure 15. Instrumentation of Concrete specimen ...................................................... 32
Figure 16. Test setup for concrete specimens. ............................................................ 33
Figure 17. Failure modes of low- and high-strength circular columns. ...................... 35
Figure 18. Failure modes of low- and high-strength square columns......................... 37
Figure 19. Stress-strain curve behavior of confined columns: (a) Low-strength
circular specimens; (b) low-strength square specimens; (c) high-strength circular
specimens; and (d) high-strength square specimens. ................................................... 41
Figure 20. Effect of JPFRP layers on (a) strength enhancement and (b) strain
enhancement. ............................................................................................................... 44
Figure 21. The area under the load-displacement curve obtained by OriginPro. ....... 46
I
Figure 22. Performance of existing strength and strain models: (a) Strength models of
circular specimens; (b) strain models of circular specimens; (c) strength models of
square specimens; and (d) strain models of square specimens. ................................... 61
1
A STUDY ON SUSTAINABLE AND ECO-FRIENDLY FIBER
REINFORCED POLYMER COMPOSITE FOR THE
STRENGTHENING OF CONCRETE MEMBERS
INTRODUCTION
Reinforced concrete structures may experience structural deficiencies and
damages from the start of the construction stage until the end of its service life.
During the construction stage, the structure may experience structural defects because
of the structural design errors, low-quality concrete production, and bad workmanship
in execution processes. Moreover, in its service life, damages can be attributed to
various reasons such as cracking and peeling of the concrete cover, occurrence of an
earthquake, increase in live loads, or an accident, for example, fire and explosions [1].
Besides, corrosion of steel reinforcement and carbonation of concrete are also some of
the problems which arise worldwide depending on the exposure of the structure to its
external environment [2]. Hence, to maintain the structural integrity and safety
requirements, the damaged and structurally deficient buildings need to be
strengthened or rebuilt. However, demolition and rebuilding of a structure are not
considered an economically viable solution because of its high cost, environmental
impacts, and low time efficiency [2]. Therefore, structural strengthening and repairing
of a deficient reinforced concrete (RC) structure is a preferable option in terms of
economic and environmental aspects. In the past, several techniques were employed
for the up-gradation of reinforced concrete structures to sustain its structural integrity
and to ensure the serviceability. Reinforced concrete jacketing and steel jacketing
have been used to repair and upgrade the structural properties of the reinforced
concrete structures. Reinforced concrete jacketing can enhance the compressive
strength and ductility of the structure. However, this method is labor-intensive, time-
consuming, and sometimes in situ implementation is difficult [3]. Besides, corrosion
of internal steel may occur due to prolonged exposure to moisture, and penetration of
chlorides can also result in concrete cracking and spalling. Hence, such methods are
considered appropriate in an environment where the corrosion intensity is under
2
acceptable limits. Therefore, an advanced, easy to install, and durable strengthening
system is needed to replace out of date methods.
To replace the old strengthening methods, extensive research was carried out
for finding an alternative material for strengthening purpose. As a result, fiber-
reinforced polymer (FRP) composites have emerged as a potentially promising
material for rehabilitation and strengthening of deficient reinforced concrete structural
elements, particularly RC columns, which essentially supports the entire bridge
structures and buildings. The most attractive features of FRPs are the ease of
installation, non-corrodibility, high tensile strength, high versatility, and high-
strength-to-weight ratio. These remarkable properties of FRPs make them one of the
most predominant materials for structural rehabilitation. Many experimental studies
reported that external wrapping by FRP composite could remarkably increase the
load-carrying ability of circular, square, and rectangular concrete columns [4-6].
Different types of FRPs have been used for the lateral confinement of concrete
columns, which includes carbon [7-13], glass [6, 14-17], aramid [5, 18-20],
polyethylene napthalate (PEN) [21, 22], polyethylene terephthalate (PET) [21-25],
and basalt [26-28]. These FRP composites are effectively used in enhancing the load-
bearing capacity and energy absorption of masonry and reinforced concrete
infrastructures. In Japan, carbon fiber has been successfully utilized in strengthening
and rehabilitation of columns, freeway piers, and chimneys [29]. Jing et al. [30]
summarised that carbon and glass FRP jacketing is very effective in improving the
ultimate compressive strength and axial strain of concrete columns with circular
cross-sections. Due to its linear elastic nature, the lateral confinement system by FRPs
has been proved very successful by enhancing the compressive strength and axial
strain of concrete members without a substantial increase in its cross-section and
weight. Despite its advantages, currently, the wider application of C/GFRP is
restricted by their high cost, lack of enough performance-based long-term data
availability, fire risk, and environmental impacts [21-25].
3
Therefore, nowadays, efforts have been made to replace the conventional
composite materials, i.e., C/GFRP, with alternate materials for civil infrastructure
applications, to gain economic and environmental benefits. Studies were conducted on
the use of economical and environmentally friendly fibers including hemp [31, 32],
sisal [33-35], jute [33, 34, 36], coir, and flax [37, 38]. Previous studies exemplify that
natural fibers have high potential in the reinforcement of the cement matrix [33β40].
In comparison with conventional C/GFRP, natural fibers have a low density, low to
moderate tensile properties, low cost, easy availability, and non-abrasive nature.
Ghalieh et al. [31] carried out experimental testing to study the performance of
circular concrete specimens wrapped with a hemp fiber reinforced polymer jacket and
concluded that hemp fiber could increase the axial strength and ductility of jacketed
specimens. In another study, Sen and Reddy [34] highlighted that jute and sisal fiber
could significantly enhance the load-bearing capacity as well as energy absorption of
concrete columns. Pimanmas et al. [33] investigated the ultimate axial strength and
deformability of small-scale low and high-strength circular and square concrete
specimens confined with locally manufactured sisal fiber. The study revealed that the
enhancement in axial compressive strength and the axial strain was more significant
in low strength circular concrete columns as compared with square columns.
Consequently, the use of natural fibers can be considered a very workable option
towards sustainable development in the construction industry. However, natural fibers
have some drawbacks such as the low strength and durability compared with the
synthetic fibers. In order to solve this problem, a few experimental studies were
performed on hybridization of two or more different types of FRP sheets [30, 39-43].
These studies have shown that upon hybridization the superior features of both the
FRP sheets are utilized. Very limited published literature is available on the
hybridization of natural fibers with synthetic fibers. Ramnath et al. [44] and Ramesh
et al. [45] studied the hybridization of natural and artificial fibers in different
combinations and configurations, such as Sisal GFRP, AbacaβJute GFRP, SisalβJute
GFRP, and Jute GFRP composite systems. From the obtained test results, the authors
concluded that the hybrid composites showed superior mechanical properties than
4
individual FRP composites and can be utilized as a replacement material for the
GFRP composite system. Padanattil et al. [46] experimentally examined the behavior
of circular concrete specimens confined by Sisal GFRP hybrid sheets. The authors
compared the behavior of hybrid confinement with GFRP and CFRP confined
concrete. They inferred that the hybrid configuration of natural and synthetic fibers
has the potential of replacing the high-cost carbon fibers in retrofitting applications of
civil infrastructure.
A better understanding is needed for the hybridization of different types of
natural and synthetic fibers, such as a jute and polyester fiber-reinforced polymer
composite. Jute is a low-cost natural fiber occurring in abundance in many countries,
having low to moderate mechanical properties. Sen and Paul [34] studied the axial
compressive behavior and energy absorption ability of jute FRP (JFRP) wrapped
concrete columns. They stated that lateral jacketing by JFRP could enhance the
compressive strength and energy absorption of concrete members. On the other hand,
polyester fiber, which belongs to the category of synthetic fibers has a low price, large
rupture strain, water-resistant nature, and non-corrodibility. Huang et al. [47]
conducted an experimental investigation to oversee the confinement effects of a low
cost, high ductile polyester sheet. The authors found that a polyester-FRP has
significantly improved the deformability of the standard confined specimens, whereas
a slight improvement was observed in the compressive strength.
In comparison with fibers like nylon, sisal, jute, and flax, polyester fiber has
well-shaped preservation, lower relative density, and better fire resistance. Thus,
using polyester as hybrid fiber with jute will overcome the deficiencies of the natural
fiber, as there are synergies of the superior features of the two-hybrid fibers. Because
of the competitive price and good mechanical characteristics, a juteβpolyester FRP
(JPFRP) hybrid confinement can be used in the strengthening of structures. To the
best of our knowledge, no published literature about the behavior of circular and
square concrete short columns confined with a juteβpolyester hybrid FRP is available.
5
We hypothesized that the hybrid-confinement enhance the structural performance of
the concrete. Hence, the objective of this research work is to investigate the viability
and performance of the juteβpolyester FRP as an external wrapping material for
circular and square plain concrete columns under uniaxial compressive loading. The
effects of JPFRP confinement on strength, ductility, the stress-strain response, and
energy absorption are also investigated. The test parameters considered in this study
are the cross-sectional shape, concrete strength, and a number of composite layers.
Moreover, the test results are used to evaluate the efficiency of the existing strength
and strain models, established for conventional fiber-reinforced polymer composites.
6
OBJECTIVES
The aim of this research work is to develop a low cost, easily available, and
sustainable strengthening technique by using Jute fiber sheet in combination with
polyester fiber sheet.
The present study is mainly focused on:
β’ The investigation of the compressive behavior of low strength small-scale
circular and non-circular concrete columns confined with the hybridization of
Jute and Polyester FRP composites.
β’ The investigation of the compressive behavior of high strength small-scale
circular and non-circular concrete columns confined with the Jute and
Polyester hybrid FRP composites.
β’ The evaluation of the applicability of the existing stress-strain models for the
Jute-Polyester Hybrid FRP confinement.
7
LITERATURE REVIEW
The worldβs infrastructure consists of a comprehensive variety of structures,
built over many years as well as from a diverse range of materials. To use such types
of structures continuously, they might be needing strengthening to overcome the
deficiency in their structural strength. There can be many reasons for the structural
deficiency of the structures such as the structure shall need modification in load-
carrying capacity. Therefore, it can bear different types of loads other than the
originally designed or the structure might be degraded due to different environmental
conditions or due to aging or it might need repairing/retrofitting against different
earthquake loads. Hollaway and Teng defined the three most frequently used
terminologies that are repaired, strengthening and retrofitting. These three terms are
often mistakenly interchanged, but they define three diverse structural conditions. The
term repairing is used to define the functional improvement of a structure or structural
member. While the term strengthening refers to the enhancement in the current design
by the addition or application of the composite material. Whereas the word retrofit is
referred to the up-gradation of the seismic load-carrying ability of the structural
components, such as columns, etc.
Demolishing and dismantling the existing structures will cost a huge amount
of money and it cannot be considered an economical option. Avoiding reconstruction
and taking the best use out of the old structures, is the need of the time. During the
19th century, there were no building codes or seismic codes, and the structures were
built without taking care of the durability issues. Now the old aged structures need to
be strengthened rather than demolition. Another need for the repairing, strengthening
and retrofitting of structures is due to the climate change, many disasters have
occurred in the near past throughout the world. These disasters are in the shape of
Floods, Tsunamis, Earthquakes, and cyclones. The effects of these disasters have
compelled the researchers to work intensively on the strengthening, repairing and
retrofitting techniques. The strengthening, repairing and retrofitting will also help us
8
in the conservation of the resources as well as the environment. A very massive
amount of carbon dioxide (CO2) is produced in the production of cement and to
overcome the carbon footprint we need to control the use of cement, which is the
major component of the cement industry. The world is moving towards sustainability
for which we need to use our resources wisely and carefully.
2.1 General Strengthening Methods
The researchers had developed many strengthening techniques. Some
conventional methods include concrete jacketing and steel jacketing. These traditional
methods of strengthening are cheap, but they pose some disadvantages, such as they
are susceptible to corrosion, and chloride attack, as well as these techniques, are very
labor-intensive. To overcome the weaknesses of the conservative methods of
strengthening, repair and retrofitting, the researchers have developed some modern
methods and materials such as fiber-reinforced polymers (FRP). The Fiber Reinforced
Polymer (FRP) composites possess excellent mechanical properties which make them
compelling, unique and practical substitute of the conventional methods. Some of
their essential features include Impact resistance, high Strength Stiffness, flexibility
and ability to carry loads. Currently, there are different types of FRPβs being used in
the construction industry. Carbon FRP, Glass FRP, Aramid FRP are the most
common and widely used in the strengthening. Although these fibers have many
benefits, they also have some cons, as they are expensive, and the availability varies
from country to country. FRP external confinement can provide high strength and
ductility to the confined structure, thus enhance the structural stability and strength.
Natural fiber-reinforced polymers are receiving consideration from researchers
and academician due to their eco-friendly nature and sustainability. Experimental
studies about the use of Natural Fiber Reinforced Polymer Composites in a
strengthening of concrete structures show satisfactory results in enhancing the
strength and ductility, by the external confinement of NFRPs. The general application
9
of NFPCs is due to its low specific weight, resistance to corrosion, fairly high strength
and reasonably low production cost.
To bring some innovation in the field of strengthening, the researchers are
working on different material and different techniques. Limited research data is
available on the use of the rope. Ropes are being used for decades in different
applications such as in anchoring big ships, construction industry and other types of
heavy work. Rope can be made of synthetic fibers as well as natural fibers. Ropes are
cheap and easily available. The synthetic fiber ropes are stronger than natural fiber
ropes; however, NFR is environmentally friendly. There are numerous varieties of
ropes available such as hemp, sisal, jute, cotton, aramid, polypropylene, polyester, etc.
The researchers have done extensive research work on different types of
strengthening techniques, which includes the strengthening of concrete structures by
RC jacketing, Steel Jacketing, CFRP, GFRP, AFRP, BFRP, and PET-FRP. According
to researchers, the RC jacketing can enhance the structural strength, it can also
improve the ductility of the buildings. The reinforced concrete jacketing is cheap but
it is very labor-intensive and time-consuming [48]. Ferrocement Jacketing can also
enhance the strength of the concrete structures, especially columns. Different types of
steel plates and metal strips were also used for strengthening and repair. These metal
strips can improve the ability of the structure to take more load, so the strength is
increased along with the ductility of the concrete. Some researchers have already
tested different types of fibers in a hybrid configuration. The hybridization enhances
the strength and ductility of the confined concrete. In a hybrid configuration, the
superior properties of both the fibers are utilized due to the synergy between them.
2.1.1 Reinforced Concrete Jacketing
Rodriguez and Park [48] carried out experimental testing on rectangular
columns repaired and strengthened by reinforced concrete jackets. Four reinforced
10
concrete columns were tested under the compressive axial loading as well as lateral
loading to investigate repair and strengthening technique. Two arrangements of
transverse reinforcement in the jacket were investigated. Rebar hoops are added as the
retrofit reinforcement for the concrete jackets. Two as-built columns were first
exposed to seismic loading, which showed low ductility, as well as significant
degradation of strength, is observed, whereas the jacketed column behaved in a
ductile manner, with higher strength. After jacketing the strength and stiffness of the
as-built (un-retrofitted) columns is increased about three times. It is also shown that
damage before the retrofit has no substantial impact on the performance of the
jacketed columns. The results of this experiment also show that generally, concrete
jackets with reinforcement bars significantly improve strength, ductility, and stiffness
of typical reinforced concrete columns, however, the method is a very labor-intensive
job and sometimes on-site implementation is difficult.
Julio and Branco [49] carried out experimental research, to evaluate the effects
of the interface curing on the structural behavior of the reinforced concrete columns
strengthened with RC jackets under monotonic loading. The experimental program
consists of seven full-scale columns having a cross-section of 0.20 x 0.20 m2 and a
height of 1.35 m. The thickness and height of the jacket were 0.35m and 0.90 m,
respectively. The concrete compressive strength and description of models of all the
seven columns are mentioned in Table 1. All the concrete columns were tested under
monotonic axial compression load. From the experimental results, it has been
concluded that the behavior of all the models was monolithic independent of the
adopted interface preparation technique. Strengthened columns show significantly
higher resistance than that of the original column and marginally higher than that of
the monolithic column.
2.1.2 Steel Jacketing
Belal et al., (2015) carried out an experimental investigation to observe the
behavior and failure load of Reinforced Concrete columns strengthened by steel
11
jacketing technique. The author considered three variables (using angles, C-sections
and plates) and evaluated the effect of these parameters on the strengthened columns.
Seven specimens were examined in the experiment, which was separated into two
groups, five specimens were strengthened, having different steel jacket configuration,
as shown in Figure 1. From the experimental results, it was concluded that steel
jacketing could expand the strength capacity of the columns by 20%. The control
specimens showed a brittle mode of failure while the failure behavior of the jacketed
columns was more ductile. Failure load was high for column confined with steel
angles and channel section with battens plates as compared with steel plates.
Figure 1. Strengthened RC columns after casting and jacketing
Source: Belal et al. (2015)
Frangou et al., [50] conducted experimental research on reinforced concrete
columns confined by metal strips. The purpose of the study was to observe the effects
of the metal confinement on the strength of the columns as well as ductility. Column
specimens were confined externally by metal strips. The width was 12.7 mm while
thickness was kept 0.5 mm. A standard strapping machine was used to create post-
tension metal strips. The strips were tensioned to just below their yield stress. The
specimens were tested under axial loading and bending. The experimental results
12
show that the confinement of the columns by metal strips is a useful technique in
increasing the ductility, stiffness, and strength of the members under axial and
bending load. The post-tensioning of metal strips enhanced the strength and improved
the effectiveness of this technique. The low cost of material and ease of use make this
strengthening/repair technique very competitive.
Amulya and Kumar [51] used steel angle section for the retrofitting of
reinforced concrete columns. The RC column was confined by steel angle section
welded with each other forming a cage. Six RC columns were used during the
experimental investigation. Three as control columns while the other three were
confined by angular steel section. The columns behavior was observed under
monotonic axial compression load. It was concluded that the steel jacketing could
upgrade the columns load-carrying ability by 20%. The failure behavior of RC
columns confined by steel angle section was more ductile than the normal specimen.
The increase in the strength and ductility was observed to be significant due to the
confinement by steel angular section.
2.1.3 Fiber Reinforced Polymers
Saleem et al., [25] studied the compressive behavior of circular and non-
circular concrete confined with Polyethylene Terephthalate (PET) FRP. A total of 54
specimens were tested under monotonic axial compression loading. The experimental
results showed that the axial strength and ductility of the concrete specimens is
significantly improved by PET FRP wrapping. Furthermore, the wrapping was more
effective in circular specimens as compared with square and rectangular specimens.
Suon et al., [28] carried out experimental testing of circular and non-circular
concrete columns confined with Basalt FRP. The tested parameters included were the
cross-section shape, no of FRP plies and corner radius. A total of 56 specimens were
tested under the effect of axial compressive loading. The test results indicated that the
13
increase in compressive strength of circular specimens was more significant.
Moreover, a remarkable increase in strength was recorded, when the number of FRP
layers was increased from 3 to 9. In addition, the experimental results were used to
assess the performance of existing stress-strain models. Out of seven models, only the
models of Shehata et al. predicted the stress-strain values of BFRP relatively closer to
the experimental results.
Mourad et al., [52] conducted an experimental investigation to note the
durability and mechanical properties of the two types of composite material that is
glass/epoxy and glass/polyurethane which was exposed to the marine atmosphere, at
different atmospheric temperatures. The experimental parameters include the
exposure duration, absorption time and the permeability of the different types of salts
and other contaminants present in the seawater. The effects of the mentioned
parameter on the behavior of a physical and chemical bond between the composite
materials were observed. The specimen was immersed in saline water, which ranges
from three months to one year. Two different types of temperatures were used for the
experimental investigation that is room temperature and a higher temperature of 65
degree Celsius. Due to longer immersion time and high temperature, the absorption
rate of seawater increased. In both specimens, the composite played a vital role in
keeping the fiber protected from the corrosive components present in the seawater.
The experimental results concluded that the higher temperature and longer emersion
time caused the excessive degradation of the glass/polyurethane. Furthermore, the
tensile properties of the composite material were greatly affected by the high
temperature and exposure time; the fiber and matrix failed in a brittle mode.
Alsaad and Hassan [8] investigated the environmental effects on the reinforced
concrete columns externally confined by carbon fiber reinforced polymer (CFRP)
composites. The main parameters included in the research work is the exposure time
of the RC columns in the crude oil and seawater. The experimental work consists of
the testing of 32 circular-shaped specimens wrapped by CFRP sheets having a
14
diameter of 150 mm and a height of 550 mm. Both the control specimen and CFRP
confined specimen were exposed to the marine environment for a short period. The
specimens were loaded under axial monotonic compression till failure. The
experimental observations of the CFRP wrapped RC cylinders show that there were
no substantial effects due to the exposure to seawater and crude oil on the ultimate
loading ability. However, lateral strain and axial displacement were significantly
reduced. Overall the immersion has a negative impact on the ductility of the
specimens.
The harsh environment such as seaside areas, hot and cold regions can cause
weathering and ageing of concrete structures as well as it causes the corrosion of the
steel reinforcement, which results in the deterioration of concrete structures. El-Hacha
et al., [53] conducted an experimental study to understand the efficiency of the plain
concrete cylinders confined by carbon fiber reinforced composite, exposed to the
extreme varying environment. The diverse environment includes the exposure to high
temperature, i.e. 45 degree Celsius and low temperature, i.e. 23 degree Celsius.
Cycles of freezing and thawing are also included in environmental exposure. The
experimental program consists of casting 36 concrete cylinders of standard size (150
X 300 mm), wrapped with two layers of unidirectional CFRP sheets. The specimens
were tested under monotonic axial compression load until failure. The results indicate
that the strength of the CFRP confined do not decrease with higher temperature.
Kumutha et al., [17] studied three different aspect ratios of rectangular-shaped
reinforced concrete column confined externally by Glass Fiber Reinforced
Composites (GFRP). A total of nine reinforced concrete cylinder were externally
wrapped by GFRP sheet with the help of resin and were tested under concentric
compression loading. It was concluded from the experimental results, that the
compressive strength of the concrete columns could be enhanced by the use of
effective confinement with GFRP sheets. The increase in the number of GFRP sheets
results in better confinement; therefore, caused the improvement in the loading
15
capacity of the column, as well as ductility enhancement, was also achieved. The
variation in the aspect ratios affect the loading capability of the column, and the
capacity of the column was reduced with the rise in the cross-sectional aspect ratio.
Hussain et al., [54] investigated the performance of externally bonded concrete
cylinders confined externally by sprayed fiber-reinforced polymer (SFRP) to increase
the compressive strength and ductility. The experimental work includes the testing of
a total of 24 specimens, 12 circular and the same number of square columns. A better
bond between SFRP and concrete was achieved by the scratching of the concrete
surface. Both SRFP and resin was applied at the same time onto the surface of the
concrete specimens. SRFP was applied in different thickness that is, 3, 6 and 9mm
while the length is 13 and 26mm. All the specimens were tested under uniaxial
monotonic loading by UTM. The typical failure mode of the specimens is shown in
Figure 2. From the test results, it was observed that for both square and circular
columns, the ultimate strength and strain was improved by the effective confinement
of SRFP. For circular columns the recorded value of axial strain and compressive
strength was greater; however, the compressive stress and strain of square columns
are generally less than circular columns. The experimental results acquired, were used
to assess the performance of present strengthening models established for
unidirectional carbon, glass, and aramid FRP fibers.
In general, the compressive strengths computed from the suggested equations
are found relatively proximate to the experimental results. Lower values of
compressive strength were produced by using the suggested model, which is lower
than the values acquired from the experimental data, as a result, the models which
were proposed was conservative.
16
Figure 2. The typical mode of failure of tested SFRP confined circular specimen
Source: Hussain et al., [54]
Muthuramu et al., [55] used Glass Chopped Strand Mats (GCSM) sheets for
the external confinement of the reinforced concrete beam. The primary purpose of this
research work was to analyses different modes of failure and to observe the flexure
strengthening performance of the different layers of GCSM. For the experimental
testing, a rectangular reinforced concrete beam having a cross-section of 150 x 200
and length of 2200 mm was cast. The GCSM sheet and the confined beam is shown in
Figure 3 and 4, respectively. The beam was subjected to a bending load. The
experimental results conclude that the GCSM performed very well up to two layers,
and no de-bonding occurs between beam surface and GCSM sheet. The glass fiber
confined beam shows a significant increase in the flexure strength and the load-
carrying ability of the beam. Pre-mature failure occurred due to de-bonding of the
sheets when it was increased from two layers.
17
Figure 3. Sheet of Glass Chopped Strand Mats Fiber.
Figure 4. Beam confined by GCSM Sheet.
Source: Muthuramu et al., [55]
Ispir [24] conducted an experimental study to examine the compressive
behavior of concrete specimens externally confined with PET-FRP, under the effect
of monotonic axial and cyclic compression load. The experimental program includes
the casting of fourteen standard size cylinders. The specimens were examined under
the effect of compression loading. Three different parameters were studied in the
experiment, which includes the loading type that is cyclic and monotonic and the
thickness of the PET-FRP layers. From the tests results and visual observations, it was
concluded that the confinement of concrete specimens with PET-FRP was an
effective method in increasing the compressive strength capacity and the ultimate
axial strain. The PET FRP can be termed as a suitable alternative material for the
seismic retrofitting because of its higher deformation capacity. The stress-strain
18
behavior of PET FRP is somehow identical to other types of FRPβs material used for
the confinement of concrete.
Sen and Reddy [56] conducted experimental research to evaluate the
mechanical characteristics of natural woven jute fiber-reinforced polymer (FRP)
composite exposed to three different pre-treatments, alkali, benzyl chloride, and
finally heat treatment. The experimental results show that the mechanical properties
of jute FRP were significantly improved by the heat treatment as it was the most
suitable technique. The tensile strength of jute FRP was also analyzed under common
environmental effects such as the effect of normal water and thermal ageing. The
experimental results show that the woven jute FRP can significantly enhance the
reinforced concrete (RC) beamβs flexural strength.
Pimanmas et al. [33] studied the axial behavior of concrete confined with sisal
fiber-reinforced polymer composites. The test parameters include concrete strength,
cross-sectional shape, and the number of FRP plies. A total of 24 specimens were
tested under monotonic axial loadings. The test results showed that sisal wrapping has
significantly increased the compressive strength and ductility of the concrete
specimens. Low strength circular columns showed a more considerable increase in the
axial strength and deformation. Moreover, the test results were used to assess the
workability of the existing strength and strain models. None of the existing strength
and strain models predicted the values accurately.
Padanattil et al. [46] highlighted the use of hybrid fiber-reinforced polymer
composites in the external wrapping of small scale circular concrete columns. Glass
FRP and sisal FRP was hybridized and applied simultaneously on the circumference
of concrete specimens. The specimens were tested until failure by monotonically
increasing axial compressive loads. The test results showed remarkable enhancement
in the compressive strength and strain of the confined specimens. Moreover, strength
19
models were developed for hybrid confinement. Furthermore, it was concluded that
the hybrid confinement could be used as a replacement of carbon FRP confinement.
Ispir et al. [41] carried an experimental investigation on the hybridization of
PET, Glass and Carbon FRP. The author concluded that the hybrid confinement could
be useful if one of the fiber has large rupture strain. The hybridization of glass and
PET FRP shows an enormous increase in the strength and ductility enhancement of
the concrete.
Wu et al. [43] studied the behavior of circular concrete columns confined with
hybrid FRP composites. A total of 35 standard size circular specimen were tested by
axial compressive loads. From the experimental results, stress-strain models were
developed for a different number of FRP layers and materials. The proposed multi-
linear strength-strain models predicted the compressive strength and strain values
closer to the experimental results.
Rosakis [42] studied the mechanical characteristics of concrete cylinders
confined by a thin layer of glass fiber reinforced polymer composites, as well as
polypropylene fiber ropes on the surface of the cylinder. The use of both GFRP sheets
and FR made it hybrid confinement. In this hybrid confinement method, the fiber rope
was wrapped around the surface of the cured FRP sheets as external un-bonded
reinforcement. The FR was wrapped around the specimen without any resin, and its
ends were fixed with steel collars. Concrete having different strength values were
used for the casting of 16 specimens. The specimens were divided into two series and
confined by various layers of GFRP and FR. The displacement increases with the
repetition of the axial compression cycles. The concrete specimen having normal
strength and effectively confined by PPFR shows higher strength and the load
dropped was in a controlled manner. The efficiency of the columns which were made
from high strength concrete and having low PPFR confinement level results in lower
ultimate stress and strain.
20
Rousakis [57] conducted experimental research to assess the compressive
behavior of concrete cylinders confined externally by un-bonded polypropylene and
vinylon ropes. The experimental program includes the concrete specimens having a
strength of 11.04 MPA at 28 days. The ropes were wrapped around the standard
concrete cylinder by hand without any resin or any binder, in three different
volumetric ratios. Steel collars were used to provide mechanical anchorage, at top and
bottom, as given in Figure 5 and 6. The concrete specimens were examined under the
effect of monotonic axial compression load as well as cyclic loading. The
experimentation presents that the ropes posed low sensitivity to concrete cracking and
could be reused for further experiments. A significant upgrade of concrete strain at
failure reaching values of 13% strain was achieved by the fiber rope confinement of
concrete cylinders. The efficiency of enhancing the ultimate strength of concrete
confined by FR is higher than FRP.
Figure 5. Concrete specimen confined by vinylon rope with two steel collars at top
and bottom.
21
Figure 6. Concrete cylinder confined by Polypropylene rope after applying axial
loading.
Source: Rousakis [57]
22
MATERIALS AND METHODS
3.1 Specimen Design
The experimentation in this study included the testing of 48 plain concrete
specimens under quasi-static monotonic axial loading, out of which 36 concrete
specimens were confined, leaving the remaining 12 as control specimens for
reference. Out of 36 specimens, 18 of them were classified as circular, and the
remaining half were categorized as square specimens with a standard corner radius of
26 mm. The test parameters included the cross-sectional shape (that is, square and
circular), concrete compressive strength (i.e., low strength below 20 MPa), medium
strength of around 30 MPa (in this study for simplicity this strength is denoted as high
strength), and the number of FRP layers (five, ten, and fifteen). Table 1 and Figure 7
shows the specimens detail. The dimensions of the circular concrete columns were
150 mm Γ 300 mm while that of square specimens was 150 mm Γ 150 mm Γ 300 mm.
The circular concrete columns were casted in standard circular steel molds, whereas
special steel molds were prepared with a standard well-rounded corner radius of 26
mm to cast square specimens in accordance to the recommendation of the American
Concrete Institute (ACI) committee 440 [58], a 13 mm corner radius was considered
as the minimum value, whereas 26 mm was considered as a well-rounded corner. The
labeling of specimens was done in an alphabetical manner followed by a number (e.g.,
L-C-2J3P). The first alphabet letter represents the strength of concrete (L denotes low
strength, and H represent high strength). The second alphabet letter denotes the cross-
sectional shape of the specimens, (C for circular columns or S for square columns),
whereas the first digit and preceding alphabet letters show the number of jute FRP
layers (i.e., 2J), followed by the number of layers of polyester FRP (i.e., 3P).
23
Table 1. Details of test specimens.
Group Specimen Specimen Shape JFRP Ply
(Inside)
PFRP Ply
(Outside)
Total
Hybrid Ply
No. of
Specimen
L-C-CON Circular 0 0 0 3
A L-C-2J3P Circular 2 3 5 3
L-C-4J6P Circular 4 6 10 3
L-C-6J9P Circular 6 9 15 3
H-C-CON Circular 0 0 0 3
B H-C-2J3P Circular 2 3 5 3
H-C-4J6P Circular 4 6 10 3
H-C-6J9P Circular 6 9 15 3
L-S-CON Square 0 0 0 3
C L-S-2J3P Square 2 3 5 3
L-S-4J6P Square 4 6 10 3
L-S-6J9P Square 6 9 15 3
H-S-CON Square 0 0 0 3
D H-S-2J3P Square 2 3 5 3
H-S-4J6P Square 4 6 10 3
H-S-6J9P Square 6 9 15 3
Figure 7. Dimensions of circular and square specimens.
24
3.2. Material Properties
3.2.1. Concrete
The test specimens were cast using locally available ordinary Portland cement
with a specific gravity of 3.15, fine aggregates with a specific gravity of 2.5, and
coarse aggregates of the size of 13 mm. All the concrete specimens were prepared in
different batches with a target strength of 20 MPa and 30 MPa at 28-days strength
following the guidelines of ASTM C192 [59]. The constituents used in the mix design
of the concrete are given in Table 2.
Table 2. Concrete mix proportion (kg per cubic meter).
Composition Low-Strength Concrete High-Strength Concrete
Cement 242.0 517.64
Water 212.0 188.25
Fine Aggregate 727.0 847.00
Coarse Aggregate 1212.0 847.00
3.2.2. Epoxy Resin
A two-part high-performance, easily applicable epoxy resin used in this
experimentation was obtained from a local distributor based in Bangkok, Thailand
(Smart and Bright Co., Ltd.). The high-performance two-part epoxy resin consists of
resin (Part A) and hardener (Part B). They are mixed in a 2:1 (Part A: Part B) by
weight as prescribed by the manufacturer. It can be applied with the help of roller or
brush. The manufacturer provided the mechanical properties of the epoxy resin and
are shown in Table 3.
25
Table 3. Mechanical properties of epoxy resin.
SMART CF RESIN Properties
Tensile Strength 50 MPa
Compressive Strength 650 kgf/cm2
Flexural Strength 75 MPa
Bond Strength 2.11 N/mm2
Elongation at break 2.50%
3.2.3. Jute Fiber
A bi-directional jute fabric having a plain-woven structure was used as an
inner layer external reinforcement for concrete specimens. The jute fabric was
obtained in the form of a continuous sheet having a density of 1.46 g/cm3 and width
of 102 cm, which was cut into a designated size for each specimen. The easy
availability, low price, and moderate stiffness of the jute fabric make it a favorable
selection. The tensile properties of the jute fabric were determined by preparing and
testing five flat coupons of jute FRP composites, by ASTM D3039-M08 [60]. The flat
coupon length was 250 mm, the width was 20 mm, and the nominal thickness was
0.55 mm. Before testing, the flat coupons were dried for seven days under normal
laboratory conditions. A constant tensile loading rate of 1 mm/min was applied to the
specimen. The tensile strain of each specimen was determined by fixing a strain gauge
of 30 mm gauge length at the center of the coupons. Whereas, the tensile force was
obtained directly from the UTM. The tensile stress of jute FRP was calculated from
the nominal area of the coupon. The coupons failed at the center after reaching its
maximum strain capacity. The mechanical properties and stress-strain curve of jute
fabric are given in Table 4 and Figure 8, respectively.
26
Table 4. Mechanical properties of the jute fiber reinforced polymer (JFRP).
Tensile Strength 65 MPa
Ultimate Strain 1.7%
Elastic Modulus 4000 MPa
Density 1.46 g/cm3
Figure 8. Stress-strain curve of the jute FRP composite.
3.2.4. Polyester Fiber
A commercially available bi-directional, plain weave structured polyester fiber
sheet was used as an outer layer reinforcement in confining concrete specimens. The
tensile stresses in both the weft and warp direction were the same due to the uniform
distribution of the fibers in both directions. The ASTM D3039-M08 [52] standard
testing method was used to obtain the mechanical properties of the polyester FRP
composite. According to the standard, the thickness and width of the flat coupon are
determined as required, whereas the minimum length is considered as the sum of the
grips, twice the width, and gauge length. The length, nominal thickness, and width of
the coupon were 250 mm, 0.22 mm, and 20 mm, respectively. To hold the coupons,
aluminum tabs were used having a 50 mm length and a 20 mm width. During tensile
testing, the tabs provided a stable and uniform transfer force. Figure 10 shows the
dimensions of a flat FRP coupon prepared for tensile test. An M500-50-AT
computer-controlled universal materials testing-machine (Testometric, Rochdale, UK)
as shown in Figure 12, was used to conduct the tensile testing of JFRP coupons. The
0
15
30
45
60
75
0 0.005 0.01 0.015 0.02
Str
ess
(MP
a)
Strain
27
longitudinal strain was determined with the help of a 30 mm gauge length, strain
gauge, instrumented vertically at the center of each specimen. Tensile stresses were
calculated from the nominal area of the coupon. The tensile properties of the polyester
coupons are shown in Table 5. The test investigation shows that the flat coupons
fractured at the mid-height, additionally the failure crack was perpendicular to the
direction of applied tensile stress. The same failure mode for the polyester FRP
coupon was reported by [47]. Figure 9 shows the stress-strain curve of the PFRP
composite. A similar response was previously recorded by Woldemarium [61] for un-
plasticized polyvinyl chloride pipes (uPVC). The attained mechanical properties
indicate a large breaking strain but comparatively low tensile strength in comparison
with traditional FRP composites.
Table 5. Mechanical properties of the polyester FRP.
Tensile Strength Ultimate Strain Elastic Modulus Density
100 MPa 4.4% 2.4 GPa 1.39 g/cm3
Figure 9. Stress-strain behavior of the polyester FRP composite.
0
30
60
90
120
0 0.01 0.02 0.03 0.04 0.05
Str
ess
(MP
a)
Strain
28
Figure 10. Specifications of the FRP flat coupon for tensile testing
3.2.5. Hybrid FRP Wrapping
After the curing period of 28 days, concrete specimens were dried under
laboratory temperature; its surface was cleaned and polished to remove the dirt and
contaminants. Taking a hybrid confinement configuration into consideration, the jute
FRP sheet having a low elongation capacity was carefully wrapped first (inner sheet).
Afterwards, the polyester fiber (outer sheet), which has a very large breaking strain,
was applied cautiously over the JFRP sheet. Herein, some modification was made to
the earlier technique reported by [45] for synthetic hybrid fibers. In the present study,
the jute layer was carefully wrapped first; the second sheet was then wrapped on an
alternate day. This technique was applied to adjust the high number of layers and to
prevent any void formation. An appropriate bond among the inner and outer FRP
sheet was attained using high-performance epoxy resin and proper inner surface
treatment. Superglue was used to bond the starting end of jute fabric onto the
specimen surface, making it as an initial position and preventing the fiber sheet from
slippage during the wrapping process. The two-part high-performance epoxy resin
was thoroughly mixed with a high-speed drill mixer until a homogenous mixture was
achieved. Moreover, a thin layer of epoxy resin was thoroughly applied on the
concrete specimen surface and, subsequently, the continuous jute sheet, matching to a
29
specific number of layers, was thoroughly impregnated with epoxy. Following the
wet-layup process, the FRP fabric was carefully wrapped around the perimeter of
concrete specimens by hand. Any slippage or debonding amongst the FRP layers was
countered by providing an overlap length of 160β180 mm for circular columns and
150 mm for square columns. Furthermore, the top and bottom end of high strength
circular specimens were additionally wrapped with 3 layers of carbon fibers. The
surface of the JFRP was smoothened, and extra epoxy was squeezed out with the help
of a roller brush. Moreover, the roller brush helped in removing the entrapped air
bubbles between the fiber sheet and concrete surface. Extra care was taken to clean
and smoothen the inner sheet surface. The fiber wrapping configuration of circular
and square specimens is given in Figure 13 and 14, respectively. The stress-strain
curve of the hybrid FRP composite obtained by the tensile coupon test is also given in
Figure 5a. To ensure the uniform axial compression loading application only on the
concrete core and to evade any eccentric loading conditions, high-strength plaster was
applied on both ends of the specimens to achieve a uniform capping. Furthermore, the
plaster capping was trimmed and shaped precisely to the specimen cross-section to
avert the possibility of axial load transmission onto the FRP shell area.
Figure 11. The stress-strain curve of the juteβpolyester fiber reinforced polymer
(JPFRP) hybrid composite.
0
20
40
60
80
100
0 0.01 0.02 0.03 0.04
Str
ess
(MP
a)
Strain
(a)
30
Figure 12. The M500-50-AT computer-controlled UTM.
Figure 13. Hybrid Fiber wrapping configuration of circular specimens
31
Figure 14. Hybrid fiber wrapping configuration of square specimens
3.3. Instrumentation
In this experimental study, the instrumentation used for the measurement of
axial and lateral strains of all the concrete specimens consisted of linear variable
differential transducer (LVDTs) with a capacity of 50 mm and strain gauges of
different gage length (i.e., 30 mm and 5 or 10 mm). The 30 mm gage length strain
gauges were mounted horizontally and vertically to measure the lateral and axial
strains, respectively. Hoop strain was recorded by a strain gauge having 10 mm size.
The instruments setup is shown in Figure 15. In case of circular specimens, two pairs
of strain gauges were mounted horizontally and vertically at center height and outside
the overlay region at an interval of 180 degrees to record lateral and axial strain,
respectively. Besides strain gauges, 6 LVDTS were assigned to each column for the
measurement of radial and axial deformation. To measure lateral deformation, 3 of
them were installed horizontally at an angle of the 120 degrees while the other 3 were
installed to record the deformation in the axial direction. A special circular and square
steel frame were used to support the vertical LVDTs.
32
Figure 15. Instrumentation of Concrete specimen
3.4. Test Procedure
All of the circular and square concrete columns were tested until failure by
monotonically increasing the concentric axial compression load in a 2000 KN
capacity universal testing machine. To make sure the loading application was on the
concrete core only, two high-strength steel plates with a thickness of 5 mm, having
the same cross-sectional shape as that of the circular and square (corner radius of 26
mm) specimens, were positioned at both ends of concrete columns. The unconfined
specimens were tested with a constant displacement rate of 0.2 mm per minute,
whereas, due to the high expected axial displacement of the confined specimens, the
loading rate was kept at 1 mm per minute. As stated by Demir et al. [25] and Saleem
et al. [25], the variation in this loading rate has an insignificant effect on the confined
concrete response. Test data from a load cell and the LVDTs were recorded with the
help of a data acquisition system (data logger TDS-530) connected to a computer. The
test setup is given in Figure 16.
33
Figure 16. Test setup for concrete specimens.
34
Results and Discussions
4.1 Failure Modes
4.1.1. Circular Columns
Low-strength JPFRP-confined circular concrete specimens failed due to the
cracking of the JPFRP laminate, which is caused by the lateral deformation of the
concrete core under the effect of high axial compressive loading. The ultimate failure
of the JPFRP wrapped concrete columns resulting from the fracture of the JPFRP
laminate was abrupt, brittle, and characterized by an explosive sound. Before the
failure, infrequent cracking sounds were heard which indicated the progressive
fracturing of the inner fibers sheet and hardened epoxy. The JPFRP shell failed in a
straight continuous crack from top to bottom, parallel to the direction of axial
compressive stress and perpendicular to the alignment of the JPFRP confining layers,
as shown in Figure 17. A similar failure pattern has been previously reported in the
literature [33, 34]. When the concrete core cracked, the confinement of the JPFRP
laminate had activated and effectively resisted the lateral expansion. However, the
lateral expansion of the circular columns produced transverse strain in the JPFRP
shell, and upon reaching the ultimate state, the rupture of the FRP shell occurred.
Unlike the JPFRP-confined low-strength columns, the failure of the high-strength
concrete columns was quiet, and the narrow vertical crack did not propagate
throughout the entire jacket. It was because of the additional wrapping of 20 mm of
carbon fiber at the top and bottom ends of the specimens. This carbon wrapping was
done after the author observed load drop in the high strength circular specimens,
which were different from the low-strength circular columns. However, upon reaching
the peak strength, load drop still occurs, but it has changed the failure mode of the
specimens. The specimen H-C-6J9P shows bulging in the lateral direction without any
crack. Besides, the cracking pattern of high strength specimens also changed. Figure 8
shows the failure mode of circular specimens. No debonding between jute and
35
polyester layers was observed by using our technique, which shows a solid bond
between jute and polyester fibers. Also, the hybrid JPFRP shell could not be separated
from the surface of concrete columns, indicating a strong and robust interfacial bond
between them. It is worth mentioning that the failure pattern of hybrid JPFRP-
confined concrete columns is different from the concrete specimens wrapped with the
conventional fiber-reinforced polymer composites such as glass, aramid, and carbon.
In the case of synthetic fibers, the rupture of the FRP laminate is in the form of single
or multi-shape rings.
Figure 17. Failure modes of low- and high-strength circular columns.
36
4.1.2. Square Columns
Likewise, the hybrid JPFRP-confined square concrete specimens failed by
cracking of the JPFRP shell. The failure mode showed a fully vertical rupture of the
FRP laminate started from the center and reaching both ends. Before the rupture of
the FRP sheet, an intermittent snapping noise was heard which was followed by a
sudden single crack accompanied by a very loud popping sound. In contrast, high-
strength square specimens failed with a low sound, indicating a ductile failure. Due to
stress accumulation at the corners, almost all square specimens developed the JPFRP
rupture at or near the corner. Concrete crushing and disruption were observed along
the unconfined portion of the specimen created by the FRP fracture. Supporting our
results, [25, 33, 54] also reported this type of failure for different types of FRP
confined square specimens. Figure 18 shows the failure modes of low- and high-
strength square specimens. No cracks were observed in the H-S-6J9P specimen
confined by 15 layers. The knife action of the corners was restrained by the large
rupture strain of polyester fiber. Bulging in the lateral direction was observed in the
specimen H-S-6J9P, which reflects the dominance of the large strain capacity of the
polyester fiber in hybrid confinement. As stated by Ispir et al. [41], the increase in the
number of outer layers of the FRP increases the ultimate axial deformability of the
concrete. Hence the 15-layer specimens showed a very high axial deformation
capacity.
37
Figure 18. Failure modes of low- and high-strength square columns.
4.2. Stress-Strain Response
The observed test results are presented in terms of normalized axial
compressive stress versus axial strain and normalized axial stress versus lateral strain
curves, as shown in Figure 19 (aβd). Due to the difference in unconfined concrete
compressive strengths (fβco), the compressive strength of JPFRP-confined concrete
(fβcc) for group (A, B, C, and D) is normalized by dividing it with the unconfined
concrete strength, recorded for its corresponding batch and plotted against axial and
lateral strain to clearly understand the effects of the considered parameters. The stress
results were obtained by a load cell, whereas the strain values were derived from the
LVDTs linked to the computer via a data acquisition system. LVDTs were able to
record the deformation of the confined concrete columns until failure occurred. The
average data of tested specimens are given in Table 6. Together, Figure 19 (aβd) and
Table 6 shows that the lateral jacketing with the hybrid JPFRP can improve the
structural properties of confined concrete specimens, particularly ductility
enhancement. No strain-softening was observed for JPFRP-confined low-strength
specimens, which demonstrates that the specimens were sufficiently confined.
Similarly, Wu et al. [43] stated that enough lateral confinement of concrete
specimens with an FRP sheet yields no strain-softening behavior. The hybrid JPFRP-
38
confined low-strength circular and square specimens showed bi-linear stress-strain
behavior similar to the behavior of sisalβglass hybrid FRP confinement of circular
columns reported by [46]. The stress-strain curve of the hybrid JPFRP-confined
specimens could be characterized into two main regions connected by a transitional
zone. The first region of the stress-strain curve exhibits almost similar initial stiffness
as that of an un-strengthened concrete specimen with peak stress (fβco) and axial strain
(Ιco). In this region, JPFRP jacketing has less effect because of insignificant lateral
deformation of the concrete core. When the level of applied stress reached the
ultimate compressive strength of un-strengthened concrete specimens, a transitional
zone appeared, indicated by the development of micro-cracks in the concrete core.
Subsequently, the confinement effect of the JPFRP laminate was fully activated. Thus
the second part of the stress-strain curve of jacketed concrete columns showed a linear
trend reaching the peak compressive strength (fβcc) with the corresponding
compressive strain (βcc), and a reduced elastic modulus until the failure of the hybrid
sheet. The second part is usually governed by the material properties of the FRP and
the concrete. Ispir et al., [41] utilized two or more FRPs (such as glass, carbon, and
PET) in hybrid confinement of circular concrete. The authors further studied the
stress-strain behavior of these fibers in a different hybrid configuration; that is, they
observed different types of mechanisms in hybrid FRP confined concrete: (1) inner
and outer FRP sheet rupture at the same time and (2) fiber sheet having low
elongation capacity will rupture first. Therefore, in the present study JPFRP sheets
ruptured simultaneously in case of the low-strength specimens, and its stress-strain
behavior is similar to the stress-strain curve of the hybrid-confined concrete
specimens observed by [41, 46]. However, the stress-strain curves of high-strength
JPFRP-jacketed concrete columns showed a sudden load drop after reaching the peak
compressive strength and decreased gradually until the ultimate strain of the
specimens was reached. This phenomenon can be attributed to the dominance of the
polyester fiber as it has a large rupture strain to accommodate the released energy
after the breaking of the inner jute sheet. Similar behavior was previously observed by
Huang et al. [47] for high-strength concrete specimens strengthened by polyester
39
FRP. Furthermore, Rosakis [42] conducted experimental testing of low- and high-
strength concrete jacketed with two or more different types of fibers with different
elongation capacities (i.e., glass-FRP, vinylon, and polypropylene fiber ropes) in a
hybrid configuration. The author observed a significant load drop after the inner
GFRP sheet fractured and energy was released, however this energy was absorbed by
fiber rope and enable the specimens to fail in a ductile manner. The load drop was
significantly higher in high-strength concrete than low-strength jacketed specimens.
Woldemariam et al. [61] used un-plasticized polyvinyl chloride (uPVC) tubes for
concrete confinement; the authors observed similar load drops varying with the
thickness-to-diameter ratio and concrete strength. A detailed description of the effects
of the considered parameters is discussed in the next sections.
Table 6. Average values of tested specimens.
Group Specimens fβco
(MPa)
fβcc
(MPa)
βco or βcc
(%)
βcu
(%)
βhrup
(%) fβcc/fβco βcc/βco
A L-C-CON 13.17 0.34 - 1.00 - L-C-2J3P - 19.41 1.40 2.2 1.35 1.47 4.00 L-C-4J6P - 29.85 2.90 3.2 2.72 2.27 8.53 L-C-6J9P - 34.38 3.05 3.6 2.90 2.61 8.97
B H-C-CON 30.00 0.43 1.00
H-C-2J3P - 37.2 0.80 2.00 2.01 1.24 1.86 H-C-4J6P - 40.6 1.00 3.80 3.70 1.35 2.32 H-C-6J9P - 47.36 1.10 4.89 2.30 1.58 2.55
C L-S-CON 18.42 0.30 - 1.00 - L-S-2J3P - 24.70 1.00 0.80 1.63 1.34 3.33 L-S-4J6P - 32.94 1.30 1.29 2.02 1.79 4.33 L-S-6J9P - 39.50 1.35 1.40 2.10 2.14 4.50
D H-S-CON 29.23 0.60
H-S-2J3P - 34.54 0.83 2.90 4.00 1.18 1.38 H-S-4J6P - 38.18 1.20 5.70 7.00 1.31 2.00 H-S-6J9P - 40.91 1.30 8.03 9.20 1.41 2.17
40
Fig 19 Cont.
0
0.5
1
1.5
2
2.5
3
3.5
4
-0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04
No
rmal
ized
Str
ess
(fβ c
c /f
β co)
Lateral Strain Axial Strain
(a)
L-C-CONL-C-2J3PL-C-4J6PL-C-6J9PSeries1
0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
-0.03 -0.02 -0.01 0 0.01 0.02
No
rmal
ized
S
tres
s (f
'cc/
f'co
)
Lateral Strain Axial Strain
(b)
L-S-CONL-S-2J3PL-S-4J6PL-S-6J9PSeries5
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
-0.04 -0.02 0 0.02 0.04 0.06
No
rmal
ized
Str
ess
(fβ c
c /fβ c
o)
Lateral Strain Axial Strain
(c)
H-C-CONH-C-2J3PH-C-4J6PH-C-6J9PSeries2
41
Figure 19. Stress-strain curve behavior of confined columns: (a) Low-strength
circular specimens; (b) low-strength square specimens; (c) high-strength circular
specimens; and (d) high-strength square specimens.
4.2.1. Circular Concrete Specimens
The results obtained from the experimental testing of all the specimens under
concentric compression loadings are given in Table 6. Together Figure 19 (aβd) and
Table 6 imply that the hybrid confinement enhanced the overall response of low- and
high-strength circular columns. The peak axial compressive strength and strain of the
confined specimen increases with the increase in the number of FRP layers. Whereas
the effectiveness of lateral jacketing is more dominant in low-strength circular
concrete than high-strength circular concrete columns. The increase in the JPFRP
plies increased the axial strength and strain of the circular concrete columns. This
reflects that the thicker the JPFRP shell, the larger will be the peak compressive stress
as well as ultimate strain. For low-strength circular specimens, when the number of
JPFRP layers increased from 5 to 10, a substantial increase in both peak strength and
strain was observed. However, when the JPFRP plies were increased from 10 to 15, a
less significant enhancement was observed. The axial strength and strain of the
specimen L-C-2J3P increased to 47% and 300%, respectively, compared to the
unconfined reference concrete column. When the number of layers was increased to
10, for specimen L-C-4J6P, a substantial increase in axial strength and strain was
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
No
rmal
ized
Str
ess
(fβ c
c /fβ c
o)
Lateral Strain Axial Strain
(d)
H-S-CON
H-S-2J3P
H-S-4J6P
H-S-6J9P
42
recorded, that is, 127% and 753%, respectively. For the specimen L-C-6J9P, having
15 layers of confinement with a thickness of 5.28 mm, the highest compressive stress
of 161% and axial strain of 797% was achieved. Previous studies highlighted that the
effectiveness of lateral wrapping with FRP is more significant in low-strength
concrete columns as compared to high-strength concrete columns [54, 62]. In the case
of JPFRP-confined high-strength circular specimens, it is evident from Figure 19 and
Table 6 that the enhancement in compressive strength and strain is higher in low-
strength JPFRP-confined concrete columns. The specimen H-C-2J3P displayed a 24%
and 89% rise in the peak axial compressive strength and strain, respectively. The 10-
layer specimen, H-C-4J6P with the JPFRP thickness of 3.52 mm, showed an
enhancement of 35% and 135% in terms of axial compressive stress and strain,
respectively. The 15-layer specimen H-C-6J9P exhibited an improvement of 58% and
93% in peak strength and strain, respectively.
4.2.2. Square Concrete Specimens
The typical stress versus strain response of JPFRP-confined square concrete
columns are shown in Figure 19 (b and d). The mean values obtained from the
experimental testing are given in Table 6. Test results showed that the jacketed high-
strength square specimens exhibited a remarkable enhancement in its load-carrying
capacity along with axial deformability. From Figure 19 (b and d) it was also
observed that the increase in the number of JPFRP plies enhances the peak
compressive strength and strain of the confined concrete. For example, the specimen
L-S-2J3P with five FRP layers having a shell thickness of 1.76 mm failed at 34%
higher strength and 233% higher axial strain as compared to the unjacketed square
specimen. In the case of specimen L-S-4J6P, an increase of 79% and 333% was
achieved in axial compressive strength and strain, respectively. The low-strength
square concrete specimen, L-S-6J9P, achieved the highest axial stress and strain of
114% and 350%, respectively, compared to the un-jacketed concrete specimen.
43
In contrast to the low-strength concrete column, the JPFRP-confined high-
strength square column yields less improvement in the axial strength and strain. The
high-strength specimen, H-S-2J3P, shown an increase of 18% in strength and 38%
enhancement in axial strain. When the FRP layers were increased to 10, the specimen
H-S-4J6P shown an increase of 31% in axial strength and a 100% increase in axial
strain compared to the un-wrapped specimen. Similarly, the enhancement of 41% and
117% in strength and strain, respectively, was observed for the square concrete
column, H-S-6J9P. Furthermore, the relation between the JPFRP layers and its effect
on the normalized axial compressive strength and strain enhancement is graphically
presented in Figure 20 (a and b). The figure clearly shows that the strength and strain
of the confined concrete increased linearly with the increase in the number of JPFRP
layers. However, the increase is significant in low-strength circular columns.
Overall, the improvement in axial compressive stress and strain of square
specimens, especially high-strength specimens, are found to be less as compared to
circular specimen because it has been observed in the previous literature that
confining pressure is non-uniformly distributed in square specimens, decreasing the
effectiveness of the FRP shell. Also, confinement effectiveness is more pronounced in
low-strength concrete columns than in high-strength concrete columns. The results of
the present experimental study are in agreement with the findings of Chaallal et al.
[25], as the authors observed an insignificant increase in compressive strength and
strain of FRP-wrapped high-strength concrete columns. They stated that the
deformation capacity of the high-strength concrete is less as compared to low-strength
concrete, which results in a smaller lateral confining pressure.
44
Figure 20. Effect of JPFRP layers on (a) strength enhancement and (b) strain
enhancement.
4.3. Ductility and Fracture Energy of the JPFRP-Confined Specimens
Ductility can be described as the property of composite material, structural
system, or structural element to withstand the inelastic deformation before the
ultimate failure, without causing significant loss in structural resistance. Ductility is
offered by the steel bars in the conventional reinforced concrete structures, and it is
well-defined as the ratio of the ultimate deformation to that of yield deformation [37,
38]. Unlike steel reinforcement, which deforms significantly once it starts to yield, a
0.0
0.5
1.0
1.5
2.0
2.5
3.0
5 10 15
f''c
c/f'
co
No. of Layers
(a) L-CircularL-SquareH-CircularH-Square
0.0
1.5
3.0
4.5
6.0
7.5
9.0
10.5
5 10 15
βcc
/ β
co
No. of Layers
(b)
L-CircularL-SqaureH-CircularH-Square
45
juteβpolyester hybrid FRP composite is elastic until failure, as shown in Figure 5a.
Consequently, the necessity arises to find an appropriate method to assess the ductility
of the JPFRP jacketed specimens. One approach towards the solution of this problem
is to calculate the fracture energy of the JPFRP-confined concrete specimens and
compare it with fracture energy of the unconfined specimen, represented as the energy
ductility index. Fracture energy of a concrete specimen is defined as the energy
absorbed by the concrete columns during breaking; this absorbed energy is equal to
the area under the load-displacement curve of concrete specimens having units of KN
and mm, respectively, which leads to Nm [33]. The data analysis software OriginPro
was used for the calculation of area under the load versus displacement curve, as
graphically presented in Figure 21. The fracture energy and ductility index of all
tested concrete specimens are given in Table 7. Apart from an increase in compressive
strength, JPFRP wrapping can increase the fracture energy and ductility of the
jacketed concrete specimens. It is concluded from Table 7 that JPFRP-confined
concrete specimens exhibited a significant increase in fracture energy as compared
with the control specimen. In addition, the energy absorption is highly reliant on the
amount of JPFRP. Hence, the increase varies with the number of JPFRP plies, cross-
sectional shape, and concrete strength. An enormous increase was observed in the
fracture energy and ductility index of 5-, 10-, and 15-layer JPFRP-reinforced low-
strength circular specimens in which the ductility index increased from 13.42 to
26.56. However, in high-strength, circular concrete specimens for the same number of
layers the ductility index recorded is 4.94 to 14.56. Very high fracture energy was
recorded for high-strength square specimens and its corresponding ductility index.
The low-strength square specimens yield low fracture energy and its ductility index
ranging from 4 to 10. The ductility index for 5-, 10- and 15-layer high-strength square
specimens range from 5.20, 14.01, and 20.79, respectively.
46
Table 7. Average values of all tested specimens.
Group Specimens Fracture Energy N-m Ductility Index
A L-C-CON 150.00 1.00 L-C-2J3P 2013.00 13.42 L-C-4J6P 3007.00 20.05 L-C-6J9P 3984.35 26.56
B H-C-CON 603.16 1.00 H-C-2J3P 2982.10 4.94 H-C-4J6P 5843.44 9.69 H-C-6J9P 8783.13 14.56
C L-S-CON 214.00 1.00 L-S-2J3P 965.55 4.51 L-S-4J6P 1635.27 7.64 L-S-6J9P 2067.00 9.66
D H-S-CON 768.69 1.00 H-S-2J3P 3993.78 5.20 H-S-4J6P 10,777.00 14.01 H-S-6J9P 15,986.87 20.79
Figure 21. The area under the load-displacement curve obtained by OriginPro.
47
4.4. Review of Strength and Strain Models
Since 1990s researchers have done extensive experimental and analytical work
in developing several confinement models for the estimation of compressive strength
and strain of FRP-strengthened circular, square, and rectangular concrete members
tested under concentric loading. The majority of the proposed models have been
derived by testing unreinforced concrete columns under uniaxial monotonic
compression loadings. Few models were predicted by experimentally testing FRP
wrapped reinforced concrete specimens. Most of the confinement models are based on
the empirical investigation; these models can further be divided into two types,
design-oriented and analysis-oriented models [63]. In design-oriented models, the
compressive stress and strain of FRP-strengthened concrete columns are estimated by
the regression analysis of the test data obtained from the experiments [63-65]. An
increasing type of axial compressive stress versus strain curve was predicted by most
of these design-oriented models. The second type is an analysis-oriented model in
which the stress-strain response of the FRP-confined concrete column is predicted by
using an incremental numerical technique. As stated by Lam and Teng [25], the
stress-strain behavior of effectively confined circular or square concrete columns is
usually characterized by a parabolic curve, consisting of two main segments
ascending linearly. The first portion displays an ascending parabolic curve almost
similar to that of un-strengthened concrete, linked with the second linear segments
through an even transition zone. In the first region, the FRP reinforcement provides
insignificant effects on the lateral confinement of the concrete core. However, the
second linear segment of the curve is significantly influenced by the FRP wrapping,
type of FRP material, shape of cross-section, and concrete strength. This part stretches
continuously until the failure of the specimens occurs.
To achieve a reliable design, it is very important that the existing models
should accurately predict the strength and axial strain of FRP-jacketed specimens.
Currently, numerous stress-strain models are available to predict the axial
48
compressive stress and strain of different types of FRP-jacketed circular and non-
circular concrete columns [13, 17, 22, 63, 65-68].
4.4.1. Existing Strength Models
Most of the existing stress models, developed for different types of FRP-
jacketed concrete members employ the following general form:
πβ²ππ
πβ²ππ
= 1 + π1
ππ
πβ²ππ
(1)
where the compressive strength of the confined specimen is denoted by πβ²ππ
and the compressive strength of the unconfined specimen is denoted by πβ²ππ
; π1 is the
confinement effectiveness coefficient while the lateral confining pressure of the FRP
jacket is represented by ππ. This equation was originally proposed by Richart et al.
[69] for active confinement of concrete by a fluid with π1 = 4.1. Subsequently,
Richart et al. [70] stated that the use of this strength model can be extended for steel-
confined concrete. Later, experimental studies were conducted by Fardis and Khalili
[71], they suggested that the model proposed by Richart et al. can be used for fiber-
reinforced polymers confined concrete. The lateral confining pressure could be
directly correlated to the volumetric ratio and the tensile strength of the FRP
composite as follows:
For circular specimens,
ππ = 2. πΈπππ. π‘. ππ
π· (2)
ππππ = πΈπππ. ππ
(3)
49
ππ = 2πππππ‘
π·
(4)
where πΈπππ is the elastic modulus of FRP; ππ, and ππππ are the lateral strain and
tensile strength of FRP sheet, respectively; t represents the thickness of the FRP; and
D represents the diameter of the jacketed circular columns.
For square specimens,
ππ = 2πππππ‘
π·ππ
(5)
where the diagonal length of the square concrete column is represented by D.
For square cross-sections with rounded shaped corners (ππ), D can be given as
π· = β2 π β 2ππ(β2 β 1) (6)
and where ππ is the shape factor. Masia et al. [72] and Pessiki et al. [73]
defined shape factor as the ratio of the effective confined area Ac, to the total cross-
sectional area of concrete expressed in terms of gross-sectional area Ag, expressed as
ππ =π΄π
π΄π (7)
By limiting the Pessiki et al. [66] expression to plain concrete and square
cross-sections allows ππ to be defined as a function of corner-radius (ππ) and cross-
section dimension (b) as
ππ = 1 β2
3[
(1 β 2ππ
π)
2
1 β (4 β π) (ππ
π)
2] (7)
50
4.4.2. Existing Strain Models
Previous studies suggested that in steel-jacketing concrete, the axial strain πππ
achieved at the corresponding compressive strength can be directly associated with
the confining pressure of FRP [69]. The expression can be written as
πππ = πππ (1 + π2
ππ
πβ²ππ
) (9)
where πππ symbolizes peak strain of the jacketed concrete specimens
corresponding to the peak strength πβ²ππ
, πππ denotes the strain of unconfined
concrete, and π2 is the experimentally determined confinement coefficient. Richart et
al. [69] proposed the value of π2 equal to 5π1 for concrete jacketed by steel
reinforcement. Researchers have proposed different k values ranging from 2 to 15,
based on different parameters such as types of concrete and the spacing between
spiral reinforcement. Later, for FRP wrapped concrete, researchers proposed that the
lateral confining pressure can also be associated with the axial strain. Table 7 shows
the selected strain models from the available literature.
4.5. Evaluation of Strength and Strain Models
The test data obtained from the experimental program of the current study
were used to evaluate the effectiveness of the several existing strength and strain
models in predicting the peak axial strength and strain of the JPFRP-confined circular
and square concrete specimens. The selected stress-strain models for circular and non-
circular columns are given in Tables 8 and 9, respectively. These include the models
of [17, 33, 46, 47, 54, 63, 64, 67, 71, 74-77]
51
Tab
le 8
. E
xis
ting s
tren
gth
and s
trai
n m
odel
s fo
r F
RP
-confi
ned
cir
cula
r sp
ecim
ens.
Mod
els
Str
ength
S
train
Huan
g e
t al
. [4
7]
πβ² π
π
πβ² π
π
=1
+1
.40
(π
π
πβ² π
π
)2
π
ππ
ππ
π=
1+
3.8
0(
ππ
πβ² π
π
)0.2
Kar
bh
ari
and
Gao
[6
4]
πβ² π
π
πβ² π
π
=1
+2
.10
(π
π
πβ² π
π
)0.8
7
π
ππ
ππ
π=
1+
0.0
1(
ππ
πβ² π
π
)
Pim
anm
as e
t al
. [3
3]
πβ² π
π
πβ² π
π
=1
+3
.00
(π
π
πβ² π
π
)
ππ
π
ππ
π=
2+
6.7
0(
ππ
πβ² π
π
)
Ilki
et a
l. [
78
] π
β² ππ
πβ² π
π
=1
+2
.22
7(
ππ
πβ² π
π
)
ππ
π
ππ
π=
1+
15
.15
6(
ππ
πβ² π
π
)0.7
53
Spoel
stra
and
Mo
nti
[7
4]
πβ² π
π
πβ² π
π
=0
.2+
3.0
0(
ππ
πβ² π
π
)0.5
0
π
ππ
ππ
π=
2+
1.2
5(
ππ
πβ² π
π
)0.5
Lam
an
d T
eng [
67]
πβ² π
π
πβ² π
π
=0
.2+
2.0
0(
ππ
πβ² π
π
)
__
Miy
auch
i 1
99
7
πβ² π
π
πβ² π
π
=1
+3
.50
(π
π
πβ² π
π
)
ππ
π
ππ
π=
1+
10
.6(
ππ
πβ² π
π
)0.3
73
Pad
anat
til
et a
l. [
46]
πβ² π
π
πβ² π
π
=1
+2
.21
(π
π
πβ² π
π
)
__
Far
dis
and
Kh
alil
i [7
1]
πβ² π
π
πβ² π
π
=1
+4
.1 (
ππ
πβ² π
π
)
__
52
Tab
le 9
. E
xis
ting s
tren
gth
and s
trai
n m
odel
s fo
r F
RP
-confi
ned
squar
e sp
ecim
ens.
M
od
els
Str
ength
S
train
Kum
uth
a et
al.
[17
] π
β² ππ
πβ² π
π
=1
+0
.93
(π
π
πβ² π
π
)
__
Sheh
ata
et a
l. [
75
] π
β² ππ
πβ² π
π
=1
+0
.85
(π
π
πβ² π
π
)
ππ
π
ππ
π=
1+
13
.5(
ππ
πβ² π
π
)
Al-
Sal
lou
m [
76
] π
β² ππ
πβ² π
π
=1
+0
.85
(π π·
)(
ππ
πβ² π
π
)
__
Huss
ain
et
al.
[54]
πβ² π
π
πβ² π
π
=1
+5
.90
(π
π
πβ² π
π
)
__
Mir
mir
an e
t al
. [7
5]
πβ² π
π
πβ² π
π
=1
+6
.0(2
π π π)
(ππ0
.7
πβ² π
π
)
__
Pim
anm
as e
t al
. [3
3]
πβ² π
π
πβ² π
π
=1
+2
.50
(π
π
πβ² π
π
)
ππ
π
ππ
π=
2+
7.0
0(
ππ
πβ² π
π
)
Lam
an
d T
eng [
63]
πβ² π
π
πβ² π
π
=1
+3
.30
(π
π
πβ² π
π
)
ππ
π
ππ
π=
1.7
5+
10
.6(
ππ
πβ² π
π
)(π
ππ
ππ
π)0
.45
Wei
an
d W
u [
77
] π
β² ππ
πβ² π
π
=1
+3
.30
(2π π π
)0.4
(π
π
πβ² π
π
)0.7
3
(β π)β
1
ππ
π
ππ
π=
1.7
5+
12
(π
π
πβ² π
π
)0.7
5
(π 3
0
πβ² π
π
)0.6
2
(0.3
62
π π π+
0.6
4)
(β π)β
0.3
53
Huang et al. [47] proposed a stress-strain model by using the experimental
results obtained from circular polyester FRP-wrapped concrete specimens. The
unreinforced concrete strength (πβ²ππ
) ranged from 22.1 to 39.3 MPa. The expression
is given in Table 8, where ππ represents the lateral confining pressure which is equal to
2. πΈπππ. π‘. ππ π·β ; here πΈπππ is the elasticity modulus of polyester jacket and t denotes
PFRP thickness. The lateral strain of the PFRP jacket is denoted by ππ and the
diameter of the specimen is denoted by D.
The model of Karbhari and Gao [64] is based on the analytical study of three
data sets, which includes the test results of [79, 80], and the authorβs experimental
results obtained from testing of 15 axially loaded CFRP and glass-aramid FRP-
confined concrete columns in developing this model. The derived strength model is
based on the model in [81]. However, it differs in confinement coefficients.
Pimanmas et al. [33] developed stress-strain models for sisal FRP-confined
circular and square concrete columns. The model is applicable for both low- and high-
strength concrete specimens with circular and square sections. The lateral confining
pressures were calculated by ππ = 2πππππ‘ π·β for circular section and ππ =
(2πππππ‘ π·)β ππ for square section, where ππππ is the tensile strength of the FRP-
composite jacket obtained by tensile coupon test; t represent a total thickness of the
FRP-laminate; while the diameter of the circular section is represented by D, or it is
the diagonal length of the square section; shape factor is represented by ππ ,
determined by Equation (8).
Ilki et al. [78] established stress and strain models based on the test data of
CFRP-wrapped circular concrete columns. The strength of the ready-mixed concrete
used in the preparation of concrete columns was 6.2 MPa. The experimental program
includes the testing of 14 circular concrete columns having dimensions of 150 mm Γ
300 mm, under axial compression. The expressions are given in Table 8. The effective
lateral confining pressure ππ was determined by the equation 2ππ ππ‘ π·β .
54
The proposed strength and strain model of Lam and Teng 2002 [67] was
established by the statistical analysis of the test database compiled from the existing
literature. The database included over 200 test results of plain circular concrete
jacketed by CFRP, GFRP, and aramid FRP. The unconfined concrete strength ranged
between 26 and 55 MPa. Based on the statistical analysis of the test data set, the
authors concluded that there exists a direct correlation between the compressive stress
of FRP-jacketed concrete and the lateral confining pressure of FRP-laminate. Lam
and Teng [63] proposed another stress-strain model, based on the analysis of test
results obtained from their experiments as well as from the test database they
compiled from the available literature. Their experimental program included the
testing of CFRP-wrapped square and rectangular concrete specimens. The strength of
unconfined concrete specimens ranged between 23.8 and 44 MPa. The proposed
expression for the square section is given in Table 9. Shape factor ππ is incorporated
for the stress-strain models of square concrete sections.
Padanattil et al. [46] analyzed the experimental results obtained from the axial
compressive testing of sisalβglass hybrid FRP-jacketed circular columns and
proposed a strength model, presented in Table 8. The axial strength of unwrapped
concrete was 21.1 MPa. Fifty circular concrete columns having dimensions of 100
mm Γ 200 mm were tested under concentric axial compression loads. Equation (4)
was used to determine the lateral confining pressure of hybrid FRP composite.
The model of Kumutha et al. [17] was developed for GFRP-confined
rectangular concrete columns. The unconfined concrete strength was approximately
27.45 MPa. A total of nine reinforced concrete specimens were experimentally
analyzed under concentric compression loading. Moreover, the shape factor was
incorporated because of the non-circular shape. The proposed equation is given in
Table 9.
55
Shehata et al. [75] carried out experimental testing of CFRP-wrapped concrete
columns with different cross-sectional shapes that are circular, square, and
rectangular. Based on test results, the author established strength and strain models for
the aforementioned cross-sectional shapes. The axial compressive strength of the un-
jacketed concrete varied between 23.7 to 29.7 MPa. The corners were rounded to 10
mm for square and rectangular columns. The expression is given in Table 9; the
lateral confining pressure is derived by using Equation (2).
Using the test results obtained from CFRP-confined square specimens, Al-
Salloum [76] proposed an expression for compressive stress of CFRP-wrapped
concrete specimens. The unconfined concrete strength ranged between 32 and 35
MPa. Sixteen square concrete columns with different corner radii, having a breadth
and depth of 150 mm, were tested under the effect of axial compressive loading until
the failure of specimens. The proposed model is given in Table 9. A modification
factor (b/d) was introduced to counter the non-uniformity of the confining pressure.
Where, b is the section dimension, and D is the length of the diagonal of the square
concrete column, calculated by Equation (6).
A unified strength and strain model was developed by Wei and Wu [77] for
different cross-sectional shapes. This proposed expression was established by
analyzing a large test database of AFRP, CFRP, and GFRP-jacketed circular, square,
and rectangular concrete specimens having an un-jacketed compressive strength
between 18 and 55 MPa. The reported corner radius in the test database varied
between 0β60 mm. In the proposed expression, given in Table 9, (h/b) is the cross-
section aspect ratio; π30 is the unconfined concrete strength of grade C30.
The performance of these given models (Tables 8 and 9) in predicting the peak
axial compressive strength (πβ²ππ
) and axial strain (πππ) of JPFRP-jacketed circular
and square columns are assessed by using the experimental data of the present study.
These predicted values are then compared with the test results recorded in the present
56
study in the form of predicted ratio experimental results (πβ²ππ
)π
(πβ²ππ
)π
β , and
(πππ)π (πππ)πβ for strength and strain, respectively. Tables 10 and 11 show the
predicted values of axial stress and strain ratios.
57
Tab
le 1
0. C
om
par
ison o
f m
odel
pre
dic
tions
wit
h e
xper
imen
tal
resu
lts
of
circ
ula
r sp
ecim
ens.
Sp
ecim
en
[4
7]
[64
] [3
3]
[78
] [7
4]
[77
] [6
7]
[46
] [7
1]
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² π
π) π
(πβ² π
π) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
(πβ² ππ
) π
L-C
-2J3
P
0.7
0
0.9
0
0.9
5
0.2
4
0.9
8
0.7
2
0.9
0
1.1
1
0.9
1
0.6
0
1.0
3
1.5
0
0.8
8
0.9
0
1.0
8
L-C
-4J6
P
0.5
0
0.5
1
0.7
6
0.1
2
0.8
3
0.4
6
0.7
3
0.8
2
0.8
0
0.3
1
0.8
9
0.9
0
0.7
0
0.7
3
0.9
7
L-C
-6J9
P
0.4
9
0.5
0
0.7
8
0.1
1
0.8
9
0.5
5
0.7
6
1.0
2
0.8
4
0.3
2
0.9
7
0.9
8
0.7
2
0.7
6
1.0
7
H-C
-2J3
P
0.8
1
1.7
2
0.9
6
0.5
4
0.9
6
1.3
1
0.9
2
1.5
7
0.7
7
1.2
5
0.9
9
2.5
9
0.9
1
0.9
2
1.0
2
H-C
-4J6
P
0.7
6
1.5
1
0.9
9
0.4
3
1.0
2
1.2
3
0.9
5
1.8
2
0.9
4
1.0
5
1.0
7
2.5
5
0.9
3
0.9
5
1.1
2
H-C
-6J9
P
0.6
7
1.4
6
0.9
5
0.3
9
1.0
0
1.2
9
0.9
1
2.1
1
0.9
6
0.9
9
1.0
6
2.6
3
0.8
8
0.9
0
1.1
3
58
Tab
le 1
1. C
om
par
ison o
f m
odel
pre
dic
tions
wit
h e
xper
imen
tal
resu
lts
of
squar
e sp
ecim
ens.
Sp
ecim
en
[75
] [3
3]
[58]
[77]
[54
] [1
7]
[76
] [7
3]
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊπ
π) π
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊπ
π) π
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊπ
π) π
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊπ
π) π
(π
β² ππ) π
(πβ² ππ
) π
( πΊ
ππ) π
( πΊπ
π) π
(π
β² ππ) π
(πβ² ππ
) π
(πβ² π
π) π
(πβ² ππ
) π
(π
β² ππ) π
(πβ² ππ
) π
L-S
-2J3
P
0.7
8
0.6
7
0.8
5
0.9
1
0.8
8
0.7
6
1.1
4
1.3
4
0.9
9
- 0
.78
0
.76
0.8
1
L-S
-4J6
P
0.6
1
0.7
4
0.7
2
0.8
2
0.7
7
0.6
7
1.0
5
1.4
0
0.9
4
- 0
.62
0
.59
0.6
4
L-S
-6J9
P
0.5
3
0.9
3
0.6
7
0.9
0
0.7
3
0.7
2
1.0
2
1.6
4
0.9
4
- 0
.54
0
.50
0.5
6
H-S
-2J3
P
0.8
7
1.0
7
0.9
2
1.6
3
0.9
4
1.3
6
1.1
7
1.9
5
1.0
2
0
.87
0
.86
0.8
9
H-S
-4J6
P
0.8
1
0.9
9
0.9
0
1.2
5
0.9
5
1.0
1
1.2
5
1.6
7
1.0
9
0
.82
0
.79
0.8
4
H-S
-6J9
P
0.7
8
1.1
3
0.9
1
1.2
7
0.9
7
0.9
9
1.3
2
1.8
1
1.1
7
0
.78
0
.75
0.8
0
59
The performance of the aforementioned strength and strain models in
predicting the stress and strain of the JPFRP-wrapped specimens of the current study
was further assessed by statistical indicators such as Mean, standard deviation (SD),
coefficient of variation (COV), and average absolute error (AAE), presented in
equations (10)β(13), respectively. Standard deviation indicates the accuracy of the
data points to its mean value. Coefficient of variation can be well-defined as the ratio
of the standard deviation to its statistical mean. It shows variability in the data points
around the mean value. The average absolute error indicates the difference between
the average absolute errors of predicted and experimental results. The alphabet n
represents the total number of data points obtained from test results. Also, the
performance comparison between the predicted values from stress-strain models and
the experimental results are graphically presented in Figures 22 (a-d), in which the
models predicted values below the line of equality, indicating the underestimation,
while the predicted values above the equality line imply overestimation. Furthermore,
the statistical performance of the given existing models are mentioned in Tables 12
and 13 in the form of (πβ²ππ
)π
(πβ²ππ
)π
β and (πππ)π (πππ)πβ .
Mean = 1
nβ (
Predicted
Experimental)
i
n
i, (10)
SD = β1
nβ ((
Predicted
Experimental)
i
β Mean)
2n
i=1 ,
(11)
COV =Standard Deviation
MeanΓ 100 ,
(12)
AAE =1
nβ |
Predicted β Experimental
Experimental|
n
i=1Γ 100
(13)
60
0
10
20
30
40
50
60
0 10 20 30 40 50 60
(f' c
c)p
(f'cc)e
(a)
Huang et al
Karbhari and Gao
Pimanmas et al
Spoelstra and Monti
Miyauchi et al
Lam and Teng
A.Padanattil et al
Fardis and Khalili
Ilki and Kumbasar
0
1
2
3
4
5
0 1 2 3 4 5
(Ζcc
) p
(πΊππ )π
(b)
Huang et alKarbhari and GaoPimanmas et alSpoelstra and MontiMiyauchi et alIlki and Kumbasar
61
Figure 22. Performance of existing strength and strain models: (a) Strength models
of circular specimens; (b) strain models of circular specimens; (c) strength models of
square specimens; and (d) strain models of square specimens.
0
10
20
30
40
50
60
0 10 20 30 40 50 60
(f' c
c)p
(f'cc)e
(c)
Shehata et al
Pimanmas et al
Lam and Teng
Wei and Wu
Hussain et al
Kumutha et al
Al-Salloum
Mirmiran et al
0
1
2
3
0 1 2 3
(πΊππ) p
(πΊππ )π
(d)
Shehata et al
Pimanmas et al
Lam and Teng
Wei and Wu
62
T
ab
le 1
2. S
tati
stic
al p
erfo
rman
ce o
f th
e st
ren
gth
model
vs.
ex
per
imen
tal
dat
a of
circ
ula
r sp
ecim
ens.
M
od
els
(πβ² ππ
) π(π
β² ππ) π
β
( πΊππ
) π( πΊ
ππ) π
β
Av
era
ge
SD
C
OV
(%
) A
AE
(%
) A
ver
ag
e S
D
CO
V (
%)
AA
E (
%)
Huang e
t al
. [4
9]
0.5
6
0.0
9
17
.55
56
.84
1.0
8
0.1
9
17
.54
21
.27
Kar
bhar
i an
d G
ao [
59
] 0
.90
0
.08
9.3
8
9.4
0
0.3
1
0.0
6
19
.83
78
.71
Pim
an
mas
et
al.
[36
] 0
.95
0
.06
6.4
1
6.8
6
0.5
8
0.1
3
22
.77
27
.45
Ilki
and
Ku
mb
asa
r [7
2]
0.8
6
0.0
7
8.6
6
16
.43
0.9
8
0.4
6
46
.69
17
.84
Sp
oel
stra
and
Mo
nti
[6
8]
0.8
7
0.0
7
8.0
9
13
.60
0.7
5
0.3
6
48
.37
48
.83
Miy
auch
i 1
99
7 [
76
] 1
.00
0
.06
6.0
0
1.1
7
1.8
6
0.7
5
40
.60
42
.22
Lam
and
Ten
g 2
00
2 [
61
] 0
.84
0
.09
10
.89
19
.16
- -
- -
Pad
anat
til
et a
l. [
48
] 0
.86
0
.09
9.9
3
16
.64
- -
- -
Far
dis
and
Khal
ili
[65
] 1
.07
0
.06
5.4
3
6.0
2
- -
- -
Tab
le 1
3. S
tati
stic
al p
erfo
rman
ce o
f th
e st
ren
gth
model
vs.
ex
per
imen
tal
dat
a of
squar
e sp
ecim
ens.
M
od
els
(π
β² ππ) π
(πβ² ππ
) πβ
( πΊππ
) π( πΊ
ππ) π
β
Av
era
ge
SD
C
OV
(%
) A
AE
(%
) A
ver
ag
e S
D
CO
V (
%)
AA
E (
%)
Shehat
a et
al.
[6
9]
0.7
3
0.1
2
16
.14
29
.29
0.9
2
0.1
67
18
.11
23
.48
Pim
an
mas
et
al.
[36
] 0
.83
0.0
9
11
.99
19
.21
1.1
3
0.2
8
24
.98
10
.13
Lam
and
Ten
g [
58
] 0
.88
0.0
9
10
.54
14
.32
0.9
2
0.2
4
25
.82
27
.07
Wei
and
Wu [
71
] 1
.16
0.1
8
.97
14
.44
1.6
3
0.2
1
13
.11
36
.07
Huss
ain e
t al
. [5
5]
1.0
3
0.0
8
8.1
4
1.5
6
- -
- -
Ku
mu
tha
et a
l. [
18
] 0
.74
0.1
2
15
.9
28
.8
- -
- -
Al-
Sall
ou
m [
70
] 0
.71
0.1
2
17
.49
31
.88
- -
- -
Mir
mir
an e
t al
. [7
5]
0.7
6
0.1
2
15
.54
26
.72
- -
- -
63
From Figure 22a and Table 12 it can be observed that the model of [47]
highly underestimate the compressive strength of JPFRP-wrapped circular concrete
with an AAE of 56.84%, although the variation in the data was less than 20%. The
model of [76] accurately predicted the compressive strength with an AAE of 1.17%.
Moreover, the models of [71], predicted the compressive strength of the circular
columns reasonably close, with an AAE of 6.02%, 6.86%, and 9.40%, respectively.
The strength models of [74], also performed better with an AAE of less than 20%.
Furthermore, for strain models of circular specimens, Figure 22b shows very scattered
data points, reflecting that almost all the strain models either underestimate or
overestimate the experimental results. However, the strain models of [47],[78]
predicted the strain values slightly closer to the experimental strain values with an
AAE of 17.84% and 21.27%, respectively. Similarly, Table 12 and Figure 22c
exemplifies that the strength model of [55] accurately predicts compressive strength
for square columns. The models of [63] provide underestimation with an AAE of less
than 20%. On the other hand, the model of [77] shows overestimation with an AAE of
14.44%. The stress-strain models of [75] predicted the strength and strain values with
an AAE of 30% and 24%, respectively. The strength models of [76], predicted the
axial strength with an AAE of around 30%. Table 13 and Figure 22d show that the
strain models of square specimens show better results as compared with the strain
models of circular specimens. The model of [77] provides high overestimation with
an AAE of 36.07%. The model of [33] considered the parameter of corner radius and
strength for square specimens. Thus, their strain model performed reasonably well
with an AAE of 10.13%.
64
CONCLUSIONS
This paper presents an experimental investigation of Jute-Polyester hybrid
FRP (JPFRP) confinement of circular and square standard concrete specimens to
assess its suitability and structural performance. For this purpose, a total of 32
concrete specimens were tested under monotonic axial compression loads. Based on
the experimental results of this study and analytical evaluation, the following
conclusions are made:
β’ The jute-FRP composite showed a linear stress-strain response, whereas
polyester-FRP composite showed a bi-linear stress-strain response. Moreover,
the tensile testing of hybrid JPFRP composite coupon showed a linear stress-
strain response with an ascending parabolic shape.
β’ The JPFRP hybrid confinement is effective in improving the compressive
strength and strain capacity of both circular and non-circular specimens. The
increase in strength and strain is directly dependent upon the increasing
number of FRP layers.
β’ The efficacy of the JPFRP in providing external confinement is more
dominant in concrete specimens having low strength as compared to
specimens with high strength. Furthermore, the JPFRP jacketing is more
efficient and effective in circular concrete columns than in square concrete
columns.
β’ The stress-strain behavior in high strength specimens was dominated by the
large strain behavior of polyester FRP. The inner FRP sheet ruptures after
reaching the peak strength; afterwards, the outer FRP sheet provides enough
resistance to the lateral expansion of the concrete core, until it reaches the
ultimate state.
65
β’ The hybrid confined significantly enhanced the fracture energy and ductility
index of the confined concrete. The increase in fracture energy and ductility
index varies with the number of FRP layers and concrete strength. In addition,
the external confinement helped the concrete columns to stay for an extended
period without reaching its failure after reaching the peak axial load. This
phenomenon was more dominantly observed in high strength specimens,
which results in a ductile failure.
β’ Majority of the existing strength and strain models developed for conventional
FRP-wrapped concrete tends to underestimate the compressive strength of the
JPFRP-wrapped concrete. However, the best prediction of compressive
strength was made by the models of [76] and [55]. The existing strain models
did not perform well in predicting the strain of JPFRP-wrapped circular and
square concrete specimens.
66
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71
CURRICULU M VITAE
CURRICULUM VITAE
NAME Nauman Wahab
DATE OF BIRTH 23 September 1994
BIRTH PLACE Pakistan
ADDRESS Peshawar University Campus, KPK, Pakistan
EDUCATION Master of Engineering (Sustainable Energy and Resources
Engineering)
Bachelor of Science (Civil Engineering)
WORK EXPERIENCE One Year
Civil Engineer China State Construction Engineering
Corporation Limited (CSCEC)
AWARD RECEIVED -
PUBLICATION Wahab, N.; Srinophakun, P.; Hussain, Q.; Chaimahawan,
P. Performance of Concrete Confined with a Juteβ
Polyester Hybrid Fiber Reinforced Polymer Composite: A
Novel Strengthening Technique. Fibers 2019, 7, 72.
SCHOLARSHIP 1. Japan Student Service Organization (JASSO)
2. Kanazawa University, Japan. [Science and Technology
Semester Exchange Scholarship]
3. Thailand Advance Institute of Science and Technology
and the Tokyo Institute of Technology Collaborative
Master Degree Program Scholarship ( TAIST-TOKYO
TECH)
4. Faculty of Engineering Scholarship for International
Students, Kasetsart University