Strengthening of small concrete columns by natural fiber ...

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STRENGTHENING OF SMALL CONCRETE COLUMNS

BY NATURAL FIBER REINFORCED POLYMERS

COMPOSITES (HEMP FRP AND SISAL FRP)

BY

ARISSAMAN SANGTHONGTONG

A THESIS SUBMITTED IN PARTIALFULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

(ENGINEERING AND TECHNOLOGY)

SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY

THAMMASAT UNIVERSITY

ACADEMIC YEAR 2016

Ref. code: 25595722040218TUO

STRENGTHENING OF SMALL CONCRETE COLUMNS

BY NATURAL FIBER REINFORCED POLYMERS

COMPOSITES (HEMP FRP AND SISAL FRP)

BY


A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE

(ENGINEERING AND TECHNOLOGY)

SIRINDHORN INTERNATIONAL INSTITUTE OF TECHNOLOGY

THAMMASAT UNIVERSITY

ACADEMIC YEAR 2016

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Abstract

STRENGTHENING OF SMALL CONCRETE COLUMNS BY NATURAL FIBER

REINFORCED POLYMERS COMPOSITES (HEMP FRP AND SISAL FRP)

by


Bachelor of Engineering (Civil Engineering and Technology), King Mongkut’s

University of Technology North Bangkok, 2014

Master of Science (Engineering and Technology), SIIT Thammasat University, 2016

During the last decade, the use of fiber reinforced polymer (FRP)

composites has been successfully promoted for external confinement of reinforced

concrete (RC) columns all over the world. This technique is considered superior to

conventional concrete and steel jacketing methods in terms of confinement strength,

post-retrofit ductility, sectional areas, weight, corrosion resistance, ease of

application, and overall project costs. The existing research on FRP confined concrete

column is mainly concentrated on the use of artificial fibers such as carbon, glass,

aramid, PET and PEN. This research presents results of an experimental study on the

behavior of axially loaded concrete columns that have been strengthened with natural

fiber reinforced polymer (CFRP) composites. Both hemp and sisal natural fibers were

investigated. Six series, forming a total of 90 specimens, were subjected to axial

compression. All the test specimens were loaded to failure in axial compression and

investigated in both axial and transverse directions. The parameters considered are the

shape of cross section shape (i.e. circular, square and rectangular); the size of column;

the number of fiber wrap layers (i.e., two and four layers) and the fiber material such

as hemp and sisal. The experimental results clearly demonstrate that the CFRP

confinement enhances the compressive strength and the ductility of concrete columns.

The ultimate strength and the ductility of the NFRP confined concrete increase with

increasing number of confining layers. The efficiency of NFRP confinement is very

sensitive to the column cross section geometry. When column sizes increase there is a

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reduction in ultimate stress both for NSFRP and NHFRP strengthened specimens. The

NHFRP strengthening is more efficient than NSFRP

Keywords: Compressive behavior, hemp, strengthening, deformability, Hemp Fiber,

Sisal Fiber, Reinforced polymer (FRP)

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Acknowledgements

The author would like to express his appreciation of gratitude to his

advisor, Prof. Dr. Amorn Pimanmas, for his valuable suggestions, constructive

criticism, discussion and persistent supervision. The sincere appreciation is also

extended to Assoc. Prof. Dr. Winyu Rattanapitikorn and Asst. Prof. Dr. Suniti Suparp

for their serving as members of the examination committee. The sincere thanks are

also Dr. Qudeer Hussain for his helps and suggestions.

In addition, the author would like to thanks are also extended to Asian

Institute of Technology (AIT) for supporting test facilities.

Finally, I must express my very profound gratitude to my parents and to

my friend for providing me with unfailing support and continuous encouragement

throughout my years of study and through the process of researching and writing this

thesis. This accomplishment would not have been possible without them. Thank you.

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Table of Contents

Chapter Title Page

Signature Page i

Abstract ii

Acknowledgements iv

Table of Contents v

List of Table viii

List of Figures ix

1 Introduction 1

1.1 General 1

1.2 Significant of Study 2

1.3 Statement of Problems 2

1.4 Purpose of Study 2

2 Literature Review 3

2.1 Introduction 3

2.2 Cylinder Concrete columns with CFRP 3

2.3 Square Concrete columns with CFRP 5

2.4 Cylinder and Square Concrete columns with CFRP 8

2.5 Cylinder, Square and Rectangular Concrete columns

with CFRP 11

2.6 Cylinder and square concrete columns with aramid FRP 14

2.7 Properties of Natural Fiber 16

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3 Tentative Designed-Method 19

3.1 Proposal Research program 19

3.2 Experimental Program 19

3.3 Strengthening Scheme 22

3.4 Loading setup 22

4 Specimens Preparation 23

4.1 Fiber Strengthening 23

4.2 Epoxy Resin 25

4.3 Concrete Preparation 26

4.4 Casting 28

4.5 Corners Preparation 29

4.6 Strengthening 30

4.7 Capping 31

4.8 Test set up 32

4.8.1 Strain gauge 32

4.8.2 Load setup 32

5 Conclusions and Recommendations 35

5.1 Analysis Failure Modes 35

5.1.1 Circular specimens 35

5.1.2 Square and Rectangular specimens 36

5.2 Load Capacity of Concrete Columns 36

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5.2.1 Circular column specimens 37

5.2.1.1 Circular 1 (50x100) 37

5.2.1.2 Circular 2 (100x200) 37

5.2.1.3 Circular 3 (150x300) 38

5.2.2 Square column specimens 41

5.2.2.1 Square 1 (50x50x100) 41

5.2.2.2 Square 2 (100x100x200) 42

5.2.2.3 Square 3 (150x150x300) 42

5.2.3 Rectangular column specimens 46

5.2.3.1 Rectangular 1 (50x100x100) 46

5.2.3.2 Rectangular 2 (100x150x200) 46

5.2.3.3 Rectangular 3 (150x225x300) 47

5.3 Effect of Fiber thickness 50

5.4 Effect of Fiber material 54

5.5 Effect of Section shape 54

5.6 Effect of Size 54

References 61

Appendices 64

Appendix A 65

Appendix B 80

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List of Tables

Tables Page

2.1 Properties of Hemp Fiber 16

2.2 Properties of natural and synthetic fibers 17

3.1 Properties of specimens 20

4.1 Mechanical properties of sisal FRP composites using epoxy resin 24

4.2 Mechanical properties of hemp FRP composites using epoxy resin 25

4.3 Mechanical properties of epoxy resin 27

4.4 Concrete mix composition (per cubic meter) 17

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List of Figures

Figures Page

2.1 Test and Instrumentation Configurations 4

2.2 Example of Failure Condition 4

2.3 Test Setup for Columns with Eccentric Loadings 5

2.4 (a) Creep testing machine and (b) Compression test machine 6

2.5 Specimen dimensions 6

2.6 Axial stress versus CFRP strain and axial strain for

small wrapped prisms 7


medium wrapped prisms 7


large wrapped prisms 8

2.9 (a) Effect of size on peak axial stress

(b) Effect of size on peak axial strain 8

2.10 Stress strain curves of normal strength for CFRP confined specimens 9

2.11 Stress strain curves of high strength forCFRP confined specimens 10

2.12 Failed unconfined and confined square specimens 11

2.13 Failed unconfined and confined cylindrical specimens 11

2.14 Cross section types and strengthening of tested columns 12

2.15 Obtained stress-strain curves for confined circular, square

and rectangular columns 13

2.16 Compressive strength and ultimate strains value for

the tested concrete column 14

2.17 Geometric characteristic of specimens (mm) 15

2.18 Types of axial stress-strain curves 15

2.19 Failure modes of confined specimens 16

2.20 Hemp fiber in concrete 17

2.21 (a) Tensile fracture sample of sisal FRP

(b) Sisal fabric reinforced RC beam 18

3.1 Details of test specimen (units in mm) 21

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3.2 Loading setup 22

4.1 (a) Sisal Fiber (b) Hemp fiber (c) Sisal fabric (d) Hemp fabric 23

4.2 Epoxy resin 26

4.3 Preparation of concrete mixing 27

4.4 Slump test 28

4.5 Concretes casting in the molds 28

4.6 Column specimens 29

4.7 Square and rectangular cross section columns 30

4.8 Strengthening of specimens 31

4.9 Column specimens after capping 31

4.10 Strain gauge on Column specimens 32

4.11 Strain gages setup 33

4.12 Loading set 34

4.13 Column specimens on UTM 34

5.1 Axial stress – axial deformation curves of circular columns 38

5.2 Axial stress – axial deformation curves of square columns 43

5.3 Axial stress – axial deformation curves of rectangular columns 47

5.4 Comparison of normalized stress (Effect of fiber thickness) 51

5.5 Comparison of normalized stress (Effect of fiber material) 55

5.6 Comparison of normalized stress (Effect of section shape) 58

5.7 Comparison of normalized stress (Effect of size) 59

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Chapter 1

Introduction

1.1 General

The need for strengthening or retrofitting reinforced concrete (RC) and pre-

stressed concrete (PC) structures is becoming more apparent, particularly when there

is an increase in load requirements, a change in use, a degradation problem, or some

design/construction defects. Increase in load is mainly because of natural disasters

such as earthquake. At present, earthquakes are occurring all over the world. These

earthquakes are occurring at a higher rate than ever before and pose a much greater

risk to people living nearby. The structures which were designed prior to development

of modern seismic codes maybe destroyed during any earthquake event. Potential

solutions range from replacement of the structures to strengthening with a variety of

techniques. The use of Fiber reinforced polymer (FRP) materials for structural repair

presents several advantages and had been recently investigated all over the world.

These FRP composites are usually comprised of uni-directional or bi-directional

carbon, glass, and aramid fiber with suitable epoxy resin. Other types of FRP are

sprayed FRP, in which chopped glass or carbon fibers are sprayed on the surface of

concrete structures. The resulted material is randomly distributed fibers with resin,

which can be used for strengthening purpose. The main advantages of these FRP

materials are due to their light weight, high strength and stiffness, resistance to

corrosion, flexibility, and ease of application. However these conventional FRP are

usually chemical based which are hazardous to the environment. In addition these

FRP are relatively expensive.

In contrast to the conventional chemical based FRPs, recently a new method

of strengthening “Natural Fiber Reinforced Polymer (NFRP)” has been studied.

Natural fibers are usually made from plant leaves which are environment friendly and

low price compared with chemical based FRPs. The additional advantages of natural

fibers are high toughness and acceptable engineering properties. The present research

work is aimed to investigate the potential use of different natural fibers such as Sisal

and Hemp fibers for seismic strengthening or rehabilitation of the concrete structures.

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1.2 Significant of Study

A detail review of exiting literature shows that few research efforts are

available to enhance strength and ductility of reinforced concrete member by using

natural fibers. Further these available studies were mostly conducted on small scale

reinforced concrete columns using sisal and hemp fibers. However no research

activity is found on potential use of natural fibers such as Hemp and Sisal for seismic

strengthening of RC large scale columns. The present study will provide a better

understanding about Natural Fiber Reinforced Polymer Composites.

1.3 Statement of Problems

During severe earthquake, the structure is likely to undergo inelastic

deformation and has to depend on the ductility and energy absorption capacity to

avoid collapse. Such buildings designed for gravity loading need to be strengthened to

increase strength, stiffness and ductility. In this study, the effect of NFRP composites

on strength and ductility of low rise buildings (1-2 stories) will be investigated in

detail.

1.4 Purpose of Study

The main objective of this study is to investigate the potential use of natural

fiber such as hemp fiber and sisal fiber to enhance strength and ductility of reinforced

concrete members. The research parameters included will be concrete columns such

as shape of concrete columns there are three shapes as cylinder, rectangular and

square, the size of concrete columns there are three sizes as small, medium and large

and the number of FRP layers (2 and 4 layers). All specimens were tested by axial

compressive load. These concrete columns will be strengthened using natural fibers

with epoxy resin.

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Chapter 2

Literature review

2.1 Introduction

In the past, Fiber reinforced polymer (FRP) composite have been successfully

applied to retrofit and strengthen RC members. Several composite jacketing systems

have been developed and validated in research. Many researchers are being developed

in different research centers, in order to study the behavior of concrete strengthened

with FRP. These studies mainly focused to investigate different retrofitting methods

to enhance the flexural strength, ductility and shear strength of reinforced concrete

columns confined with glass and carbon fibers. The parameters considered were

number of composite layers, the compressive strength of the unconfined concrete and

the cross-section shape.

2.2 Cylinder Concrete columns with CFRP

Y. Xiao and H. Wu [1] studied axial compression test results of 27 concrete

cylinders confined (diameter of 152 mm and a height of 305 mm) by carbon fiber.

The main parameters are unconfined concrete strength and thickness (1 to 3 layers) of

carbon fiber composite jackets. The concrete strengths were 27.6 MPa, 37.9 MPa and

48.2 MPa for lower, medium and higher strength concrete, respectively. All the

specimens were tested using a high-stiffness, high-capacity compression testing

machine. The result showed that the strength and ductility of concrete can be achieved

by carbon fiber composite jacketing. The efficiency performance is higher for lower

concrete strength than high concrete strength. The stress-strain performances of

confined concrete with carbon fiber after exceeding unconfined concrete strength

exhibit an approximately bilinear behavior which can be formulated by empirical

equations from the tests.

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Fig.2.1 Test and Instrumentation Configurations

The final failure corresponding to the rupture of the carbon fiber jacket was very

explosive. For concrete cylinders confined by carbon and E-glass, the typical failure is

shown in Fig.2.2

Fig.2.2 Example of Failure Condition

J.F. Berthet and all [2] studied the influences of the confinement level and mechanical

properties of the jacket sand the compressive strength of concrete. Five different

concretes have been tested and two level of jackets. The behaviors of different

confined specimens were compared to evaluate the influence of the jacket properties

on the stress–strain response. This result has shown that the ultimate strengths and

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strains increase with the enhancement of the number of composite layers and bilinear

behavior of confined concrete. The curvature of the transition zone and the slope of

the pseudo-plastic branch depend on the jacket stiffness. The failures of the specimens

occurred before the shell fibers reached their ultimate strain capacities at the middle

high of column.

2.3 Square Concrete columns with CFRP

Azadeh Parvin and Wei Wang [3] proposed FRP (Carbon fiber) confined

square concrete columns under axial compressive loading with small eccentricities.

FRP coupon test was conducted to estimate the tensile strength, the modulus of

elasticity. This research investigates the effect of strain gradient and FRP thickness

(one and two layers). Additionally, control specimens without FRP jackets were

tested for comparison. When the axial load was added until the columns failed, the

unconfined concrete failed by crushing of the concrete on the side with larger

compression near the column mid-height. For one and two layers, the jacket was

separated from concrete surface.

Fig.2.3 Test Setup for Columns with Eccentric Loadings

Dian Jie Zhang and all [4] proposed the results of experimental and theoretical

investigations on the stress and strain of short square concrete columns confined by

fiber reinforced polymer (FRP) after creep by the creep testing. The specimen was

placed on the machine and first compressed to 30% of the theoretical ultimate load

which was recorded by the vibrating string strain gauge.

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Fig.2.4 (a) Creep testing machine and (b) Compression test machine.

The results shown the creep does not have an effect on the stress strain curves but that

increases the elastic modulus and slightly decreases the compressive strength of

square concrete columns. The comparison between the model predictions and the

experimental results showed good agreement.

Mark J. Masia and all [5] were investigating the carbon fiber reinforced polymer

(CFRP) wrapping to strengthen plain concrete prisms. The prisms of three different

square cross-sectional sizes (100 mm×100 mm× 300 mm, 125 mm×125 mm×375

mm, 150 mm×150 mm×450 mm) were tested under axial load in compression until

failure occurred. The axial stiffness for each prism was calculated base on the

procedure for determining the elastic modulus E of concrete (AS 1012.17).

Fig.2.5 Specimen dimensions.

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When the prism cross-sectional size was increased for a fixed corner radius, the

effectiveness of confinement was reduced. The effect of size was as significant as its

effect on the increase in strength has shown in Fig.2.6-2.8 for small, medium and

large column, respectively. Thus, significant increases in strength and ductility were

achieved by wrapping. The initial failure occurred when the brittle CFRP wrapping

suddenly ruptured at a corner. At these locations, the measured strains were found

higher than other positions.

Fig.2.6 Axial stress versus CFRP strain and axial strain for

small wrapped prisms.


medium wrapped prisms.

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large wrapped prisms.

Fig.2.9 (a) Effect of size on peak axial stress (b) Effect of size on peak axial strain

2.4 Cylinder and Square Concrete columns with CFRP

R.Benzaid and all [6] proposed the behavior of axially loaded short reinforced

concrete (RC) columns that have been strengthened with carbon fiber-reinforced

polymer (CFRP). Total of 48 specimens are circular and square RC columns and

unconfined concrete were subjected to axial compression. The parameters considered

are the shape of cross section shape, the number of CFPR layers (1 and 3 layers), the

concrete strength (high and normal strength) and plain concrete (PC) and reinforce

concrete (RC) columns. For all RC specimens the diameter of longitudinal and

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transverse reinforcing steel bars were respectively 12 mm and 8 mm. The longitudinal

steel ratio was constant for all specimens and equal to 2.25%.The experimental results

demonstrate that the ultimate strength and the ductility of the CFRP confined concrete

increase with increasing number of confining layers. The increase in strength and

ductility is more significant for lower strength concrete. The efficiency of the CFRP

confinement is higher for circular than for square sections, as expected. The increase

of ultimate strength of sharp edged sections is low as shown Fig.2.10

(a) Cylinder of PC column series (b) Cylinder of RC column series

(c) Square of PC column series (d) Square of RC column series

Fig.2.10 Stress strain curves of normal strength for CFRP confined specimens

The CFRP confinement on low-strength concrete specimens produced higher results

in terms of strength and strains than for high-strength concrete as shown Fig.2.11

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(a) Cylinder of PC column series (b) Cylinder of RC column series

(c) Square of PC column series (d) Square of RC column series

Fig.2.11 Stress strain curves of high strength for CFRP confined specimens

The failure of CFRP wrapped specimens occurred in a sudden and explosive manner.

For cylindrical specimens, the fiber rupture started mainly in the central zone, but

square specimens failed near a corner, because of the high stress concentration. CFRP

strengthened specimens showed a typical bi-linear trend with a transition zone.

Yousef A. Al-Salloum and all [7] reported the influence of the radius of the cross-

sectional corners (edges) of about 1/6, 1/4, and 1/3on the strength of small scale

square concrete column specimens confined with FRP composite laminates and

cylinder column. These specimens were tested in uniaxial compression. All columns

were instrumented with strain gauges at mid-height of unconfined and confined

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specimens to measure lateral strains. To measure axial strain, each specimen was

fitted with LVDTs. The load was applied at a loading rate of 4 kN/s. The best

performance was that of the circular specimens. The performances of square

specimens are directly related to the radius of the cross-section edges. The failure of

the square columns always starts at one of the corners while the failure of cylinder

column started at the middle height. In addition, analytical model is presented to

predict the strength of FRP-confined. There are excellent agreements with the

measured ones.

Fig.2.12 Failed unconfined and confined square specimens

Fig.2.13 Failed unconfined and confined cylindrical specimens

2.5 Cylinder, Square and Rectangular Concrete columns with CFRP

I.A. E. M. Shehata and all [8] investigated the gain in strength and ductility of

concrete columns confined by CFRP. Total of 54 short column specimens with

circular column, square column and rectangular column and with the ratio of

specimens height/short size length of the section equal to 2, were tested. The number

of CFRP sheet layers applied to the models was one or two layers. The results of an

experimental program were studied on the behavior of short concrete column. These

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obtained experimental strengths and ultimate strains are compared with the equations

to estimate the confined concrete strength under a constant rate of compressive axial

load (0.22 MPa/s) until their failure.

Fig.2.14 Cross section types and strengthening of tested columns.

The obtained results as shown in Fig.2.15 can be clearly noticed that both the stress

and strain at failure for the confined columns were higher than those for the

unconfined ones and the highest values were obtained for confined circular columns

with 2 CFRP layers. The strengths and ultimate strains were increased when the

number of sheet layers increased. The failure modes for all confined columns with 1

layer have a mid height of the column and one-fourth to midway of the height with 2

layers. For the square and rectangular columns, the failure occurred at one of the

columns corners.

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Fig.2.15 Obtained stress-strain curves for confined circular, square

and rectangular columns.

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Fig.2.16 Compressive strength and ultimate strains value for

the tested concrete column

2.6 Cylinder and square concrete columns with aramid FRP.

Yuan-feng Wang and Han-liang Wu [9] studied concrete short columns

confined with aramid FRP (AFRP). In this study, 99 confined concrete short columns

wrapped with circular and square cross sections were tested under axial compressive

loading and the effect of size and concrete strength was studied. The height to width

ratio or diameter was constantly equal to 3. In each group of circular specimens, there

were three different scaling dimensions 70×210 mm for the small cylinders, 105×315

mm for the medium one, and 194×582 mm for the large one. In each group of square

specimens, the sizes were 70×210 mm for the small square column, 100×300 mm for

the medium column and 150×450 mm for the large column. The geometric

characteristic of the specimens is shown in Fig.2.17

Fig.2.17 Geometric characteristic of specimens (mm)

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The experimental nominal axial stress-strain curves of the confined specimens are

characterized by three types as in Fig.2.18. The curves are bilinear by two key points

are the kink or transitional point (TP) and the end or ultimate point on the curve (UP).

For the specimen with a higher confinement ratio (confinement pressure per strength

of unconfined concrete), the shape will be the first type where the specimen can reach

the load until point TP and increased to point UP. The third type curve was for the

specimen with a smaller confinement ratio and they failed at the ultimate point.

(a) Type I (b) Type II (C) Type III

Fig.2.18 Types of axial stress-strain curves

For the failure mode, the AFRP ruptured either at the mid height or at the end of the

circular specimens. For the square specimens, stress concentrations in the regions

close to the corners led to the rupture of the AFRP sheets as shown in Fig. 2.19.

Fig.2.19 Failure modes of confined specimens

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2.7 Properties of Natural Fiber

Zhijian Li and all [10] studied the mechanical and physical properties of hemp

fiber reinforced concrete (HFRC). The results of the physical and mechanical

properties of HFRC showed that fiber factors (fiber content) have a significant

relationship with the mechanical and physical properties of cementations materials.

The properties of hemp fiber are shown in Table 2.1.

Table2.1 Properties of Hemp Fiber.

Properties Values

Specific gravity (g/mm3)

Fiber Content (%)

1.5

30 – 40

Width (µm) 23.15+ 17.60

Water absorption (%) 85-100

Tensile strength (MPa) 900

Modulus of elasticity (GPa) 34

Natural fibers are a high tensile strength and they have a low modulus of elasticity. F.

P. TORGAL and all [11] proposed that their tensile performance compares favorably

to synthetic fibers. Tensile strength and modulus of elasticity of hemp fiber is highest.

Table2.2 Properties of natural and synthetic fibers

Properties Specific gravity

(kg/m3)

Water

absorption (%)

Tensile

strength (MPa)

Modulus of

elasticity (GPa)

Sisal 11370 110 347-378 15.2

Coconut 1177 93.8 95-118 2.8

Bamboo 1158 145 73-505 10-40

Hemp 1500 85-105 900 34

Banana 1031 407 384 20-51

Polypropylene 913 - 250 2.0

I. Merta and all [12] reported the fracture energy of concrete reinforced with natural

fibers of hemp, elephant grass, and wheat straw, there are contained in concrete

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with0.19% of fibers by weight and of 40 mm of length were uniaxially tested with the

wedge splitting test (WST) method .The hemp fiber as reinforcement produces the

enhancement of the concrete fracture energy up to 70% compared to unreinforced

concrete and up to 2% and 5% for straw and elephant grass fiber, respectively. The

beneficial effect of hemp fiber is believed to be the result of the fiber high tensile

strength.

Fig.2.20 Hemp fiber in concrete

M.Ramesh and all [13] investigated the mechanical properties such as tensile and

flexural properties of hybrid glass fiber-sisal/jute reinforced epoxy composites. The

sisal/GFRP composite samples possess good tensile strength and can withstand the

strength up to 68.55 MPa. The jute/GFRP composite specimen is holding the

maximum flexural load of 1.03KN, slightly higher than the sisal/GFRP composite

sample. Whereas, Tara Sena and all [14] investigated the mechanical characterization

of the FRP and strengthening effects provided by the bonding of sisal FRP to beams

over bonding of carbon FRP and glass FRP. For the sisal FRP, it increased the

flexural strength as well as the tensile strength. The sisal FRP composite exhibited a

tensile strength of 223.367 N/mm2, which was 24% of the tensile strength of CFRP

(923.056 N/mm2) and 33% of the tensile strength of GFRP (678.571 N/mm

2). The RC

beam strengthened by SFRC showed highest amount of ductility and delayed the

formation of cracks without rupture failure. This result showed that the reinforcement

of woven sisal fiber reinforced polymer composites created a new alternate material.

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Fig.2.21 (a) Tensile fracture sample of sisal FRP (b) Sisal fabric reinforced RC beam

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Chapter 3

Tentative Designed-Method

3.1. Proposal Research program

In order to investigate the efficiency of natural hemp fiber and sisal fiber, the

proposed research work is divided into two parts.

A) Strengthening of concrete columns using hemp fibers. The main objective of this

experimental program is to evaluate the strengthening efficiency of natural hemp

fibers to enhance strength and ductility of concrete columns.

B) Strengthening of concrete columns using sisal fibers. The main objective of this

experimental program is to evaluate the strengthening efficiency of natural sisal fibers

to enhance strength and ductility of concrete columns.

The research parameters included are shape of concrete columns, size of

columns, number of layers and type of natural fibers (i.e. sisal and hemp fiber).

3.2 Experimental Program

The experimental program will be comprised of testing of 90 concrete

columns strengthened using natural fibers. The shapes of concrete columns consisted

of three groups such as cylinder, square and rectangular and they have three sizes for

each group. Each group has 30 specimens for difference in the type of fiber and

number of confinement. In all test conditions, there will be 2 specimens for the same

condition to find average value of the results. Table 3.1 shows the detail of specimens

in this experiment.

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Table3.1 Properties of specimens

Main

Groups

Subgroups

(size – mm) Specimen

NFRP

material

NFRP

thickness

(layers)

Number of

specimens

Circular Circular 1

(50 x 100)

C1-CON - - 2

C1-S-2 Sisal 2 2

C1-S-4 Sisal 4 2

C1-H-2 Hemp 2 2

C1-H-4 Hemp 4 2

Circular 2

(100 x 200)

C2-CON - - 2

C2-S-2 Sisal 2 2

C2-S-4 Sisal 4 2

C2-H-2 Hemp 2 2

C2-H-4 Hemp 4 2

Circular 3

(150 x 300)

C3-CON - - 2

C3-S-2 Sisal 2 2

C3-S-4 Sisal 4 2

C3-H-2 Hemp 2 2

C3-H-4 Hemp 4 2

Square Square 1

(50 x 50 x

100)

S1-CON - - 2

S1-S-2 Sisal 2 2

S1-S-4 Sisal 4 2

S1-H-2 Hemp 2 2

S1-H-4 Hemp 4 2

Square 2

(100 x 100 x

200)

S2-CON - - 2

S2-S-2 Sisal 2 2

S2-S-4 Sisal 4 2

S2-H-2 Hemp 2 2

S2-H-4 Hemp 4 2

Square 3

(150 x 150 x

300)

S3-CON - - 2

S3-S-2 Sisal 2 2

S3-S-4 Sisal 4 2

S3-H-2 Hemp 2 2

S3-H-4 Hemp 4 2

Rectangular Rectangular1

(50 x 100 x

100)

R1-CON - - 2

R1-S-2 Sisal 2 2

R1-S-4 Sisal 4 2

R1-H-2 Hemp 2 2

R1-H-4 Hemp 4 2

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Main

Groups

Subgroups


NFRP

material

NFRP

thickness

(layers)

Number of

specimens

Rectangular Rectangular2

(100 x 150 x

200)

R2-CON - - 2

R2-S-2 Sisal 2 2

R2-S-4 Sisal 4 2

R2-H-2 Hemp 2 2

R2-H-4 Hemp 4 2

Rectangular3

(150 x 225 x

300)

R3-CON - - 2

R3-S-2 Sisal 2 2

R3-S-4 Sisal 4 2

R3-H-2 Hemp 2 2

R3-H-4 Hemp 4 2

Fig.3.1 Details of test specimen (units in mm)

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3.3 Strengthening Scheme

The strengthening scheme is designed for the following objectives. To study

the influence of type of confined jacket effect on the strengthening. The results will be

compared in terms of the percentage of effective strength between hemp and sisal

fibers. In addition, the study the influence of number of confined jacket layers as 2

and 4 layers on the strengthening. The results will be compared in terms of the

percentage of effective strength between 2 and 4 layers confinement layers provided

all other conditions remained the same.

3.4 Loading setup

The compressive load was applied at a constant rate of 100 N/minute with a

maximum load capacity of 200 ton and the load was recorded with an automatic data

acquisition system. The axial load, vertical displacement, lateral expansion, and strain

were recorded by a load cell mounted on a hydraulic jack. The specimen was then

loaded under the manual control until failure.

Fig.3.2 Loading setup

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Chapter 4

Specimens Preparation

4.1 Fiber Strengthening

Natural sisal fibers were obtained from farms in Cha Am district, whereas

natural hemp fibers were obtained from farms in Konken district, Thailand. Sisal and

hemp fibers were extracted from the Agava Sisalana and Cannabis plan leaves,

respectively, A thin flat strip of fiber (see Fig 4.1(a) and Fig 4.1(b)) having a constant

rectangular cross section. The average thickness of fabric was 1.5 mm as shown in

figure 4.1 (c) and 4.1 (d).

Fig.4.1 (a) Sisal Fiber

Fig.4.1 (b) Hemp Fiber

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Fig.4.1 (c) Sisal fabric

Fig.4.1 (d) Hemp fabric

The tensile strength of sisal FRP composites were determined by testing the

strip specimens of sisal FRP in accordance with ASTM Standard D638 [28]. The

mechanical properties of sisal FRP composites are given in Table 4.1.

Table4.1 Mechanical properties of sisal FRP composites using epoxy resin

Properties Value Units

Tensile strength 104 MPa

Fracturing strain 0.41 %

Ultimate strain 3.48 %

Modulus of elasticity 3.19 GPa

The tensile strength of sisal FRP composites fiber were determined by testing

the strip specimens of sisal FRP in accordance with ASTM Standard D638 [28]. The

mechanical properties of GCSM composite are given in Table 4.2.

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Table4.2 Mechanical properties of hemp FRP composites using epoxy resin

Properties Value Units

Tensile strength 156 MPa

Fracturing strain 0.505 %

Ultimate strain 2.035 %

Modulus of elasticity 6.414 GPa

4.2 Epoxy Resin

The resin used in the experimental program was “Smart CF-Resin”

manufactured by Smart and Bright Co. Ltd. Thailand. The Smart CF-Resin is two part

high performance epoxy resin. Part A is comprised of epoxy resin and part B is a

hardener. Both parts are mixed together with the mixing ratio of 2:1 (i.e. A:B = 2:1).

The resin can be easily applied using trowel, roller or brush. The mechanical

properties of resin (curing time, tensile strength, tensile modulus and elongation at

break) of the resin as provided by the manufacturer are given in Table 4.3

Table4.3 Mechanical properties of epoxy resin

Properties Smart CF-Resin Unit

Curing time 7-10 Hours

Tensile strength 40-45 MPa

Flexural strength 70-75 MPa

Elongation at break 2.5 %

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Fig.4.2 Epoxy resin

4.3 Concrete Preparation

As the first step for preparation of specimens, the concrete mix was slowly

poured into the forms to prevent segregation. The concrete columns were made using

Type I Portland cement. The concrete mix proportion consists of 272 kg of cement,

221 kg of water, 721 kg of sand and 1,187 kg of aggregates. For the concrete property

tests, the slump test (ASTM C143) [16] is an empirical test that measures the

workability of fresh concrete to check the consistency of freshly made concrete which

is being filled in the molds. The test is carried out using a mould known as a slump

cone in three layers. Each layer is tamped 25 times with a steel rod to ensure

compaction. The third layer is finished off level with the top of the cone. The cone is

carefully lifted up. The decrease in the height of the center of the slumped concrete is

called “slump”. The amount of slump is measured in inches from the bottom of the

straight edge to the top of the slumped concrete. The slump is controlled near 8 cm. If

the slump is over the control range, a new mixing is required.

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Table4.4 Concrete mix composition (per cubic meter)

Components Quantity (kg)

Cement 272.00

Water 221.00

Sand 720.00

Gravel 1187.0

Water-to-cement ratio (W/C) 0.65

Fig.4.3 Preparation of concrete mixing

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Fig.4.4 Slump test

4.4 Casting

A ready-mix concrete was used in the molds and the vibrator was used to

vibrate the concrete carefully to prevent voids from forming during concreter filling

.A ready-mix concrete was used in the molds as shown in Fig .4.5. After 28 days of

curing, the compressive strength of concrete is controlled to be20MPa (ASTMC39)

[17] by testing a standard cylinder specimen with diameter of 15cm and height of 30

cm at 28 days for curing time. The concrete specimens were cleaned and completely

dried before the resin was applied.

Fig.4.5 Concretes casting in the molds

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Fig.4.6 Column specimens

4.5 Corners Preparation

Before strengthening, corners of the square and rectangular cross section

columns were rounded off to a radius of about 10, 20, 30 mm for the small, medium

and large columns, respectively (see Fig.4.7) in order to prevent breakage of the

natural fiber sheets due to sharp edges and the concrete surface of all columns was

cleaned and completely dried before applying resin.

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Fig.4.7 Square and rectangular cross section columns

4.6 Strengthening

The natural fibers were applied to the specimens by manual wet lay-up

process. A thin layer of primer epoxy was first applied to the concrete surface. After

the primer epoxy on the concrete surface was cured at the ambient temperature for

several hours the first fiber was carefully placed into the resin with gloved hands and

smooth out any air pockets. After installing the first layer, a second layer of resin was

applied to allow the impregnation of the second layer of the HFRP and SFRP. The

following layer is applied in the same way. Finally, a layer of resin was applied to

complete the operation. [18,19]

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Fig.4.8 Strengthening of specimens

4.7 Capping

Each layer had an overlap of about 100 mm to assure the development of full

composite strength. After strengthening, all column had their ends (top and bottom)

capped with plaster to assure parallel surfaces and uniform load distribution. [10,17]

Fig.4.9 Column specimens after capping

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4.8 Test set up

4.8.1 Strain gauge

The instrumentation is completed by strain gages bonded axially and

transversely at mid-height of specimens. The gages are bonded on the concrete

surface in the case of unconfined columns and on the composite shell for confined

concrete columns.

Fig.4.10 Strain gauge on Column specimens

4.8.2 Load setup

Both ends of all strengthened specimens were additionally wrapped

with two 20 mm wide strips of Glass Fiber Reinforced Polymer (GFRP) sheets to

avoid premature failure of NFRP shells at the ends. The compressive load was applied

a constant rate increasing of 4 kN/s and strains was recorded with an automatic data

acquisition system. All of the specimens were tested under concentric compression

using a testing machine with a maximum load capacity of 1,000 kN at AIT laboratory.

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The axial load, vertical displacement, lateral expansion, and strain were recorded and

measured by a load cell mounted on a hydraulic jack. Deflections were measured at

the bottom of concrete column by electronic LVDT transducers. On the top end of the

specimens were applied by gypsum in order to avoid direct eccentric load during the

test and the steel plate with 12 mm thickness, 150x150mm was applied to avoid the

direct eccentric load. Then, the load is applied at a constant rate of4 kN/s. The

comparison of the readings of the two vertical LVDTs located at the left- and right-

hand sides was implemented. The specimen was loaded under the manual

displacement control manner until failure. All specimens were instrumented by

electrical resistance strain gauges for measuring compressive strain at the mid-height

of the column. [18]

Fig.4.11 Strain gages setup

150x150

L1

L2

Vertical Strain Gages

Horizontal Strain Gages

150x150

LVDT

Steel Frame

Clamping Screw

Ref. code: 25595722040218TUO

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Fig.4.12 Loading set

Fig.4.13 Column specimens on UTM

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Chapter 5

Conclusions and Recommendation

All concrete column specimens were tested under monotonic uniaxial

compression up to failure in a Universal Testing Machine of 1000 kN capacity. The

load was applied at a constant rate of 4 kN/s and the strains were measured at the load

interval of 120 kN by an electronic data logger. Prior to the testing, some

arrangements were made to accurately measure the data and to avoid any possibility

of premature failure. These arrangements included;

1) All NFRP-confined and unconfined specimens were capped with sulfur mortar pad

at both ends to ensure a full contact surface, which results in a uniform distribution of

load over the entire cross sectional area.

2) The sulfur mortar cap was trimmed off over the NFRP jackets to avoid the

possibility of transmitting the axial load onto the fiber shell area.

3) Both ends of all strengthened specimens were additionally wrapped with two 20

mm wide strips of Glass Fiber Reinforced Polymer (GFRP) sheets to avoid premature

failure of NFRP shells at the ends.

4) In addition to the sulfur mortar capping, steel plates of 5 mm thickness were placed

at both ends to ensure the application of load over the confined concrete area and to

avoid any accidental axial load transfer onto the fiber shell especially when the

specimen undergoes a large deformation near the failure. Linear variable differential

transducers (LVDTs) were instrumented to the specimen to record axial deformation

of concrete during loading. To ensure the safety of instruments, all LVDTs were

removed prior to the final failure of specimens.

5.1 Analysis Failure Modes

5.1.1 Circular specimens

All NFRP-confined circular specimens failed by the rupture of

NFRP composite caused by the hoop tension associated with the lateral expansion

(See the Appendix A). The failure of the confined specimens resulted from the rupture

Ref. code: 25595722040218TUO

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of NFRP composite was suddenly brittle and characterized by a large explosive

sound. Comparatively, the rupture of NHFRP composite was more explosive and

brittle than NSFRP composite. Prior to the final rupture intermittent snapping sounds

indicating progressive fracturing of fibers could be clearly observed in all confined

circular specimens. The typical failure of SFRP-confined circular specimens is shown

in the Appendix A. The failure is characterized by a fully or partly vertical splitting

rupture of SFRP shells. This failure mode indicates that the hoop tension in the SFRP

shell which is caused by the transverse strain due to the lateral expansion of the

specimen exceeds the composite strength.

5.1.2 Square and Rectangular specimens

The typical failure of NFRP-confined square and rectangular

specimens also occurred due to the rupture of NFRP composites (see the Appendix

A). Similar to the case of circular specimens, the failure of square and rectangular

specimens was sudden and explosive. When the final failure was approaching, some

snapping sounds could be heard too. In almost all square specimens, the rupture of

SFRP shell started near one of the corners of the section due to the stress

concentration except in few specimens where the rupture of SFRP shell occurred at

the column face. The rupture of confining fibers at the corners due to stress

concentration has also been reported for FRP confined square and rectangular

columns [12].

5.2 Load Capacity of Concrete Columns

The axial stress-axial deformations of all circular, square and rectangular

NFRP confined columns under uniaxial monotonic loading are shown in Figures 5.1-

5.3. Average values of tested compressive strength and deformations are given in the

Appendix B. It can be seen that the confinement from NFRP jacketing is effective to

increase the ultimate strength and deformation of circular, square and rectangular

specimens. For all NFRP-confined specimens, the increase in the ultimate stress and

deformation is observed to vary with the increase in NFRP thickness. The test results

are discussed in detail in the following sections.

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5.2.1 Circular column specimens

5.2.1.1 Circular 1 (50x100)

The dimensions of the cylinder specimens in this group were

50 mm in diameter and 100 mm in height. The axial stress and deformation curves of

all specimens are shown in Figure 5.1a and 5.1b. The control specimen failed at the

average peak stress of 15.8 MPa. Among the strengthened specimens, a maximum

increase in the peak stress of 848% over the control specimen was recorded for

specimen C1-H-4, whereas a minimum increase of 556% was measured for specimen

C1-S-2 (see the Appendix B). The remaining column specimens in this group (i.e.,

C1-H-2 and C-S-4) reached peak stresses that were 532 and 785 greater than the

control specimen, respectively. Similar to the load carrying capacity, the axial

deformations of the NFRP-strengthened concrete columns were also increased. A

maximum increase in the axial deformation was 1700%, for specimen C1-S-4.

Column specimens C1-S-2, C1-H-2 and C1-H-4 reached the peak stress at 940%,

590% and 1100% enhanced axial deformations, respectively.

5.2.1.2 Circular 2 (100x200)



all specimens are shown in Figure 5.1c and 5.1d. The control specimen failed at the

average peak stress of 22.1 MPa. The maximum stress increase of 293% was

measured for the specimen C2-H-4 with 4 layers of NHFRP composite and the

minimum increase in peak stress of 126% was measured for column C2-S-2 with 2

layers of NSFRP composite. The increases in peak stress were 162% and 216% for

column specimens C2-H-2 and C2-H-4, respectively. Similar to the load carrying

capacity, the axial deformations of the NFRP-strengthened columns were also

increased. As shown in Figure 5.1c and 11d, the increases in the axial deformations of

300%, 500%, 200% and 400% were recorded for columns C2-S-2, C2-S-4, C2-H-2

and C2-H-4, respectively.

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5.2.1.3 Circular 3 (150x300)



all specimens are shown in Figure 5.1e and 5.1f. The control specimen failed at the

average peak stress of 26.9 MPa. Among the strengthened specimens, a maximum

increase in the peak stress of 197% over the control specimen was recorded for

specimen C3-H-4, whereas a minimum increase of 86% was measured for specimen

C3-S-2 (see the Appendix B). The remaining column specimens in this group (i.e.,

C3-H-2 and C3-S-4) reached peak stresses that were 94% and 164% greater than the

control specimen, respectively. Similar to the load carrying capacity, the axial

deformations of the NFRP-strengthened concrete columns were also increased. A

maximum increase in the axial deformation was 395%, for specimen C3-S-4. Column

specimens C3-S-2, C3-H-2 and C3-H-4 reached the peak stress at 155%, 101% and

50% enhanced axial deformations, respectively.

Fig.5.1 (a) Circular 1 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)

Axial deformation (mm)

C1-S-4

C1-S-2

C1-CON

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Fig.5.1 (b) Circular 1 with NHFRP

Fig.5.1 (c) Circular 2 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Axia

l st

ress

(M

Pa)


C1-H-4

C1-H-2

C1-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


C2-S-4

C2-S-2

C2-CON

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Fig.5.1 (d) Circular 2 with NHFRP

Fig.5.1 (e) Circular 3 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


C2-H-4

C2-H-2

C2-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


C3-S-4

C3-S-2

C3-CON

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Fig.5.1 (f) Circular 3 with NHFRP

Fig.5.1 Axial stress – axial deformation curves of circular columns

5.2.2 Square column specimens

5.2.2.1 Square 1 (50x50x100)

The dimensions of the square specimens in this group were

50 mm in width, 50 mm in depth and 100 mm in height. The axial stress and

deformation curves of all specimens are shown in Figure 5.2a and 5.2b. The control

specimen failed at the average peak stress of 12.7 MPa. Among the strengthened

specimens, a maximum increase in the peak stress of 848% over the control specimen

was recorded for specimen S1-H-4, whereas a minimum increase of 421% was

measured for specimen S1-S-2 (see the Appendix B). The remaining column

specimens in this group (i.e., S1-H-2 and S-S-4) reached peak stresses that were 532%

and 652% greater than the control specimen, respectively. Similar to the load carrying

capacity, the axial deformations of the NFRP-strengthened concrete columns were

also increased. A maximum increase in the axial deformation was 1475%, for

specimen S1-S-4. Column specimens S1-S-2, S1-H-2 and S1-H-4 reached the peak

stress at 875%, 590% and 1100% enhanced axial deformations, respectively.

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


C3-H-4

C3-H-2

C3-CON

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5.2.2.2 Square 2 (100x100x200)



deformation curves of all specimens are shown in Figure 5.2c and 5.2d. The control

specimen failed at the average peak stress of 17.7 MPa. The maximum stress increase

of 293% was measured for the specimen S2-H-4 with 4 layers of NHFRP composite

and minimum increase in peak stress of 182% was measured for column S2-S-2 with

2 layers of NSFRP composite. The increases in peak stress were 210% and 216% for

column specimens S2-H-2 and S2-H-4, respectively. Similar to the load carrying


increased. As shown in Figure 5.2c and 5.2d, the increases in the axial deformations

of 400%, 500%, 200% and 400% were recorded for columns S2-S-2, S2-S-4, S2-H-2

and S2-H-4, respectively

5.2.2.3 Square 3 (150x150x300)



deformation curves of all specimens are shown in Figure 5.2e and 5.2f. The control



was recorded for specimen S3-H-4, whereas a minimum increase of 86% was

measured for specimen S3-S-2 (see the Appendix B). The remaining column

specimens in this group (i.e., S3-H-2 and S3-S-4) reached peak stresses that were 94%

and 164% greater than the control specimen, respectively. Similar to the load carrying


also increased. A maximum increase in the axial deformation was 395%, for specimen

S3-S-4. Column specimens S3-S-2, S3-H-2 and S3-H-4 reached the peak stress at

155%, 118% and 340% enhanced axial deformations, respectively.

Ref. code: 25595722040218TUO

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Fig.5.2 (a) Square 1 with NSFRP

Fig.5.2 (b) Square 1 with NHFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S1-S-4

S1-S-2

S1-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S1-H-4

S1-H-2

S1-CON

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Fig.5.2 (c) Square 2 with NSFRP

Fig.5.2 (d) Square 2 with NHFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S2-S-4

S2-S-2

S2-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S2-H-4

S2-H-2

S2-CON

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Fig.5.2 (e) Square 3 with NSFRP

Fig.5.2 (f) Square 3 with NHFRP

Fig.5.2 Axial stress – axial deformation curves of square columns

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S3-S-4

S3-S-2

S3-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


S3-H-4

S3-H-2

S3-CON

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5.2.3 Rectangular column specimens

5.2.3.1 Rectangular 1 (50x100x100)

The dimensions of the rectangular specimens in this group

were 50 mm in width, 100 mm in depth and 100 mm in height. The axial stress and

deformation curves of all specimens are shown in Figure 5.3a and 5.3b. The control



was recorded for specimen R1-H-4, whereas a minimum increase of 556% was

measured for specimen R1-S-2 (see the Appendix B). The remaining column

specimens in this group (i.e., R1-H-2 and R-S-4) reached peak stresses that were 532

and 785 greater than the control specimen, respectively. Similar to the load carrying


also increased. A maximum increase in the axial deformation was 1507%, for

specimen R1-S-4. Column specimens R1-S-2, R1-H-2 and R1-H-4 reached the peak


5.2.3.2 Rectangular 2 (100x150x200)



deformation curves of all specimens are shown in Figure 5.3c and 5.3d. The control

specimen failed at the average peak stress of 16.6 MPa. The maximum stress increase

of 162% was measured for the specimen R2-H-4 with 4 layers of NHFRP composite

and minimum increase in peak stress of 51% was measured for column R2-S-2 with 2

layers of NSFRP composite. The increases in peak stress were 75% and 111% for

column specimens R2-H-2 and R2-H-4, respectively. Similar to the load carrying


increased. As shown in Figure 5.3c and 5.3d, the increases in the axial deformations

of 167%, 300%, 100% and 233% were recorded for columns R2-S-2, R2-S-4, R2-H-2

and R2-H-4, respectively.

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5.2.3.3 Rectangular 3 (150x225x300)



deformation curves of all specimens are shown in Figure 5.3e and 5.3f. The control



was recorded for specimen R3-H-4, whereas a minimum increase of 33% was

measured for specimen R3-S-2 (see the Appendix B). The remaining column

specimens in this group (i.e., R3-H-2 and R3-S-4) reached peak stresses that were

39% and 89% greater than the control specimen, respectively. Similar to the load

carrying capacity, the axial deformations of the NFRP-strengthened concrete columns

were also increased. A maximum increase in the axial deformation was 230%, for

specimen R3-S-4. Column specimens R3-S-2, R3-H-2 and R3-H-4 reached the peak


Fig.5.3 (a) Rectangular 1 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R1-S-4

R1-S-2

R1-CON

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Fig.5.3 (b) Rectangular 1 with NHFRP

Fig.5.3 (c) Rectangular 2 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R1-H-4

R1-H-2

R1-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R2-S-4

R2-S-2

R2-CON

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Fig.5.3 (d) Rectangular 2 with NHFRP

Fig.5.3 (e) Rectangular 3 with NSFRP

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R2-H-4

R2-H-2

R2-CON

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R3-S-4

R3-S-2

R3-CON

Ref. code: 25595722040218TUO

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Fig.5.3 (f) Rectangular 3 with NHFRP

Fig.5.3 Axial stress – axial deformation curves of rectangular columns

5.3 Effect of Fiber thickness

Figure 5.1-5.3 shows the axial stress-axial deformation curves of concrete

columns with different fiber thicknesses and the comparison of normalized ultimate

stress is displayed in Figure 5.4. It can be seen that the ultimate load carrying capacity

of circular, square and rectangular column specimens were elevated with an increase

in NFRP thickness. Both types of NFRP (i.e. NSFRP and NHFRP) led to the increase

in ultimate load carrying capacity with an increase in thickness. Increase in ultimate

load carrying capacity is more highly noticeable in smaller size specimens (i.e.

Circular 1, Square 1 and Rectangular 1) than in larger specimens as shown in Figure

5.4.

0

20

40

60

80

100

120

140

160

0 2 4 6 8 10 12 14 16

Ax

ial

stre

ss (

MP

a)


R3-H-4

R3-H-2

R3-CON

Ref. code: 25595722040218TUO

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Fig.5.4 (a)

Fig.5.4 (b)

Fig.5.4 (c)

0

100

200

300

400

500

600

700

800

900

1000

Circular 1

Nor

mal

ized

Str

ess

(MP

a)

C1-CON

C1-S-2

C1-S-4

C1-H-2

C1-H-4

0

100

200

300

400

500

600

700

800

900

1000

Circular 2

Nor

mal

ized

Str

ess

(MP

a)

C2-CON

C2-S-2

C2-S-4

C2-H-2

C2-H-4

0

100

200

300

400

500

600

700

800

900

1000

Circular 3

Nor

mal

ized

Str

ess

(MP

a)

C3-CON

C3-S-2

C3-S-4

C3-H-2

C3-H-4

Ref. code: 25595722040218TUO

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Fig.5.4 (d)

Fig.5.4 (e)

Fig.5.4 (f)

0

100

200

300

400

500

600

700

800

900

1000

Square 1

Nor

mal

ized

Str

ess

(MP

a)

S1-CON

S1-S-2

S1-S-4

S1-H-2

S1-H-4

0

100

200

300

400

500

600

700

800

900

1000

Square 2

Nor

mal

ized

Str

ess

(MP

a)

S2-CON

S2-S-2

S2-S-4

S2-H-2

S2-H-4

0

100

200

300

400

500

600

700

800

900

1000

Square 3

Nor

mal

ized

Str

ess

(MP

a)

S3-CON

S3-S-2

S3-S-4

S3-H-2

S3-H-4

Ref. code: 25595722040218TUO

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Fig.5.4 (g)

Fig.5.4 (h)

Fig.5.4 (i)

Fig.5.4 Comparison of normalized stress (Effect of fiber thickness)

0

100

200

300

400

500

600

700

800

900

1000

Rectangular 1

Nor

mal

ized

Str

ess

(MP

a)

R1-CON

R1-S-2

R1-S-4

R1-H-2

R1-H-4

0

100

200

300

400

500

600

700

800

900

1000

Rectangular 2

Nor

mal

ized

Str

ess

(MP

a)

R2-CON

R2-S-2

R2-S-4

R2-H-2

R2-H-4

0

100

200

300

400

500

600

700

800

900

1000

Rectangular 3

Nor

mal

ized

Str

ess

(MP

a)

R3-CON

R3-S-2

R3-S-4

R3-H-2

R3-H-4

Ref. code: 25595722040218TUO

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5.4 Effect of Fiber material

In this experimental study, two types of natural fiber composites, namely

natural sisal fiber reinforced polymer (NSFRP) composites and natural hemp fiber

reinforced polymer (NHFRP) composites were studied. To compare the effectiveness

of different strengthening composites, a comparison of comparison of normalized

ultimate stress is displayed in Figure 5.5.As can be seen, the composite NHFRP

demonstrates a consistently superior performance over NSFRP composite for circular,

square and rectangular columns specimens. This is supposedly due to the higher

tensile strength and stiffness of NHFRP compared with NSFRP. This experimental

data indicates that the NHFRP strengthening is more efficient than NSFRP.

5.5 Effect of Section shape

In this study, three different types of cross section shapes (i.e. circular, square

and rectangular) were constructed and strengthened using NFRP composites to

investigate the efficiency of NFRP confinement on the ultimate strength. The

comparison of normalized ultimate stress is displayed in Figure 5.6. It can be seen that

efficiency of NFRP confinement is very sensitive to the column cross section

geometry i.e. circular, square and rectangular for all sizes, it is evident that change in

section shape (from circular to rectangular), regardless of the specimens size, led to

the reduction in load carrying capacity of NFRP strengthened specimens. These

findings are compliant with the results reported in the literature for artificial CFRP

composites [17].

5.6 Effect of Size

In this study, three different sizes for each cross section shape were considered

to investigate the effect of size of NFRP-confined concrete columns. A comparison of

normalized ultimate stress is displayed in Figure 5.7. Similar to the sectional shape,

the efficiency of NFRP confinement is observed to be very sensitive to the column

size. As can be seen, as the column size increases, regardless of the section shape, the

ultimate stress for both NSFRP and NHFRP strengthened specimens was reduced.

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These findings are complying with the results reported in the literature for artificial

FRP composites [18].

Fig.5.5 (a)

Fig.5.5 (b)

Fig.5.5 (c)

0

200

400

600

800

1000

Circular 1

Nor

mal

ized

Str

ess

(MP

a)

C1-CON

C1-S-2

C1-H-2

C1-S-4

C1-H-4

0

200

400

600

800

1000

Circular 2

Nor

mal

ized

Str

ess

(MP

a)

C2-CON

C2-S-2

C2-H-2

C2-S-4

C2-H-4

0

200

400

600

800

1000

Circular 3

Nor

mal

ized

Str

ess

(MP

a)

C3-CON

C3-S-2

C3-H-2

C3-S-4

C3-H-4

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Fig.5.5 (d)

Fig.5.5 (e)

Fig.5.5 (f)

0

200

400

600

800

1000

Square 1

Nor

mal

ized

Str

ess

(MP

a)

S1-CON

S1-S-2

S1-H-2

S1-S-4

S1-H-4

0

200

400

600

800

1000

Square 2

Nor

mal

ized

Str

ess

(MP

a)

S2-CON

S2-S-2

S2-H-2

S2-S-4

S2-H-4

0

200

400

600

800

1000

Square 3

Nor

mal

ized

Str

ess

(MP

a)

S3-CON

S3-S-2

S3-H-2

S3-S-4

S3-H-4

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Fig.5.5 (g)

Fig.5.5 (h)

Fig.5.5 (i)

Fig.5.5 Comparison of normalized stress (Effect of fiber material)

0

200

400

600

800

1000

Rectangular 1

Nor

mal

ized

Str

ess

(MP

a)

R1-CON

R1-S-2

R1-H-2

R1-S-4

R1-H-4

0

200

400

600

800

1000

Rectangular 2

Nor

mal

ized

Str

ess

(MP

a)

R2-CON

R2-S-2

R2-H-2

R2-S-4

R2-H-4

0

200

400

600

800

1000

Rectangular 3

Nor

mal

ized

Str

ess

(MP

a)

R3-CON

R3-S-2

R3-H-2

R3-S-4

R3-H-4

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Fig.5.6 (a) Size 1

Fig.5.6 (b) Size 2

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)

Fiber material type - Fiber thickness (layers)

Circular Square Rectangular

Control specimen

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)



Control specimen

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Fig.5.6 (c) Size 3

Fig.5.6 Comparison of normalized stress (Effect of section shape)

Fig.5.7 (a) Circular specimens

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)



Control specimen

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)


Circular 1 Circular 2 Circular 3

Control specimen

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Fig.5.7 (b) Square specimens

Fig.5.7 (c) Rectangular specimens

Fig.5.7 Comparison of normalized stress (Effect of size)

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)


Square 1 Square 2 Square 3

Control specimen

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

S-2L S-4L H-2L H-4L

No

rmal

ized

str

ess

(MP

a)


Rectangular 1 Rectangular 2 Rectangular 3

Control specimen

Ref. code: 25595722040218TUO

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References

Books and Book Articles

1. Standard Test Method for Tensile Properties of Polymer Matrix Composite

Materials. Designation: D 3039/D 3039M – 00.(1997). International Harmonization of

Composite Materials-Phase I : ASTM Institute for Standards Research.

2. Standard Method of Test for Slump of Hydraulic Cement Concrete. AASHTO

Designation: T 119M/T 119-07.ASTM Designation: C 143/C 143M-05a.

3. Standard Test Method for Compressive Strength of Cylindrical Concrete

Specimen. Agencies of the Department of Defense. Designation: C 39/C 39M – 03

Articles

4. Y. Xiao and H. Wu.(2000). Compressive behavior of concrete confined by

carbon fiber composite jackets. Journal of Materials in Civil Engineering, May 2000

(pp.139-146).

5. J.F. Berthet, E. Ferrier and P. Hamelin.(2004).Compressive behavior of

concrete externally confined by composite jackets. Part A: experimental study.

Construction and Building Materials 19, 2005 (pp. 223–232).

6. Azadeh Parvin and Wei Wang. Tests on concrete square columns confined by

composite wraps. The University of Toledo, Toledo, Ohio, U.S.A.

7. Dian Jie Zhang and all. (2010). Compressive behavior of FRP-confined square

concrete columns after creep. School of Civil Engineering, Beijing Jiaotong

University, Beijing.Engineering Structures 32, 2010 (pp. 1957-1963).

8. Mark J. Masia and all. (2003). Size effects in axially loaded square-section

concrete prisms strengthened using carbon fiber reinforced polymer wrapping.

Canada. Journal Civil Engineer 31, 2004 (pp. 1-13).

9. R. Benzaid and all. Experimental investigation of circular and square RC

columns strengthened with CFRP sheets. International Conference on Sustainable

Built Environment Infrastructures in Developing Countries ENSET Oran (Algeria),

October 2009 (pp. 12-1).

Ref. code: 25595722040218TUO

62

10. Yousef A. Al-Salloum. (2006). Influence of edge sharpness on the strength of

square concrete columns confined with FRP composite laminates. Department of

Civil Engineering, King Saud University, Saudi Arabia. Composites: Part B38, 2007

(pp. 640–650).

11. I.A.E.M. Shehata and all.(2001).Strength of short concrete columns confined

with CFRP sheets. Materials and Structures Vol. 35, January-February 2002 (pp. 50-

58).

12. Yuan-feng Wang and Han-liangWu. Size Effect of Concrete Short Columns

Confined with Aramid FRP Jackets. Journal of composite for construction @ ASCE,

July- August 2011 (pp. 535-544).

13. Zhijian Li and all. Properties of hemp fibre reinforced concrete composites

(January 2005). Composites: Part A 37, 2006 (pp. 497–505). School of Engineering

and Technology, Deakin University, Pigdons Road, Geelong, Vic. 3217, Australia.

14. F. P. TORGAL and S .JALALI. Natural fiber reinforced concrete. Woodhead

Publishing Limited, 2011 (pp. 154-167). University of Minho, Portugal.

15. I. Merta, E.K. Tschegg. (2012). Fracture energy of natural fiber reinforced

concrete. Construction and Building Materials 40, 2013 (pp. 991–997). Institute for

Building Construction and Technology, Building Construction and Maintenance,

Faculty of Civil Engineering, University of Technology Vienna, Karlsplatz.

16. M. Ramesh and all. (2012). Mechanical property evaluation of sisal–jute–glass

fiber reinforced polyester composites. Composites: Part B 48, 2013 (pp. 1–9).

17. Tara Sena and H.N. Jagannatha Reddyba.Flexural strengthening of RC beams

using natural sisal and artificial carbon and glass fabric reinforced composite system.

Sustainable Cities and Society 10, 2014 (pp. 195–206).

18. Rochette P and all. Axial testing of rectangular column models confined with

composites. J Compos Construct 2000;4(3 )(pp.129–36)

19. Fang-Yao Yeh and all. Size and shape effect on FRP confinements for

rectangular concrete columns. 13th World Conference on Earthquake Engineering

Vancouver, B.C., Canada, 1-6 August 2004 Paper No. 657

20. Riad Benzaid and all. Circular and Square Concrete Columns Externally

Confined by CFRP Composite: Experimental Investigation and Effective Strength

Models. Fiber Reinforced Polymers - The Technology Applied for Concrete Repair.

Ref. code: 25595722040218TUO

63

21. K.A. Harries and all. (1998). Axial Behavior of Reinforced Concrete Columns

Retrofit with FRPC Jackets. Proc., of 2nd int. Conf. on Compos .in Infrastructure,

(pp. 411-425). University of Ariz., Tucson, Ariz.

22. A.Mirmiran. and all. (1998). Effect of column parameters on FRP-confined

concrete, Journal of Composites for Construction ,November 1998 (pp. 175-185).

23. Silvia Rocca. (2007). Experimental and analytical evaluation of FRP confined

large size reinforced concrete columns. Dissertations Paper 1997. Faculty of the

Graduate School of the University of Missouri-Rolla Doctoral.

24. Thong M. Pham. (2014). Confinement model for FRP confined normal- and

high-strength concrete circular columns. University of Wollongong, Australia.

Faculty of Engineering and Information Sciences-Papers.

25. Ahmed Belaadi and all.(2012). Tensile static and fatigue behavior of sisal

fibers. Materials and Design 46. 2013 (pp. 76–83), from

http://www.sciencedirect.com/science/article/pii/S0261306912006796

Ref. code: 25595722040218TUO

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Appendices

Ref. code: 25595722040218TUO

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Appendix A

Failure modes

Failure modes of circular specimens

C-CON

C1-2S

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C1-4S

C1-2H

C1-4H

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C2-2S

C2-4S

C2-2H

Ref. code: 25595722040218TUO

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C2-4H

C3-2S

C3-4S

Ref. code: 25595722040218TUO

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C3-2H

C3-4H

Ref. code: 25595722040218TUO

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Failure modes of square specimens

S-CON

S1-2S

S1-4S

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S1-2H

S1-4H

S2-2S

Ref. code: 25595722040218TUO

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S2-4S

S2-2H

S2-4H

Ref. code: 25595722040218TUO

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S3-2S

S3-4S

S3-2H

Ref. code: 25595722040218TUO

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S3-4H

.

Ref. code: 25595722040218TUO

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Failure modes of rectangular specimens

R-CON

R1-2S

R1-4S

Ref. code: 25595722040218TUO

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R1-2H

R1-4H

R2-2S

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R2-4S

R2-2H

R2-4H

Ref. code: 25595722040218TUO

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R3-2S

R3-4S

R3-2H

Ref. code: 25595722040218TUO

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R3-4H

Ref. code: 25595722040218TUO

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Appendix B

Summary of test results

Main Groups Subgroups


Peak stress

(MPa)

% Increase in

peak stress

Ultimate

deformation (mm)

% Increase in

ultimate deformation

Circular

Circular 1

(50 x 100)

C1-CON 15.8 - 0.5 -

C1-S-2 88.0 456 5.2 940

C1-S-4 140.0 785 9.0 1700

C1-H-2 90.0 469 3.5 590

C1-H-4 150.0 848 6.0 1100

Circular 2

(100 x 200)

C2-CON 22.1 - 2.0 -

C2-S-2 50.0 126 8.0 300

C2-S-4 70.0 216 12.0 500

C2-H-2 58.0 162 6.0 200

C2-H-4 87.0 293 10.0 400

Circular 3

(150 x 300)

C3-CON 26.9 - 2.8 -

C3-S-2 50.0 86 7.0 155

Ref. code: 25595722040218TUO

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Circular Circular 3

(150 x 300)

C3-S-4 71.0 164 13.6 395

C3-H-2 52.3 94 0.0 100

C3-H-4 80.0 197 3.0 199

Square

Square 1

(50 x 50 x 100)

S1-CON 12.7 - 0.4 -

S1-S-2 66.0 421 3.9 875

S1-S-4 95.2 652 6.3 1475

S1-H-2 72.0 469 2.8 590

S1-H-4 120.0 848 4.8 1100

Square 2

(100 x 100 x 200)

S2-CON 17.7 - 1.6 -

S2-S-2 50.0 182 8.0 400

S2-S-4 56.0 216 9.6 500

S2-H-2 46.4 162 4.8 200

S2-H-4 69.6 293 8.0 400

Square 3

(150 x 150 x 300)

S3-CON 21.5 - 2.2 -

S3-S-2 40.0 86 5.6 155

S3-S-4 56.8 164 10.9 395

S3-H-2 41.8 94 4.8 118

S3-H-4 64.0 197 9.7 340

Ref. code: 25595722040218TUO

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Rectangular

Rectangular 1

(50 x 100 x 100)

R1-CON 11.9 - 0.3 -

R1-S-2 44.0 271 2.6 829

R1-S-4 70.0 490 4.5 1507

R1-H-2 45.0 279 1.7 516

R1-H-4 75.0 532 3.0 971

Rectangular 2

(100 x 150 x 200)

R2-CON 16.6 - 1.5 -

R2-S-2 25.0 51 4.0 167

R2-S-4 35.0 111 6.0 300

R2-H-2 29.0 75 3.0 100

R2-H-4 43.5 162 5.0 233

Rectangular 3

(150 x 225 x 300)

R3-CON 18.8 - 2.1 -

R3-S-2 25.0 33 3.5 70

R3-S-4 35.5 89 6.8 230

R3-H-2 26.1 39 3.0 45

R3-H-4 40.0 112 6.1 193

Strengthening of small concrete columns by natural fiber ...

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