STRENGTHENING OF REINFORCED CONCRETE BEAMS USING GLASS FIBER REINFORCED POLYMER COMPOSITES

STRENGTHENING OF REINFORCED CONCRETE BEAMS USING GLASS FIBER REINFORCED POLYMER COMPOSITES
 STRENGTHENING OF REINFORCED CONCRETE
BEAMS USING GLASS FIBER REINFORCED
POLYMER COMPOSITES
A THESIS SUBMITTED IN PARTIAL FULFILMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Technology
in
Structural Engineering
By
NISHIKANT DASH
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA-769008,
2009
STRENGTHENING OF REINFORCED CONCRETE
BEAMS USING GLASS FIBER REINFORCED
POLYMER COMPOSITES
A THESIS SUBMITTED IN PARTIAL FULFILMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Technology
in
Structural Engineering
By
NISHIKANT DASH
Under the guidance of
Prof. K. C. BISWAL
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA-769008,
2009
NATIONAL INSTITUTE OF TECHNOLOGY
ROURKELA – 769008, ORISSA
INDIA
CERTIFICATE
This
is
to
certify
that
the
thesis
entitled,
“STRENGTHENING OF REINFORCED CONCRETE BEAMS USING
GLASS FIBER REINFORCED POLYMER COMPOSITES” submitted by
Mr. Nishikant Dash in partial fulfillment of the requirements for the award of
Master of Technology Degree in Civil Engineering with specialization in
Structural Engineering at the National Institute of Technology, Rourkela is an
authentic work carried out by him under my supervision and guidance.
To the best of my knowledge, the matter embodied in this
thesis has not been submitted to any other University/ Institute for the award of
any degree or diploma.
Dr. K. C. Biswal
Date: 25th May 2009
Place: Rourkela
Dept of Civil Engineering
National Institute of Technology
Rourkela – 769008
ACKNOWLEDGEMENT
I express my gratitude and sincere thanks to Prof. K. C. Biswal, for his guidance and
constant encouragement and support during the course of my work in the last one year. I
truly appreciate and value his esteemed guidance and encouragement from the beginning to
the end of this thesis, his knowledge and company at the time of crisis would be
remembered lifelong.
My sincere thanks to Prof. M. Panda, Head of the Civil Engineering Department,
and Prof. M. R. Barik, Professor of Civil Engineering Department, National Institute of
Technology Rourkela, for their advice and providing necessary facility for my work.
I am also very thankful towards Prof. S. K. Sahu, Professor, my faculty adviser and
all faculty members of Civil Engineering Department for their help and encouragement
during the project.
I am also thankful to Mr. S. K. Sethi, Mr. R. Lugun and other structural laboratory
staff and administrative staff of this department for their timely help.
I also thank all my friends who have directly or indirectly helped me in my project
work and in the completion of this report.
Last but not least I would like to thank my parents, who taught me the value of hard
work by their own example. I would like to share this moment of happiness with my father
and mother. They rendered me enormous support during the whole tenure of my stay in NIT
Rourkela.
Nishikant Dash
M. Tech (Structural Engineering)
Department of Civil Engineering
National Institute of Technology
Rourkela-769008
CONTENTS
Page No.
V
ABSTRACT
LIST OF FIGURES
VI - VII
LIST OF TABLES
VIII
1-10
INTRODUCTION
CHAPTER-1
1.1 General
1
1.2
Strengthening Using FRP Composites
5
1.3
Advantages And Disadvantages Of Fiber Composite
Strengthening
7
1.3.1
Advantages
7
1.3.2
Disadvantages
9
1.4
Present Investigation
LITERATURE REVIEW
CHAPTER-2
11-52
2.1 Introduction
11
2.2
External Strengthening Using Steel Plates
11
2.2.1 Introduction
11
2.2.2 Structural Investigations
12
2.2.3 Plate Separation And Anchorage
14
2.2.4 Disadvantages Of External Strengthening Using
Steel Plates
17
External Strengthening Using Composite Materials
18
2.3.1 Introduction
18
2.3.2
Review Of Experimental Investigations
22
2.3.3
Prestressing Composite Plates For Strengthening
Concrete Beams
33
2.3
I 10
2.3.4
Strengthening Of Reinforced Concrete Members In
Shear
2.4
Applications Of FRP Strengthening
41
2.5
Structural Adhesive Bonding
43
2.5.1 Type Of Structural Adhesives
45
2.5.2
Requirements Of The Adhesive For Plate Bonding
48
2.5.3
Tests To Measure Structural Adhesive Bond
Strength
49
2.6
Critical Observations From Literature Review
MATERIALS AND METHODS
CHAPTER-3
52
53-90
3.1 Materials
53
3.1.1
Concrete
53
3.1.1.1 Cement
55
3.1.1.2 Fine Aggregate
55
3.1.1.3 Coarse Aggregate
56
3.1.1.4 Water
57
3.1.2 Reinforcement
57
3.1.3
Fiber Reinforced Polymer (FRP)
58
3.1.3.1 Fiber
59
3.1.3.2 Fiber Sheet
65
3.1.4 Types Of Matrix Materials
66
3.1.4.1
Epoxy Resin
67
3.1.4.2
Unsaturated Polyester Resins
70
3.1.4.3 Adhesives
72
3.2
Experimental Study
74
3.3
Casting Of Beams
74
II 37
3.3.1 Materials For Casting
3.3.1.1 Cement
76
3.3.1.2 Fine Aggregate
76
3.3.1.3 Coarse Aggregate
77
3.3.1.4 Water
77
3.3.1.5
77
Reinforcing Steel
3.3.2
Form Work
77
3.3.3
Mixing Of Concrete
78
3.3.4
Compaction
79
3.3.5 Curing Of Concrete
79
3.4 Strengthening Of Beams
80
3.5
Experimental Setup
82
3.5.1 Procedure
86
3.6 Fabrication Of GFRP Plate
86
3.7
89
CHAPTER-4
Determination Of Ultimate Stress, Ultimate Load And
Young’s Modulus
ANALYTICAL STUDY
91-94
4.1 Introduction
91
4.2 Flexural Strengthening Of Beams
91
CHAPTER-5
4.2.1
Assumptions
92
4.2.2
Calculation Of Moment Of Resistance Of The
Beams
93
RESULTS AND DISCUSSION
95-117
5.1 Introduction
95
5.2
95
Failure Modes
5.3 Load Deflection History
III 76
97
CHAPTER-6
5.4 Load At Initial Crack
106
5.5
108
Ultimate Load Carrying Capacity
5.6 Crack Pattern
110
5.7
117
Comparision Of Results
118-120
CONCLUSIONS
121-125
REFERENCES
IV ABSTRACT
Worldwide, a great deal of research is currently being conducted concerning the use
of fiber reinforced plastic wraps, laminates and sheets in the repair and strengthening of
reinforced concrete members. Fiber-reinforced polymer (FRP) application is a very effective
way to repair and strengthen structures that have become structurally weak over their life
span. FRP repair systems provide an economically viable alternative to traditional repair
systems and materials.
Experimental investigations on the flexural and shear behavior of RC beams
strengthened using continuous glass fiber reinforced polymer (GFRP) sheets are carried out.
Externally reinforced concrete beams with epoxy-bonded GFRP sheets were tested to failure
using a symmetrical two point concentrated static loading system. Two sets of beams were
casted for this experimental test program. In SET I three beams weak in flexure were casted,
out of which one is controlled beam and other two beams were strengthened using
continuous glass fiber reinforced polymer (GFRP) sheets in flexure. In SET II three beams
weak in shear were casted, out of which one is the controlled beam and other two beams
were strengthened using continuous glass fiber reinforced polymer (GFRP) sheets in shear.
The strengthening of the beams is done with different amount and configuration of GFRP
sheets.
Experimental data on load, deflection and failure modes of each of the beams were
obtained. The detail procedure and application of GFRP sheets for strengthening of RC
beams is also included. The effect of number of GFRP layers and its orientation on ultimate
load carrying capacity and failure mode of the beams are investigated.
V
LIST OF FIGURES
Figure No.
Title
Page No.
Fig. 3.1
Formation of Fiber Reinforced Polymer Composite
59
Fig. 3.2
Discontinuous Glass Fibers
62
Fig. 3.3
Structure of aramid fiber
65
Fig. 3.4
Structure of DGEBA
67
Fig. 3.5
The curing of epoxy resin with primary amines
68
Fig. 3.6
Reinforcement details of SET I beams
75
Fig. 3.7
Section of SET I beams
75
Fig. 3.8
Reinforcement details of SET II beams
76
Fig. 3.9
Section of SET II beams
76
Fig. 3.10
Application of epoxy and hardener on the beam
81
Fig. 3.11
Fixing of GFRP sheet on the beam
81
Fig. 3.12
Roller used for removal of air bubbles
82
Fig. 3.13
Two point loading experimental setup
84
Fig. 3.14
Shear force and bending moment diagram for two point loading
84
Fig. 3.15
Shear strengthening zone and flexure strengthening zone of the
beam
85
Fig. 3.16
Experimental setup for testing of beams
85
Fig. 3.17
Specimen for tensile testing in INSTRON 1195
88
Fig. 3.18
Experimental setup of INSTRON 1195
88
Fig. 3.19
Specimen failure after tensile test
89
Fig. 4.1
Area for flexural and shear strengthening of beams
91
VI Figure No.
Title
Page No.
Fig. 4.2
Flexural strengthening of beam using FRP sheets at the bottom
92
Fig. 4.3
Stress Strain diagram of singly reinforced beam strengthen with
FRP
93
Fig. 5.1
Load vs Deflection Curve for Beam F1
98
Fig. 5.2
Load vs Deflection Curve for Beam F2
99
Fig. 5.3
Load vs Deflection Curve for Beam F3
100
Fig. 5.4
Load vs Deflection Curves for Beams F1, F2 and F3
101
Fig. 5.5
Load vs Deflection Curve for Beam S1
102
Fig. 5.6
Load vs Deflection Curve for Beam S2
103
Fig. 5.7
Load vs Deflection Curve for Beam S3
104
Fig. 5.8
Load vs Deflection Curves for Beams S1, S2 and S3
105
Fig. 5.9
Load at initial crack of beams F1, F2 and F3
106
Fig. 5.10
Load at initial crack of beams S1, S2 and S3
107
Fig. 5.11
Ultimate load of beams F1, F2 and F3
108
Fig. 5.12
Ultimate load of beams S1, S2 and S3
109
Fig. 5.13
Crack pattern of Beam F1
111
Fig. 5.14
Crack pattern of Beam F2
112
Fig. 5.15
Crack pattern of Beam F3
113
Fig. 5.16
Crack pattern of Beam S1
114
Fig. 5.17
Crack pattern of Beam S2
115
Fig. 5.18
Crack pattern of Beam S3
116
VII LIST OF TABLES
Table No.
Title
Page No.
Table 3.1
Properties of different fibers
60
Table 3.2
Typical composition of fiberglass (% in weight)
61
Table 3.3
Typical properties of Carbon Fiber
63
Table 3.4
Properties of epoxy resin
69
Table 3.5
Properties of epoxy resin and hardener
70
Table 3.6
Typical properties of cast Thermosetting Polyesters
72
Table 3.7
Size of the specimen for tensile test
89
Table 3.8
Ultimate stress, Ultimate load and Young’s modulus of GFRP
plate
90
Table 4.1
Analytical calculations of beams F1 and F2
94
Table 5.1
Ultimate load and nature of failure for SET I and SET II beams
97
Table 5.2
Comparision of ‫ܯ‬௨ value obtained from analytical and
experimental study
117
VIII CHAPTER -1
INTRODUCTION
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
CHAPTER 1
INTRODUCTION
1.1 GENERAL
The maintenance, rehabilitation and upgrading of structural members, is perhaps one
of the most crucial problems in civil engineering applications. Moreover, a large number of
structures constructed in the past using the older design codes in different parts of the world
are structurally unsafe according to the new design codes. Since replacement of such
deficient elements of structures incurs a huge amount of public money and time,
strengthening has become the acceptable way of improving their load carrying capacity and
extending their service lives. Infrastructure decay caused by premature deterioration of
buildings and structures has lead to the investigation of several processes for repairing or
strengthening purposes. One of the challenges in strengthening of concrete structures is
selection of a strengthening method that will enhance the strength and serviceability of the
structure while addressing limitations such as constructability, building operations, and
budget. Structural strengthening may be required due to many different situations.
•
Additional strength may be needed to allow for higher loads to be placed on the
structure. This is often required when the use of the structure changes and a higher loadcarrying capacity is needed. This can also occur if additional mechanical equipment,
filing systems, planters, or other items are being added to a structure.
•
Strengthening may be needed to allow the structure to resist loads that were not
anticipated in the original design. This may be encountered when structural
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
strengthening is required for loads resulting from wind and seismic forces or to improve
resistance to blast loading.
•
Additional strength may be needed due to a deficiency in the structure's ability to carry
the original design loads. Deficiencies may be the result of deterioration (e.g., corrosion
of steel reinforcement and loss of concrete section), structural damage (e.g., vehicular
impact, excessive wear, excessive loading, and fire), or errors in the original design or
construction (e.g., misplaced or missing reinforcing steel and inadequate concrete
strength).
When dealing with such circumstances, each project has its own set of restrictions
and demands. Whether addressing space restrictions, constructability restrictions, durability
demands, or any number of other issues, each project requires a great deal of creativity in
arriving at a strengthening solution.
The majority of structural strengthening involves improving the ability of the
structural element to safely resist one or more of the following internal forces caused by
loading: flexure, shear, axial, and torsion. Strengthening is accomplished by either reducing
the magnitude of these forces or by enhancing the member's resistance to them. Typical
strengthening techniques such as section enlargement, externally bonded reinforcement,
post-tensioning, and supplemental supports may be used to achieve improved strength and
serviceability.
Strengthening systems can improve the resistance of the existing structure to internal
forces in either a passive or active manner. Passive strengthening systems are typically
engaged only when additional loads, beyond those existing at the time of installation, are
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
applied to the structure. Bonding steel plates or fiber-reinforced polymer (FRP) composites
on the structural members are examples of passive strengthening systems. Active
strengthening systems typically engage the structure instantaneously and may be
accomplished by introducing external forces to the member that counteract the effects of
internal forces. Examples of this include the use of external post-tensioning systems or by
jacking the member to relieve or transfer existing load. Whether passive or active, the main
challenge is to achieve composite behavior between the existing structure and the new
strengthening elements.
The selection of the most suitable method for strengthening requires careful
consideration of many factors including the following engineering issues:
•
Magnitude of strength increase;
•
Effect of changes in relative member stiffness;
•
Size of project (methods involving special materials and methods may be less costeffective on small projects);
•
Environmental conditions (methods using adhesives might be unsuitable for
applications in high-temperature environments, external steel methods may not be
suitable in corrosive environments);
•
In-place concrete strength and substrate integrity (the effectiveness of methods
relying on bond to the existing concrete can be significantly limited by low concrete
strength);
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
•
Dimensional/clearance constraints (section enlargement might be limited by the
degree to which the enlargement can encroach on surrounding clear space);
•
Accessibility;
•
Operational constraints (methods requiring longer construction time might be less
desirable for applications in which building operations must be shut down during
construction):
•
Availability of materials, equipment, and qualified contractors;
•
Construction cost, maintenance costs, and life-cycle costs; and
•
Load testing to verify existing capacity or evaluate new techniques and materials.
In order to avoid the problems created by the corrosion of steel reinforcement in
concrete structures, research has demonstrated that one could replace the steel reinforcement
by fiber reinforced polymer (FRP) reinforcement. Corrosion of the steel reinforcement in
reinforced concrete (RC) structures affects the strength of both the steel and the concrete.
The strength of a corroding steel reinforcing bar is reduced because of a reduction in the
cross-sectional area of the steel bar. While the steel reinforcing bars are corroding, the
concrete integrity is impaired because of cracking of the concrete cover caused by the
expansion of the corrosion products.
The rehabilitation of infrastructures is not new, and various projects have been
carried out around the world over the past two decades. One of the techniques used to
strengthen existing reinforced concrete members involves external bonding of steel plates by
means of two-component epoxy adhesives. By this way, it is possible to improve the
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
mechanical performance of a member. The wide use of this method for various structures,
including buildings and bridges, has demonstrated its efficiency and its convenience. In spite
of this fact, the plate bonding technique presents some disadvantages due to the use of steel
as strengthening material. The principal drawbacks of steel are its high weight which causes
difficulties in handling the plates on site and its vulnerability against corrosive
environments. Moreover, steel plates have limited delivery lengths and, therefore, they
require joints.
1.2 STRENGTHENING USING FRP COMPOSITES
Only a few years ago, the construction market started to use FRP for structural
reinforcement, generally in combination with other construction materials such as wood,
steel, and concrete. FRPs exhibit several improved properties, such as high strength-weight
ratio, high stiffness-weight ratio, flexibility in design, non-corrosiveness, high fatigue
strength, and ease of application. The use of FRP sheets or plates bonded to concrete beams
has been studied by several researchers. Strengthening with adhesive bonded fiber
reinforced polymers has been established as an effective method applicable to many types of
concrete structures such as columns, beams, slabs, and walls. Because the FRP materials are
non-corrosive, non-magnetic, and resistant to various types of chemicals, they are
increasingly being used for external reinforcement of existing concrete structures. From the
past studies conducted it has been shown that externally bonded glass fiber-reinforced
polymers (GFRP) can be used to enhance the flexural, shear and torsional capacity of RC
beams. Due to the flexible nature and ease of handling and application, combined with high
tensile strength-weight ratio and stiffness, the flexible glass fiber sheets are found to be
highly effective for strengthening of RC beams. The use of fiber reinforced polymers (FRPs)
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
for the rehabilitation of existing concrete structures has grown very rapidly over the last few
years. Research has shown that FRP can be used very efficiently in strengthening the
concrete beams weak in flexure, shear and torsion. Unfortunately, the current Indian
concrete design standards (IS Codes) do not include any provisions for the flexural, shear
and torsional strengthening of structural members with FRP materials. This lack of design
standards led to the formation of partnerships between the research community and industry
to investigate and to promote the use of FRP in the flexural, shear and torsional
rehabilitation of existing structures. FRP is a composite material generally consisting of high
strength carbon, aramid, or glass fibers in a polymeric matrix (e.g., thermosetting resin)
where the fibers are the main load carrying element.
Among many options, this reinforcement may be in the form of preformed laminates
or flexible sheets. The laminates are stiff plates or shells that come pre-cured and are
installed by bonding them to the concrete surface with a thermosetting resin. The sheets are
either dry or pre-impregnated with resin (known as pre-preg) and cured after installation
onto the concrete surface. This installation technique is known as wet lay-up. FRP materials
offer the engineer an outstanding combination of physical and mechanical properties, such
as high tensile strength, lightweight, high stiffness, high fatigue strength, and excellent
durability. The lightweight and formability of FRP reinforcement make FRP systems easy to
install. Since these systems are non-corrosive, non-magnetic, and generally resistant to
chemicals, they are an excellent option for external reinforcement. The properties of FRP
composites and their versatility have resulted in significant saving in construction costs and
reduction in shut down time of facilities as compared to the conventional strengthening
methods (e.g., section enlargement, external post-tensioning, and bonded steel plates).
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
Strengthening with externally bonded FRP sheets has been shown to be applicable to
many types of RC structural elements. FRP sheets may be adhered to the tension side of
structural members (e.g., slabs or beams) to provide additional flexural strength. They may
be adhered to web sides of joists and beams or wrapped around columns to provide
additional shear strength. They may be wrapped around columns to increase concrete
confinement and thus strength and ductility of columns. Among many other applications,
FRP sheets may be used to strengthen concrete and masonry walls to better resist lateral
loads as well as circular structures (e.g., tanks and pipelines) to resist internal pressure and
reduce corrosion. As of today, several millions of square meters of surface bonded FRP
sheets have been used in many strengthening projects worldwide.
1.3 ADVANTAGES AND DISADVANTAGES OF FIBER COMPOSITE
STRENGTHENING
1.3.1 ADVANTAGES
Fiber composite strengthening materials have higher ultimate strength and lower
density than steel. When taken together these two properties lead to fiber composites having
a strength/weight ratio higher than steel plate in some cases (though it is often not possible
to use this fully). The lower weight makes handling and installation significantly easier than
steel. This is particularly important when installing material in cramped locations. Work on
soffits of bridges and building floor slabs can often be carried out from man-access
platforms rather than full scaffolding. Steel plate requires heavy lifting gear and must be
held in place while the adhesive gains strength. Bolts must be fitted through the steel plate
into the parent concrete to support the plate while the adhesive cures and to reduce the
effects of peeling at the ends. On the other hand, the application of FRP plate or sheet
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
material has been likened to applying wallpaper; once it has been rolled on carefully to
remove entrapped air and excess adhesive it may be left unsupported. In general, no bolts
are required; in fact, the use of bolts would seriously weaken the material unless additional
cover plates are bonded on. Furthermore, because there is no need to drill into the structure
to fix bolts or other mechanical anchors there is no risk of damaging the existing reinforcement. Fiber composite materials are available in very long lengths while steel plate is
generally limited to 6 m. The availability of long lengths and the flexibility of the material
also simplify installation:
•
Laps and joints are not required
•
The material can take up irregularities in the shape of the concrete surface
•
The material can follow a curved profile; steel plate would have to be pre-bent to the
required radius
•
The material can be readily installed behind existing services
•
Overlapping, required when strengthening in two directions, is not a problem
because the material is thin.
The materials fibers and resins are durable if correctly specified, and require little
maintenance. If they are damaged in service, it is relatively simple to repair them, by adding
an additional layer. The use of fiber composites does not significantly increase the weight of
the structure or the dimensions of the member. The latter may be particularly important for
bridges and other structures with limited headroom and for tunnels. In terms of
environmental impact and sustainability, studies have shown that the energy required to
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
produce FRP materials is less than that for conventional materials. Because of their light
weight, the transport of FRP materials has minimal environmental impact.
These various factors in combination lead to a significantly simpler and quicker
strengthening process than when using steel plate. This is particularly important for bridges
because of the high costs of lane closures and possession times on major highways and
railway lines. It has been estimated that about 90% of the market for plate strengthening in
Switzerland has been taken by carbon plate systems as a result of these factors.
1.3.2 DISADVANTAGES
The main disadvantage of externally strengthening structures with fiber composite
materials is the risk of fire, vandalism or accidental damage, unless the strengthening is
protected. A particular concern for bridges over roads is the risk of soffit reinforcement
being hit by over-height vehicles . However, strengthening using plates is generally
provided to carry additional live load and the ability of the unstrengthened structure to carry
its own self-weight is unimpaired. Damage to the plate strengthening material only reduces
the overall factor of safety and is unlikely to lead to collapse.
Experience of the long-term durability of fiber composites is not yet available. This
may be a disadvantage for structures for which a very long design life is required but can be
overcome by appropriate monitoring.
A perceived disadvantage of using FRP for strengthening is the relatively high cost
of the materials. However, comparisons should be made on the basis of the complete
strengthening exercise; in certain cases the costs can be less than that of steel plate bonding.
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Department of Civil Engineering, NIT Rourkela Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
A disadvantage in the eyes of many clients will be the lack of experience of the
techniques and suitably qualified staff to carry out the work. Finally, a significant
disadvantage is the lack of accepted design standards.
1.4 PRESENT INVESTIGATION
The purpose of this research is to investigate the flexural and shear behavior of
reinforced concrete beams strengthened with varying configuration and layers of GFRP
sheets. More particularly, the effect of the number of GFRP layers and its orientation on the
strength and ductility of beams are investigated. Two sets of beams were fabricated and
tested up to failure. In SET I three beams weak in flexure were casted, out of which one is
controlled beam and other two beams were strengthened using continuous glass fiber
reinforced polymer (GFRP) sheets in flexure. In SET II three beams weak in shear were
casted, out of which one is the controlled beam and other two beams were strengthened by
using continuous glass fiber reinforced polymer (GFRP) sheets in shear.
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Department of Civil Engineering, NIT Rourkela CHAPTER -2
LITERATURE REVIEW
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
This chapter provides an introduction to the strengthening of reinforced concrete
(RC), prestressed concrete and steel members using externally bonded steel plate or fiber
reinforced polymer (FRP) composites sheets and plates by reviewing the most significant
investigations reported in the literature. In addition, a section is devoted to the strengthening
of RC members in shear utilising FRP plates and sheets. However, since the external plating
and its application as a strengthening technique has only been made possible by the
development of suitable adhesives, consideration is also given to the types of adhesive
which may be used for external plate bonding and their requirements for this application.
After considering reported plate bonding studies, a brief review of surface preparation
techniques applicable to FRP and concrete adherends is presented.
2.2 EXTERNAL STRENGTHENING USING STEEL PLATES
2.2.1 INTRODUCTION
A review of some significant experimental investigations conducted using steel
plates is presented to demonstrate some of the structural implications of external plating.
Research work into the performance of members strengthened with steel plates was
pioneered simultaneously in South Africa and France in the 1960s (L’Hermite and Bresson,
1967; Fleming and King, 1967; Lerchenthal, 1967; Gilibert et al., 1976). Continued
development of suitable adhesives and the increased use of the technique in practice
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Department of Civil Engineering, NIT Rourkela
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
stimulated further research work. Eberline et al. (1988) present a literature review on
research and applications related to steel plate bonding.
2.2.2 STRUCTURAL INVESTIGATIONS
The history of bonded external reinforcement in the UK goes back to 1975 with the
strengthening of the Quinton Bridges on the M5 motorway. This scheme followed a number
of years of development work by the Transport and Road Research Laboratory (TRRL),
(now TRL), in association with adhesive manufacturers and the Department of Transport. In
terms of testing programmes, research and development work continued at the TRRL and at
several academic institutions in the UK, most notably at the University of Sheffield.
Theoretical investigations and the evaluation of suitable adhesives were allied to the
extensive beam testing programmes which were undertaken.
Preliminary studies were conducted by Irwin (1975). Macdonald (1978) and
Macdonald and Calder (1982) reported four point loading tests on steel plated RC beams of
length 4900mm. These beams were used to provide data for the proposed strengthening of
the Quinton Bridges (Raithby, 1980 and 1982), and incorporated two different epoxy
adhesives, two plate thicknesses of 10.0mm and 6.5mm giving width-to-thickness (b/t)
ratios of 14 and 22, and a plate lap-joint at its centre. In all cases it was found that failure of
the beams occurred at one end by horizontal shear in the concrete adjacent to the steel plate,
commencing at the plate end and resulting in sudden separation of the plate with the
concrete still attached, up to about mid-span. The external plate was found to have a much
more significant effect in terms of crack control and stiffness. The loads required to cause a
crack width of 0.1mm were increased by 95%, whilst the deflections under this load were
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substantially reduced. The post cracking stiffness was found to be increased by between 35–
105% depending upon the type of adhesive used and the plate dimensions. The features of
this work became the subject of a more detailed programme of research at the TRRL
(Macdonald, 1982; Macdonald and Calder, 1982), in which a series of RC beams of length
3500mm were tested in four point bending. The beams were either plated as-cast or plated
after being loaded to produce a maximum crack width of 0.1mm. The effect of widening the
plate whilst maintaining its cross-sectional area constant was studied. It was found that the
plated as-cast and the pre-cracked beams gave similar load/deflection curves, demonstrating
the effectiveness of external plating for strengthening purposes.
An extensive programme of research work carried out at the University of Sheffield
since the late 1970s has highlighted a number of effects of external, epoxy-bonded steel
plates on the serviceability and ultimate load behaviour of RC beams. A brief summary of
some of the research findings is presented by Jones and Swamy (1995).
Steel plate strengthening of existing structures has also been investigated in
Switzerland at the Swiss Federal Laboratories for Material Testing and Research (EMPA)
(Ladner and Weder, 1981). Bending tests were carried out on RC beams 3700mm in length,
and the plate width-to-thickness (b/t) ratio was studied whilst maintaining the plate crosssectional area constant. The external plate continued through and beyond the beam supports,
with which they were not in contact, for a distance such that the bonded area (48000mm2)
was the same for each plate width. The external plate was not bonded to the concrete beam
except in the anchorage areas beyond the supports. The results clearly showed that thin
plating was more effective than thick narrow plating, as noted in studies conducted in the
UK. The effective anchorage length la which allowed the plate to reach yield before shear
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failure adjacent to the bonded areas was found to be inversely proportional to the b/t ratio.
Therefore, as b/t increased (wide, thin plates), the anchorage length decreased.
2.2.3 PLATE SEPARATION AND ANCHORAGE
The ultimate behaviour of steel plated RC beams appears to be closely related to the
geometry of the plated cross-section. For thin plates, failure usually occurs in flexure.
However, if the plate aspect ratio falls below a certain value, separation of the plate from the
beam can occur, initiating from the plate end and resulting in the concrete cover being
ripped off.
These observations are consistent with the fact that simple elastic longitudinal shear
stresses are inversely proportional to the plate width. Consequently, as the steel plate width
decreases, the longitudinal shear stresses increase. In addition, the bending stiffness of the
plate increases, thereby increasing the peeling stresses normal to the beam.
However, the levels of stress at the steel plate ends are thought to be well in excess
of those due to simple elastic considerations (Macdonald, 1982). Concentrations of shear
and normal stress arise at the plate ends of beams subjected to flexure as a result of stiffness
incompatibility between the plate and concrete, which can only be accommodated by severe
distortion of the adhesive layer. The sudden transition from the basic unplated members to
the plate reinforced member is usually situated in a region of high shear and low bending
moment. The changing bending moment and distortion in the adhesive layer causes a buildup of axial force at the end of the external plate; this induces high bond stresses on the
adhesive/plate and adhesive/concrete interfaces which may reach critical levels, thereby
initiating failure. The magnitude of these plate end stresses for externally strengthened
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beams depends upon the geometry of the plate reinforcement, the engineering properties of
the adhesive and the shear strength of the original concrete beam (Swamy and
Mukhopadhyaya, 1995). The existence of peak peeling and shear stresses at the plate end, in
addition to bending stresses, results in a biaxial tensile stress state which forces the crack
initiated at the plate end to extend horizontally at the level of the internal steel.
When failure occurs in this way, the use of a more flexible adhesive is advantageous,
since the region over which the tensile strain builds up in the external steel plate is extended,
thereby resulting in a lower peak stress. This has been verified experimentally by Jones et al.
(1985), where beams strengthened using an adhesive with an elastic modulus of around 1.0
× 103 Nmm-2 gave slightly improved strengths when failure occurred by plate separation
than strengths given by an adhesive with a modulus of around 10 × 103 Nmm-2.
As the structural benefits of external plating with steel are enhanced by the use of
larger, thicker plates, an alternative to limiting the areas (or perhaps as a safeguard against
separation), would be the provision of some form of plate anchorage. Jones et al. (1988)
presented theoretical and experimental studies into the problem of anchorage at the ends of
steel plates. A series of RC beams 2500mm in length, strengthened with epoxy-bonded steel
plates of 6.0mm thickness were tested to investigate different plate end anchorage schemes.
Four 6.0mm diameter bolts at each end of the plate, which penetrated to a depth of 75mm,
were used in one configuration, whilst different sizes of angle plates were also tried, one of
which covered the extent of the shear span, and compared with those of a beam plated with a
single unanchored steel plate of b/t ratio 21, which failed suddenly by plate separation at a
load which was below that of the unplated control beam. It was found that the anchorage
detail had no apparent effect on the deflection performance of the beams. The use of bolts
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did not prevent debonding, but complete separation was avoided and increases in strength up
to 8% over the unplated beam were achieved. The bonded anchor plates were more
effective, producing yielding of the tensile plates and allowing the full theoretical strength to
be achieved, 36% above that of the unplated beam. The anchorage detail was also found to
affect the ductility of the beams near the ultimate load. Unanchored, the beams failed
suddenly with little or no ductility. The beams with bolts or anchor plates all had similar
ductilities, at least as high as the unplated control.
Hussain et al. (1995) investigated the use of anchor bolts at the ends of steel plated
beams, in an attempt to prevent brittle separation of the plate. In agreement with Jones et al.
(1988) the bolts, which were 15mm in diameter and penetrated to half the depth of the beam,
were found to improve the ductility of the plated beams considerably, but to have only a
marginal effect on the ultimate load. The percentage improvement in ductility due to the
addition of bolts was found to decrease as the plate thickness increased. The end anchorage
could not prevent premature failure of the beams, although in this case failure occurred as a
result of diagonal shear cracks in the shear spans.
It will be realised that in providing anchorage to the steel plated beams, considerable
extra site work is involved and this in turn will increase the cost of the plate bonding
technique considerably. However, with steel plate bonding this anchorage is completely
necessary.
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2.2.4 DISADVANTAGES OF EXTERNAL STRENGTHENING USING STEEL
PLATES
The in situ rehabilitation or upgrading of RC beams using bonded steel plates has
been proven in the field to control flexural deformations and crack widths, and to increase
the load-carrying capacity of the member under service load for ultimate conditions. It is
recognised to be an effective, convenient and economic method of improving structural
performance. However, although the technique has been shown to be successful in practice,
it also has disadvantages. Since the plates are not protected by the concrete in the same way
as the internal reinforcement, the possibility of corrosion exists which could adversely affect
the bond strength, leading to failure of the strengthening system. Uncertainty remains
regarding the durability and the effects of corrosion. To minimise the possibility of
corrosion, all chloride-contaminated concrete should be removed prior to bonding and the
plates must be subjected to careful surface preparation, storage and the application of
resistant priming systems. After installation, the integrity of the primer must be periodically
checked, introducing a further maintenance task to the structure. The plates are generally
prepared by grit blasting which, unless a minimum thickness of typically 6mm is imposed,
can cause distortion.
Steel plates are difficult to shape in order to fit complex profiles. In addition, the
weight of the plates makes them difficult to transport and handle on site, particularly in areas
of limited access, and can cause the dead weight of the structure to be increased significantly
after installation. Elaborate and expensive false work is required to maintain the steelwork in
position during bonding. The plates are required to be delivered to site within flatness
tolerances to prevent stresses being introduced normal to the bondline during cure. The
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weight of the plates and this flatness requirement generally restricts the maximum plate
length to between 6–8m. Since the spans requiring strengthening are often greater than this
length, joints are required. Welding cannot be used in these cases since this would destroy
the adhesive bond. Consequently, lapped butt joints have to be formed, adding further
complications to the design of the system.
Studs are required to assist in supporting the steel plates during installation and under
service loading conditions. This is especially true towards the ends of the plates where
anchorages are required due to the high bending stiffness of the plate. The position of these
studs must therefore be established prior to bonding. This process can involve a considerable
amount of site work. Finally, if the plates are loaded in compression, buckling may occur,
causing the plates to become detached. The process involved in strengthening with steel
plates can therefore be considered as relatively time consuming and labour intensive.
2.3
EXTERNAL
STRENGTHENING
USING
COMPOSITE
MATERIALS
2.3.1 INTRODUCTION
To overcome some of the shortcomings that are associated with steel plate bonding,
it was proposed in the mid-1980s that fiber reinforced polymer (FRP) plates could prove
advantageous over steel plates in strengthening applications (Meier, 1987; Kaiser, 1989;
Meier and Kaiser, 1991). Unlike steel, FRPs are unaffected by electrochemical deterioration
and can resist the corrosive effects of acids, alkalis, salts and similar aggressive materials
under a wide range of temperatures (Hollaway, 1993). Consequently, corrosion-resistant
systems are not required, making preparation prior to bonding and maintenance after
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installation less arduous than for steel. The reinforcing fibres can be introduced in a certain
position, volume fraction and direction in the matrix to obtain maximum efficiency,
allowing the composites to be tailor made to suit the required shape and specification. The
resulting materials are non-magnetic, non-conductive and have high specific strength and
stiffness in the fiber direction at a fraction of the weight of steel. They are consequently
easier to transport and handle, require less falsework, can be used in areas of limited access
and do not add significant loads to the structure after installation. Continuous lengths of FRP
can be readily produced which, because of their low bending stiffness, can be delivered to
site in rolls. The inclusion of joints during installation is thus avoided. With the exception of
glass fiber composites, FRPs generally exhibit excellent fatigue and creep properties and
require less energy per kilogram to produce and transport than metals. As a result of easier
installation in comparison to steel, less site disruption should be experienced in the process,
allowing faster, more economical strengthening.
The benefits of utilising FRP materials over steel in plate bonding applications are
thus clear. The drawbacks are the intolerance to uneven bonding surfaces which may cause
peeling of the plate, the possibility of brittle failure modes (Swamy and Mukhopadhyaya,
1995) and the material cost, since fiber composites are between 4–20 times as expensive as
steel in terms of unit volume. However, in a rehabilitation project, where material costs
rarely exceed 20% of the overall project cost, the installation savings can offset the higher
material costs (Meier, 1992). Peshkam and Leeming (1994) have considered the commercial
viability of FRP plate bonding for bridge strengthening. In a straight comparison with steel
plate bonding for a typical application, despite the fact that material costs will be increased,
labour and equipment costs will be reduced, construction times will be shorter and durability
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will be improved. It is shown that 2kg of FRP could replace 47kg of steel on an equal
strength basis. The costs of installing both materials are shown to be similar; however, when
traffic management, traffic delay and maintenance costs are included, the use of FRP
provides a saving of 17.5% over steel. There are situations where steel plate bonding is not a
viable option because of the extent of chloride contamination of the concrete. In such cases,
the use of FRP may avoid the need for demolition and replacement. Peshkam and Leeming
(1994) presented a cost comparison of bridge replacement against strengthening with FRP,
in which possible savings of 40% are demonstrated. These cost comparisons were made
before true manufacturing and installation costs were known and were at the best estimates.
Subsequently the tendering process for real installation projects has shown carbon fiber
reinforced polymer (CFRP) plate bonding to be very competitive against steel plate bonding
in first cost, before even future maintenance costs are added to the whole life cost equation.
The general versatility of composite materials makes them a viable alternative to
steel plates in strengthening applications, resulting in both short term and long term savings.
Meier and Winistorfer (1995) consider that for applications in which the possibility of
corrosion is minimal and the length of the strengthening is less than 8m, steel will remain
the most favourable option. However, more recent work by other researchers (ROBUST)
and trends in costs, show that this position is changing and the indications are that FRP is
more economical than steel whatever the length. This is the case mainly in building
construction, although plate thickness may be important from an aesthetic viewpoint. In
applications where corrosion, length of the required strengthening and handling on site are
of greater significance, for example bridge rehabilitation, fiber composites become a more
attractive alternative.
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Concerns have been expressed regarding the behaviour of FRP strengthened
members when exposed to fire. A series of tests has been carried out at EMPA in
Switzerland in which the performance of steel and CFRP plated beams was compared when
exposed to extreme high temperatures (Deuring, 1994). It was found that a steel plate
became detached after a matter of minutes of exposure, whereas the CFRP laminates
progressively lost cross-sectional area due to burning at the surface, causing a gradual loss
of stiffness of the member, before final detachment after over an hour. This superior
behaviour is a consequence of the low thermal conductivity of the composite. In addition,
detachment of a heavy steel plate from a structure for any reason presents a far greater
hazard than that of a lightweight FRC material. Aspects of the effects of fire on resin
compounds are considered by Tabor (1978) and Hollaway (1993a).
Glass, aramid and carbon fiber composites may be considered for strengthening
applications. In common usage, glass is the most popular reinforcing fiber since it is
economical to produce and widely available. Carbon fibers exhibit better resistance to
moisture, solvents, bases and weak acids, and can withstand direct contact with concrete
(Santoh et al., 1983). Composite materials produced from them are light in weight, with
strengths higher than steel and stiffnesses higher than either glass or aramid composites. For
example, laminates fabricated from glass fiber must be three times thicker than CFRP
laminates to achieve the same tensile stiffness for the same fiber volume fraction. CFRP has
excellent fatigue properties and a very low (or even negative) linear thermal coefficient of
thermal expansion in the fiber direction. Quality assurance can be performed by non
destructive testing, for example infrared inspection in the field, if CFRP laminates are used;
this is not possible with steel plates. This technique allows fast and accurate judgement on
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the quality of the strengthening work. Despite the higher cost, carbon composites appear to
provide the best characteristics for structural strengthening.
2.3.2 REVIEW OF EXPERIMENTAL INVESTIGATIONS
The following section reviews, on a geographical basis, experimental work reported
to investigate the flexural strengthening of RC members using non-prestressed FRP plates.
These studies have utilised fibrous materials in various forms, including pultruded plates,
precured prepreg plates, prepreg sheets or tapes cold laminated in place, and dry fiber sheets
impregnated at the time of bonding.
Some investigations in Europe
Recent work on the use of FRP materials as a replacement for steel in plate bonding
applications was pioneered at the EMPA in Switzerland. Four point loading tests were
initially performed on RC beams 2000mm (Meier, 1987; Kaiser, 1989) or 7000mm (Ladner
et al., 1990) in length. Strengthening was achieved through the use of pultruded carbon
fiber/epoxy laminates up to 1.0mm thick bonded with the same epoxy adhesives used in
earlier steel plating work (Ladner and Weder, 1981). For the 2000mm length beams, the
ultimate load was almost doubled over the unplated control beam, although these beams
were designed with a low proportion of internal steel, and hence the strength of the unplated
beam was low. In the case of the 7000mm length beam, strengthened with a 1.0mm CFRP
laminate, the increase in the ultimate load was about 22% (Ladner and Holtgreve, 1989).
However, for both beam lengths the ultimate deflection was considerably reduced, although
it was claimed that there was still sufficient rotation to predict impending failure.
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The following modes were observed either individually or in combination in the tests
carried out at the EMPA:
• sudden, explosive, tensile failure of the CFRP laminates
• compressive failure in the concrete
• slow, continuous peeling of the laminate during loading resulting from an uneven concrete
bond surface
• sudden peeling of the laminate during loading due to relative vertical displacement across a
shear crack in the concrete
• horizontal shearing of the concrete in the tensile zone
• interlaminar shear within the CFRP sheet.
The CFRP plate was found to reduce the total width of cracks and produce a more
even crack distribution over the length of the beam (Meier and Kaiser, 1991). Meier et al.
(1992) recommended that in strengthening applications, the external CFRP should fail in
tension after yielding the internal steel but before failure of the concrete in the compressive
zone, since this would ensure a more ductile failure mode.
Deblois et al. (1992) investigated the application of unidirectional and bidirectional
glass fiber reinforced polymer (GFRP) sheets for flexural strengthening. A series of RC
beams 1000mm long were tested after strengthening. The use of bidirectional sheets
increased the ultimate load by up to 34%, whereas unidirectional GFRP resulted in an
increase of only 18%. The authors of this current chapter feel that this is an unexpected
conclusion and emphasise that the FRP material used was GFRP. The additional bonding of
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bidirectional GFRP to the sides of the beam increased the load carried with unidirectional
sheets to 58%. To further the programme of study, Deblois et al. (1992) epoxy-bonded a
bidirectional GFRP sheet to the soffit of a 4100mm long RC beam. Bolts were also used as
additional anchorage at the plate ends. The maximum load increased by 66% over the
unplated control beam. It was noted that for all tests the application of GFRP reduced the
ductility of the beam.
Research into external FRP plating has been conducted at Oxford Brookes
University (Hutchinson and Rahimi, 1993). The effect of plate-end geometries on the stress
concentrations at the plate ends was of primary interest in this investigation, for which RC
beams 2300mm in length were used. Two beams were preloaded to 80% of their ultimate
strength, before plating, to cause cracking of the concrete and yielding of the steel
reinforcement. Unidirectional carbon fiber/epoxy prepreg tape of total thickness 0.78mm
was used for the plating, with the various plate end geometries, tapering in either plan or
section, cut whilst the composite was in the uncured state. Several different two-part cold
cure epoxy adhesives were evaluated using a modified Boeing wedge cleavage test, of
which no detail is given, developed to measure adhesion to concrete surfaces. The adhesive
selected as most suitable was Sikadur 31 PBA, an epoxy which has been used in both steel
and FRP plate bonding applications in Switzerland (Meier and Kaiser, 1991) and in steel
plating applications in the UK (Shaw,1993).
It was found that the flexural performance of all strengthened beams was
significantly better than the unplated specimens, in terms of both strength and stiffness. The
ultimate load-carrying capacity was increased by as much as 230%; however, it should be
pointed out that the actual increase is dependent upon the degree of internal reinforcement in
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the beam before plating. The increased stiffness resulted in an increased load to first
cracking, but a substantial decrease in ductility to failure. After first cracking, cracks grew
progressively in number, covering most of the test span. Most of these were of hairline
width even close to the ultimate load level. In all cases, failure was sudden and catastrophic,
characterised by a shear crack running from the tensile zone towards the loading point and
delamination of the concrete cover along the tensile reinforcement. This type of failure has
been identified in steel plating work as described above. Tapering of the plate end in either
plan or section appeared to have no effect on the flexural performance or failure mode for
the cases considered. As with steel plates, the beams which had been precracked before
bonding had an equivalent performance to the other test beams, indicating the effectiveness
of the plate bonding technique for repair. The load/deflection behaviour was similar for all
different plate configurations, except for those with laminates bonded to the full length of
the beam, clamped by the reaction at the supports, which resulted in an increase in strength
over the other plated beams. It was concluded that for these particular beams and plates the
ultimate loading capacity of the system appeared to have been reached, being governed by
the shear capacity of the concrete beams.
The tests at Oxford Brookes University continued (Hutchinson and Rahimi, 1996),
under the ROBUST programme of research, by utilizing both glass and carbon fiber/epoxy
laminates of different thicknesses built up from prepreg tapes. Three internal steel
reinforcement ratios were examined. All beams with external reinforcement performed
significantly better than their unplated counterparts in terms of stiffness and strength. The
use of GFRP was found to provide significant ductility and reasonable strength, whilst
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enhancements were greater with CFRP but at the expense of a loss of ductility. Greater
enhancements were achieved with lower steel ratios.
A limited programme of experimental testing has been carried out at the University
of Bologna (Arduini et al., 1994; Arduini et al., 1995) in which small scale steel fiber
reinforced concrete specimens of length 500mm or 600mm have been tested in three point
bending after being strengthened with unidirectional aramid fiber/epoxy or glass fiber/epoxy
composites of thickness between 2.0–5.0mm. These small scale tests were used to
demonstrate that the load-carrying capacity of the basic unplated beam could be increased
through external plating with FRC but that different failure modes, often brittle, were
involved. It was noted that peeling and shear cracks at the plate ends were responsible for
causing premature, brittle failure. The use of thicker FRC plates was found to increase the
occurrence of peeling failure. Ductility was increased and peeling failure delayed through
the use of plates bonded to the sides of the beams in the plate end regions; the effects were
enhanced by coupling the side and soffit plates together, in which case failure was observed
to occur by diagonal shearing at the highest attained loads.
The University of Surrey (Quantrill et al., 1995) under the ROBUST programme of
research, undertook a parametric study on flexurally strengthened RC beams using GFRP
bonded plates. The study involved varying the concrete strength, the pultruded composite
plate area and its aspect ratio (b/t), and as discussed above in steel plating applications, thick
narrow plates with aspect ratios of less than 50 have been associated with brittle peeling
failure modes. Consequently, ratios of 38 and 67 were tested in the study. The effect of the
b/t ratio was isolated in these tests by maintaining a constant plate cross-sectional area. The
tests showed that plating can considerably enhance both the strength and stiffness of RC
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members, although at the expense of ductility at failure. It appeared that the higher strength
concrete produced the greatest increase in strength over the unplated section and the aspect
ratio of the plate has little effect on the overall behaviour.
The above programme continued with further investigations at the University of
Surrey (Quantrill et al., 1996a) into the experimental and analytical strengthening of
reinforced concrete beams with fiber reinforced polymer plates, and analysed the effects of
different plate parameters on the overall behaviour of the system. It was shown that testing
relatively small scale 1m long specimens can reveal useful information on strengthened
beam behaviour. By reducing the plate area the expected reduction in strengthening and
stiffening caused the ductility and the plate strains for a given load to increase; the aspect
ratio for the values tested had little effect on the overall response. Plating with CFRP
components increased the serviceability, yield and ultimate loads and increased the
strengthened member stiffness after both cracking and yielding; ductility was reduced. The
iterative analytical model accurately predicted the tensile plate strain and compressive
concrete strain responses of the beam for a partially cracked section.
Quantrill et al. (1996b) continued with tests on small scale specimens and showed
that when the CFRP plated beams were uncracked at their extremities the theoretical shear
stress reached 11.15Nmm-2 and the peel stress 6.37Nmm-2. The anchored CFRP plated
beams were able to sustain higher levels of shear and peel stress before failure occurred
around 14.1Nmm-2 in shear and 8.10Nmm-2 in peel stress.
In France, a programme of small scale tests has also been carried out to study the
effects of different adhesive and FRC combinations when used for external strengthening
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(Varastehpour and Hamelin, 1995). A series of plain concrete specimens 280mm in length
were tested to failure in four point bending after being strengthened with glass or carbon
fiber/epoxy sheets bonded to both the tension and side faces of the specimen with one of
four different epoxy or acrylic adhesives. The composite plate on the tension face was
anchored by the reactions at the supports in all cases. Failure occurred either by FRP
rupture, interface failure or by debonding of the plate from the concrete. In all cases the
flexural and shear capacity of the beams was increased by plating, although this was found
to be dependent on the choice of adhesive; in general, the epoxies performed better than the
acrylics, the tests demonstrating that a rubber-toughened epoxy was superior.
Under the ROBUST research programme, Garden et al. (1996) showed that the
ultimate capacity of the CFRP beams falls with reducing the width thickness b/t and beam
shear span/depth ratios. Failure under low shear span/beam depth ratios is associated with
high plate strains (the value being in the region of 70% of the plate ultimate strain) and
relatively high longitudinal shear stresses at the adhesive/concrete interface, and although
the concrete failed in the cover concrete area, debonding from the concrete was not
observed. Plate end anchorage delays failure by resisting plate separation but does not
increase stiffness until the internal reinforcement has yielded.
He et al. (1997a), at the University of Sheffield, used steel and CFRP plates with the
same axial stiffness-to-strength precracked reinforced concrete beams in which a new, but
unspecified, plate anchorage system was adopted. The basic improvement in structural
performance due to plating was verified and it was found that the CFRP plates produced a
greater improvement in ultimate load than the steel plates. The authors (He et al., 1997b)
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noted that the high stress and strain potential of the CFRP will not be utilised unless the
plate is prestressed.
Bencardino et al. (1997) tested CFRP plated beams at the University of Calabria,
Italy, recording reductions in member ductility due to plating without end anchorage; the
ductility was restored when anchorage was fitted in the form of externally bonded U-shaped
steel stirrups. The method of CFRP plating was used successfully to strengthen an
experimental portal structure.
Some investigations in North America
During the late 1980s, a pilot study was carried out at the University of Arizona to
establish the feasibility of post strengthening concrete bridge beams with GFRP plates
(Saadatmanesh and Ehsani, 1989 and 1990a). Selection of a suitable epoxy adhesive for
plate bonding purposes was the main subject of investigation. Five RC beams 1675mm in
length were tested in four point bending to determine their static strength. None of the
beams contained shear reinforcement, which resulted in premature failure in the first tests.
To prevent shear cracks causing separation of the plate from the beam, external shear
reinforcement was thus provided in the end regions by means of several large G-clamps.
One beam was unplated, while the remainders were strengthened with a 6.0mm thick GFRP
plate bonded with one of four different types of two-part cold cure epoxy with a range of
shear strengths from 13MPa to 16MPa using aluminium substrates. It was found that
flexible epoxies did not allow any measurable shear to be transferred between the plate and
the beam, and no increase in the ultimate strength was achieved in comparison to the
unplated control beam. For the most rigid epoxy, after the concrete had cracked in tension,
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the plate was observed to separate from the beam in a very brittle manner, again resulting in
no increase in ultimate capacity. The beam strengthened using a relatively viscous rubbertoughened epoxy was found to perform best in the tests carried out. This beam was
significantly stronger and stiffer than the unplated control beam. Substantial force developed
in the plate indicated good shear transfer and composite action between the plate and the
concrete beam. The cracks were found to be considerably smaller throughout the range of
loading and distributed more evenly along the length of the beam. Failure occurred when a
layer of concrete delaminated about 10mm above the bondline, indicating satisfactory
performance of the epoxy.
Following on from this initial study, a further experimental research project was
undertaken at the University of Arizona (Saadatmanesh and Ehsani, 1990b and 1991). In this
project, five rectangular beams and one T-beam were tested to failure in four point bending
over a clear span of 4570mm. All beams were strengthened with a GFRP plate 6.0mm thick
bonded to the concrete with the epoxy adhesive identified as most suitable for the
application from the previous study. Three different reinforcement ratios were used for the
tension steel in the beams. The majority of the beams were overdesigned for shear to prevent
premature shear failure. The tests indicated that significant increases in the external load, at
which the steel yielded, and an increase in flexural strength, could be achieved by bonding
GFRP plates to the tension face of RC beams. The gain in the ultimate flexural strength was
found to be significant in beams with lower steel reinforcement ratios, as noted later by
Hutchinson and Rahimi (1996). In addition, plating reduced the crack size in the beams at all
load levels. For several of the beams tested, failure occurred as a result of sudden
longitudinal shear failure of the concrete between the plate and internal steel reinforcement.
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The flexural stiffness was increased, although the ductility of the beams and curvature at
failure were reduced by the addition of the GFRP plates.
An experimental programme was undertaken by Chajes et al. (1994) at the
University of Delaware, in which RC beams 1120mm in length were loaded to failure in
four point bending after the majority had been externally strengthened with composite fabric
of either bidirectional woving aramid, E-glass or carbon fiber reinforcement. In each case,
the fabrics had a tensile capacity close to the yield strength of the steel. Fabrics were used as
an alternative to plates to exploit their ability to conform to irregular surface geometries,
thus reducing the possibility of the continuous peeling failures observed in testing at the
EPMA. No shear reinforcement was provided in the beams. A variety of layers of each
fabric were epoxy bonded to the concrete. A set of three beams were also prepared and
tested with twice the amount of internal steel reinforcement. It was found that the mode of
failure of the strengthened beams varied depending upon the fabric used; those externally
strengthened with E-glass and carbon fibers failed by tensile rupture of the fabric. The first
aramid strengthened beam exhibited a fabric debonding mode of failure and consequently,
for the remaining specimens, end tabs were included, bonded to the sides of the beam and
enclosing the soffit reinforcement in the end regions. The extent of the end tabs in the shear
spans is not clear from the publication, but their use prevented debonding, allowing failure
of the concrete in compression to occur. For each of the fabric types used, increases in
flexural strength similar to those found in the beams with additional steel reinforcement
were achieved, in the range 34–57%. The fabric reinforcement beams also exhibited
increases in flexural stiffness within the range 45–53%. Both of the failure modes observed
were said to yield a reasonable amount of ductility, although this was around half that
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obtained from the unstrengthened beams. The research carried out at the University of
Delaware forms part of a wider study concerned with the possibility of rehabilitating
deteriorated prestressed concrete box beam bridges using transversely bonded advanced
composite materials (Chajes et al., 1993; Finch et al., 1994; Chajes et al., 1995b; Chajes et
al., 1996).
The US Navy has been studying the possibility of using external FRP plating for
upgrading waterfront structures affected by reinforcement corrosion (Malvar et al., 1995).
Enhancements of both bending and shear strength are being considered through the use of
unidirectional CFRP tow sheets. RC beams 1680mm long have been tested in an
experimental investigation, none of which contained shear reinforcement. Beams
strengthened longitudinally demonstrated that the flexural strength could be significantly
enhanced, but failure occurred, not surprisingly, in shear.
When additional CFRP was wrapped onto the sides and soffit of the beam over its
full span, to provide shear reinforcement and additional anchorage for the longitudinal
CFRP sheets, sufficient shear strength was provided to revert to a bending failure in which
the steel yielded, the concrete crushed and then the CFRP material ruptured. However, this
occurred at a ductility which was somewhat less than that of the unplated control beam.
In addition to upgrading reinforced concrete beams, research into the feasibility of
externally reinforcing continuous RC slab bridges in response to observed longitudinal
cracking was initiated in South Dakota (Iyer et al., 1989). To close the observed cracks, the
possibility of bonding the external reinforcement whilst the beam was relieved of dead load
was examined. The use of both steel (Iyer et al., 1989) and CFRP plates (Iyer, 1988) has
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been reported. Initial results on small scale beams showed that the strains in the concrete and
internal steel were considerably reduced by the introduction of external reinforcement, while
the stiffness was increased and cracking was controlled.
2.3.3
PRESTRESSING
COMPOSITE
PLATES
FOR
STRENGTHENING
CONCRETE BEAMS
The utilisation of prestressed composite plates, at the time of bonding, for
strengthening concrete members has been studied only relatively recently in comparison
with investigations of non-prestressed plates, although the benefits of external prestressing
with plate materials have been recognized for many years. For example, Peterson (1965)
considered the external prestressing of timber beams using prestressed steel sheets and
found significant improvements in bending stiffness and ultimate capacity. External
prestressing with composite plates also provides these benefits as well as cost savings.
Triantafillou and Deskovic (1991) noted that this method of prestressing is a more
economical alternative to conventional prestressing methods used in new construction.
Initial research on the strengthening of reinforced concrete beams by external plate
prestressing at EMPA in Switzerland has been widely reported (Meier and Kaiser, 1991;
Meier et al., 1993; Deuring, 1994). This work included the cyclic loading of a beam whose
plate was prestressed to 50% of its strength. Although this prestress ensures the mean stress
level in the cyclic loading was high, there was no evidence of damage to the plate after 30 ×
107 cycles and the cracking of the concrete was well controlled.
The non-prestressed beam loading tests reported by Deuring (1993) revealed failures
by the initiation of plate separation from the base of a shear crack. It was found that the
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compression transfer into the concrete by the plate prestress could delay or even prevent this
type of failure, thereby allowing the plate to reach its ultimate tensile strain so that the beam
failed in flexure rather than by premature plate separation (Deuring, 1993). The ability of the
plate to alter the failure mode from premature plate separation to flexure is influenced by the
prestressing force and the cross-sectional area of the plate. One of the conclusions of the
work was that the greatest flexural resistance of a strengthened section is reached when the
plate fractures in tension, either after or at the same time as yield of the internal steel rebars.
Saadatmanesh and Ehsani (1991) conducted an experimental study of the
strengthening of reinforced concrete beams using non-prestressed and prestressed GFRP
plates. One of the two prestressed beams contained a relatively small amount of internal
tensile steel reinforcement, while the other contained larger bars and was precracked prior to
bonding of the plate. The plate prestress in the precracked case closed some of the cracks,
indicating the benefit of prestressing from a serviceability point of view. The beam with
little original reinforcement before plating experienced a large improvement in ultimate
capacity due to the additional moment couple provided by the plate prestress. In both cases,
the prestress was generated by cambering the beam before bonding the plate so that a tensile
load was transferred to the plate when the camber was released. Improved concrete crack
control was observed with prestressed plates, a clear advantage from a serviceability point of
view. A previous experimental study by Saadatmanesh and Ehsani (1990) also included the
prestressing of a beam by cambering; this particular specimen had a low internal
reinforcement area ratio of 0.32% (based on effective depth) so that a very high increase in
ultimate load (323%) was observed as a result. The GFRP plate was not anchored at its ends,
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and failure occurred by premature plate separation associated with the removal of a layer of
concrete from the tension face of the beam.
Triantafillou et al. (1992) tested reinforced concrete beams in three point bending
with various quantities of internal reinforcement and magnitudes of CFRP plate prestress.
Improved control of concrete cracking was brought about not only by a greater internal
reinforcement provision, but also by higher plate prestress, indicating the serviceability
advantage gained by prestressing the composite. It was noted that prestressed composite
plates can potentially act as the sole tensile reinforcement in new concrete construction and
prefabrication is also possible due to the simplicity with which composites may be handled
and applied. The confinement imposed by the initial compressive stress at the base of the
beam was thought to be capable of improving the shear resistance of the member. Also, an
advantage from a cost point of view is that the same strengthening to failure may be
achieved with a prestressed plate of relatively small cross section, like that achieved with a
larger non-prestressed plate (Triantafillou et al., 1992).
Char et al. (1994) conducted an analytical parameter study to determine the effects of
varying the cross-sectional area and material type of the composite plate and the prestress in
the plate. The parameter study revealed that prestressing a GFRP plate would not necessarily
increase the ultimate moment capacity over that of a beam with a non-prestressed plate, for
the particular beam configuration and prestress level considered. This was because both the
non-prestressed and prestressed beams failed by plate fracture. Garden and Hollaway (1997)
showed that prestressing with CFRP plates increases the ultimate capacity of a beam but the
magnitude of the increase depends on the failure modes of the beams with and without
prestress; the failure mode of the prestressed beam depends on the prestress magnitude.
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Wight et al. (1995) reported data on the strengthening and stiffening achieved with
prestressed CFRP plates. The control of concrete crack widths and numbers of cracks was
improved by prestressing the plates. The beam with an initial non-stressed plate failed by
concrete fracture in the cover thickness within one of the shear spans of the four point
loaded beam, whereas the prestressed plated beams failed by plate fracture in the constant
moment region. The compression generated in the concrete near the beam soffit, due to the
plate prestress, was sufficient to reduce the magnitudes of vertical displacements across
shear cracks and to transfer failure into the plate. The avoidance of concrete failure in the
shear spans was associated with a much improved ultimate load.
The testing at the University of Surrey, under the ROBUST programme, continued
(Quantrill and Hollaway 1998) by pretensioning the ROBUST pultruded composite plates,
prior to bonding to the concrete. The prestressing technique employed was developed and
refined on small scale 1.0m long specimens before being applied to larger 2.3m long beams.
Pretensioning the plate prior to bonding to the concrete beam considerably increased the
external applied load at which cracking of the concrete occurred, reduced overall member
stiffness and also the load at which visible cracking occurred. The observation of crack
control is of significant importance to serviceability based design criteria. It was generally
concluded that this technique has the potential to provide a more efficient solution to
strengthening problems.
Furthermore, Garden and Hollaway (1997) tested 1.0m and 4.5m lengths of
reinforced concrete beams in four point bending after strengthening them with externally
bonded prestressed CFRP plates. The plates were bonded without prestress and with
prestress levels ranging from 25–50% of the plate strength. The ultimate capacities of the
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plated non-prestressed beams were significantly higher than those of the unplated members
and plate prestress brought about further strengthening. The non-prestressed beams failed by
concrete fracture in the cover to the internal rebars, whilst most of the prestressed beams
failed by plate fracture. The plate prestress prevented cracking of the adhesive layer, a
phenomenon associated with shear cracking in the concrete. The plates of the prestressed
beams had an initial tensile strain before any external load was applied to the beam system
and consequently at this stage, the beams had a relatively high stiffness. It was found that
prestressed plates were utilised more efficiently than non-prestressed plates since a given
plate strain was associated with a lower plated beam deformation in a prestressed member.
Prestressing the composite plates lowers the position of the neutral axis so more of the
concrete section is loaded in compression, making more efficient use of the concrete.
All the above experiments were carried out in the laboratory on relatively small scale
beams and the method of prestressing could not have been used on site on a real structure
where the plate would have to be stressed before bonding within the confines of the
abutments or supports. Within the ROBUST project, two 18m long beams recovered from a
real bridge structure, which had to be demolished and reconstructed, were strengthened with
plates that were prestressed under conditions that were little different from those that would
occur on a real bridge structure (Lane et al., 1997).
2.3.4 STRENGTHENING OF REINFORCED CONCRETE MEMBERS IN SHEAR
Some research work has been conducted on the use of fibre reinforced composite
plates for strengthening structures in shear. Al-Sulaimani et al. (1994) experimentally
studied the use of GFRP plates for the shear strengthening of initially shear-cracked concrete
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with a shear capacity 1.5 times lower than their flexural capacity. A low shear span/beam
depth ratio of 2.7 was used, which would have ensured that shear was dominant in the beam
behaviour. The shear repair comprised three different systems, with and without the soffit
plate in each case. The first repair involved the external bonding of 20mm wide strips over
the side and soffit of the beam at regular intervals throughout each shear span. The second
repair utilised the bonding of side plates, throughout each shear span, covering 80% of the
beam depth and located centrally in the depth. The third method involved the bonding of a
U-shaped jacket covering the sides of the beam and the soffit plate throughout each shear
span. The beams repaired with side strips and side plates failed by diagonal tension, with
dominant cracks at failure following the cracks initially present in the beams from the
preloading stage. Concrete compression failure occurred in the beams with the jackets.
The programme of experimental work by Chajes et al. (1994), on small scale
specimens, concentrated on GFRP composites as the external reinforcing medium (Chajes et
al., 1995a). Increases in flexural and shear capacity of beams 1120mm in length were
examined when tested to failure in four point bending. These small scale beams, which
again had no shear reinforcement, were externally strengthened with unidirectional CFRP
tow sheets to the basic control beam configuration. To evaluate the effect of composite shear
reinforcement, a CFRP sheet was wrapped around the section; again, the extent of this
reinforcement along the span is unclear. It was found that the control beam was increased by
158% by adding a single CFRP sheet to the tensile face of the beam. Increases in the load
cracking of the concrete and yielding of the internal steel were also noted. In addition to the
increase in capacity, a 115% increase in stiffness, a change in failure mode from flexural to
shear, and a decrease in ductility were observed. By wrapping the beam with a CFRP sheet,
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shear failure was prevented and tensile failure of the composite occurred. Finally, by adding
a second CFRP sheet to the tensile face, a 292% increase in capacity and a 178% increase in
stiffness were achieved. It should be stressed, however, that these large percentages are a
function of the initial structural capacities of the beam.
Chajes et al. (1995b) tested beams reinforced externally with CFRP plates bonded to
their soffit and sides to study flexural and shear behaviours. The fiber orientation in the
shear plates was in the vertical direction of the beams only. This orientation was believed to
be the reason for the similarity in the load–deflection responses of flexurally strengthened
beams with and without external material; the vertical fibers had little effect on the flexural
behaviour of the beams. The composite material used by Chajes et al. (1995b) was a
unidirectional CFRP tow sheet having a dry thickness of 0.11mm and a tensile modulus of
elasticity of 227.37GPa. The continuous strips were able to control shear crack opening due
to their greater axial stiffness, resulting in reduced shear deflection. This result showed that,
unlike the flexural soffit reinforcement, a thin sheet covering as much of the concrete as
possible will not necessarily produce the greatest improvement in crack control where shear
is concerned, but the sheets were able to avoid concrete shear failure, the failure mode
observed without the sheets. The tests showed a logical progression of failure modes as
more and more external reinforcement was added. There was an increase in capacity of
115% in stiffness, a change in failure mode from flexure to shear and a decrease in ductility.
When a further single CFRP sheet was applied to the beam, shear failure was prevented and
a flexural failure initiated as a tensile failure of the composite occurred. Finally by adding a
second single layer of CFRP sheet to the tensile face a 292% increase in capacity and a
178% increase in stiffness were achieved.
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Taljsten (1997) studied the shear force capacity of beams when these had been
strengthened by CFRP composites applied to the beams by four different techniques. These
were:
• hand-lay-up, by two different systems
• prepreg in combination with vacuum and heat
• vacuum injection.
The results of the four point loaded tests showed, in all cases, a good strengthening
effect in shear when the CFRP composites were bonded to the vertical faces of concrete
beams. The strengthening effect of almost 300% was achieved and it was possible to reach a
value of 100% with an initially completely fractured beam. Generally it was easier to apply
the hand-lay-up system and Taljstenn suggested that although the prepreg and vacuum
injection methods gave higher material properties than those of the hand-lay method, the site
application technique seemed to be more controllable for the hand-lay process.
Hutchinson et al. (1997) has described tests that were undertaken at the University of
Manitoba to investigate the shear strengthening of scaled models of the Maryland bridge
which required shear capacity upgrading in order to carry increased truck loads. The bridge
had an arrangement of stirrups which caused spalling off of the concrete cover followed by
straightening of the stirrups and sudden failure. CFRP sheets were effective in reducing the
tensile force in the stirrups under the same applied shear load. The CFRP plates were
clamped to the web of the Tee beams in order to control the outward force in the stirrups
within the shear span. This allowed the stirrups to yield and to contribute to a 27% increase
in the ultimate shear capacity. Hutchinson showed that diagonal CFRP sheets are more
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efficient than the horizontal and vertical CFRP sheet combination in reducing the tensile
force in the stirrups at the same level of applied shear load.
2.4 APPLICATIONS OF FRP STRENGTHENING
Of the applications of FRP strengthening reported in the literature, the majority occur
in Switzerland where the concept was first proposed and developed. In these cases, which
are considered in more detail by Meier (1995), pultruded carbon fibre/epoxy laminates have
been used exclusively. The first reported application was the repair in 1991 of the Ibach
Bridge in the canton of Lucerne, for which several prestressing tendons had been severed
during the installation of traffic signals. The bridge was repaired with three CFRP sheets of
dimensions 150 mm wide by 5000 mm long and of thickness 1.75 mm or 2.0 mm. The total
weight of the CFRP used was only 6.2 kg, compared with the 175 kg of steel which would
have been required for the repair. In addition, all work was carried out from a mobile
platform, eliminating the need for expensive scaffolding. A loading test with an 840 kN
vehicle demonstrated that the rehabilitation work had been satisfactory. The wooden bridge
at Sins in Switzerland was stiffened in 1992 to meet increased traffic loading (Meier et al.,
1993). Two of the most highly loaded cross beams were strengthened using 1.0mm thick
CFRP laminates. The appearance of the historic structure was unaltered by the strengthening
technique. Other CFRP strengthening applications in Switzerland include slab reinforcement
around a newly installed lift shaft in the City Hall of Gossau St. Gall, the upgrading of a
supermarket roof using laminates 15.5m in length to allow the removal of a supporting wall,
ground floor strengthening of the Rail Terminal in Zurich, and the strengthening of a
multistory car park in Flims. A chimney wall at the nuclear power plant in Leibstadt has also
been poststrengthened for wind and seismic loading after the installation of ducts.
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Rostasy et al. (1992) report the use of GFRP plates at the working joints of the
continuous multispan box girder Kattenbusch Bridge in Germany to reduce fatigue stresses
in the prestressing tendons and transverse cracking due to thermal restraint. A representative
specimen of the joint was tested in the laboratory to verify the technique prior to field
application. Ten joints required rehabilitation; eight of these were strengthened with steel
plates 10mm thick, whilst the remaining two utilised GFRP plates 30mm thick to provide
the same area stiffness as the steel plates. The installation of such plates, of which twenty
were used at each joint, took place in 1987 and was found to reduce the stress amplitude at
the joints by 36% and the crack widths by around 50%.
Greenfield (1995) describes applications of composite strengthening in the United
States, in which the integrity of a sewage treatment basin was restored with carbon
fiber/epoxy laminates 1.65mm thick. The laminates were also used to relieve overstress in
areas of the basin due to lack of reinforcing steel. The seismic retrofit of bridge columns in
California using GFRP jackets has been reviewed by Priestley et al. (1992). An existing roof
structure at Kings College Hospital, South London has been strengthened using epoxybonded, 1.0mm thick, 11m long pultruded CFRP laminates (NCE, 1996). An extra floor was
added to the building such that the existing roof was strengthened to meet new floor
requirements. The installation took place quickly and conveniently, 2kg of CFRP being used
instead of 60kg of steel.
Nanni (1995) reported the findings of a visit to Japan to determine the scale of FRC
use as external reinforcement. He concluded that a greater number of field applications in
Japan in recent years have used thinner FRC sheets than the plates used in Europe, Saudi
Arabia and North America. The use of FRC sheets for the structural strengthening of
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2009
concrete in Japan has addressed problems in bridges, tunnels, car parks and other structures
(Greenfield, 1995). The following five examples of FRC strengthening were cited by Nanni
(1995), carbon fibre composites having been used in all cases:
• Strengthening of a cantilever slab of the Hata Bridge along the Kyushu Highway in order
to accommodate large parapet walls which caused elevated bending moments due to the
higher wind force;
• Increase of the load rating of the Tokando Highway bridge at Hiyoshikura, a reinforced
concrete deck supported on steel girders, causing a 30–40% reduction of stress in the
internal rebars;
• Arrest of the internal steel reinforcement corrosion of the concrete beams in the waterfront
pier at the Wakayama oil refinery;
• Strengthening and stiffening of the concrete lining of the Yoshino Route tunnels on
Kyushu Island, necessary due to cracking which arose from unexpected fluctuations in the
underground water pressure. No loss of tunnel cross-sectional area occurred and the road
remained open during the bonding work;
• Longitudinal strengthening of the sides and soffit of a culvert at the Fujimi Bridge in
Tokyo.
2.5 STRUCTURAL ADHESIVE BONDING
Structural adhesives are generally accepted to be monomer composites which
polymerise to give fairly stiff and strong adhesive uniting relatively rigid adherends to form
a load-bearing joint (Shields, 1985). The feasibility of bonding concrete with epoxy resins
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2009
was first demonstrated in the late 1940s (ACI, 1973), and the early development of structural
adhesives is recorded by Fleming and King (1967). Since the early 1950s adhesives have
become widely used in civil engineering (Mays, 1985). However, although the building and
construction industries represent some of the largest users of adhesive materials, many
applications are non-structural in the sense that the bonded assemblies are not used to
transmit or sustain significant stresses (e.g. crack injection and sealing, skid-resistant layers,
bonding new concrete to old). Truly structural application implies that the adhesive is used
to provide a shear connection between similar or dissimilar materials, enabling the
components being bonded to act as a composite structural unit.
A comprehensive review of applications involving the use of adhesives in civil
engineering is given by Hewlett and Shaw (1977), Tabor (1982) and Mays and Hutchinson
(1992). Assessment of an adhesive as a suitable product for structural use must take into
account the design spectrum of loads, the strength and stiffness of the material under short
term, sustained or cyclic loads and the effect on these properties of temperature, moisture
and other environmental conditions during service (Mays, 1993). Concern regarding the
durability properties of adhesive joints has meant that resistance to creep, fatigue and
fracture are considered of greater importance than particularly high strength (Vardy and
Hutchinson, 1986). Temperature is important at all stages in the use and performance of
adhesives, affecting viscosity and therefore workability, usable life and contact time, rate of
cure, degree of cross-linking and final cured performance (Tu and Kruger, 1996). Controlled
conditions are therefore generally required during bonding. This applies equally during the
surface treatment procedures if a durable system is to be achieved. Adhesives, which are
workable and cure at ambient temperatures, have been used and are able to tolerate a certain
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amount of moisture without a marked reduction in performance. These must have adequate
usable time under site conditions and a cure rate which does not hinder the construction
programme. Workmanship under conditions prevalent on site is less conducive to quality
control than in other industries, and thus ability to tolerate minor variations in proportioning
and mixing, as well as imperfect surface treatment, is important. In addition, the products
involved are more toxic, require more careful storage and, bulk for bulk, are considerably
more expensive than traditional construction materials. Nondestructive test methods for
assessing the integrity of bonded joints are now available for civil engineering applications.
Despite some drawbacks, structural adhesives have enormous potential in future
construction applications, particularly where the combination of thick bondlines, ambient
temperature curing and the need to unite dissimilar materials with a relatively high strength
joint are important (Mays and Hutchinson, 1992).
2.5.1 TYPE OF STRUCTURAL ADHESIVES
The principal structural adhesives specifically formulated for use in the construction
industry are epoxy and unsaturated polyester resin systems, both thermosetting polymers.
The formulation of adhesives is considered in detail by Wake (1982), whilst Tabor (1978)
offers guidance on the effective use of epoxy and polyester resins for civil engineering
structures. Two-part epoxies, first developed in the 1940s (Lee and Neville, 1967), consist of
a resin, a hardener or cross-linking agent which causes polymerisation, and various additives
such as fillers, tougheners or flexibilisers, all of which contribute to the physical and
mechanical properties of the resulting adhesive. Formulations can be varied to allow curing
at ambient temperature, the so-called cold cure epoxies, the most common hardeners for
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which are aliphatic polyamines, whose use results in hardened adhesives which are rigid and
provide good resistance to chemicals, solvents and water (Mays and Hutchinson, 1992).
Correct proportioning and thorough mixing are imperative when using epoxy resin systems.
The rate of curing doubles as the temperature increases by 10°C and halves as the
temperature drops by 10 °C and many of the formulations stop curing altogether below a
temperature of 5 °C. Fillers, generally inert materials such as sand or silica, may be used to
reduce cost, creep and shrinkage, reduce exotherm and the coefficient of thermal expansion,
and assist corrosion inhibition and fire retardation. Fillers increase the viscosity of the
freshly mixed system but impart thixotropy, which is useful in application to vertical
surfaces.
Unmodified epoxy systems tend to be brittle when cleavage or peel forces are
imposed. Toughening of the cured adhesive can be achieved by the inclusion of a dispersed
rubbery phase which absorbs energy and prevents crack propagation. Epoxies are generally
tolerant of many surface and environmental conditions and possess relatively high strength.
They are preferred for bonding to concrete since, of all adhesives, they have a particularly
high tolerance of the alkalinity of concrete, as well as moisture. By suitable formulation,
their ability to wet out the substrate surfaces can even be achieved in the presence of water,
the resin being able to disperse the water from the surface being bonded (Tabor, 1978).
Unsaturated polyester resins were discovered in the mid-1930s and have adhesive
properties obtained by cross-linking using a curing agent. They are chemically much more
simple than epoxy resins but have a 10% contraction by volume during curing due to a
volume change during the transition from the uncured liquid phase to the hardened resin
resulting in further curing shrinkage. As a result of these factors, there are usually strict
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limits on the volume of material that can be mixed and applied at any one time and as a
general rule polyester resins do not form as strong adhesive bonds as do epoxy resins. In
storage, the polyester resins are also somewhat less stable and present a greater fire hazard
than epoxies. These limitations significantly restrict their applications.
The advantages of epoxy resins over other polymers as adhesive agents for civil
engineering use can be summarised as follows (Mays and Hutchinson, 1992):
• High surface activity and good wetting properties for a variety of substrates.
• May be formulated to have a long open time (the time between mixing and closing of the
joint).
• High cured cohesive strength, so the joint failure may be dictated by the adherend strength,
particularly with concrete substrates.
• May be toughened by the inclusion of a dispersed rubbery phase.
• Minimal shrinkage on curing, reducing bondline strain and allowing the bonding of large
areas with only contact pressure.
• Low creep and superior strength retention under sustained load.
• Can be thixotropic for application to vertical surfaces.
• Able to accommodate irregular or thick bondlines.
• Formulation can be readily modified by blending with a variety of materials to achieve
desirable properties.
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These various modifications make epoxy adhesives relatively expensive in
comparison to other adhesives. However, the toughness, range of viscosity and curing
conditions, good handling characteristics, high adhesive strength, inertness, low shrinkage
and resistance to chemicals have meant that epoxy adhesives have found many applications
in construction, for example, repair materials, coatings and as structural and non-structural
adhesives.
2.5.2 REQUIREMENTS OF THE ADHESIVE FOR PLATE BONDING
There are many features of an adhesive product, in addition to its purely adhesive
properties, which will form the basis for the selection of a particular bonding system. Mays
(1985) has considered requirements for adhesives to be used for external plate bonding to
bridges under conditions prevalent in the UK. These requirements are extended and refined
in a later publication referred to as a proposed Compliance Spectrum (Mays and Hutchinson,
1988), which addresses the general engineering requirements of adhesives, bonding
procedures and test methods for structural steel-to-concrete bonding, based on research work
at the University of Dundee (Hutchinson, 1986). The requirements proposed for the
adhesive itself can be considered to be equally applicable to fibre reinforced polymer (FRP)
plate bonding. An epoxy resin and polyamine hardener are recommended.
Choice of a suitable adhesive is only one of a number of requirements for a
successfully bonded joint. Other factors also affect the joint strength and performance (Mays
and Hutchinson, 1988) namely:
• appropriate design of the joint
• adequate preparation of the adherend surfaces
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• controlled fabrication of the joint
• protection from unacceptably hostile conditions in service
• postbonding quality assurance.
Both short term and long term structural performance are likely to be improved by
using an appropriately designed joint and suitably preparing the surface of the substrate
materials. A review of factors important to the satisfactory design of joints is given by
Adams and Wake (1984) and Lees (1985) and will not be considered here. Full account
must be taken of the poor resistance of adhesives to peel and cleavage forces; shear strength
itself is unlikely to be a limiting factor. With concrete structures, the tensile/shear, or tear-off
strength of the concrete should be the critical design factor if a suitable adhesive formulation
is selected and appropriate methods of surface preparation implemented. This has been
demonstrated through detailed shear testing on site and in the laboratory (Moustafa, 1974;
Hugenschmidt, 1975; Schultz, 1976).
2.5.3 TESTS TO MEASURE STRUCTURAL ADHESIVE BOND STRENGTH
A number of tests are available for testing adhesive and thin films (Adams and
Wake, 1984; Kinloch, 1987). However, appropriate tests for assessing bond strength in
construction are complicated by the fact that the loading condition in service is difficult to
simulate, and one of the adherends, namely concrete, tends to be weaker in tension and shear
than the adhesives which may be used, making discrimination between adhesive systems
difficult. As a result, confirmation of the suitability of a proposed adhesive system is
generally limited to demonstrating that, when the bondline is stressed in the test
configuration chosen, the failure surface occurs within the concrete substrate. Such tests
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may also be used to exhibit the adequacy of the surface preparation techniques employed,
since it is difficult to separate the individual effects on adhesion of the adhesive type and
method of surface treatment.
Several possible test methods have evolved to measure the bond strength between
adhesive and concrete substrates, mainly for applications in concrete repair (Franke, 1986;
Naderi et al., 1986). The Réunion Internationale des Laboratoires d’Essais et de Recherches
sur les Matériaux et les Constructions (RILEM) Technical Committee 52-RAC lists some
currently used laboratory and field test methods for assessing the bond between resin and
concrete (Sasse and Friebrich, 1983). Procedures are mentioned on the strength of adhesion
in tension, shear and bending, as well as shrinkage and thermal compatibility in the context
of coatings, concrete repair, concrete/ concrete and steel/concrete bonds.
Variations of the slant shear test (Kreigh, 1976), in which two portions of a standard
cylinder or prism are joined by a diagonal bondline and then tested in compression, have
been found to produce discriminating and consistent results (Kreigh, 1976; Naderi, 1985;
Wall et al., 1986). Tu and Kruger (1996) used such a configuration to demonstrate that a
flexible, tough epoxy provided improved adhesion compared to a more brittle material
because it allows redistribution of forces before fracture. However, Tabor (1985) concluded
that the slant shear test is of little use in assessing adhesion between resin and concrete
because the interfaces are not subjected to tensile forces.
In assessing the shear connection in steel/concrete composite construction, tests at
the Wolfson Bridge Research Unit at the University of Dundee employed a kind of double-
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lap joint configuration as described by Solomon (1976), in which fracture was characterised
by shear failure of the concrete adjacent to the interface with the adhesive.
The University of Surrey (Quantrill et al., 1995) have reported a programme of small
scale tests to investigate three different adhesives, two of which were two-part cold cure
epoxies and the third a two-part acrylic. The tests involved subjecting an adhesive/concrete
joint to tensile force and a composite/adhesive/concrete joint to shear, to verify the adequacy
of the surface preparation of the concrete and composite bond surfaces. In these tests the
Sikadur 31 PBA epoxy adhesive was superior to the two other products and demonstrated
strengths in both tension and shear which exceeded those of the concrete. The acrylic
adhesive failed within the adhesive under very small ultimate loads.
Chajes et al. (1996) used a single-lap specimen, in which a strip of carbon composite
was bonded to a concrete prism, to study the bond strength of composite plate materials
bonded to concrete. Four different adhesives were used to bond the composite strip; three
two-part cold cure structural epoxies and a two-part cold cure urethane. Three methods of
surface preparation were studied, varying in severity from untreated to mechanically
abraded to expose the coarse aggregate. It was found that all epoxy-bonded joints failed as a
result of the concrete shearing directly beneath the bond surface at similar loads. The final
strength was therefore a function of the concrete strength. The surface treatment which
involved exposing the coarse aggregate produced the highest average strengths. The
urethane adhesive, which was much less stiff and had a much higher ductility to failure in
tension than the epoxies, failed within the adhesive at lower ultimate loads. It is of interest to
note that a silane surface primer was used on two of Chajes’ adhesives (the primer used was
Chemglaze 9926) and it improved the bond performance of the joints compared with a joint
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not treated thus; when used on concrete the primer tends to improve the bond by
strengthening the surface of the concrete and making it water repellent.
Karbhari and Engineer (1996) describe the use of a modified peel test for
investigation of the bond between composite and concrete, in which a composite strip is
pulled away from the concrete at a known angle and at a controlled rate. The test is said to
provide a good estimate of interfacial energy and could be used in durability assessment.
2.6 CRITICAL OBSERVATIONS FROM LITERATURE REVIEW
The review given in above is based on steel plate and composite sheet and plate
bonding and has been covered extensively but not exhaustively. It has demonstrated the
improvement in structural strength and stiffness brought about by externally bonded
material. The worldwide level of interest in the technique reflects its potential benefits and
also the current importance placed on economical rehabilitation and upgrading methods.
Although the level of experience in the bonding technique of composite plates is limited, the
investigations reported in this chapter have gone some way to illustrate its potential and to
establish a basic technical understanding of short term and long term behaviour. Despite the
growing number of field applications, there remain many material and structural
implications that need to be addressed, in particular with regard to long term performance
under loads.
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OBJECTIVES OF THE STUDY
•
To study the flexural behaviour of reinforced concrete beams.
•
To study the effect of GFRP strengthening on ultimate load carrying capacity and
failure pattern of reinforced concrete beams.
•
To study the shear behaviour of reinforced concrete beams.
•
To study the effect of GFRP strengthening on the shear behaviour of reinforced
concrete beams.
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CHAPTER -3
MATERIALS AND
METHODS
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CHAPTER 3
MATERIALS AND METHODS
3.1 MATERIALS
3.1.1 CONCRETE
Concrete is a construction material composed of portland cement and water
combined with sand, gravel, crushed stone, or other inert material such as expanded slag or
vermiculite. The cement and water form a paste which hardens by chemical reaction into a
strong, stone-like mass. The inert materials are called aggregates, and for economy no more
cement paste is used than is necessary to coat all the aggregate surfaces and fill all the voids.
The concrete paste is plastic and easily molded into any form or troweled to produce a
smooth surface. Hardening begins immediately, but precautions are taken, usually by
covering, to avoid rapid loss of moisture since the presence of water is necessary to continue
the chemical reaction and increase the strength. Too much water, however, produces a
concrete that is more porous and weaker. The quality of the paste formed by the cement and
water largely determines the character of the concrete. Proportioning of the ingredients of
concrete is referred to as designing the mixture, and for most structural work the concrete is
designed to give compressive strengths of 15 to 35 MPa. A rich mixture for columns may be
in the proportion of 1 volume of cement to 1 of sand and 3 of stone, while a lean mixture for
foundations may be in the proportion of 1:3:6. Concrete may be produced as a dense mass
which is practically artificial rock, and chemicals may be added to make it waterproof, or it
can be made porous and highly permeable for such use as filter beds. An air-entraining
chemical may be added to produce minute bubbles for porosity or light weight. Normally,
the full hardening period of concrete is at least 7 days. The gradual increase in strength is
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due to the hydration of the tricalcium aluminates and silicates. Sand used in concrete was
originally specified as roughly angular, but rounded grains are now preferred. The stone is
usually sharply broken. The weight of concrete varies with the type and amount of rock and
sand. A concrete with trap rock may have a density of 2,483 kg/m3. Concrete is stronger in
compression than in tension, and steel bar, called rebar or mesh is embedded in structural
members to increase the tensile and flexural strengths. In addition to the structural uses,
concrete is widely used in precast units such as block, tile, sewer, and water pipe, and
ornamental products.
Portland slag cement (PSC) – 43 grade (Kornak Cement) was used for the
investigation. It was tested for its physical properties in accordance with Indian Standard
specifications. The fine aggregate used in this investigation was clean river sand, passing
through 4.75 mm sieve with specific gravity of 2.68. The grading zone of fine aggregate was
zone III as per Indian Standard specifications. Machine crushed granite broken stone angular
in shape was used as coarse aggregate. The maximum size of coarse aggregate was 20 mm
with specific gravity of 2.73. Ordinary clean portable water free from suspended particles
and chemical substances was used for both mixing and curing of concrete.
For concrete, the maximum aggregate size used was 20 mm. Nominal concrete mix
of 1:1.5:3 by weight is used to achieve the strength of 20 N/mm2. The water cement ratio 0.5
is used. Three cube specimens were cast and tested at the time of beam test (at the age of 28
days) to determine the compressive strength of concrete. The average compressive strength
of the concrete was 31N/mm2.
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3.1.1.1 Cement
Cement is a material, generally in powder form, that can be made into a paste usually
by the addition of water and, when molded or poured, will set into a solid mass. Numerous
organic compounds used for adhering, or fastening materials, are called cements, but these
are classified as adhesives, and the term cement alone means a construction material. The
most widely used of the construction cements is portland cement. It is a bluish-gray powder
obtained by finely grinding the clinker made by strongly heating an intimate mixture of
calcareous and argillaceous minerals. The chief raw material is a mixture of high-calcium
limestone, known as cement rock, and clay or shale. Blast-furnace slag may also be used in
some cements and the cement is called portland slag cement (PSC). The color of the cement
is due chiefly to iron oxide. In the absence of impurities, the color would be white, but
neither the color nor the specific gravity is a test of quality. The specific gravity is at least
3.10. Portland slag cement (PSC) – 43 grade (Kornak Cement) was used for the
investigation.
3.1.1.2 Fine aggregate
Fine aggregate / sand is an accumulation of grains of mineral matter derived from the
disintegration of rocks. It is distinguished from gravel only by the size of the grains or
particles, but is distinct from clays which contain organic materials. Sands that have been
sorted out and separated from the organic material by the action of currents of water or by
winds across arid lands are generally quite uniform in size of grains. Usually commercial
sand is obtained from river beds or from sand dunes originally formed by the action of
winds. Much of the earth’s surface is sandy, and these sands are usually quartz and other
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siliceous materials. The most useful commercially are silica sands, often above 98% pure.
Beach sands usually have smooth, spherical to ovaloid particles from the abrasive action of
waves and tides and are free of organic matter. The white beach sands are largely silica but
may also be of zircon, monazite, garnet, and other minerals, and are used for extracting
various elements.
Sand is used for making mortar and concrete and for polishing and sandblasting.
Sands containing a little clay are used for making molds in foundries. Clear sands are
employed for filtering water. Sand is sold by the cubic yard (0.76 m3) or ton (0.91 metric
ton) but is always shipped by weight. The weight varies from 1,538 to 1,842 kg/m3,
depending on the composition and size of grain. Construction sand is not shipped great
distances, and the quality of sands used for this purpose varies according to local supply.
Standard sand is a silica sand used in making concrete and cement tests. The fine aggregate
obtained from river bed of Koel, clear from all sorts of organic impurities was used in this
experimental program. The fine aggregate was passing through 4.75 mm sieve and had a
specific gravity of 2.68. The grading zone of fine aggregate was zone III as per Indian
Standard specifications.
3.1.1.3 Coarse aggregate
Coarse aggregate are the crushed stone is used for making concrete. The commercial
stone is quarried, crushed, and graded. Much of the crushed stone used is granite, limestone,
and trap rock. The last is a term used to designate basalt, gabbro, diorite, and other darkcolored, fine-grained igneous rocks. Graded crushed stone usually consists of only one kind
of rock and is broken with sharp edges. The sizes are from 0.25 to 2.5 in (0.64 to 6.35 cm),
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although larger sizes may be used for massive concrete aggregate. Machine crushed granite
broken stone angular in shape was used as coarse aggregate. The maximum size of coarse
aggregate was 20 mm and specific gravity of 2.78. Granite is a coarse-grained, igneous rock
having an even texture and consisting largely of quartz and feldspar with often small
amounts of mica and other minerals. There are many varieties. Granite is very hard and
compact, and it takes a fine polish, showing the beauty of the crystals. Granite is the most
important building stone. Granite is extremely durable, and since it does not absorb
moisture, as limestone and sandstone do, it does not weather or crack as these stones do. The
colors are usually reddish, greenish, or gray. Rainbow granite may have a black or darkgreen background with pink, yellowish, and reddish mottling; or it may have a pink or
lavender background with dark mottling. The density is 2,723 kg/m3, the specific gravity
2.72, and the crushing strength 158 to 220 MPa.
3.1.1.4 Water
Water fit for drinking is generally considered fit for making concrete. Water should
be free from acids, oils, alkalies, vegetables or other organic Impurities. Soft waters also
produce weaker concrete. Water has two functions in a concrete mix. Firstly, it reacts
chemically with the cement to form a cement paste in which the inert aggregates are held in
suspension until the cement paste has hardened. Secondly, it serves as a vehicle or lubricant
in the mixture of fine aggregates and cement.
3.1.2 REINFORCEMENT
The longitudinal reinforcements used were high-yield strength deformed bars of 12
mm diameter. The stirrups were made from mild steel bars with 6 mm diameter. The yield
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strength of steel reinforcements used in this experimental program was determined by
performing the standard tensile test on the three specimens of each bar. The average proof
stress at 0.2 % strain of 12 mm φ bars was 437 N/mm2 and that of 6 mm φ bars was 240
N/mm2.
3.1.3 FIBER REINFORCED POLYMER (FRP)
Continuous fiber-reinforced materials with polymeric matrix (FRP) can be
considered as composite, heterogeneous, and anisotropic materials with a prevalent linear
elastic behavior up to failure. They are widely used for strengthening of civil structures.
There are many advantages of using FRPs:
lightweight, good mechanical properties,
corrosion-resistant, etc. Composites for structural strengthening are available in several
geometries from laminates used for strengthening of members with regular surface to bidirectional fabrics easily adaptable to the shape of the member to be strengthened.
Composites are also suitable for applications where the aesthetic of the original structures
needs to be preserved (buildings of historic or artistic interest) or where strengthening with
traditional techniques can not be effectively employed.
Fiber reinforced polymer (FRP) is a composite material made by combining two or
more materials to give a new combination of properties. However, FRP is different from
other composites in that its constituent materials are different at the molecular level and are
mechanically separable. The mechanical and physical properties of FRP are controlled by
its constituent properties and by structural configurations at micro level. Therefore, the
design and analysis of any FRP structural member requires a good knowledge of the
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material properties, which are dependent on the manufacturing process and the properties of
constituent materials.
FRP composite is a two phased material, hence its anisotropic properties. It is
composed of fiber and matrix, which are bonded at interface. Each of these different phases
has to perform its required function based on mechanical properties, so that the composite
system performs satisfactorily as a whole. In this case, the reinforcing fiber provides FRP
composite with strength and stiffness, while the matrix gives rigidity and environmental
protection.
Fig. 3.1 Formation of Fiber Reinforced Polymer Composite
Reinforcement materials
A great majority of materials are stronger ad stiffer in fibrous form than as bulk
materials. A high fiber aspect ratio (length: diameter ratio) permits very effective transfer of
load via matrix materials to the fibers, thus taking advantage of there excellent properties.
Therefore, fibers are very effective and attractive reinforcement materials.
3.1.3.1 Fiber
A fiber is a material made into a long filament with a diameter generally in the order
of 10 tm. The aspect ratio of length and diameter can be ranging from thousand to infinity
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in continuous fibers. The main functions of the fibers are to carry the load and provide
stiffness, strength, thermal stability, and other structural properties in the FRP.
To perform these desirable functions, the fibers in FRP composite must have:
i) high modulus of elasticity for use as reinforcement;
ii) high ultimate strength;
iii) low variation of strength among fibers;
iv) high stability of their strength during handling; and
v) high uniformity of diameter and surface dimension among fibers.
There are three types of fiber dominating in civil engineering industry-glass, carbon
and aramid fibers, each of which has its own advantages and disadvantages.
Table 3.1 Properties of different fibers
Types of fibers used in fiber reinforced polymer composites
¾ Glass fibers
¾ Carbon fibers
¾ Aramid fibers
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Glass fibers
These are fibers commonly used in the naval and industrial fields to produce
composites of medium-high performance. Their peculiar characteristic is their high strength.
Glass is mainly made of silicon (SiO2) with a tetrahedral structure (SiO4). Some aluminium
oxides and other metallic ions are then added in various proportions to either ease the
working operations or modify some properties (e.g., S-glass fibers exhibit a higher tensile
strength than E-glass).
Table 3.2 Typical composition of fiberglass (% in weight)
The production technology of fiberglass is essentially based on spinning a batch
made of sand, alumina, and limestone. The constituents are dry mixed and brought to
melting (about 1260 °C) in a tank. The melted glass is carried directly on platinum bushings
and, by gravity, passes through ad hoc holes located on the bottom. The filaments are then
grouped to form a strand typically made of 204 filaments. The single filament has an
average diameter of 10 µm and is typically covered with a sizing. The yarns are then
bundled, in most cases without twisting, in a roving.
Glass fibers are also available as thin sheets, called mats. A mat may be made of
both long continuous and short fibers (e.g., discontinuous fibers with a typical length
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between 25 and 50 mm), randomly arranged and kept together by a chemical bond. The
width of such mats is variable between 5 cm and 2 m, their density being roughly 0.5 kg/m2.
Glass fibers typically have a Young modulus of elasticity (70 GPa for E-glass) lower
than carbon or aramid fibers and their abrasion resistance is relatively poor; therefore,
caution in their manipulation is required. In addition, they are prone to creep and have low
fatigue strength. To enhance the bond between fibers and matrix, as well as to protect the
fibers itself against alkaline agents and moisture, fibers undergo sizing treatments acting as
coupling agents. Such treatments are useful to enhance durability and fatigue performance
(static and dynamic) of the composite material. FRP composites based on fiberglass are
usually denoted as GFRP.
Fig 3.2 Discontinuous Glass Fibers
Carbon fibers
Carbon fiber is the most expensive of the more common reinforcements, but
in space applications the combination of excellent performance characteristics coupled
with light weight make it indispensable reinforcement with cost being of secondary
importance.
Carbon fibers consist of small crystallite of turbostratic graphite. These
resemble graphite single crystals except that the layer planes are not packed in a regular
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fashion along the c-axis direction. In a graphite single crystal the carbon atoms in a
basal plane are arranged in hexagonal arrays and held together by strong covalent
bonds. Between the basal planes only weak Van-der-waal forces exist. Therefore the single
crystals are highly anisotropic with the plane moduli of the order of 100 GPa whereas the
molecules perpendicular to the basal plane are only about 75 GPa. It is thus evident that to
produce high modulus and high strength fibers, the basal planes of the graphite must be
parallel to the fiber axis. They have lower thermal expansion coefficients than both the glass
and aramid fibers. The carbon fiber is an anisotropic material, and its transverse modulus is
an order of magnitude less than its longitudinal modulus. The material has a very high
fatigue and creep resistance. Since its tensile strength decreases with increasing
modulus, its strain at rupture will also are much lower. Because of the material
brittleness at higher modulus, it becomes critical in joint and connection details, which
can have high stress concentrations. As a result of this phenomenon, carbon composite
laminates are more effective with adhesive bonding that eliminates mechanical fasteners.
Table 3.3 Typical properties of Carbon Fiber
Kevlar fibers
Kevlar (poly-paraphenylene terephthalamide) is the DuPont Company’s brand
name for a synthetic material constructed of para-aramid fibers that the company
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claims is five times stronger than the same weight of steel, while being lightweight,
flexible and comfortable. It is also very heat resistant and decomposes above 400 °C
without melting. It was invented by Stephanie Kwolek of DuPont from research into high
performance polymers, and patented by her in 1966 and first marketed in 1971. Kevlar is a
registered trademark of E.I. du Pont de Nemours and Company.
Originally intended to replace the steel belts in tires, it is probably the most well known
name in soft armor (bulletproof vests). It is also used in extreme sports equipment, hightension drumhead
applications,
animal
handling
protection,
composite
aircraft
construction, fire suits, yacht sails, and as an asbestos replacement. When this polymer is
spun in the same way that a spider spins a web, the resulting commercial para-aramid
fiber has tremendous strength, and is heat and cut resistant. Para-aramid fibers do not rust
or corrode, and their strength is unaffected by immersion in water. When woven together,
they form a good material for mooring lines and other underwater objects. However, unless
specially waterproofed, para-aramid fiber’s ability to stop bullets and other projectiles is
degraded when wet. Kevlar is a type of aramid that consists of long polymeric chains with
a parallel orientation. Kevlar derives its strength from inter-molecular hydrogen bonds
and aromatic stacking interactions between aromatic groups in neighboring strands.
These interactions are much stronger than the Van der Waals interaction found in other
synthetic polymers and fibers like Dyneema. The presence of salts and certain other
impurities, especially calcium, would interfere with the strand interactions and has to be
avoided in the production process. Kevlar consists of relatively rigid molecules, which
form a planar sheet-like structure similar to silk protein.
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These properties result in its high mechanical strength and its remarkable heat
resistance. Because it is highly unsaturated, i.e. the ratio of carbon to hydrogen atoms is
quite high, it has a low flammability. Kevlar molecules have polar groups accessible for
hydrogen bonding. Water that enters the interior of the fiber can take the place of bonding
between molecules and reduce the material's strength, while the available groups at the
surface lead to good wetting properties. This is important for bonding the fibers to other
types of polymer, forming a fiber reinforced plastic. This same property also makes
the fibers feel more natural and "sticky" compared to non-polar polymers like
polyethylene. In structural applications, Kevlar fibers can be bonded to one another or to
other materials to form a composite. Kevlar's main weaknesses are that it decomposes under
alkaline conditions or when exposed to chlorine. While it can have a great tensile strength,
sometimes in excess of 4.0 GPa, like all fibers it tends to buckle in compression.
Fig. 3.3 Structure of aramid fiber
3.1.3.2 Fiber sheet
Fiber sheet used in this experimental investigation was E-Glass, Bi directional
woven roving mat. It was not susceptible to atmospheric agents. It was also chemically
resistive and anticorrosive.
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3.1.4 TYPES OF MATRIX MATERIALS
Fibers, since they cannot transmit loads from one to another, have limited use
in engineering applications. When they are embedded in a matrix material, to form a
composite, the matrix serves to bind the fibers together, transfer loads to the fibers, and
damage due to handling. The matrix has a strong influence on several mechanical
properties of the composite such as transverse modulus and strength, shear properties,
and properties in compression. Physical and chemical characteristics of the matrix such
as melting or curing temperature, viscosity, and reactivity with fibers influence the
choice of fabrication process. The matrix material for a composite system is selected,
keeping in view all these factors. Thermoset resins are the most commonly used matrices for
production of FRP materials. They are usually available in a partially polymerized state with
fluid or pasty consistency at room temperature. When mixed with a proper reagent, they
polymerize to become a solid, vitreous material. The reaction can be accelerated by
adjusting the temperature. Thermoset resin have several advantages, including low viscosity
that allows for a relative easy fiber impregnation, good adhesive properties, room
temperature polymerization characteristics, good resistance to chemical agents, absence of
melting temperature, etc. Disadvantages are limited range of operating temperatures, with
the upper bound limit given by the glass transition temperature, poor toughness with respect
to fracture (“brittle” behavior), and sensitivity to moisture during field applications. The
most common thermosetting resins for civil engineering are the epoxy resin. Polyester or
vinylester resins are also used. Considering that the material is mixed directly at the
construction site and obtains its final structural characteristics through a chemical reaction, it
should always be handled by specialized personnel.
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Commonly used matrix materials are described below:
3.1.4.1 Epoxy resin
Epoxy resins are relatively low molecular weight pre-polymers capable of
being processed under a variety of conditions. Two important advantages of these
resins are over unsaturated polyester resins are: first, they can be partially cured and
stored in that state, and second they exhibit low shrinkage during cure. However, the
viscosity of conventional epoxy resins is higher and they are more expensive compared to
polyester resins. The cured resins have high chemical, corrosion resistance, good mechanical
and thermal properties, outstanding adhesion to a variety of substrates, and good and
electrical properties. Approximately 45% of the total amount of epoxy resins produced is
used in protective coatings while the remaining is used in structural applications such as
laminates and composites, tooling, moulding, casting, construction, adhesives, etc.
Epoxy resins are characterized by the presence of a three-membered ring containing
two carbons and an oxygen (epoxy group or epoxide or oxirane ring). Epoxy is the
first liquid reaction product of bisphenol-A with excess of epichlorohidrin and this
resin is known as diglycidylether of bisphenol A (DGEBA). DGEBA is used extensively
in industry due to its high fluidity, processing ease, and good physical properties of the
cured of resin.
Fig. 3.4 Structure of DGEBA
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A range of epoxy resins is now available, varying from relatively tough low
temperature epoxies for use in construction industry to brittle epoxies for use in
construction
industry
to brittle epoxies useful in aerospace sector. This widespread
application of epoxy resin is primarily due to the availability of resins with different
backbone structures and molecular weights to give products with low viscosity (liquids) to
low melting point solids. The ease of processibility, good melting characteristics, excellent
adhesion to various types of substrates, low shrinkage during cure, superior mechanical
properties of cured resin, and good thermal and chemical resistance have made epoxy
resin a material of choice in advanced fiber reinforced composites. Ethylene diamines are
most widely used aliphatic amines for cured epoxy resins. These are highly reactive, low
molecular weight curing agents that result in tightly cross-linked network. One primary
amino group reacts with two epoxy groups. The primary and secondary amines are reactive
curing agents. The primary amino group is more reactive towards epoxy than secondary
amino groups are consumed (95%), whereas only 28% of secondary amino groups are
consumed.
Fig. 3.5 The curing of epoxy resin with primary amines
The primary amino-epoxy reaction results in linear polymerization while secondary
amino-epoxy reaction leads to branching and cross-linking. The cured epoxy resins find
a variety of applications as adhesives, laminates, sealants, coatings, etc. The optimum
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curing temperature and the thermal stability of epoxy resin depend on the type of curing
agent. The anhydride cured epoxy resins have excellent electrical, chemical, and
mechanical properties and are used for electrical and electronic applications. Epoxies
are used as binders in materials for construction. Filling of cracks in concrete structures is
achieved by epoxies. In construction industry, for bonding and coating purposes, low
temperature curing of epoxies is achieved by using thiols that exhibit higher curing rates.
Epoxy based prepregs have been used in numerous aircraft components such as
rudders, stabilizers, elevators, wing tips, launching gear doors, radomes, ailerons, etc.
The composite materials constitute 3-9% of total structural weight of the commercial
aircrafts such as Boeing 767 or Boeing 777. Composite and laminate industry uses 28%
of epoxy resins produced. Besides these applications, the applications, the major user of
epoxy is coating industry.
Table 3.4 Properties of epoxy resin
The success of the strengthening technique critically depends on the performance of the
epoxy resin used. Numerous types of epoxy resins with a wide range of mechanical
properties are commercially available. These epoxy resins are generally two part systems, a
resin and a hardener. The resin and hardener are used in this study is Araldite LY 556 and
Hardener HY 951, respectively. Araldite LY-556, an unmodified epoxy resin based on
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Bisphenol-A and the hardener (Ciba-Geig, India) HY 951 (8% of total Epoxy taken) an
aliphatic primary amine, were mixed properly.
Properties
Araldite LY 556
Hardener HY 951
Color
Clear
Colorless
Odor
Slight
Ammonia
Physical State
Liquid
Liquid
Solubility in water
Insoluble
Miscible
Vapor Pressure
< 0.01 Pa at 20⁰C
<0.01 mmHg at 20⁰C
Specific Gravity
1.15 – 1.2 at 25⁰C
1 at 20⁰C
Boiling Point
>200⁰C
>200⁰C
Decomposition Temperature
>200⁰C
>200⁰C
Table 3.5 Properties of epoxy resin and hardener
3.1.4.2 Unsaturated polyester resins
A polyester resin is an unsaturated (reactive) polyester solid dissolved in a
polymerizable monomer. Unsaturated polyesters are long-chain linear polymers containing a
number of carbon double bonds. They are made by a condensation reaction between a glycol
(ethylene, propylene, diethylene glycol) and an unsaturated dibasic acid (maleic or fumaric).
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The polymerizable (reactive) monomer such as styrene, which also contains carbon double
bonds, acts as a cross-linking agent by bridging adjacent polyester molecules at their
unsaturated points. The monomer also acts as a diluent, reduces viscosity, and makes it
easier to process. The curing or cross-linking process is initiated by adding a small quantity
of a catalyst such as organic peroxide or an aliphatic azo compound. Since there is no byproduct of the reaction, the curing is done at room temperature of elevated temperature with
or without application of pressure. A typical polyester resin made from reaction of maleic
acid and diethylene glycol is shown below:
HOOC-CH=CHCOOH + [HOCH2CH2OCH2OH]
OH-[CH2CH2OCH2CH2OC-OCH=COHCO]n-H+H2O
The length of the molecule or degree of polymerization n may vary. The resin
will generally be a solid but is dissolved in a monomer such as styrene. The solution
viscosity can be controlled by the percent styrene and is generally quite fluid (less than the
viscosity of honey). The conversion from liquid to solid occurs through the use of a
free-radical initiator (e.g., benzoyl peroxide) or curing agent. Te styrene monomer crosslinks or reacts with the double bond in the polyester backbone above to form a network
polymer. The reaction does not produce any by-product and is exothermic reaction. Thus,
the curing process is accompanied by shrinkage as well as temperature increase.
Capabilities of modifying or tailoring the chemical structure of polyesters by processing
techniques and raw materials selection make them versatile.
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Table 3.6 Typical properties of cast Thermosetting Polyesters
3.1.4.3 Adhesives
The implementation of FRP-based structural strengthening (e.g., pultruded laminate)
requires the use of adhesives. The choice of the most suitable adhesive as well as the type of
surface treatment to be carried out prior to FRP application shall be made on the basis of
available substrate and properties of the selected FRP system. Technical data sheets for FRP
materials usually report the indications of the adhesive to be used as a function of the
structure to be strengthened. Even the application of dry fabrics impregnated on-site might
be considered as an assembling operation using adhesives. The type of surface treatment to
be carried out prior to FRP application is important for the correct use of adhesives.
An adhesive is a material quite often of a polymeric nature capable of creating a link
between at least two surfaces and able to share loads. There are many types of natural and
synthetic adhesives (elastomers, thermoplastics, and mono- or bi-component thermosetting
resins); the most suitable adhesives for composite materials are based on epoxy resins.
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Epoxy adhesives usually are bi-component viscous mixture; once hardened, through a crosslinking chemical reaction, they become suitable for structural applications.
There are several advantages in the use of adhesive bonding compared to mechanical
anchorage. They include the possibility of connecting different materials, providing greater
stiffness, uniform distribution of loads, and avoiding holes dangerous for stress
concentrations. On the other hand, adhesives are sensitive to environmental conditions, such
as moisture, and are not appropriate when exposed to high temperatures (fire resistance).
The efficiency of adhesion depends on many factors, such as surface treatment,
chemical composition and viscosity of the adhesive, application technique, and hardening or
cross-linking process of the adhesive itself. Adhesion mechanisms primary consist of
interlocking of the adhesive with the surface of the support with formation of chemical
bonds between polymer and support. As a result, adhesive strength is enhanced by surface
treatments that improve interfacial properties of the support by increasing the roughness of
the surface to be strengthened.
The main objective of surface treatment is “cleaning” of the surface by removal of
all possible surface contaminations, such as oxides, foreign particles, oil, laitance, dust,
moisture, etc. The adopted treatment usually modifies the chemistry of the surface,
enhancing the formation of stronger bonds with the adhesive such to resist environmentally
aggressive agents, which would degrade the adhesion over time. Finally, treatments should
ensure adequate surface roughness.
In some instances, prior to adhesive application, a layer of primer acting as a
coupling agent is applied. The use of FRP pultruded laminate requires an additional cleaning
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of the laminate face prior to bonding. In some cases, laminates have a protective film that
prevents external contamination. Such films shall be removed before the laminate is applied.
Any surface treatment shall be carried out immediately before FRP application takes place
to avoid surface recontamination. Presence of moisture in the support shall be avoided
during FRP application; the surfaces of the support shall be perfectly dry prior to application
of the adhesive.
3.2 EXPERIMENTAL STUDY
The experimental study consists of casting of two sets of reinforced concrete (RC) beams. In
SET I three beams weak in flexure were casted, out of which one is controlled beam and
other two beams were strengthened using continuous glass fiber reinforced polymer (GFRP)
sheets in flexure. In SET II three beams weak in shear were casted, out of which one is the
controlled beam and other two beams were strengthened by using continuous glass fiber
reinforced polymer (GFRP) sheets in shear. The strengthening of the beams is done with
varying configuration and layers of GFRP sheets. Experimental data on load, deflection and
failure modes of each of the beams were obtained. The change in load carrying capacity and
failure mode of the beams are investigated as the amount and configuration of GFRP sheets
are altered. The following chapter describes in detail the experimental study.
3.3 CASTING OF BEAMS
Two sets of beams were casted for this experimental test program. In SET I three
beams (F1, F2 and F3) weak in flexure were casted using same grade of concrete and
reinforcement detailing. In SET II three beams (S1, S2 and S3) weak in shear were casted
using same grade of concrete and reinforcement detailing. The dimensions of all the
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specimens are identical. The cross sectional dimensions of the both the set of beams is 250
mm by 200 mm and length is 2300 mm. In SET I beams 2, 12 mm φ bars are provided as the
main longitudinal reinforcement and 6 mm φ bars as stirrups at a spacing of 75 mm center to
center where as in SET II beams 3, 12 mm φ bars are provided as the main longitudinal
reinforcement and without any stirrups.
Fig. 3.6 Reinforcement details of SET I beams
Fig. 3.7 Section of SET I beams
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Fig. 3.8 Reinforcement details of SET II beams
Fig. 3.9 Section of SET II beams
3.3.1 MATERIALS FOR CASTING
3.3.1.1 Cement
Portland slag cement (PSC) – 43 grade (Kornak Cement) was used for the
investigation. It was tested for its physical properties in accordance with Indian Standard
specifications.
3.3.1.2 Fine aggregate
The fine aggregate obtained from river bed of Koel, clear from all sorts of organic
impurities was used in this experimental program. The fine aggregate was passing through
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4.75 mm sieve and had a specific gravity of 2.68. The grading zone of fine aggregate was
zone III as per Indian Standard specifications.
3.3.1.3 Coarse aggregate
The coarse aggregates used were of two grades, non-reactive and available in local
quarry. One grade contained aggregates passing through 4.75 mm sieve and retained on 10
mm size sieve. Another grade contained aggregates passing through 10 mm sieve but
retained on 20 mm sieve.
3.3.1.4 Water
Ordinary tap water used for concrete mix in all mix.
3.3.1.5 Reinforcing steel
HYSD bars of 12 mm φ were used as main reinforcement. 6 mm φ mild steel bars
were used for shear reinforcement.
3.3.2 FORM WORK
Fresh concrete, being plastic requires some kind of form work to mould it to the
required shape and also to hold it till it sets. The form work has, therefore, got to be suitably
designed. It should be strong enough to take the dead load and live load, during construction
and also it must be rigid enough so mat any bulging, twisting or sagging due to the load if
minimized, Wooden beams, mild steel sheets, wood, and several other materials can also be
used. Formwork should be capable of supporting safely all vertical and lateral loads that
might be applied to it until such loads can be supported by the ground, the concrete
structure, or other construction with adequate strength and stability. Dead loads on
formwork consist of the weight of the forms and the weight of and pressures from freshly
placed concrete. Live loads include weights of workers, equipment, material storage, and
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runways, and accelerating and braking forces from buggies and other placement equipment.
Impact from concrete placement also should be considered in formwork design.
Horizontal or slightly inclined forms often are supported on vertical or inclined
support members, called shores, which must be left in place until the concrete placed in the
forms has gained sufficient strength to be self-supporting. The shores may be removed
temporarily to permit the forms to be stripped for reuse elsewhere, if the concrete has
sufficient strength to support dead loads, but the concrete should then be reshored
immediately.
The form work used for casting of all the specimen consists of mould prepared with
two Channel sections of iron bolted by iron plates at the ends. The form work was
thoroughly cleaned and all the corners and junctions were properly sealed by plaster of Paris
to avoid leakage of concrete through small openings. Shuttering oil was then applied to the
inner face of the form work. The reinforcement cage was then placed in position inside the
form work carefully keeping in view a clear cover of 20 mm for the top and bottom bars.
3.3.3 MIXING OF CONCRETE
Mixing of concrete should be done thoroughly to ensure that concrete of uniform
quantity is obtained. Hand mixing is done in small works, while machine mixing is done for
all big and important works. Although a machine generally does the mixing, hand mixing
sometimes may be necessary. A clean surface is needed for this purpose, such as a clean,
even, paved surface or a wood platform having tight joints to prevent paste loss. Moisten the
surface and level the platform, spread cement over the sand, and then spread the coarse
aggregate over the cement. Use either a hoe or a square-pointed D-handled shovel to mix the
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materials. Turn the dry materials at least three times until the color of the mixture is
uniform. Add water slowly while you turn the mixture again at least three times, or until you
obtain the proper consistency. Usually 10% extra cement is added in case of hand mixing to
account for inadequency in mixing.
3.3.4 COMPACTION
All specimens were compacted by using needle vibrator for good compaction of
concrete. Sufficient care was taken to avoid displacement of the reinforcement cage inside
the form work. Finally the surface of the concrete was leveled and finished and smoothened
by metal trowel and wooden float.
3.3.5 CURING OF CONCRETE
The concrete is cured to prevent or replenish the loss of water which is essential for
the process of hydration and hence for hardening. Also curing prevents the exposure of
concrete to a hot atmosphere and to drying winds which may lead to quick drying out of
moisture in the concrete and thereby subject it to contraction stresses at a stage when the
concrete would not be strong enough to resists them. Concrete is usually cured by water
although scaling compounds are also used. It makes the concrete stronger, more durable,
more impermeable and more resistant to abrasion and to frost. Curing is done by spraying
water or by spending wet heissian cloth over the surface. Usually, curing starts as soon as
the concrete is sufficiently hard. Normally 14 or more days of curing for ordinary concrete is
the requirement. However, the rate of hardening of concrete is very much reduced with the
reduction of ambient temperature.
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3.4 STRENGTHENING OF BEAMS
Before bonding the composite fabric onto the concrete surface, the required region of
concrete surface was made rough using a coarse sand paper texture and cleaned with an air
blower to remove all dirt and debris. Once the surface was prepared to the required standard,
the epoxy resin was mixed in accordance with manufacturer’s instructions. Mixing was
carried out in a plastic container (Araldite LY 556 – 100 parts by weight and Hardener HY
951 – 8 parts by weight) and was continued until the mixture was in uniform colour. When
this was completed and the fabrics had been cut to size, the epoxy resin was applied to the
concrete surface. The composite fabric was then placed on top of epoxy resin coating and
the resin was squeezed through the roving of the fabric with the roller. Air bubbles
entrapped at the epoxy/concrete or epoxy/fabric interface were to be eliminated. Then the
second layer of the epoxy resin was applied and GFRP sheet was then placed on top of
epoxy resin coating and the resin was squeezed through the roving of the fabric with the
roller and the above process was repeated. During hardening of the epoxy, a constant
uniform pressure was applied on the composite fabric surface in order to extrude the excess
epoxy resin and to ensure good contact between the epoxy, the concrete and the fabric. This
operation was carried out at room temperature. Concrete beams strengthened with glass fiber
fabric were cured for 24 hours at room temperature before testing.
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Fig. 3.10 Application of epoxy and hardener on the beam
Fig. 3.11 Fixing of GFRP sheet on the beam
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Fig. 3.12 Roller used for removal of air bubbles
3.5 EXPERIMENTAL SETUP
All the specimens were tested in the loading frame of the “Structural Engineering”
Laboratory of National Institute of Technology, Rourkela. The testing procedure for the
entire specimen was same. After the curing period of 28 days was over, the beam as washed
and its surface was cleaned for clear visibility of cracks. The most commonly used load
arrangement for testing of beams will consist of two-point loading. This has the advantage
of a substantial region of nearly uniform moment coupled with very small shears, enabling
the bending capacity of the central portion to be assessed. If the shear capacity of the
member is to be assessed, the load will normally be concentrated at a suitable shorter
distance from a support.
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Two-point loading can be conveniently provided by the arrangement shown in
Figure. The load is transmitted through a load cell and spherical seating on to a spreader
beam. This beam bears on rollers seated on steel plates bedded on the test member with
mortar, high-strength plaster or some similar material. The test member is supported on
roller bearings acting on similar spreader plates.
The
loading
frame
must
be
capable
of
carrying
the
expected
test
loads without significant distortion. Ease of access to the middle third for
crack
observations,
deflection
readings
and
possibly
strain
measurements
is
an important consideration, as is safety when failure occurs.
The specimen was placed over the two steel rollers bearing leaving 150 mm from the
ends of the beam. The remaining 2000 mm was divided into three equal parts of 667 mm as
shown in the figure. Two point loading arrangement was done as shown in the figure.
Loading was done by hydraulic jack of capacity 100 KN. Three number of dial gauges were
used for recording the deflection of the beams. One dial gauge was placed just below the
center of the beam and the remaining two dial gauges were placed just below the point loads
to measure deflections.
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Fig. 3.13 Two point loading experimental setup
Fig. 3.14 Shear force and bending moment diagram for two point loading
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Fig. 3.15 Shear strengthening zone and flexure strengthening zone of the beam
Load Cell
Spreader beam Steel roller
bearing on mild
steel plates
Beam F1
Dial Gauges
Fig. 3.16 Experimental setup for testing of beams
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3.5.1 PROCEDURE
Before testing the member was checked dimensionally, and a detailed visual
inspection made with all information carefully recorded. After setting and reading all
gauges, the load was increased incrementally up to the calculated working load, with loads
and deflections recorded at each stage. Loads will then normally be increased again in
similar increments up to failure, with deflection gauges replaced by a suitably mounted scale
as failure approaches. This is necessary to avoid damage to gauges, and although accuracy is
reduced, the deflections at this stage will usually be large and easily measured from a
distance. Similarly, cracking and manual strain observations must be suspended as failure
approaches unless special safety precautions are taken. If it is essential that precise
deflection readings are taken up to collapse. Cracking and failure mode was checked
visually, and a load/deflection plot was prepared.
3.6 FABRICATION OF GFRP PLATE
To meet the wide range of needs which may be required in fabricating composites,
the industry has evolved oven a dozen separate manufacturing processes as well as a number
of hybrid processes. Each of these processes offers advantages and specific benefits which
may apply to the fabricating of composites. Hand lay-up and spray-up are two basic molding
processes. The hand lay-up process is the oldest, simplest, and most labour intense
fabrication method. The process is most common in FRP marine construction. In hand layup method liquid resin is placed along with reinforcement (woven glass fiber) against
finished surface of an open mould. Chemical reactions in the resin harden the material to a
strong, light weight product. The resin serves as the matrix for the reinforcing glass fibers,
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much as concrete acts as the matrix for steel reinforcing rods. The percentage of fiber and
matrix was 50:50 in weight.
The following constituent materials were used for fabricating the plate:
1. E-glass woven roving as reinforcement
2. Epoxy as resin
3. Hardener as diamine (catalyst)
4. Polyvinyl alcohol as a releasing agent
Contact moulding in an open mould by hand lay-up was used to combine plies of
woven roving in the prescribed sequence. A flat plywood rigid platform was selected. A
plastic sheet was kept on the plywood platform and a thin film of polyvinyl alcohol was
applied as a releasing agent by use of spray gun. Laminating starts with the application of a gel
coat (epoxy and hardener) deposited on the mould by brush, whose main purpose was to
provide a smooth external surface and to protect the fibers from direct exposure to the
environment. Ply was cut from roll of woven roving. Layers of reinforcement were placed on
the mould at top of the gel coat and gel coat was applied again by brush. Any air which may
be entrapped was removed using serrated steel rollers. The process of hand lay-up was the
continuation of the above process before the gel coat had fully hardened. Again, a plastic
sheet was covered the top of plate by applying polyvinyl alcohol inside the sheet as releasing
agent. Then, a heavy flat metal rigid platform was kept top of the plate for compressing
purpose. The plates were left for a minimum of 48 hours before being transported and cut to
exact shape for testing.
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Fig. 3.17 Specimen for tensile testing in INSTRON 1195
Fig. 3.18 Experimental setup of INSTRON 1195
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Fig. 3.19 Specimen failure after tensile test
3.7 DETERMINATION OF ULTIMATE STRESS, ULTIMATE LOAD
AND YOUNG’S MODULUS
The ultimate stress, ultimate load and Young’s modulus are determined
experimentally by performing unidirectional tensile tests on specimens cut in longitudinal
and transverse directions, and at 45° to the longitudinal direction, as described in ASTM
standard: D 638-08 and D 3039/D 3039M - 2006. A thin flat strip of specimen having a
constant rectangular cross section was prepared in all cases. The dimension of the specimen
was taken as below:
Length(mm)
200
Width(mm)
Thickness(mm)
24
Table 3.7 Size of the specimen for tensile test
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The specimens were cut from the plates themselves by diamond cutter or by hex saw.
After cutting in the hex saw, it was polished in the polishing machine. At least three
replicate sample specimens were tested and mean values adopted.
Coupons were machined carefully to minimize any residual stresses after they were
cut from the plate and the minor variations in dimensions of different specimens are carefully
measured. For measuring the Young's modulus, the specimen is loaded in INSTRON 1195
universal testing machine monotonically to failure with a recommended rate of extension (rate
of loading) of 5 mm/minute. Specimens were fixed in the upper jaw first and then gripped in
the movable jaw (lower jaw). Gripping of the specimen should be as much as possible to
prevent the slippage. Here, it was taken as 50mm in each side. Initially strain was kept at zero.
The load, as well as the extension, was recorded digitally with the help of a load cell and an
extensometer respectively. From these data, engineering stress vs. strain curve was plotted;
the initial slope of which gives the Young's modulus. The ultimate stress and ultimate load
were obtained at the failure of the specimen.
Ultimate Stress
(MPa)
GFRP plate of 2layers
334.5
Ultimate load (KN)
4.817
Young's modulus
(MPa)
11310
Table 3. 8 Ultimate stress, Ultimate load and Young’s modulus of GFRP plate
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CHAPTER -4
ANALYTICAL STUDY
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
CHAPTER 4
ANALYTICAL STUDY
4.1 INTRODUCTION
This chapter is devoted to the development of an analytical model to analyse and
design reinforced concrete beams strengthened in flexural by means of externally bonded
glass fiber reinforced polymer composite sheets. The purpose of this analytical model is to
accurately predict the flexural behavior of reinforced concrete beams strengthened with
GFRP sheet.
4.2 FLEXURAL STRENGTHENING OF BEAMS
To increase flexural strength, FRP fabrics are bonded as an external reinforcement
on the tension side of steel-reinforced concrete beams with fiber orientation along the
member length. Depending on the ratio of FRP reinforcement area to the beam's crosssectional area and the area of internal steel reinforcement, the increase in flexural strength
can be more than 100%. However, a flexural strength increase up to 50% would be more
realistic, which depends on practical considerations such as the concrete member
dimensions, serviceability limits, ductility, and effective thickness of FRP fabric
reinforcement. The design philosophy of strengthening rectangular RC beams, is equally
applicable to other shapes such as T- and I-sections having non-prestressed reinforcement.
Fig. 4.1 Area for flexural and shear strengthening of beams
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Elevation
Section A-A
Fig. 4.2 Flexural strengthening of beam using FRP sheets at the bottom
4.2.1 ASSUMPTIONS
The following assumptions are made in calculating the flexural resistance of a section
strengthened with an externally applied FRP system:
1) Design calculations are based on the actual dimensions, internal reinforcing steel
arrangement, and material properties of the existing member being strengthened;
2) The strains in the reinforcement and concrete are directly proportional to the distance
from the neutral axis, that is, a plane section before loading remains plane after
loading;
3) There is no relative slip between external FRP reinforcement and the concrete;
4) The shear deformation within the adhesive layer is neglected since the adhesive layer
is very thin with slight variations in its thickness;
5) The maximum usable compressive strain in the concrete is 0.003;
6) The tensile strength of concrete is neglected; and
7) The FRP reinforcement has a linear elastic stress-strain relationship to failure.
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Fig. 4.3 Stress Strain diagram of singly reinforced beam strengthen with FRP
4.2.2 CALCULATION OF MOMENT OF RESISTANCE OF THE BEAMS
The moment of resistance of the SET I beams are obtained from the following
calculations:
31 200 415 /
226.19 As per IS: 456 : 2000, Clause 38.1, ANNEX G, the total force due to compression is equal
to the total force due to tension, hence
0.36 0.87 36.59 93
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0.36 0.42 17.12 Hence, the moment of resistance of beam F1 is 17.12 .
Now considering the effect of strengthening of beam F2 using two layers of GFRP
sheets, so along with the tensile force
The value of
an addition tensile force
i.e. stress of GFRP
will be also acting.
area of GFRP. The value of
is
obtained from the experimental testing.
334.5 /
120 0.36 0.87 54.54 0.36 24.6 0.42 Hence the moment of resistance of beam F2 is 24.6 . Due to application of
two layers of GFRP sheet at the soffit of the F2 beam, the moment of resistance of the beam
F2 is higher than moment of resistance of beam F1. The depth of neutral axis and the
moment of resistance of both the beams is presented in table 4.1.
Beam mm
F1 36.59
17.12 F2 54.54
24.6 Table 4.1 Analytical calculations of beams F1 and F2
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CHAPTER -5
RESULTS AND
DISCUSSIONS
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
CHAPTER 5
RESULTS AND DISCUSSION
5.1 INTRODUCTION
This chapter describes the experimental results of SET I beams (weak in flexure) and
SET II beams (weak in shear). Their behavior throughout the static test to failure is
described using recorded data on deflection behavior and the ultimate load carrying capacity.
The crack patterns and the mode of failure of each beam are also described in this chapter.
Two sets of beams were tested for their ultimate strengths. In SET I three beams (F1,
F2 and F3) weak in flexure are tested. In SET II three beams (S1, S2 and S3) weak in shear
are tested. The beams F1and S1 were taken as the control beams. It was observed that the
beams F1 and S1 had less load carrying capacity when compared to that of the externally
strengthened beams using GFRP sheets. In SET I beams F2 is strengthened only at the soffit
of the beam and F3 is strengthened up to the neutral axis of the beam along with the soffit of
the beam. SET II beams S2 is strengthened only at the sides of the beam in the shear zone
and S3 is strengthened by U-wrapping of the GFRP sheets in the shear zone of the beam.
Deflection behavior and the ultimate load carrying capacity of the beams were noted. The
ultimate load carrying capacity of all the beams along with the nature of failure is given in
Table 5.1.
5.2 FAILURE MODES
The flexural and shear strength of a section depends on the controlling failure mode.
The following flexural and shear failure modes should be investigated for an FRPstrengthened section:
• Crushing of the concrete in compression before yielding of the reinforcing steel;
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• Yielding of the steel in tension followed by rupture of the FRP laminate;
• Yielding of the steel in tension followed by concrete crushing;
• Shear/tension delamination of the concrete cover (cover delamination); and
• Debonding of the FRP from the concrete substrate (FRP debonding).
A number of failure modes have been observed in the experiments of RC beams
strengthened in flexure and shear by GFRPs. These include flexure failure, shear failure,
flexural failure due to GFRP rupture and crushing of concrete at the top. Concrete crushing
is assumed to occur if the compressive strain in the concrete reaches its maximum usable
strain. Rupture of the FRP laminate is assumed to occur if the strain in the FRP reaches its
design rupture strain before the concrete reaches its maximum usable strain. Cover
delamination or FRP debonding can occur if the force in the FRP cannot be sustained by the
substrate. In order to prevent debonding of the FRP laminate, a limitation should be placed
on the strain level developed in the laminate.
The GFRP strengthened beam and the control beams were tested to find out their
ultimate load carrying capacity. It was found that the control beams F1 and S1 failed in
flexure and shear showing that the beams were deficient in flexure and shear respectively. In
SET I beam F2 failed due to fracture of GFRP sheet in two pieces and then flexural-shear
failure of the beam took place. Beam F3 failed due to delamination of the GFRP sheet after
that fracture of GFRP sheet took place and then flexural-shear failure of the beam. In SET I
beams F2 and F3, GFRP rupture and flexural-shear kind of failure was prominent when
strengthening was done using both the wrapping schemes. In SET II beams S2 and S3 failed
due to flexural failure and crushing of concrete on the top of the beam. The SET II beams S2
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and S3 developed major flexural cracks at the ultimate loads. In SET II beams S2 and S3 the
flexural kind of failure was prominent when strengthening was done using both the
wrapping schemes.
Sr.
No.
1
2
Type of Beam
Beams weak in
flexure (SET I)
Beams weak in
shear (SET II)
F1
Load at
initial
crack
(KN)
30
F2
34
104
GFRP rupture +
Flexure-shear
failure
F3
Not
visible
112
GFRP rupture +
Flexure-shear
failure
S1
35
82
Shear failure
S2
39
108
Flexural failure +
Crushing of
concrete
S3
40
122
Flexural failure +
Crushing of
concrete
Beam
designation
Ultimate
load
(kN)
Nature of failure
78
Flexural failure
Table 5.1 Ultimate load and nature of failure for SET I and SET II beams
5.3 LOAD DEFLECTION HISTORY
The load deflection history of all the beams was recorded. The mid-span deflection
of each beam was compared with that of their respective control beams. Also the load
deflection behaviour was compared between two wrapping schemes having the same
reinforcement. It was noted that the behaviour of the flexure and shear deficient beams when
bonded with GFRP sheets were better than their corresponding control beams. The mid-span
deflections were much lower when bonded externally with GFRP sheets. The graphs
comparing the mid-span deflection of flexure and shear deficient beams and their
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2009
corresponding control beams are shown in Figs 5.4 and 5.8. The use of GFRP sheet had
effect in delaying the growth of crack formation. In SET I when both the wrapping schemes
were considered it was found that the beam F3 with GFRP sheet up to the neutral axis along
with the soffit had a better load deflection behaviour when compared to the beam F2 with
GFRP sheet only at the soffit of the beam. In SET II when both the wrapping schemes were
considered it was found that the beam S3 with U wrapping of GFRP sheet had a better load
deflection behaviour when compared to the beam S2 with GFRP sheet only at the sides of
the beam.
Load vs Deflection Curve for Beam F1
60
50
Load in KN
40
Left
30
Middle
Right
20
10
0
0
1
2
3
4
5
6
Deflection in mm
Fig. 5.1 Load vs Deflection Curve for Beam F1
Beam F1 was the control beam of SET I beams which were weak in flexure but
strong in shear. In beam F1 strengthening was not done. Two point static loading was done
on the beam and at the each increment of the load, deflection at the left, right and middle
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dial gauges were taken. Using this load and deflection of data, load vs deflection curve is
ploted. At the load of 30 KN initial cracks started coming on the beams. Further with
increase in loading propagation of the cracks took place. The beam F1 failed completely in
flexure.
Load vs Deflection Curve for Beam F2
80
70
Load in KN
60
50
40
Left
30
Middle
Right
20
10
0
0
2
4
6
8
10
12
Deflection in mm
Fig. 5.2 Load vs Deflection Curve for Beam F2
Beam F2 of SET I beams which were weak in flexure but strong in shear. In beam
F2 strengthening is done by application of GFRP sheet only at the soffit of the beam. Two
point static loading was done on the beam and at the each increment of the load, deflection
at the left, right and middle dial gauges were taken. Using this load and deflection of data,
load vs deflection curve is ploted. At the load of 34 KN initial cracks started coming on the
beams. Initial cracks started at a higher load in beam F2 compared to beam F1. Further with
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increase in loading propagation of the cracks took place. The beam F2 failed in flexural
shear. Beam F2 carried a higher ultimate load compared to beam F1.
Load vs Deflection Curve for Beam F3
90
80
70
Load in KN
60
50
Left
40
Middle
30
Right
20
10
0
0
2
4
6
8
10
12
14
Deflection in mm
Fig. 5.3 Load vs Deflection Curve for Beam F3
Beam F3 of SET I beams which were weak in flexure but strong in shear. In beam
F3 strengthening is done by application of GFRP sheet upto the neutral axis along with the
soffit of the beam. Two point static loading was done on the beam and at the each increment
of the load, deflection at the left, right and middle dial gauges were taken. Using this load
and deflection of data, load vs deflection curve is ploted. Initial cracks are not visible on the
beams. Further with increase in loading propagation of the cracks took place but it had poor
visibility of cracks due to the covering of the GFRP sheet. The beam F3 also failed in
flexural shear like beam F2 but beam F3 carried a higher ultimate load compared to both
beam F1 and F2.
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Load vs Deflection Curve for Beams F1, F2 and F3
90
80
70
Load in KN
60
50
Beam F1
40
Beam F2
30
Beam F3
20
10
0
0
2
4
6
8
10
12
14
Deflection in mm
Fig. 5.4 Load vs Deflection Curves for Beams F1, F2 and F3.
From the load and deflection of data of SET I beams F1, F2 and F3, load vs
deflection curve is ploted for all the three beams. From this load vs deflection curve, it is
clear that beam F1 has lower ultimate load carrying capacity compared to beams F2 and F3.
Beam F1 had also undergone higher deflection compared to beams F2 and F3 at the same
load. Beam F2 had higher ultimate load carrying capacity compared to the controlled beam
F1 but lower than beam F3. Beam F3 had higher ultimate load carrying capacity compared
to the beams F1 and F2. Both the beams F2 and F3 had undergone almost same deflection
upto 65 KN load. After 65 KN load beam F3 had undergone same deflection as beam F2 but
at a higher load compared to beam F2. The deflection undergone by beam F3 is highest.
Beam F2 had undergone higher deflection than beam F1.
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Load vs Deflection Curve for Beam S1
80
70
Load in KN
60
50
40
Left
30
Middle
Right
20
10
0
0
1
2
3
4
5
6
Deflection in mm
Fig. 5.5 Load vs Deflection Curve for Beam S1
Beam S1 was the control beam of SET II beams which were weak in shear but strong
in flexure. In beam S1 strengthening was not done. Two point static loading was done on the
beam and at the each increment of the load, deflection at the left, right and middle dial
gauges were taken. Using this load and deflection of data, load vs deflection curve is ploted.
At the load of 35 KN initial cracks started coming on the beams. Further with increase in
loading propagation of the cracks took place. At first in beam S1 only flexural cracks were
developed but ultimately the beam failed in shear.
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Load vs Deflection Curve for Beam S2
90
80
70
Load in KN
60
50
Left
40
Middle
30
Right
20
10
0
0
2
4
6
8
10
12
14
Deflection in mm
Fig. 5.6 Load vs Deflection Curve for Beam S2
Beam S2 of SET II beams which were weak in shear but strong in flexure. In beam
S2 strengthening is done by application of GFRP sheet only on the two sides of the beam.
Two point static loading was done on the beam and at the each increment of the load,
deflection at the left, right and middle dial gauges were taken. Using this load and deflection
of data, load vs deflection curve is ploted. At the load of 39 KN initial cracks started coming
on the beams. Initial cracks started at a higher load in beam S2 compared to beam S1.
Further with increase in loading propagation of the cracks took place. In beam S2 only
flexural cracks were developed and finally the beam failed by flexural failure and crushing
of concrete. Beam S2 carried a ultimate load higher than beam S1 but lower than beam S3.
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Load vs Deflection Curve for Beam S3
100
90
80
70
Load in KN
60
50
Left
40
Middle
30
Right
20
10
0
0
2
4
6
8
10
12
14
16
18
Deflection in mm
Fig. 5.7 Load vs Deflection Curve for Beam S3
Beam S3 of SET II beams which were weak in shear but strong in flexure. In beam
S3 strengthening is done by application of GFRP sheet as U-wrap on the beam. Two point
static loading was done on the beam and at the each increment of the load, deflection at the
left, right and middle dial gauges were taken. Using this load and deflection of data, load vs
deflection curve is ploted. At the load of 39 KN initial cracks started coming on the beams.
Initial cracks started at a higher load in beam S3 compared to beams S1 and S2. Further with
increase in loading propagation of the cracks took place. In beam S3 similar to beam S2 only
flexural cracks were developed and finally the beam failed by flexural failure and crushing
of concrete, but beam S3 carried a higher ultimate load compared to both beam S1 and S2.
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Load vs Deflection Curve for Beams S1, S2 and S3
100
90
80
70
Load in KN
60
50
Beam S1
40
Beam S2
30
Beam S3
20
10
0
0
2
4
6
8
10
12
14
16
18
Deflection in mm
Fig. 5.8 Load vs Deflection Curves for Beams S1, S2 and S3.
From the load and deflection of data of SET II beams S1, S2 and S3, load vs
deflection curve is ploted for all the three beams. From this load vs deflection curve, it is
clear that beam S1 has lower ultimate load carrying capacity compared to beams S2 and S3.
Beam S1 had also undergone higher deflection compared to beams S2 and S3 at the same
load. Beam S2 had higher ultimate load carrying capacity compared to the controlled beam
S1 but lower than beam S3. Beam S3 had higher ultimate load carrying capacity compared
to the beams S1 and S2. Both the beams S2 and S3 had undergone almost same deflection
upto 70 KN load. After 70 KN load beam S3 had undergone same deflection as beam S2 but
at a higher load compared to beam S2. The deflection undergone by beam S3 is highest.
Beam S2 had undergone higher deflection than beam S1.
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5.4 LOAD AT INITIAL CRACK
Two point static loading was done on both SET I and SET II beams and at the each
increment of the load, deflection and crack development were observed. The load at initial
crack of all the beams was observed, recorded and is shown in figure 5.9
.9 and 5.10.
Load at Initial Crack of SET I beams
34
35
30
Load in KN
30
25
20
15
10
5
NOT VISIBLE UP TO 90 KN
40
0
F1
F2
SET I Beams
Set I Beams
Details
F1 : Control Beam
F2 : Strengthen
only at the soffit
of the beam
F3 : Strengthen at
the soffit and upto
the neutral axis of
the beam
F3
Fig. 5.9 Load at initial crack of beams F1, F2 and F3.
Under two point static loading of SET I beams, at each increment of load,
load deflection
and crack development were observed. In beam F1 initiation of the crack takes place at a
load of 30 KN which is lower than beam F2 in which crack initiation started at 34 KN. The
crack initiation of beam F3 was not visible due to application of GFRP sheet up to the
neutral axis of the beam. The cracks were only visible after a load of 90 KN.
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Load at Initial Crack of SET II beams
41
40
40
39
39
Set II Beams Details
S1 : Control Beam
S2 : Strengthen only
at the sides of the
beam in shear zone
S3 : Strengthen by Uwrapping of the beam
in the shear zone
Load in KN
38
37
36
35
35
34
33
32
S1
S2
SET II Beams
S3
Fig. 55.10 Load at initial crack of beams S1, S2 and S3.
Under two point static loading of SET II beams, at each increment of load, deflection
and crack development were observed. In beam S1 initiation of the crack takes place at a
load of 35 KN which is lower than beam F2 in which crack initiation started at 39 KN and
further lower than beam F3 in which crack initiation started at 40 KN. There was not much
difference in load for crack initiation in beam S2 and S3.
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5.5 ULTIMATE LOAD CARRYING CAPACITY
The load carrying capacity of the control beams and the stren
strengthen
gthen beams were
found out and is shown in fig. 5.11 and 5.12. The control beams were loaded up to their
ultimate loads. It was noted that of all the beams, the strengthen beams F2, F3 and S2, S3
had the higher load carrying capacity compared to the contro
controlled
lled beams F1 and S1. An
important character to be noticed about the usage of GFRP sheets is the high ductile
behaviour of the beams. The shear failure being sudden can lead to huge damage to the
structure. But the ductile behaviour obtained by the use of GFRP can give us enough
enoug
warning before the ultimate failure. The use of FRP can delay the initial cracks and further
development of the cracks in the beam.
Ultimate load of Set I Beams
120
112
104
100
Load in KN
80
78
Set I Beams Details
F1 : Control Beam
F2 : Strengthen
only at the soffit of
the beam
F3 : Strengthen at
the soffit and upto
the neutral axis of
the beam
60
40
20
0
F1
F2
F3
Set I Beams
Fig. 5.11 Ultimate load of beams F1, F2 and F3.
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SET I beams F1, F2 and F3 were loaded under two point static loading. As the load
was increased incrementally development of cracks takes place and ultimately the beam
failed. The ultimate load of F1 beam was 78 KN which is lower than F2 beam which carried
an ultimate load of 104 KN and further lower than F3 beam which carri
carried
ed an ultimate load
of 112 KN.
Ultimate load of Set II Beams
140
122
120
108
100
Set II Beams Details
S1 : Control Beam
S2 : Strengthen only
at the sides of the
beam in shear zone
S3 : Strengthen by Uwrapping of the beam
in the shear zone
Load in KN
82
80
60
40
20
0
S1
S2
S3
Set II Beams
Fig. 5.12 Ultimate load of beams S1, S2 and S3.
SET II beams S1, S2 and S3 were loaded under two point static loading. As the load
was increased incrementally development of cracks takes place and ultimately the beam
failed. The ultimate load of S1 beam was 82 KN which is lower than S2 beam which carried
an ultimate load of 108 KN and further lower than S3 beam which carried an ultimate load
of 122 KN.
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5.6 CRACK PATTERN
The crack patterns at collapse for the tested beams of SET I and SET II are shown in
Fig. 5.13 to 5.18. In SET I the controlled beam F1 exhibited widely spaced and lesser
number of cracks compared to strengthened beams F2 and F3. The strengthened beams F2
and F3 have also shown cracks at relatively close spacing. This shows the enhanced concrete
confinement due to the GFRP strengthening. This composite action has resulted in shifting
of failure mode from flexural failure (steel yielding) in case of controlled beam F2 to peeling
of GFRP sheet in case of strengthened beams F2 and F3. The debonding of GFRP sheet has
taken place due to flexural-shear cracks by giving cracking sound. A crack normally initiates
in the vertical direction and as the load increases it moves in inclined direction due to the
combined effect of shear and flexure. If the load is increased further, cracks propagate to top
and the beam splits. This type of failure is called flexure-shear failure.
In SET II beam S1 the shear cracks started at the centre of short shear span. As the
load increased, the crack started to widen and propagated towards the location of loading.
The cracking patterns show that the angle of critical inclined crack with the horizontal axis
is about 45°. For strengthened reinforced concrete beams S2 and S3, the numbers of vertical
cracks were increased compared to controlled beam S1.
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Fig. 5.13 Crack pattern of Beam F1
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Fig. 5.14 Crack pattern of Beam F2
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Fig. 5.15 Crack pattern of Beam F3
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Fig. 5.16 Crack pattern of Beam S1
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Fig. 5.17 Crack pattern of Beam S2
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Fig. 5.18 Crack pattern of Beam S3
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Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
5.7 COMPARISION OF RESULTS
The results of the two set of beams tested are shown in Table 5.1. The failure mode,
load at initial crack and ultimate load of the control beams without strengthening and the
beams strengthen with two layers GFRP sheet are presented. The difficulties inherent to the
understanding of strengthen structural member behavior subjected to flexure and shear have
not allowed to develop a rigorous theoretical design approach. The complexity of the
problem has then made necessary an extensive experimental research. Moment of resistance
of the SET I beams was calculated analytically and was compared with the obtained
experimental results
SET I Beams
from analytical study
from experimental study
F1
17.12 KN-m
26.00 KN-m
F2
24.60 KN-m
34.68 KN-m
Table 5.2 Comparision of value obtained from analytical and experimental study
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CHAPTER -6
CONCLUSIONS
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
CHAPTER 6
CONCLUSIONS
In this experimental investigation the flexural and shear behaviour of reinforced concrete
beams strengthened by GFRP sheets are studied. Two sets of reinforced concrete (RC)
beams, in SET I three beams weak in flexure and in SET II three beams weak in shear were
casted and tested. From the test results and calculated strength values, the following
conclusions are drawn:
A)>SET I Beams (F1, F2 and F3)
1. Initial flexural cracks appear at a higher load by strengthening the beam at soffit. The
ultimate load carrying capacity of the strengthen beam F2 is 33 % more than the
controlled beam F1.
2. Load at initial cracks is further increased by strengthening the beam at the soffit as
well as on the two sides of the beam up to the neutral axis from the soffit. The
ultimate load carrying capacity of the strengthen beam F3 is 43 % more than the
controlled beam F1 and 7 % more than the strengthen beam F2.
3. Analytical analysis is also carried out to find the ultimate moment carrying capacity
and compared with the experimental results. It was found that analytical analysis
predicts lower value than the experimental findings.
4. When the beam is not strengthen, it failed in flexure but after strengthening the beam
in flexure, then flexure-shear failure of the beam takes place which is more
dangerous than the flexural failure of the beam as it does not give much warning
before failure. Therefore it is recommended to check the shear strength of the beam
and carry out shear strengthening along with flexural strengthening if required.
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5. Flexural strengthening up to the neutral axis of the beam increases the ultimate load
carrying capacity, but the cracks developed were not visible up to a higher load. Due
to invisibility of the initial cracks, it gives less warning compared to the beams
strengthen only at the soffit of the beam.
6. By strengthening up to the neutral axis of the beam, increase in the ultimate load
carrying capacity of the beam is not significant and cost involvement is almost three
times compared to the beam strengthen by GFRP sheet at the soffit only.
B) SET II Beams (S1, S2 and S3)
1. The control beam S1 failed in shear as it was made intentionally weak in shear.
2. The initial cracks in the strengthen beams S2 and S3 appears at higher load
compared to the un-strengthen beam S1.
3. After strengthening the shear zone of the beam the initial cracks appears at the
flexural zone of the beam and the crack widens and propagates towards the neutral
axis with increase of the load. The final failure is flexural failure which indicates that
the GFRP sheets increase the shear strength of the beam. The ultimate load carrying
capacity of the strengthen beam S2 is 31 % more than the controlled beam S1.
4. When the beam is strengthen by U-wrapping in the shear zone, the ultimate load
carrying capacity is increased by 48 % compared to the control beam S1 and by 13%
compared the beam S2 strengthen by bonding the GFRP sheets on the vertical sides
alone in the shear zone of the beam.
5. When the beam is strengthen in shear, then only flexural failure takes place which
gives sufficient warning compared to the brittle shear failure which is catastrophic
failure of beams.
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6. The bonding between GFRP sheet and the concrete is intact up to the failure of the
beam which clearly indicates the composite action due to GFRP sheet.
7. Restoring or upgrading the shear strength of beams using GFRP sheet can result in
increased shear strength and stiffness with no visible shear cracks. Restoring the
shear strength of beams using GFRP is a highly effective technique.
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REFERENCES
Strengthening of Reinforced Concrete Beams using Glass Fiber Reinforced Polymer Composites
2009
REFERENCES
1) M. A. Shahawy, M. Arockiasamy, T. Beitelman, R. Sowrirajan “Reinforced concrete
rectangular beams strengthened with CFRP laminates” Composites: Part B 27B
(1996) 225-233
2) Victor N. Kaliakin, Michael J. Chajes and Ted F. Januszka “Analysis of concrete
beams reinforced with externally bonded woven composite fabrics” Composites:
Part B 27B (1996) 235-244
3) Koji Takeda, Yoshiyuki Mitsui, Kiyoshi Murakami, Hiromichi Sakai and
Moriyasu
Nakamura “Flexural
behaviour
of
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2009
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2009
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