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REPAIR AND STRENGTHENINGTKENING OF COLUMNS WITH FIBRE REINFORCED COMPOSITES
GRACE YAU
A Thesis submitted in confonnity with the requirements for the Degree of Master o f Applied Science
in the University of Toronto
O Grace Yau 1998
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ACKNOWLEDGEMENTS
The author would like to express her sincere appreciation to Professor S.A.
Sheikh for his support and guidance over the extended period required to conduct this
work. The tirne, efforts and suggestions of Professor F.J. Vecchio are also much
appreciated.
Special thanks are due to Oguzhan Bayrak? whose contributions to this work is
gratefully acknowledged. The author would also like to thank the technical staff of the
Structures Lab and Machine Shop, especially Renzo Basset, Mehmet Bahadir Citak. Joel
Babbin, Giovanni Buaeo and Peter Heliopoulos, for their conmbutions to the
development and success of the experimentai program.
The project is primarily supporied by a Strategic grants to Professor S.A. Sheikh
fiorn Natural Sciences and Engineering Research Council of Canada. Additional
financial support and technical assistance fiom Hexcel Fyfe of Del Mar California, R.J.
Wastson of E. Amherst New York, Road Savers of Toronto and Petro Canada of Toronto
are gratefully acknowledged.
Finally, the author would like to thank her family and fiends who offered support
and encouragement throughout the project. Above dl, the author thanks God, for the
courage and the strength.
ABSTRACT
Repair, retrofitting and rehabilitation of existing concrete structures have become
a large part of the construction activity in North America By some eshates . the money
spent on retrofitting of existing structures in recent years has exceeded that spent on new
structures. Bridge failures in recent earthquakes (Loma Pneta, 1989: and Northridge,
1994) have attracted the attention of the e n g i n e e ~ g community to the large number of
bridges built before 1970, which had substandard seisrnic design details. As a
consequence, a substantiai research effort has been put into seismic retrofit of bridge
structures.
In the research presented in this thesis, relatively new materials, fibre reinforced
plastics (FRP), have been used to retrofit circular columns. Contïnuous fibers of g l a s
and carbon were used in the circurnferential direction to confine the columns. Resuits
fiom twelve specimens tested under constant axial load and reversed cyclic lateral load
are presented. Each specimen consisted of a 356 mm (14 in.) diameter and 1473 mm (58
in.) long column cast integraily with a 5 10 x 760 x 810 mm (20 x 30 x 32 in.) stub. The
test specimens can be divided into three categories. The fint category consisted of four
columns that were conventionally reuiforced with longitudinal and spiral reinforcement;
the second category contained six columns which were strengthened with carbon or giass
FRP before testing; and the last category included two columns that were damaged to a
certain extent, repaired with FRP under load, and then tested to failure.
The main purpose of this study was to evaluate the effectiveness of FRP
reinforcement in strengthening deficient columns or repairing damaged columns. This
was achieved by comparing the behaviour of FRP-retrofitted columns with that of
conventionally reinforced columns. The main variables were axial load level, spacing of
spirais, and thickness and type of FRP. From the test results, it c m be concluded that
carbon and glass FRP c m be used effectively to sirengthen deficient colurnns such that
their behaviour under simulated earthquake loads matches or exceeds the performance of
colurnns designed according to the seisrnic provisions of the AC1 Code (1995). Use of
FRP si@ ficantiy enhances strength, ductility and energy absorption capacity of columns.
TABLE OF CONTENTS
Page
TABLX OF CONTENTS
LLST OF FIGURES
LIST OF TABLES
NOMENCLATURE
CHAPTER 1 INTRODUCTION
1.1 G E N E W
1.2 PROBLEM
1.3 OBJECTIVE A . SCOPE OF THE PRESENT RESEARCH
1.4 ORGANLZATION
CHAPTER 2 CONCRETE CONFINEMENT
2.1 GENERAL
2.2 MECHANISM OF CONFINEMENT
2.3 EFFECTS OF DIFFERENT VARIABLES ON CONFINEMENT
2.4 STRESS-STRAIN MODELS FOR CONFINED CONCRETE
2.4.1 Sheikh (1978) and Sheikh and Uaimeri (1982) 2.4.2 Mander, Priestley and Park (1 988) 2.4.3 Saatciogiu and Rami (1992)
Table of Contents
CHAPTER 3 ADVANCEID COMPOSITE MATERIAL
3.2 MATERIAL PROPERTES OF ACM
3 -2.1 Fiber Properties 3 -2.2. Matrix Properties 3 -2.3. Composite Properties
3.3 FACTORS AFFECTING THE MATERlAL PROPERTIES
3.3.1 Effkt of Loading Duration 3.3 -2 Environmental Effects 3.3.3 Temperature Effects 3.3.4 Moisture Effects 3 -3.5 Effécts o f Weather 3 -3 -6 Fie Resistance
3 -4 FUTURE OF ACM
CHAPTER 4 LITERATURE REVIEW
4.1 GENERAL
4.2 PREVIOUS RESEARCH ON COLUMNS RETROFITTED BY STEEL REINFORCEMENT
4.2.1 Chai, Priestley and Seible (1 99 1) 4.2.2 Cofkan, Marsh and Brown (1993)
4.3 RESEARCH OF COLUMNS RETROHTTED WITH FRP COMPOSITE
4.3.1 Priestley, Seible and Fyfe (1 992) 4.3 -2. Saadatmanesh, Ehsani and Li (1994) 4.3.3 Saadatmanesh, Ehsani and Jin ( 1 996) 4.3.4 Saadatmanesh, Ehsani and Jin (1 997)
CHAPTER 5 EXPERllMENTAL PROGlRAlM
5.1 GENERAL
Table of Contents
5.2.1 Concrete 5.2.2 Patching Materials 5.2-3 Steel 5 -2.4 Fiber Reinforcd Plastics (FRP)
5.3 TEST SPECIMENS
5.4 CONSTRUCTION OF SPEClMENS
5 -4.1 Reinforcing Cages 5.4.2 Forms 5.4.3 Casting and Curing
5.5 INSTRUMENTATION
5.5.1 Strain Gauges 5.5.2 Linear Variable Differential Transducers (LVDTs)
TEST
5.6.1 Test Setup 5.6.2 S pecimen P reparation 5.6.3 Testing Procedure 5.6.4 Repais of Damaged Columns
CIB[APTER 6 RESULTS AND DISCUSSIONS
6.2 TEST OBSERVATIONS
6.3 ANALYSIS RESULTS
6.3.1 Behaviour of Specimens 6.3 -2 Ductility Parameters
6.4 DISCUSSIONS
6.4.1 Effect of Axial Load 6.4.2 Efféct of Spacing of Spiral Reinforcement 6.4.3 Effect of FRP Wraps on Deficient Columns 6.4.4 Effect of FRP Wraps on Damaged Columns
Table of Contents
6-45 Stub Effect 6.4.6 Equivaient Plastic Hbge Length
CHAPTER 7 CONCLUSIONS AND RECOMMENDA'ITONS
7.1 SUMMARY
7.2 CONCLUSIONS
7.3 RECOMMENDATIONS
LJST OF REFERENCES
APPENDICES
Figure
Confinement from Transverse Reinforcement
Stress-Strain Curve of Confhed Concrete
Proposed Stress-Strain Mode1
Lateral Pressure in Circular Columns
Stress-Suain Curves for Typical Fibres
Effect of Loading Rate on Matrix
Cross Section of Columns
Stress-Strain Curves for Fibers
Layout of Specimens
Test Setup
Strength Development Curve of Concrete
Stress-Strain Curves for Reinforcing Bars
Detaiis of Tende Coupons
Stress-Strain Curves for FRP Composites
Layout of Test Specirnens
Reinforcing Cages of Specimens
Formwork Used for Casting of Specimens
Locations of Strain Gauges on Longitudinal and Spiral Reinforcement
Generai LVDT Arrangement
5.10 Test Setup
Page
5
7
9
11
13
14
25
26
29
29
34
36
37
37
39
42
43
45
47
48
List of Fimues
Figure
5.1 1 Damage Regions of Specimen R-2NT
5.12 Specimens R-1NT and R-ZNT after Patchuig of Concrete
6. la Specimen S-1NT D u ~ g and at the End of Test
6. lb Specimen S-2NT During and at the End of Test
6. lc Specimen S 3 N T During and at the End of Test
6. Id Specimen S4NT DuMg and at the End of Test
6. le Specimen ST-INT at the End of the Test
6. l f Specimen ST-2NT at the End of the Test
6. lg Specimen ST-3NT at the End of the Test
6.1 h S pecimen ST-4NT at the End of the Test
6. l i Specimen ST-SNT at the End of the Test
6. lj Specimen ST-6NT at the End of the Test
6. lk Specirnen R-1NT at the End of the Test
6.11 Specimen R-2NT at the End of the Test
6.2 Idealization of Test Specimens
6.3 Applied Load vs. Displacement Behaviour of Specimen S-INT
6.4 Applied Load vs. Displacement Behaviour of Specimen S-2NT
6.5 Applied Load vs. Displacement Behaviour of Specimen S-3NT
6.6 Applied Load vs. Displacement Behaviour of Specimen S4NT
6.7 Applied Load vs. Displacement Behaviour of Specimen ST-1NT
6.8 Applied Load vs. Displacement Behaviour of Specimen ST-2NT
Page
52
53
56
57
58
59
60
61
62
63
64
65
66
67
70
71
72
Figure
6.9 Applied Load vs. Displacement Behaviour of Specimen ST-3NT
6.10 Applied Load vs. Displacement Behaviour of Specirnen ST4NT
6.1 1 Appiied Load vs. Displacement Behaviour of Specimen ST-SN?'
6.12 Applied Load vs. Displacement Behaviour of Specimen ST-6NT
6.13 Appiied Load vs. Displacement Behaviour of Specirnen R-1NT
6.14 Appiied Load vs. Displacement Behaviour of Specimen R-2NT
6.15 Shear vs. Tip Deflection Behaviour of Specirnen S- INT
6.16 Shear vs. Tip Deflection Behaviour of Specimen S-2NT
6.17 Shear vs. Tip Deflection Behaviour of Specimen S3NT
6.18 Shear vs. Tip Deflection Behaviour of Specirnen S-4NT
6.19 Shear vs. Tip Defldon Behaviour of Specimen ST-1NT
6.20 Shear vs. Tip Deflection Behaviour of Specimen ST-2NT
6.21 Shear vs. Tip Deflection Behaviour of Specimen ST3NT
6.22 Shear vs. Tip Deflection Behaviour of Specimen ST-4NT
6.23 Shear vs. Tip Deflection Behaviour of Specimen ST-SNT
6.24 Shear vs. Tip Deflection Behaviour of Specimen ST-oNT
6.25 Shear vs. Tip Deflection Behaviour of Specimen R-1NT
6.26 Shear vs. Tip Deflection Behaviour of Specimen R-2NT
6.27 Moment vs. Curvature Behaviour of Specimen S-1NT
6.28 Moment vs. Curvature Behaviour of Spechen S-2NT
6.29 Moment vs. Curvature Behaviour of Specimen S-3NT
List of Fimires
Page
77
78
79
80
8 1
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
- -
Figure
6.30 Moment vs. Curvature Behaviour of Specimen S4NT
6.3 1 Moment vs. Curvature Behaviour of Specimen ST-1NT
6.32 Moment vs. Cwature Behaviour of Specimen ST-2NT
6.33 Moment vs. Curvature Behaviour of Specimen ST-3NT
6.34 Moment vs. Cumature Behaviour of Specirnen ST-4NT
6.35 Moment vs. Curvature Behaviour of Specimen ST-SNT
6.36 Moment vs. Cwature Behaviour of Specimen ST-6NT
6.37 Moment vs. Curvature Behaviour of Specimen R-INT
6.3 8 Moment vs. Curvature Behaviour of Specirnen R-2NT
6.3 9 Definitions of Member Ductiiïty Parameters
6.40 Definitions of Section Ductiiity Parameters
6.4 1 Extensively Damaged Regions in S pecimens
6.42 Cantilever Column with Laterai Point Loading
A. 1 Specimen S- INT
A-2 Specimen S-2NT
A3 Specimen S-3NT
A 4 Specimen S 4 N T
A5 SpecimenST-1NT
A.6 Specimen ST-2NT
A7 Specimen ST-3NT
A8 Specimen ST-4NT
List of Figures
Page
98
99
100
101
102
103
104
105
1 O6
107
108
120
121
13 1
132
133
134
135
136
List ofFimues
Figure Page
A9 SpecimenST-SNT 139
A 10 Specimen ST-6NT 140
k l 1 Original S pecimen R- 1NT 141
A 12 Repaired Specirnen R- 1NT 142
A 13 Original Specimen R-2NT 143
A 14 Repaired Specimen R-2NT 144
LIST OF TABLES
Table
3.1
4.1
4.2
4.3
4.4
4.5
4.6
4.7
5.1
5 -2
5.3
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
Typical Mechanical Properties of ACM
Details Of Test Specimens
Details of Specirnens
Details of Specimens for Flexurai Tests
Details of Columns
Detds of Column Spechnens
Material Properties of Columns
Mechanical Properties of GFRP
Mechanical Properties of Reinforcing Steel
Material Properties of FRP Composites
Details of Test Specimens
Ductility Factors of Test Specimens
Cumulative Ductility Ratios of Test Specimens
Damage Indicators of Test Specimens
Member Ductility Parameters of ' S' Series Specimens
Seaion Dudity Parameters of 3' Series Specimens
Effea of FRP Wraps on Member Ductility Parameters
Effect of FRP Wraps on Section Ductility Parameters
Member Ductility Parameters of Repaired Columns
Section Ductility Panuneters of Repaired Columns
Page
15
20
22
23
24
28
31
31
35
38
40
1 O9
110
110
112
112
115
116
117
118
List of TabIes
Table Page
6.10 Maximum Moment of Specimens 119
6.1 1 Equivalent Plastic Hinge Length of Specimens 122
NOMENCLATURE
area of spuais
total cross-sectional area of laterai steel within spacing s
diarneter of core concrete
diarneter of rebar
energy-darnage indicator
modulus elasticity of concrete
area enclosed in i* cycle by the moment-cuwature loop
modulus of elasticity of steel
compressive strength of confined concrete
compressive strength of plain concrete
lateral pressure in circular columns
equivaient laterd pressure
yield strength of steel
ultirnate strength of steed
compressive strength of 6 x 12 in. cylinder
compressive arength of contined concrete
compressive strength of confined concrete
depth of section
strength gain factor
coefficient
equivalent plastic hinge length
Nomenclature
maximum experimental moment of most damaged section
maximum experimental moment at section adjacent to stub
moment calculated accordimg to 1995 AC1 Code
cumulative displacement ductiiity ratio
cumulative curvature ductility ratio
spiral spacing
clear spacing between strap
work-damage indicator
area enclosed in the i" cycle by the shear force-tip defiedon Ioop
strain corresponding to compressive strength of confined concrete
ultimate compressive concrete strain
strain at onset of strain hardening
minimum arain corresponding to compressive strength of confined concrete
maximum strain corresponding to compressive strength of confined concrete
strain corresponding to 85% of compressive strength of confined concrete
yield strain of steel
strain at ultimate stress in steel
yield deflection
lateral deflection corresponding to 80% of P, on descending portion of P d curve
yield curvature
curvature corresponding to 80% to 90% of M.- on descending portion of M+ cuve
Nomenclature
pt ratio of area of longitudinai steel to that of cross section
CL ductility Wor
displacement ductility factor
p+ mature ductifity factor
CHAPTER 1
INTRODUCTION
1.1 GENERAL
Repair, rehabilitation and strengthening of existing structures has become a major
part of construction activity in North Arnerica By some estimates, the money spent on
retrofitting of existing structures in recent years has exceeded that spent on new
structures. There are more than 200,000 bridges in North Amenca representing about
40% of the available inventory, that are deemed deficient and require some form of
rehabilitation or replacement.
Bridge failures in recent earthquakes (Loma Prieta, 1989; and Northridge, 1994)
have attracted the attention of the engineering community to the large nurnber of bridges
built before 1970, which had substandard seismic design details. As a consequence. a
substantial research effort has been put into seismic retrofit of bridge structures.
1.2 PROBLEM
The work of many researchers has indicated that increasing the confiement in the
potential plastic hinge regions of columns will increase their strength and ductility.
Therefore, strengthening techniques typically involve methods for increasing the
confinement in the potential plastic hinge zone. Recently, a new repair method using
FRP wraps has been developed. Although this retrofit scheme has been recently used in
the field, there is very little information available with regard to the seismic behaviour of
structures repaired or strengthened by FRP composites. There is a need for more
experimental investigations to provide designers with the required uiformation.
1.3 OBJECTIVE AND SCOPE OF THE PRESENT RESEARCH
The present research aimed to evaiuate the effectiveness of FRP reinforcement in
strengthening deficient columns or repairing damaged columns. This was achieved by
cornparing the behaviour of FRP-retrofitted columns with that of conventionally
reinforced columns. A total of tweive columns were tested under inelastic cyclic loading
while simultaneously being subjected to a constant axiai load. Each specirnen consisted
of a 356 mm (14 in.) diameter and 1473 mm (58 in.) long column cast integraily with a
510 x 762 x 813 mm (20 x 30 x 32 in.) stub. The testing variables included axiai load
level, spacing of spirals, types and amount of FRP.
1.1 ORGNIZATION
Chapter 2 explains the concept of confinement. Three stress-strain models for
confined concrete are also given. in chapter 3, the material properties of advanced
composite materials (ACM) are discussed in details. Chapter 4 is devoted to literature
review. The experimental program is presented in Chapter 5. Chapter 6 summarizes the
expenmental results. and includes an examination of the effects of different variables on
the behaviour of columns. Finally, conclusions and recommendations for fiiture research
are listed in Chapter 7.
CHAPTER 2
CONCRETE CONFINEMENT
2.1 GENERAL
in seismic design, the behaviour of a reuiforced concrete structural member
subjected to significant deformations in the inelastic range is highly infiuenced by the
behaviour of the confined concrete. Confinement of concrete by sufficient and suitable
arrangement of lateral reinforcement in the f o m of spirals or circular hoops or
rectangular ties results in a significant increase in both the strength and ductility of
compressed concrete. In order to predict the behaviour of concrete rnembers with
confinement throughout their loading range, the knowledge of the complete stress-strain
relationship of confhed concrete is needed. With the introduction of lateral
reinforcement as confinement, the behaviour of concrete changes and is af5ected by a
nurnber of factors that comprise the lateral reinforcement. The longitudinal
reinforcernent in columns M e r complicates the concrete behaviour. As a result. stress-
strain charactenstics of confined concrete are distinctly different fiom those of uniaxially-
stressed concrete. in this chapter, the mechanisms of confinement and factors afTecting
the behaviour of confined concrete are discussed. In addition, three different stress-strain
models for confined concrete including Sheikh and Uzumeri (1982), Mander, Priestley
and Park (1 988)- and Saatciogiu and Rani (1 992) are presented.
Concrete Confinement
2.2. MECHANISMS OF CONFINEMENT
Concrete under uniaxial compression tends to expand laterally and the
longitudinal strains generated by such loading give rise to transverse tensile strains. which
cause vertical cracking and failure in concrete. Lateral pressure that confines the concrete
counteracts the laterai expansion, and results in a significant increase in ductility dong
with the strength.
in practice, concrete is cornrnonly codined by transverse reinforcement in the
fom of spirals or circular hoops or recbnguiar ties. Sheikh (1978) has stated that if the
concrete member is effectively co f i ed , the core and the cover will respond differently
under the application of axial load. At low level of longitudinal strains in concrete. the
lateral expansion of concrete will be small; hence the lateral confinement provided by the
transverse reinforcement will be negligible. As the longitudinal mains increase, the
lateral strains of concrete also increase. The core concrete is restrained fiom expansion
by the transverse reinforcement, resulting in the confinement of core and separation of the
cover h m the core. The cover concrete behaves as unconfined concrete and wiil become
ineffective afier the compressive strength is anained while the core concrete will continue
to cany stress at high strains. After the cover spalls, the load carrying capacity of the
concrete core will depend on the nature of confinement. Therefore, the compressive stress
distributions for the core and cover concrete follow the confined and unconfmed concrete
stress-strain relations, respectively.
Effectiveness of confinement is dependent on the contiguration of the laterai
reinforcement. Confinement pressure exerted by spirals is different fiom that by
rectangular ties. Circular spirals, due to their shape, are in axial hoop tension and provide
Concrete Confinement
a uniform conhning pressure on the concrete core (Park and Pauley 1975; Shiekh 1978).
Therefore, circular hoops or spirais provide an efficient confinement of the concrete core.
However, the confining pressure provided by ties is not unifom and depends on the
restraining force developed Ui the hoop steel. The hoop steel develops hi& restraining
forces at the corners where it is supported by longitudinal reinforcement, and low
restraining action between the comers. This is because as concrete expands lateraily
under axial compression, the lateral concrete pressure tends to bend the sides of the ties
outward due to their low stiffhess and results in higher reactive pressures building up at
the comers than at locations away fiom the corners. See Figure 2.1.
Figure 2.1 Confinement from Transverse Reinforcement (Park and Pauley, 1975)
Since the confining pressure exerted by rectilinear reinforcement is nonuniform, a
portion of the core concrete is not effectively confined. According to the mode1
developed by Sheikh (1978), the separation between the effectively confined concrete and
the unconfined concrete is in the form of a series of arcs spanning between the bars. The
area of the effectively confined concrete core reduces M e r away fkom the ties and is
minimum midway between two tie sets.
Concrete Confinement
2.3 EFFECTS OF DLII'FERENT VARIABLES ON CONFINEMENT
Concrete codmed by lateral reinforcement exhibits a significant increase in both
strength and ductility. According to the experiments conducted by Sheikh and Unimeri
(1980), the behaviour of confined concrete is af5ected by a number of variables which
Uiclude:
1) Amount of lateral reinforcement-The strength and ductility of confined
concre te increase as the lateral steel content increases.
2) Distribution of Longitudinal Reinforcement-Tie Configuration-in case of
rectilinear confuiing steel, as the number of longitudinal bars supported by ties
increase, the effectively confined concrete area increases. For the sarne amount of
longitudinal reinforcement, better distribution of the reinforcement around the
core perimeter and hence the tie configuration results in higher strength and
ductility of concrete. Overlapping hoops and supplernentary cross-ties with hooks
anchored inside the core provide effective configuration for confinement
(Saatcioglu and Razvi 1992; Sheikh and Khoury 1993).
3) Tie Spacing--Tie spacing is another important factor which determines the area
of effectively confined concrete. The strength gain and ductility of the concrete
core decrease as the tie spacing increases, even with the same volumetric ratio of
tie steel. Tie spacing also controls the buckling of longitudinal reinforcement.
4) Characteristics of Lateraf Steel-The stress-strain relationship of the steel
determines the state of the confinùig pressure at any level of the applied load.
The yield strength of the laterd reinforcement defines the upper Iimit of the
confining pressure capability.
Concrete Confinement
2.4. STRESS-STRAIN MODELS FOR COMFLNED CONCRETE
2.4.1 Sheikh (1978) and Sheikh and Uzmueri (1982)
This analytical model for the confînement mechanisms in tied columns is based
on the results of tests on twenty-four short square tied columns. The strength of confined
concrete is caiculated by using the concept of the effectively confïned concrete area
within the concrete core. The model accounts for a number of variables, such as
volumetric ratio of lateral reinforcement, tie spacing, characteristics of steel and
distribution of longitudinal steel around the core perimeter, and the resulting tie
configuration. The proposed stress-scrain curve for confined concrete is shown in Figure
Figure 2.2 StressStnin Curve of Confmed Concrete (Sheikh & Uzmueri 1982)
ï he curve consists of three parts. Part OA is a parabola with point A at fcc, E,! .
Tem f, is the compressive strength of confined concrete and is equal to &f,, in which
f,, = compressive strength of plain concrete; % = strength gain factor. The and es2 are
the minimum and maximum saaui values corresponding to fcc. The is the strain
corresponding to 85% of fcc on the unloading branch of the curve. Parts AB and BC are
Concrete Confinement
straight lines. Beyond point C, the cuve continues in the same straight line until the
stress is about 30% off,,, Le. point D. M e r point D, the stress-strain curve is assurned to
be a horizontal line. Detail formulations of the four parameters f,, E,, , cs2 and Ergs that
defmed the stress-strain relationship of confined concrete may be seen in the references.
The stress-& c w e obtained fiom this model can be applied to members
subjected to either axiai load only or combined bending and axiai load. Experimental
results have shown that the analytical results display good agreement with the
experimentai data According to the model, unsupported longitudinal bars will not be
veiy effective in providing confinement but may help increase ductility somewhat. The
model can also be applied to circular confinement by considering the entire concrete core
as effectively confiied at the level of lateral reinforcement.
2.4.2 Mander, Priestley and Park (1988)
The theoreucal stress-strain model for confined concrete developed by Mander,
Priestley and Park in 1988 is applicable to members with either circular or rectangular
sections. under static or dynamic axial loading, either monotonically or cyclically applied.
The concrete section may contain any kind of confinement with spirais. circular hoops or
rectangular hoops with or without cross ties. The influence of various types of
confinement is taken into consideration by d e f i n g an effective lateral confining
pressure, which is dependent on the area of effectively confied concrete core as
proposed by Sheikh and Unimen (1982).
Con frned
Compress~ve Strom , Ec
Figure 2.3 Proposed StressStrain Mode1 (Mandar, Priestley & Park 1988)
The proposed stress-strain relationships for monotionic loading of confined and
unconfmed concrete are illustrated in Figure 2.3. It is expressed in terms of three control
parameters: the conhed concrete compressive strength f',, the strain at confined
compressive strength and modulus of elasticity of concrete Ec. The ultimate
compressive concrete strain E,, is defined as the strain at which first hoop fracture occun
and is determined by equating available strain energy of the transverse steel and the strain
energy stored in the conhed concrete. At this point, the section is considered to have
reached its ultimate defomation. For dynamic loading, the three control parameters are
modified by dynamic magnification factors. Unloading and reloading c w e s are also
developed for cyclic loading response. Full details of the proposed mode1 are discussed
in the Iiterature.
Concrete Confinement
Thirty-one neady Ml-size reinforced concrete columns, of different cross section.
and containing various arrangements of longitudid and tramverse reinforcement were
loaded concentrically with W rates up to 0.01 67/s to check the accuracy of the model.
The experimentai results reported by Mander shows that the proposed anaiyticai model
gives excellent prediction of the enhanced strength and general shape of the stress-strain
curves for confïned concrete.
2.43 Saatcioglu and Rami (1992)
The analytical model developed by Saatcioglu and Rami is based on equivalent
uniform confinement pressure generated by reinforcement cage. The equivaient uniform
pressure is obtained from average laterai pressure computed from sectional and materiai
properties, and results of expenmental observations. The authors state that the model is
applicable to circular, square, and rectangular sections confined with spirais. rectilinear
hoops. cross ties and combinations of different types of lateral reinforcement and can be
used to predict concrete behaviour under concentric and eccentric loading, and slow and
fast strain rates.
The triaxiai strength of concrete can be expressed in tems of uniaxial strength
and laterai confinement pressure as follow:
f 'cc = f *Co + kif l (2- 1)
where /', and f, are the conhed and unconfined strengths of concrete respectively.
The coefficient ki is obtained fkom regression analysis of test data,
ki = 6.7(fi (2-2)
where fi = uniform confining pressure in MPa.
Concrete Confinement
Since the lateral pressure provided by closely spaced circular spirals and vertical
column reinforcement is uniform around the perimeter of core as shown in Figure 2.4, the
pressure can be computed fiom static as follow:
where A, = area of spirais,& = yield strength of steel and s = spiral spacing.
The confined concrete strength can be established for spirally reinforced circular
columns by applying Eq. 2.2 and 2.3 into Eq. 2.1. For other cross-sections. the confining
pressure, fi is modified to be equivalent confinkg pressure. &. The accuracy of the
formulations was examined through experimentd studies of a large number of columns
with either circular, square or rectangular sections. Further details of the mode1 may be
seen in the literature.
Figure 2.4 Lateral Pressure in Circular Columns (Saatciogiu & Rani, 1992)
CHAPTER 3
ADVANCED COMPOSITE MATERIALS
3.1 GENERAL
Advanced composite materials (ACM) that have been extensively used in
aerospace and milita^^ applications are now king actively considered for use in civil
engineering structures. ACM are composed of synthetic fibres embedded in a resin
matrix. Typical combinations are glass, aramid or carbon fibres in a polymer or epoxy
maaix. Ln this chapter, a brief introduction is given to the properties of ACM. The
various factors which affect the material properties of composites are reviewed. The
future of ACM in civil e n g i n e e ~ g is also discussed.
3.2 MATERIAL PROPERTIES OF ACM
3.2.1 Fibre Properties
Several types of fibres are now available, including different varieties of glass,
aramidKevlar and carbodgraphite. They display a wide range of stnicnual properties,
including strength, stifiess and durability. Fibres have very hi& tensile strength but
show briale behavior. The stress-strain curves for typical fibres are shown in Figure 3.1.
It is the near-perfect crystal alignrnent in the fibres that results in high tensile strength
(Neale and Labossiere, 1991). Fibres provide strength and stiffhess to the composite and
carry the majority of the applied loads. ACM can be made up of short fibres or long and
continuous fibres called filaments embedded in resin matrix. Many civil engineering
Advanced Comwsite Materials
products, such as cable and reinforcing rods, are made of filaments. The orientations of
fibres can significantly infiuence the strength of ACM. Aiso, precise fibre placement can
increase the arnount of fibres in a composite, resulting in an increase in strength.
Among al1 the fibre matenals, glass fibres are the most widely used. This is
because their specific characteristics are relatively weil-known and they c m be produced
at relatively low cost.
STRESS ( MPo 1
( O 1 HlGH MOOULUS CARBON
( b ) BORON
[ C 1 HKiH STRENGTH C2EOON
( d l K E V L M 49
( e l S - GLaSS
( f 1 E - G L A S S
f
z. 3. l 0.2 0.3
STRA IN taA)
Figure 3.1 Stress-Strain Curves for Typical Fibres (Neale & Labossiere, 1991)
3.22 Matrix Propeties
The matrix serves as a bonding agent of the composite. Its main function is to
protect the fibres from environmental attack and damage due to handling. It aiso transfers
applied loads between fibres through shearing stresses. The most common matrix
material is resin, which includes polyrners and epoxies. Resin matrix generally has low
strength, low modulus and poor mechanical charactenstics. Its behaviour is dependent on
the duration of load the rate and fiequency of loading, and the ambient temperature.
When load is maintained over a long period of time, creep will appear. At high rate of
loading, the stress-strain curve appears to be linear, whereas at low rate of loading the
behaviour is nonlinear. See Figure 3.2. At high temperatures, the behaviour is similar to
that at a low rate of loading.
Figure 3.2 Effect of Loading Rate on Matrix (Neale & Labossiere, 1991)
3.2.3 Composite Properties
In the current civilian applications, advanced composite materials are most likely
to be found in the foxm of bars, rods, cables and laminates. A laminate consists of a
series of laminae stacked together. with a prescribed sequence of orientations for the
individuai laminae. A lamina is a layer of unidirectional fibres in a rnatrix material
(Neale and Labossiere, 1991). In addition, the fibres can be assembled in a fabric form
and applied as wraps to structurai components by impregnating them with epoxies. in
general, ACM are anisotropic and are characterized by hi& strength, light weight, non-
corrosive, good fatigue resistance and electromagnetically neutral.
The tensile strength of some fibres under consideration exceeds the tensile
strength of steel by two to three times. However, they do not exhibit yielding, but instead
are linearly elastic up to failure. ACM is aiso characterized by low (glas) to high
Advanced Cornwsite Materials
(carbon) moddus of elasticity in tension but low compressive properties. The tende
strength and moduius of elasticity of composite is smailer than that of the fibre itself.
Typical mechanical properties for giass(GFRP) and carbon(CFRP) are presented in Table
3.1. According to Neale and Labossiere (1 99 1 ), the strength of anisotropic larninae
depends on the fibre orientation. For unidirectional lamuiae, the maximum strength
occurs in the fibre direction. In bi-directional larninae, the maximum strength occurs in
the directions of fibres. For the case of short fibres distributed randomly in a matrix. the
composite performs isotroopically. The unidirectional Iaminae has the greatest strength
while the isotropic one has the least.
ACM is very light in weight. typicdly one-fifth that of steel (Saadatmanesh,
1994), which makes handling and installation much easier and greatly reduces
construction cost. This feature dso makes it very attractive as a rehabilitation material.
Since ACM has a high strength to weight ratio. it allows structural mernbers to bear more
live load and therefore, a more efficient use of capacity. It is a very important
characteristic to structures with long spans, since large proportions of capacity are
required to resist dead loads.
Table 3.1 Typical Mechanical Properties of ACM (Neal & Labossiere, 1991)
Material Density Tensile Modulus Tensile Strength ( k g h (GPa) (MPa)
Unidirectional GFRP/polyester 1600-2000 20-50 400- 1250
laminate
Advanced Composite Matenals
One of the major advantage of ACM over steel is its excellent corrosion
resistance. The potential result is lower maintenance cost and longer service life.
According to Chajes (1994), among the three fibers tested aramid, E-glas and graphite.
graphite fiber is l e s t afZected by environmental conditions and may be used in
applications involving wet/dry and fieezelthaw cycling in the presence of chlorides.
Generally, ACM exhibits good fatigue resistance. M e r many millions of cycles,
carbon fibres maintain 80% of its static strength, aramid fibres 40%, and glas fibres 25%
(Neale and Labossiere, 199 1 ).
3.3 FACTORS AFFECTING MATERIAL PROPERTIES
3.3.1 Effect of Loading Duration
Generally, the stress-strain curve of an ACM can be approximated as linearly
elastic. In most cases, the fibres fracture in a brinle manner. However, many polymers
used as matrices exhibit Iinear behavior at Iow stresses, but behave as visco-elastic
materials at higher stress levels. Therefore, with sustained loading. the stress-strain curve
of ACM will become slightly nonlinear. Expenments has shown that deviation of the
stress-strain curve corresponds to the microdamage in the matrix and debonding between
the fibres and matrix.
As for concrete, long terni deformations due to creep are significant in ACM. The
effects are dependent on the applied stress and strain, ma& type and its stress history.
For uni-directional fibres, the matrix contributes little to the lamina properties and the
effect of creep c m be neglected.
Advanced Comwsite Materials
3.3.2 Environmental Effects
Polymer-based matrices may be affécted by environmental conditions, which can
lead to the loss of strength and failure of the ACM. These effects include photo-
degradation, degradation by X-rays or gamma rays, chemical and biodegradable
degradation, and mechanical degradation through the application of loads to the fibre.
M e n ACM is used outdoors, degradation c m be clearly indicated by change of color.
3 3 3 Temperature Effects
Fluctuations of temperature cause deterioration of material. Since the fibres and
resin have different coefficients of thermal expansion, fluctuations in temperature may
cause a weakening of the material, and possible debonding. At high temperature,
discoloration of the laminate may occur.
3.3.4 Moisture Effects
Absorption of water has a plasticizing effect on the material. It modifies the
mechanical properties of the resin and reduces the elastic modulus of the ciry composites
by up to 2530% (Neale and Labossiere, 199 1 ). It also causes swelling and warping. In
addition, water cm fills any voids in a lamina and cause blisters to appear at fibre-resin
interfaces. As a result, the bond between the constituents is reduced.
3.3.5 Effects of Weather
The constant attack by weather can produce mechanical corrosion like punctures
or cracks. Solar radiation can cause discoloration, and the action of ultra-violet rays will
cause chemical reactions leading to breakage of the molecular chains of the polymer. It
Advanced Com~osite Materiais
has been reported forgiass fibres that weather is responsible for a loss of 12-20% of
flexural strength over 1 5 years (Neaie and Labossiere, 1 99 1 ).
3.3.6 Fire Resistance
The polymer rnatrix is very susceptible to f i e due to its high content of carbon.
hydrogen and nitrogen which are ail flammable materids. Depending on the chemical
composition of the matrix, large arnounts of very dense black toxic smoke rnay be
produced during a fue. However, additives can be used to irnprove the behaviour during
fire.
3.4 FUTURE OF ACM
The fust application of ACM in bridge engineering was a GFRP highway bridge
built in 1982 in Beijing, China (Mufti, Erki and Jaeger. 1991). Today in North America
the most common use of ACM for bridges are prestressing tendons. cable and fibre
reinforced plastic (FRP) sheets for strengthening of concrete girders. In addition, a new
concept of rehabilitation of bridge coiumns with FRP wraps has aiso developed.
Despite d i the advantages of ACM, many designers are still reluctant to
recommend their use. Several obstacles to M e r development of FFW applications to
bridges have been recognized by a number of researchea and are listed as follows:
(a) cost,
(b) the lack of codes and specifications that govem the use of FRP, and
(c) incomplete understanding of material properties and long term behaviour.
Without doubc M e r research will solve these problems, and the applications of
ACM in structures will certainly be increased.
CHAPTER 4
LITERATURE REVIEW
4.1 GENERAL
To the author's knowledge, there has not yet been extensive study of seismic
retrofit of concrete columns with fiber reinforced plastics (FRP). Some of the research in
this area has been reported (e.g. Saadatmanesh, Ehsani and Li 1994), while a Lot of work
is in progress. Some of the available work relevant to the curent study is surnmarized in
the following sections.
4.2 PREVIOUS RESEARCH ON COLUMNS REINFORCEMENT
RETROFITTED BY STEEL
4.2.1 Chai, Priestley and Seibe, 1991
To iinvestigate the performance of columns retrofitted with steel jacketing in
plastic hinge regions, six large-scale circular columns were tested at the University of
California at San Diego. The columns were 6 10 mm (24 in.) in diameter and 3657 mm
(1 2 fi.) in height. They were considered to be 0.4-scaie models of a prototype 1524 mm
(60 in.) diameter column. The columns were constructed with a footing to allow the
foundation interaction to be monitored. The longitudinai steel reinforcement ratio was
2.53% while lateral reinforcement ratio was 0.174%. Transverse reinforcement consisted
of #2 circdar hoops with center to center spacing of 127 mm (5 in.). The hoops were
closed by lap spiices in the concrete cover. A 6.3 mm (0.25 in.) gap was provided
between the column and jacket and was pressurized with watedcement grout. Design
Literature Review
variables between specimens are given in Table 4.1. For colurnn 5, a thin sheet of
styrofoam was added between the column and injected grout to dlow a controlled dilation
of cover concrete at large displacement. Al1 the columns were tested under an axial load
of 1779 kN (400 kips) and revened cyclic loading.
Table 4.1 Detaiis of Test Specimens
Col.
l
1
2
3
4
5
6
1 -R
Column and Footing Detaiis I Remarks
20db lap for longitudinal bars without steel jacket 20db iap for longitudinal bars with steel iacket Continuous column bars with steel iacket
.- - - -
Continuous column bars without steel iacket 20db lap for longitudinal bars, % in.
styrofoam wrap and steel jacket 20db lap for longitudinal bars with steel jacket 20db lap for longitudinal bars, repaired by steel jacket
Weak footing I Re ference II Weak footing I Fullre&Ofit II Strong footing I Reference II Strong footing
Strong footing
Weak footing, ;p"p, 1 1
Full retrofit
Partial retrofit
Strong footing
The experimental program indicated that retrofitting circular bridge columns by
steel jacketing resulted in enhancement of flexural strength and ductility. The followuig
conclusions were drawn fiom the results of the study:
O A lap length of 20 times the longitudinal bar diameter was uisufficient to
develop yield stress of longitudinal bars. The strength of unretrofitted
columns degraded rapidly due to bond failure.
I Full retrofit I
Literanire Review
Footings designed prior tol970 might be susceptible to joint shear failure in
the region right under the column.
The steel jacket enabled a displacement ductility factor of 7 to be achieved.
The columns failed by low-cycle fatigue of longitudinal reinforcement. No
bond failures occurred.
Steel jacketing increased the column stifkess by 10 to 20% due to
additional confinement fiom the jacket.
4.2.2 Coffman, Marsh & Brown, 1993
The seismic performances of four half-scale, circular, reinforced-concrete
columns were investigated. The columns were 3048 mm (10 fi.) high with 456 mm (18
in.) diarneter. The longitudinal reinforcement was spliced to the foundation dowels with
a lap l e n a of 660 mm (26 in.) (35 diarneten of the longitudinal rebar). The dowels were
screwed and weided to a thick column base plate. Transverse reinforcement was
provided with #3 hoops at 305 mm (12 in.) centers, with 356 mm (14 in.) lap splices in
the concrete cover. Three of the columns were retrofitted with prestressed, extemal
circular hoops at intervais dong the lower 12 19 mm (4 fi.), and the fourth was unaltered.
The retrofit hoops were grade 60 bars fonned into semicircular pieces connected by
swaged opposing threaded coupling. The details of the specimens are present in Table
4.2-
Al1 the columns were tested under an axial load of 700 1ù\1 and reversed cyclic
lateral loading until failure. The following results were reported:
Literame Review
a For cyciing at u = 4' the control column sustained ody one cycle before
losing structural integrity. The retrofitted columns sustained a minimum of
twelve cycles.
O The total energy dissipated depended on the sizes and spacing of the hoops.
Column C-4, which had the smallest hoop size combined with the greatest
hoop spacing, produced the highest energy dissipation.
O The retrofit did not change the column stifhess or significantly increase the
strength.
Table 4.2 Details of Specimens
4.3 RESEARCH OF COLUMNS RETROFITTED W T H FRP COMPOSITES
43.1 Priestley, Seible and Fyfe, 1992
Priestley, Seible and Fyfe investigated the behaviour of columns retrofitted using
a combination of active and passive confinement provided by jackets of fiberglasdepoxy
composites. Seven tests were conducted: three on circular columns with lapspliced
longitudinal reinforcement dominated by flexural action, and two tests each of rectangular
and circular columns subj ected to double bending dominated by shear failure.
Literature Review
For the flexud tests, the columns were 610 mm (24 in.) in diarneter and 3660
mm (144 in.) long to the point of load application. The specimens were designed to
approxirnately model typical 1950-70 details, at a scale of 0.4 : 1. They were retrofitted
using active confinement. The details of the three specimens are given in Table 4.3.
Table 4 3 Details of Specimens for Flexunl Tests
For the shear tests, the two circular columns had the same dimension as the
flexural-test columns; the two rectangular columns had 620 x 406 m m (24.4 x 16 in.)
L
cross sections. T'he two circuiar columns were retrofitted using active confinement while
SE)--
C - l
C - 2
C - 3
the two rectangular columns were retrofined with passive confinement only. Al1 four
f~ (MPa)
34.5
34.5
34.5
columns were subjected to double bending.
Grout
It was concluded that the use of fiberglasslepoxy composite jackets inhibited lap
Type
ePoxY grouted ePox3'
grouted cernent grouted
GFRP
splice failure and enhanced the flexural ductility. It also increased the shear strength of
Thickness M m 3.25
3 -25
3 -25
the rectangular columns to the extent that b&ie shear failure modes were converted to
Thickness mm 2.44
1 -22
1-83
Height mm 305
305
305
ductile inelastic f l e x d deformation modes.
Pressure MPa 1.72
0.69
1.38
4.3.2 Saadatmanesh, Ehsani and Li, 1994
Saadatmanesh Ehsani and Li proposeci an analytical model to investigate the
dectiveness of strengthening concrete wlumns with high-strength fibre composite straps.
The variables that were exarnined hcludes concrete compressive strength, thickness and
spacing of straps, and type of strap.
A pacametric analytical study was wnducted on the behaviour of circular and
rectangular columns strengthened with composite straps under monotonie ioading. The
cross sections of the columns are shown in Figure 4.1. The stress-strain curves for both
E-glas and carbon fiber straps are shown in Figure 4.2. The analytical study was the
same for both circular and rectangular colurnns and it was divided into three parts. For
eac h part, the columns were analyzed as unretro fitted, E-giass fib re-wrap ped and carbon
fibre-wrapped. The details of the specimens are presented in Table 4.4, where t =
thickness of swap and s' = clear spacing between straps. The width of the strap was 152
mm (6 in.).
Table 4.4 Details of Columns
Part 1
t=5mm,s '=Omrn
f, = 20.67 MPa 1
f, = 27.56 MPa
f, = 34.45 MPa
- Part 2
f, = 34.45 MPa, t = 5 mm
S' = 0.0 mm
s' = 152.4 mm
s' = 305.0 mm
Part 3
f. = 34.45 MPa, s' = O mm
t = 5mm J
t=10mm
t = 1 5 m m
Literature Review
Figure 4.1 Cross Section of Columns
Literanire Review
l I carbon fiber strap
Figure 4.2 Stress-Strain Curves for Fibers
The stress-strain models for confined concrete, developed by Mander. Priestley
and Park (1988), and based on an equation proposed by Popovics (1973). were adapted in
the analysis of circular and rectangular colums confined with composite straps.
According to the analytical studies, the strength and ductility of concrete columns
increased significantly by wrapping fiber straps around them. nie following conclusions
were reported by the researchers:
The stress-srrain models for concrete confined with composite straps
indicated significant increases in compressive strength and s a at failure
when compared with that of unconfined concrete.
O Carbon fiber had a larger energy-absorbing capacity. Based on an energy
balance approach the increase in uitimate axial load and ductility as a result
of strengthening with carbon fiber is larger that that with E-glas, if the
same volume of straps was used.
Literature Review
The increase in the maximum moment capacity was less than that in the
ultimate axial load and ductility factor.
The gain due to confuiement by FRP, in the uitimate axial load. ductility
and maximum moment capacity decreased with increasing concrete
strength.
The ductility factor increased linearly as the strap thickness increased.
however, the rate of increase in ductility factor decreased as strap spacing
increased,
4.3.3 Saadatmanesh, Ehsani and Jin, 1996
Saadatmanesh, Ehsani and Jin conducted an experirnental program to study the
seismic behavior of circular columns strengthened with E-glas fiber reinforced plastic
(GFRP) composite straps. Five reinforced concrete bridge colurnn footing assemblages
were constnicted with a 0.2-dimensionai scale factor. Only single column bent was
considered in this study. The layout of the specimens is shown in Figure 4.3 and the
details of column specimens are presented in Table 4.5. For specimens C- 1, C-2 and C-3,
the longitudinal reinforcement was extended into the footing using starter bars overlapped
with the main longitudinal bars over a length of 20 bar diameters, Le., 254 mm (10 in.).
For specimens C-4 and C-5, the reinforcement extended into the footing and was
anchored with a standard 90" hook.
The composite straps used for this project were 0.8 mm (0.03 in) thick and had a
tende strength and modulus of elasticity of 532 MPa and 18.6 GPa (77 and 2700 ksi)
respectively. The GFRP was applied in the potential plastic hinge region; i.e., 635 mm
Literanw Review
(25 in.) long pomon of the column above the top face of the footing prior to testing. Both
active and passive retrofit methods were tested in this experiment. For the passive retrofit
scheme, the composite with fiber orientation in the circuderential direction was directly
wrapped ont0 the column. For the active retrofit scheme, the composite straps were
slightly oversized for the column and the resuiting gap was injected with pressurized
epoxy resin. For both retrofit schemes, the columns were wrapped with six layers of
composite stmps. An epoxy was applied to the straps while wrapping for interlaminar
bond.
The specirnens were tested in a steel reaction frame, as shown in Figure 4.4. Fint, the
axial load of 445 kN (100kips) was applied by prestressing the hi&-strength steel rods.
Then, the reversed cyclic laterai load was applied by an MTS H89 kN (+Il0 kips)
hydraulic acniator. Ten electrical inclinometers were distributed over both opposite faces
of the column within the plastic hinge region to measure the plastic hinge rotations. Four
displacement transducers were used to measure the column deflection. Strains in the
column bars and hoops were measured using twelve strain gauges. In addition, for each
retrofit column, twelve strain gages were used to measure the sirains in the composite
smps.
Table 4.5 Detaih of Column Specimens
Lateral Steel Retro fit (MPa) No. of Size* Size Spaeing fy,,
bars (%) (MPa) (mm) (mm) (MPa) . .
301 Passive
Literanire Review
Vertical Load 445 kN
Col. 4 No.4 Square Hoops
No.4 Hoops @ 76mm Centre
14 No.4 Longitudinal Bars
9 Gage Wire Hoops @ 89mm Centre
i 1 1 - 1 N0.6 Bars < 1 7 ; Plus hio.6 Straight
Il a m 4
No.4 Hoops @ 76mm Centre I
N0.6 h Bars ! 1 .07m
Figure 4.3 Layout of Specimens
Bars
\ 2 Elgdmulic Jrlu
Figure 4.4 Test Setup
Literature Review
it was concluded that the strength and displacement ductility of circular column
extemally wrapped with GFRP composite straps improved significantiy as a result of the
confining action of the straps. The straps are highly effective in c ~ ~ n i n g the core
concrete and preventing the longitudinal reinforcement from buckling under cyclic
loading. It was also reported that both active and passive retrofit schemes provided
additional confinement; however, additional studies are necessary to M e r investigate
the benefits of active over passive confinement.
43.4 Saadatmanesh, Ehsani and Jin, 1997
An investigation was conducted to evaluate the flexurai behavior of earthquake-
darnaged reinforced concrete columns repaired with g l a s fiber reinforced plastic (GFRP)
wraps. Four columns were tested in this study. Columns C-l and C-2 were circular
while R-l and R-2 were rectangular. A11 coiumns were 0.2 scale of prototype bridge
columns. The design details of test specimens are shown in Figure 4.3. Columns C-i
and R-1 had starter bars with a lap length of 30 times the bar diameter, while Columns C-
2 and R-2 had continuous reinforcement. The material properties of the specimens are
presented in Table 4.6. The mechanical properties of GFRP wraps were determined
according to ASTM D3039-76 and are given in Table 4.7. Al1 the columns were tested to
a certain damage level under revesed inelastic cyclic loading. They were then repaired
with composite wraps and tested again.
Before repair work began, the damaged columns were pushed back to the original
position, i.e. zero lateral displacement. The repair procedures included chipping out loose
concrete, filling the cavity with quick-settuig concrete and applying an active retrofit
Literafure Review
scheme. The active retrofit scheme consisted of wrapping columns with slightly
oversized straps and filling the gap with pressurized epoxy. The repaired columns were
subjected to the same loading sequence, approximately one week after repair operation
was completed.
Table 4.6 Material Properties of Columns
Table 4.7 Mechanical Properties of GFRP
Column
C- l C-I/R C-2
C-2/R R- 1 R-1/R
R-2 R 3 / R
Fiber Volume 1 ht io
The following concIusions were drawn fiom the test results:
O GFRP composite wraps were effective in restoring the flexural strength and
ductility of earthquake-damaged concrete columns.
f,
MPa
36.5 36.5 36.6 36.6 34.9 34.9 33.4
Longitudinal Steel .
8
5 MPa 358 358 358 358 359
0.8
P ' %
2.48 2.48 2.48 2.48 2.70
33.4 f 359
- Transverse Steel
8 -
GFW Wraps l
il MPII 30 1 30 1
0.8 -
359 359
Layers
- 6 - 6 -
I
fa MPa - 532
Pn YO
0.1704 0.1704
5.45
tllayer mm - 0.8 - 0.8 -
30 1 301 30 1
Spacing mm 88.9 88.9
O. 133 30 I
2.70 5.45
O. 1704 0.1704 0.133 0.133 0.133
114.3
301 ,
301 532
88.9 88.9 114.3 114.3 114.3
- 532 - 532 -
Literature Review
O Originally, columns C-1 and R-1 failed as a resdt of debonding of
longitudinal reinforcement in the lapped region. C-2 failed by buckling of
longitudinal bars and R-2 failed in shear. M e r repair, columns with lapped
starter bars developed stable hysteresis loops up to displacement ductility of
u = *4. For columns with continuous reinforcement, u = 6 was achieved
without any sign of stnichiral degradation.
O The rate of stiffiness degradation in repaired columns under large reversed
cyclic loading was lower than that of corresponding original columns.
However, the initiai stiffhess of repaired columnç was lower than that of
original columns due to pre-existing damage.
4.4 SUMMARY
A review of literature, which deais with the experimental research on seismic
retrofit of columns is presented in this chapter. Special attention is given to research
focused on seismic strengthening of columns with FRP composite.
CHAPTER 5
5.1 GENERAL
An experimental program was conducted to invesfigate the effectiveness of
strengthening deficient columns or retrofitting damaged columns with fiber reinforced
polyrners (FRP). A total of twelve specimens were tested. The test specimens can be
divided into three categones. The first category consisted of four columns that were
reinforced with conventional longitudinal steel and spiral only. Two of these columns
contained the arnount of spiral reinforcement which met AC1 Code (1995) requirements
for seisrnic resistance while the other two contained much less spiral reinforcement. The
second category included six specimens which contained less than the required arnount of
spiral reinforcement for seisrnic design (AC1 1995) and were strengthened with glas or
carbon FRP before testing. The third category consisted of two columns that were
darnaged to a certain extent, repaired under load and then tested to failure. The
specimens consisted of 356 mm (14 in.) diameter and 1473 mm (58 in.) long columns
with 508 x 762 x 813 mm (20x30~32 in.) stubs. Al1 columns were tested under lateral
cyclic loading while shultaneously subjected to a constant axial load. The testing
variables included axial load level, spacing of spirals, thickness and types of FRP.
In this chapter, the properties of materials used, configuration of specimens,
construction phase, instrumentation, test setup, and testing details are presented.
ExDerimental Pro-
5.2 M A T E U S
5 . 1 Concrete
Ready mix concrete was used for dl the specimens. Fde concrete mix contained
Type 10 Portland Cernent and 10 mm (0.4 in.) maximum size aggregate and had 76 mm
(3 in.) slump. The specified 28 days compressive concrete strength was 30 MPa. The
strength development curve of concrete as obtained from 150 x 300 mm long cyiinders is
presented in Figure 5.1. All the cylinders were cured with the specimens. Each plotted
value of the concrete strength is the average of at least three cylinder tests.
Figure 5.1 Strength Development Cuwe of Concrete
5.2.2 Patching Materiais
Two types of patching materials, hi& early strength mortar and EMACO S77-CR
structural repair mortar were used, when needed, for repair of columns. The high early
strength mortar consisted of fine sand and Type 10 Portland Cernent at a mWng ratio of
one to one by weight. The waterkement ratio was 0.15. The compressive strength of the
mortar reached 40 MPa in two days.
The commercially available EMACO S77-CR was very flowable and shrinkage-
cornpensated. It can be mixed with clean water at a ratio of 14 to 18.5% by weight
depending on the workability required. A slow speed ârill(400 to 600 rpm) was used for
mixing small batches and typical mixing Ume was three to five minutes. The
compressive strength reached 26 MPa in one day and 57 MPa in seven days.
5.2.3 Steel
Three different types of reinforcing steel were used to construct the twelve
specimens. The mechanicd properties of the reinforcing steel are given in Table 5.1. and
stress-strain curves are presented in Figure 5.2.
Table 5.1 Mechanical Properties of Reinforcing Steel
Size Y ield Stress, fy V a )
Y ield Strain, E,
Elastic Modulus,
E
Strain @ Ültimnte S train I Stress, f,
Hardening, (MPa)
Ultimate Strain, E,
UmnBar(Am3-500 mm2) - r C r C - - ~ - ~ - " ' ' - - - - - -
10 mm Bar (Area = 1 00 mm2)
Figure 5.2 Stress-Strnin Curves for Reinforcing Bars
5.2.1 Fi ber Reinforced Plastics (FRP)
Three types of FRP composites were used to strengthen deficient columns or
repair darnaged columns. The strength of the composites was detennined from tensile
tests of coupons made From the composite fabncs impregnated with epoxy adhesive
( T W O ~ S ) and cured to harden. The details of the tes- coupon are shown in Figure
5 . The material properties of FRP are given in Table 5.2 and the stress-strain curves are
presented in Figure 5.4. The value of FRP strength is the average of at l e s t three coupon
tests. Since the thickness of composite depends on the amount of epoxy, and the
mechanicd properties do not change appreciably by the amount of epoxy used, the tensile
strength is represented in force per unit width instead of stress.
FRP and Steel Plates FRP Only
- s
L ongi tudinul f i b r e s in Ihis
, direction.
.-il/ Dimensions in mm
Figure 5 3 Details of Tensüe Coupons
0.00 0.0 1 0.02 0 .O3
Stmi. (mdmm)
Figure 5.4 Sbess-strain Curves for FRP Composites
Table 5.2 Material Properties of FRP Composites
5.3 TEST SPECIMENS
A total of twelve specimens were consmicted. A11 the specimens consisted of 356
mm (14 in.) diameter and 1473 mm (58 in.) long colurnns with 508 x 762 x 813 mm
(20x30~32 in.) stubs. See Figure 5.5. Clear concrete cover of 20 mm was provided for
al1 the specimens. The layout of the specimen is shown in Figure 5.5. The column
represented the part of a bridge column or a building column between the section of
maximum moment and the point of contraflexure. The stub represented a discontinuity
such as a bearn column joint or a footing. In al1 specimens, the ratio of the core area
measwd to the center-line of spiral to the gross area of the column section was kept
constant at 74%.
Table 5.3 gives the details of the test specimens. Al1 the columns contained six
25M (500 mm2) longitudinal steel bars, resulting in a longitudinal reinforcement ratio of
3.0 1 %. The test specimens are divided into three categories. The fm category consisted
of columns S-INT, S-ZNT, S-3NT and S-4NT. These columns contained only steel spiral
as lateral reinforcement. Specimens S-1NT and S-2NT contained the amount of spiral
reinforcement which satisfied AC1 Code (1995) provisions for seismic resistance whereas
Specimens S J N T and S-4NT contahed much less spirai reinforcement. These four
FRP
1
1.25 mm GFRP L O O mm CFRP -
Tensile ~kength (Force / Unit Width)
(N/mdiayer) 518 912
Rupture Sîrain
0.0 197 0.0 1 42
columns were tested to failure to establish the standard behaviour against which other
columns could be compared The second category coosisted of six columns which
contained the same amount of spiral reinforcement as Specimens SJNT and S-4NT but
were strengthened with GFRP or CFRP before testing. Specimens ST-1NT to ST-6NT
fall in this category. The third category included Specimens R-INT and R-2NT. These
two columns were damaged to a certain extent, repaired under load with FRP and then
tested to failure.
mm-
Figure 5.5 Layout of Test Specimens
Table 5 3 Details of Test Specimens
Strengthened with 1 layer of 1.25 mm GFRP
Strengthened with 2 layers of 1.25 mm GFRP
Strengthened with 1 layer of 1 .O0 mm CFRP
l
Spec.
.- - -
Strengthened with 1 layer of 0.50 mm CFRP
Strengthened with 1 layer of 1.25 mm GFRP
Strengthened with 1 .O0 mm CFRP Bands
For Specimens ST-INT and ST-2NT. the FRP composite was wrapped within the
potentiai plastic hinge zone of the c o l u x ~ , for approximately 800 mm (3 1.5 in.) length
fiom the stub's face and the failure occurred in the test zone. However, during the testing
of Specirnen ST-3NT, crushing of concrete was observed outside the test region,
therefore, in order to ensure that the failure takes place within the plastic hinge zone, it
was decided to wrap the whole column for the rest of the specimens. For colurnn ST-
Lateral Reinforcement
Category 1
Category III
Treatment
Size
S-1 NT S-2NT S-3NT S4NT
R-INT
R-2NT d
Axial Load P m Spacing
(mm)
Category II
US#3 US#3 US#3 US#3
C L
US#3
US#3
PS
80 80
300 300
160
160
0.56
0.56
0.54 0.27 0.54 0.27
1.12 1.12 0.30 0.30
Control Control Control Control
Tested and repaired with 2 layers of 1.25 mm GFRP
Tested and repaired with 1 layer of 1 .O0 mm CFRP
0.54
0.54
Exwrimental Pro- -
6NT, it was strengthened with four 100 mm (3.9 in.) wide CFRP bands at a clear spacing
of 10 mm (3.9 in.. The first band was wrapped at a distance of 50 mm (2 in.) away fkom
the stub face.
The alphanumenc characters in the names of the specimens have the following
significance. The letien 'S', 'ST'. 'R', respectively, represent the Spiral columns that
served as control, columns STrengthened with FRP and columns Repaired with FRP.
The number in the designation is the sequence number of the test specimens. The letter
'N' shows that the specimens were made of Normal strength concrete. The 1s t letter 'T'.
hdicates the specimen is constructed and tested in the University of Toronto Structures
Testing Laboratory.
5.4 CONSTRUCTION OF THE SPECIMENS
5.4.1 Reinforcing Cages
Each reinforcing cage was cornposed of two parts: the cages for columns and the
cages for stubs. They were assembled separately and comected to each other before
being placed in the form. The reinforcement for the stub contained 1 0M horizontal and
vertical stimps at 64 mm (2.5 in.) spacing. In addition, 10M bars with 135" hooks were
placed at top and bottom sides of the mib at the same spacing. The longitudinal bars in
columns were compietely extended into the stub whereas the spiral reinforcement was
extended into the stub for only 100 mm (3.9 in.), and the ends of the spiral were bent
around the longitudinal bars. The design of the specimens aimed at forcing the failure in
the potential plastic h g e region, Le. within 800 mm (31.5 in.) fiom the face of stub.
Outside the test region, the spacing of spiral relliforcement was reduced to around two-
third of the sprcitird spacing in the test zone. Figure 5.6 shows the reinforcing cages of
al1 specimens. Spacrrs were anached to the cages to provide a constant clear cover
thickness of 30 mm (0.79 in.).
Figure 5.6 Reinforcing Cages of Specimens
5.4.2 Forms
The fomwork for the specirnens consisted of two parts: the base for the stubs and
the sonotubes for the coIumns. The base was constnicted with 19 mm (3/4 in.) plywood
and 51x102 mm (2x4 in.) spruce studs. In order to prevent any significant movement
during casting, steel angles were installed around the base to provide extra lateral support.
Before placing the reinforcing cages inside the base, the inner surfaces of the fomwork
were lightly coated with a thin layer o f oïl to avoid bond betwern the çoncrete and
fomwork.
The sonotubes were dividrd into groups of threc: with the bottom rides screwed
into a woodrn frame. Xfrrr placing the reinforcing cages inside the Form. the sonotubes
were slid down to the base and thcir position was centered by adjusting the framr.
Another wooden h r : w u thrn attached to the top sides of the sonotubes. Afirr that. the
sonotubes were plumbrd to mdcc sure the colurnns were straight and the center of the
colurnns lined up with the center of the stub. The top h e of the sonotubes was then
c o ~ r c t e d to the base using spnice snids placed diagonally.
Figure 5.7 Formwork Used for Casting of Specimens
Each specimen had six anchon screwed to the base and another 6 screwed to a
piece of circular 356 mm (14 in.) diameter plywood which wodd be placed on the top of
the sonotubes at the tune of casting. Furthemore, 10 mm threaded rods used to install
the LVDT mounts, were cast in the specimens using the holes dnlled on the sides of the
sonotubes. The corners of the fonnwork and the holes around the threaded rods were
sealed using silicone to prevent leakage. Figure 5.7 shows the formwork used.
5.4.3 Casting and Curing
Ail twelve columns were cast verticaiiy fiom one batch of concrete. The initial
slump of the ready mix concrete was 50 mm (2 in.); superplasticizer was added to
increase the slurnp to 75 mm (3 in.). The stubs were cast first and then the columns. Al1
the specimens were thoroughly vibrated using rod vibrators. At the same time, thirty-two
152 x 304 mm (6 x 12 in.) cylinders were aiso cast to monitor concrete strength
At the end of casting, the 356 mm (14 in) diameter, 19 mm (3/4 in.) thick
plywood with 6 anchors was placed on top of each sonotubes. Wet burlap and plastic
sheet were used to cover the top surface of the base formwork. Al1 the cylinders were
kept with the specimens until the seventh day when the formwork was removed, and the
cylinders were demolded. The cylinders were air cured with the columns until they were
tested.
5.5 INSTRUMENTATION
5.5.1. Strain Gauges
Ail the specimens had a total of eighteen strain gauges installeci on the
- longitudinal reinforcement. Moreover, the spiral reinforcement within the test region was
insmimented with three straui gauges on each tum. Specùnens S-INT and S-2NT had
nine strain gauges attached to the spirai reinforcement and the rest had six. Figure 5.8
shows the locations of the main gauges.
âam L2 and L5
All dimensions in miliirne4ms.
Figure 5.8 Locations of Strain Gauges on Longitudinal and Spiral Reinforcement
The generai procedure for installing stmin gauges is described as follows. First of
dl, the ribs of the deformed bars were removed using a power grinder. The surface was
then smoothened by special equipment with a coasse sanded belt followed by a fine one.
The surface was further smoothened by conditioner and water paper. When a reasonably
smooth surface was achieved neutralizer was applied to clean the surface. Srrain gauge
adhesive and tape were used to attach the strain gauge to the steel surface. Two layers of
a coating material -M-Coat A- were applied to the face of the suain gauge for
waterproofing. Wire of 4.0 m (157.5 in.) length was soldered to the terminals for
connection to the data acquisition system. Mer soldering was completed several layers
of waterproof coating were applied to the surface of the gauge and the terminals. The
wire was then taped to the rebar, the surfaces of the strain gauge and terminai were
covered with wax and sel f-adhesive aluminum foil.
5.5.2 Linear Variable Differential Transducers (LVDTs)
The concrete core deformations were measured using eighteen LVDTs with ten on
the north side and eight on the south side. See Figure 5.9. The gauge lengths varied from
75 to 120 mm (3 to 4.7 in.) and covered a length of about 5 15 mm (20.3 in.). These
LVDTs were mounted between the threaded rods which were previously placed in the
specimens before concrete casting. Vertical displacements of each specimen were also
measured at six different locations dong its length using LVDTs.
(a) North Face
(b) South Face
Figure 5.9 General LVDT Arrangement
5.6 TESTING
5.6.1 Test Setup
A hydraulic jack with a capacity of 4450 kN (1000 kips) was used to apply the
axial ioad which was measured by a load cell. The cyclic lateral load was applied by an
MTS achüitor having 1000-kN (220-kips) load capacity and 152 mm (6 in.) stroke
capacity. A displacement control mode of loading was used in ail the tests to apply
predetermined displacement history. The testing apparatus was specidly designed to
allow in plane rotation of testing specimens. Figure 5.10 gives the schematic drawing of
the test setup.
In order to instail a specimen in the test frame, a 64 mm (2.5 in.) thick steel plate
was attached to each end of the specimen using the six anchors cast in it. The specimen
was then forklified up and connected to the hinges in the f k n e using high strength bolts.
t 7.52 m
Figure 5.10 Test Setup
5.63 Specimen Preparation
The 'ST' series specimens were strengthened with GFRP or CFRP wrap before
installation according to the following procedure. First of dl , the MO" S Epoxy for
the ?'YFoTM Composite FIBERWRAP~~ System was prepared. This epoxy consisted of
two components, A and B, which were mixed for five minutes with a mixer at a speed of
400-600 RPM. The rnixing ratio was 100 parts of A to 42 parts of B by volume. The
glass or carbon fabnc was saturated in the epoxy prior to being wrapped around the
column. in order to have a better bond between the fabric and concrete, a layer of epoxy
was also applied on the surface of the column. The composite was then wrapped around
the column with fiber orientation in the circumferential direction, with an overlap length
of 102 mm (4 in.). The thickness of epoxy was not connolled but excess amount was
squeezed out.
According to the supplier. approximately 90% of the epoxy strength is gained in
the fint twenty-four hours. The epoxy was allowed to cure for at least three days to gain
full strength before testing. The R series specimens were repaired with FRP under load
utiiizing the sarne method.
5.63 Testing Procedure
Pnor to testing, each specimen was aiigned both vertically and horizontally
following the sarne procedure. In vertical plane, using engineering levels, the position of
the specimen was adjusted until its center-line matched the line of action of axial load. in
the horizontal plane, the center-line of the specimen was aligned to match the line of
action of axial load defined by a string using plumb-bobs. After this initial alignrnent, the
specimen was loaded up to 50% of the specified axial load for testing with a load
increment of 250 kN, and the displacements on four sides of the column, over a gauge
length of 520 mm (20.5 in) fiom the stub face were recorded at each increment. If the
difference between the average and the maximum or minimum displacement was more
than 5%. the specimen was unloaded, adjusted and reloaded until this 5% cnteria was
met.
Afier alignrnent. a special loading mechanism was used to linked the specimen to
the MTS actuator- This arrangement included placing two sets of steel plates on the top
and bottom of the stub, and comecting the upper and lower plates with four 32 mm ( 1.25
in.) diameter high strength all-threaded rods (Figure 5.10).
AI1 the specimens were subjected to inelastic cyclic loading while simultaneously
carrying a constant axial load throughout the test. The lateral load sequence consisted of
one cycle to a displacement of 0.75Al followed by two cycles each to AI, 2Ai, 3hl ... so
on, until it was unable to maintain the applied axial load. Deflection hi was defined as
the lateral deflection corresponding to the maximum lateral load along a line that
represented the initiai stifiess of the specimen. The lateral deflection Al was calculated
using the theoreticai sectional behaviour of the column and integrating curvatures along
the length of the specimen. This loading sequence is sirnilar to the one used earlier by
Sheikh and Khoury (1 993).
Specimen R-INT was subjected to three load cycles, i.e. maximum displacement
of Al, when cracks fonned in both the top and bottom covers. The specimen was m e r
darnaged with two cycles of 1.4 Ai before it was repaired. Specimen R-2NT was loaded
up to the fifth cycle, Le. maximum displacement of 2Ai. Before repairing each specimen,
the laterai displacement was brought back to zero and the axial load was lowered to two-
third of its original level. Both specimens were then repaired and tested to failure by
subjecting them to lateral load excursions starting fiom 0.754 while under the originally
applied axial load.
Al1 the data was collected automaticaily at specified intervals using Hewlett
Packard data acquisition system and stored in a micro-cornputer.
5.6.4 Repair of Damaged Columns
Specimen R-lNT was moderately damaged before it was repaired. Vertical
(perpendicular to the longitudinal axis of the mernber) flexural cracks were observed in
the hinging zone at a distance of approximately 100 to 400 mm (4 to 15.7 in.) fiom the
stub face. Spalling of top concrete cover at a distance of 435 to 685 mm (1 7 to 27 in.)
fiom the stub occurred due to the voids present in the column. Specirnen R-2NT was
damaged more extensively with spalling of both top and bottom covers, and yielding of
longitudinal and spiral reinforcement. The top concrete cover spalled at a distance of 150
to 550 mm (5.9 to 21.7 in.) fiom the stub face while the bottom cover spalled for a
distance of 500 mm (21.7 in.) fiom close to the stub. Figure 5.1 1 shows the damaged
regions of Specimen R-2NT.
The two damaged columns were repaired with patching materials and FRP. Al1
the loose concrete were first chipped out and the surfaces of the columns were cleaned. A
high early strength mortar was used for patching Column R-INT. For Specimen R-2NT,
in order to maintain its shape, a dit open sonotubes was tied around the bottom of the
column, and the structural repair mortar EMACO S77-CR (water = 18.5% by weight of
EbIACO) was poured from the sides until excess rnortar tlowed out. A stiffer mix of
EMACO (water = 14% by weight of EklACO) was used to patch the top of the column.
The matèrial proprrties of both patching materials are described in Section 5.2.3. For
both sprcimens. the mortar was cured for at l e s t two days beîbre FRP was wnpped
around the columns. Specirnen R- I NT was repaired wi th two Iayers of 1 2 5 mm GFRP
whilr Specimen R-INT was repaired with one layer of 1.00 mm CFRP. The rrpaird
columns after patching are shown in Figure 5-12.
Figure 5.1 1 Damage Regions of Specimen R-2NT
52
Ex~erirnentai Prornm
(a) Specimen R-INT
(b) Specimen R-2NT
Figure 5.12 Specimens R-LNT and R-2NT after Patching of Concrete
CHAPTER 6
MSULTS AND DISCUSSIONS
6.1 GENERAL
Observations recorded during the tests are first reported. The resuits of the
experimentd program are then presented by providing the load-defoxmation and moment-
curvature responses of specimens, based on the collected data. In sections following the
presentation of results, the performances of specimens are evaluated using seveml
ductility parameten. The effects of axial load levei, spiral spacing, thickness and types of
FRP wraps and the existence of stub are also discussed.
6.2 TEST OBSERVATIONS
The first signs of distress in al1 test specimens were the cracks in the cover
concrete at the top and the bottom. For the 'S' series specimens, it was at the first peak of
the fourth cycle. Le. A = 2Al, that the cover at the top spalled followed by spalling of the
cover at the bottom at the second peak. For Specimens S-INT and S-2NT. cracks
propagated to the sides of the columns during the fia cycle, i.e. A = 3Ai and spailing of
cover on the sides was observed at later stages. in al1 the 'S' series specimens, vertical
flexural cracks formed first in hinging zone at a distance of approximately 400 to 450 mm
(15.7 to 17.7 in.) from the face of the stub and extended towards the stub. The most
extensive damage concentrated at about 295 to 350 mm (1 1.6 to 13.8 in.) fiom the stub
face. However, spalling of cover extended £iom close to stub for a distance of about 585
to 740 mm (23 to 29 in.). During the last cycle, buckling of longitudinal bars was
Results and Discussions
observed after yielding of spiral reinforcement, which indicated the commencement of
failure. ui Specimens S3NT and S-4NT, the spiral reinforcement did not yield. Fracture
of the spirai reinforcement occurred in Specixnens S-INT and S-2NT and brought about
the temination of the tests.
For the 'ST' series specimens and the two repaired columns, R-INT and R-ZNT.
popping sound of epoxy was heard throughout testing. For most of the specirnens.
separation of fabric in the circumferential direction as indicated by the change of FRP
colour, was observed within the hinging zone during the fourth or fifth cycle when the
concrete crushed. As the applied displacement increased, this separation in the FRP
wraps extended for a distance of 200 to 400 mm (7.9 to 15.7 in.) from close to the stub.
During testing of Specimen ST-3NT, crushg of concrete outside the test region was
observed in the ninth cycle (A = 4A1). The test was stopped immediately by bringing the
specimen to zero displacement and reducing the axial load to half of its original level.
The end of the column was then strengthened with two layers of CFRP. After that, the
test was continued by bnnging the specimen back to its original position and increasing
the axial load to the original level.
in most cases, rupture of FRP fibres at the bottom of the columns occurred during
the Iast loading cycle after the buckling of longitudinal rebars; this was au indication of
the commencement of failure. In the case of Specimen R-2NT, rupture of fiben was
caused by hcture of spiral reinforcement during the 1st cycle.
Specimen ST-INT, failed in an unpredictable manner. The GFRP composite split
dong the extruded LVDT bars at a distance of 390 to 560 mm (15.4 to 22 in.) fiom the
stub face. It is believed that during wrapping of the colunm, the GFRP was weakened by
Results and Discussions
(i) Dunng the Test
(ii) At the End of the Test
Figure 6.1 a Specimen S-1NT
Rcsults and Discussions
(i) During the Test
(ii) At the End of the Test
Figure 6.1 b Specimen S-2NT
Results and Discussions
(i) During the Test
(ii) At the End of the Test
Figure 6 . 1 ~ Specimen S3NT
Rrsults and Discussions
(i) Duriog the Test
(ii) At the End of the Test
Figure 6. l d Specimen S J N T
Resulrs and Discussions
(i) North Side
(ii) South Side
Figure 6.le Specimen ST-1NT at the End of the Test
Results and Discussions
Figure
(i) North Side
(ii)
6.lf Specimen
South Side
ST-2NT at the End of the Test
Results and Discussions
Figure 6.lg Specimen ST3NT at the End of the Test
Figure 6.lh Specimen ST-4NT at the End of the Test
Results and Discussions
Figure 6.li Specimen ST-SNT at the End of the Test
Figure 6.lj Specimen ST-6NT at the End of the Test
ResuIts and Discussions
i) North Side
ii) South Side
Figure 6.1 k Repaired Specimen R-1NT at the End of the Test
Results and Discussions
Figure 6.11 Repaired Specimen R-2NT at the End of the Test
Resulîs and Discussions
the exmided LVDT bars, which in turn caused prematine rupture of the composite. To
avoid this type of failure, one additional FRP strip of 75 mm (3 in.) width was installed
dong the extruded LVDT bars on all other specimens. For Specimen ST-6NT, failure
was initiated by delamination of the CFRP bands. During the eighth cycle (A = 4Ai), the
first CFRP band adjacent to the stub debonded followed by the second one in the next
cycle which brought about the termination of the test. The most extensive damage for al1
the columns with FRP wraps concentrated at about 250 to 300 mm (9.9 to 1 1.8 in.) from
the stub face. which is the location of the fint fibre rupture. Failure mode for al1 testing
specimens was dominated by flexural effects. No cracking was seen in the stub in any
specimen. Figure 6.1 shows the specimens during and at the end of the tests.
6 3 ANALYSIS RESULTS
63.1 Behaviour of Specimens
Responses of each specimen are presented graphically in the fom of applied
lateral load-displacement at column-stub comection, shear force-tip deflection and
moment-curvature relationships. Each specimen represented a portion of a bridge column
or a building colurnn between the section of maximum moment and the point of
contrflexure. Figure 6.2 shows the idealization of test specimens. The tip deflection of
the column, h is calculated using the foxmula
where
Results and Discussions
where a = 1 04 1 mm (4 1 in.), b = 2007 mm (9 in.), c = 368 mm (1 4.5 in.) and d is the
deflection at the column-stub interface computed using displacements measured by
vertical LVDTs located dong the specimens. The shear force in the column is given as
where PL is the applied lateral load. Knowing the vaiues of d and V, the moment at the
colurnn-stub comection can be obtained using the equation
where P is the applied axial load. The moment M inchdes two components: the primary
moment caused by the lateral load and the secondary moment caused by the axial load.
In al1 the specimens, failure did not occur at the column-stub comection, although
the interface was subjected to the maximum moment. Due to the additionai confinement
provided by the stub to the adjacent concrete, the failure shifted away From the interface.
The deflection at failed section was cornputed from the deflected shape of the colurnn.
and was used to calculate the secondary moment at that section. The curvature was
computed using the deformation readings measured by upper and Iower LVDTs located at
the most damaged zone within the hinging zone. The graphs of applied load-
displacernent at column-stub interface and the V-A relationships are presented in Figures
6.3 to 6.26. Figures 6.27 through 6.38 show the M-a reiationships for the failed sections
of al1 the specimens. Load-deflection curves plotted during the tests are given in
Appendk A.
Results and Discussions
Figure 6.2 Ideaiizsttion of Test Specimens (Sheikh & Khoury 1993)
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yieldingofspiral
Buckling of longitudinal rebars + Fracture of spiral
#3 Spiral @ 80 mm pitch p, = 1.12% P = 0.54 Po
- 1 O0 -75 -50 -25 O 25 50 75 1 O0
Displacement @ Col,-Stub Interface, 6 (mm)
Figure 6.3 Applied Load vs. Displacernent Behaviour of Sprcirnen S-INT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars
#3 Spiral @ 80 mm pitch p, = 1.12 % P = 0.27 Po
- 1 O0 -75 -50 -25 O 25 50 75 1 O0
Displacement @ Col.-Stub Interface, d (mm)
Figure 6.4 Applied Load vs. Displacement Behaviour of Specimen S-2NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.54 Po
- 100 -75 -50 -2 5 O 25 50 75 1 O0
Displacement @ Col.-Stub Interface, & (mm)
Figure 6.5 Applied Load vs. Displacernent Behaviour of Specimen S-3NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral 4 Buckling of longitudinal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.27 Po
-100 -75 -50 -2 5 O 25 50 75 100
1)isplacement @ Col.-Stub Interface, 6 (mm)
Figure 6.6 Applied Load vs. Displacement Bchaviour of Specimen S-4NT
A Yielding of spiral Buckling of longitudinal rebars Rupture of fibre
#3 Spiral @ 300 mm p itch + 1 layer of 1.25 mm GFRP wrap p, = 0.30 % P = 0.54 Po
-100 -75 -50 -25 O 25 50 75 1 O0
Displacement @ Col.-Stub Interface, 6 (mm)
Figure 6.7 Applied Load vs. Displacement Behaviour of Specimen STlNT
Figure 6.8 Applied Load vs. Displacement Behaviour of Specimen ST-2N1'
A Yielding of spiral ~uckling of longitudinal rebars Rupture of fibre
#3 Spiral / Q 300 mm pitch + 1 layer of 1 mm CFRP wrap p, = 0.30 % P = 0.54 Po
- 1 O0 -75 -50 -25 O 25 50 75 1 O0
Displacement @ CoLStub Interface, d (mm)
Figure 6.9 Applied Load vs. Dis placement Behaviour of Specimen ST-3NT
Results and Discussions
A Yielding of spiral Buckling of longitudinal rebars Ruptureoffibre
#3 Spiral @ 300 mm pitch + 1 layer of 1.25 mm GFRP wrap p, = 0.30 % P = 0.27 Po
-100 -75 -50 -2 5 O 25 50 75 1 O0
Displacement @ Col.-Stub Interface, d (mm)
Figure 6.1 1 Applied Load vs. Displacement Behaviour of Specimen ST-SNT
Results and Discussions
-- - -- - -- - .- .- - - . - - - - . - - -
A Yielding of spiral Buckling of longitudinal rebars
#3 Spiral @ 160 mm pitch + 2 layers of 1.25 mm GFRP wraF p, = 0.56 Oh P = 0.54 Po
-100 -75 -50 -25 O 25 50 75 1 O0
Displacement @ Col.-Stub Interface, 6 (mm)
Figure 6.13 Applied Load vs. Displacement Behaviour of Repaired Spccimen H-1 NT
A Yielding of spiral Buckling of longitudinal rebars
#3 Spiral @ 160 mm pitch + 1 layer of 1 mm CFRP wrap p, = 0.56 % P = 0.54 Po
-100 -75 -50 -25 O 25 50 75 1 O0
Dis placement @ Col.-Stub Interface, 6 (mm)
Figure 6.14 Applied Load vs. Displacement Behaviour of Repaired Specimen R-2NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars + Fracture of spiral
#3 Spiral @ 80 mm pitch p, = 1.12% P = 0.54 Po
-200 -1 50 -100 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.15 Shear vs Tip Deflection Behaviour oCSpecimen S-INT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars
#3 Spiral @ 80 mm pitch p, = 1.12 % P = 0.27 Po
-200 -150 -1 O0 -50 O 50 100 150 200
Tip Deflection, A (mm)
Figure 6.16 Shear vs. Tip Deflection Behaviour of Specimen S-2NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral 4 Buckling of longitudinal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.54 Po
-200 -1 50 -100 -50 O 50 100 150 200
Tip Deflection, A (mm)
Figure 6.1 7 Shear vs. Tip Deflection Behaviour of Specimen S-3NT
. --- -- . - -
Spalling of top concrete cover Spalling of bottom concrete cover Yielding of spiral Buckling of longitud inal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.27 Po
-200 - 1 50 - 1 O0 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.18 Shear va. Tip Deflection Bchaviour of Specimen S-4NT
- .
Yielding of spiral Buckling of longitudinal rebars Rupture of fibre
#3 Spiral @ 300 mm pitch + 2 layers of 1.25 mm GFRP wrar p, = 0.30 % P = 0.54 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.20 Shear vs. Tip Deflection Behaviour of Specimen ST-2NT
Rtsults and Discussions
Resuits and Discussions
. ----- - - - - - - _ - - - - - _ .
A Yielding of spiral Buckling of longitudina Ruptureoffibre
I rebars
#3 Spiral @ 300 mm pitch + 1 layer of 1.25 mm GFRP wrap p, = 0.30 % P = 0.27 Po
-200 - 150 - 1 O0 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.23 Shear vs. Tip Deflection Behaviour of Specimen ST-SNT
+..--.-- - - - . - . . - - - - - . . . - . . - - . . -.- - - -.------- -
A Yielding of spiral Buckling of longitudinal rebars Delamination of CFRP band
'1
@ 300 mm pitch + 1 layer of 1 mm CFRP band p, = 0.30 % P = 0.27 Po
f l 1
Tip Deflection, A (mm)
Figure 6.24 Shear vs. Tip Deflection Behaviour of Specimen ST-6NT
A Yielding of spiral Buckling of longitudinal rebars Rupture of fibre
#3 Spiral @ 160 mm pitch + 2 layers of 1.25 mm GFRP wrar p, = 0.56 % P = 0.54 Po
-200 -150 - 1 O0 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.25 Shear vs. Tip Deflection Behaviour of Repaired Specimen H-I NT
Yielding of spiral Buckling of longitudinal rebars Rupture of fibre
#3 Spiral @ 160 mm pitch + 1 layer of 1 mm CFRP wrap p, = 0.56 % P = 0.54 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Tip Deflection, A (mm)
Figure 6.26 Shear vs. Tip Defiection Behaviour of Repaired Specimen R-2NT
+ Spallingoftopconcretecover * Spatling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars + Fracture of spiral
#3 Spiral @ 80 mm pitch p, = 1.12% P = 0.54 Po
-200 - 1 50 -100 -50 O 50 1 O0 150 200
Curvature, 0 (10' rad)
Figure 6.27 Moment vs Curvature Behaviour of Specimen S-I NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars + Fracture of spiral
1 @ 80 mm pitch
-200 - 1 50 - 1 O0 -50 O 50 1 O0 150 200
Curvature, 8 (106 rad)
Figure 6.28 Moment vs. Curvature Behaviour of Specimen S-2NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.54 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Curvature, g (106 rad)
Figure 6.29 Moment vs. Curvature Behaviour of Spccimen S-3NT
+ Spalling of top concrete cover * Spalling of bottom concrete cover A Yielding of spiral
Buckling of longitudinal rebars
#3 Spiral @ 300 mm pitch p, = 0.30 % P = 0.27 Po
-200 - 1 50 - 1 O0 -50 O 50 1 O0 150 200
Curvature, (106 rad)
Figure 6.30 Moment vs. Curvature Behaviour of Specirnen S-4NT
400
300
200
ê ioo is,
O J'
H ~uckling of longitudinal rebars
#3 Spiral @ 300 mm pitch + 2 layers of 1.25 mm GFRP wrap p, = 0.30 % P = 0.54 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Curvature, @ (106 rad)
Figure 6.32 Moment vs. Curvature Behaviour of Specimen ST-2NT
A Yielding of spiral Buckling of longitudinal rebars
0 Rupture of fibre
/ #3 Spiral @ 300 mm pitch + 1 layer of 1 mm CFRP wrap p, = 0.30 % P = 0.54 Po
-200 - 1 50 - 1 O0 -50 O 50 1 O0 150 200
Curvature, B (106 rad)
Figure 6.33 Moment vs. Curvature Behaviour of Specimen ST-3NT
A Yielding of spiral Buckling of longitudinal rebars Ruptureoffibre
#3 Spiral @ 300 mm pitch + 1 layer of 0.5 mm CFRP wrap p, = 0.30 % P = 0.27 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Curvature, 8 (106 rad)
Figure 6.34 Moment vs. Curvature Behaviour of Specimen ST-4NT
A Yielding of spiral Buckling of longitudinal rebars Rupture of fibre
#3 Spiral @ 300 mm pitch + 1 layer of 1.25 mm GFRP wrap p, = 0.30 % P = 0.27 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Curvature, 6 (106 rad)
Figure 6.35 Moment vs. Curvature Behaviour of Specimen ST-SNT
A Yielding of spiral Buckling of longitudinal rebars Delamination of CFRP band
#3 Spiral @ 300 mm pitch + 1 layer of 1 mm CFRP band p, = 0.30 % P = 0.27 Po
-200 - 1 50 -100 -50 O 50 1 O0 150 200
Curvature, 6 (la6 rad)
Figure 6.36 Moment vs. Curvature Behaviour of Specimen ST-6NT
A Yielding of spiral I Buckling of longitudinal rebars
Rupture of fibre
#3 Spiral @ 160 mm pitch + 2 layers of 1.25 mm GFRP wrap p, = 0.56 % P = 0.54 Po
- 150 -100 -50 O 50 1 O0 150 200
Curvature, rad)
Figure 6.37 Momemt vs. Curvaturo Behaviour of Repaired Specimen R-1 NT
A Yielding of spiral Buckling of longitudinal rebars
0 Rupture of fibre
#3 Spiral @ 160 mm pitch + 1 layer of 1 mm CFRP wrap p. = 0.56 % P = 0.54 Po
-200 -150 -100 -50 O 50 1 O0 150 200
Curvature, (106 rad)
Figure 6.38 Momemt vs. Curvature Behaviour of Repaired Specimcn R-2NT
Results and Discussions
63.2 Ductiiity Parameters
Generally, the behaviour of a reinforced concrete member is not elasto-plastic,
and therefore there is no universal definition for ductility. in evaluating the performance
of test specimens and studying the effects of different variables, ductility and toughness
are defined using various parameters, which are assumed to give a reasonable b a i s for
consistent evaiuation of section and member behaviour.
Figure 639 Defmitions of Member Ductiiity Parameters (Sheikh & Khoury, 1993)
Results and Discussions
ai- B&
Figure 6.40 Defmitions of Section Ductiiity Parameters (Sheïkh & Khoury, 1993)
Figure 6.39 defines the member ductility parameters, which include displacement
ductility factor p ~ , cumulative displacement ductility ratio N , and work-damage indicator
W. The section ductility pararneters which include curvature ductili ty factor p,
cumulative curvature ductility ratio N,, and energy-damage indicator E, are described in
Figure 6.40. Subscripts t and 80 added to N, N,.W and E indicate the value of each
parameter until the end of the test or up to the end of a cycle in which the lateral load
carrying capacity or moment capacity drops by 20% of the maximum value as an average
of both directions, respectively. Al1 terms are defined in Figures 6.39 and 6.40 except Lf
and t, which represent the length of the most damaged zone measured fiom the test and
depth of column section, respectively. The ductility factors and cumulative ductility
ratios describe the extent to which the section or member can deform in the inelastic
Results and Discussions
range, while the damage indicators represent the energy absorption and dissipation
capacity of the member or the section.
Table 6.1 gives the ductility factors for al1 the specimens. The curvature ductility
factors p, are calculated for both 10Y0
cumulative ductility ratios and damage
and 20Y0 drops in
indictors of ail test
the moment capacity. The
specimens are presented in
Tables 6.2 and 6.3 respectively. By means of these ductiiity
different variables on the inelastic performance and energy
specimens are studied.
parameters. the effects of
dissipation ability of test
Table 6.1 Ductility Factors of Test Specimens
Displacement ~ u c t i l i t ~ Curvature Ductility Factors Factor
Specimen PA @ 0.8 Pm= F, @ 0.8 Mm. P* @ 00.9 Mm,
* Repaired specimens + Values calculated using A, and @, of repaired specimens a Values calculated using A, and @, of original specimens
Results and Discussions
Table 6.2 Cumulative Duetüity Ratios of Test Specimens
Cumulative Dispiacement Cumulative Curvature Ductility Specimeo Ductiüty Ratio Ratio
Table 6 3 Damage Indicators of Test Specimens
Energy Damage Indicators Work Damage Indicators
* Repaired specimens + Values calculated using Al and el of repaired spechens + Values calculated using AI and $ 1 of original specimens
Results and Discussions
6.4 DISCUSSIONS
6.4.1 Effect of Axial Load
The effect of axial load is evaluated by comparing the responses of Specimens S-
1 NT and S-2NT. Specimen S- INT was tested under an axial load of 0.54P0 while in S-
2NT, the axial load was 0.27Po. Both specimens were identical in ail other aspects. The
member ductility parameters and section ductility parameters of these two specimens are
presented in Tables 6.4 and 6.5 respectively. It is evident that an increase in axial load
results in reduced ductility and energy dissipation of the column section. Section ductility
appears to be more sensitive to the level of axial load than member ductility. The most
effected parameters are the energy-damage indicators. By comparing the values of Et, it
can be seen that the energy dissipated by S-2NT is about seven times the energy
dissipated in S- I NT.
The only variable differing between Specimens ST-1NT and ST-SNT was also
axial load level. Since Specimen ST-INT failed prematurely as discussed in Section 6.2,
a direct cornparison of the two specimens can not be made.
Table 6.4 Member Ductility Parameters of 'S' Series Specimens
Results and Discussions
Table 6.5 Section Ductiüty Parameters of 'S9 Series Specimens
6.4.2 Effect of the Spacing of Spiral Reinforcement
The effect of the spacing of spiral reinforcement is examined by comparing the
behaviour of Specimen S-1NT with that of S3NT and the behaviour of S-2NT with that
of S-4NT. Specimens in each pair were similar in ail respects except for the spacing of
spiral reinforcement. It is obvious that a decrease in the spiral spacing significantly
improves the behaviour of the specimen. The responses of Specimens S- INT and SdNT
are a lot more ductile and stable than those of S-3NT and S4NT as illustrated in Table
6.4. The section ductility parameters of al1 four specimens are available in Table 6.5.
The cumulative ductility ratios and the energy damage indicators of Specimens S-INT
and S-2NT are significantly greater than those of S3NT and S-4NT.
6.43 Effect of FRP Wraps on Deficient Columns
The effectiveness of strengthening deficient columns with FRP is evaluated by
considering two sets of specimens with the first set tested under an axial load of 0.54Po
Results and Discussions
while the axial load for the second set is 0.27Po. The fïrst set includes Specimens S-INT.
S-3NT, ST-INT, ST-2NT and ST-3NT. Specimen SdNT was similar to Specimens ST-
INT, ST-2NT and ST-3NT in dl respects except the lack of FRP. The member and
ductility parametee given in Tables 6.6 and 6.7 respectively, and the P-6 (Figures 6.5 &
6.7), V-A (Figures 6.17 & 6.19) and M-) (Figures 6.29 & 6.3 1) relationships indicate that
both Specimens S-3NT and ST- INT behaved in a very brinle manner and the energy
dissipation capacity is poor. As mentioned earlier , failure of Spechen ST-1NT was
caused by premature rupture of the GFRP composite dong the extmded LVDT bars. The
cornparisons of the behaviour of Specimens S3NT with ST-2NT and ST-3NT show the
beneficial effects of FRP wrapping on strength and ductility of colurnns. Al1 the section
and member ductility parameters of Specimens ST-2NT and ST-3NT are remarkably
greater than those of S-3NT. The adverse effect of large spiral spacing is compensated by
the additional confinement provided by the FRP wraps. It shouid be noted that
Specimens ST-2NT and ST-3NT had no strength degradation, the laterai load carrying
capacity (Figures 6.20 & 6.21) and section moment capacity (Figures 6.32 & 6.33) kept
increasing until failure. Behaviour of the two Specimens was even better than that of
Specimen S- INT in which the spiral reinforcement satisfied the seismic code provisions
of the AC1 Code (1995). The energy dissipated in Specimens ST-2NT and ST3NT is 2.6
to 2.9 times the energy dissipated in Specimen S-1NT as measured by the energy damage
indicator, Et. A cornparison of Specimens ST-2NT and ST-3NT shows that two layers of
GFRP results in similar improvement of column behaviour compared with that obtained
using one layer of CFRP.
Results and Discussions
The second set of columns which were tested under an axial load of 0.27P0,
includes Specirnens S-2NT, S-4NT, ST-4NT, ST-SNT and ST-6NT. Specimen S-4M'
was identical to Specimens ST-QNT, ST-SNT and ST-6NT in al1 respects except the lack
of FRP. Similar to the first set, specimens strengthened with FRP have much greater
member and section ductility parameters than that of S-4NT as shown in Tables 6.6 and
6.7 respectively. The seismic resistance of retrofitted columns improves significantly as a
resdt of the confinhg action of the FRP composite wraps. The overall responses of
Specimens ST-4NT (Figures 6.1 O, 6.22 & 6.34) and ST-SNT (Figures 6.1 1, 6.23 Br 6.35)
are similar to or better than that of Specimen S-2NT Figures 6.4, 6.16 & 6.28) in which
the spiral reinforcement was designed according to the seismic code provisions of the
AC1 Code (1995). It should be noted that Specimens ST4NT and ST-SNT do not show a
significant descending part in their responses. The cumulative curvature ductility ratio,
N, of these two specimens is approxirnately 46% to 56 % larger than that of Specimen S-
?NT and their energy dissipation capacity is 2.0 times that of S-2NT, as measured by the
energy-damage indicator Et. Specimens ST4NT and ST-5NT have reasonably sirnilar
member and section ductility parameters which indicates that a column retrofitted with
one layer of 0.50 mm CFRP performs as well as that with one layer of 1 -25rnx-n GFRP.
The behaviour of Specimen ST-6NT (Figures 6.12, 6.24 & 6.36) is more ductile
and stable than that of Specimen S-4NT but not as good as S-2NT. As mentioned before,
failure of Specimen ST-6NT was induced by delamination of the first two CFRP bands
adjacent to the column-stub interface. As the first CFRP band debonded, the column
started to deteriorate due to the loss of confinement. When delamination of the second
band occurred, the column was unable to maintain the axial load and failed rapidly.
Results and Discussions
From the cornparison of Specimens ST3NT with ST4NT, the amount of
confinement required to produce comparable ductile behaviour depends on the level of
axial load. Specimen ST-4NT, with 0.5mm thick CFRP wrap and axial load of 0.27Po.
displayed more ductile behaviour than Specimen ST-3NT in which CFRP wrap was Imrn
thick and the axial was 0.54Po. A similar conclusion c m be drawn by comparing
Specimens ST-2NT and ST-SNT.
Table 6.6 Effect of FRP Wraps on Member Ductiiity Parameters
Lateral Steel -
Axial Load
P - Po -
0.54
0.54
Treatmtnt
Control
1 Iayer 1.25mm GFRP
2 layers 1 25mm GFRP 1 layers 1 .oom CFRP I
Control
Control
1 layer 125mm GFRP 1 layer
0.5Omm CFRP Bands of 1 .OOmm CFRP
Factor
Results and Discussions
Table 6.7 Effect of FRP Wraps on Section Ductility Parameters
6.4.4. Effect of FRP Wraps on Damaged Columns
The original Specimens R-INT and R-2NT were identical in al1 respects and were
tested under an axial load of 0.54P0. They were damaged to a certain extent, repaired
with FRP under load and then tested to failure. Specimen R-lNT was repaired with two
layen of GFRP while Specimen R-2NT was wrapped with one layer of CFRP. The
repaired specimens were tested under the same hi& axial load level until failure. The
member and section ductility parameters of the two repaired columns are listed in Tables
Results and Discussions
6.8 and 6.9 respectively. The behaviour of repaired Specimen R-1NT exceeds the
performance of Specimen S-1NT and is similar to that of Specimens ST-2NT and ST-
3NT. The response of Specimen R-lNT is also more ductile than that of Specimen R-
2NT. This appean to be due to the fact that Specimen R-2NT was more extensively
damaged than R-INT as mentioned in section 5.6.4. The values of member and section
ductility parameters of Specimen R-2NT are even lower than that of Specimen S-I NT.
This is because the column was sofiened due to preexisting darnage including cracks in
concrete cover and yielding of spiral and longitudinal reidorcement. Using the values of
A and @ I of the original columns, the ductility parameters of Specirnen R-2NT are
greater than those of Specimen S-INT, while the ductility parameten of Specimen R-
INT exceeds that of Specimens ST-2NT and ST-3NT. it should be noted the laterai load
carrying capacity (Figures 6.25 & 6.26) and section moment capacity (Figures 6.37 &
6.3 8) of both repaired columns kept increasing with every ioad cycle until failure.
Table 6.8 Member Ductility Parameters of Repaired Columns
Resuits and Discussions
Table 6.9 Section Ductüity Parameters of Repaired Columns
* Repaired specimens *
Values calculated using At and t$, of repaired specimens * Values calculated using AI and of original specimens
6.4.5 Stub Effect
It is obvious that the maximum moment in the coiumn occurs at the column-stub
comection. However, in dl specimens, the failure initiated at a section away fiom the
stub face. Figure 6.41 shows the sketches of the most damaged regions in ail twelve
specimens. It is believed that the additional confinement provided by the stub caused a
delay of propagation of cracks in concrete and reduced the tendency of lateral expansion.
As a result, the moment capacity of the critical section increased and the failure was
pushed away form the stub. Table 6.10 listed the maximum moment at the column-stub
interface Msmw, the maximum moment of the most damaged section and the moment
Results and Discussions
capacity using the AC1 Code (1995) for ail test specimens. The locations of the most
darnaged sections measured from the snib face for ali specimens are also listed in Table
6.10. Specïmen ST- 1 NT has the lowest moment capacity at the most darnaged section. It
failed premanirely at a distance of 353 mm from the stub face, due, most Iikely. to the fact
that the GFRP was weakened due to the protruding LVDT bars.
Table 6.10 Maximum Moment of Specimens
Repaired specimens
Results and Discussions
Specimen S l N T Specimen ST-3NT
Specimen S-2 NT Specimen ST-4NT
Specimen S-3NT Specimen ST-SNT
Specimen S-4NT
Specirnen ST-1 NT
Specimen ST-2NT
Specimen STdNT
Repaired Specimen R-1NT
Repaired Specimen R-2NT
Ali dimension in millimeters.
Figure 6.41 Exteasively Damaged Regions in Specimens
Results and Discussions
6.4.6 Equivalent Plastic Hinge Length
If the equivalent plastic length is defined as the length over which the plastic
curvature is assumed to be constant, it can be computed for a cantilever column as shown
in Figure 6.42, using the following equation taken fiom Sheikh and Khou~y (1 993)
&m'-Ay = (@mu-%)Lp(L-o-sLp) (6.5)
where Ay and 8y are the yield displacement and yield curvature, respectively.
The preceding equation assumes the plastic hinge located right at the base of the
column. However, as mentioned earlier, the plastic hinge is pushed away fiom the stub
face due to the additional confinement provided by the stub. Since the offset distance is
smail compared to L, it is ignored in the equation. Using Equation 6.5. the equivalent
plastic hinge length 4, was calculated for each specimen tested for the last two cycles.
The average L, for each specimen is listed in Table 6-41.
Equivalcnt Plastic
(a) Colurnn (b) Dcfknon Prof* (c) Moment (d) C u m m Dkmmaari Disarbution
Figure 6.42 Cantiiever Column With Lateral Point Loading
Results and Discussions
Table 6. If Equivalent Plastic Hinge Length of Specimens
Specimen Cycle No. Equivatent Plastic Hinge Length L,(mm) Average Ldh
S I N T 7 468 8 376 422 1.19
S-2NT 11 285 12 344 315 0.88
'b
ST-4NT 14 571 15 528 550 1.55
ST-SNT 14 453 15 450 452 1.27
STdNT 7 38 1 8 529 505 1.42
r
R-1 NT* 12 443 +/45 5. 13 443 +/455* 1 .24'/ 1.27.
* Repaired specimens * Values calculated using A,, and #, of repaired specimens O Values calculated using 4, and @, of original specimens
For Specimens S- 1 NT and S-2NT, which contained spiral reinforcement
according to seismic provisions of AC1 Code (1995), the equivalent plastic hinge length
is approxirnately equai to the diarneter of the columns. Specimens S3NT and S-4NT,
which had large spiral spacing, failed in brittle manner without significant inelastic
behaviour. Therefore, the plastic hinge in these columns may not have appropnately
Results and Discussions
developed. For most of columns with FRP wraps, the equivalent plastic Iength varies
from 1.12 to 1.47 times the diameter of columo which is somewhat larger than that in
unwrapped columns. This may be due to the relatively tougher behaviour of plastic hinge
in wrapped columns, which resuited in smaller #,, and hence longer plastic hinge length.
For Specirnen ST-1NT which failed outside the test region. the calculated plastic hinge
length may not well represented the behaviour of the colurnn.
CHAPTER 7
CONCLUSIONS A N D RECOMMENDATIONS
The main purpose of the experimental program was to evaluate the effectiveness
of FRP composites in strengthening deficient columns or retrofitting damaged columns.
This was achieved by comparing the behaviour of FRP-retrofitted columns with that of
conventionally reinforced columns. A total of twelve specimens each consisting of a
circdar column and a snib were tested under constant axial load and reversed cyclic
lateral load. Conclusions made fiom this study are given below followed by a set of
recornmendations.
7.2 CONCLUSIONS
The following conclusions are drawn from the results of the tests:
1 . Use of carbon and giass FRP resulted in remarkable improvement in the
behaviour of columns resulting in significant increase in ductility, energy
absorption capacity and strength.
2. The CFRP or GFRP wraps are highly effective in confining the core concrete.
The behaviour of appropriately retrofitted columns under simdated
earthquake load matches or exceeds the performance of columns designed
according to the seismic provisions of the AC1 CODE (1995).
3. The spacing of spirais and hence the amount of laterai reinforcement have a
pronounced effect on both strength and ductility of reinforced concrete
Conclusions and Recommendations
subjected to axial load and lateral cyclic loading. As the spacing increases.
both the section and member ducdity decrease significantly. The adverse
effect of large spiral spacing can be compensated by the additional
corfimement provided by the FRP composites.
4. Both section and rnember ductility deteriorates as the level of axial load
increases. The arnount of FRP reinforcement needed to improve column
behaviour depends on the level of axial load. Under an axial load of 0.54P0,
two layen of 1.25 mm GFRP results in similar improvement of column
bebaviour as one layer of 1 .O0 mm CFRP, while under an axial load of 0.27P0,
the behaviour of a column with one layer of 1.25 mm GFRP is very sirnilar to
that of a similar column with one layer of 0.5 mm CFRP. It can be also
concluded that the arnount of CFRP reinforcement needed in a column under
an axiaI load of 0.54Po is more than twice what is needed for an axial load of
0.27P0 for sirnilar performance enhancement.
5. The F W composites are very effective for retrofitting damaged columns. The
amount of FRP needed depends on the extent of damage.
7.3 RECOMMENDATIONS
Only a lirnited number of tests have been conducted in this study to evaluate the
behaviour of circular columns reîrofitted with FRP composites. Further tests should be
carried out to ascertain and confirm the effects of variables exarnined in this study and
additionai variables such as concrete strength, different varieties of FRP and different
loading conditions. The following matters appear to rnerit M e r investigation:
Conclusions and Recommendations
1. To study the effectiveness of FRP wraps for strengthening or retrofitthg
square or rectangular columns.
2. To study the effectiveness of FRP composites for strengthening columns with
lap-sliced longitudinal reinforcement in the potentid plastic hinge zone.
. It is observed in most cases, the strength of retrofitted columns kept increasing
until failure. Since the behaviour of FRP is linear elastic to failure, it gives no
sign of waming before it ruptures. Research is needed to improve its mode of
failure.
4. The GFRP and CFRP are susceptible to environmental effects such as fkeeze
and thaw. temperature variation and moisnire. Further work is needed to
evaluate the long tem behaviour of the composites.
LIST OF REFERENCES
"Code for the Design of Concrete Structure for Building (CAN 3-A23.3M84)." Canadian Standards Association, Rexdale, Ontario. 1995, 28 1 p.
Ballinger, Craig A., "Specification Needs for FRP Composite Products," Proceedings of the Third Materials Engineering Conference, American Society of Civil Engineers, 1 994, pp.56-63.
Ba& Oguzhan, "High Strength Concrete Columns Subjected to Earthquake Type Loading", Thesis submitted in conformity with the requirements for the Degree of A master of Applied Science in the University of Toronto, 1995,239 p.
C o h a n . Harvey L.; Marsh, M. Lee; and Brown, Colin B, "Seismic Durability of Retrofitted Reinforced-Concrete Columns," Journal of Structural Division. ASCE, Vol. 119, No. 5, May 1992, pp. 1643-1661.
Chai. Yuk Hon; Priestley, M. J. Nigel; and Seibie, Frieder, "Seismic Retrofit of Circular Bridge Columns for Enhanced Flexural Performance," AC1 Structural Journal. Vol. 88. No. 5, Sept-Oct 1991, pp. 572-584.
Mander, J. B.; Priestly, J. N.; and Park, R., "Theoretical Stress-Strain Mode1 for Confineci Concrete, " Journal of Structurai Engineering, Vol. 1 14, No. 8, August. 1988, pp. 1804- 1 826.
Mander, J. B.; Priestly, J. N.; and Park, R., "Observed Stress-Strain Behaviour of Confilneci Concrete," Journal of Structural Engineering, Vol. 1 14. No. 8, August. 1988, pp. 1827- 1829.
Morgan, D. R.; Razaqpur, A. G.; and Crimi, J., "Fiber Reinforced Concrete Products." Advanced Comwsite Materials in Bridges and Structures in Japan, published by The Canadian Society for Civil Engineering, Montreal, 1992, pp. 18-30.
Muffti, Aftab A.; Erki, Marie-Anne; and Jaeger, Leslie G., "Introduction and Overview," Advanced Com~osite Materials with Av~lication to Bridges, published by The Canadian Society for Civil Engineering, Montreal, 199 1, pp. 1-20.
N b , Antoinio, "Concrete Repair with Extemally bonded FRP Reinforcement," Concrete International, June 1995, pp. 22-26.
Neale, K. W.; and Labossiére, P., "Materiai Properties of Fiber-Reinforced Plastics," Advanced Comwsite Materials with A~~l ica î ion to Bridges, published by The Canadian Society for Civil Engineering, Monmal, 199 1, pp. 2 1-60.
List of References
Park, R., and Pauley, T., "Reinforced Concrete St~~ctures", John Wiley & Sons, New York, London, Sydey, Toronto, 1975.
Priestly, M. J. N.; Seible, F.: and Fyfe, E, bbColumns Seisrnic Retrofit Using FiberglassiEpoxy Jackets," Proceedings of Advanced Composite Materials in Bridges and Structures, Canadian Society for Civil Engineering, 1992. pp.287- 298.
Saadatmanesh, H.; Ehsani, M. R.; and Jin, Limin, "Seismic Strengthening of circular Bridge Pier Models with Fiber Composites," AC1 Structural Journal, Vol. 93. No.6, Nov.-Dec. 1996, pp.639-647.
Saadatmanesh, H.; Ehsani, M. R.; and Jin, Limin, "Repair of Earthquake-Damaged RC columns with FRP Wraps," AC1 Structurai Journal, Vol. 94, No.2, March-Apnl 1 997, pp.206-2 1 5 .
Saadatmanesh, H.; Ehsani, M. R; and Li, M. W., "Strength and Ductility of Concrete Columns Externally Reinforced with Fiber Composite Straps,' AC1 Structurai Journal, Vol. 9 1. No.4, Jul-Aug. 1994, pp.434-447.
Saatcioglu, Murat; and Razvi, Salim R., "Strength and Ductility of Confïned Concrete," Journal of Structural Engineering, Vol. 1 18, No. 6, June 1992, pp. 1590- 1607.
Saatcioglu. Murat, and Razvi, Sdim R., "Strength and Ductility of Co&ned Concrete." Journal of Structural Engineering, Vol. 1 1 8, No. 6, June 1992, pp. 1 590- 1607.
Sheikh, Shamim A ., "Effectiveness of Rectangular Ties as Confinement Steel in Reinforced Concrete Columns," Thesis subrnitted in conformity with the requirernents for the Degree of Doctor of Philosophy in the University of Toronto, 1978,256 p.
Sheikh, S.&; Khoury, S.S., "Confued Concrete Columns with Stubs", AC1 Structural Journal , Vol. 90, No. 4., July-August 1993, pp.4 14-43 1.
Sheikh, Shamim A. and Uzumeri, S. M.. "Strength and Ductility of Tied Concrete Columns, "Journal of the Structurai Division, ASCE, Vol. 106, No. ST5, May 1980, pp. 107% 1 102.
Sheikh, Shamim A. and Unimeri, S. M., "Andyticd Mode1 for Concrete Confinement in Tied Columns," Journal of the Structural Division, ASCE, Vol. 108, No. ST12, December, 1982, pp. 2703-2722.
List of References
Wallenberger, Fredenck, T., "High Modulus Glass-Ceramic Fiber Reuiforced Composites for Currently Emerging Idkstmcture Applications," Proceedings of the Third Materials Engineering Conference, American Society of Civil Engineers, 1994, pp.272-279.
APPENDICES
LOAD-DEFLECTION CURVES PLOTTED DURING THE ACTYAL TEST
Amendices
IMAGE EVALUATION TEST TARGET (QA-3)
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