2015Yin - Compressive Behavior of Concrete Confined by CFRP

7/21/2019 2015Yin - Compressive Behavior of Concrete Confined by CFRP

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O R I G I N A L A R T I C L E

Compressive behavior of concrete confined by CFRP

and transverse spiral reinforcement. Part A: experimentalstudy

Peng Yin Liang Huang Libo Yan Deju Zhu

Received: 4 August 2014 / Accepted: 2 February 2015

RILEM 2015

Abstract This study presents the results of an

experimental investigation of 18 short concrete

columns confined by carbon fiber-reinforced polymer

(CFRP) and transverse spiral reinforcement (TSR)

under uniaxial compression. Longitudinal rebars are

not installed in the specimens in order to eliminate their

confinement effect to concrete which affects the

analysis of 3-D compression of concrete. The paper

only consider for FRP and spiral reinforcement con-

finement in transverse direction. Two key experimen-

tal parameters were investigated: the thickness of theCFRP tube (0.167, 0.334, and 0.501 mm) and the

spacing of the TSR (25 and 50 mm). The failure mode,

axial and transverse stressstrain relationship, con-

finement effectiveness, Poissons ratio and dilatation

performance of the specimens were discussed. Test

results show that the ultimate strength of concrete has a

linear proportional enhancement with an increase in

the FRP layer in each TSR category and a decrease in

the TSR spacing in each FRP layer category. The

ultimate load carrying capacity of the confined con-

crete depends on the confinement pressure during

failure in terms of ultimate strength and axial strain.

Keywords CFRPTransverse spiral reinforcementLongitudinal rebar Experimental study

1 Introduction

Fiber-reinforced polymer (FRP) composite materials

have been widely used in the field of civil engineering in

the last two decades. Because of their high strength-to-

weight ratio, good corrosion behavior, and electromag-

netic neutrality, FRP have been successfully used to

rehabilitate and upgrade existing reinforced concrete

structures through wrapping of the FRP sheet on

structures surface. Another attractive application ofFRP materials is the fabrication of new concrete

columns in FRP tubes or shells. FRP tubes or shells

offer several advantages, such as their increased trans-

verse confinement, as well as their use as formwork.

To develop these applications, the compressive

stressstrain behavior of FRP-confined concrete has

been studied, and many models have been proposed

and developed for analysis and design [18]. Research

shows that FRP composites have a linear elastic

P. Yin (&) L. Huang D. ZhuDepartment of Civil Engineering, Hunan University,

Yuelu Montain, Changsha 410082, Chinae-mail: [email protected]

L. Huang

e-mail: [email protected]

D. Zhu


L. Yan

Department of Construction & Structural Engineering,

Fraunhofer Wilhelm-Klauditz Institution,

Bienroder Weg 54E, 38108 Brunswick, Germany


Materials and Structures

DOI 10.1617/s11527-015-0554-1


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stressstrain response with brittle rupture failure at a

small rupture strain [9]. Current North American

codes and design guidelines provide design equations

for short circular columns, which are strengthened or

retrofitted with FRP-confinement wrapping [10]. All

codes and designed guidelines focus on the compres-

sive strength of FRP wrapped concrete but neglect theeffect of transverse steel reinforcement. In reality,

however, the actual concrete columns are under two

actions of confinement: the action of the FRP and that

of steel reinforcement. Therefore, existing codes and

guidelines cannot provide accurate instructions. The

minimum amount of steel reinforcement must also be

added to the FRP-confined concrete to avoid gener-

ating a brittle failure mode. In this regard, researchers

note the need for a concrete confinement model that

considers the interaction between internal steel rein-

forcement and external FRP sheets [1113].Some researchers have investigated concrete column

confined by an FRP, spiral or hoop, and longitudinal

reinforcement simultaneously [1417]. In their

experiment, they designed the longitudinal rebar in

specimens. In this paper, it does not deny the importance

of longitudinal rebar in the column. However, this study

aims to investigate the behavior of concrete under the

condition of 3D pressure by FRP and Spiral reinforce-

ment. The experiment is designed to create a pure

transverse confinement circumstance for concrete. Lon-

gitudinal reinforcement will influence the analysis, soanalyzing the 3D compression of concrete confined by an

FRP and TSR will be inaccurate. The specimens without

longitudinal rebar in this paper can avoid this issue. The

goal is to make sure the concrete specimens can have a

relative accurate experiment data. The experiment data

can be compared with other researchers model and

proposed a new model with more accuracy.

Therefore, this study investigate the compressive

behavior of concrete by dual confinement provided by

CFRP and TSR, which terms as CFRPTSR-confined

concrete. Stressstrain behavior, ultimate condition,

and other properties subjected to monotonic axial

compression were investigated. These results were

recorded to verify future studies that develop related

models.

2 Experimental program

2.1 Test matrix

A total of 18 CFRP-TSR-confined concrete cylinders

with a diameter of 150 mm and a height of 300 mm

were constructed and tested under uniaxial compres-

sion. Three plain concrete cylinders with the same

diameter and height were also tested for comparison.

The cylinder specimens were designed on the basis

of the following variables: (1) the number of CFRPlayers,nf, and (2) the spacing of TSR,s. Each variable

is classified into several characters: the number of

CFRP layers is classified into one, two, and three

layers labeled as 1L, 2L, and 3L, respectively; the

spacing of TSR is classified into 25 and 50 mm, which

are labeled as -25 and -50, respectively. For each

combination of testing parameters, three identical

specimens were fabricated and tested, and they were

labeled as N1, N2, and N3, respectively. i.e., 3L-25-

N2 indicates that the specimen is confined by three

layers of CFRP with a 25 mm spacing of TSR (thesecond specimen in this combination of testing

parameters). The plain concrete column is represented

by 0L-N. The CFRP-TSR-confined concrete speci-

mens occupy a 5 mmconcrete cover (c). The details of

these specimens are summarized in Table1.

2.2 Preparation of the specimens

Before the CFRP tubes are fabricated, the PVC tubes

with a diameter of 150 mm were prepared as the

Table 1 Test matrix and

details of the specimensSpecimen group f0c (MPa) D (mm) c (mm) /h (mm) s (mm) qs (%) nf t(mm)

1L-50 30.6 150 5 6 50 1.416 1 0.167

1L-25 30.6 150 5 6 25 2.682 1 0.167

2L-50 30.6 150 5 6 50 1.416 2 0.334

2L-25 30.6 150 5 6 25 2.682 2 0.334

3L-50 30.6 150 5 6 50 1.416 3 0.501

3L-25 30.6 150 5 6 25 2.682 3 0.501



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mould. It is noted that this mould is only valid for

CFRP tubes, not for concrete. The PVC tubes were

covered with a layer of thin plastic films, which enable

the cured CFRP tubes to be easily detached later. The

carbon fiber sheets were cut and trimmed into

appropriate lengths for each layer of wraps, with an

overlap length of 150 mm. Then epoxy resin wasapplied. The CFRP sheet was saturated with epoxy

with the use of a brush before the sheet was wrapped

around the PVC tube. Extra CFRP strips with width of

40 mm were placed at the top and bottom ends of each

cylinder to enhance its strength and ensure that failure

occurs in the middle portion of the specimen. Addi-

tional epoxy was applied as an overcoat to ensure that

the entire fabric was wet. Excess epoxy and air bubble

were squeezed out of the CFRP sheet. After 3 h, all

FRP tubes were pulled out from the PVC tubes. All

FRP tubes should be dried for at least 7 days. Duringthis period, the TSR was prepared and placed into the

CFRP tubes, and the concrete was cast into the FRP

tubes. The fabrication procedure is shown in Fig.1.

2.3 Material properties

2.3.1 Concrete

The concrete mix design is shown in Table 2. Three

plain concrete cylinders were tested to determine the

average maximum strength of the unconfined con-crete,f0co, and its corresponding strain, e

0co. The average

concrete compressive strength at 28 days is 30.6 MPa.

2.3.2 Steel reinforcement

The transverse spiral reinforcement was made with the

diameter of 6 mm deformed bars that have an average

yield strength of fyt = 335 MPa. Tension tests were

performed on steel samples. The average yield

strength values were calculated from five tension tests.

2.3.3 Fiber reinforced polymer

To obtain the material mechanical property of the

CFRP, related tensile tests were conducted according to

ASTM specification D3039-M08 (ASTM 2008b). The

tensile coupons were cut from an FRP sheet along the

fiber. Aluminum flat plates were glued to the ends of the

coupons before they were tested to prevent them from

premature failure. TheCFRP coupon is shown in Fig.2.

The strength and modulus were calculated accordingto the gross sectional area of the coupons. The ultimate

strain was obtained from the stain gauge stick at the

middle portion of the coupons. The main mechanical

properties obtained from the average values of five

tensile coupon tests were as follows: thickness (one

layer of CFRP) =0.167 mm; ultimate strength,

ffu = 3,200 MPa; ultimate strain, efu = 0.0150; and

modulus,Efu =213 GPa.

2.4 Instrumentation and loading

For each CFRPTSR-confined specimen, four strain

gauges with a length of 20 mm in the axial direction

Fig. 1 Fabrication procedure of the CFRP tube:a CFRP sheet, epoxy, and PVC tube;b CFRP tubes on the PVC mold; cCFRP tube;

dCFRP tube with the fixed TSR

Table 2 Concrete mixture proportions

f0co (MPa) e0co Cement (kg/m

3) Water (kg/m3) Fine aggregates (kg/m3) Coarse aggregates (kg/m3) w=c

30.6 0.002 290 195 1,024 898 0.67



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and four strain gauges with a length of 10 mm in the

transverse direction were installed at the middle

portion of the specimen. One axial strain gauge and

one transverse gauge were used as one unit. Each unitwas symmetrically installed 90 apart on the surface of

the specimens (see Fig.3a). For each unconfined

concrete specimen, two axial strain gauges and two

transverse strain gauges, all with a gauge length of

50 mm, were placed at the middle portion of the

specimen to measure the strain gauges in two direc-

tions. One axial strain gauge and one transverse gauge

were used as one unit. Each unit was symmetrically

installed 180 apart on the surface of the specimens

(see Fig.3a). The installation of the TSR strain gauges

is shown in Fig.3b.In addition, the average axial strains of the cylin-

ders were also measured with four LVDTs installed at

four edges of the compression board of the testing

machine. Two high-stiff steel plates with a diameter

slightly smaller than that of the specimen were placed

at the top and bottom of the specimens to avoid direct

loading of the CFRP tube when the specimen is

subjected to compressive load, as shown in Fig. 4. All

the specimens were tested in a 5,000 kN testing

machine under displacement control with a constant

rate of 0.01 mm/s. All test data were automaticallyrecorded with a data acquisition system.

3 Results

3.1 Failure mode

Specimens failed when the tensile fiber ruptured. The

confinement level can affect the damage level of core

56 138 56

15

1.5

0.1

67

CFRP coupon

Aluminlum flat bar

CFRP coupon Aluminlum flat bar

Strain gauges

SG1

SG2SG3

SG1 & SG2

SG3

Fig. 2 Details of the CFRP tension coupon (unit of mm)

(a) (b)

SG5

OverlappingZone

CFRP Tube

SG6SG1SG2

SG3SG4

SG7 SG8

Plain ConcreteSG1SG3

SG4

SG2

D=130 mm

h=

300mm

s=25mm

D=130 mm

h=

300mm

s=50mm

Strain gauge location

Fig. 3 Installation of the strain gauges:a Strain gauges on the CFRP tube and unconfined concrete; b Strain gauges on the TSR

LVDT LVDT

Strain Gauges

Steel Plate

Steel Plate

Compression Board

Compression Board

Fig. 4 Test setup in the axial compression test



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concrete, i.e. a higher confinement level caused more

damage. During the testing, debonding or shear failure

of the specimens was not detected. The rupture strain

of the CFRP was lower than the ultimate strain

capacity obtained in the coupon test, and this condition

led to the premature failure of the CFRP. The

appearance of the specimens slightly changed beforethe ultimate compression load reached an average of

95 %. When it reached 95 %, the sound of CFRP

tearing was heard. The CFRP tube was completely

torn and produced a very loud sound when the ultimate

compression load was reached. The appearance of the

column after testing is shown in Fig. 5.

3.2 Stressstrain behavior

Figure6illustrates the average axial stress versus the

axial and transverse strain of the cylinder specimens in

each category. The figure shows that the CFRPTSR-

confined concrete bi-linearly behaved, with two linear

regions connected by a transition zone. The axial

stress of the cylinder specimens was obtained by

division of the measured axial load by the cross-

section area of the cylinder. The axial and transverse

strains of the cylinder specimens were obtained fromLVDTs and strain gauges, respectively.

Table3 shows the test data obtained from the

specimens. The maximum axial load is defined as

Pmax, and the relative maximum axial stress and axial

strain of the columns are defined as fcmax and ecmax,

respectively. The actual FRP rupture strain, efu;a, is

less than the ultimate tensile strain, efu, obtained from

the standard tension coupon test. The value fcmax

f0corepresents confinement effectiveness. The confine-

ment effectiveness became larger when increasing the

layer of CFRP or decreasing the spacing of TSR.

The recorded CFRP ultimate strain was less than

the rupture strain obtained from the coupon tests. The

possible reasons for this phenomenon are considered

below [1822]. The rupture strain obtained from the

coupon test is the result of a pure tensile experiment.

However, in the axial compression test, the CFRP

tubes may be subjected to axial stress and transverse

stress, a condition that is different from that in the pure

tensile test in coupons. Concrete fragment impales the

interface of the CFRP tube, and this leads to a local

stress concentration as a result of the non-uniform

deformation of cracked concrete. The CFRP tubeFig. 5 Failure modes of the specimens

-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.0200

20

40

60

80

100

120

140

160

AxialLoad(kN)

AxialStress(MPa)

Axial Strain

-6.0 -4.5 -3.0 -1.5 0.0 1.5 3.0 4.5 6.0

0

353

706

1059

1412

1765

2118

2471

2824

0L-N

3L-25

3L-50

2L-25

2L-50

1L-251L-50

3L-50

3L-25

2L-25

2L-50

1L-25

1L-50

Displacement (mm)

Transverse Strain

Fig. 6 Stressstrain

behavior of the specimens



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compression. In this region, the axial load begins with

a load defined asPc2, and the corresponding stress and

strain are defined asfc2and ec2, respectively. The curve

continues to be represented by an ascending line until

the maximum axial stress is reached; this stress also

corresponds with the maximum axial and transverse

strain. At this point, the CFRP tube ruptures with avery loud sound similar to an explosion.

3.3 Poissons ratio

The Poissons ratio of the specimens at different

confinement levels is shown in Fig. 8. When the axial

stress of the specimen is lower than the ultimate

strength of unconfined concrete (30.6 MPa), the

Poissons ratio of all types of specimens approximate-

ly equals 0.2, which represents the typical initial

Poissons ratio of unconfined concrete. In this region,the confinement effect provided by the CFRP and TSR

is not activate to affect the specimen. The transverse

expansion of the concrete core is negligible because

the micro crack inside the core concrete occupies a

very small space, so a constant value of the initial

Poissons ratio is reached.

When the axial stress of the specimen exceeds the

ultimate strength of unconfined concrete, the Pois-

sons ratio of the specimen shows a linear increase. In

this region, the micro cracks inside the concrete core

gradually expand. The CFRP sheet outside the columnand the TSR inside the column begin to provide the

transverse confinement. The level of transverse con-

finement determines the slope of the curve in the

second region. When the column is subjected to weak

confinement with insufficient transverse stiffness, the

transverse strain rapidly increases, so the Poissons

ratio rapidly increases too. As the column is subjectedto strong confinement with sufficient transverse stiff-

ness, the transverse strain slowly increases, so is the

Poissons ratio.

When the axial stress continues to increase, the

Poissons ratio finally stabilized to its maximal value

until the specimen fails. The expansion of micro

cracks inside the core concrete is subjected to a high

level of confinement. The loading capacity of the

specimen, as well as the tensile strain of the CFRP and

TSR, also increases until the CFRP ruptures.

3.4 Dilatation performance

In a triaxial state of stress, the dilatation (also called

volumetric strain ev) is defined as follows:

ev ec2er

where ec is the axial strain and er is the transverse

strain. Figure9 shows the curve of the axial stress

versus the volumetric strain for all types of specimens.

Initially, volume change is in the form of reduction

and is nearly linear until the ultimate unconfinedconcrete stress is reached. After this point, the

specimens reverse the direction of volume change in

such a way that the volume expands, a phenomenon

called dilatancy. With the increase in axial stress, the

specimens with one and two layers of FRP continue to

conduct volume expansion, whereas the specimens

with three layers of FRP reverse the direction of

volume change again to a reduction manner. As the

confinement increases (increase in FRP layers or

20 40 60 80 100 120 140 1600.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

3L-25

3L-50

2L-25

2L-50

1L-25

PoissonRatio

Axial Stress (MPa)

5

1L-50

Fig. 8 Influence of the confinement level on the Poissons ratio

-0.020 -0.015 -0.010 -0.005 0.000 0.005 0.0100

20

40

60

80

100

120

140

160

Volume Reduction

3L-253L-50

2L-25

2L-50

1L-25

AxialStress

Volumertric Strain

1L-50

Volume Expansion

Fig. 9 Axial stressvolumetric strain of the specimens



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decrease in TSR spacing), the dilation zone shrinks, so

an effective confinement response is achieved.

4 Analysis

4.1 Interactive function of the CFRP and TSR

Thedual confinement mechanism of the CFRP and TSR

is shown in Fig.10. In the initial part of loading (axial

strain is smaller than 0.002), the tensile strain provided

by TSR,es, is similar to that developed by CFRP, ef.

However, when the axial strain level approaches or

larger than the unconfined concrete strain (0.002), the

relationship between the FRP transverse strain and

TSR transverse strain can be divided into two

categories. One category is for Column 1L-50, 1L-

25, 2L-50. These column indicate that the FRPtransverse strain is larger than the TSR transverse

strain after 0.002 in axial strain. Another category is

for Column 2L-25, 3L-25, 3L-50. These column

indicate that the FRP transverse strain is smaller than

the TSR transverse strain after 0.002 in axial strain. In

can be found that with the increasement of FRP layer,

the TSR transverse strain would increase. Before

CFRP rupture, the TSR reaches its yield strain, and

this result indicates that the specimens are subjected to

the maximum confining pressure provided by the TSR

and CFRP tube at the CFRP rupture state.

4.2 Influence of ultimate confinement pressure

The confinement pressure provided by the CFRP and

TSR determines the ultimate capacity of the confined

concrete in terms of compressive strength and axial

strain. The strength of concrete generated at failure

linearly increases with the confinement ratio, as indi-

cated in Fig.11a. Parameterac is defined as the ratio

between the ultimate confinement pressure (flu) andthe compressive strength of unconfined concrete (f0co).

ac flu

f0co

where flu flf fls; flf is the ultimate confinementpressure provided by FRP, and fls is the ultimate

confinement pressure provided by transverse steel

reinforcement. The calculation of flf and fls is as

follows:

flf2Efuefu;at

d

where d is the diameter of the entire concrete cross

section,Efuis the Youngs modulus of the FRP, tis the

thickness of the FRP, and efu;a is the actual tensile

strain of the FRP.

fls 2keEselAs

sdsel\esy

fls 2kefsyAs

sdsel[ esy

where Es is the Youngs modulus of the spiral

reinforcement, els is the actual tensile strain of the

spiral reinforcement,esyis the yield strain of steel,Asis

the cross-sectional area of the spiral reinforcement,dsis the diameter of the spiral between bar centers, and keis the confinement effectiveness coefficient. The

previous ke is defined as ke 1 s

0

2ds

2

1q0s. In this study,

the specimens do not contain longitudinal rebar

(q0s 0. Therefore, ke should be revised as

ke 1 s0

2ds

2, wheres0 is the clear vertical spacing

between spiral reinforcement.

For strain capacity, when the confinement ratio

reaches a high value, the curve stabilizes to a platform,

as indicated in Fig. 11b.

It is noted that the regression analysis is limited tothe range of confinement ratios used in experimental

tests, and it should not be extended to zero confine-

ment ratio, hence to the unconfined point.

5 Conclusion

This paper presents an experimental study on the

compressive behavior of concrete cylinder column

confined by both CFRP and TSR. The effects of main

variables, such as the CFRP tube layer and TSR

volumetric ratio, were investigated. The main conclu-

sions are as follows:

Experimental results show that increasing the layer

of the CFRP tube or the TSR volumetric ratio

enhances the ultimate strength of concrete and its

corresponding ultimate strain. Test results indicate

that when the layer of FRP in each different

spacing of TSR group is increased, the ultimate



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strength of concrete demonstrates a linear propor-

tional enhancement.

The volume changes in the specimens are controlled

by theconfinement level. Thedilatation zonedecreases

when the confinement level increases, and this condi-

tion leads to an effective confinement response.

The Poissons ratio remains at 0.2 before the

compression stress reaches the unconfined

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020

Actual CFRP rupture strain

Axial Strain

Column 1L-50

TSR

CFRP

CFRP coupon rupture strain

TSR yield strain

TransverseS

train

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020


Axial Strain

Column 1L-25

TSR

CFRP


TSR yield strain

TransverseStrain

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020


Axial Strain

Column 2L-50

TSR

CFRP


TSR yield strain

TransverseStrain

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020


Axial Strain

Column 2L-25

TSR

CFRP


TSR yield strain

TransverseStrain

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020


Axial Strain

Column 3L-50

TSR

CFRP


TSR yield strain

TransverseStrain

-0.016

-0.014

-0.012

-0.010

-0.008

-0.006

-0.004

-0.002

0.0000.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020


Axial Strain

Column 3L-25

TSR

CFRP


TSR yield strain

TransverseStrain

Fig. 10 Interaction function of the CFRP and TSR



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concrete strength. Then, it undergoes a propor-

tional increase and eventually stabilizes at a

constant value until the CFRP tube ruptures.

The ultimate capacity of the confined concrete

depends on the confinement pressure during failure

in terms of ultimate strength and axial strain.

Acknowledgments This research was funded by the Natural

Science of China (project codes: 51078132) and China 973 Plan

(Project codes: SQ2011CB076458). The experimental work were

supported by the Structure Laboratory of Hunan University. The

authors also acknowledge the technical instruction and assistance

of Professor Yan Xiao and Professor Giorgio Monti.

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(a) (b)

0.0 0.1 0.2 0.3 0.4 0.50

50

100

150

200

250

300

350

400

450

1L-501L-252L-502L-25

3L-503L-25

IncreaseofStrength(%)

Confinement Ratio c

y = 1931.3x - 374.62

0.25 0.30 0.35 0.40 0.45500

520

540

560

580

600620

640

660

680

700

720

740

1L-501L-252L-502L-253L-503L-25

Increaseof

Strength(%)

Confinement Ratio c

y = -152929x3+ 159146x

2- 53320x + 6361.9

Fig. 11 a Typical relationship between strength gain and confinement ratio; b Typical relationship between strain gain and

confinement ratio



11/11

20. Luca AD, Matta F, Nanni A (2011) Behavior of full-scale

glass fiber-reinforced polymer reinforced concrete columns

under axial load. ACI Struct J 107(5):589596

21. Matthys S, Taerwe L, Audenaert K (1999) Test on axially

loaded concrete columns confined by fiber reinforced

polymers sheet wrapping. In Proceedings of FRPRCS-4,

Baltimore, pp 217218

22. Zinno A, Lignola GP, Prota A, Manfredi G, Cosenza E

(2010) Influence of free edge stress concentration on ef-

fectiveness of FRP confinement. ELSEVIER Compos Part

B 41(7):523532


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