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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
e-mail: [email protected]
L. Yan
Department of Construction & Structural Engineering,
Fraunhofer Wilhelm-Klauditz Institution,
Bienroder Weg 54E, 38108 Brunswick, Germany
e-mail: [email protected]
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
Actual CFRP rupture strain
Axial Strain
Column 1L-25
TSR
CFRP
CFRP coupon rupture strain
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
Actual CFRP rupture strain
Axial Strain
Column 2L-50
TSR
CFRP
CFRP coupon rupture strain
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
Actual CFRP rupture strain
Axial Strain
Column 2L-25
TSR
CFRP
CFRP coupon rupture strain
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
Actual CFRP rupture strain
Axial Strain
Column 3L-50
TSR
CFRP
CFRP coupon rupture strain
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
Actual CFRP rupture strain
Axial Strain
Column 3L-25
TSR
CFRP
CFRP coupon rupture strain
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
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