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Research ArticleAxial Behaviour of Slender RC Circular
ColumnsStrengthened with Circular CFST Jackets
Yiyan Lu ,1 Tao Zhu ,1 Shan Li ,1 Weijie Li,1 and Na Li2
1School of Civil Engineering, Wuhan University, Wuhan 430072,
China2School of Civil Engineering and Architecture, Wuhan
University of Technology, Wuhan 430070, China
Correspondence should be addressed to Shan Li;
[email protected]
Received 2 July 2018; Accepted 23 August 2018; Published 12
September 2018
Academic Editor: Eric Lui
Copyright © 2018 Yiyan Lu et al. 'is is an open access article
distributed under the Creative Commons Attribution License,which
permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
'is paper investigates the axial behavior of slender reinforced
concrete (RC) columns strengthened with concrete filled steel
tube(CFST) jacketing technique. It is realized by pouring
self-compacting concrete (SCC) into the gap between inner original
slenderRC columns and outer steel tubes. Nine specimens were
prepared and tested to failure under axial compression: a
controlspecimen without strengthening and eight specimens with
heights ranging between 1240 and 2140mm strengthened with
CFSTjacketing. Experimental variables included four different
length-to-diameter (L/D) ratios, three different
diameter-to-thickness(D/t) ratios, and three different SCC
strengths. 'e experimental results showed that the outer steel tube
provided confinement tothe SCC and original slender RC columns and
thus effectively improved the behavior of slender RC columns. 'e
failure mode ofslender RC columns was changed from brittle failure
(concrete peel-off) into ductile failure (global bending) after
strengthening.And, the load-bearing capacity, material utilization,
and ductility of slender RC columns were significantly enhanced.
'estrengthening effect of CFST jacketing decreased with the
increase of L/D ratio andD/t ratio but showed little variation with
higherSCC strength. An existing expression of load-bearing capacity
for traditional CFST columns was extended to propose a formulafor
the load-bearing capacity of CFST jacketed columns, and the
predictions showed good agreement with theexperimental results.
1. Introduction
Strengthening of RC structures is critically important
forseveral reasons [1–7]. One is to restore the
load-bearingcapacity of deteriorated concrete infrastructures
because ofaging or damage. Another reason is to enhance the
ser-viceability and capacity of structures in response to a
loaddemand increase beyond the original design. A third reasonis to
improve the load-bearing capacity for deficientmembers as a result
of design or construction errors.
Common strengthening methods such as section en-largement
(concrete jacketing) [8], externally bonded steelplates [9, 10],
and externally bonded fiber-reinforcedpolymer (FRP) [11, 12] have
been used for many years toimprove structural service performance
and ultimate ca-pacity of concrete structures. However, their
disadvantagesmay limit the further application. 'e concrete
jacketing
method obviously enlarges the cross section of concretemembers,
and the construction of steel cage and formworkcosts a lot of labor
and time. 'e steel plate strengtheninghardly changes the appearance
of concrete structures, but itrequires a large amount of steel and
antirust work [13–15].'e FRP strengthening degrades the deformation
of con-crete structures and the effective utilization of FRP
typicallyranges from 30 to 35% [16–19]. 'erefore, new
strength-ening methods for concrete structures are still
causingconcern.
Recently, a novel technique, CFST jacketing, has becomean option
to strengthen concrete columns because of thesuperior performance
of CFST columns (e.g., high load-bearing capacity and good
ductility). It has been successfullyapplied in the strengthening of
RC bridge piers [20, 21].CFST jacketing is realized by the
installation of in-fieldwelded steel tube around the original RC
column and
HindawiAdvances in Civil EngineeringVolume 2018, Article ID
7923575, 11 pageshttps://doi.org/10.1155/2018/7923575
mailto:[email protected]://orcid.org/0000-0001-7999-2484http://orcid.org/0000-0001-5124-2059http://orcid.org/0000-0002-9515-578Xhttps://doi.org/10.1155/2018/7923575
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pouring concrete into the gap between the inner originalcolumn
and outer steel tube.'e CFST jacketing method hasmany preferable
advantages. In addition to a significantincrease of load-bearing
capacity and ductility, the CFSTjacketing method is quick and easy
to apply because theouter steel tube could serve as formwork and
steel re-inforcement, and thus, it requires less temporary
formworkand reduces the usage of steel. Moreover, it could
betterutilize properties of each material with little change
incolumn section size.
To better understand the performance of RC
structuresstrengthened with CFST jacketing, many studies have
beenconducted. Priestley et al. [22, 23] conducted experiments
toverify the effectiveness of CFST jacketing approach for RCcolumns
and concluded that the shear strength and ductilityof RC columns
were increased significantly. Sezen andMiller[24] compared the
performances of bridge piers strength-ened with FRP jacketing,
concrete jacketing, and CFSTjacketing. 'e results showed that the
CFST jacketingmethod was much more effective in enhancing the
load-bearing capacity and ductility. Wang [25] and He et al.
[26]studied the effects of preloading on the compressive strengthof
CFST jacketed columns. 'e results showed that thepreloading level
had little effect on the load-bearing capacityof specimens. He et
al. [27] also studied the axial com-pressive behavior of CFST
jacketed columns with recycledaggregate concrete. 'e results showed
that the influence ofthe recycled coarse aggregate replacement
ratio on thecompressive strength might be negligible. Recently,
theauthors of the present paper have also carried out experi-mental
and theoretical studies on the axial and eccentricperformance of
CFST jacketed RC stub columns [28–30].'e influence of initial
eccentricity on the strength of CFSTjacketed RC column had been
addressed.
Nevertheless, most of the previous studies are limited tostub
columns, very limited research has been conducted toverify the
effectiveness of CFST jacketing strengthening onslender RC columns.
'is paper presents an experimentalstudy of slender RC columns
strengthened with CFSTjacketing under axial compression, and the
SCC was usedinstead of normal concrete.'e test program consists of
ninespecimens, one of which is unstrengthened and serves ascontrol
specimen, the remaining eight are strengthened withCFST jacketing.
'e main parameters in the test are the L/Dratio (5.7, 7.1, 8.4, and
9.8), D/t ratio (56.2, 67.4, and 121.7),and compressive strength of
SCC (40, 50, and 60MPa). Amodified model is applied to predict the
load-bearing ca-pacity of strengthened slender RC columns.
2. Experimental Programme
2.1. Test Specimens. Nine columns were tested to failureunder
axial compression, including one control specimen(Ref) and eight
CFST jacketed specimens which are namedin the form of tx-Cy-z. 'e
number after “t” represents thenominal thickness of steel tube. 'e
number after “C” do-nates the design cubic compressive strength of
SCC.'e lastnumber indicates the slenderness of columns. Table
1summarizes the details of each specimen.
2.2. Preparation of Test Specimens. All original columns
arecircular with a diameter of 154mm and variation of
length(1240mm, 1550mm, 1850mm, and 2140mm). 'e re-inforcement
consists of 6 longitudinal steel rebars (12mmdiameter) and stirrups
(6mm diameter) spacing at 150mm.'e internal reinforcement ratio of
the original column is3.6% which meets the 0.6–5.0% range
requirement [31]. 'eoriginal columns are cast using C25-grade
normal concreteand cured for 28 days in the laboratory. Afterward,
the CFTSjacketing strengthening is followed.
(i) 'e original column is sandblasted by a handgrinder to remove
the irregularities and debris
(ii) 'e outer steel tube is carefully placed on thedesigned
region leaving a uniform gap
(iii) 'e SCC is poured into the gap between originalcolumns and
steel tube at three intervals
(iv) 'e strengthened column is then cured for 28 daysin the
laboratory
'e reinforcement ratios of CFST jacketed columnsrange from 5.2%
to 9.5%. 'e ratios are in the range of3.0–20.0% which are commonly
used by others’ researches[32].
2.3. Material Properties. 'e original concrete and SCC aremade
from 42.5-grade Portland cement, aggregates witha maximum diameter
of 20mm, and river sand. Moreover,the water reducer, expansive
agent, and fly ash are added tothe SCC. 'e cubic compressive
strength is determined withthree 150×150×150mm concrete cubes after
28 days ofcure. 'e mix design, cubic strength, and slump of
concreteare summarized in Table 2.
'e material properties of steel tubes, steel rebar, andstirrup
are tested according to the Chinese code GB/T50081-2002 [33]. 'e
results are shown in Table 3.
2.4.Test SetupandProcedure. Eight strain gauges are bondedevenly
on the exterior surface of the steel tube to measure
thelongitudinal and transverse strains at midheight. After
thespecimen is placed on the hinge supports, two linear
variabledifferential transducers (LVDTs) are used to measure
theaxial shortening and three other LVDTs are placed tomeasure the
lateral deflection along the specimen’s height(0.25 L, 0.50 L, and
0.75 L). 'e test is carried out witha universal hydraulic testing
machine (capacity of 5000 kN).'e load is applied in increments of
one-tenth of the the-oretical load-bearing capacity (Nu,theo)
before the steel yieldand in increments of one-fifteenth of Nu,theo
afterward. Eachload interval is maintained for about 2minutes as
perChinese code GB/T 50152-2012 [34]. 'e test setup
andinstrumentation are shown in Figure 1.
3. Experiment Results and Discussion
3.1. Failure Modes of Specimens. Specimen Ref exhibiteda brittle
failure. Concrete crack occurred near the top at557 kN (about 85%
of Nu). And the concrete cracks widened
2 Advances in Civil Engineering
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and propagated downwards with the increase of load. At
thefailure load (656 kN), the concrete on the top began to peelo�
without obvious axial shortening or lateral de�ection, asshown in
Figure 2(a). In comparison, all CFST jacketedcolumns failed in
excessive lateral de�ection, showing muchbetter ductility. As the
load increased, rstly the axialshortening and lateral de�ection
developed invisibly; sec-ondly obvious axial shortening and lateral
de�ection were
observed after the load reached around 85% of Nu;
nally,excessive lateral de�ection was obtained with the
localbuckling of outer steel tube at midheight, as shown inFigure
2(b). is observation is consistent with the researchnding of
traditional CFST columns [8, 35], where localbuckling of steel
tubes was also reported. It should bementioned that our team has
also conducted experiments ofCFST jacketing strengthening on
slender RC square columns
Table 2: Mixes and properties of concrete.
Concrete Cement Sand Gravel Water Water reducer Expansive agent
Fly ash Cubic strength (MPa) Slump (mm)RC C25 1.000 2.103 4.082
0.635 0.000 0.000 0.000 32.83 N/ASCC C40 1.000 2.239 3.075 0.522
0.014 0.143 0.284 43.01 670SCC C50 1.000 1.892 2.838 0.459 0.017
0.143 0.286 52.58 675SCC C60 1.000 1.667 3.000 0.423 0.022 0.146
0.282 57.29 672
Table 3: Material properties of steel.
Steel Diameter (mm) ickness (mm) Young’s modulus (GPa) Yield
strength (MPa) Tensile strength (MPa)Steel tube 219 1.80 191 390
587Steel tube 219 3.25 211 352 425Steel tube 219 3.90 234 342
522Steel rebar 12 — 190 365 527Stirrup 6 — 203 214 278
LVDT
Load cell
Axial strain gauges
Hoop strain gauges
Strain gauges
Figure 1: Test setup and instrumentation.
Table 1: Specimen details.
Specimen D2(D1)× t× L (mm) L/D1 L/D2 D2/t fcu1 (MPa) fcu2 (MPa)
fsl (MPa) fs2 (MPa) Nu (kN) Nu,theo (kN) Nu,theo/NuRef 154× 0×1850
12.0 — — 32.83 — 365 — 656 677 1.03t3-C50-5.7 219× 3.25×1240 8.1
5.7 67.4 32.83 52.58 365 352 2980 2622 0.88t3-C50-7.1 219×
3.25×1550 10.1 7.1 67.4 32.83 52.58 365 352 2810 2574
0.92t3-C50-8.4 219× 3.25×1850 12.0 8.4 67.4 32.83 52.58 365 352
2751 2525 0.92t3-C50-9.8 219× 3.25× 2140 13.9 9.8 67.4 32.83 52.58
365 352 2703 2474 0.92t2-C50-8.4 219×1.80×1850 12.0 8.4 121.7 32.83
52.58 365 390 2319 2621 1.13t4-C50-8.4 219× 3.90×1850 12.0 8.4 56.2
32.83 52.58 365 342 2932 2500 0.85t3-C40-8.4 219× 3.25×1850 12.0
8.4 67.4 32.83 43.01 365 352 2633 2373 0.90t3-C60-8.4 219×
3.25×1850 12.0 8.4 67.4 32.83 57.29 365 352 2845 2600 0.91D1 andD2
are the diameters of unstrengthened and strengthened column; t is
the measured thickness of outer steel tube; L is the length of
column; fcu1 and fcu2are the cubic compressive strengths of
original concrete and SCC; fs1 and fs2 are the tensile strengths of
steel rebar and steel tube; Nu and Nu,theo are theexperimental and
theoretical load-bearing capacities of specimens.
Advances in Civil Engineering 3
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[36]. After the specimens failed, the outer steel tubes werecut
o� and the crush of SCC at midheight was observed.When the SCC
jackets were removed, obvious de�ection oforiginal columns and
small cracks near the midheight wereobserved. It indicated that
CFST jacketing could e�ectivelychange the failure mode of slender
RC columns from brittlefailure to ductile failure. And the failure
at the similar lo-cation of each part (original column, SCC jacket,
and outersteel tube) indicated they worked well together under
theconne of outer CFST jackets.
3.2. Axial Load-Lateral Deection Response. Figure 3 showsthe
typical curves of axial load and lateral de�ection alongheight for
CFST jacketed column. e curves along theheight were basically a
half-sine shape, and the biggestde�ection was obtained at the
midheight. e observablelateral de�ection atNu and unloading to 85%
ofNu indicatedgood ductility of the specimen after CFST
jacketingstrengthening.
Figure 4 shows the axial load (N) versus the lateralde�ection at
midheight (Δ) response. e specimen Refexhibited an almost linear
curve, the load and midheightde�ection at limit state were very low
(656 kN and 0.47mm),whereas the CFST jacketed columns exhibited an
initiallinear elastic phase before the load reached about 80% of
Nu,followed by a curved ascending phase when outer steel tubebegan
yielding and a smooth descending phase after Nu dueto the buckling
of columns. In general, all the CFST jacketedcolumns experienced an
obvious lateral de�ection at muchhigher load-bearing capacity.
Figure 4(a) shows the in�u-ence of L/D ratio on the N-Δ response. e
curves ofspecimens t3-C50-5.7, t3-C50-7.1, and t3-C50-8.4 hada
similar linear phase but specimen t3-C50-9.8 behaved in
a softer manner with a larger de�ection at the same load.
Itmeans that when the L/D ratio exceeds a certain value,
thesecondary moment could a�ect the behavior of CFSTjacketed
columns signicantly. At the limit state, there wasa lager de�ection
at a smaller ultimate strength when the L/Dratio increased from 5.7
to 9.8. Figure 4(b) shows the in-�uence of D/t ratio on the N-Δ
response. e specimen withthicker steel tube attained much higher
ultimate strength.Figure 4(c) shows the in�uences of SCC strength
on the N-Δresponse. All the specimens exhibited similar curves. is
isbecause that the �exural sti�ness correlates with the
elasticmodulus of materials which varies slightly when the SCC
Concrete peel-off
(a)
Local buckling
(b)
Figure 2: Typical failure modes of specimens. (a) Specimen Ref.
(b) Specimen t3-C50-9.8.
0.00
0.25
0.50
0.75
1.00
Rela
tive h
eigh
t
10 20 30 40 500Lateral deflection (mm)
0.74Nu1.00NuUnloading to 0.93Nu
Unloading to 0.85NuHalf sine waves
Figure 3: Typical load-lateral de�ection curves (specimen
t3-C50-8.4).
4 Advances in Civil Engineering
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strength increases from 40 to 60MPa. However, a slightincrease
in ultimate strength was also observed.
3.3. Axial Load-Strain Response. Figure 5 shows the axialload
(N) versus axial strain (εv) of outer steel tube response
atmidheight. e negative strain indicates compression andthe
positive strain indicates tension. Figure 5(a) shows thein�uence of
L/D ratio on the development of axial strainduring loading. In the
initial loading stage, the maximumaxial strain and the minimum
axial strain of all the CFSTjacketed columns were almost the same,
indicating the axialstrains were nearly uniform around the cross
section. Withthe increase of the axial load, the L/D ratio a�ected
thedistribution of axial strain signicantly. For specimens
t3-C50-5.7 and t3-C50-7.1, the maximum axial strain grewmuch faster
than the minimum axial strain when the loadapproached about 90% of
Nu. While for specimens t3-C50-8.4 and t3-C50-9.8, this diversion
happened at about 70%and 35% of Nu, and the minimum axial strain
even changedfrom negative to positive. is nonuniform distribution
ofaxial strain is because of the development of lateral de-�ection
and the consequent secondary moment. Figure 5(b)shows the in�uence
of D/t ratio on the N-εv response. Aftera linear increase of axial
strain around the cross section,specimen t2-C50-8.4 experienced a
curved ascending phaseat about 1600 kN while specimens t3-C50-8.4
and t3-C50-8.4 kept increasing linearly up to about 2500 kN. Figure
5(c)shows the in�uence of SCC strength on the N-εv response.All the
specimens exhibited similar curves with a slightvariation of axial
values at the limit state. e developmentof axial strain for
specimens with di�erent SCC strengthalmost overlapped during the
loading, indicating that theSCC strength did not a�ect the behavior
of the CFST col-umns signicantly.
Figure 6 shows the relationship between the axial load(N) and
hoop strain (εh) of outer steel tube at midheight interms of L/D
ratio, D/t ratio, and SCC strength. In general,the hoop strain near
the compression side was much higherthan that near the opposite
side which even decreasedto zero. is is because the dilation of SCC
and original
concrete was most active in the compression side and thusthe
most e�ective connement was provided there. Asshown in Figure 6(a),
the maximum hoop strain of the outersteel tube showed a decreasing
trend with the increase of L/Dratio, indicating a gradual
decreasing connement. Asshown in Figure 6(b), specimen t2-C50-8.4
experienceda nonlinear increased hoop strain much earlier than
spec-imens t3-C50-8.4 and t4-C50-8.4. As shown in Figure
6(c),specimens with di�erent SCC strength exhibited similarcurves,
and only little di�erences were found after ultimatestrength.
4. Discussion
4.1. Con�nement. e outer steel tube connes the SCC andoriginal
concrete and thus increases their compressivestrengths while the
inner concrete suppresses the inwardbulking of the outer steel
tube.
is interaction enhances the load-bearing capacity,material
utilization, and ductility of the CFST jacketedcolumns. e
transverse deformation coe¤cient of the outersteel tube is adopted
to evaluate the level of connement anddened as εh/εv. Figure 7
shows the development of trans-verse deformation coe¤cient on the
most compressive side(εhmax/εvmax) and the least-compressive side
(εhmin/εvmin). Inthe initial loading stage (before 40% of Nu), the
values ofεhmax/εvmax and εhmin/εvmin ranged from 0.25 to 0.35,
whichwere higher than that of concrete (usually 0.17–0.20).
Itindicated that the connement was negligible in this stagebecause
the dilation of inner concrete was smaller than thatof the steel
tube under the same axial deformation.When theaxial load increased
to 60–90% of Nu, the values ofεhmax/εvmax and εhmin/εvmin for
several specimens increasedsignicantly beyond their initial values,
indicating that theinner concrete was subjected to good connement
becausethe dilation was constrained by the outer steel tube.
FromFigure 7(a), the values of εhmax/εvmax of specimens with
L/Dratio from 5.7 to 9.8 were 0.91, 0.86, 0.64, and 0.66 while
thevalues of εhmin/εvmin were 0.44, 0.32, 0.28, and 0.49 at
theultimate limit state. is decreasing trend indicated theconnement
was less e�ective with the increase of L/D ratio.
5 10 15 20 25 30 350∆ (mm)
0
500
1000
1500
2000
2500
3000N
(kN
)Nu
85%Nu
Reft3-C50-5.7t3-C50-7.1
t3-C50-8.4t3-C50-9.8
(a)
Reft2-C50-8.4
t3-C50-8.4t4-C50-8.4
Nu85%Nu
0
500
1000
1500
2000
2500
3000
N (k
N)
5 10 15 20 25 30 350∆ (mm)
(b)
Reft3-C40-8.4
t3-C50-8.4t3-C60-8.4
Nu85%Nu
0
500
1000
1500
2000
2500
3000
N (k
N)
5 10 15 20 25 30 350∆ (mm)
(c)
Figure 4: Axial load-lateral de�ection response. (a) L/D ratio.
(b) D/t ratio. (c) SCC strength.
Advances in Civil Engineering 5
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It should be mentioned that although the value of εhmin/εvminof
specimen t3-C50-9.8 was 0.49, it seemed that the con-nement of the
least-compressive side was more e�ectivethan other specimens. In
fact, the value of εhmin/εvmin forspecimen t3-C50-9.8 was kept at
0.35 between 80 and 99.5%ofNu, indicating a negligible connement.
From Figure 7(b),on the most compressive side, the values of
εhmax/εvmax forspecimens t2-C50-8.4, t3-C50-8.4, and t4-C50-8.4
were 0.53,0.64 and 0.54 at Nu, indicating e�ective connement.
Whileon the least-compressive side, the values of εhmin/εvmin
were0.39, 0.28, and 0.47. It means that the connement wasnegligible
for specimens t2-C50-8.4 and t3-C50-8.4, but theconnement was
e�ective for specimen t4-C50-8.4. FromFigure 7(c), the curves of
specimens with di�erent SCCstrength agreed quite closely with each
other except that
stage after 85% of Nu, the εhmax/εvmax and εhmin/εvmin
ofspecimen t3-C60-8.4 increased at a faster rate. It may bebecause
that higher strength SCC is prone to formingsplitting cracks and
therefore, the circumferential stress inthe steel tube increased
rapidly.
4.2. Load-Bearing Capacity. e load-bearing capacities(Nu) of
specimens are shown in Table 1 and compared inFigure 8 in terms of
L/D ratio, D/t ratio, and SCC strength.
e Nu of t3-C50-8.4 was 2751 kN, which was 4.06 timesNu,theo
(calculated by equations in [31]) and 4.19 times Nu ofspecimen Ref.
is signicant enhancement indicated thatthe CFST jacketing method
was e�ective to improve theload-bearing capacity of slender RC
columns under axial
0500
10001500200025003000
N (k
N)
0 2000 4000 6000
t3-C50-5.7 Hoop maxt3-C50-7.1 Hoop maxt3-C50-8.4 Hoop
maxt3-C50-9.8 Hoop maxt3-C50-5.7 Hoop mint3-C50-7.1 Hoop
mint3-C50-8.4 Hoop mint3-C50-9.8 Hoop min
8000 10000–2000εh(με)
(a)
t2-C50-8.4 Hoop maxt3-C50-8.4 Hoop maxt4-C50-8.4 Hoop
maxt2-C50-8.4 Hoop mint3-C50-8.4 Hoop mint4-C50-8.4 Hoop min
0 2000 4000 6000 8000 10000–2000εh(με)
0500
10001500200025003000
N (k
N)
(b)
t3-C40-8.4 Hoop maxt3-C50-8.4 Hoop maxt3-C60-8.4 Hoop
maxt3-C40-8.4 Hoop mint3-C50-8.4 Hoop mint3-C60-8.4 Hoop min
0 2000 4000 6000 8000 10000–2000εh(με)
0500
10001500200025003000
N (k
N)
(c)
Figure 6: Axial load-hoop strain of outer steel tube response.
(a) L/D ratio. (b) D/t ratio. (c) SCC strength.
–10000
t3-C50-5.7 Axial maxt3-C50-7.1 Axial maxt3-C50-8.4 Axial
maxt3-C50-9.8 Axial maxt3-C50-5.7 Axial mint3-C50-7.1 Axial
mint3-C50-8.4 Axial mint3-C50-9.8 Axial min
–5000 0 5000–15000εv(με)
0
500
1000
1500
2000
2500
3000N
(kN
)
(a)
t2-C50-8.4 Axial maxt3-C50-8.4 Axial maxt4-C50-8.4 Axial
maxt2-C50-8.4 Axial mint3-C50-8.4 Axial mint4-C50-8.4 Axial min
–10000 –5000 0 5000–15000εv(με)
0
500
1000
1500
2000
2500
3000
N (k
N)
(b)
t3-C40-8.4 Axial maxt3-C50-8.4 Axial maxt3-C60-8.4 Axial
maxt3-C40-8.4 Axial mint3-C50-8.4 Axial mint3-C60-8.4 Axial min
–10000 –5000 0 5000–15000εv(με)
0
500
1000
1500
2000
2500
3000
N (k
N)
(c)
Figure 5: Axial load-axial strain of outer steel tube response.
(a) L/D ratio. (b) D/t ratio. (c) SCC strength.
6 Advances in Civil Engineering
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compression. On the other hand, the load-bearing
capacitydecreased progressively with larger L/D ratio. e Nu
ofspecimens t3-C50-7.1, t3-C50-8.4, and t3-C50-9.8 were5.7%, 7.7%,
and 9.3% lower than that of specimen t3-C50-5.7, respectively. e
load-bearing capacity decreased sig-nicantly with larger D/t ratio.
It means that the load-bearing capacity decreased signicantly with
decreasing ofsteel CFST jacket thickness with the constant external
di-ameter. e Nu of specimen t3-C50-8.4 and specimen t2-C50-8.4 were
6.2% and 20.9% lower than that of speciment4-C50-8.4.e load-bearing
capacity increased slightly withhigher SCC strength. e Nu of
specimen t3-C50-8.4 andspecimen t3-C60-8.4 were 4.5% and 8.1%
higher than that ofspecimen t3-C40-8.4. e load-bearing capacity
increasedwith higher concrete strength as expected.
4.3. Strength Index. e utilization of the full plastic
com-pressive resistance of a CFST column can be assessedthrough its
strength index (SI) [8, 37]. Similarly, for CFST
jacketed columns, SI is adopted to evaluate the e�ectivenessof
material utilization and dened as
SI �Nu
fs1As1 + fs2As2 + fc1Ac1 + fc2Ac2, (1)
where fc1 (� 0.80 fcu1) and fc2 (� 0.80 fcu2) are the
compressivestrengths of original concrete and SCC;As1,As2,Ac1,
andAc2are the cross-sectional areas of steel rebar, outer steel
tube,original concrete, and SCC, respectively.
For specimen Ref, SI can also be calculated usingEquation (1)
when taking As2 and Ac2 as zero.
e SI of specimen Ref was only 0.89 while that ofspecimen
t3-C50-8.4 was 1.24. It indicated that the slenderRC column did not
take full use of steel rebar and originalconcrete but the CFST
jacketed column exhibited 124%utilization of materials.e 24%
increase can be explained asfollows:
(i) Under axial compression, the original concrete andSCC dilate
laterally with the increase of load.However, the original concrete
is conned by the
2980 2810 2751 2703
0
1000
2000
3000
4000
Nu (
kN)
6 7 8 9 10 115L/D
(a)
2932 27512391
50 75 100 125 15025D/t
0
1000
2000
3000
4000
Nu (
kN)
(b)
2633 2751 2845
40 45 50 55 60 6535SCC strength (MPa)
0
1000
2000
3000
4000
Nu (
kN)
(c)
Figure 8: Load-bearing capacity of specimens. (a) L/D ratio. (b)
D/t ratio. (c) SCC strength.
0.2 0.4 0.6 0.8 1.0
t3-C50-5.7 most-compressed sidet3-C50-7.1 most-compressed
sidet3-C50-8.4 most-compressed sidet3-C50-9.8 most-compressed
sidet3-C50-5.7 least-compressed sidet3-C50-7.1 least-compressed
sidet3-C50-8.4 least-compressed sidet3-C50-9.8 least-compressed
side
1.2 1.40.0εh/εv
0.0
0.2
0.4
0.6
0.8
1.0
N/N
u
(a)
t2-C50-8.4 most-compressive sidet3-C50-8.4 most-compressive
sidet4-C50-8.4 most-compressive sidet2-C50-8.4 least-compressive
sidet3-C50-8.4 least-compressive sidet4-C50-8.4 least-compressive
side
0.2 0.4 0.6 0.8 1.0 1.2 1.40.0εh/εv
0.0
0.2
0.4
0.6
0.8
1.0
N/N
u
(b)
t3-C40-8.4 most-compressive sidet3-C50-8.4 most-compressive
sidet3-C60-8.4 most-compressive sidet3-C40-8.4 least-compressive
sidet3-C50-8.4 least-compressive sidet3-C60-8.4 least-compressive
side
0.2 0.4 0.6 0.8 1.0 1.2 1.40.0εh/εv
0.0
0.2
0.4
0.6
0.8
1.0
N/N
u
(c)
Figure 7: Normalised load-transverse deformation coe¤cient
response. (a) L/D ratio. (b) D/t ratio. (c) SCC strength.
Advances in Civil Engineering 7
-
SCC jacket and the outer steel tube while the SCC isconned by
the outer steel tube. It means that boththe original concrete and
SCC are under triaxialcompression. e compressive strength of
concreteunder triaxial stress (fcc) is higher than
concretecompressive strength without connement (fc), andit can be
written as [31]
fcc � fc +(4.5 ∼ 7.0)fL, (2)
where fL is the radial stress.
(ii) e existence of original concrete and SCC couldavoid the
inward local buckling of the outer steeltube and thus the material
properties are betterexploited.
is behavior of CFST jacketed columns also indicatesthat the
signicant increase in the load-bearing capacity isnot only due to
enlargement and additional steel re-inforcement in cross section
but also because of steel jacket’sconnement.
Figure 9 compares the e�ects of L/D ratio, D/t ratio, andSCC
strength on the SI of specimens. e SI decreasedgradually with
larger L/D ratio. e SI of specimens t3-C50-7.1, t3-C50-8.4, and
t3-C50-9.8 were 5.2%, 7.5%, and 9.0%lower than that of t3-C50-5.7,
respectively. e SI decreasedsignicantly with larger D/t ratio. e SI
of specimens t3-C50-8.4 and t2-C50-8.4 were 6.8% and 24.1% lower
than thatof t4-C50-8.4. But the SCC strength showed little
in�uenceon the SI of specimens. e SI of specimens t3-C50-8.4
andt3-C60-8.4 were only 1.6% and 0.8% lower than that of
t3-C40-8.4.
4.4. Ductility. To qualify the ductility of a column,
ductilityindex (DI) is often adopted by many researchers [8,
38].Similarly, for CFST jacketed columns, DI is dened as
DI �Δ85%Δu
, (3)
where Δ85% is the midheight de�ection when the load dropsto 85%
of the ultimate load on the unloading branch and Δuis the midheight
de�ection at the ultimate load.
e DI of specimen Ref was zero because there was nodescending
branch in the N-Δ curve while that of speciment3-C50-8.4 was 3.48.
It indicated that the CFST jacketingmethod was e�ective to improve
the ductility of slender RCcolumns. Figure 10 shows the e�ects of
L/D ratio, D/t ratio,and SCC strength on the DI of specimens. It
should bementioned that value of DI was not available for
speciment3-C50-7.1 due to the test having not been continued
forsu¤cient deformation for the load to reduce to 85% of
theultimate load. Although there was �uctuation in the com-parison
of DI in terms of L/D ratio, the trend of decreasingDI with
increasing slenderness might be clearly observed bylinear tting the
experimental data. On the other hand, it wasobserved that there was
a signicant reduction in ductilitywith increasing D/t ratio and
that the SCC strength did not
have a large in�uence on the ductility of specimens. e DIof
specimens t3-C50-8.4 and t2-C50-8.4 were 38.9% and48.2% lower than
that of t4-C50-8.4.e DI of specimens t3-C50-8.4 and t3-C60-8.4 were
1.8% and 4.4% higher than thatof t3-C40-8.4.
4.5. Load-Bearing Capacity Prediction. Because the innerconcrete
and SCC worked well together under the con-nement of outer steel
tube, the CFST jacketed slendercolumn failed similarly as the
traditional CFST slendercolumn.
us the load-bearing capacity prediction model for thetraditional
slender column may be also applied to CFSTjacketed slender column
when considering the di�erentcompressive strengths of original
concrete and SCC and thecontribution of longitudinal steel rebar.
With reference toChinese code GB 50936-2014 [39], a modied formula
isproposed to predict the load-bearing capacity for CFSTjacketed
slender column:
1.34 1.27 1.24 1.22
6 7 8 9 10 115L/D
0.00
0.25
0.50
0.75
1.00
1.25
1.50
SI
(a)
1.331.24
1.01
50 75 100 125 15025D/t
0.00
0.25
0.50
0.75
1.00
1.25
1.50
SI
(b)
1.26 1.24 1.25
40 45 50 55 60 6535SCC strength (MPa)
0.00
0.25
0.50
0.75
1.00
1.25
1.50
SI
(c)
Figure 9: SI of specimens. (a) L/D ratio. (b) D/t ratio. (c)
SCCstrength.
8 Advances in Civil Engineering
-
Nu,theo � φ fscAsc + fs1As1( ), (4)
where φ is the slenderness reduction factor, fsc is
thecompressive strength of CFST jacketed column, Asc is thesum of
the area of original concrete, SCC and outer steeltube.
fsc can be written as
fsc � 1.212 +0.176fs2
213 + 0.974( )ξ +−0.104fc
14.4 + 0.031( )ξ2[ ]fc,
(5)
where ξ is the connement index and fc is the averageconcrete
strength of original concrete and SCC, which aredened as
ξ �As2fs2
Ac1 + Ac2( )fc,
fc �Ac1fc1 + Ac2fc2Ac1 + Ac2
.
(6)
φ can be calculated by a modied formula for traditionalCFST
slender column:
φ �1
2λ2sc1λ2sc1 + 0.95 + 0.5λsc1( )
·��������������������������λ2sc1 + 0.95 + 0.5λsc1( )( )
2− 4λ2sc1
√,
(7)
where λsc1 is the regular slenderness ratio, which is dened
as
λsc1 � 0.01λsc 0.001fs2 + 0.781( ),
λsc �μli,
i �
���IscAsc
√
�D24,
(8)
where λsc is the slenderness ratio and μl and i are the
cal-culating length and radius of gyration of CFST
jacketedcolumn.
Table 1 compares the predicted strength (Nu,theo) withthe
experimental results (Nu) and the ratio Nu,theo/Nu is 0.93
with a coe¤cient of variation (CV) of 0.09. e goodagreement
indicates the accuracy of the proposed model inpredicting the
load-bearing capacity for CFST jacketedslender column.
5. Conclusions
is paper presents an experiment of slender circular RCcolumns
strengthened with CFST jacketing. Nine slenderspecimens were tested
under axial compression, and thefollowing conclusions can be drawn
within the scope of thisstudy:
(1) e CFST jacketed columns exhibited good ductilebehavior under
axial compression, and all experi-enced ductile failure mode
(global bending), whilethe slender RC columns showed a brittle
failuremode (concrete peel-o�).
(2) e outer steel tube provided e�ective connementon the SCC and
original slender columns, and thusenhanced the load-bearing
capacity, material utili-zation, and ductility of slender RC
columns. e L/Dratio and D/t ratio showed obvious in�uence on
theperformance of CFST jacketed columns. e Nu, SI,and DI decreased
9.3%, 9.0%, and 51.9% when theL/D ratio increased from 5.7 to 9.8
and dropped20.9%, 24.1%, and 48.2% when the D/t ratio in-creased
from 56.2 to 121.7.
(3) e SCC strength had a slight e�ect on the per-formance of
CFST jacketed columns, in which thespecimen t3-C60-8.4 showed 8.1%,
−0.8% and 4.4%increase in the Nu, SI, and DI compared to
speciment3-C40-8.4.
(4) A modied model was proposed to predict the load-bearing
capacity for CFST jacketed slender columnsbased on the model for
traditional CFST columns.Comparison between the prediction and the
ex-perimental results showed good agreement.
It is worth mentioning that the CFST jacketed column inpractice
may not behave similarly to specimens in this studybecause the
external steel jacket may be discontinuous at thecolumn top and
bottom. For the CFST jacketed specimenunder compression over the
existing column and retrot
8.77
4.22
3.48
6 7 8 9 10 115L/D
0
2
4
6
8
10D
I
(a)
5.70
3.48
2.95
50 75 100 125 15025D/t
0
2
4
6
8
10
DI
(b)
3.42 3.48 3.57
40 45 50 55 60 6535SCC strength (MPa)
0
2
4
6
8
10
DI
(c)
Figure 10: DI of specimens. (a) L/D ratio. (b) D/t ratio. (c)
SCC strength.
Advances in Civil Engineering 9
-
area, the elastic stiffness increased more than 100% andstrength
increased up to 30% compared to the specimencompressed on the
existing column only [24]. 'e structuralsteel collars were placed
around the gaps at the top andbottom of the column and tied to the
adjacent elements withpostinstalled anchors aiming to increase
shear strength lo-cally [40]. 'e results showed this technique can
transfer thecolumn shears to the footing and adjacent elements and
thusenhance the blast resistance to bridge columns
seismicallyretrofitted using steel jackets. 'erefore, the
additionalstrengthening technique should be applied at the
bottomand top of the column, such as structural steel collars,
totransfer the whole axial load and shear force over the
entirecross section. Otherwise, a relative reduction
coefficientshould be carefully considered before the application
ofCFST jacketing.
Conflicts of Interest
'e authors declare that they have no conflicts of interest.
Acknowledgments
'e authors are grateful for the financial supports providedby
the Natural Science Foundation of China (51678456), theFundamental
Research Funds for the Central Universities ofChina
(2015210020201), and the Hubei Provincial NaturalScience Foundation
of China (2016CFB271).
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