Experimental Study on CFRP-Confined Circularized Concrete ...Nov 17, 2020  · tested under axial compression using a pressure testing ... Stress-Strain Models of CFSST Columns 4.1.

Research ArticleExperimental Study on CFRP-Confined CircularizedConcrete-Filled Square Steel Tube Short Columns

Yuchuan Wen Zhongjun Hu Anningjing Li Quanheng Li Xuepeng Li and Yan Xu

College of Construction Engineering Jilin University Changchun 130026 China

Correspondence should be addressed to Zhongjun Hu huzjjlueducn

Received 17 November 2020 Revised 17 December 2020 Accepted 4 March 2021 Published 17 March 2021

Academic Editor Peng Zhang

Copyright copy 2021 YuchuanWen et alis is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

is study investigates the suitability of the circularization technique for strengthening square concrete-filled square steel tube(CFSST) short columns A total of 16 specimens were tested under axial compression e main parameters under investigationwere concrete strength the thickness of arc cement mortar layer components (CAM) and the layers of carbon fiber-reinforcedpolymer (CFRP) sheets Test results indicated that the failure mode of CFRP-confined circularized CFSST (C-C-CFSST) columnswas similar to that of CFRP-confined concrete columns e CFRP-confined circularized strengthening method can increaseconfinement efficacy and reduce the stress concentration at the corners of CFSSTcolumnsree existing CFRP-confined concretestress-strain models were evaluated using the test results e predictions of the Lam and Teng stress-strain model agree well withthe test data

1 Introduction

Concrete-filled steel tube (CFST) has the advantages of highbearing capacity and ductility and is widely used in civilstructures [1 2] However the exposed CFST structures areeasily failed by corrosion in engineering practice especiallyin the moisture environment [3ndash5] When the corrosionoccurs on steel tube the durability and bearing capacity ofCFST will degrade [6] Consequently finding a method toenhance the corrosion resistance of CFST columns is nec-essary [7]

Fiber-reinforced polymer (FRP) composites have beenwidely used in retrofitting existing columns [8 9] FRP-confined concrete has been proven to be feasible in theoreticalresearch and engineering practice [10] Strengthening CFSTcolumns with FRP material has the dual advantages of im-proving bearing capacity and durability [11ndash13]

In recent years concrete-filled square steel tube (CFSST)has been increasingly applied in various building structuresbecause of its advantages of easy joint connection andconstruction [14ndash16]

Tao Han and Wang studied the section shape influenceon the axial compression performance of CFRP-confined

CFST short columns e results indicated that the CFRPconfinement efficacy of a CFSST column is lower than acircular CFST column because of the stress concentration atthe corners of the square steel tube and the reduction of theeffective area of the confined section [17 18]

To improve the confinement efficiency of CFSST col-umns by CFRP jackets and to reduce stress concentrationthe circularizing technique has been proven to be an effectivemethod before FRP wrapping by some scholars e ex-perimental study of Priestley and Seible first indicated thatshape modification by bonding concrete segments canimprove the confinement efficiency of CFRP jackets [19]Hadi et al carried out relevant research on the CFRP-confined arc-treated concrete rectangular columns the re-sults show that using precast concrete arc-treated compo-nents as transitions between CFRP and rectangular columnscould significantly reduce stress concentration and enhancethe effective constraint area of the cross section [20 21] eabove studies showed that circularizing concrete columns bybonding precast segments can increase the axial load ca-pacity and change the stress-strain curve from softening tohardening the branch of reinforced concrete (RC) columns[20]

HindawiAdvances in Materials Science and EngineeringVolume 2021 Article ID 6620577 13 pageshttpsdoiorg10115520216620577

Yang et al performed an experimental study on the axialcompressive performance of rectangular concrete-filledFRP-steel composite tube columns for various corner ra-diuses and proposed that FRP-confined CFSST concretestress-strain curve can be divided into four phases (ieinitial linear phase transition to yield phase hardeningphase and residual phase) Different corner radii signifi-cantly affect the confinement effectiveness and the thirdphase of the FRP-confined CFSST stress-strain curve [22]Most of the existing CFSST columns have sharp cornerswhich cause stress concentration and the CFSST columnscannot round the corners similar to RC columns in practicalengineering erefore shape modification before FRPwrapping may effectively reduce stress concentration Tosolve this problem further an experimental study was un-dertaken to investigate the suitability of the circularizationtechnique for strengthening CFSST short columns isstudy utilizes the bonding CAM components between FRPand the CFSST column as a circularizing method Sixteenspecimens were tested under axial compression to study theinfluence of different CAM thicknesses (the middle height ofCAM component) layers of CFRP and concrete strength onbearing capacity deformation performance and stress-strain relationship

2 Experimental Program

21 Test Specimens All of the specimens were100mmtimes 100mm in cross section and 300mm in heightTwelve specimens were C-C-CFSSTcolumns two specimenswere CFRP-confined CFSST columns and two specimenswere CFSST columns Sixteen specimens were divided intotwo groups according to the concrete strength e speci-mens of group 1 were C20 and those of group 2 were C30Each group had six C-C-CFSST specimens with 5 10 and15mm CAM thickness e corresponding radius ofrounded corners was 20 30 and 40mm T700 CFRP sheetswith 172 elongation were used in this test e perfor-mances of steel tubes epoxy adhesive and CFRP sheets weretested in a structural laboratory as shown in Table 1e testspecimens are listed in Table 2 e cross section and FRPbonding position of specimens are shown in Figure 1

e concrete specimens were prepared with Portlandcement e diameter of the coarse aggregate was 5ndash10mmwhich was used for fine aggregate in continuous grading andmedium sand e axial compressive strength measuredvalues of two groupsrsquo specimens were 207 and 271MPa

e main processes of specimen preparation are as follows(1) PrefabricatedCAMwas configuredwith a highmark cementmortar and the CAM mold was made with a PVC tube and aplate e specimen and the CAM should have similar strengthvalues to satisfy the equal-strength principle e test usingCAM strength was slightly higher than the specimen (2) eepoxy resin was smeared on the surface of the steel tube and theCAM was bonded on corresponding positions (3) e CAMcomponents were fixed to dry using adjustable circular steelrings for 48h (4) e specimens were wrapped with CFRPsheets after circularizing e overlap length of the CFRP was100mmemaking process of specimen is shown in Figure 2

22 Test Setup and Instrumentation All specimens weretested under axial compression using a pressure testingmachine with 2000 kN capacity Four axial strain gauges andfour hoop strain gauges were pasted on the mid-height of thespecimen Four linear variable differential transducers(LVDTs) were used to monitor the axial deformation ofspecimens e LVDTs were installed at the corner of thespecimen and covered the mid-height of the specimen elayout of the test setup and measuring point are shown inFigure 3e loading programwas based on standard for testmethod of concrete structures (GBT 50152ndash2012) [23] Toavoid the premature failure of specimens the top andbottom ends of specimens were wrapped with additional twolayers of CFRP with 50mm width

3 Test Results and Discussion

31 Main Test Results e key test results of axial com-pression specimens are shown in Table 3 fc0 is the com-pressive strength of unconfined concrete columns fco

prime is thecompressive strength calculated value of CFSST andN is theultimate bearing capacity of specimens

32 Test Failure Modes e typical failure mode of speci-mens is shown in Figure 4e tested CFRP-confined CFSSTcolumns failed by CFRP jacket rupture near the cornersese ruptures occurred in the mid-height region of all thespecimens e failure mode of C-C-CFSST columns wassimilar to CFRP-confined concrete columns

e specific breakpoint locations of test specimens afterloading are shown in Figure 5 L is the horizontal distancefrom the breakpoint to the corner e CFRP-confinedCFSST column failed by CFRP rupture at the corner of thesteel tube because of stress concentration e CFRPbreakpoints of C-C-CFSSTcolumns occurred away from thecorner when CAM thickness increases from 5mm to 15mme changing position of CFRP breakpoints showed that thestress concentration of the steel tube corner gradually re-duced with the increasing CAM thickness

33 Load-Strain Response Figure 6 shows the load-straincurves of C-C-CFSST specimens (group 2) e axial andhoop strains were obtained by the average of four axial straingauges and four hoop strain gauges respectively Forspecimens C30-5-1-11 C30-10-1-13 and C30-15-1-15 theultimate load is 7383 8439 and 8934 kN respectively Forspecimens C30-5-2-12 C30-10-2-4 and C30-15-2-16 theultimate load is 8671 9503 and 10698 kN respectivelySimilar to those FRP-confined concrete load-strain curvesall specimensrsquo curves showed the same trend with a bilinearshape and a monotonically ascending characteristic Whenthe load was less than 80 of the ultimate load the axial andhoop strain developed slowly When the load was more than80 of the ultimate load the curves came into the plasticstage and the deformation grew rapidly e hoop rupturefailure of CFRP occurred when reaching the ultimate loadsof specimens and the ultimate hoop strain slightly increasedwith the increasing CAM thickness

2 Advances in Materials Science and Engineering

Table 1 Material properties

Materials ickness (mm) Yield strength (MPa) Tensile strength (MPa) Tensile elastic modulus (MPa)Steel tube 2 3139 3926 207times105

CFRP 0167 mdash 3094 244times105

Epoxy resin mdash mdash 58 2584times103

Table 2 Test parameters of specimens

Specimen CAM thickness Confinement conditionC20-0-0-1 mdash mdashC20-0-1-2 mdash 1-layer CFRP confinementC20-5-1-3 5mm 1-layer CFRP confinementC20-5-2-4 5mm 2-layer CFRP confinementC20-10-1-5 10mm 1-layer CFRP confinementC20-10-2-6 10mm 2-layer CFRP confinementC20-15-1-7 15mm 1-layer CFRP confinementC20-15-2-8 15mm 2-layer CFRP confinementC30-0-0-9 mdash mdashC30-0-1-10 mdash 1-layer CFRP confinementC30-5-1-11 5mm 1-layer CFRP confinementC30-5-2-12 5mm 2-layer CFRP confinementC30-10-1-13 10mm 1-layer CFRP confinementC30-10-2-14 10mm 2-layer CFRP confinementC30-15-1-15 15mm 1-layer CFRP confinementC30-15-2-16 15mm 2-layer CFRP confinement

ConcreteR = 6

R = 4

100

100

(a)

Square steel tube CFRP sheet

(b)

CAM

(c)

Figure 1 Cross sections of specimens (a) CFSST column (b) CFRP-confined CFSST column (c) C-C-CFSST column

(a) (b) (c) (d)

Figure 2 Production process of specimens (a) CFSST column (b) CAM bonding (c) CAM fixing (d) CFRP wrapping

Advances in Materials Science and Engineering 3

34 CAM Influence on Bearing Capacity and VerticalDisplacement Figures 7 and 8 show vertical displacement andbearing capacity of test specimens with different CAM thick-nessWhen the CAM thickness increases from 5mm to 15mmthe bearing capacity and vertical displacement increase grad-ually Compared with C20-0-1-2 the bearing capacity of C20-5-1-3 C20-10-1-5 and C20-15-1-7 increased by 199 402and 475 respectively Compared with C30-0-1-10 thebearing capacity of C30-5-1-11 C30-10-1-13 and C30-15-1-15increased by 169 336 and 414 respectively e in-creasing bearing capacity and vertical displacement showed thegreat effectiveness of CAM between CFRP sheets and CFSSTcolumns e increase of the CAM thickness generally leads toan increase in the bearing capacity and vertical displacementwhich indicated that the circularization technique forstrengthening CFSSTshort columns is a suitable and alternativestrengthening method in engineering

4 Stress-Strain Models of CFSST Columns

41 Axial Stress-Strain Curves e axial stress-axial straincurves of all the test specimens are shown in Figure 9 All thecurves had an obvious bilinear shape with two segments efirst-segment slope of the curve was much bigger than the

second-segment slope e CAM thickness affected mainly thesecond segment of the stress-strain curve e second-segmentslopes of C-C-CFSSTspecimens became slightly larger with theincrease of CAM thickness e ultimate axial stress fcc

prime wasaffected by the CAM thickness and the layers of CFRP As forspecimens with no CAM fcc

prime and the second-segment slopewere the smalleste stress-strain curve of C20-5-1-3 andC30-5-1-11 was close to the stress-strain curve of C20-0-1-2 andC30-0-1-11 respectively showing that CFRP wrapping was lesseffective for specimens with a CAM thickness of 5mm especimens with a CAM thickness of 10 or 15mm increased theeffectiveness of CFRP confinement To ensure the strengtheningeffectiveness in practical engineering the CAM thicknessshould be large e layers of CFRP affected fcc

prime and ductilitye ultimate axial strain εcc

prime and ultimate axial stress fccprime of

specimens with two layers of CFRP were significantly largerthan those of specimens with one layer of CFRP

42 Existing Stress-Strain Models e existing FRP-con-fined concrete stress-strain models are mainly separatedinto two types e first type uses a single function toexpress the stress-strain relationship and includes theMander model Samaan model Yu model and Yang andFeng model [24ndash27] e second type uses piecewisefunction to express the stress-strain relationship andincludes the Lam and Teng model Lai model Miyauchimodel and Wei and Wu model [28ndash34] Among allexisting models the Lam and Teng model Lai model andYang model are appropriate to predict the stress-strainrelationship of CFRP-confined circularized concretecolumns according to the published literature [8 27]

421 Lam and Teng Model e first segment of the Lamand Teng model is a parabolic type and the second segmentis a linear type is model has a high degree of accuracy inpredicting FRP-confined concrete strength e model isdescribed by the following equation

σc Ecεc minusEc minus E2( 1113857

4f0ε2c 0le εc le ε( 1113857t

σc fcoprime + E2εc εt le εc le εcu( 1113857

⎧⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎩

(1)

where σc and εc are the axial stress and axial strain re-spectively e ultimate axial stress and the ultimate strainare calculated as follows

fccprime

fcoprime

1 + 33k1fl

fcoprime

εcu

εco

175 + 12ks

fl

fco

εru

εco

1113888 1113889

045

(2)

where εru is the confinement effectiveness coefficient k1 andtransition strain εt is given as follows

LVD

T4

LVD

T2

LVD

T3

300m

m

LVD

T1

Straingauges

500mm

15mmthick steel

loadingplate

Figure 3 Loading apparatus and LVDTs

Table 3 Experimental results of specimens

Specimen fc0 (MPa) fcoprime (MPa) N (kN)

C20-0-0-1 207 4582 4880C20-0-1-2 207 4582 5427C20-5-1-3 207 4582 6505C20-5-2-4 207 4582 7650C20-10-1-5 207 4582 7600C20-10-2-6 207 4582 8756C20-15-1-7 207 4582 8004C20-15-2-8 207 4582 10193C30-0-0-9 271 5339 5399C30-0-1-10 271 5339 6316C30-5-1-11 271 5339 7383C30-5-2-12 271 5339 8671C30-10-1-13 271 5339 8439C30-10-2-14 271 5339 9503C30-15-1-15 271 5339 8934C30-15-2-16 271 5339 10698

4 Advances in Materials Science and Engineering

C30-5-1-11C30-10-1-13C30-15-1-15

N (kN)

0

200

400

600

800

1000

ndash4000 ndash2000 0 2000 4000 6000ndash6000

εc εh

(a)

C30-5-2-12C30-10-2-14C30-15-2-16

N (kN)

0

200

400

600

800

1000

1200

ndash6000 ndash3000 0 3000 6000 9000ndash9000

εc εh

(b)

Figure 6 Test load-strain curves (a) Specimens warped with one-layer CFRP (b) Specimens warped with two-layer CFRP

Figure 4 Failure mode of some specimens

1

14

10

15

22

35

30

3

10

1820

26 25

20

0

10

20

30

40L

(mm

)

C30-

15-2

-16

C30-

10-2

-14

C30-

15-1

-15

C30-

5-2-

12

C30-

10-1

-13

C30-

0-1-

10

C30-

5-1-

11

C20-

15-1

-7

C20-

15-2

-8

C20-

10-1

-5

C20-

10-2

-6

C20-

5-1-

3

C20-

5-2-

4

C20-

0-1-

2

Specimen designation

Figure 5 Influence of CAM thickness on CFRP breakpoint location

Advances in Materials Science and Engineering 5

ks 1 minus(b minus 2r)

2+(h minus 2r)

2

3Acor

εt 2f0

Ec minus E2

(3)

where Ec 4730

fcoprime

1113969

is the elastic modulus of the uncon-fined concrete E2 (fcc

prime minus f0)εcu is the slope of the linearsecond portion εco 0002 is the axial strain with ultimateload and ks is the constraint coefficient

422 Lai Model e Lai model has a high accuracy topredict the CFRP-confined rectangular concrete columnwith corner radii which is described by the followingequation

σz εz

A + Bεz + Cε2z 0le εz le εzb

σz σzb + E2 εz minus εzb( 1113857 εzb le εz le εzc

⎧⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎩

(4)

A B and C are given as follows

A 1

E0

B 1

Ep

minus2

E0+

1Ep

E2

Ep

1113888 1113889

C 1

E0minus

1Ep

E2

Ep

1113888 11138891ε2t

⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

(5)

where Ep σzbεzbFRP-confined concrete E2Ec in different sections is

given as follows

E2

Ec

00331 ln βj1113872 1113873 minus 00564 βj ge 56

01217 ln βj1113872 1113873 minus 02091 βj lt 56

⎧⎪⎪⎨

⎪⎪⎩(6)

Constraint stiffness βj is given as follows

βj Eftf

fcR (7)

where R represents the radii of equivalent circle 2(b+ h)2π

Transition stress and strain are given as followsσzb

fcm

00568β046j + 1

εt

εcu

0011βj + 1

(8)

423 Yang and Feng Model e Yang and Feng model isdifferent from the two above models which have no obvioustransition segment is model has high accuracy to predictthe CFRP-confined concrete column which is given asfollows

σc εcεlowastcc( 1113857flowastccr

r minus 1 + εcεlowastcc( 1113857r

r Ec

Ec minus flowastccεcc( 1113857

flowastcc

εcc

1 + 333σl

fcoprime1113888 1113889

09

εlowastcc

εcc

1 + 174σl

fcoprime

1113888 1113889

107

(9)

C20-1C20-2

C30-1C30-2

40

45

50

55

60

65D

ispla

cem

ent (

mm

)

6 8 10 12 14 164Thickness of CAM (mm)

Figure 7 e effects of CAM thickness on vertical displacement

C20-1C20-2

C30-1C30-2

600

700

800

900

1000

1100

Bear

ing

capa

city

(kN

)

6 8 10 12 14 164Thickness of CAM (mm)

Figure 8 e effects of CAM thickness on bearing capacity

6 Advances in Materials Science and Engineering

where flowastcc and εlowastcc are the peak axial stress and correspondingaxial strain of concrete under a specific level of lateralconfining stress

43CFSSTColumnStrengthfcoprime CFSSTcolumn strength fco

primeis given in technical code for concrete filled steel tubularstructures (GB50936-2014) [35] It is calculated as follows

fcoprime 1212 + Bθsc + Cθ2sc1113872 1113873fco

B 0131fy

235+ 0723

C minus007fck

1441113888 1113889 + 0026

θsc αsc

f

fc

αsc As

Ac

(10)

where As and Ac are the steel tube area and core concretearea respectively fy and f are the tensile strength standardvalue and design value respectively αsc and θsc are the steelratio and confinement coefficient of the specimen respec-tively B and C are the influence coefficients

44 Effective CFRP Confinement Coefficient ke e theo-retical fracture strain εfu can be calculated as fiber strengthdivided by the elastic modulus However the actual fracturestrain εru is much smaller than the theoretical fracture strain

Lam and Teng suggested that εru can be calculated from εfu

as follows

εru keεfu (11)

where the effective CFRP confinement coefficient k of thecircular column is approximately 0586 [15] and was taken as05 053 and 056 corresponding with CAM thickness of 510 and 15mm respectively according to the test results

45 Intercept of the StressAxis by theLinear SecondPortionf0Lam and Teng showed f0fco

prime 109 from the test andsuggested that f0 fco

prime approximately [15] However thismethod ignores the influence of FRP confinement eintercept f0 was affected by the confinement effectivenesswhich can be calculated by confinement stiffness ratio βj andconfining factor ξ Yu [26] suggested that f0 can be cal-culated as follows

f0 (1 + 11ξ)fcprime (12)

However for FRP-confined CFSST columns confiningfactor ξ should value FRP and steel tube confining factorand f0 is given as follows

f0

frsquoco

1 + kξ (13)

where k is modified based on the test results e linearfitting between f0fco

prime and ξ is shown in Figure 10 andk 004854

46 Stress-Strain Model Verification e experimentalstress-strain curves of 12 C-C-CFSST columns were com-pared with the calculation curves of the Lam and Teng

C20-0-1-2C20-5-1-3C20-5-2-4C20-10-1-5

C20-10-2-6C20-15-1-7C20-15-2-8

0

20

40

60

80A

xial

stre

ss σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

(a)

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

C30-0-1-10C30-5-1-11C30-5-2-12C30-10-1-13

C30-10-2-14C30-15-1-15C30-15-2-16

(b)

Figure 9 Test stress-strain curves (a) Specimens of group C20 (b) Specimens of group C30

Advances in Materials Science and Engineering 7

10

10

08

09

10

11

12

06 09 1203

ξ

f0fcoprime

Figure 10 Relationship between f0fcoprime and ξ

Experiment curve (C20-5-1-3)Teng et alLai et alYang et al

0

10

20

30

40

50

60

70

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 00080000Axial strain εc

(a)

Experiment curve (C20-5-2-4)Teng et alLai et alYang et al

0

20

40

60

80A

xial

stre

ss σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

(b)

Figure 11 Continued

8 Advances in Materials Science and Engineering

model Lai model and Yang and Feng model e com-parison of calculated stress-strain curves with experimentcurve is shown in Figures 11 and 12

e Lam and Teng model had the highest fitting gradeon C-C-CFSST column stress-strain curves among threemodels e Yang and Feng model fitted well in somesituations such as C20-10-2-6 C20-15-1-7 C30-5-1-11and C30-15-2-16 e Lai model had a big deviationcompared with other models e stress-strain curves ofthe C-C-CFSST column can be separated into threesegments as shown in Figure 13

In segment I the Lam and Teng model shows a greatprediction on the stress-strain relationship of the C-C-CFSSTcolumne Lai model and the Yang and Fengmodel generallyunderestimated the axial stress of specimens in segment I eLam and Teng model has an unobvious segment II slightlyoverestimating the axial stress e Yang and Feng model has asimilar trend to the experiential curves of segment II Althoughsimilar to segment I this model underestimates the axial stressof specimens e Lai model also greatly underestimates theaxial stress because of the inaccuracy of the calculated transitionstrain In segment III the ultimate axial stress frsquo

cc calculated

Experiment curve (C20-10-1-5)Teng et alLai et alYang et al

0

10

20

30

40

50

60

70A

xial

stre

ss σc (

MPa

)

0002 0004 0006 00080000Axial strain εc

(c)

Experiment curve (C20-10-2-6)Teng et alLai et alYang et al

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

(d)

Experiment curve (C20-15-1-7)Teng et alLai et alYang et al

0

10

20

30

40

50

60

70

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 00080000Axial strain εc

(e)

Experiment curve (C20-15-2-8)Teng et alLai et alYang et al

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

(f )

Figure 11 Comparison of calculated stress-strain curves with the experiment curve of group C20 (a) C20-5-1-3 (b) C20-5-2-4 (c) C20-10-1-5 (d) C20-10-2-6 (e) C20-15-1-7 (f ) C20-15-2-8

Advances in Materials Science and Engineering 9

0

20

40

60

80A

xial

stre

ss σc (

MPa

)

0001 0002 0003 0004 0005 00060000Axial strain εc

Experiment curve (C30-5-1-11)Teng et alLai et alYang et al

(a)

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

Experiment curve (C30-5-2-12)Teng et alLai et alYang et al

(b)

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0001 0002 0003 0004 0005 00060000Axial strain εc

Experiment curve (C30-10-1-13)Teng et alLai et alYang et al

(c)

0

20

40

60

80

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

Experiment curve (C30-10-2-14)Teng et alLai et alYang et al

(d)

Figure 12 Continued

10 Advances in Materials Science and Engineering

from the Lam and Teng model and the Yang and Fengmodel iswell estimated to the test value e stress-strain curves cal-culated from the Lam and Teng model are dramatically close tothe test curves In conclusion the Lam and Teng model has thegreatest prediction for the stress-strain relationship of the C-C-CFSSTcolumn among the three models e Yang model has amore accurate prediction while CAM thickness is increasinge transition strain of the Lai model is always larger than thetest transition strain which leads to the inaccuracy of thecalculated curves A more accurate stress-strain model for C-C-

CFSST columns should be developed in experimental andtheoretical research

5 Conclusions

is study presents the results from an experimentalinvestigation on the compressive behavior of 16 C-C-CFSST short columns Based on test data the effect ofCAM thickness on the failure model and the axial stress-strain relationship is discussed e following conclu-sions can be made

(1) CFRP confinement using the CAM could effectivelyenhance the ductility and axial load carrying capacityof CFSST short columns For the increasing CAMthickness the bearing capacity of C20-5-1-3 C20-10-1-5 and C20-15-1-7 is improved by 199402 and 475 respectively

(2) e increasing CAM thickness leads to the CFRPbreakpoint gradually moving far from the specimencorner which led to the decrease of stress concen-tration of CFRP hoop stress in the corner of thesquare steel tubee increasing CAM thickness alsomade the lateral stress well distributed e effectivefracture strain and constraint efficiency of CFRPincreased well

(3) e stress-strain curve characteristic of the C-C-CFSSTcolumn is similar to the CFRP-confined circular con-crete columne applicability of the existingmodel forFRP-confined concrete was evaluated and comparedwith the test data e predictions of the Lam and Tengmodel agree well with the test data

0

20

40

60

80A

xial

stre

ss σc (

MPa

)

0001 0002 0003 0004 0005 00060000Axial strain εc

Experiment curve (C30-15-1-15)Teng et alLai et alYang et al

(e)

0

20

40

60

80

100

Axi

al st

ress

σc (

MPa

)

0002 0004 0006 0008 00100000Axial strain εc

Experiment curve (C30-15-2-16)Teng et alLai et alYang et al

(f )

Figure 12 Comparison of calculated stress-strain curves with the experiment curve of group C30 (a) C30-5-1-11 (b) C30-5-2-12 (c) C30-10-1-13(d) C30-10-2-14 (e) C30-15-1-15 (f) C30-15-2ndash16

Axial strain

Axi

al st

ress

σc

Segment I

Segment II

Segment III

fccprime

f0

fcoprime

Figure 13 Proposed stress-strain model for C-C-CFSST

Advances in Materials Science and Engineering 11

Data Availability

e data used to support the findings of this study are listedin this paper

Conflicts of Interest

e authors declare no conflicts of interest regarding thepublication of this paper

Acknowledgments

is research work was financially supported by the NationalNatural Science Foundation of China (Grant no 42072296)

References

[1] X Zhou D Gan J Liu and S Zhang ldquoExperiment and analysison square tubed reinforced concrete stub columns under axialcompressionrdquo Journal of Building Structures vol 32 no 1pp 68ndash74 2011

[2] J Wang Q Shen F Wang and W Wang ldquoExperimental andanalytical studies on CFRP strengthened circular thin-walledCFST stub columns under eccentric compressionrdquo in-WalledStructures vol 127 pp 102ndash119 2018

[3] Y-X Hua L-H Han Q-L Wang and C Hou ldquoBehaviour ofsquare CFST beam-columns under combined sustained loadand corrosion experimentsrdquoin-Walled Structures vol 136pp 353ndash366 2019

[4] CHou L-HHan andX-L Zhao ldquoFull-range analysis on squareCFST stub columns and beams under loading and chloridecorrosionrdquo in-Walled Structures vol 68 pp 50ndash64 2013

[5] F Zhang J Xia G Li et al ldquoDegradation of axial ultimateload-bearing capacity of circular thin-walled concrete-filledsteel tubular stub columns after corrosionrdquoMaterials vol 13no 3 2020

[6] H Huang Y Xu L Guo et al ldquoAxial compressive behavior ofCFRP-reinforced circular concrete-filled steel tubular stubcolumns with local corrosionrdquo Journal of Building Structuresvol 41 pp 109ndash116 2020

[7] S Zhong ldquoApplication and development in China of concretefilled steel tubular structurerdquo Architecture Technology vol 02pp 80ndash82 2001

[8] J Zhu G Lin J Teng et al ldquoFRP-confined square concretecolumns with section curvilinearization under axial com-pressionrdquo Journal of Composites for Construction vol 24no 2 2020

[9] Y Wang G Chen B Wan et al ldquoBehavior of circular ice-filledself-luminous FRP tubular stub columns under axial compres-sionrdquo Construction and Building Materials vol 232 2020

[10] Y Guo and Y Zhang ldquoComparative study of CFRP-confinedCFST stub columns under axial compressionrdquo Advances inCivil Engineering vol 18 Article ID 7109061 8 pages 2018

[11] H Elsanadedy T Almusallam Y Al-Salloum and R IqballdquoEffect of high temperature on structural response of rein-forced concrete circular columns strengthened with fiberreinforced polymer compositesrdquo Journal of Composite Ma-terials vol 51 no 3 pp 333ndash355 2017

[12] J LiangW Zou ZWang and D Liu ldquoCompressive behaviorof CFRP-confined partially encased concrete columns underaxial loadingrdquo Composite Structures vol 229 2019

[13] X Song X Gu Y Li T Chen and W Zhang ldquoMechanicalbehavior of FRP-strengthened concrete columns subjected to

concentric and eccentric compression loadingrdquo Journal ofComposites for Construction vol 17 no 3 pp 336ndash346 2013

[14] T Zhong ldquoState of the art of concrete filled square steelcolumnsrdquo Building Structure vol 7 pp 20ndash23 2003

[15] Z Tao and L-H Han ldquoBehaviour of fire-exposed concrete-filled steel tubular beam columns repaired with CFRP wrapsrdquoin-Walled Structures vol 45 no 1 pp 63ndash76 2007

[16] J Dong Q Wang and Z Guan ldquoStructural behaviour ofrecycled aggregate concrete filled steel tube columnsstrengthened by CFRPrdquo Engineering Structures vol 48pp 532ndash542 2013

[17] Z Tao L-H Han and J-P Zhuang ldquoAxial loading behaviorof CFRP strengthened concrete-filled steel tubular stub col-umnsrdquo Advances in Structural Engineering vol 10 no 1pp 37ndash46 2007

[18] Q Wang Y Xue Y Shao and X Yan ldquoStudy of staticperformance of axially compressed concrete filled square steeltubular stub columns confined by CFRPrdquo China Civil En-gineering Journal vol 41 pp 32ndash39 2011

[19] F Seible and M J N Priestley ldquoStrengthening of rectangularbridge columns for increased ductilityrdquo in Proceeding of theSymposium on Practical Solutions for Bridge Strengthening ampRehabilitation pp 239ndash248 DesMoines IA USA April 1993

[20] M N S Hadi T M Pham and X Lei ldquoNew method ofstrengthening reinforced concrete square columns by circu-larizing and wrapping with fiber-reinforced polymer or steelstrapsrdquo Journal of Composites for Construction vol 17 no 2pp 229ndash238 2013

[21] M T Jameel M N Sheikh and M N S Hadi ldquoBehaviour ofcircularized and FRP wrapped hollow concrete specimensunder axial compressive loadrdquo Composite Structures vol 171pp 538ndash548 2017

[22] W Yang Y Zhang J Chai G Wu and Z Dong ldquoExperi-mental investigation of rectangular concrete-filled fiberreinforced polymer (FRP)-steel composite tube columns forvarious corner radiirdquo Composite Structures vol 244 2020

[23] National Standard of the Peoplersquos Republic of China StandardFor test Method of Concrete Structures GBT 50152-2012China Architecture amp Building Press Beijing China 2012

[24] J B Mander M J N Priestley and R Park ldquoeoreticalstress-strain model for confined concreterdquo Journal of Struc-tural Engineering vol 114 no 8 pp 1804ndash1826 1988

[25] M Samaan A Mirmiran and M Shahawy ldquoModel ofconcrete confined by fiber compositesrdquo Journal of StructuralEngineering vol 124 no 9 pp 1025ndash1031 1998

[26] Q Yu ldquoStress-strain relationship of FRP-confined concretesubjected to axial compressionrdquo Industrial Construction vol4 pp 5ndash8 2001

[27] L Lam and J Teng ldquoDesign-oriented stressndashstrain model forFRP-confined concreterdquo Construction amp Building Materialsvol 17 no 6-7 pp 471ndash489 2003

[28] J G Teng T Jiang L Lam and Y Z Luo ldquoRefinement of adesign-oriented stress-strain model for FRP-confined con-creterdquo Journal of Composites for Construction vol 13 no 4pp 269ndash278 2009

[29] J Yang and P Feng ldquoAnalysis-oriented models for FRP-confinedconcrete 3D interpretation and general methodologyrdquo Engi-neering Structures vol 219 2020

[30] W Lai J Pan and X Jin ldquoCompressive stress-strain behaviorof concrete confined by fiber reinforced polymerrdquo IndustrialConstruction vol 10 pp 81ndash84 2004

[31] X Jin J Pan G Liu and W Lai ldquoResearch of stress-straincurve of concrete confined by fiber reinforced plastics under

12 Advances in Materials Science and Engineering

axial compressionrdquo Journal of Building Structures vol 04pp 47ndash53 2003

[32] J Pan YWang andW Lai ldquoEffect of sectional shape of concretecolumn on the bearing capacity of short columns wrapped withFRPrdquo Industrial Construction vol 06 pp 17ndash19 2001

[33] K Miyauchi S Inoue T Kuroda et al ldquoStrengthening effectsof concrete column with carbon fiber sheetrdquo Transactions ofthe Japan Concrete Institute vol 21 pp 143ndash150 1999

[34] Y-Y Wei and Y-F Wu ldquoUnified stress-strain model ofconcrete for FRP-confined columnsrdquo Construction andBuilding Materials vol 26 no 1 pp 381ndash392 2012

[35] National Standard of the Peoplersquos Republic of China Tech-nical Code for Concrete Filled Steel Tubular Structures GB50936-2014 China Architecture amp Building Press BeijingChina 2014

Advances in Materials Science and Engineering 13