EffectofCross-SectionalAspectRatioonRectangular FRP-Concrete … · 2020. 5. 27. · [17, 18], which displays the excellent energy dissipation ... ASTM C39/C39M [32]. ... 2.2.2. FRP.

Research ArticleEffect of Cross-Sectional Aspect Ratio on RectangularFRP-Concrete-Steel Double-Skin Tubular Columns underAxial Compression

Bing Zhang 12 Xia-Min Hu2 Wei Wei23 Qian-Biao Zhang2 Ning-Yuan Zhang2

and Yi-Jie Zhang 2

1School of Civil and Environmental Engineering Harbin Institute of Technology (Shenzhen) Shenzhen China2College of Civil Engineering Nanjing Tech University Nanjing China3Powerchina Real Estate Group Ltd Beijing China

Correspondence should be addressed to Bing Zhang zhangbnjtecheducn

Received 23 October 2019 Accepted 12 March 2020 Published 27 May 2020

Guest Editor Tianyu Xie

Copyright copy 2020 Bing Zhang et al is 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

Hybrid FRP-concrete-steel double-skin tubular columns (hybrid DSTCs) are novel hollow columns consisting of an outer FRP tube aninner steel tube and the concrete between the two tubes Hybrid DSTCs possess important advantages such as excellent corrosionresistance as well as remarkable seismic resistance However existing studies are mainly focused on hybrid DSTCs with a circular crosssection or a square cross section When a column is subjected to different load levels in the two horizontal directions a rectangularcolumn is preferred as it can provide different bending stiffness and moment capacity around its two axes of symmetry is paperpresents an experimental study on rectangularDSTCswith a particular focus on the effect of the cross-sectional aspect ratio (ie the ratioof the breadth to the width of the rectangular cross section) e effect of the cross-sectional shape of the inner steel tube (ie bothelliptical and rectangular inner steel tubes were used) and the effect of FRP tube thickness were also investigated experimentallyExperimental results show that a larger aspect ratio will have no negative effect on the confinement effect in rectangular DSTCs arectangular DSTCwith a larger aspect ratio generally has a larger ultimate axial strain and a higher axial stress at the ultimate axial strainrectangular DSTCs with an elliptical steel tube generally have better performance than corresponding specimens with a rectangular steeltube An existing model which was developed based on a model for rectangular FRP-confined concrete columns and a model forcircular DSTCs is verified using the test results of the present study e model generally provides close predictions for the peak axialstress of the confined concrete but yields conservative predictions for the ultimate axial strain for rectangular DSTCs

1 Introduction

In the field of civil engineering FRP composites have foundincreasing applications for the retrofitting of existingstructures as well as the construction of new structures [1ndash3]FRP composites are particularly attractive for use in com-bination with traditional construction materials (ie con-crete steel and timber) to create novel hybrid structures [4]Hybrid FRP-concrete-steel double-skin tubular columns(hybrid DSTCs) are novel hollow columns that combineconcrete steel and FRP together in an optimal manner [5]Hybrid DSTCs have an outer FRP tube an inner steel tubeand a layer of concrete between the two tubes (Figure 1)e

inner steel tube is the primary longitudinal reinforcementwhereas the outer FRP tube provides confinement to theconcrete e two tubes could be used as the in situformwork for the concrete casting thus saving the con-struction cost and advancing the construction scheduleHybrid DSTCs are quite suitable for structural members inharsh environments due to their excellent corrosion resis-tance which is enabled by the use of the outer FRP tubeHybrid DSTCs are also particularly attractive for use inseismic regions due to their excellent energy dissipationability under earthquake loading [6 7]

Teng et al [5] presented the first-ever experimental studyon hybrid DSTCs under axial compression to explain the

HindawiAdvances in Polymer TechnologyVolume 2020 Article ID 1349034 15 pageshttpsdoiorg10115520201349034

rationale of hybrid DSTCs and to demonstrate their ad-vantages Han et al [8] commented that this new memberform ldquocombines the advantages of all three constituentmaterials and those of the structural form of double-skintubular columnsrdquo Hollaway [9] introduced this new form ofhybrid members in detail in his review paper and com-mented that it ldquois relatively easy to construct and is highlyresistant to corrosion and earthquakesrdquo In recent yearshybrid DSTCs have already received extensive researchattention Existing studies are mainly focused on hybridDSTCs under loading conditions as follows (1) monotonicaxial compression [5 10ndash12] which confirms that theconcrete is confined effectively by the steel tube and the FRPtube leading to a ductile behavior (2) cyclic axial com-pression [13 14] which shows that the loading history hasan accumulative effect on the stress-strain response of re-peated unloadingreloading cycles (3) eccentric compres-sion [15 16] which indicates that the axial load capacitydecreases with the increase of load eccentricity (4) com-bined axial compression and cyclic lateral loading [6 7]which demonstrates that hybrid DSTCs possess excellentductility under cyclic lateral loading and (5) impact loading[17 18] which displays the excellent energy dissipationability of hybrid DSTCs Existing studies however aremostly focused on hybrid DSTCs with a circular crosssection (ie the cross section of the outer FRP tube iscircular) [5 8ndash18] and hybrid DSTCs with a square crosssection (ie the cross section of the outer FRP tube is square)[6 19ndash22] Although circular columns and square columnsare attractive as bridge piers rectangular columns arepreferred if such columns are subjected to different loadlevels in the two horizontal directions (Figure 1) Rectan-gular DSTCs could be designed to provide different bendingstiffness and moment capacity around the two axes ofsymmetry according to the engineering requirements [23]Rectangular FRP-confined concrete columns have receivedextensive research attention in the last decade [24ndash30] butexperimental research on rectangular DSTCs is rather rareTo the best knowledge of the authors there is only oneexperimental study on rectangular DSTCs subjected to axialcompression In Cavill and Yursquos [31] study the effect of theaspect ratio of the rectangular cross section was not in-vestigated All rectangular DSTCs had a breadth of 185mm

a width of 105mm and a height of 370mm and two circularsteel tubes were used for each specimen Cavill and Yu [31]confirmed the concrete in rectangular DSTCs was effectivelyconfined by the FRP tube and the steel tube leading to a veryductile response

e present study extends the existing work to rectan-gular DSTCs subjected to axial compression with a partic-ular focus on the effect of the aspect ratio which is the ratioof the breadth (the longer side) to the width (the shorterside) of the rectangular cross section e effect of the cross-sectional shape of the inner steel tube (ie both rectangularand elliptical inner steel tubes were used) and the effect ofthe FRP tube thickness are also investigated experimentallyRectangular FRP-confined solid concrete columns are fab-ricated and tested for comparison with rectangular DSTCs

2 Experimental Program

21 Specimen Details In the present study rectangularspecimens with four types of cross-sectional aspect ratioswere fabricated and tested ese specimens all had a heightof 600mm and a corner radius of 30mm on the outerrectangular cross section Specimen details are summarizedin Table 1 and the cross-sectional configurations are shownin Figure 2 For all specimens the breadth l of the outerrectangular cross section was 300mm while the width w was300mm 250mm 200mm or 150mm leading to fourdifferent cross-sectional aspect ratios lw (ie 10 12 15 or20) ese specimens with an aspect ratio lw of 10 aresquare DTSCs As shown in Table 1 and Figure 2 thesespecimens could be divided into three groups based on theircross-sectional configurations (1) group 1 rectangularDSTCs with an elliptical inner steel tube (referred to as RE-DSTCs) (2) group 2 rectangular DSTCs with a rectangularinner steel tube (referred to as RR-DSTCs) and (3) group 3rectangular FRP-confined solid concrete columns (referredto as R-CFFTs) For RE-DSTCs in group-1 four types ofelliptical steel tubes were used which had the same majoraxis 2as (ie 204mm) but four different minor axes 2bs (ie204mm 170mm 136mm and 102mm) (Table 2 andFigures 2 and 3) For RR-DSTCs in group 2 four types ofrectangular steel tubes were used which had the same cornerradius of 20mm and the same breadth ls for the steel

FRPConcreteSteel

(a)

FRPConcreteSteel

(b)

FRPConcreteSteel

(c)

Steel

FRPConcrete

(d)

Figure 1 Cross section of square and rectangular DSTCs (a) Square DSTCs with a circular steel tube (b) square DSTCs with a square steeltube (c) rectangular DSTCs with an elliptical steel tube and (d) rectangular DSTCs with a rectangular steel tube

2 Advances in Polymer Technology

Table 1 Specimen details

Specimen type Specimen nameSectional dimensions

Steel tube type FRP layersthickness (mm)l (mm) w (mm) lw

RE-DSTCs

RC1-ec1-F6 300 300 10 ec1 6 layers210RC2-ec2-F3 300 250 12 ec2 3 layers105RC2-ec2-F6 300 250 12 ec2 6 layers210RC3-ec3-F3 300 200 15 ec3 3 layers105RC3-ec3-F6 300 200 15 ec3 6 layers210RC4-ec4-F6 300 150 20 ec4 6 layers210

RR-DSTCs

RC1-rc1-F6 300 300 10 rc1 6 layers210RC2-rc2-F3 300 250 12 rc2 3 layers105RC2-rc2-F6 300 250 12 rc2 6 layers210RC3-rc3-F3 300 200 15 rc3 3 layers105RC3-rc3-F6 300 200 15 rc3 6 layers210RC4-rc4-F6 300 150 20 rc4 6 layers210

R-CFFTs

RC1-F6 300 300 10 mdash 6 layers210RC2-F3 300 250 12 mdash 3 layer105RC2-F6 300 250 12 mdash 6 layers210RC3-F3 300 200 15 mdash 3 layers105RC3-F6 300 200 15 mdash 6 layers210RC4-F6 300 150 20 mdash 6 layers210

204300

204

300

(a)

204

300

170250

(b)

136200

204

300

(c)

150102

204

300

(d)

180300

180

300

(e)

180

300

150250

(f )

180

300

120200

(g)

180

300

90150

(h)

300

300

(i)

250

300

(j)

200

300

(k)

150

300

(l)

Figure 2 Cross-sectional configurations (a) RC1-ec1-F6 (b) RC2-ec2-F6 RC2-ec2-F3 (c) RC3-ec3-F6 RC3-ec3-F3 (d) RC4-ec4-F6 (e)RC1-rc1-F6 (f ) RC2-rc2-F6 RC2-rc2-F3 (g) RC3-rc3-F6 RC3-rc3-F3 (h) RC4-rc4-F6 (i) RC1-F6 (j) RC2-F6 RC2-F3 (k) RC3-F6 RC3-F3and (l) RC4-F6

Advances in Polymer Technology 3

rectangular cross section (ie 180mm) but four differentwidths ws (ie 180mm 150mm 120mm and 90mm)(Table 3 and Figures 2 and 4) Compared with R-CFFTsrectangular DSTCs can save concrete significantly due to theinner void which could be indicated by the void area ratio(ie the ratio of the concrete void area to the area of thegross rectangular concrete section) ese rectangularDSTCs were designed to have the same void area ratio of046 thus saving around 46 concrete compared withrectangular R-CFFTs in the present study

As shown in Table 1 each specimen was given a name forease of reference (1) ldquoRC1rdquo ldquoRC2rdquo ldquoRC3rdquo and ldquoRC4rdquo areused to indicate the cross-sectional aspect ratio to be ldquo10rdquoldquo12rdquo ldquo15rdquo and ldquo20rdquo respectively (2) ldquoecrdquo or ldquorcrdquo is used torepresent the cross section of the inner steel tube to beelliptical or rectangular followed by a digit to indicate thetype of the steel tube as shown in Tables 2 and 3 (3) the letterldquoFrdquo and one digit are representing the fiber sheet layers of theFRP tube e nominal thickness of each layer fiber sheetwas 035mm leading to two FRP tube thicknesses for thepresent study (ie 3-layer FRP tube with a nominalthickness of 105mm and 6-layer FRP tube with a nominalthickness of 210mm) (Table 1) For each rectangular DSTCwith a rectangular inner steel tube there was a corre-sponding rectangular DSTC with an elliptical inner steeltube for comparison which had the same FRP tube thicknessand the same void area ratio as the former one (eg RC2-rc2-F6 and RC2-ec2-F6 are a pair of rectangular DSTCs for

comparison) All steel tubes in the present study had thesame thickness (ie 45mm) e FRP tube of all specimenswas formed by wrapping continuous unidirectional glassepoxy laminates on the hardened concrete surface with thefibers oriented in the hoop direction For each FRP tubethere was an overlapping zone spanning a circumferentialdistance of around 150mm along the longer side of therectangular cross section (Figure 5) Additional FRP stripswith a width of 40mm were provided near the two ends ofthe specimens to prevent premature failure there A thinlayer of high-strength plaster was used for capping to achievea flat end before the compressive test

22 Material Properties

221 Concrete In order to guarantee the casting quality ofthe concrete self-compacting concrete (SCC) was adopted forthe present study Plain concrete cylinders with a height of300mm and a diameter of 150mm were tested followingASTM C39C39M [32] A displacement-controlled loadingrate of 018mmmin was adopted for the testing of concretecylinders e elastic modulus Ec the peak stress fcoprime and theaxial strain at the peak stress εco averaged from these concretecylinder tests are 336GPa 504MPa and 026 respectively

222 FRP Tensile tests were conducted on flat couponsfollowing ASTM D3039 [33] to obtain the material

Table 2 Details of elliptical steel tubes

Type of steel tube 2as (mm) 2bs (mm) asbs fy (MPa) Es (GPa) fu (MPa)

ec1 204 204 10

3026 2010 441ec2 204 170 12ec3 204 136 15ec4 204 102 20

ec1 ec2 ec3 ec4

600

204

204

204

170

204

136

204

102

(a)

350

300

250

200

150

100

50

0

Axi

al st

ress

(MPa

)

0 0005 001 0015 002 0025 003 0035Axial strain

Steel tube ec1Steel tube ec2

Steel tube ec3Steel tube ec4

(b)

Figure 3 Axial compression test of elliptical hollow steel tubes (a) Elliptical hollow steel tubes and (b) axial stress-axial strain curves

4 Advances in Polymer Technology

properties of the FRP tube e FRP coupon which con-tained two layers of fiber sheets was fabricated using thesame wet-layup technique as the FRP tube for rectangularDSTCs e elastic modulus Efrp the ultimate strength andthe ultimate strain averaged from these FRP flat coupons are801GPa 18362MPa and 229 respectively

223 Steel Tube As shown in Tables 2 and 3 there were fourtypes of elliptical steel tubes and four types of rectangularsteel tubes in the present study All rectangular steel tubeswere fabricated using the same batch of raw materialswhereas all elliptical steel tubes were manufactured usinganother batch of raw materials All these steel tubes were

Table 3 Details of rectangular steel tubes

Type of steel tube ls (mm) ws (mm) lsws fy (MPa) Es (GPa) fu (MPa)

rc1 180 180 10

3080 2003 459rc2 180 150 12rc3 180 120 15rc4 180 90 20

600

rc1 rc2 rc3 rc4

180

180

150

180

12018

090

180

r = 20r = 20

r = 20

r = 20

(a)

300

250

200

150

100

50

0

Axi

al st

rcss

(MPa

)

0 0005 001 0015 002 0025 003 0035Axial strain

Steel tube rc1Steel tube rc2

Steel tube rc3Steel tube rc4

(b)

Figure 4 Axial compression test of rectangular hollow steel tubes (a) Rectangular hollow steel tubes and (b) axial stress-axial strain curves

LVDT-600LVDT-300Strain gauge in the hoop directionStrain gauge in the longitudinal direction

R-CFFTs

Overlapping zone

RE-DSTCs

Overlapping zone

RR-DSTCs

Overlapping zone

A

A

A

A

(a) (b)

Figure 5 Experimental setup and instrumentation (a) Planar view of strain gauges and LVDTs and (b) experimental setup

Advances in Polymer Technology 5

manufactured following these four steps (1) cutting the flatsteel plate to designed dimensions (2) bending the flat steelplate to form half part of a steel tube (3) welding twoidentical half parts together by two longitudinal welds and(4) milling the two ends of each steel tube to achieve flat endswhich are perpendicular to its axis

Tensile tests on steel coupons were conducted followingBS 18 [34] for elliptical steel tubes and rectangular steeltubes respectively Test results showed the tensile stress-strain curves of these steel coupons had a long yield plateauand then a hardening branch before the final rupture eaverage elastic modulus Es the average yield stress fy andthe average ultimate tensile strength fu are shown in Ta-bles 2 and 3 for elliptical and rectangular steel tubes re-spectively In addition for each type of these steel tubes twohollow steel tubes which had the same height as those usedin rectangular DSTCs (ie 600mm) were tested undermonotonic axial compression Four LVDTs were installed tomeasure the overall axial shortening of each steel tube efailed steel tubes after axial compression tests and the axialstress-axial strain curves are all shown in Figures 3 and 4 withthe axial strain being obtained from LVDTs All steel tubessuffered severe buckling after the axial compression test

23Experimental SetupandInstrumentation Figure 5 showsthe experimental setup and instrumentation for all speci-mens Six LVDTs were installed to measure the axial de-formation of each specimen Of the six LVDTs four (ieLVDT-300) were used to measure the shortening of the300mm midheight region while the other two (ie LVDT-600) were used to measure the total shortening of thespecimens For the outer FRP tube four hoop strain gaugesand four axial strain gauges (gauge length 20mm) wereinstalled at the midpoint on each side of the rectangularcross section while two additional hoop strain gauges wereattached at the corner of the rectangular FRP tube At themidheight of the inner steel tube four hoop strain gaugesand four axial strain gauges with a gauge length of 10mmwere installed and distributed evenly as shown in Figure 5 Alarge column testing facility with a maximum capacity of10000 kN was used to conduct axial compression tests witha displacement control rate of 036mmmin All the testdata including strains loads and displacements wererecorded simultaneously by a data acquiring system

3 Test Results and Discussion

31 General At the initial stage of the loading test readingsof the four axial strains on the FRP tube were quite uniformand there was no obvious phenomenon on the FRP tubeWhen the axial strain readings exceeded around 030 aloud noise emitted from the specimen and a substantial loaddropfluctuation occurred simultaneously suggesting thatsevere damage had occurred in the concrete which wasmainly due to the insufficient confinement provided by therectangular FRP tube As the loading process progressed thehoop strain readings of the two strain gauges at the FRP tubecorner generally increased faster compared with other hoop

strain gauges indicating the FRP tube provided more ef-fective confinement at the corner of the FRP tube Noticeabledamages were then observed on the outer surface of the FRPtube which were generally at the corner or close to thecorner of the rectangular cross section At the final stage ofthe loading test the progressive snapping noise of fibers wasnoticed Finally the explosive rupture of the FRP tube oc-curred associated with a big rupture noise

After the test the damage of the FRP tube and the innersteel tube was carefully examined (Figure 6)e damage of theFRP tube which was mainly due to the hoop tension inducedby the dilation of the inner concrete was generally localized atthe corner of the rectangular cross section As expected theconcrete also suffered severe crushing at the location of the FRPrupture Severe inward deformation and local buckling whichwas generally close to the localized rupture of the FRP tube wasobserved for both elliptical and rectangular steel tubes

32 Axial Load-Axial Strain Curves As the axial straingauges were on the outer surface of the FRP tube or the innersteel tubes their readings may not closely reflect the strainstate of the confined concrete especially after the devel-opment of significant localized damage on the FRP tubeeaxial strain obtained from the LVDTs covers the full heightof the specimen (ie LVDT-600) may not reflect the strainstate of the confined concrete especially at the early stage ofthe axial compression as there may be initial gaps betweenthe loading plates and the two ends of the specimen In thispaper the axial strain found from LVDT-300 which reflectsthe average axial strain of the midheight 300mm region isused to represent the axial strain of these specimens

Axial load-axial strain curves of all specimens are shownin Figure 7 in three groups As shown in Figure 7(c) the axialload-axial strain curves of R-CFFTs have an initial linearascending branch and then a severe axial load drop at theaxial strain of around 030 followed by an ascendingbranch until the final failure For RR-DSTCs (Figure 7(b))the axial load drop at the axial strain of around 030 ismuch smaller than the corresponding R-CFFTs whereascorresponding RE-DSTCs have only small axial load fluc-tuation (Figure 7(a))e sudden axial load dropfluctuationwas due to the insufficient confinement of rectangular FRPtubes e existence of the inner steel tube in rectangularDSTCs mitigates the sudden axial load drop as the concreteis confined by both tubes and the steel tube also contributesdirectly to the axial load of rectangular DSTCs

33 KeyTest Results For R-CFFTs the average axial stress ofthe confined concrete is found as the load resisted by theconcrete divided by the cross-sectional area of the concreteAs mentioned before the FRP tube had fibers oriented onlyin the hoop directionerefore the direct load contributionof the FRP tube is ignored for all specimens For rectangularDSTCs the direct load contribution of the inner steel tubeshould be considered e load resisted by the concrete inrectangular DSTCs is assumed to be equal to the loadresisted by the specimen subtracted by the load resisted bythe inner steel tube at the same axial strain e load carried

6 Advances in Polymer Technology

by the inner steel tube was assumed to be the same as thehollow steel tube under axial compression in Figures 3 and 4e axial loads of the specimen the concrete and the steeltube are illustrated for typical rectangular DSTCs in Figure 8

e method used above may introduce some errors to theaxial stress of the confined concrete in rectangular DSTCswhich is believed to be small before the severe buckling ofthe inner steel tube

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

(j) (k) (l)

Figure 6 Typical failed specimens after test (a) RC1-ec1-F6 (b) RC2-ec2-F6 (c) RC3-ec3-F6 (d) RC4-ec4-F6 (e) RC1-rc1-F6 (f ) RC2-rc2-F6 (g) RC3-rc3-F6 (h) RC4-rc4-F6 (i) RC1-F6 (j) RC2-F6 (k) RC3-F6 and (l) RC4-F6

Advances in Polymer Technology 7

e key test results of all specimens are summarized inTable 4 In this table Pmax is the peak axial load of thespecimen fcc is the peak axial stress of the confined con-crete εcu is the ultimate axial strain of the specimen when theFRP tube ruptured εhrup is the rupture strain of FRP tubeaveraged from the hoop strain gauges outside of the over-lapping zone and fccfco and εcuεco are the strength en-hancement ratio and the strain enhancement ratio of theconfined concrete

34 Effect of Cross-Sectional Aspect Ratio As shown inFigure 9 the axial stress-strain curves of three groups ofspecimens are compared to evaluate the effect of the cross-sectional aspect ratio As shown in Figure 9(c) all R-CFFTsexhibit axial stress-strain curves with an ascending branch

and a sudden stress drop at the axial strain of around 030followed by an ascending branch until the final failure eaxial stress at the ultimate axial strain is however muchlower than the axial stress at the axial strain of around 030As indicated in Table 4 the average strength enhancementratio fccfco of these four specimens in Figure 9(c) is almostthe same (ie 13) while specimen RC1-F6 had the largeststrain enhancement ratio εcuεco Although these specimensin Figure 9(c) had different cross-sectional aspect ratios theaxial stress-strain curves of R-CFFTs show a good agreementwith each other As shown in Figure 9(b) similar toR-FCSCs all RR-DSTCs exhibited a linear ascending branchand followed by a stress drop at the axial strain of around030 e axial stress is then stabilized for specimens RC1-rc1-F6 and RC2-rc2-F6 In contrast the axial stress forspecimens RC3-rc3-F6 and RC4-rc4-F6 recovered with an

3500

3000

2500

2000

1500

1000

500

0

Axi

al lo

ad (k

N)

Axial strain0 0005 0015 002 0025 003 0035001

RC2-ec2-F3RC1-ec1-F6

RC2-ec2-F6

RC3-ec3-F3

RC4-ec4-F6RC3-ec3-F6

(a)

3500

4000

3000

2500

2000

1500

1000

500

0

Axi

al lo

ad (k

N)

Axial strain0 0005 0015 002 0025 003001

RC2-rc2-F3RC1-rc1-F6

RC2-rc2-F6

RC3-rc3-F3

RC4-rc4-F6RC3-rc3-F6

(b)

Axi

al lo

ad (k

N)

6000

5000

4000

3000

2000

1000

0

Axial strain0 0005 0015 002 0025001

RC2-F3RC1-F6

RC2-F6

RC3-F3

RC4-F3RC3-F6

(c)

Figure 7 Axial load-axial strain curves (a) RE-DSTCs (b) RR-DSTCs and (c) R-FCSCs

8 Advances in Polymer Technology

ascending curve until the final failure As indicated in Ta-ble 4 the average strength enhancement ratio fccfco ofthese four specimens in Figure 9(b) is around 12 which areslightly smaller than corresponding R-CFFTs As shown inFigure 9(a) RE-DSTCs display similar stress drop at theaxial strain of around 030 Specimens RC3-ec3-F6 andRC4-ec4-F6 have higher axial stress and larger ultimate axialstrain than specimens RC1-ec1-F6 and RC4-ec4-F6 Asindicated in Table 4 specimens RC3-ec3-F6 and RC4-ec4-F6also have higher strength enhancement ratio fccfco andstrain enhancement ratio εcuεco than specimens RC1-ec1-F6and RC4-ec4-F6 It is evident that a larger aspect ratio willhave no negative effect on the confinement effect in rect-angular DSTCs On the contrary a rectangular DSTC with alarger aspect ratio generally has a larger ultimate axial strain

and higher axial stress at the ultimate strain is obser-vation is inconsistent with the test observation for rectan-gular FRP-confined concrete columns made in [28 29] thatthe confinement effect of the rectangular FRP tube decreaseswith the increase of the aspect ratio As shown in Figure 5with the increase of the aspect ratio the concrete in theregion A is under more effective confinement due to the localconfinement effect provided by the steel tube and the FRPtube leading to higher strength enhancement there

35 Effect of FRP7ickness As shown in Figure 10 six pairsof rectangular DSTCs which have the same aspect ratio andthe same void ratio but different FRP tube thicknesses arecompared to investigate the effect of FRP tube thickness For

Axi

al lo

ad (k

N)

3500

3000

2500

2000

1500

1000

500

00 0005 001 0015 002

Axial strain

Axial load of specimenAxial load of concreteAxial load of steel

(a)

Axi

al lo

ad (k

N)

3500

3000

2500

2000

1500

1000

500

00 0005 001 0015 002 0025 003 0035

Axial strain

Axial load of specimenAxial load of concreteAxial load of steel

(b)

Figure 8 Axial load taken by the concrete and the steel tube (a) RC1-ec1-F6 and (b) RC3-ec3-F6

Table 4 Key test results

Specimen name Pmax (kN) fcc (MPa) εcu () εhrup () fccfco εcuεcoRC1-ec1-F6 3384 499 184 060 10 71RC2-ec2-F3 2763 459 201 054 09 77RC2-ec2-F6 2916 462 217 065 09 84RC3-ec3-F3 2784 570 194 096 11 75RC3-ec3-F6 3023 662 323 132 13 124RC4-ec4-F6 2254 614 254 122 12 98RC1-rc1-F6 3648 545 122 036 11 47RC2-rc2-F3 2907 527 133 027 10 51RC2-rc2-F6 3700 648 229 104 13 88RC3-rc3-F3 2750 600 139 122 12 53RC3-rc3-F6 2868 620 216 095 12 83RC4-rc4-F6 2340 662 287 105 13 110RC1-F6 5846 655 218 186 13 84RC2-F3 4751 640 119 092 13 46RC2-F6 4561 614 161 108 12 62RC3-F3 3799 641 070 083 13 27RC3-F6 4169 704 138 085 14 53RC4-F6 2806 634 165 103 13 64

Advances in Polymer Technology 9

FRP-confined concrete columns the axial stress-strain be-havior of the confined concrete is significantly affected by theconfinement stiffness and the hoop rupture strain of the FRPtube [27 35 36] As shown in Table 4 the strength en-hancement ratio fccfco and the strain enhancement ratioεcuεco of rectangular DSTCs with a 6-layer FRP tube aremuch higher than those of corresponding specimens with a3-layer FRP tube As shown in Figure 10 a thicker FRP tubegenerally leads to a larger stiffness for the second branch ofthe axial stress-strain curves a larger strength enhancementratio and a larger ductility enhancement ratio

36 Effect of Cross Section of Inner Steel Tube Six pairs ofrectangular DSTCs are compared in Figure 11 to evaluate theeffect of the cross sectional shape of the inner steel tube eaxial stress-strain curves of R-CFFTs are also included inFigure 11 for comparison It is evident that the axial stress-

axial strain curves of R-CFFTs have an initial linear as-cending branch and then a severe axial stress drop at theaxial strain of around 030 followed by an ascendingbranch until the final failure is phenomenon is consistentwith the observation in [30] which is believed to be asso-ciated with the brittle nature of the concrete when theconfinement is insufficient For RE-DSTCs the axial stress-strain curves have an initial ascending branch and then afluctuationdrop in the axial stress followed by a secondascending branch In contrast RR-DSTCs experienced asudden drop in the axial stress starting right at the transitionpoint at their axial stress-strain curves e sudden drop inthe axial stress of RR-DSTCs is smaller than that of cor-responding R-CFFTs but much larger than that of corre-sponding RE-DSTCs erefore the confinement effect ofRR-DSTCs is less efficient than that of RE-DSTCs For RE-DSTCs their cross section can be regarded as two arcs due tothe existence of the elliptical steel tube e arc effect may

70

60

50

40

30

20

10

0

Axi

al st

ress

(MPa

)

0 0005 001 0015 002 0025 003 0035Axial strain

RC1-ec1-F6RC2-ec2-F6

RC3-ec3-F6RC4-ec4-F6

(a)

70

60

50

40

30

20

10

0

Axi

al st

ress

(MPa

)

0 0005 001 0015 002 0025 003Axial strain

RC1-rc1-F6RC2-rc2-F6

RC3-rc3-F6RC4-rc4-F6

(b)

70

80

60

50

40

30

20

10

0

Axi

al st

ress

(MPa

)

0 0005 001 0015 002 0025Axial strain

RC1-F6RC2-F6

RC3-F6RC4-F6

(c)

Figure 9 Effect of cross-sectional aspect ratio (a) RE-DSTCs (b) RR-DSTCs and (c) R-FCSCs

10 Advances in Polymer Technology

exist on the cross section of RE-DSTCs when the concretelayer is under axial compression and under the confinementof the FRP tube leading to better performance than RR-DSTCs It is evident that the cross sectional shape of theinner steel tube has a significant effect on the axial stress-strain behavior of the confined concrete in rectangularDSTCs As illustrated in Figure 11 and Table 4 rectangularDSTCs with an elliptical steel tube generally have betterductility than corresponding specimens with a rectangularsteel tube

4 Comparisons with Stress-Strain Model

Lam and Teng [35] developed a stress-strain model forcircular FRP-confined concrete columns in which a numberof important issues including the actual hoop rupture strainthe effect of the hoop stiffness and the sufficiency of the FRPtube were carefully examined Based on the model for cir-cular FRP-confined concrete columns Lam and Teng [27]

further developed a stress-strain model for rectangular FRP-confined concrete columns in which the aspect ratio of therectangular cross section was considered In 2009 Teng et al[36] refined Lam and Tengrsquos [35] model for circular FRP-confined concrete columns in which more accurate ex-pressions for the ultimate axial strain and the compressivestrength were employed Based on Teng et alrsquos [36] modelfor circular FRP-confined concrete Yu et al [10] proposed asimple stress-strain model for the confined concrete incircular DSTCs with a circular inner steel tube e effect ofthe inner void in circular DSTCs was considered for theultimate axial strain of the concrete using the void ratio φwhich was defined as the ratio of the steel tube diameter tothe outer diameter of the circular concrete section In 2013Yu and Teng [19] conducted an experimental study onhybrid DSTCs with a square outer FRP tube and a circularinner steel tube Based on the experimental results in Yu andTeng [19] a stress-strain model for concrete in squareDSTCs was proposed by combining Lam and Tengrsquos [27]

70

60

50

40

30

20

10

00 0005 001 0015 002 0025 003 0035

Axial strain

Axi

al st

ress

(MPa

)

RC2-ec2-F3RC2-ec2-F6

RC3-ec3-F3RC3-ec3-F6

(a)

0 0005 001 0015 002 0025Axial strain

70

60

50

40

30

20

10

0

Axi

al st

ress

(MPa

)

RC2-rc2-F3RC2-rc2-F6

RC3-rc3-F3RC3-rc3-F6

(b)

0 0005 001 0015 002Axial strain

80

70

60

50

40

30

20

10

0

Axi

al st

ress

(MPa

)

RC2-F3RC2-F6

RC3-F3RC3-F6

(c)

Figure 10 Effect of FRP tube thickness (a) RR-DSTCs (b) RE-DSTCs and (c) R-FCSCs

Advances in Polymer Technology 11

RC1-F6RC1-rc1-F6RC1-rc1-F6 prediction

RC1-ec1-F6RC1-ec1-F6 prediction

0

10

20

30

40

50

60

70 A

xial

stre

ss (M

Pa)

0 001 0015 002 00250005 Axial strain

(a)

RC2-F6RC2-rc2-F6RC2-rc2-F6 prediction

RC2-ec2-F6RC2-ec2-F6 prediction

0

10

20

30

40

50

60

70

Axi

al st

ress

(MPa

)

0 001 0015 002 00250005 Axial strain

(b)

RC3-F6RC3-rc3-F6RC3-rc3-F6 prediction

RC3-ec3-F6RC3-ec3-F6 prediction

0

10

20

30

40

50

60

70

80

Axi

al st

ress

(MPa

)

0005 001 0015 002 0025 003 00350 Axial strain

(c)

RC4-F6RC4-rc4-F6RC4-rc4-F6 prediction

RC4-ec4-F6RC4-ec4-F6 prediction

0

10

20

30

40

50

60

70 A

xial

stre

ss (M

Pa)

0005 001 0015 002 0025 0030 Axial strain

(d)

RC2-F3RC2-rc2-F3RC2-rc2-F3 prediction

RC2-ec2-F3RC2-ec2-F3 prediction

0

10

20

30

40

50

60

70

Axi

al st

ress

(MPa

)

0005 001 0015 002 00250 Axial strain

(e)

RC3-F3RC3-rc3-F3RC3-rc3-F3 prediction

RC3-ec3-F3RC3-ec3-F3 prediction

0

10

20

30

40

50

60

70

Axi

al st

ress

(MPa

)

0005 001 0015 0020 Axial strain

(f )

Figure 11 Effect of inner steel tubes

12 Advances in Polymer Technology

model for rectangular FRP-confined concrete and Yu et alrsquos[10] model for circular DSTCs with a circular inner steeltube is model also adopted Lam and Tengrsquos [27] equa-tions to consider the effect of the aspect ratio of the rect-angular cross section

Yu and Tengrsquos [19] model consists of a parabolic firstportion and a linear second portion for the stress-straincurve of confined concrete in hybrid DSTCs

σc Ecεc minusEc minus E2c( 1113857

2

4fo

ε2c middot 0le εc le εt

σc fo + E2cεc middot εt le εc le εcu

(1)

where σc and εc are the axial stress and the axial strain ofconfined concrete respectively fo is the intercept of thestress axis by the linear second portion which is taken to befcoprime Ec is the initial elastic modulus of confined concrete E2c

is the slope of the linear second portion of the stress-straincurve εcu is the ultimate axial strain of confined concreteand εt is the axial strain at the smooth transition point wherethe parabolic first portion meets the linear second portion

e parabolic first portion and the linear second portionare connected with a smooth transition at the transitionstrain εt

εt 2fo

Ec minus E2c

(2)

e slope of the linear second portion of the stress-straincurve E2c is given by

E2c fccprime minus fo

εcu (3)

where fccprime is the compressive strength of confined concretee compressive strength fccprime and the ultimate axial

strain εcu of confined concrete are shown in equations (4)and (5)e effect of the aspect ratio for the rectangular crosssection is considered for fccprime and εcu using the cross-sectionalshape factor for the strength enhancement ks1 and the cross-sectional shape factor for the strain enhancement ks2 evoid area ratio φA (ie the ratio of the concrete void area tothe area of the gross rectangular concrete section) ratherthan the void ratio φ (ie the ratio of the steel tube diameterto the outer diameter of the circular concrete section) wasused in Yu and Tengrsquos [19] model

fccprime

fcoprime

1 + 35ks1 ρK minus 001( 1113857ρε ρK ge 001

1 ρK lt 001

⎧⎨

⎩ (4)

εcuεco

175 + 65ks2ρ08K ρ145

ε 1 minusφA

radic( 1113857

minus 022 (5)

φ φA

radic (6)

ρK Efrptfrp

EsecoRo

(7)

ρε εhrup

εco (8)

Eseco fcoprime

εco (9)

where fcoprime and εco are the compressive strength and the axialstrain at peak axial stress of unconfined concrete respec-tively φ is the void ratio for circular DSTCs with a circularinner steel tube which is defined as the ratio of the steel tubediameter to the outer diameter of the circular concretesection φA is the void area ratio for square and rectangularDSTCs which is defined as the ratio of the concrete void areato the area of the gross concrete section ρK and ρε are theconfinement stiffness ratio and the strain ratio respectivelyEseco is the secant modulus of unconfined concrete Efrp isthe elastic modulus of FRP in the hoop direction tfrp is thethickness of the FRP tube and εhrup is the hoop strain of FRPat the rupture of the tube due to hoop tensile stresses

e cross-sectional shape factors ks1 and ks2 for rect-angular FRP-confined concrete columns in Lam and Tengrsquos[27] model are given by equations (10) and (11) For hybridDSTCs with a square cross section ks1 and ks2 are the sameas used in Yu and Tengrsquos [19] model In the followingpredictions ks1 and ks2 from Lam and Tengrsquos [27] model areemployed to consider the effect of the cross-sectional aspectratio

ks1 b

h1113888 1113889

2Ae

Ac

(10)

ks1 h

b1113888 1113889

2Ae

Ac

(11)

Ae

Ac

1 minus (bh) h minus 2Rc( 1113857

2+(hb) b minus 2Rc( 1113857

21113872 11138733Ag minus ρsc

1 minus ρsc

(12)

Ag bh minus (4 minus π)R2c (13)

where b and h are the width (the shorter side) and thebreadth (the longer side) of the rectangular cross sectionrespectively Ro is the outer radius of the circular section ofconcrete for circular DSTCs which is taken as

bhπ

radicfor

rectangular DSTCs in the present study AeAc is the ef-fective confinement area ratio for rectangular FRP-confinedconcrete columns Ag is the gross area of the rectangularcolumn section and ρsc is the cross-sectional area ratio of thelongitudinal steel reinforcement

e test results from the present study are comparedwith Yu and Tengrsquos [19] model in Figures 11 and 12 Due tothe existence of the axial stress drop at the axial strain ofaround 030 Yu and Tengrsquos [19] model could not capturethe complicated shape of the axial stress-strain curves ofrectangular DSTCs However Yu and Tengrsquos [19] modelgenerally provides close predictions for the peak axial stressof the confined concrete in rectangular DSTCs

Advances in Polymer Technology 13

(Figure 12(a)) As shown in Figures 11 and 12(b) Yu andTengrsquos [19] model yields conservative predictions for theultimate axial strain of the confined concrete in rectangularDSTCs Yu and Tengrsquos [19] model which was originallybased on Lam and Tengrsquos [27] model for rectangular FRP-confined concrete and Yu et alrsquos [10] model for circularDSTCs with a circular inner steel tube does not consider thecomplicated mechanism in such rectangular DSTCs Furtherresearch is needed for the development of a more reliablestress-strain model for the confined concrete in rectangularDSTCs when more test data are available Such a stress-strain model should take due account of various factorsincluding the cross-sectional aspect ratio the void area ratiothe thickness of the steel tube and the cross-sectional shapeof the inner steel tube

5 Conclusions

is paper presents an experimental study on rectangularDSTCs under axial compression e test results and dis-cussions allow the following conclusions to be drawn

(1) A larger aspect ratio will have no negative effect onthe confinement effect in rectangular DSTCs Arectangular DSTC with a larger cross-sectional as-pect ratio generally has a larger ultimate axial strainand higher axial stress at the ultimate axial strain

(2) Rectangular DSTCs with an elliptical steel tube ex-hibit better performance than corresponding spec-imens with a rectangular steel tube

(3) Rectangular DSTCs with a thicker FRP tube gen-erally have a larger stiffness for the second branch ofthe axial stress-strain curve a larger strength en-hancement ratio and a ductility enhancement ratio

(4) Yu and Tengrsquos model generally provides close pre-dictions for the peak axial stress of the confinedconcrete but yields conservative predictions for theultimate axial strain of the confined concrete inrectangular DSTCs

Data Availability

All tests were conducted by the authors Resuests for data 12months after publication of this article will be considered bythe corresponding author

Conflicts of Interest

e authors declare that they have no conflicts of interest

Acknowledgments

e authors are grateful for the financial support receivedfrom the National Natural Science Foundation of China(grant nos 51978332 and 51608263) and the Natural ScienceFoundation of Jiangsu Province (grant no BK20160998)

References

[1] L C Hollaway and J G Teng Strengthening and Rehabili-tation of Civil Infrastructures Using Fibre Reinforced Polymer(FRP) Composites Woodhead Publishing Cambridge UK2008

[2] J-J Xu Z-P Chen Y Xiao C Demartino and J-H WangldquoRecycled aggregate concrete in FRP-confined columns areview of experimental resultsrdquo Composite Structuresvol 174 pp 277ndash291 2017

[3] Y Wang G Cai Y Li D Waldmann A Si Larbi andK D Tsavdaridis ldquoBehavior of circular fiber-reinforcedpolymer-steel-confined concrete columns subjected to

0

20

40

60

80

100 P

eak

stres

s pre

dict

ion

(MPa

)

20 40 60 80 1000 Peak stress test (MPa)

(a)

0

001

002

003

004

Ulti

mat

e str

ain

pred

ictio

n

001 002 003 0040 Ultimate strain test

(b)

Figure 12 Comparisons with Yu and Tengrsquos [19] model (a) Peak stress and (b) ultimate strain

14 Advances in Polymer Technology

reversed cyclic loads experimental studies and finite-elementanalysisrdquo Journal of Structural Engineering vol 145 no 9Article ID 04019085 2019

[4] J G Teng ldquoNew-material hybrid structuresrdquo China CivilEngineering Journal vol 51 no 12 pp 1ndash11 2008 in Chinese

[5] J G Teng T Yu Y L Wong and S L Dong ldquoHybrid FRP-concrete-steel tubular columns concept and behaviorrdquoConstruction and Building Materials vol 21 no 4 pp 846ndash854 2007

[6] B Zhang J G Teng and T Yu ldquoExperimental behavior ofhybrid FRP-concrete-steel double-skin tubular columns un-der combined axial compression and cyclic lateral loadingrdquoEngineering Structures vol 99 pp 214ndash231 2015

[7] I A Omar M A ElGawady G Ahmed A Sujith andAMohanad ldquoSeismic performance of innovative hollow-coreFRP-concrete-steel bridge columnsrdquo Journal of Bridge Engi-neering vol 22 no 2 p 04016120 2017

[8] L-H Han Z Tao F-Y Liao and Y Xu ldquoTests on cyclicperformance of FRP-concrete-steel double-skin tubular col-umnsrdquo7in-Walled Structures vol 48 no 6 pp 430ndash439 2010

[9] L C Hollaway ldquoA review of the present and future utilisationof FRP composites in the civil infrastructure with reference totheir important in-service propertiesrdquo Construction andBuilding Materials vol 24 no 12 pp 2419ndash2445 2010

[10] T Yu J G Teng and Y L Wong ldquoStress-strain behavior ofconcrete in hybrid FRP-concrete-steel double-skin tubularcolumnsrdquo Journal of Structural Engineering vol 136 no 4pp 379ndash389 2010

[11] B Zhang J-L Zhao T Huang N-Y Zhang Y-J Zhang andX-M Hu ldquoEffect of fiber angles on hybrid fiber-reinforcedpolymer-concrete-steel double-skin tubular columns undermonotonic axial compressionrdquo Advances in Structural En-gineering 2020

[12] B Zhang J G Teng and T Yu ldquoCompressive behavior ofdouble-skin tubular columns with high-strength concrete anda filament-wound FRP tuberdquo Journal of Composites forConstruction-ASCE

[13] T Yu B Zhang Y B Cao and J G Teng ldquoBehavior of hybridFRP-concrete-steel double-skin tubular columns subjected tocyclic axial compressionrdquo 7in-Walled Structures vol 61pp 196ndash203 2012

[14] T Ozbakkaloglu and E Akin ldquoBehavior of FRP-confinednormal- and high-strength concrete under cyclic axialcompressionrdquo Journal of Composites for Construction vol 16no 4 pp 451ndash463 2012

[15] P Xie ldquoBehavior of large-scale hybrid FRP-concrete-steeldouble-skin tubular columns subjected to concentric and ec-centric compressionrdquo PhDesiseHong Kong PolytechnicUniversity Hong Kong China 2018

[16] T Yu Y L Wong and J G Teng ldquoBehavior of hybrid FRP-concrete-steel double-skin tubular columns subjected to ec-centric compressionrdquo Advances in Structural Engineeringvol 13 no 5 pp 961ndash974 2010

[17] R Wang L-H Han and Z Tao ldquoBehavior of FRP-concrete-steel double skin tubular members under lateral impactexperimental studyrdquo 7in-Walled Structures vol 95pp 363ndash373 2015

[18] I A Omar andM A ElGawady ldquoPerformance of hollow-coreFRPndashconcretendashsteel bridge columns subjected to vehiclecollisionrdquo Engineering Structures vol 123 pp 517ndash531 2016

[19] T Yu and J G Teng ldquoBehavior of hybrid FRP-concrete-steeldouble-skin tubular columns with a square outer tube and acircular inner tube subjected to axial compressionrdquo Journal ofComposites for Construction vol 17 no 2 pp 271ndash279 2012

[20] T Ozbakkaloglu B A L Fanggi and J Zheng ldquoConfinementmodel for concrete in circular and square FRP-concrete-steeldouble-skin composite columnsrdquoMaterials amp Design vol 96pp 458ndash469 2016

[21] B A L Fanggi and T Ozbakkaloglu ldquoEffect of inner steel tubecross-sectional shape on compressive behavior of square FRP-concrete-steel double-skin tubular columnsrdquo Applied Me-chanics and Materials vol 752-753 pp 578ndash583 2014

[22] B A L Fanggi and T Ozbakkaloglu ldquoInfluence of inner steeltube diameter on compressive behavior of square FRP-HSC-steel double-skin tubular columnsrdquo Advanced Materials Re-search vol 1119 pp 688ndash693 2015

[23] B Zhang W Wei G S Feng Q B Zhang N Y Zhang andX M Hu ldquoExperimental study of rectangular FRP-concrete-steel double-skin tubular column under axial compressionrdquoIndustrial Construction vol 49 no 12 pp 195ndash200 2019 inChinese

[24] X Li J Lu D-D Ding andWWang ldquoAxial strength of FRP-confined rectangular RC columns with different cross-sec-tional aspect ratiosrdquo Magazine of Concrete Research vol 69no 19 pp 1011ndash1026 2017

[25] H Toutanji M Han J Gilbert and S Matthys ldquoBehavior oflarge-scale rectangular columns confined with FRP com-positesrdquo Journal of Composites for Construction vol 14 no 1pp 62ndash71 2009

[26] T Ozbakkaloglu and D J Oehlers ldquoConcrete-filled squareand rectangular FRP tubes under axial compressionrdquo Journalof Composites for Construction vol 12 no 4 pp 469ndash4772008

[27] L Lam and J G Teng ldquoDesign-oriented stress-strain modelfor FRP-confined concrete in rectangular columnsrdquo Journal ofReinforced Plastics and Composites vol 22 no 13pp 1149ndash1186 2003

[28] H-X Liu G-J Liu X-Z Wang and X-Q Kong ldquoEffect ofcross-sectional aspect ratio and basalt fiber-reinforced poly-mer-confined number on axial compression behavior of shortcolumnsrdquo Journal of Reinforced Plastics and Compositesvol 34 no 10 pp 782ndash794 2015

[29] Y-F Wu and Y-Y Wei ldquoEffect of cross-sectional aspect ratioon the strength of CFRP-confined rectangular concrete col-umnsrdquo Engineering Structures vol 32 no 1 pp 32ndash45 2010

[30] T Ozbakkloglu ldquoUltra-high-strength concrete-filled FRPtubes compression tests on square and rectangular columnsrdquoKey Engineering Materials vol 575 pp 239ndash244 2014

[31] G Cavill and T Yu ldquoRectangular hybrid FRP-concrete-steeldouble-skin tubular columns stub column testsrdquo in Pro-ceedings of the 23rd Australasian Conference on the Mechanicsof Structures and Materials (ACMSM23) pp 521ndash526 BryonBay Australia December 2014

[32] ASTM C39C39M Standard Test Method for CompressiveStrength of Cylindrical Concrete Specimens American Societyfor Testing and Materials Philadelphia PA USA 2008

[33] ASTM D3039D3039M Standard Test Method for TensileProperties of Polymer Matrix Composite Materials AmericanSociety for Testing and Materials Philadelphia PA USA 2017

[34] BS 18 Tensile Testing of Metals (Including Aerospace Mate-rials) British Standards Institution London UK 1987

[35] L Lam and J G Teng ldquoDesign-oriented stressndashstrain modelfor FRP-confined concreterdquo Construction and Building Ma-terials vol 17 no 6-7 pp 471ndash489 2003

[36] 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

Advances in Polymer Technology 15