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0950-0618/03/$ - see front matter� 2003 Published by Elsevier Science Ltd.doi:10.1016/S0950-0618(03)00048-5

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2

3 Large scale testing and analysis of hybrid concreteycomposite tubes for4 circular beam-column applications

5

6 Amir Fam , Sami Rizkalla*a b,

78 Queen’s University, Kingston, Ontario, Canada K7L 3N6a

9 North Carolina State University, Centennial Campus, Raleigh NC 27695-7533, USAb

1011 Received 24 April 2003; accepted 14 July 2003

1221

22 Abstract23

24 Concrete-filled fiber reinforced polymer(FRP) circular tubes provide an effective structural system for a variety of applications25 such as piles, columns, overhead sign structures and utility poles. This paper discusses the behavior of concrete-filled Glass-FRP26 tubes ranging in diameter from 90 to 942 mm, using test results of eight beams, five columns and ten beam-column specimens.27 The effects of concrete fill, laminate structure of the tube, reinforcement ratio based on the wall thickness, as well as different28 failure modes are examined. Analytical models have been established and used in a parametric study to examine the effects of29 fiber orientation within the FRP tubes, thickness of the FRP tube, and the diameter of a central hole, which could be used to30 reduce the self-weight of the member. The benefits of concrete fill as well as the confinement effects have been demonstrated31 experimentally and analytically.32 � 2003 Published by Elsevier Science Ltd.3334 Keywords: Fibre reinforced polymer; Tubes; Concrete-filled; Confinement; Beam; Column35

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38 1. Introduction

39 The use of FRP materials in new structures is still40 limited to few demonstration projects. Innovative hybrid41 systems such as concrete-filled FRP tubes are proven to42 be effective in facing the great demand for corrosion-43 resistant piling, poles, highway overhead sign structures44 and bridge componentsw1–3x. The FRP tube provides45 lightweight structural component, permanent formwork,46 non-corrosive characteristics and saving of construction47 time and effort. The fibers in the circumferential direc-48 tion are utilized to provide confinement of the concrete,49 while the fibers in the axial direction provides the50 flexural strength and stiffness. For long time, concrete-51 filled steel tubes are used as structural members and52 have been extensively studiedw4,5x. Steel tubes are,53 however, susceptible to corrosion and could be less54 efficient in confinement at low load levels due to the601

602 *Corresponding author. Tel.:q1-919-513-1733; fax:q1-919-513-603 1765.604 E-mail address: sami_rizkalla@ncsu.edu(S. Rizkalla).

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higher Poisson’s ratio of steel, compared to that of56concretew6x. However, the laminate structure of FRP57tubes could be optimized by controlling the proportions58of fibers in the axial and hoop directions to suit the59application. For flexural members, larger stiffness would60be required in the axial direction while for axial mem-61bers, larger stiffness is required in the hoop direction as62well as a minimum Poisson’s ratio in order to produce63the maximum confinement of concrete.

642. Experimental program

65This paper discusses the test results of a large exper-66imental program, which included testing of different67beams, short columns and beam-columns. Eight different68filament-wound GFRP tubes, fabricated using E-glass69fibers and either epoxy or polyester resin, as well as70one steel tube, were used in this investigation.Table 171provides the details of the tubes including diameter,72thickness, stacking sequence of different layers and73mechanical properties of the tubes in the axial and hoop74directions. a shows the characteristics of test beams,75including the cross-section configurations, span, spacing

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3Table 1

4 Details and mechanical properties of the tubes used to fabricate the specimens5

Tube Diameter- Thickness Stacking Arial direction Hoop direction67 no. (mm) (mm) sequence

f (ten.)u f (comp.)u E ys f (ten.)u E8(MPa) (MPa) (GPa) (MPa) (GPa)

916171 89 2.05 Iq15yy82yy82yq15yy82yq15yy82yq15yy82x NyA 241 17.6a 0.15a NyA 27.6a182 90 2.0 wq30yy30yq30yy30yq30yy30x NyA NyA 22a NyA NyA 9.7a193 100 3.08 wy87yq3yy87yq3yy87yq3yy87yq3yy87x 449 NyA 29 0.1a 398a 23204 168 2.56 wq8yy86yy86yq8yy86yq8yy86yq8yy86x 283 224b 19.8 0.066 548 33.4215 320 5.96 wq34yy34yq80yq34yy34: 178 NyA 14.7 0.34a NyA 17.1a226 326 6.4 wy88yq3yy88yy88yq3yy88yq3yy88yq3yy88x 237 276b 17 0.11 NyA 24.2a237 626 5.41 wq34yy34yq85yq34yy34: 206 NyA 14.3 0.32a NyA 17.7a248 942 8.93 wq34yy34yq86yq34yy3yq34yy34x 207b NyA 15.2b 0.39a NyA 15a259 169 4.09 wsteel tubex 305 305 203 0.3 305 203

2627 Lamination theory; Manufacturer data; Angles arc measured wi(h respect to longitudinal direction.a b c28

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32

33

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between the applied loads, tube identification number77 based onTable 1 and the concrete strength.Table 2b78 and c provide similar details of the columns and the79 beam-column specimens, respectively.

80 2.1. Fabrication of specimens

81 The tubes were placed in an inclined position as82 shown in Fig. 1 and concrete was cast from the top.

83

The central hole in column C3 was achieved using a84cardboard tube, which was left inside the specimen.85Wooden plugs were used to seal the ends. The concrete86mix was designed to provide pressure fit to the tubes87by adding an expansive agent in order to prevent88separation due to shrinkage. External vibration was89applied. The columns and beam-column specimens were90cut from some of the concrete-filled tubes using a91diamond blade saw.

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38

39 Fig. 1. Casting set-up of GFRP tubes.

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44

45 Fig. 2. Test set-up of beams, columns and beam-columns.

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2.2. Beam tests

93 Fig. 2a shows a typical test set-up for beams tested94 in bending using stroke control and four-point load95 configuration. Details of test beams are given inScheme96 1a. Beam B1 is a hollow FRP tube, while B2 is concrete-97 filled tube, which is identical to the tube of B1. The98 two beams were used to study the effect of concrete99 filling. Beams B3 and B4 were tested to study the effect100 of two different FRP laminate structures. The tubes of101 B3 and B4 are very close in diameter and wall thickness.102 B3 has 33 and 67% of the fibers oriented at 15 and 828,103 respectively, with the axial direction, while B4 has all104 fibers oriented at"308 with the axial direction. Beams105 B5 and B6 are used to determine the pure flexural106 strength of two different concrete-filled FRP tubes, and107 are used to establish one point of the beam–column108 interaction diagrams of the two types of tubes. The two109 tubes of B5 and B6 are similar in size and wall thickness,110 however, B5 has almost equal fiber ratios at 3 and 888

111 with the axial direction, while B6 has 70% of fibers112 oriented at"348 and 30% at 808 with the axial direction.113 B6, B7 and B8 are used to study the effect of different

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reinforcement ratios since all beams have similar lami-115nate structure but different diameter-to-wall thickness116ratios, which provided reinforcement ratios of 7.4, 3.5117and 3.8%, respectively, where the reinforcement ratior118is defined as the ratio of four times of the thicknesst119to the diameterD, (4tyD). For all test beams, mid-span120deflections, axial and hoop strains at compression and121tension sides, and applied loads have been measured.122Other details of the beam tests can be found elsewhere123w7x.

1242.3. Short column tests

125Fig. 2b shows a typical test set-up for short columns126under axial compression using stroke control. Details of127the specimens are given inScheme 1b. Columns C1128and C2 are used to compare FRP-confined concrete to129steel-confined concrete. Both tubes have the same diam-130eter, however, C2 is a concrete-filled GFRP tube with131wall thickness 37% smaller than that of the steel tube132of C1, C2 and C3 are similar concrete-filled FRP tubes,133however, C3 has a concentric central hole, which could134be used to reduce the self-weight of the member. C4

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50 Fig. 3. Moment–curvature response and failure modes of B1 and B2.

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55Fig. 4. Load–deflection response and failure modes of B3 and B4.

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and C5 are used to determine the pure axial strength for136 two types of tubes, which will be used to establish the137 beam–column interaction curves. Both columns have138 FRP tubes similar to those of beams B5 and B6,139 respectively. Axial and lateral strains at four points140 around the perimeter as well as the applied load were141 measured. Other details of the short column tests can be142 found elsewherew1x.

143 2.4. Beam-column tests

144 Fig. 2c shows a typical test set-up for the beam-145 column specimens. The specimens were tested under an146 eccentric axial load using rigid steel caps at the ends to147 apply the axial compression using different eccentrici-148 ties. The steel caps consisted of two half-tubes bolted149 together from both sides through flanges to grip the150 specimen. The two half cylinders supported a flat steel151 plate through a bolted flange to form a bracket, which152 allows for the application of axial load at different153 eccentricities. Stiffeners were provided to ensure a stiff154 bracket.Scheme 1c provides the details of the specimens155 including the maximum eccentricity measured at ulti-156 mate, which account for the P-D effect. Specimens BC1157 to BC5 as well as B5 and C4 were used to establish the158 complete interaction diagram for concrete-filled FRP159 Tube number 6. Specimens BC6 to BC10 as well as B6160 and C5 are used to establish the interaction diagram of161 concrete-filled FRP Tube number 5.

162 3. Test results

163 3.1. Beam tests

164 Fig. 3 shows the moment–curvature behavior of165 hollow and concrete-filled GFRP filament-wound tubes166 B1 and B2, respectively. The figure indicates that the167 strength and stiffness are both significantly increased by

168

filling the tube with concrete. The strength gain is169212%. The presence of concrete has contributed to the170stiffness and moment resistance of the section in the171compression zone of the beam. The concrete also pro-172vided internal support to the tube and prevented ovali-173zation and local buckling.Fig. 3 also shows the failure174mode of B1, which was due to local buckling and175crushing of the hollow tube, while B2 had flexural176tension failure due to rupture of the fibers in the tension177side.178Fig. 4 compares the load-deflection behavior of B3179and B4, which are similar in size and wall thickness,180but different in laminate structure. The relative stiffness181after cracking is almost proportional to the relative182effective elastic modulus of the tubes in the axial183direction. Although both beams achieved similar flexural184strength, the failure modes were quite different. B4185developed a gradual compression failure due to matrix186cracking and fiber buckling in the compression zone,187which contributed to the non-linear behavior. This is188attributed to the absence of fibers in the hoop direction189to confine the other layers. B3 had flexural tension190failure. It should be noted that B3 has only 33% of the191fibers oriented in the axial direction, which is relatively192low for flexural members.193Due to the difference in span of beams B6, B7 and194B8, their behavior is compared using the moment–195curvature response as shown inFig. 5. The behavior is196normalized with respect to the outer diameterD , dueo

197to the difference in beams size. The moment is divided198by in order to be presented using sress units and the3Do

199curvature is multiplied byD in order to be presentedo

200in a dimensionless form. The normalized behavior of201B7 and B8 is almost identical due to the similar202reinforcement ratios, 3.5 and 3.8%, respectively. B6203showed higher strength and stiffness due to the higher204reinforcement ratio of 7.4%. All beams had similar

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60 Fig. 5. Normalized moment–curvature response of B6, B7 and B8.

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65Fig. 6. Load–axial strain behavior of C1 and C2.

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70Fig. 7. Stress–strain response of confined concrete of C2 and C3.

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laminate structure and all failed in tension by rupture of206 the fibers as shown inFig. 5.

207 3.2. Short column tests

208 The behavior of column C2 is compared to that of209 C1 in Fig. 6. The plain concrete behavior is also shown210 for comparison. Although the wall thickness of the211 GFRP tube of C2 is 37% less than that of the steel tube212 of C1, they both achieved the same axial strength. The213 behavior of C2 is characterized by a bi-linear response214 with a transition zone near the vicinity of unconfined215 concrete strength. The stiffness beyond this point, sec-216 ond slope, is governed by the stiffness of the GFRP217 tube. Once the tube reached its tensile strength, in218 presence of the axial compressive stresses, it fractured219 and the stub failed as shown inFig. 6. However, steel220 confined concrete behaved almost linearly untill the221 steel tube yielded, followed by a plastic plateau with222 large deformations. The continuous increase of the load223 resistance of FRP confined concrete is attributed to the224 continuous increasing of the confining pressure due to225 the linear behavior of FRP. However, once the steel tube226 yields, the confining pressure is stable regardless of the227 degree of concrete expansion until the column finally228 fails due to yielding and bulging of the steel tube as229 shown inFig. 6.230 Fig. 7 shows the stress–strain response of the FRP231 confined concrete of C2(totally filled with concrete),232 and that of partially filled FRP tube, C3(with central233 hole). It is evident that providing a central hole reduces234 the confinement effect due to the reduction of the235 internal radial stresses in presence of the central hole.236 In large diameter piles, central holes could, however, be237 used to reduce the self-weight of the member. C3 also238 failed by fracture of the tube as shown inFig. 7.

239

3.3. Beam-column tests

240For beam-column specimens, the bending moment at241ultimate was estimated based on the ultimate axial load242and the total eccentricity, which includes the initial243eccentricity and the lateral deflection at mid height.Fig.2448 shows the axial load-bending moment interaction245diagram for two types of concrete-filled FRP tubes246(tubes number 5 and 6 inTable 1). The curves are of a247similar shape to those of conventional reinforced con-248crete beam-columns. The first portion of the curves249shows an increased bending moment as the axial load250increases, which corresponds to the tension failure251region. In this region, specimen failure is governed by252the rupture of the fibres in the GFRP shell in tension as253shown in Fig. 8. The curve reaches a balanced point,254after which, increasing axial force is accompanied by255decreasing bending moment. In this region, failure is256governed by crushing of the fibres in the GFRP shell at257the extreme compression face as also shown inFig. 8.258It is evident fromFig. 8 that concrete-filled FRP tube

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76 Fig. 8. Axial load–bending moment interaction diagrams and failure modes of beam-column specimens.

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number 6 has a larger size interaction curve than that260 of concrete-filled FRP tube number 5, despite the similar261 size and the comparable wall thickness. This is attributed262 to the better laminate structure of tube number 6 in263 comparison to tube number 5, which resulted in better264 strength and stiffness in the axial and hoop directions265 and much lower Poisson’s ratio, and, therefore, results266 in less separation from concrete. It should be noted that267 the concrete strength of tube number 5 is 12% higher268 than that of tube number 6.

269 4. Analytical modeling

270 For axially loaded FRP-confined concrete, a confine-271 ment model based on equilibrium and radial displace-272 ment compatibility between the outer shell and the273 concrete core is developed. The model accounts for the274 continuously increasing confining pressures due toR

275 the linear material properties of the FRP tubew8x and276 utilizes the following equation, which is driven based277 on radial displacement compatibility at the interface278 between the concrete core and the tube:279280

281 (y yy )c ss s ´ (1)R ccR 1yycq

E t Es c

282

283 Where y and y are Poisson’s ratio of concrete andc s

284 FRP tube, respectively.R is the radius of the concrete285 core.E and E are the elastic moduli of concrete andc s

286 the tube in the hoop direction, respectively.´ is thecc

287 axial strain. For a tube with a central hole, the following288 equation is used to calculate the confining pressuresR

289 as a function of the inner radiusR and the outer radiusi

290 R .o

293

R yR y yyŽ .Ž .o i c ss s ´ (2)R cc2 2B ER qRo iC FR yyo c2 2 2

D GR R yRo o iqE ts Ec

294

295The model also accounts for the bi-axial state of296stresses developed in the FRP tube and utilizes the Tsai–297Wu bi-axial failure criteria. The model predicts the298stress–strain response of the confined concrete by com-299bining Eq. (1) with Mander’s confinement modelw9x.300The model can also account for the central hole inside301the concrete core, utilizing an expression similar to that302in Eq. (1), but modified to account for the radius of the303central hole.Fig. 7 shows the predicted stress–strain304response of the confined concrete of specimens C2 and305C3, with and without a central hole, which showed good306agreement.307For flexural members, an analytical model based on308strain compatibility, equilibrium and effective mechani-309cal properties of the FRP tube in the axial direction has310been developed. The model utilizes an unconfined311stress–strain response with extended strain softening of312concrete in the compression zonew10x, based on the313fact that in bending, there is insignificant confinement314effect in the compression zone. This in fact is demon-315strated experimentally inFig. 9, which shows the meas-316ured strain in the hoop direction of column C4 vs. the317measured axial strain. Also, shown on the same figure,318the hoop strains vs. the axial compressive strains, meas-319ured on the compression side of beam B5 of the same320FRP tube. The figure clearly shows that in columns, the321hoop strains increases significantly at a certain level of322axial strain, due to concrete internal microcracking and323expansion of concrete, which activates the FRP tube324and develops significant confinement. However, in

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81 Fig. 9. Axial strain vs. hoop strains in columns and beams.

85

86

87 Fig. 10. Effect of loading the FRP tube in the axial direction.

325

beams, the rate of increase of lateral strains remains326 stable, almost similar to the Poisson’s ratio of the tube,327 which indicates lack of confinement in the compression328 zone. However, the ductility of concrete is increased as329 evident by the large axial strains achieved. In the330 analytical model, different values of the neutral axis331 depth are assumed for a given value of the axial332 compressive strain at the extreme compression fibers,333 until the equilibrium of internal forces is achieved. By334 varying the level of axial compressive strains, the335 corresponding moments and curvatures are used to336 establish the moment–curvature response of the section.337 Other details of the analytical model could be found338 elsewherew11x. Fig. 3 shows the predicted moment–339 curvature response of B2, which showed good agreement340 with the measured response. Also, shown on the same341 graph, the predicted response of the hollow FRP tube,342 B1.

343

As the eccentricity of the axial load increases from344zero (columns) to infinity (beams), the size of com-345pression zone of concrete is gradually reduced and a346strain gradient is developed, therefore, the confinement347effect is also gradually reduced. The confinement level348of concrete in the compression zone at any given349eccentricity varies between an upper bound(fully con-350fined concrete) and a lower bound(unconfined con-351crete) w12x.

3525. Parametric study

3535.1. Axial members

354The confinement model for axially loaded members355was used in a parametric study to examine the effects356of: (1) loading the concrete core only in comparison to357loading the core and the tube;(2) the effect of the wall358thickness of the tube; and(3) the effect of the size of359the central hole. In this study, a typical 150-mm diameter360concrete-filled GFRP tube with 40 MPa concrete and3610.18 initial Poisson’s ratio was used. The GFRP tube362has all fibers oriented in the hoop direction. For the E-363glass/epoxy GFRP tube, the major and minor elastic364moduli (parallel and perpendicular to the fibers) were36539 and 8.6 GPa and the major and minor Poisson’s366ratios were 0.28 and 0.06, respectively. Major Poisson’s367ratio is defined as the ratio of transverse to axial strain368due to loading in the axial direction, parallel to the369fibers. The tube thickness was varied from 0.5 to 8 mm.370The diameter of the central hole was varied from 0371(totally filled tube) to 125 mm for the case of the 2 mm372GFRP tube.373Fig. 10 shows the confined stress–strain curves of374concrete for the case of loading the concrete core only375of the hybrid system,(a) and loading both the concrete

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92

93 Fig. 11. Effect of stiffness of the tube and inner hole size on confinement.

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core and the FRP tube,(b) in comparison to the377 unconfined stress–strain curve of the same concrete.378 When the load is applied to the core only, the 2-mm379 thick tube is fully utilized in the hoop direction and380 develops the full hoop tensile strength as shown in the381 failure criteria, which is also shown inFig. 10, where382 the stress path(a) represents hoop tensile stresses only.383 When the load is applied to both the core and the tube,384 significant reduction in strength and stiffness is385 observed. The reduced strength is attributed to the bi-386 axial state of stress developed in the tube as illustrated387 by the bi-axial stress path(b), which intersects the388 failure envelope at a point with lower hoop strength389 than that of case(a). The slightly reduced stiffness is390 attributed to the outward expansion of the tube due to391 its Poisson’s ratio effect, which reduces the confining392 pressure.393 Fig. 11shows the variation of the strength of confined394 concretef , normalized with respect to the unconfined9

cc

395 strength f , with the stiffness of the tube in the hoop9c

396 direction (E tyR) as controlled by the wall thickness,s

397 whereR and t are the radius and thickness of the FRP398 tube, respectively, andE is the elastic modulus of thes

399 tube in the hoop direction. The figure also shows the400 variation of the ratio(f y f ) with the inner-to-outer9 9

cc c

401 diameter ratio of columns with central holes(D yD ).i o

402 The figure indicates that increasing the stiffness of the403 tube in the hoop direction would increase the strength404 of confined concrete, however, the rate of increase is405 non-linear. The figure also shows that below a certain406 stiffness level, there is no gain in strength, due to the407 post-peak softening behavior. Also, increasing the size408 of the central hole would reduce the confinement effect409 due to the reduced radial confining pressure.

410 5.2. Flexural members

411 The strain compatibility model for analysis of flexural412 members was used in a parametric study to examine the

413

effects of laminate structure of the FRP tube, thickness414of the tube and effect of filling both high stiffness415(thick) and low stiffness(thin) FRP tubes. A 300-mm416diameter GFRP tube with aw0y90x symmetric cross-s

417ply E-glassyepoxy laminate is used in the analysis. The418laminate structure is changed by varying the proportions419of fibers in the axial w0x and hoop w90x directions420including 9:1, 3:1, 1:1 and 1:3 ratios, respectively. A4213:1 laminate indicates that 75% of fibers is oriented in422the axial direction. The different proportions resulted in423effective elastic moduli and tensile strengths of 35.2 to42411.5 GPa and 976 to 319 MPa, respectively, in the axial425direction. The wall thickness varied from 2 to 16 mm,426which is equivalent to reinforcement ratios of 2.67 to42721.33%. The concrete strength was 50 MPa.428Fig. 12 shows the effect of increasing the wall429thickness as well as the ratio of fibers in the axial430direction, which are represented in terms of the rein-431forcement indexv, on the normalized flexural strength432. The reinforcement index is given inEq. (3) in termsM̄433of the wall thicknesst, the outer diameterD , FRP axialo

434tensile strengthf , and concrete strengthf 9.u c 435436

4374t fuvs (3)

D f9o c

438

439The ultimate momentM is normalized with respectu

440to D and f 9. For the case of the concrete-filled GFRPo c

441tubes with thickness ranging from 2 to 16 mm, the442laminates of all the tubes were maintained(1:1). Also,443for the case of the concrete-filled GFRP tubes with four444different laminate structures ranging from(1:3) to (9:1),445the wall thickness was maintained 4 mm.Fig. 12clearly446shows that increasing either the wall thickness of the447tube (for a given laminate structure), or increasing the448ratio of fibers in the axial direction(for a given wall449thickness), have the same effect. They both result in450increasing the flexural strength of the tube, however, the

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99 Fig. 12. Variation of the flexural strength with the reinforcement index.

103

104 Fig. 13. Effect of concrete filling on flexural behavior of tubes with105 high and low stiffness.

451

failure modes could change. Tubes with low stiffness in452 the axial direction or smaller wall thickness tend to fail453 in tension, while thicker wall tubes or tubes with large454 ratio of fibers in the axial direction tend to fail in455 compression. As a result, the balanced condition is not456 only dependent on the reinforcement ratio(wall thick-457 ness of the tube), but also on the laminate structure458 (ratio of fibers in the axial direction).459 Fig. 13 shows the moment–curvature response of460 hollow and concrete-filled(1:1) GFRP tubes of two461 different wall thickness, 2 and 16 mm. The 2 mm tube462 represents a case of low stiffness tube while the 16 mm463 tube represents a very stiff tube. The failure of the 16464 mm hollow tube was governed by the compressive465 strength of the shell, while the failure of the 2 mm

466

hollow tube was governed by local buckling of the thin467tube rather than material failure of the FRP tubew11x.468The figure clearly shows that the concrete filling signif-469icantly improves the strength and ductility of low stiff-470ness tubes, while its effect is relatively small in stiff471tubes. The gain in flexural strength by filling the tubes472with concrete was 20% in the 16 mm tube and 500%473in the 2 mm tube. The effect of concrete filling in low474stiffness tubes is reflected in two aspects. It prevents475the premature local buckling failure in compression. In476fact, it could change the failure mode to tension failure.477Concrete also contributes significantly to the internal478moment resistance of the section by providing large479compressive strength in the compression side of the480beam.

4816. Conclusions

482Based on a comprehensive experimental program483using large-scale concrete-filled FRP tubes as well as484analytical modeling and parametric studies, the behavior485of concrete-filled FRP tubes as flexural members, axial486members and beam-column members have been inves-487tigated. The following conclusions are drawn:488

4891. In bending, concrete filling is more efficient for thin-491walled tubes or tubes with low stiffness in the axial492direction than it is for stiff or thick-walled tubes. It493prevents local buckling and increases the flexural494strength and stiffness.4962. Flexural strength can be increased either by increasing498the wall thickness or the ratio of fibers in the axial499direction of the tube, however, the failure mode could500change to compression. The balanced reinforcement501ratio is dependent on the laminate structure of the

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tube and generally is smaller for tubes with higher503 stiffness in the axial direction.505 3. Using unconfined stress–strain curve with extended507 strain softening for concrete in the analytical model-508 ing, predicts very well the flexural behavior of con-509 crete-filled FRP tubes.511 4. In axial members, ignoring the effect of axial loading513 of the FRP tube under compression and assuming the514 development of its full hoop strength overestimate515 the confinement effectiveness.517 5. Totally filled GFRP tubes provide the most effective519 confinement for columns. Although an inner hole520 offers material saving and reduced self-weight, it521 reduces the confinement. The slope of the second522 branch of the bi-linear stress–strain response is gov-523 erned by the stiffness of the tube and the size of the524 inner hole. The strength of the column is governed525 by failure of the FRP jacket. Unlike steel tubes, the526 FRP tubes fracture in a brittle manner.528 6. The confinement model accounts for both totally530 filled tubes and tubes with central holes. It also531 accounts for axial load applied to both the concrete532 and FRP tubes. The model predicts well the stress–533 strain response of FRP confined concrete.535 7. Axial load – bending moment interaction curves of537 concrete-filled FRP tubes of moderate diameter-to-538 thickness ratios are similar to that of reinforced539 concrete members. It shows a tension region governed540 by fracture of the FRP tube, a balanced point, and a541 compression region governed by crushing of the tube.543 8. For a given wall thickness of the FRP tube, the545 laminate structure significantly affects the shape and546 size of the interaction diagram. There are several547 combinations of wall thickness and laminate structure548 that satisfies a particular combination of moment and549 axial load capacity.

551 Acknowledgments

552 The authors wish to acknowledge financial support553 provided by the Network of Centres of Excellence on

554

Intelligent Sensing for Innovative Structures(ISIS Can-555ada), the Natural Science and Engineering Research556Council of Canada(NSERC), Lafarge Canada and557Lancaster Composite. The authors are also grateful to558Bart Flisak for his assistance during the experimental559program.

560References

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