1

STATE OF THE ART OF CONCRETE-FILLED FRP TUBULAR STRUCTURAL MEMBERS

SUMMARY Concrete-filled fiber reinforced polymer (FRP) circular tubes provide an effective structural system for a variety of applications such as piles, columns, overhead sign structures and utility poles. This paper discusses the behavior of concrete-filled Glass-FRP tubes ranging in diameter from 90 to 942 mm, using test results of eight beams, five columns and ten beam-column specimens. The effects of concrete fill, laminate structure of the tube, reinforcement ratio based on the wall thickness, as well as different failure modes are examined. Analytical models have been established and used in a parametric study to examine the effects of fiber orientation within the FRP tubes, thickness of the FRP tube, and the diameter of a central hole, which could be used to reduce the self-weight of the member. The benefits of concrete fill as well as the confinement effects have been demonstrated experimentally and analytically. Keywords: FRP, tube, concrete-filled, confinement, beam, column INTRODUCTION The use of FRP materials in new structures is still limited to few demonstration projects. Innovative hybrid systems such as concrete-filled FRP tubes are proven to be effective in facing the great demand for corrosion-resistant piling, poles, highway overhead sign structures and bridge components [1], [2] and [3]. The FRP tube provides lightweight structural component, permanent formwork, non-corrosive characteristics, and saving of construction time and effort. The fibers in the circumferential direction are utilized to provide confinement of the concrete, while the fibers in the axial direction provides the flexural strength and stiffness. For long time, concrete-filled steel tubes are used as structural members and have been extensively studied [4 and 5]. Steel tubes are, however, susceptible to corrosion and could be less efficient in confinement at low load levels due to the higher Poisson’s ratio of steel, compared to that of concrete. On the other hand, the laminate structure of FRP tubes could be optimized by controlling the proportions of fibers in the axial and hoop directions to suit the application. For flexural members, larger stiffness would be required in the axial direction while for axial members, larger stiffness is required in the hoop direction as well as a minimum Poisson’s ratio in order to produce the maximum confinement of concrete.

Amir Fam Assistant Professor Department of Civil Engineering Queen’s University Kingston, ON, Canada K7L 3N6

Sami Rizkalla Distinguished Professor Civil Engineering Department North Carolina State University Raleigh, NC, USA 27695-7533

Sami Rizkalla, FACI, is a Distinguished Professor ofCivil Engineering and Construction and Director ofthe Constructed Facilities Laboratory (CFL) at NorthCarolina State University. He is Chairman of ACICommittee 440, Fiber reinforced PolymerReinforcement and is a member of Joint ACI-ASCECommittee 550, Precast Concrete Structures.

ACI member Amir Fam is an Assistant Professor atQueen’s University. He is a member of ACICommittee 440, Fiber reinforced PolymerReinforcement. His research interests include the useof fiber-reinforced polymers for reinforced andprestressed concrete structures as well as stay-in-place formwork for concrete members.

2

EXPERIMENTAL PROGRAM This paper discusses the test results of a large experimental program, which included testing of different beams, short columns, and beam-columns. Eight different filament-wound GFRP tubes, fabricated using E-glass fibers and either epoxy or polyester resin, as well as one steel tube, were used in this investigation. Table 1 provides the details of the tubes including diameter, thickness, stacking sequence of different layers and mechanical properties of the tubes in the axial and hoop directions. Table 2(a) shows the characteristics of test beams, including the cross-section configurations, span, spacing between the applied loads, tube identification number based on Table 1, and the concrete strength. Tables 2(b) and 2(c) provide similar details of the columns and the beam-column specimens respectively.

Fabrication of Specimens The tubes were placed in an inclined position as shown in Fig. 1 and concrete was cast from the top. The central hole in column C3 was achieved using a cardboard tube. Wooden plugs were used to seal the ends. The concrete mix was designed to provide pressure fit to the tubes by adding an expansive agent in order to prevent separation due to shrinkage. External vibration was applied. The columns and beam-column specimens were cut from some of the concrete-filled tubes using a diamond blade saw. Beam Tests Fig. 2(a) shows a typical test set-up for beams tested in bending using stroke control and four-point load configuration. Details of test beams are given in Table 2(a). Beam B1 is a hollow FRP tube, while B2 is concrete-filled tube, which is identical to the tube of B1. The two beams were used to study the effect of concrete filling. Beams B3 and B4 were tested to study the effect of two different FRP laminate structures. The tubes of B3 and B4 are very close in diameter and wall thickness. B3 has 33 and 67 percent of the fibers oriented at 15 and 82 degrees respectively with the axial direction, while B4 has all fibers oriented at ±30 degrees with the axial direction. Beams B5 and B6 are used to determine the pure flexural strength of two different concrete-filled FRP tubes, and are used to establish one point of the beam-column interaction diagrams of the two types of tubes. The two tubes of B5 and B6 are similar in size and wall thickness, however, B5 has almost equal fiber ratios at 3 and 88 degrees with the axial direction, while B6 has 70 percent of fibers oriented at ±34 degrees and 30 percent at 80 degrees with the axial direction. B6, B7 and B8 are used to study the effect of different reinforcement ratios since all beams have similar laminate structure but different diameter-to-wall thickness ratios, which provided reinforcement ratios of 7.4, 3.5 and 3.8 percent respectively, where the reinforcement ratio ρ is defined as the ratio of four times of the thickness t to the diameter D, (4t/D). For all test beams, mid-span deflections, axial and hoop strains at compression and tension sides, and applied loads have been measured. Other details of the beam tests can be found elsewhere [6].

Table 1 Details and mechanical properties of the tubes used to fabricate the specimens

Tubeno.

Diam-eter

(mm)

Thick-ness(mm)

stacking sequence

4 168 2.56 [+8/-86/-86/+8/-86/+8/-86/+8/-86]3 100 3.08 [-87/+3/-87/+3/-87/+3/-87/+3/-87]

1 89 2.05 [+15/-82/-82/+15/-82/+15/-82/+15/-82]

5 320 5.96 [+34/-34/+80/+34/-34]

449

N/A

Hoop directionAxial directionfu

(ten.) fu

(comp.) E fu

(ten.) E

283 224 19.8 0.066 548 33.4

MPa MPa GPa MPa GPa

14.7 0.34 N/A 17.1

N/A 29 398 23

υ

178

241 17.6 27.6

N/A

N/A0.152 90 2.0 [+30/-30/+30/-30/+30/-30] N/A N/A 22 9.7N/AN/A

0.1

6 326 6.4 [-88/+3/-88/-88/+3/-88/+3/-88/+3/-88] 17 0.11 N/A 24.2237 276

7 626 5.41 [+34/-34/+85/+34/-34] 14.3 0.32 N/A 17.7206 N/A

8 942 8.93 [+34/-34/+86/+34/-34/+34/-34] 15.2 0.39 N/A 15207 N/A

9 169 4.09 [steel tube] 203 0.3 305 203305 305

a a aaa

a a

a aaa

aaa

b

b

b b

Lamination theory Manufacturer dataa b

sc

c Angles are measured with respect to longitudinal direction

Table 1 Details and mechanical properties of the tubes used to fabricate the specimens

Tubeno.

Diam-eter

(mm)

Thick-ness(mm)

stacking sequence

4 168 2.56 [+8/-86/-86/+8/-86/+8/-86/+8/-86]3 100 3.08 [-87/+3/-87/+3/-87/+3/-87/+3/-87]

1 89 2.05 [+15/-82/-82/+15/-82/+15/-82/+15/-82]

5 320 5.96 [+34/-34/+80/+34/-34]

449

N/A

Hoop directionAxial directionfu

(ten.) fu

(comp.) E fu

(ten.) E

283 224 19.8 0.066 548 33.4

MPa MPa GPa MPa GPa

14.7 0.34 N/A 17.1

N/A 29 398 23

υ

178

241 17.6 27.6

N/A

N/A0.152 90 2.0 [+30/-30/+30/-30/+30/-30] N/A N/A 22 9.7N/AN/A

0.1

6 326 6.4 [-88/+3/-88/-88/+3/-88/+3/-88/+3/-88] 17 0.11 N/A 24.2237 276

7 626 5.41 [+34/-34/+85/+34/-34] 14.3 0.32 N/A 17.7206 N/A

8 942 8.93 [+34/-34/+86/+34/-34/+34/-34] 15.2 0.39 N/A 15207 N/A

9 169 4.09 [steel tube] 203 0.3 305 203305 305

a a aaa

a a

a aaa

aaa

b

b

b b

Lamination theory Manufacturer dataa b

sc

c Angles are measured with respect to longitudinal direction

3

Short Column Tests Fig. 2(b) shows a typical test set-up for short columns under axial compression using stroke control. Details of the specimens are given in Table 2(b). Columns C1 and C2 are used to compare FRP-confined concrete to steel-confined concrete. Both tubes have the same diameter, however, C2 is a concrete-filled GFRP tube with wall thickness 37 percent smaller than that of the steel tube of C1. C2 and C3 are similar concrete-filled FRP tubes, however, C3 has a concentric central hole, which could be used to reduce the self-weight of the member. C4 and C5 are used to determine the pure axial strength for two types of tubes, which will be used to establish the beam–column interaction curves. Both columns have FRP tubes similar to those of beams B5 and B6 respectively. Axial and lateral strains at four points around the perimeter as well as the applied load were measured. Other details of the short column tests can be found elsewhere [1]. Beam-Column Tests Fig. 2(c) shows a typical test setup for the beam-column specimens. The specimens were tested under an eccentric axial load using rigid steel caps at the ends to apply the axial compression using different eccentricities. Table 2(c) provides the details of the specimens including the maximum eccentricity measured at ultimate, which account for the P-∆ effect. Specimens BC1 to BC5 as well as B5 and C4 were

(a) Details of beams

Beamno.

Span(m)

Spacingbetweenloads (m)

TubefromTable 1

Config-uration(MPa)

B5 5.5 1.5 6 60

B6 5.5 1.5 5 67

B7 5.0 1.5 7 33

B8 10.4 1.5 8 58

Col-umn no.

Config-uration

Tube from

Table 1

C1 9

Height(mm)

336 58

(MPa)fc

’fc’

(b) Details of columns

B1 1.3 0.2 3 N/AB2 1.3 0.2 3 37B3 1.07 0.2 1 37B4 1.07 0.2 2 28

C2 4336 58

C3 4336 58

C4 61800 60

C5 51750 67

(c) Details of beam-columns

Spec.no.

height(m)

TubefromTable 1

Eccen-tricity(mm)

(MPa)

BC1BC2BC3

1.8 655145250 60

BC6BC7BC8BC9BC10

1.75 5

1130120231

67

329

e

Config-uration

fc’

BC4 342BC5 839

Table 2 Details and configurations of test specimens

89mm

(a) Details of beams

Beamno.

Span(m)

Spacingbetweenloads (m)

TubefromTable 1

Config-uration(MPa)

B5 5.5 1.5 6 60

B6 5.5 1.5 5 67

B7 5.0 1.5 7 33

B8 10.4 1.5 8 58

Col-umn no.

Config-uration

Tube from

Table 1

C1 9

Height(mm)

336 58

(MPa)fc

’fc’

(b) Details of columns

B1 1.3 0.2 3 N/AB2 1.3 0.2 3 37B3 1.07 0.2 1 37B4 1.07 0.2 2 28

C2 4336 58

C3 4336 58

C4 61800 60

C5 51750 67

(c) Details of beam-columns

Spec.no.

height(m)

TubefromTable 1

Eccen-tricity(mm)

(MPa)

BC1BC2BC3

1.8 655145250 60

BC6BC7BC8BC9BC10

1.75 5

1130120231

67

329

ee

Config-uration

fc’

BC4 342BC5 839

Table 2 Details and configurations of test specimens

89mm

Fig. 1 Casting setup of GFRP tubes

End plug

External vibrator

Fig. 1 Casting setup of GFRP tubes

End plug

External vibrator

End plug

External vibrator

4

used to establish the complete interaction diagram for concrete-filled FRP Tube number 6. Specimens BC6 to BC10 as well as B6 and C5 are used to establish the interaction diagram of concrete-filled FRP Tube number 5.

TEST RESULTS Beam Tests Fig. 3 shows the moment-curvature behavior of hollow and concrete-filled GFRP filament-wound tubes, B1 and B2 respectively. The figure indicates that the strength and stiffness are both significantly increased by filling the tube with concrete. The strength gain is 212 percent. The presence of concrete has contributed to the stiffness and moment resistance of the section in the compression zone of the beam. The concrete also provided internal support to the tube and prevented ovalization and local buckling. Fig. 3 also shows the failure mode of B1, which was due to local buckling and crushing of the hollow tube, while B2 had flexural tension failure due to rupture of the fibers in the tension side.

Fig. 4 compares the load-deflection behavior of B3 and B4, which are similar in size and wall thickness, but different in laminate structure. The relative stiffness after cracking is almost proportional to the relative effective elastic modulus of the tubes in the axial direction. Although both beams achieved similar flexural strength, the failure modes were quite different. B4 developed a gradual compression failure due to matrix cracking and fiber buckling in the compression zone, which contributed to the non-linear behavior. This is

B7

(a) Beam tests

Fig. 2 Test setup of beams, columns, and beam-columns

(b) Column tests (c) Beam-column tests

C4

BC

7

B7

(a) Beam tests

Fig. 2 Test setup of beams, columns, and beam-columns

(b) Column tests (c) Beam-column tests

C4

BC

7

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

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500

Curvature (1/m) x (10-3)

Mom

ent (

kN.m

)

ExperimentalModel

B2

B1Local crushing incompression side

B1

Rupture of fibers in tension side

B2

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

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500

Curvature (1/m) x (10-3)

Mom

ent (

kN.m

)

ExperimentalModel

B2

B1Local crushing incompression side

B1

Rupture of fibers in tension side

B2

0

2

4

6

8

10

12

14

16

0 100 200 300 400 500

Curvature (1/m) x (10-3)

Mom

ent (

kN.m

)

ExperimentalModelExperimentalModel

B2

B1Local crushing incompression side

B1

Rupture of fibers in tension side

B2

5

attributed to the absence of fibers in the hoop direction to confine the other layers. B3 had flexural tension failure. It should be noted that B3 has only 33 percent of the fibers oriented in the axial direction, which is relatively low for flexural members. Due to the difference in span of beams B6, B7 and B8, their behavior is compared using the moment-curvature response as shown in Fig. 5. The behavior is also normalized with respect to the outer diameter Do, due to the difference in their sizes. The normalized behavior of B7 and B8 is almost identical due to the similar reinforcement ratios, 3.5 and 3.8 percent respectively. B6 showed higher strength and stiffness due to the higher reinforcement ratio of 7.4 percent. All beams had similar laminate structure and all failed in tension by rupture of the fibers as shown in Fig. 5.

Short Column Tests The behavior of column C2, is compared to that of C1 in Fig. 6. The plain concrete behavior is also shown for comparison. Although the wall thickness of the GFRP tube of C2 is 37 percent less than that of the steel tube of C1, they both achieved the same axial strength. The behavior of C2 is characterized by a bi-linear response with a transition zone near the vicinity of unconfined concrete strength. The stiffness beyond this point, second slope, is governed by the stiffness of the GFRP tube. Once the tube reached its

0

5

10

15

20

25

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

B4

B3

B3

B4

Tension failure

Compression failure

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

0

5

10

15

20

25

0 10 20 30 40 50 60Deflection (mm)

Load

(kN

)

B4

B3

B3

B4

Tension failure

Compression failure

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

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25

Normalized curvature

Norm

aliz

ed m

omen

t(M

Pa) %4.7ρ =

%5.3ρ =

%8.3=ρ

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

3 oD

M

[ ] ( )3-10xD.Ψ o

B6

B7

B8

B7

Tension failure

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25

Normalized curvature

Norm

aliz

ed m

omen

t(M

Pa) %4.7ρ =

%5.3ρ =

%8.3=ρ

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

3 oD

M3 o

DM

[ ] ( )3-10xD.Ψ o

B6

B7

B8

B7

Tension failure

6

tensile strength, in presence of the axial compressive stresses, it fractured and the stub failed as shown in Fig. 6. On the other hand, steel confined concrete behaved almost linearly untill the steel tube yielded, followed by a plastic plateau with large deformations. The continious increase of the load resistance of FRP confined concrete is attributed to the continuous increasing of the confining pressure due to the linear behavior of FRP. On the other hand, once the steel tube yields, the confining pressure is stable regardless of the degree of concrete expansion until the column finally fails due to yielding and bulging of the steel tube as shown in Fig. 6.

Fig. 7 shows the stress-strain response of the FRP confined concrete of C2 (totally filled with concrete), and that of partially filled FRP tube, C3 (with central hole). It is evident that providing a central hole reduces the confinement effect due to the reduction of the internal radial stresses in presence of the central hole. In large diameter piles, central holes could however be used to reduce the self-weight of the member. C3 also failed by fracture of the tube as shown in Fig. 7.

0

500

1000

1500

2000

2500

0 5 10 15 20 25

Load

(kN)

Axial strain x 10-3

C1

C2

Plain concrete

C1 C2

Fig. 6 Load-axial strain behavior of C1 and C2

0

500

1000

1500

2000

2500

0 5 10 15 20 25

Load

(kN)

Axial strain x 10-3

C1

C2

Plain concrete

C1 C2

Fig. 6 Load-axial strain behavior of C1 and C2

0

10

20

30

40

50

60

70

80

90

100

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Axia

l stre

ss (M

Pa)

C2

Experimental

Analytical

C3

C3

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

Axial strain

0

10

20

30

40

50

60

70

80

90

100

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014

Axia

l stre

ss (M

Pa)

C2

Experimental

Analytical

C3

C3

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

Axial strain

7

Beam-Column Tests For beam-column specimens, the bending moment at ultimate was estimated based on the ultimate axial load and the total eccentricity, which includes the initial eccentricity and the lateral deflection at mid height. Fig. 8 shows the axial load-bending moment interaction diagram for two types of concrete-filled FRP tubes (tubes number 5 and 6 in Table 1). The curves are of a similar shape to those of conventional reinforced concrete beam-columns. The first portion of the curves shows an increased bending moment as the axial load increases, which corresponds to the tension failure region. In this region, specimen failure is governed by the rupture of the fibres in the GFRP shell in tension as shown in Fig. 8. The curve reaches a balanced point, after which, increasing axial force is accompanied by decreasing bending moment. In this region, failure is governed by crushing of the fibres in the GFRP shell at the extreme compression face as also shown in Fig. 8. It is evident from Fig. 8 that Concrete-filled FRP Tube number 6 has a larger size interaction curve than that of concrete-filled FRP Tube number 5, despite the similar size and the comparable wall thickness. This is attributed to the better laminate structure of Tube number 6 in comparison to Tube number 5, which resulted in better strength and stiffness in the axial and hoop directions and much lower Poisson’s ratio, and therefore results in less separation from concrete. It should be noted that the concrete strength of Tube number 5 is 12 percent higher than that of Tube number 5. ANALYTICAL MODELING For axially loaded FRP-confined concrete, a confinement model based on equilibrium and radial displacement compatibility between the outer shell and the concrete core is developed. The model accounts for the continuously increasing confining pressure σR due to the linear material properties of the FRP tube [7] and utilizes the following Equation, which is driven based on radial displacement compatibility at the interface between the concrete core and the tube:

cc

c

c

s

scR

E1

tER

ευυυσ−

+

−=

)( (1)

Where υc and υs are Poisson’s ratio of concrete and FRP tube respectively. R is the radius of the concrete core. Ec and Es are the elastic moduli of concrete and the tube in the hoop direction respectively. εcc is the axial strain. The model also accounts for the bi-axial state of stresses developed in the FRP tube and utilizes the Tsai-Wu bi-axial failure criteria. The model predicts the stress-strain response of the confined concrete by combining Equation 1 with Mander’s confinement model [8]. The model can also account for the central hole inside the concrete core, utilizing an expression similar to that in Equation 1, but modified to account for the radius of the central hole. Fig. 7 shows the predicted stress-strain response of the confined concrete of specimens C2 and C3, with and without central hole, which showed good agreement.

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250 300

Moment (kN.m)Fig. 8 Axial load – bending moment interaction diagrams and failure modes of beam-column specimens

C4C5

BC1

BC2

BC6

BC3BC4 BC5

B5

B6

BC7

BC8BC9

BC10

Axia

l load

(kN)

Tension failureCompression failure

BC6 BC10

BC1 BC5

Tube no. 5Tube no. 6

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250 300

Moment (kN.m)Fig. 8 Axial load – bending moment interaction diagrams and failure modes of beam-column specimens

C4C5

BC1

BC2

BC6

BC3BC4 BC5

B5

B6

BC7

BC8BC9

BC10

Axia

l load

(kN)

Tension failureCompression failure

BC6 BC10

BC1 BC5

Tube no. 5Tube no. 6

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250 300

Moment (kN.m)Fig. 8 Axial load – bending moment interaction diagrams and failure modes of beam-column specimens

C4C5

BC1

BC2

BC6

BC3BC4 BC5

B5

B6

BC7

BC8BC9

BC10

Axia

l load

(kN)

Tension failureCompression failure

BC6 BC10

BC1 BC5

Tube no. 5Tube no. 6

8

For flexural members, an analytical model based on strain compatibility, equilibrium and effective mechanical properties of the FRP tube in the axial direction has been developed. The model utilizes an unconfined stress-strain response with extended strain softening of concrete in the compression zone, based on the fact that in bending, there is insignificant confinement effect in the compression zone. This in fact is demonstrated experimentally in Fig. 9, which shows the measured strain in the hoop direction of column C4 versus the measured axial strain. Also shown on the same figure, the hoop strains versus the axial compressive strains, measured on the compression side of beam B5 of the same FRP tube. The figure clearly shows that in columns, the hoop strains increases significantly at a certain level of axial strain, due to concrete internal microcracking and expansion of concrete, which activates the FRP tube and develops significant confinement. On the other hand, in beams, the rate of increase of lateral strains remains stable, almost similar to the Poisson’s ratio of the tube, which indicates lack of confinement in the compression zone. However, the ductility of concrete is increased as evident by the large axial strains achieved. Other details of the analytical model could be found elsewhere [9]. Fig. 3 shows the predicted moment-curvature response of B2, which showed good agreement with the measured response. Also shown on the same graph the predicted response of the hollow FRP tube, B1. As the eccentricity of the axial load increases from zero (columns) to infinity (beams), the size of compression zone of concrete is gradually reduced and a strain gradient is developed, therefore, the confinement effect is also gradually reduced. The confinement level of concrete in the compression zone at any given eccentricity varies between an upper bound (fully confined concrete) and a lower bound (unconfined concrete). This variable confinement model is currently being developed by the authors.

PARAMETRIC STUDY Axial Members The confinement model for axially loaded members was used in a parametric study to examine the effects of loading the concrete core only in comparison to loading the core and the tube, the effect of the wall thickness of the tube, and the effect of the central hole size. For this study, a typical 150 mm diameter concrete-filled GFRP tube using 40 MPa concrete and 0.18 initial Poisson’s ratio was used. The GFRP tube has all fibers oriented in the hoop direction. For the E-glass/epoxy GFRP tube, the major and minor elastic moduli were 39 and 8.6 GPa and the major and minor Poisson’s ratios were 0.28 and 0.06, respectively. The tube thickness was varied from 0.5 mm to 8 mm. The diameter of the central hole was varied from zero (totally filled tube) to 125 mm for the case of the 2 mm GFRP tube. Fig. 10 shows the confined stress-strain curves of concrete for the case of loading only the concrete core of the hybrid system, (a), and loading both the concrete core and the FRP tube, (b), in comparison to the unconfined stress-strain curve of the same concrete. When the load is applied to the core only, the 2 mm thick tube is fully utilized in the

Fig. 9 Axial strain versus hoop strains in columns and beams

Axial strain x 10-3

C4

0

1

2

3

4

5

6

-8 -7 -6 -5 -4 -3 -2 -1 0

υs 1

B5 Hoop

stra

in x

10-

3

Fig. 9 Axial strain versus hoop strains in columns and beams

Axial strain x 10-3

C4

0

1

2

3

4

5

6

-8 -7 -6 -5 -4 -3 -2 -1 0

υs 1

B5 Hoop

stra

in x

10-

3

Axial strain x 10-3

C4

0

1

2

3

4

5

6

-8 -7 -6 -5 -4 -3 -2 -1 00

1

2

3

4

5

6

-8 -7 -6 -5 -4 -3 -2 -1 0

υs 1

B5 Hoop

stra

in x

10-

3

9

hoop direction and develops the full hoop tensile strength as shown in the failure criteria, which is also shown in Fig. 10, where the stress path (a) represents hoop tensile stresses only.

When the load is applied to both the core and the tube, significant reduction in strength and stiffness is observed. The reduced strength is attributed to the bi-axial state of stress developed in the tube as illustrated by the bi-axial stress path (b), which intersects the failure envelope at a point with lower hoop strength than that of case (a). The slightly reduced stiffness is attributed to the outward expansion of the tube due to its Poisson’s ratio effect, which reduces the confining pressure. Fig. 11 shows the variation of the strength of confined concrete fcc

’, normalized with respect to the unconfined strength fc

’, with the stiffness of the tube in the hoop direction (Est/R) as controlled by the wall thickness, where R and t are the radius and thickness of the FRP tube respectively, and Es is the elastic modulus of the tube in the hoop direction. The figure also shows the variation of the ratio (fcc

’/ fc’) with the

inner-to-outer diameter ratio of columns with central holes (Di/Do). The figure indicates that increasing the stiffness of the tube in the hoop direction would increase the strength of confined concrete, however, the rate of increase is nonlinear. The figure also shows that below a certain stiffness level, there is no gain in strength, due to the post-peak softening behavior. Also, increasing the size of the central hole would reduce the confinement effect due to the reduced radial confining pressure.

0

20

40

60

80

100

120

140

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04Axial strain

Axia

l stre

ss (M

Pa)

unconfined concreteonly core is loadedcore and tube are loaded

fx (MPa)

0

50

100

150

0 600 1200

f y(M

Pa) Bi-axial failure

envelope

Stress path

failure

fx

fy

a

a

bb

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

0

20

40

60

80

100

120

140

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04Axial strain

Axia

l stre

ss (M

Pa)

unconfined concreteonly core is loadedcore and tube are loaded

fx (MPa)

0

50

100

150

0 600 1200

f y(M

Pa) Bi-axial failure

envelope

Stress path

failure

fx

fy

a

a

bb

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

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

(Di / Do)

(Es t /R)

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

s

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

(Di / Do)

(Es t /R)

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

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5

0 0.2 0.4 0.6 0.8 1

Inner-to-outer diameter ratio (Di / Do)

Stiffness of the tube (E t /R)

Con

finem

ent e

ffect

iven

ess

(f cc’

/ fc’)

(Di / Do)

(Es t /R)

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

s

10

Flexural Members The strain compatibility model for analysis of flexural members was used in a parametric study to examine the effects of laminate structure of the FRP tube, thickness of the tube and effect of filling both high stiffness (thick) and low stiffness (thin) FRP tubes. A 300 mm diameter GFRP tube with a [0/90]s symmetric cross ply E-glass/epoxy laminate is used in the analysis. The laminate structure is changed by varying the proportions of fibers in the axial [0] and hoop [90] directions including 9:1, 3:1, 1:1, and 1:3 ratios respectively. A 3:1 laminate indicates that 75 percent of fibers is oriented in the axial direction. The different proportions resulted in effective elastic moduli and tensile strengths of 35.2 to 11.5 GPa and 976 to 319 MPa respectively in the axial direction. The wall thickness varied from 2 to 16 mm, which is equivalent to reinforcement ratios of 2.67 to 21.33 percent. The concrete strength was 50 MPa. Fig. 12 shows the effect of increasing the wall thickness as well as the ratio of fibers in the axial direction, which are represented in terms of the reinforcement index ω, on the normalized flexural strength M . The reinforcement index is given in Equation 2 in terms of the wall thickness t , the outer diameter oD , FRP

axial tensile strength uf , and concrete strength 'cf .

'c

u

o ff

Dt4ω = (2)

The ultimate moment uM is normalized with respect to oD and '

cf . For the case of the concrete-filled GFRP tubes with thickness ranging from 2 to 16 mm, the laminates of all the tubes were maintained (1:1). Also for the case of the concrete-filled GFRP tubes with four different laminate structures ranging from (1:3) to (9:1), the wall thickness was maintained 4 mm. Fig. 12 clearly shows that increasing either the wall thickness of the tube (for a given laminate structure), or increasing the ratio of fibers in the axial direction (for a given wall thickness), have the same effect. They both result in increasing the flexural strength of the tube, however, the failure modes could change. Tubes with low stiffness in the axial direction or smaller wall thickness tend to fail in tension, while thicker wall tubes or tubes with large ratio of fibers in the axial direction tend to fail in compression. As a result, the balanced condition is not only dependent on the reinforcement ratio (wall thickness of the tube), but also on the laminate structure (ratio of fibers in the axial direction).

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250 300

Norm

aliz

ed u

ltimat

e m

omen

t

Reinforcement index 'c

u

o ff

Dt4

=ω (%)

t = 16 mm

t = 8 mm

t = 4 mmt = 2 mm

(9:1)(3:1)

(1:1)(1:3)

Ten.failure

Comp.failure

Balanced failure

' c3 o

u fDM

M=

Effect of wall thickness

Effect of laminate structure

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

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200 250 300

Norm

aliz

ed u

ltimat

e m

omen

t

Reinforcement index 'c

u

o ff

Dt4

=ω (%)

t = 16 mm

t = 8 mm

t = 4 mmt = 2 mm

(9:1)(3:1)

(1:1)(1:3)

Ten.failure

Comp.failure

Balanced failure

' c3 o

u fDM

M=

Effect of wall thickness

Effect of laminate structure

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

Norm

aliz

ed u

ltimat

e m

omen

t

Reinforcement index 'c

u

o ff

Dt4

=ω (%)

t = 16 mm

t = 8 mm

t = 4 mmt = 2 mm

(9:1)(3:1)

(1:1)(1:3)

Ten.failure

Comp.failure

Balanced failure

' c3 o

u fDM

M=

' c3 o

u fDM

M=

Effect of wall thickness

Effect of laminate structure

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

11

Fig. 13 shows the moment-curvature response of hollow and concrete-filled (1:1) GFRP tubes of two different wall thickness, 2 and 16 mm. The 2 mm tube represents a case of low stiffness tube while the 16 mm tube represents a very stiff tube. The failure of the 16 mm hollow tube was governed by the compressive strength of the shell, while the failure of the 2 mm hollow tube was governed by local buckling of the thin tube rather than material failure of the FRP tube [9]. The figure clearly shows that the concrete filling significantly improves the strength and ductility of low stiffness tubes, while its effect is relatively small in stiff tubes. The gain in flexural strength by filling the tubes with concrete was 20 percent in the 16 mm tube and 500 percent in the 2 mm tube. The effect of concrete filling in low stiffness tubes is reflected in two aspects. It prevents the premature local buckling failure in compression. In fact it could change the failure mode to tension failure. Concrete also contributes significantly to the internal moment resistance of the section by providing large compressive strength in the compression side of the beam.

CONCLUSIONS Based on a comprehensive experimental program using large-scale concrete-filled FRP tubes as well as analytical modeling and parametric studies, the behavior of concrete-filled FRP tubes as flexural members, axial members and beam-column members have been investigated. The following conclusions are drawn: 1. In bending, concrete filling is more efficient for thin-walled tubes or tubes with low stiffness in the axial direction than it is for stiff or thick-walled tubes. It prevents local buckling and contributes to the inertia and the internal forces, which increases the flexural strength and stiffness. 2. Load-deflection behavior of concrete-filled GFRP tubes is almost linear after cracking. Cracking load is significantly small compared to ultimate load. Stiffness after cracking is governed by the laminate structure and the diameter-to-thickness ratio of the tube. 3. Flexural strength can be increased either by increasing the wall thickness or the ratio of fibers in the axial direction of the tube. However, the failure mode could change to compression. Absence of fibers in the hoop direction could also lead to compression failure. 4. Using unconfined stress-strain curve with extended strain softening for concrete in the analytical modeling, predicts very well the flexural behavior of concrete-filled FRP tubes. Test results have shown insignificant confinement of concrete in flexural members, however, ductility of concrete is improved. 5. In flexural members, the balanced reinforcement ratio is dependent on the laminate structure of the tube and generally is smaller for tubes with higher stiffness in the axial direction.

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140

Mom

ent (

kN.m

)

D = 300 mm

t = 16 mmComp.failure

(1:1) laminate

Curvature (1/m) x (10-3)

Local bucklingfailure

Ten.failure

Materialfailure(comp.)

t = 2 mm

50 MPa='cf

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

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140

Mom

ent (

kN.m

)

D = 300 mm

t = 16 mmComp.failure

(1:1) laminate

Curvature (1/m) x (10-3)

Local bucklingfailure

Ten.failure

Materialfailure(comp.)

t = 2 mm

50 MPa='cf

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140

Mom

ent (

kN.m

)

D = 300 mm

t = 16 mmComp.failure

(1:1) laminate

Curvature (1/m) x (10-3)

Local bucklingfailure

Ten.failure

Materialfailure(comp.)

t = 2 mm

50 MPa='cf

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

12

6. In axial members, ignoring the effect of axial loading of the FRP tube under compression and assuming the development of its full hoop strength, overestimate the confinement effectiveness. The tube is bi-axially loaded under axial compressive and hoop tensile stresses, therefore, the material strength is governed by a biaxial failure criteria. Loading the tube also causes lateral expansion due to its Possion’s ratio, which results in less confining pressure. 7. Totally filled GFRP tubes provide the most effective confinement for columns. Although an inner hole offers material saving and reduced self-weight, it reduces the confinement. 8. GFRP tubes provide similar confined concrete strength to that provided by steel tubes, using a significantly smaller wall thickness of the tube, however, lower level of ductility is achieved. 9. The slope of the second branch of the bi-linear stress-strain response of FRP-confined concrete is governed by the stiffness of the tube as well as the size of the inner hole. 10. Initiation of confinement can be detected from the bi-linear axial-lateral strain behavior of the tube. The slope of the first part represents Poisson’s ratio of the tube. A change of slope occurs when concrete expands, producing more lateral strains in the tube. 11. The ultimate axial strength of the columns is governed by failure of the FRP jacket. Unlike steel tubes, the FRP tubes fracture in a brittle manner. 12. A confinement model has been developed. It accounts for totally filled tubes or tubes with central holes, as well as axial load applied to both the concrete and FRP tubes. The model predicts well the stress-strain response of FRP confined concrete. 13. Axial load – bending moment interaction curves of concrete-filled FRP tubes of moderate diameter-to-thickness ratios are similar to that of reinforced concrete members. It shows a tension region governed by fracture of the FRP tube, a balanced point, and a compression region governed by crushing of the tube. 14. For a given wall thickness of the FRP tube, the laminate structure significantly affects the shape and size of the interaction diagram. 15. There are several combinations of wall thickness and laminate structure that satisfies a particular combination of moment and axial load capacity. REFERENCES

[1] Fam AZ, Rizkalla SH. Behavior of axially loaded concrete-filled circular fiber reinforced polymer tubes. ACI Structural Journal 2001; Vol.98.No.3:280-289.

[2] Mirmiran A, Shahawy M. Behavior of concrete columns confined by fiber composites. Journal of Structural Engineering 1997; May:583-590.

[3] Seible F. Advanced composites materials for bridges in the 21st century. Proceeding of the First International Conference on Composites in Infrastructure (ICCI’96), Tucson, Arizona, 1996. p.17-30.

[4] Furlong RW. Strength of steel-encased concrete beam columns. Proceeding of the ASCE 1967; Vol. 93.No.ST5:113-124.

[5] Kilpatrick AE, Rangan BV. Tests on high-strength composite concrete columns. Research Report No. 1/97, School of Civil Engineering, Curtin University of Technology, Perth, Western Australia, 1997.

[6] Fam AZ, Rizkalla SH. Flexural behavior of concrete-filled FRP circular tubes. ASCE Journal of Composites for Construction, May, 2002, Vol. 6, No. 2, pp.123-132. [7] Fam AZ, Rizkalla SH. Confinement model for axially loaded concrete confined by FRP tubes. ACI Structural Journal 2001; Vol.98.No.4:451-461.

[8] Mander JB et al. Theoretical stress-strain model for confined concrete. Journal of Structural Engineering 1988; Vol.114.No.8:1804-1826.

[9] Fam AZ. Concrete-filled fiber reinforced polymer tubes for axial and flexural structural members. Ph.D. Thesis 2000, The University of Manitoba. p. 261.