Behavior of Axially Loaded Concrete-Filled Circular FRP Tubes

Amir Z. Fam and Sami H. Rizkalla

Department of Civil Engineering, North Carolina State University, Raleigh, NC 27695-7533

ABSTRACT

This paper describes the behavior of concrete-filled gla s fiber reinforced polymer (GFRP) tubes,

under axial compression loading conditions. The study i eludes totally filled and partially filled

tubes with central hole as well as a tube-in-tube system with concrete filling between the two

tubes. GFRP tubes were designed to provide strength in both axial and transverse directions and

were also axially loaded with the concrete core. The study showed that the strength and ductility

of concrete are improved due to confinement using GFRP tubes. The highest confinement level

was achieved for totally filled tubes. Using central hole reduces the confinement effect,

however, using inner tube can enhance the confinement for this type of members. Test results

indicate that loading of the GFRP tubes reduces the confinement effectiveness. The effects of

laminate structure, hole size, interface condition between the tube and the concrete core, stiffness

of the tube and failure modes are discussed.

·Key words: FRP tube, confinement, partially filled, stubs, concrete, totally filled, stress, strain.

/

INTRODUCTION

There is a great demand for columns and piles to be constructed using more durable materials in

comparison to traditional construction materials. The new products have to withstand aggressive

corrosive environments, such as the splash zone in the case of marine piles [Stapleman 1997].

1

Similarly, bridge columns have to retain their structural integrity in. cold regions where salt is

used for deicing roads. One promising innovative structural system is concrete-filled fiber

reinforced polymer (FRP) tubes, which provide many unique advantages [Seible 1996]. The

FRP tube acts as a stay-in-place structural formwork to contain the fresh concrete, which may

save the costs of formwork and labor used by cast-in-place or pre-cast industries. At the same

time, the FRP tube acts as a non-corrosive reinforcement for the concrete for flexure and shear.

More importantly, the FRP tube provides confinement to the concrete in compression, which

significa.rttly improves the strength and ductility. The contained concrete is protected from

severe environmental effects and deterioration resulting from moisture intrusion [Mirmiran

1995].

Concrete-filled steel tubes have been used for years as piles and columns. Extensive research

~ has been established in this direction [Furlong 1967, Knowles and Park 1969, and Kilpatrick and

Rangan 1997]. It should be noted, however, that in addition to the corrosion problems of steel

tubes, the confinement effectiveness is reduced at low levels of loading if the tube is also loaded

in the axial direction. This is attributed to the fact that Poisson's ratio of concrete at low levels of

loading, 0.15 to 0.2, is smaller than the 0.3 value of steel [Prion and Boehme 1994 and Wei et al

1995]. On the other hand, Poisson's ratio ofFRP tubes can be controlled through selected design

of the laminate structure to provide more confinement effect [Shahawy and Mirmiran 1998]. /

The confining pressure provided by steel tubes is limited to a constant value once the tube yields,

whereas FRP tubes provide a continuously increasing confining pressure, which adds to both the

ultimate confined strength and ductility [Samaan et aI1998].

2

For practical applications, concrete-filled FRP tubes are used to carry axial compression loads

and may also be designed to resist bending moments. Most of the reported experimental and

analytical research in the area of concrete confinement using FRP [Mirmiran et al 1998] included

FRP tubes totally filled with concrete and the fibers were mainly oriented in the hoop direction to

provide maximum stiffness and strength for confinement. Other studies considered the case of

applying the axial load to the concrete core only for an optimum use of the FRP tube in the hoop

direction for confinement [Mirmiran and Shahawy 1997]. In such a case, slip could take place

between ,the concrete and the outer tube, and consequently the member cannot resist bending.

Confinement of concrete cylinders wrapped with FRP sheets and SUbjected to axial loading

conditions was also studied by other researchers [Nanni and Bradford 1995 and Picher et al

1996].

OBJECTIVES

-~';;,a;~ective of this paper is to describe the structural behavior of concrete-filled GFRP

tubular short columns (length to diameter ratio of 2:1) subjected to axial loads based on an

experimental investigation recently completed by the authors. A companion paper by the authors

[Fam and Rizkalla 2000] describes the analytical model proposed to predict the measured

behavior. The specific objectives of the paper are:

1. Compare the behavior of a concrete-filled GFRP tube to that ofa concrete-filled steel tube. /

2. Evaluate the behavior of concrete-filled GFRP tubes using central hole with and without an

inner GFRP tube.

3. Evaluate the effect of the condition of the GFRP tube/concrete interface. This includes a

smooth (natural) and roughened interface.

3

4. Evaluate confinement as affected by the stiffuess ofthe tube.

RESEARCH SIGNIFICANCE

This paper extends the knowledge in the area of concrete-filled FRP tubes, used as structural

members, by addressing new parameters intended to simulate practical applications and loading

conditions. The paper is focused on the behavior under axial compression loading conditions

and addresses the following aspects:

1. Axial loading applied to both the tube and the concrete simultaneously. This lo~ding

condition represents the use of the system for pile applications, which requires also resisting

bending moments. (

2. FRP tub s with a laminate structure designed to provide strength and stiffuess in both the

axial and transverse directions, rather than all fibers oriented only in the hoop direction.

3. Concret -filled FRP tubes with central hole. The hole could be maintained by a non-

structural tube such as a cardboard tube, or FRP tube to enhance the strength and

confinement. The central hole provides large reduction in self-weight as well as material

saving for large diameter piles.

EXPERIMENTAL PROGRAM

A total of 12 hybrid stubs were fabricated and subjected to axial compression loads. Table 1 /

provides details of the stubs including the cross-section configurations, diameter, height, and the

tube number used to fabricate the stubs as given in Table 2. The figure also provides material

type of the tubes, condition of the interface surface, and the unconfined concrete strengths. Stubs

1 and 7 were used to examine the effect of material type of the tube. Stubs 11 and 12 were used

4

to examine the effect oflaminate structure. Stubs 1,3, and 5 were used to examine the effect of

cross-section configuration. The effect of the GFRP/concrete interface condition was examined

using stubs (1 and 2), (3 and 4), and (5 and 6). The effect of hole size was studied using stubs 2

and 4 as well as stubs 8, 9, and 10. The effect of the stiffness of the tube in the hoop direction

was examined using stubs 2,8, 11, and 12.

Materials

Five different GFRP tubes and one steel tube were used to fabricate the stubs. The GFRP tubes

were standard products fabricated by Ameron International in Burkburnett, Texas, United States.

The continuous filament winding process [Murphy 1998] was used to fabricate the cylindrical

GFRP tubes using E-glass fibers and epoxy resin with fiber weight fraction of 68 percent. Table

2 provides details of the steel and GFRP tubes including the diameter, thickness, number of

layers and stacking sequence including the direction of fibers. For example, tube 2 has multiple

layers oriented towards the axial and hoop directions at angl s of 8 and 86 degrees respectively,

while tube 3 has multiple layers oriented at 4 and 88 degrees. All angles are measured with

respect to the longitudinal axis of the tube.

Tension tests were performed to evaluate the GFRP properties in both the axial and hoop

directions. In the hoop direction the split-disk method was used, where a narrow ring was cut /

from the hollow tube and tested as shown in Fig. la using two semi-circular rigid plates of the

same curvature as the GFRP rings to apply the tension force. Strain gauges were applied in the

hoop and axial direction within the expected failure zone. In the longitudinal direction, tension

coupons were tested as shown in Fig. 1 b. Since the stress concentration at the gripping location

5

can severely influence the strength of FRP coupons, the two ends of the GFRP strip were

embedded inside hollow steel tubes, 305 mm long each, and epoxy resin was used to bond the

coupon to the steel tubes. Gripping was applied to the steel tubes, and the tested part of the

GFRP coupon between the two steel tubes was 153 mm x 25 mm. Axial compression tests were

also performed on short GFRP rings, 25 mm in height, to prevent buckling of the specimen. The

load was increased until the specimens were crushed. The mechanical prope ies of the steel

tube were evaluated from a bending test performed on the hollow steel tube. Table 2

summarizes the mechanical properties of the tubes.

Two concrete mixes were designed to induce pressure fit into the tubes after hardening of the

concrete. Although sealed concrete is expected to have much lower shrinkage compared to

exposed concrete, the coefficients of thermal expansion of FRP tubes and concrete cores are

different, therefore, a contact pressure is established to avoid possible separation under different

temperatures. This was achieved by adding expansive agents, commercially known as CONEX

or CPD, to the concrete mixes. The first mix, Mix 1, contains the following materials per unit

cubic meter: 360 kg of type 10 cement, 89 kg of CONEX, 803 kg of sand, 998 kg of 3/8 in.

stone, 154 kg of water, 4.8 kg of superplasticizer, and 5 percent of air content. The designed

slump was 8 inches to provide good workability conditions. This mix has been used in real

applications involving concrete-filled FRP tubes for piles in marine environments and was used /

for stubs 1 to 10 of this study. Mix 2, used for stubs 11 and 12, contains the following materials

per cubic meter: 566 kg of type 10 cement, 5.7 kg of CPD, 768 kg of sand, 715 kg of 3/8 in.

stone, and 228 kg of water. Three cylinders were tested from each mix according to ASTM C39

6

standards using a rate of loading of 0.1 mmlmin. Fig. 2 shows the stress-strain curves in the

axial direction of the two types of concrete used in this study.

Fabrication of Stubs

In order to provide a rough interface surface between the concrete and the tubes, as in the case of

stubs 1,3, and 5, the inner surface of the GFRP tubes 1 and 2 (see Table 2) and the outer surface

of GFRP tube 6 were roughened. This was achieved by applying a thin layer of epoxy to the

surface, and silica sand was sprayed later on top of the tacky epoxy. The tubes were phlced

inclined at an angle of 16 degrees as shown in Fig. 3a to facilitate casting concrete. The central

holes of stubs 3, 4, 9, and 10 was achl ved using cardboard tubes, while that for stubs 5 and 6

was achieved using GFRP tube 6, which we secured in position through wooden end plugs. In

practice, central holes may be constructed using mandrels that are pulled out after concrete

hardening, or by spinning the FRP tube while it is partially filled with fresh concrete, similar to

the technique currently used in fabricating spun-cast concrete poles. Strain gauges were installed

at mid-height in the hoop direction of the tubes to monitor the expansion of concrete during

curing in room temperature. Immediately after casting, hoop strains were monitored for seven

weeks. Later, the stubs were cut to lengths twice the diameter using a diamond saw in order to

maintain a smooth and square surface as shown in Fig. 3b.

/

Instrumentation and Testing

At four locations around the perimeter of each stub, at a section located at mid-height, strains

were measured using both strain gauges and 100 mm displacement transducer PI-gauges. The

PI-gauges were attached in the axial direction on two opposite sides. Strain gauges were

7

installed on the other two opposite sides on both the axial and lateral directions, as shown in Fig.

4a. This pattern of gauges allows for axial strain measurements at four sides around the

perimeter for more accuracy as well as to check for any eccentricity, which is not intended

during loading. Stubs 5 and 6 included additional strain gauges installed in the lateral direction

on the inner and outer GFRP tubes 40 mm below the upper surface.

The axial load was applied to the entire cross-section surface including the concrete core and the

tube. Stubs were tested as shown in Fig. 4b, using a closed-loop MTS 5000 kN testing machine

under stroke control with a 0.1 mmlmin. rate of loading. A thin layer of quickset plaster was

placed between the end faces of the stub and both the loading and supporting steel plates to

ensure uniform distribution of the applied press e.

TEST RESULTS A D DISCUSSION

The following sections provide the test results and a discussion of the different aspects of the

study including the concrete expansion behavior during curing, behavior of confined concrete

under aXial loads, the effect of various parameters on the behavior, and failure modes.

Concrete Expansion during Curing

Fig. Sa shows the history of hoop strains developed after casting and during curing of the /

concrete for stubs 1, 3, 5, and 7. The behavior reflects the resultant of two mechanisms taking

place over time, the internal pressure developed due to expansion of concrete, as well as the

drying shrinkage. The behavior shows that the hoop strains reached the maximum values at

about 20 days regardless of the cross-section configuration or the material type of the tube

8

(GFRP or steel). The strains were almost stabilized with little reduction over time, which

indicates that the shrinkage process could have lasted longer than the expansion process.

Knowing the stiffness of the tubes in the hoop direction, (EtIR), one can estimate the sustained

radial pressure at the interface between concrete and the tube, a, using the following equation:

Et (1'=-8

R (1)

Where E is the elastic modulus of the tube in the hoop direction, t is the thickness of the tube, R

is the radius of the outer tube, and 8 is the stabilized hoop strain measured on the tube as given in

Fig. 5a. The estimated pressures were 0.63, 0.49, 0.42, and 5.13 MPa for stubs 1, 3, 5, and 7

respectively. The higher pressure in stub 7 is attributed to the higher stiffness of the tube, mainly

due to the higher elastic modulus of steel, 203 GPa compared to that ofGFRP, 33.4 GPa, as well

as the difference in wall thickness. The behavior also shows that less pressure is built up in the

case of central holes, which is also illustrated in Fig. 5b by comparing the history of concrete

expansion of totally filled tube, stub 8, to partially filled tubes with small and large holes, stubs 9

and 10 respectively. This is attributed to the fact that the central space provides some relief of

the internal pressure, as well as the fact that in partially filled tubes, there is less volume of

concrete to produce expansion as high as that in totally filled tubes. The negative hoop strain

developed during curing of Stub 10, as shown in Fig. 5b, is very small, 0.0001. In this stub, the ~,.,

effect of concrete shrinkage, even though is smaU'iv. sealed concrete, has overcome the

/

expansion effect of concrete after the first 10 days due to the large hole size. The bond between

concrete and tube was capable of transferring the very small hoop compressive strain to the tube

in this case. The effect of hole size on the sustained pressure ratio (0/0'"0) is also shown in Fig. 6

based on stubs 1 and 3 as well as stubs 8, 9, and 10. 0'" and 0'"0 are the pressures built in the

partially and totally filled tubes respectively, for the same GFRP tube. D j and Do are the inner

9

hole diameter and the outer diameter respectively. Curve fitting was imposed on the limited

measured values, mainly to provide a trend rather than specific equations. The figure clearly

indicates significant reduction of the pressure with increasing the size of the inner hole. The

trend is consistent for the two values of the stiffness (EtIR) used in the investigation. It should be

noted that the measured pre-strain in the FRP tubes was only in the range of 4 percent of the

ultimate strain of the material, therefore, its effect was ignored in this study.

Confinement Effect on Stress-Strain Behavior of Concrete

In order to evaluate the beneficial effect of confinement on the total load carrying capacity of the

proposed system, the measured axial load-strain behavior of stub 11 is compared to that obtained

by superposition of both the measured axial load-strain behavior of the hollow GFRP tube, and

of the plain concrete core individually, as shown in Fig. 7. At strain of about 0.004, the hollow

GFRP tube suffered local damage at. the loading end when tested in compression without

concrete filling. The figure clearly indicates that the capacity of the composite stub significantly

exceeds the load sharing capacity of the two individual materials. In fact, the predicted behavior

using superposition matches the measured response of the composite stub very well up to the

vicinity of the unconfined plain concrete strength, which strongly indicates the beneficial effect

of confinement beyond this stage, when concrete starts to expand excessively at a stress level of . "~

about 87 percent of the unconfined strength [A vram et a) 1981] and the confining mechanism is /

activated through the GFRP tube.

10

Effect of Material Type of the Tube

The behavior of GFRP confined concrete, stub 1, is compared to that of steel confined concrete,

stub 7, in Fig. 8. The plain concrete behavior is also shown for comparison. Although the

structural wall thickness ofthe GFRP tube is 37 percent less than that of the steel tube, they both

achieved the same axial strength. The behavior of GFRP confined concrete 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 tensile strength, in presence of the axial compressive stresses, it fractured

and the stub failed. 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 ofFRP confined concrete is attributed to the continuous increasing

of the confining pressure due to the linear nature ofFRP. On the other hand, once the steel tube

yields, the confining pressure is quasi-constant regardless ofthe degree of concrete expansion.

It is also shown that the measured strength of the GFRP/concrete hybrid system, stub 1, is 41

percent higher than the combined load capacities of the unconfined concrete and the GFRP tube

added together, whereas the increase in the capacity for the steeVconcrete system, stub 7, is only

18 percent. This behavior is attributed to the fact that a steel tube attracts more axial load as /

compared to a GFRP tube under the same axial strain level, mainly due to the higher elastic

modulus of steel as compared to GFRP in axial direction. It should also be noted that the wall

thickness of the steel tube is 1.59 times that of the GFRP tube.

11

Effect of Laminate Structure

Stubs 11 and 12 were identical except that the GFRP tube in stub 11 is filament wound, where

fibers are oriented in both longitudinal and hoop direction with a ratio of about 2:1, whereas the

GFRP tube of stub 12 is pultruded with all fibers oriented in the axial direction. Fig. 9 compares

the load-axial and lateral strain behavior for both' stubs. The figure clearly illustrates the

advantage of fibers in the hoop direction for confinement, resulting in much higher strength. The

pultruded tube fails to confine concrete at the vicinity of peak unconfined strength, when

concrete starts to expand excessively and the tube splits immediately due to the lack of stiffness

and strength in the hoop direction. This observation confirms similar findings reported by

Kanathrana and Lu (1998). In some cases, pultruded tubes are however used as permanent

formwork and also to provide additional axial load resistance.

Effect of Cross-Section Configurations and Interface Condition

Fig. lOa shows the load-strain behavior of stubs 1 to 6 including totally and partially filled GFRP

tubes, as well as stubs with tube-in-tube configuration. The confinement effectiveness can be

detected by the slope of the second part of the bi-linear response. It is evident that the totally

filled tube is more effective in confinement compared to the tubes with inner hole. However, it

is also evident that providing an additional inner GFRP tube, as in stubs 5 and 6, could enhance

the confinement. The behavior of stubs 5 and 6 shows a similar flat response to that of stubs 3 /

and 4 at the unconfined strength level, but soon the stiffness increases and the slope of the

second part of the response approaches a similar value to that of stubs 1 and 2 due to activation

of the inner tube in the confinement mechanism. This is also evident by the lateral strain

response on both the inner and outer GFRP tubes, as shown in Fig. lOb. At early stages of

12

loading, before concrete expan~bes were under lateral tensile strains due to Poisson's

ratio effect for tubes under axial compression. Once the concrete starts to expand excessively, it

presses against both tubes. At this point the outer tube is subjected to higher hoop tensile strains,

whereas the hoop strain of the inner tube starts to reverse toward the compression side. At this

point the stiffness is comparable to a totally filled tube. In general, from Fig. i Oa, the interface

condition had an insignificant effect on the behavior of stubs 1 to 4. A more pronounced effect

is recognized for stubs 5 and 6 due to the larger surface area of GFRP in contact with concrete,

yet the maximum strength was not affected.

Effect of Hole Size

The stress-strain response of confined concrete is compared in Fig. 11 a for stubs with various

hole sizes. Stubs 8, 9, and 10 are confined using the same GFRP tube with stiffness (EtIR) of

680 MPa. Stubs 2 and 4 are confined using a similar GFRP tube with stiffness of 1040 MPa.

The only variable among each group is the hole size, which clearly affects the behavior and the

confined strength. The confined strengths of the partially filled tubes are normalized with

respect to the strength of totally filled tubes and were related to the inner-to-outer diameter ratio

(Di IDo) as shown in Fig. lIb. The reduction in strength with increasing the hole size is

attributed to the fact that voided concrete cores results in reduction of the radial stresses under ~

axial compression, consequently reducing the contact pressure with the tube, therefore reducing /

the confinement effect.

13

Effect of Tube Stiffness on the Confinement

The confinement effectiveness, (I;) I;), is defined in this paper as the ratio between the

maximum strength of the ybrid stub to the strength of the unconfined concrete. This ratio is

affected by both the stre gth and the stiffuess (EtIR) of the tube in the hoop direction, which

control the level of confi ement. Stubs 1,2, 8, 11, and 12 were selected to examine the effect of

stiffuess of the tube on the confinement effectiveness, as shown in Fig. 12. The stubs consist of

totally filled GFRP tubes with different stiffuess values including 1040,680, 1462, and 550 MPa

for stubs 1 and 2, 8, 11, and 12, respectively. The figure shows the increase in the strength of

confined concrete by increasing the stiffuess of the tube. This could be achieved by using a

higher thickness-to-diameter ratio or using GFRP tubes with higher elastic moduli in the lateral

direction.

In 1997, Mirmiran and Shahawy tested concrete-filled GFRP tubes with a range of stiffuesses

(EtIR) very similar to that experienced in this study for stubs 1,2, 8, 11, and 12. The strength of

the GFRP tubes in the hoop direction was also very similar to that of stubs 1, 2, and 8. The

152.5 x 305 mm stubs included GFRP tube with 1.3, 2.1, and 3.0 mm wall thicknesses, filled "\

with 32 MPa concrete, and tested in compression by loading the concrete core only. The tubes

consisted of a filament-wound angle ply of polyester resin with E-glass fibres at ±15 degrees

with the hoop direction, resulting in elastic moduli of 37.2 to 40.7 GPa and strengths of 524 to /

641 MPa in the hoop direction, for the 1.3 to 3.0 mm thick tubes. The results of their study are

also plotted in Fig. 12 for comparison. It is clearly shown that the present study showed less

confinement effectiveness under the same stiffuess level. This is mainly attributed to the fact

that the GFRP tubes in the present study were axially loaded, therefore, they expanded outward

14

due to Poisson's ratio effect under their own share of axial load, which results in less contact

pressure with concrete. Another factor is that the tubes were also bi-axially loaded under axial

compression and lateral tension in the present study, which reduces their tensile strength in the

hoop direction, whereas the GFRP tubes in the other study were fully utilized in the hoop

direction only under uniaxial tensile stresses, which allows the development of the full tensile

strength.

This comparison shows that loading the concrete core only in order to preserve the strength an~

stiffuess of the tube to be fully utilized in the hoop direction, could overestimate the beneficial

effect of confinement. Most practical applications require a full composite action as well as

longitudinal strength and stiffuess for combined loading cases.

Axial-Lateral Strain Behavior

The axial-lateral strain behavior of the tube is a good indicator of the initiation of the

confinement mechanism. Fig. 13 shows the axial-lateral strain behavior of totally filled GFRP

and steel tubes, stubs 2 and 7, as well as stubs with inner hole and tube-in-tube, stubs 4 and 6.

The behavior of totally filled tubes is generally characterized by a bi-linear response, with the

first slope slightly higher than Poisson's ratio of the tube, because at this stage, the concrete

undergoes small elastic lateral, expansion corresponding to stable micro-cracking, which induces /

little tensile strains in the tube. Once the concrete starts to expand excessively near the vicinity

of the unconfined strength, the tube is fully activated in confinement and the rate of lateral

straining increases as shown by the second slope. The figure also shows that under the same

axial strain level, the lateral strain in stub 4 is less than that of stub 2, which indicates less

15

confinement due to the presence of inner hole as explained before. The figure also confirms that

using an additional GFRP tube to maintain the inner hole improves the confinement as indicated

by the slope of the second branch for stub 6. Steel tube shows a similar behavior with a higher

lateral strain level due to the higher Poisson's ratio of steel to start with.

Failure Modes

Fig. 14 shows the stubs after failure. The GFRP confined concrete failed by fracture of the tube

under a bi-axial state of stress. The tubes were subjected to axial compressive stresses and

lateral tensile stresses. In most cases, the apparent failure mode is rupture of the tube in the hoop

direction, and few stubs showed additional signs of crushing in the axial direction. Once the tube

fails, the concrete core immediately fails due to the loss of containment, as was also reported by

other researchers [Fardis and Khallili 1981 and Mirmiran et al 1998]. Steel con~, on the other hand, did not fail once the steel jacket yielded but showed a ductile response where

the concrete core kept expanding, causing the stub to bulge and sustain large axial strains.

SUMMARY AND CONCLUSIONS

This study described the behavior of axially loaded concrete-filled GFRP tubes with emphasis on

the beneficial effects of confinement in practical applications and realistic loading conditions.

The experimental study investigated in particular three aspects including; (i) partially filled /

GFRP tubes with inner holes, (ii) tube-in-tube system, and (iii) fully composite sections, where

the tube and the core are bonded to resist the total applied load. Laminate structure of the GFRP

tubes were designed to provide resistance in the axial and hoop directions

16

The study showed that the confinement effect is highly dependent on the boundary conditions,

laminate structure of the tube, and the size of the central hole. Experimental modeling in

previous studies could over estimate the confinement benefits by assuming ideal cases such as

tubes with maximum strength and stiffness in the hoop direction or tubes which are not axially

loaded in order to be fully utilized in the hoop direction. The following conclusions were drawn

from this study:

1. Ignoring the effect of axial loading of the tube in a concrete-filled GFRP tube under

compressIOn and assuming the development of the full hoop strength overestimate the

confinement effectiveness. The tube is bi-axially loaded under the effect of axial

compressive and hoop tensile stresses, therefore, the material strength is governed by the

biaxial strength envelope. Also loading the tube causes lateral expansion and results in less

contact (confining) pressure with concrete.

2. Totally filled GFRP tubes provide the most effective confinement. Although an inner hole

offers material saving and reduced self-weight, it reduces the confinement effect, even

though, high level of ductility is maintained.

3. If the hole is maintained by an inner GFRP tube, the confinement effectiveness is improved

and could approach that of a totally filled tube, depending on the stiffness of the inner tube.

The outer and inner tubes are subjected to hoop tensile and compressive stresses respectively.

4. 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.

5. Filament wound GFRP tubes are superior to pultruded tubes in confinement. The pultruded

tubes split immediately once the concrete starts to expand excessively and the stub fails.

17

6. Stress-strain response of GFRP confined concrete is bilinear with the transition zone near the

peak strength of the unconfined concrete. The slope of the second branch is governed by the

stiffness of the tube as well as the inner hole size.

7. Initiation of the confinement mechanism can be detected from the bi-linear axial-lateral strain

behavior of the tube. The slope of the first part almost represents Poisson's ratio of the tube.

A change of slope occurs when concrete expands, producing more lateral strains in the tube.

8. The interface condition between the tube and concrete has an insignificant effect on the

behavior when both the concrete core and the tube are axially loaded.

9. The ultimate strength of the stub is governed by failure of the GFRP jacket. Unlike a steel

tube, the GFRP tube fractures in a brittle manner.

ACKNOWLEDGEMENT

The authors wish to acknowledge financial support provided by the Network of Centres of

Excellence on Intelligent Sensing for Innovative Structures (ISIS Canada), the Natural Science

and Engineering Research Council of Canada (NSERC), Lancaster Composite and Ameron

International for fabricating and providing the FRP tubes. The authors are also grateful to Moray

McVey and Grant Horeczy of the University of Manitoba for their assistance during the

experimental program.

Di =

Do =

E =

/

NOTATIONS

diameter of inner hole of partially filled tube

diameter of the outer tube

elastic modulus of the tube in the hoop direction

18

compressive strength of the unconfined concrete

= compressive strength of the confined concrete

h(ten.) tensile strength of the material of the tube

h(comp.) = compressive strength of the material of the tube

R radius of the outer tube

t structural wall thickness of the outer tube

strain in the hoop direction in the confining tube

sustained radial pressure at the interface between the concrete core and the tube

due to expansion of concrete during curing

= sustained radial pressure due to expansion of concrete during curing in totally-

filled tubes

u = major Poisson's ratio of the tube, which causes strain in the hoop direction under

axial loading

REFERENCES

1. Avram, C., Facaoaru, I., Mirsu, 0., Filimon, I., and Tertea, I. "Concrete Str~ngth and

Strains", Ed. by Elsevier Scientific Publishing Company, 1981.

2. Daniel, I. M. and Ishai, O. "Ep.gineering Mechani~.s of Composite Materials", Ed. By

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Oxford University Press, New York, 1994.

3. Fam, Amir Z. and Rizkalla, Sami H. "Confinement Model for Axially Loaded Concrete

Confined by Circular FRP Tubes", Submitted to ACI Structural Journal, March, 2000.

19

4. Fardis, M. N. and Khalili, H. "Concrete Encased in Fibreglass-Reinforced Plastic", ACI

Structural Journal, Title No. 78-38, Nov.-Dec., 1981, pp. 440-446.

5. Furlong, R. W. "Strength of Steel-Encased Concrete Beam Columns", Proc. of the ASCE,

Vol. 93, No. ST5, Oct. 1967, pp. 113-124.

6. Furlong, R. W. "Strength of Steel-Encased Concrete Beam Columns ", Proc. of the ASCE,

Vol. 93, No. ST5, October 1967, pp. 113-124.

7. Kanatharana, J. and Lu, L. "Strength and Ductility of Concrete Columns Reinforced by FRP

Tubes", Proc. of the First International Conference on Composites in Infrastructure (ICCI'96),

Tucson, Puizona, Jan. 1998,pp.370-384.

8. Kilpatrick, A. E., and Rangan, B. V. "Deformation-Control Analysis of Composite Concrete

Columns", Research Report No.3/97, School of Civil Engineering, Curtin University of

Technology, Perth, Western Australia, July 1997.

9. Knowles, R. B. and Park, R. "Strength of Concrete Filled Steel Tubular Columns", Proc. of

the ASCE, Vol. 95, No. STI2, Dec. 1969, pp. 2565-2587.

10. Mirmiran, Amir "Concrete Composite Construction for Durability and Strength,"

Symposium on Extending Life span of Structures, International Association for Bridge and

Structural Engineering (IABSE), San Francisco, CA, Aug. 1995, pp. 1155-1160.

11. Mirmiran, A., Shahawy, M., Michel, S., El Echary, H., Mastrapa, J. c., and Pico, O. /

"Effect of Column Parameters on FRP-Confined Concrete", Journal of Composites for

Construction, ASCE, Vol.2, No.4, Nov. 1998, pp. 175-185.

12. Mirmiran, A., and Shahawy, M. "Behavior of Concrete Columns Confined by Fiber

Composites", Journal of Structural Engineering, May 1997, pp. 583-590.

20

13. Murphy, J. "Reinforced Plastic Handbook." Second Edition, Copyright 1998 Elsevier

Science Ltd.

14. Nanni, A., and Bradford, N. M. "FRP Jacketed Concrete Under Uniaxial Compression",

Constr. and Build. Mat., 1995,9(2), pp. 115-124.

15. Picher, F., Rochette, P., and Labossiere, P. "Confinement of Concrete Cylinders with

CFRP", Proc. Of the Ft Int. Con! Composites in Infrastructure, H. Saadatmanesh and M. R.

Ehsani, eds., University of Arizona, Tucson, Ariz., 1996, pp. 829-841.

16. Prion, H. G. L., and Boehme, J. "Beam-Column Behavior of Steel Tubes Filled with High

Strength Concrete", Canadian Journal of Civil Engineering, Vol. 21, 1994, pp. 207-218.

17. Samaan, M., Mirmiran, A., and Shahawy, M. "Model of Concrete Confined by Fiber

Composites", Journal of Structural Engineering, Sept. 1998, pp. 1025-1032.

i8. Seible, F. "Advanced Composite Materials for Bridges in the 21 st Century." Proc., of the

Advanced Composite Materials in Bridges and Structures, M. EI-Badry, editor, published by

CSCE, Montreal, 1996, pp.17-30.

19. Shahawy, M., and Mirmiran, A. "Hybrid FRP-Concrete Beam-Columns", Proc. of ICCEI5

Fifth International Conference on Composites Engineering, Las Vegas, July 5-11, 1998, pp. 619-

620.

20. Stapleman, J. "Pile On The Abuse", Composite Technology, Sept/Oct. 1997. /

21. Wei, S., Mau, S. T., and Mantrala, S. K. "Performance of New Sandwich Tube under Axial

Loading: Experiment", Journal of Structural Engineering, Vol. 121, No. 12, Dec. 1995, pp.

1806-1821.

21

List of Tables

Table 1. Details and configurations of test stubs

Table 2. Details and mechanical properties of the tubes used to fabricate the stubs

/

List of Figures

Fig. 1. Details of GFRP tension specimens in axial and hoop directions

Fig. 2. Typical stress-strain curves for the two concrete mixes

Fig. 3. Fabrication oftest stubs

Fig. 4. Instrumentation and test setup

Fig. 5. Development of hoop strains over time due to expansion of concrete

Fig. 6. Effect of the hole size on the pressure due to expansion of concrete

Fig. 7. Confinement effect on load-strain behavior of concrete

Fig. 8. Comparison between GFRP-confined concrete and steel-confined concrete

Fig. 9. Comparison between concrete confined using both filament wound and pultruded GFRP

tubes

Fig. 10. Effect of cross-section configuration and interface condition on load-strain behavior

Fig. 11. Effect of void size on confinement level

Fig. 12. Effect of stiffness of the tube on confinement effectiveness

Fig. 13. Variation oflateral strain at different axial strains

Fig. 14. Failure mode of different stubs

/

Table 1. Details and configurations of test stubs

Stub C fi Diam. Height !Ube Sh II Interface Concrete on Ig. rom e d.ti strength

no. (mm) (mm) Table 2 con I on MPa

1 168 336 2 GFRP roughened 58

2 168 336 2 GFRP natural 58

3 168/ 336 2 GFRP roughened 58 95

4 168/ 336 2 GFRP natural 58 95

5 168/ 336 2,6 GFRP roughened 58 89

6 168/ 336 2,6 GFRP natural 58 89

7 169 336 1 steel roughened 58

8 219 438 3 GFRP roughened 58

9 219/ 438 3 GFRP roughened 58 95

10 219/ 438 3 GFRP roughened 58 133

11 €I. 100 200 5 GFRP roughened 37 /

12 • 100 200 4 GFRP roughened 37

Table 2. Details and mechanical properties of the tubes used to fabricate the stubs

Tube Diam. Thickness Material Number sstacking

" equence no. (nun) (nun) . type of layers (d )' egrees·

1 169 4.09 steel

2 168 3.73 (including GFRP 9 [+8/-86/-86/+8/-86 1.17 liner) /+8/-86/+8/-86]

219 3.7 (including GFRP 9 [-88/-88/+4/-88/-88 1.49 liner) /+4/-88/+4/-88]

3

4 100

5 100

3.09

3.08

GFRP 1 [0] pultruded

GFRP 9 [-87/+3/-87/+3/-87 /+3/-87/+3/-87]

Thickness Axial direction Hoop direction of layers lu (ten.) lu (comp.) E u lu (ten.) E

(nun) (MPa) (MPa) (GPa} (MPa) (GPa)

[4.09] 3?5 305 203 0 3 305 203 (YIeld) (yield) . (yield)

[0.08/0.36/0.28/0.25/0.38 /0.25/0.36/0.25/0.35] 282.9 224.1** 19.8 0.066 548 33.4

[0.23/0.23/0.23/0.25/0.25 /0.28/0.25/0.24/0.25] 201.3 182.6** 19.8 0.055 548 33.4

[3.09] 629.5 161.3 37.7* 0.242* 39* 8.7*

[0.32/0.35/0.27/0.48/0.26 444 115 29 0.101* 398* 23* /0.5110.19/0.5110.19]

6 89 2.28 (including GFRP 9 [+15/-82/-82/+15/-82 [0.18/0.33/0.25/0.18/0.25 209* 241 17.6* 0.155* 261* 27.6* 0.22liner) /+15/-82/+15/-82] /0.18/0.25/0.18/0.25]

* Classical lamination theory ** Manufacturer data

Angles are measured with respect to the longitudinal axis of the tube

semi -circular steel plate

test zone instrumented with strain gauges III two directions

25 mm wide GFRPring

(a) Hoop direction

305

153

coupon

305

cross-section

(b) Axial direction

Fig. 1. Details of GFRP tension specimens in axial and hoop directions

70

60 Mix 1

- 50 ca c.. :i!: 40 t -In In ~ 30 Mix2 -en

20

10

0 0 2 4 6 8 10 12

Strain x 10-3

Fig. 2. Typical stress-strain curves for the two concrete mixes

(b) Stubs were cut using a diamond saw

Fig. 3. Fabrication of test stubs

(a) (b)

Fig. 4. Instrumentation and test setup

700 Stub 1 -~ 600

'l! -C() 500 e (,)

'E 400 Stub 5 -tn C ,- 300 I! -tn c.. 200 0 0

::z:: 100

0 0 5 10 15 20 25 30 35 40 45 50

Time (days)

(a) Stubs 1, 3, 5 and 7

600

-tn 500 c Stub 8 'm !: 400 tn • 0 t; 300 E ";;200 c

I! 100 -tn

g. 0 0 20 25 30 35 40 45 0 ::z::

-100

-20 Time (days)

(b) Stubs 8, 9 and 10

Fig,S, Development of hoop strains over time due to expansion of concrete

1.2

1 Stubs 1,8 (EtlR) = 1040 MPa

..- Stub 3 b

O 0.8 ~ -- 0.6 0 .. C'CS 0.4 l-

e :::J

0.2 tn tn e

D- o 0.1 0.2 0.3 0.4 .7

-0.2 Stub 10

-0.4 Inner-to-outer diameter ratio (Dj DJ

Fig. 6. Effect of the hole size on the pressure due to expansion of concrete

700

600

500 -z :. 400 "C

~ 300 ...J

200

100

0 0

Confinement effect

2 4 6 Strain x 10-3

8 10 12

Fig. 7. Confinement effect on load-strain behavior of concrete

2500

Stub 7 2000 + steel

1830 kN -~ 1500 1531 kN -"C ca 0

...J 1000 plain concrete

500

lateral axial 0

·15 -10 ·5 0 5 10 15 20 25 Strai n x 10-3

Fig. 8. Comparison between GFRP-confined concrete and steel-confined concrete

700~----------------------~------------~

600

500 - axial load Z ::. 400 contribution

"C of the GFRP CU pultruded tube ..9 300 261 kN

concrete core only

200

100 lat~ral Axial

0 -18 -15 -12 -9 -6 -3 0 3 6 9 12

Strain x 10-3

Fig. 9. Comparison between concrete confined using both filament wound and pultruded GFRP tubes

-

2500~----------------~------------------~

2000

Stub 1

\ Stub 2

~ 1500 -"C ca o

...J 1000 Stub 6/

500

o

Stub 3 I Stub 4

lateral

-15 -12 -9 -6 -3 0 3 Strain x 10-3

axial

6 9

(a) Load-strain behavior of stubs 1 to 6

1400

1200 /

12 15

_1000 Gauge A Gauge B z .lII:: -"C ca 0

...J

800

600

400

200 / Stub 5

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

Lateral strai n x 1 0-3

(b) Load-lateral strain behavior of stub 5

Fig. 10. Effect of cross-section configuration and interface condition on load-strain behavior

100r-------------------------------------~

-C'G

90

80

70

D... 60 :E -en 50 en ~ 40 -en

30

20

10

0 0

Plain concrete

2 4

Stub 2

Stub 8

Stub 9 Stub 4

Stub 10 f;c' .. , ............................... .

, confined f'"''''''

c unco~ned

6 8 10 12 14 Strai n x 10-3

(a) Stress-strain curves of stubs 2,4,8,9, 10 and plain concrete

1.1r-----------------------------------------~

~ ~ ~ 3, 1

-~ -~

o 0.9 ~ .s::. m 0.8 c ~ -en 0.7

"'C (1)

C Q: c 0.6 o

(.)

Stubs 2 and 8 ............. .......... .. ........

" , -' ••• , Stub 9 '" .. · .. t

" , Stub 10 .....

Stub 4 .' ' •• ,

f;c (solid) ..... ..................................... ' "

f;c (voided)" ............................ .

f;""""""""" ...... .

0.51-----..:.~.....-----.-~=~~~---.------l o 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Inner-to-outer diameter ratio (Dj DJ

(b) Reduction in confined strength due to increase in central hole size

Fig. 11. Effect of void size on confinement level

3

1

0.5

• Mirmiran and Shahawy (1997) Shell is only utilized in the hoop direction

• •

Lower Stub 12 S:'b S limit

•

• Stub 1 Stub 2

• • Stub 11

• Present study Shell is utilized in the hoop and axial directions

o ~------------------~==~==========~ o 0.2 0.4 0.6 O.S 1 1.2 1.4 1.6 1.S

Hoop stiffness of shell E t (GPa) R

Fig. 12. Effect of stiffness of the tube on confinement effectiveness

Axial strain x 10-3

o 0 2 4 6 S 10 12 14

<? -2 0 "I"'"

><-4 c

Stub 4

l! u; -6 ca ... (1)

- -S ca ..J

-10

-12 Steel

-14

Fig. 13. Variation of lateral strain at different axial strains

Fig. 14. Failure mode of different stubs