concrete

Prestressed Concrete–Filled Fiber-Reinforced Polymer Circular Tubes Tested in Flexure

Amir Fam, Ph.D., P.Eng. Associate Professor and Canada Research Chair in Innovative and Retrofitted Structures Department of Civil Engineering Queen’s University Kingston, Ontario, Canada

This paper presents test results of an experimental investigation carried out on 12.68-in.-diameter (322 mm) and 13.8-ft-long (4.2 m) prestressed concrete–filled fiber-reinforced polymer tubes (PCFFTs). A total of five PCFFT specimens and one control specimen, manufactured with steel spiral reinforcement instead of the fiber-reinforced polymer (FRP) tube, were tested in four-point bending. Four PCFFTs were pretensioned and one was post-tensioned using unbonded strands. Prestressing strands were oriented in a circular pattern symmetric about the tube’s longitudinal axis. Other parameters investigated were prestress level, laminate structures of the FRP tubes, and number of strands. One PCFFT was also compared with a concrete-filled FRP tube without any steel reinforcement. Test results indicate that prestressing not only improves the strength and serviceability of the system substantially but it also activates a confinement mechanism of the concrete core restrained by the FRP tube. In these specimens, the FRP tubes confine more concrete than the steel spirals and also contribute longitudinally as reinforcement, leading to significant enhancement of the members’ flexural strengths.

C

Siddhwartha Mandal, M.Sc. Bridge Engineer Delcan Corp. Calgary, Alberta, Canada and former graduate student Queen’s University

42

oncrete-filled fiber-reinforced polymer tubes (CFFTs) provide an emerging and promising system for a variety of structural applications in which the tubes serve as structural formwork. Circular CFFTs have already been implemented in the field as bridge piers,1 marine piles,2 and girders.3 In applications where a structural member’s design is governed by large

flexural loads, such as in the case of marine (fender) piles, laboratory and field bending tests have shown that CFFTs can match the bending strength of similarly sized conventional prestressed and reinforced concrete members. After cracking, however, CFFTs have lower flexural stiffness, which can result in large deflections of the member. The lower flexural stiffness is due to the low Young’s modulus of the fiber-reinPCI JOURNAL

Table 1. Properties of Glass Fiber-Reinforced Polymer Tubes Properties

Type I

Type II

Type III

12.76 (324)

12.76 (324)

12.68 (322)

Structural wall thickness, in. (mm)

0.2 (5.0)

0.21 (5.3)

0.18 (4.5)

Stacking sequence and fiber angles*

[-88/+8/-88/+8/ -88/+8/-88/+8/-88]

[-88/+8/-88/+8/ -88/+8/-88/+8/-88]

[-88/+6/-88/+6/ -88/+6/-88/+6/-88]

45.7 (315)

43.6 (301)

34.5 (238)

52 (359)

48.4 (334)

39.9 (275)

Hoop tensile strength, ksi (MPa)†

40.8 (281)

43.9 (303)

67.4 (465)

Elastic modulus-axial, ksi (GPa)

3220 (22.2)

3278 (22.6)

2857 (19.7)

2741 (18.9)

2872 (19.8)

3785 (26.1)

0.133‡

0.139‡

0.095‡

Outer diameter, in. (mm)

Axial tensile strength, ksi (MPa) Axial compressive strength, ksi (MPa)

†

Elastic modulus-hoop, ksi (GPa) Poisson’s ratio†

* Angles measured relative to the longitudinal direction of the tube. † Values are predicted using classical lamination theory.13 ‡ Based on circumferential strain-to-axial strain ratio due to axial loading.

forced polymer (FRP) tubes, which is particularly true for glass FRP (GFRP) tubes. One way to counteract the effects of this lack of flexural stiffness is to introduce longitudinal prestressing. While there has been considerable research in the past decade on CFFTs, very little research has focused on improving the flexural stiffness of the system. Prestressed CFFTs (PCFFTs) have been proposed as an alternative to corroded steel mooring piles, and one field investigation has been reported.4 In this field investigation, a 12.5-in.-diameter (317 mm), 45-ft-long (13.7 m) CFFT was post-tensioned using dywidag bars to produce an effective prestress of

4.5 ksi (31 MPa). This prestress was almost equal to the unconfined concrete strength, which took advantage of the tube’s confining effect. The pile was successfully driven 25 ft (7.6 m) into a riverbed and was tested under a lateral load of 2 kip (8.9 kN). The lateral deflection recorded was 1.3 in. (33 mm). Mirmiran and Shahawy conducted an analytical study on circular CFFTs partially prestressed with eccentric tendons.5 They recommended limiting the effective prestress to between 10% and 25% of the unconfined concrete strength and also limiting the tendon’seccentricity to the kern distance in order to avoid tensile stresses

at the interface of the concrete core and FRP tube. Zhu, Mirmiran, and Shahawy investigated a number of modular construction methods for a bridge pier system using CFFT beam and column components.6 They concluded that the posttensioned system exhibited the most robust and ductile behavior. This paper reports the test results from an experimental investigation to evaluate the effects of longitudinal prestressing on the flexural performance of CFFTs. It is concluded that the prestressing improves a CFFT’s serviceability and strength and also activates a confinement mechanism between the

Table 2. Properties of Test Specimens

Specimen Tube or Identification Spiral

Prestressing Type

No. of Strands


p

As Ac

(%)

PCSS

Steel spiral

PCFFT-1

Type I

PCFFT-2 PCFFT-3

1.23

Pretensioned

8 1.01

Type II

PCFFT-4 Type III PCFFT-5

Unbonded, post-tensioned

4

Concrete Strength

Jacking Force, At Test At kip Transfer, Date, ksi ksi

Effective Prestress Level Force in Steel, kip

Concrete Stress, ksi

Percentage Losses

3.78

6.6

238

178

1.8

25.3

4.09

6.1

238

204

1.6

14.5

3.54

5.6

109

89

0.7

18.0

4.09

6.1

238

201

1.57

15.7

3.78

6.1

124

107

0.84

13.8

5.49

6.1

135

108

0.86

19.97

0.51

Note: 1 ksi  6.895 MPa; 1 kip  4.45 kN.

July–August 2006

43

PCSS

5.75

12.68

12.76

2.2 3.2 2.2

11.2

Steel spirals @ 3 in. c/c

12.68

GFRP tube

Eight 1/2 in. strands

Four 1/2 in. strands

GFRP tube

PCFFT-1, PCFFT-2, and PCFFT-3

PCFFT-4

PCFFT-5

(Dimensions are in inches)

GFRP tube Lightweight GFRP tube can be lifted without a crane

Steel strands

Fig. 1. Details of test specimens. Note: 1 in.  25.4 mm.

concrete and the FRP tube. Six fullscale prestressed concrete specimens— five PCFFTs and one control specimen with conventional steel spirals—were tested in bending. Test parameters included jacking stress, fiber proportions in longitudinal and hoop directions (of the FRP tubes), number of steel strands, and pretensioned versus unbonded post-tensioned strands. On the FRP tube’s compression side, it behaved as a biaxially loaded membrane subjected to longitudinal compressive and hoop tensile stresses due to confinement pressure. Flexural responses, failure modes, and changes in stiffness under cyclic loading of the test specimens are discussed in the following sections.

EXPERIMENTAL INVESTIGATION Five full-scale PCFFT specimens and one control prestressed specimen were tested. The PCFFT specimens were fabricated using GFRP tubes of three different laminate structures, as shown in Table 1. The control specimen had no GFRP tube but contained conventional steel spirals and is designated as PCSS. Table 2 shows the properties of the test specimens, including their designations, type of tube, type of prestressing, reinforcement ratio
p (as a function of number of strands), concrete strength, 44

jacking force, effective prestress in the steel, effective stress in the concrete, and prestress losses. Detailed calculations of prestress losses are reported elsewhere.7 In conventional prestressed concrete piles, the effective prestress in the concrete typically varies from 0.39 ksi to 1.2 ksi (2.7 MPa to 8.3 MPa).8 In the present study, the effective prestress in the concrete core of the specimens varies from 0.7 ksi to 1.8 ksi (4.8 MPa to 12.4 MPa). The strands in all specimens were oriented in a circular pattern symmetric about the tube’s longitudinal axis with a clear cover of 2 in. (50 mm). The PCFFT beam specimens had span-to-depth ratios of approximately 11, while the ratio for the control specimen was 12.4 due to a slight difference in diameter. Figure 1 shows the cross-sectional details of the test specimens and pictures of a hollow tube and a pretensioned PCFFT specimen. Test specimens were designed to study the effects of several parameters. Specimens PCSS and PCFFT-1 had similar prestress levels and number of strands and were used to compare the flexural performances of PCFFTs and PCSSs. The effect of adding the prestressed reinforcement was studied by comparing specimen PCFFT-4 with a similar CFFT, without prestressed reinforcement, from the literature.9 The effect of prestress level was studied through specimens PCFFT-1 and PCFFT-2, which had

identical tubes and numbers of strands but different prestress levels. The effect of the amount of reinforcement, in terms of a total reinforcement index of both steel strands and GFRP tubes, was studied through specimens PCFFT2 and PCFFT-4, which had different tubes and numbers of strands but comparable prestress levels. The effect of the prestressing methods—bonded pretensioned versus unbonded post-tensioned—was studied through specimens PCFFT-4 and PCFFT-5, which had identical tubes and numbers of strands and similar effective prestress levels. Types I and II FRP tubes were almost similar in wall thickness and mechanical properties. As such, specimen PCFFT-3, with a prestress level and numbers of strands similar to specimen PCFFT-1, serves as a duplicate to confirm repeatability of test results. Materials The filament-wound GFRP tubes had a 51% fiber volume fraction and a smooth inner surface. The tubes were used as received (without providing bond enhancement measures before filling them with concrete). Table 1 lists the properties of the three GFRP tubes, including diameter, structural wall thickness, and stacking sequence of different layers, in terms of angles between the fibers and longitudinal axis of the tubes. The proportions of fibers oriented close to the longitudinal axis (+8 degrees or +6 degrees) and hoop direction (-88 degrees), for tubes I, II, and III are 1.7:1, 1.5:1, and 1:1.6, respectively. Mechanical properties of the GFRP tubes were determined by testing coupons cut from the tubes in the longitudinal direction using an MTS testing machine with wedge-type hydraulic grips. Although the ASTM D 3039 standard recommends long coupons for tension tests of composite materials, both short (7 in. = 1 in. [175 mm = 25 mm]) and long (12 in. = 1 in. [300 mm = 25 mm]) coupons were tested in order to examine the effect of coupon length.10 Compression tests were performed on 7 in. = 1 in. (175 mm = 25 mm) coupons as per the ASTM D 3410 standard.11 PCI JOURNAL

For all coupons, 3-in.-long (75 mm) end tabs, made of two layers of epoxy-impregnated GFRP fabric and bonded on both sides of the coupons, were used to eliminate the effect of the tube’s curvature and ensure flat surfaces to facilitate a uniform grip in the testing machine. Figure 2 shows details of the test coupons. To ensure a smooth flow of concrete inside the tubes, the concrete was designed to produce a slump of 7 in. (175 mm). The mixture had a water–cementitious materials ratio of 0.38 and contained 648 lb/yd3 (385 kg/m3) of Type 30 cement, 1620 lb/yd (960 kg/m3) of gravel, 1566 lb/yd3 (925 kg/m3) of sand, 25.9 oz/yd3 (1.0 L/m3) of Pozzolith® 100-XR, 77.5 oz/yd3 (3.0 L/m3) of accelerator, 9 oz/yd3 (0.345 L/m3) of Micro Air®, 9.2 gal./yd3 (145 L/m3) of water, and 6% air. The concrete was designed to have a compressive strength of 3.6 ksi (25 MPa) at prestress transfer, which is typically 24 hours after casting, and 5 ksi (35 MPa) at 28 days. Cylinders were tested in compression at the day of prestress transfer and at the day of testing, according to ASTM C 39.12 Concrete strengths are given in Table 2. Steel strands were 0.5-in.-diameter (13 mm), seven-wire, low-relaxation strands with a cross-sectional area of 0.152 in.2 (97.9 mm2). The strands had a Young’s modulus of 28.6 = 103 ksi (197.4 GPa) and ultimate strength of 286 ksi (1971 MPa). The strand yield strength, corresponding to a 1% elongation, was 261 ksi (1800 MPa), and the rupture strain was 7%. Steel spirals used to fabricate specimen PCSS consisted of a bundle of four 0.132-in.-diameter (3.35 mm) helical smooth wires with an inner diameter of 8.82 in. (224 mm) and a pitch of 3.15 in. (80 mm). The pitch and crosssectional area (based on four-wire bundle) were designed to provide the same circumferential stiffness to the PCSS as that of the GFRP tube used in specimen PCFFT-1. The yield and ultimate strengths of the wires were 52 ksi and 71 ksi (358 MPa and 489 MPa), respectively, based on tension tests. July–August 2006

2.9

(a) Short coupons

1.0

2.9

A

Strain gauge (axial)

A

GFRP tabs Coupon

2.9

(1.0 q 0.02) Section A - A 5.9

(b) Long coupons

2.9

A

A

(Dimensions are in inches)

Fig. 2. Details of (a) short coupons and (b) long coupons of glass fiber–reinforced polymer tubes. Note: 1 in.  25.4 mm.

Fabrication of Test Specimens All specimens were fabricated in a precast concrete plant (Fig. 3). Stressing of specimens PCSS and PCFFT-1 through PCFFT-4 was carried out in a conventional prestressing bed, as shown in Fig. 3a. Strands were inserted into the tubes

through holes in the wooden bulkheads, which were used to seal the ends of the tubes and were anchored against two steel abutments. For specimen PCSS, steel strands were first passed through the steel spirals and a small force was applied to the strands to prevent sagging. The spirals were then tied to

Cardboard tube

PCFFT-1 & PCFFT-2 Steel spirals @ 3 in. c/c

GFRP tubes

PCSS

(b) PCFFT-5

Steel strands

(a)

Hydraulic jack

(c)

Fig. 3. Fabrication of test specimens. Note: 1 in.  25.4 mm. 45

Strands were inserted and anchored at one end using standard monostrand anchors and 0.39-in.-thick (10 mm), 4 in. = 4 in. (100 mm = 100 mm) steel bearing plates. The strands were tensioned from the opposite end, one at a time, using a hydraulic jack, and were anchored using another set of chucks and steel plates (Fig. 3c). Prestressing forces were applied in five increments in rotation to control the tensile stresses in the concrete. Increments were applied to two opposite strands. The prestressing force was monitored using a pressure gauge and was also verified with strain gauge readings and strand elongation.

1.64 ft Load cell

Strain gauge

Dial gauge

Spreader beam

PI - gauge Potentiometer

11.81 ft

Instrumentation and Test Setup

Fig. 4. Test setup and instrumentation. Note: 1 ft  0.3048 m.

the steel strands and a cardboard tube was slid over the reinforcement cage (Fig. 3b). Prestressing was carried out by pulling one strand at a time using a hydraulic jack. Forces were monitored using a pressure gauge and were confirmed using the elongations of the strands at the end of prestressing. Then, concrete was pumped into the tubes through holes in the wooden bulkheads and specimens were steam cured for about 12 hours before the transfer of prestress. Once the concrete compressive strength reached about 3.6 ksi (25 MPa), based on concrete cylinder

tests, the prestress force was transferred to the specimens by cutting the strands with torches at both ends simultaneously. Forces were released in a specific sequence, by cutting each of the two strands positioned at 180 degrees apart, such that, due to the temporary eccentricities, the developed tensile stresses remained below the cracking strength. For the unbonded post-tensioned specimen PCFFT-5, four 0.71-in.diameter (18 mm) polyethylene tubes were used to accommodate the strands. The concrete pumping procedure was similar to the one previously described. The specimen was air-dried for seven days before post-tensioning.

Side view

58.0 Long Coupons Short Coupons b1 b2

e

Stress (ksi)

b1  b2 Short

Type I

Plain view

43.5

e

29.0 coupon

Type I

Long coupon

Type III Typical failure pattern

Type III 14.5

Premature failure (long coupons)

0

Typical failure pattern 0

5

10

15 20 Strain ( × 10-3 ) (a)

25

0

5

10 15 Strain ( × 10-3 )

20

(b)

Fig. 5. Stress-strain b›avior of glass fiber–reinforced polymer tubes in (a) axial tension and (b) axial compression. Note: 1 ksi  6.895 MPa. 46

25

Before transferring prestress, circular demountable mechanical (DEMEC) discs were mounted on the concrete and tube surfaces at three locations: midspan and 47.2 in. (1200 mm) from each end. The latter DEMEC discs were beyond the transfer length and also far from the maximum moment region. Small holes were drilled through the tube to install the DEMEC strain gauge discs on the concrete surface. The holes were located near the anticipated neutral axis level to minimize stress concentrations. After prestress transfer, elastic shortening losses were measured using an 8 in. (203 mm) DEMEC gauge. Strains measured on the FRP tube and concrete surfaces were very similar. Before testing, all specimens were instrumented at midspan and at both ends (Fig. 4). Electrical resistance strain gauges were used to measure strains in the axial and circumferential directions of the tubes. Displacement-type strain transducers (8 in. [200 mm] position indicator gauges) were also used on the extreme top and bottom surfaces to measure longitudinal strains. Midspan deflection was measured using two potentiometers. Mechanical dial gauges were used at both ends to monitor any slippage between concrete and the GFRP tube. Specimens were tested in four-point bending over a span of 11.8 ft (3.6 m) (Fig. 4). The load was applied using a 110 kip (490 kN) closed-loop Instron® machine under displacement control at a rate of 0.079 in./min (2 mm/min). The speciPCI JOURNAL

mens were unloaded and reloaded at three different load levels, namely, before yielding, near yielding, and after yielding of the strands. This scheme of cyclic loading was adopted to examine any change in flexural stiffness at various load levels.

In the following sections, results obtained from the six full-scale beam tests are presented, along with test results of the GFRP coupons. The flexural behaviors of the PCFFTs and PCSS are evaluated in terms of load-deflection and load-strain responses. The effects of various parameters, including prestress level, reinforcement ratio, laminate structure of the tube, and prestressing method, are discussed in detail. Change in stiffness at various load levels is evaluated. The behavior of a PCFFT has also been compared with the control PCSS specimen and with a CFFT without prestressing from the literature. GFRP Tube Coupon Tests Figure 5a shows the stress-strain curves obtained from tension tests conducted on one long and three short coupons for each tube Type I and Type III, respectively. Type II coupons had a similar stress-strain response to those from the Type-I tube. For the long coupons, the measured strains at failure were much lower than those measured in the beam specimens at failure. This is attributed to the helical geometry of the fibers. When narrow (1 in. [25 mm]) coupons are cut in the longitudinal direction and tested, some fibers become discontinuous between grips (due to the edge effect), particularly when a long coupon is used. This is illustrated schematically in the top left corner of Fig. 5a. This discontinuation results in underestimation of the tensile strength of the tube because the band width b1 of the continuous fibers between grips in the long coupon is smaller than that in the short coupon (b2) for a given fiber angle Ö. Therefore, long coupons were not reliable in determining the full tensile strength of the filament-wound tubes. The stress-strain curves obtained July–August 2006

67.2

Transverse splitting of tube at top

Crushing of tube at top

56.0 44.8

Crushing of concrete cover

Rupture of bottom strand

33.6

Transverse splitting of tube at top

Crushing of tube at top

22.4

Load (kip)

TEST RESULTS AND DISCUSSION

78.4

11.2

(a)

PCSS

0

Transverse splitting of tube at top

67.2

(b)

PCFFT-1

Rupture of bottom strand

56.0

(c)

PCFFT-2

PCFFT-4

Rupture of bottom strand

PCFFT-5

44.8 33.6 Crushing of tube at top

22.4

Crushing of tube at top

(d)

11.2

PCFFT-3

0 0

1.2

2.4

3.6

4.8

6.0

0

1.2

2.4

3.6

Crushing of tube at top

(e) 4.8

6.0

0

1.2

2.4

3.6

(f) 4.8

6.0

7.2

Deflection (in.)

Fig. 6. Load-deflection responses of the full-scale test specimens. Note: 1 in.  25.4 mm; 1 kip  4.45 kN.

from the short coupon tests showed a nonlinear behavior, and failure strains were quite similar to those measured in actual beams. The nonlinear curves depict the progressive failure of different layers, which was initiated by splitting of the transverse (-88 degrees) layers due to matrix cracking. This splitting was followed by rupture of the longitudinal layers (+8 degrees or +6 degrees), as shown in the bottom right corner of Fig. 5a. The compression stress-strain curves of three identical coupons, from the Types I and III tubes, are shown in Fig. 5b. Also shown in the same figure is the typical failure mode, which occurred due to combined crushing and splitting of fibers in different layers. Detailed mechanical properties of the tubes are also given in Table 1. General Response and Cyclic Loading B›avior of Full-Scale Beam Specimens Figure 6 shows the load-deflection responses of the six beam specimens. In general, all specimens exhibited a reduction in stiffness after the first cracking of the concrete core. The nonlinear behavior increased after steel yielding and continued until failure. Only specimen PCSS exhibited a near plastic response after yielding, whereas PCFFT

beams showed an ascending nonlinear trend due to the presence of the GFRP tubes. Figure 6 indicates that upon unloading at a load slightly higher than the cracking load (first cycle), the unloading-reloading path is bilinear. The stiffness becomes identical to the “uncracked stiffness” below the decompression load, which corresponds to zero stress at the bottom fiber due to crack closure. The beam recovers most of the deflection; a small residual deformation remains, however, due to the unrecoverable cracking strength of concrete. Similar behavior was observed when unloading near yielding of the strands (second cycle) but with a slightly higher residual deformation. When unloading at a higher load (beyond yielding), significant reduction in stiffness was observed due to excessive yielding of bottom strands and growth of cracks in the concrete core. It can be concluded that the change in stiffness mainly depends on concrete cracking and strand yielding. No slippage was observed between the concrete core and FRP tube during testing of the specimens. This suggests that sufficient bond strength was developed and may have been further enhanced by the radial pressure exerted by the tube through confinement of the prestressed concrete. In the following 47

Buckling of strands

Crushing of concrete cover

Splitting due to failure of hoop fibers

(a) PCSS

(b) PCFFT-1

Splitting due to failure of hoop fibers Splitting due to failure of hoop fibers

(c) PCFFT-2

(d) PCFFT-3

Single crack

Multiple cracks

Rupture of bottom fiber

(e) PCFFT-4

Rupture of bottom fiber

(f) PCFFT-5

Fig. 7. Failure modes of beam specimens (a) PCSS, (b) PCFFT-1, (c) PCFFT-2, (d) PCFFT-3, (e) PCFFT-4, and (f) PCFFT-5.

sections, test results and failure modes, which are shown for all specimens in Fig. 7, are discussed in detail along with the effects of various parameters. Effect of GFRP Tube In order to study the effect of the GFRP tube, the flexural behavior of the PCSS control specimen was compared with specimen PCFFT-1. The pitch s of the steel spirals in specimen PCSS was determined by equating the stiffness of the GFRP tube of specimen PCFFT-1 in the circumferential direction (Ef(hoop)tf /Rf) to that of the steel spirals in specimen PCSS, as illustrated in Fig. 8a and given in Eq. (1) and (2):

48

Es t s E f (hoop)t f = Rs Rf

(1)

ts  As /s

(2)

where As  cross-sectional area of steel spiral Es  elastic modulus of steel spiral Rs  radius of steel spiral Ef(hoop)  hoop modulus of the FRP tube Rf  radius of FRP tube to the middle of the wall thickness tf  thickness of FRP tube ts  thickness of a fictitious steel

tube equivalent to the steel spirals Although specimens PCSS and PCFFT-1 had the same number of strands, the small difference in their diameters resulted in slightly different reinforcement ratios
p of 1.23% and 1.01%, respectively, where
p is defined as the ratio of area of steel strands to the area of concrete core. Both specimens, however, had similar effective prestressing ratios fce' / fc' of 27.3% and 26%, respectively. This ratio is defined as the ratio of the effective prestress level in the concrete core fce to the unconfined concrete compressive strength fc'. The load-deflection behaviors of specimens PCSS and PCFFT-1 are compared in Fig. 8b. To eliminate the effects of slight differences in outer di' ameters Do and concrete strengths fc , the applied load P is normalized with respect to Do and fc' . Initially, both specimens exhibited linear behavior and similar stiffness until cracking, followed by nonlinear behavior, especially as the bottom strands yielded. Specimen PCSS showed a near plastic behavior once it reached peak strength, whereas specimen PCFFT-1 exhibited a nonlinear ascending behavior until it reached the maximum load, beyond which, the load dropped as the specimen failed progressively. Figure 8 clearly shows that the maximum load of PCFFT-1 is higher than that of PCSS. For any given load beyond cracking, the deflection of PCFFT-1 was smaller than that of PCSS. Thus, the GFRP tube has increased the ultimate strength and stiffness significantly and also improved the energy absorption capacity. This is attributed to two important aspects: The tube has a reinforcing effect in the longitudinal direction, which is not provided by the steel spirals, and also has a confining effect on the concrete core that is more effective than the steel spirals because the tube is located at the outermost surface and thus confines a larger concrete area. The concrete cover in specimen PCSS, which is about 44% of the area of compression zone based on neutral axis position at ultimate, was unconfined by the spirals and therefore vulnerable to spalling. Specimen PCSS failed by crushing and spalling of the concrete cover after PCI JOURNAL

July–August 2006

(a) FRP tube

t

Unit length

Rf fr

DO

2Rf

T

Steel Strand

T Unit length

Steel spiral

As fr

s RS

DO

2Rs

T

T

Steel strand

(b)

P f 'c D 2

.01

Normalized Load

yielding of the tension strands, followed by buckling of the strands on the compression side and eventually rupture of the tension strands, as shown in Fig. 7a. The load-axial strain response, shown in Fig. 9a (for all specimens), indicates that once the concrete strain at the extreme compression fiber of specimen PCSS reached 0.004, the load reached its peak level and stabilized after spalling of the crushed concrete cover. The specimen ultimately failed in a ductile manner. Specimen PCFFT-1 failed on the compression side, as shown in Fig. 7b. Its loadaxial strain response, shown in Fig. 9a, indicates that the axial compressive strain at extreme compression fiber at failure was 0.01, which is only 50% of the ultimate strain reported in Fig. 5b for the Type I tube. The axial versus hoop strain behavior at the extreme compression side of the tube of specimen PCFFT-1 is plotted in Fig. 10a and is characterized by almost bilinear response with an initial slope similar to the Poisson’s ratio of the tube (0.13). As the load increases, the concrete is highly stressed in compression and tends to dilate excessively in the transverse direction. At this point, the GFRP tube retains concrete expansion and becomes activated in confinement. As such, the hoop tensile strains in the tube increase rapidly, as evident from the second slope of the curve in Fig. 10a. The hoop and axial strain distributions on the cross section of specimen PCFFT-1 at midspan under peak load are shown in Fig. 10b. In general, Fig. 10 clearly shows that the extreme compression side of the tube was biaxially stressed through hoop tensile and axial compressive stresses. The measured hoop strain at failure in specimen PCFFT-1 was 0.0052, which corresponds to only 25% of the ultimate hoop strength reported in Table 1 for a Type I tube. The observed lower strengths of the tube in both directions, compared with the uniaxial full strengths, are a result of the biaxial state of stress in the tube. This agrees with the concept of Tsai-Wu biaxial strength failure theory of composites.13 The biaxial state of stresses caused the tube to fail under a combined crushing in the axial direction and splitting in the hoop direction, as shown in Fig. 7b. The bottom strands

Transverse splitting of GFRP tube at top

Yielding of bottom strands

0.08

Crushing of tube at top 0.06 Crushing of concrete cover PCFFT-1 (tube)

0.04 Rupture of bottom strands

0.02 First cracking

PCSS (spiral)

0 0

1

2

3

4

5

6

Deflection (in.)

Fig. 8. (a) Concept of equivalent stiffness of glass fiber–reinforced polymer (GFRP) tube compared with steel spiral and (b) nominal load versus deflection b›avior of GFRP tube compared with steel spiral. Note: 1 in.  25.4 mm.

of PCFFT-1 yielded sufficiently at ultimate load, but did not fracture as in the PCSS specimen. (Fig. 9b) Effect of Prestressed Reinforcement To assess the improvement in flexural performance of the prestressed specimens compared with nonprestressed specimens, PCFFT-4 was compared with a CFFT specimen tested by Fam and Rizkalla.9 The 12.83-in.-diameter (326 mm) GFRP tube of the CFFT specimen had a 0.25 in. (6.4 mm) wall thickness and an ultimate tensile strength of 34.4 ksi (237 MPa) and was filled with concrete with an 8.7 ksi (60 MPa) design compressive strength. The Fam and Rizkalla specimen did not include any internal steel reinforcement and was tested in four-point bending over a span of 18 ft (5.5 m). To eliminate the effect of differences in span lengths and concrete strengths

in this comparison, the moment-curvature responses of the specimens were compared rather than their loaddeflection responses. The responses were normalized with respect to each specimen’s respective outer diameter Do and concrete strength fc' . The FRP reinforcement ratios
f in specimens PCFFT-4 and CFFT were 3.12% and 3.92%, respectively. For a large diameter Do–to–thickness t ratio, the FRP reinforcement ratio
f is defined as 4t
f = Do . The normalized moment-curvature response curves, indicate that the CFFT specimen has a substantially lower cracking strength and initial stiffness than specimen PCFFT-4 (Fig. 11). The stiffness of CFFT is mainly controlled by the tube and exhibits a rather linear behavior. Specimen PCFFT-4 showed 49