Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017 pp. 477-487
No. 43-41
Modified split disk test for characterization of frp composites G. Ramesh*,, Ravindra Gettu** and B.H. Bharatkumar* Email:
[email protected]
*CSIR-Structural Engineering Research Centre, CSIR-Campus, Taramani, Chennai - 600 113, India. **Indian Institute of Technology Madras, Chennai - 600 036, India. Received: 28 March 2016; Accepted: 13 April 2016
Fiber reinforced polymers (FRP) are used to wrap reinforced concrete (RC) elements to increase their load carrying capacity and, in such applications, the mode of failure of the retrofitted element is usually governed by the rupture of the FRP when it reaches its ultimate strain. This paper reports the split disk test (as in ASTM D2290) modified to characterize the stress-strain behaviour of FRP using a simple specimen preparation methodology. The results obtained from the modified split-disk tests on glass and carbon-FRP composites are compared with those from uniaxial FRP coupon tests. It is found that the elastic modulus obtained from both the methods are similar but the ultimate strains and the tensile strengths from the split-disk tests are lower. To compare the split disk ultimate strains, FRP confined concrete cylinders of 150 mm in diameter and 300 mm in height were prepared and tested under uniaxial compression, after wrapping with carbon and glass FRP sheets. The average ultimate strains of FRP with split-disk test are lower than the ultimate hoop strains observed in FRP-confined cylindrical specimens. Consequently, strain efficiency factors have been determined for application in design. Keywords: Carbon fiber; glass fiber; FRP; split disk test.
Characterization of FRP composites is commonly carried out using the uniaxial flat coupon test, say, in accordance with ASTM D 30391. However, during the failure of wrapped concrete elements, the FRP also undergoes bending, resulting in lower ultimate stresses and strains2-5. Consequently, the behaviour of FRP under hoop stresses has been studied using methods adopted from those used for filament-wound fibrous composites, including the split disk test, hydrostatic hoop burst pressure test with polytetrafluoroethylene (PTFE) rings, and tests with expandable bladder systems and mechanical quadrants6-10. More recently, the ultimate strain of FRP has been evaluated by rupturing the composite using the expansive force of ice; De Caso y Basalo et al.11 filled an instrumented FRP shell with water, which is then frozen causing the shell to rupture. The authors state the main merit of this
method as the application of a uniform internal pressure without any friction. However, this method cannot be used for the determination of the modulus of elasticity or failure stress. The split-disk test was developed based on the US Naval Ordnance Laboratory ring test12, which was designed for obtaining the apparent tensile strength of reinforced thermosetting resin pipes, and extruded and moulded thermo-plastic pipes13. It has been used by a few researchers4,14,15, who have found that the ultimate hoop stress and strain are lower than those obtained from flat coupon tests. This paper reports about a methodology for a modified split-disk test, with the objective of characterizing the ultimate circumferential (hoop) strain in FRP sheets, the elastic modulus and tensile strength. A novel specimen preparation methodology is also developed. The motivation for Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
477
this is to adequately characterize FRP for confinement applications, where the design requires the ultimate strain values, as in ACI 440R16. Split-disk tests have been done on FRP with carbon and glass fibers, with one and two layers, and the results have been compared with those of uniaxial FRP coupon tests and uniaxial compression tests on FRP-confined concrete cylinders.
(a)
(b)
(c)
(h)
(g)
(f)
(d)
Proposed methodology for the split-disk test
The step-by-step preparation of the FRP rings for the modified split disk testis illustrated in Fig. 1. A split disk with a diameter of 150 mm is used instead of 50 mm specified in ASTM D229013, to be closer to typical strengthening applications. Figure 1 (a) shows a cast iron mould of 150 mm diameter and 300 mm height, which is filled with a mixture of plaster of Paris and water, having a flowing consistency. The setting of the plaster starts within ten minutes after mixing, and hardening completes in 45 minutes, with no shrinkage. The plaster cylinder is demoulded after two hours and allowed to dry for a few minutes (Fig.1 (b)). The circumferential surface of the cylinderis wrapped with a polyethylene sheet of 1 micron thickness to prevent the penetration of epoxy glue into the plaster (Fig.1 (c)). Over the polyethylene sheet, a two component epoxy saturant, prepared as per the manufacturer’s recommendations,is applied as shown in Fig.1(d). The carbon and glass sheets (with the properties given in Table 1) are measured and cut to the required length; lap lengths of 150 mm have been provided. Before wrapping, the FRP sheet is saturated with epoxy resin and wrapped onto the surface of the plaster cylinder carefully to avoid misalignment (Fig. 1(e)). Ribbed rollers are used to remove air voids between the sheet and plaster. To prevent the sagging of FRP sheet or settling of epoxy resin by gravity, and to maintain uniform thickness, a polyethylene sheet of 1 mm thickness is wrapped around the FRP (Fig.1 (f)). After curing, the required width (here, 25 mm) of the rings is marked and the wrapped cylinder issliced (Fig.1 (g)); the slicing is carried out with the hardened gypsum plaster to avoid any damage to the FRP. The gypsum plaster is then removed to extract the ring. The dimensions are verified before testing (Fig.1 (h)).This procedure is simpler and more reliable than fabricating individual rings by wrapping the FRP around a metal disk4. 478
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
(e)
Fig. 1 Split ring specimen preparation sequence Table 1 Properties
of
fibres (as provided manufacturer)
Data given by the manufacturer Thickness of the fibre, GSM Modulus of elasticity, kN/mm
by the
E-Glass
Carbon
900
230
73
240
3400
3800
Density, g/cm3
2.6
1.7
Ultimate strain, %
4.5
1.6
2
Tensile strength, N/mm2
(a)
(b)
(f)
(c)
(e)
(d)
Fig. 2 Split disk testing sequence
The fixtures for the split disk testare similar to those described in ASTM D 229013, where the top and bottom pieces are self-aligning. The galvanized steel end fixtures are clamped in the grips of the testing machine using 30 mm diameter bars of 100 mm length. The sequence of setting up the test for the FRP rings is shown in Fig. 2 (a-f). The FRP rings are placed around the greased split disk (Fig. 2 (b)). The end rods are
6
Axial force
1
SG-4(lap)
150
SG-1
SG-2 SG-3
SG-3
Axial force (kN)
SG-3 SG-1
2 0
0
2000 4000 6000 8000 10000 12000 14000 16000 Strain (microstrain)
Fig. 4 Load vs. strain plots of one and two layered CFRP rings SG-6 (lap) SG-3 SG-5 (lap) SG-4 (lap) SG-2 SG-1
7 6 5
SG-1 SG-2 SG-3 SG-4 (lap) SG-5 (lap) SG-6 (lap)
4 3 2 0
(b) Two layer split ring
Fig. 3 Schematic diagram of strain gauge locations adopted in split disk test
One layer GFRP
0
2000 4000 6000 8000 10000 12000 14000 16000 Strain (microstrain)
20
SG-5 (lap)
18 16
Axial force (kN)
Figures 4 and 5 show the strain evolution in the split and overlapping regions until rupture. The results show that the strains are non-uniform around the circumference and the maximum values are observed at the split; the strains in the lap region are about 2538% of the ultimate tensile strain at the split, reflecting the effect of the higher thickness of FRP at the lap. The strains at the two ends of the split are similar while the strain at the bottom is slightly lower due to friction between the FRP ring and the disk.It was concluded that the lap region should be positioned, during testing, at the upper or lower part of the disk to ensure rupture
Two layer CFRP
4
1 (a) One layer split ring
SG-1 SG-2 SG-3 SG-4 (lap) SG-5 (lap) SG-6 (lap)
SG-5 (lap)
6
8 SG-6(lap)
SG-6 (lap)
8
9
SG-2
SG-1
SG-2
SG-4 (lap)
10
10
SG-4(lap) SG-6(lap)
2000 4000 6000 8000 10000 12000 14000 16000 Strain (microstrain)
12
SG-5(lap)
mm
0
14
Axial force (kN)
SG-5(lap)
SG-1 SG-2 SG-3 SG-4 (lap) SG-5 (lap) SG-6 (lap)
2
16
Axial force
SG-2 SG-1
3
Study of the hoop strain distribution in the ring
To analyse the strain distribution over the FRP ring and to decide the location of the strain gauges, one ring each with one and two layers of CFRP and GFRP was instrumented with six special electrical resistance strain gauges of 5 mm length (with a strain limit of 3% - appropriate for FRP tests) - two in the split region (SG-1 and SG-2), one away from the split region (SG-3) and three in the lap region (SG-4, SG-5, and SG-6), as shown in Fig. 3; the orientation of the strain gauges was along the fibre direction.
One layer CFRP
SG-3
SG-5 (lap) SG-6 (lap)
4
0
Results and Discussion
SG-4 (lap)
5
Axial force (kN)
gripped in a 500 kN MTS servohydraulic test machine. The load is applied at a displacement rate of 2 mm/ min till failure and measured using a strain gauge based load cell of 500 kN capacity. The load, displacement and strain gauge signals are acquired in an automatic data logging system. Before placing the FRP rings, the specimens were instrumented with strain gauges to measure the hoop strain (Fig. 2 (f)).
SG-6 (lap) SG-4 (lap)
SG-2 SG-3 SG-1
14
Two layer GFRP
12
SG-1 SG-2 SG-3 SG-4 (lap) SG-5 (lap) SG-6 (lap)
10 8 6 4 2 0
0
2000 4000 6000 8000 10000 1200014000 1600018000 Strain (microstrain)
Fig. 5 Load vs. strain plots of one and two layered GFRP ring
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
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Figures 6 and 7 show the in-plane force versus strain plots of one and two layered CFRP and GFRP rings, respectively; the in-plane force in the FRP is taken as half the load applied to the split-disk fixtures. The force-strain relationship in the CFRP specimens is almost linear up to failure, while in the case of GFRP there is some initial nonlinearity due to thicker rings, which leads to cracking during the straightening of the ring at the split. The failure is generally brittle, with the load-carrying capacity dropping rapidly after the peak due to rupture of the fibers. Figures 8 (a) and (b) shows the failure patterns of CFRP and GFRP rings; failure initiates by resin cracking near the split at the edges of the ring, parallel to the fibers, followed by the rupture of the fibers.The failure pattern of one and two layer CFRP and GFRP rings are similar. 10
Split CFRP-I-1 Split CFRP-I-2 Split CFRP-I-3 Split CFRP-I-4 Split CFRP-I-5
Axial force/layer (kN)
9 8 7 6 5
7 6 5 4 3 2 1 0
2
2000 4000 6000 8000 10000 12000 14000 16000 18000 Strain (microstrain) Split GFRP-II-1 Split GFRP-II-2 Split GFRP-II-3 Split GFRP-II-4 Split GFRP-II-5
9 8 7 6 5 4 3 2
0
2000 4000 6000 8000 10000 12000 14000 16000 18000 Strain (microstrain)
Fig. 7 Axial force vs. strain plot of one and two layer GFRP rings 0
2000
4000 6000 8000 Strain (microstrain)
10000
12000
8 7
Type
6 5 4 3 2 1
0
2000
4000 6000 8000 Strain (microstrain)
Table 2 Average test results from the split disk and uniaxial tests
Split CFRP-II-1 Split CFRP-II-2 Split CFRP-II-3 Split CFRP-II-4 Split CFRP-II-5
9
0
0
10
0
10
Axial force/layer (kN)
8
3
0
Split GFRP-I-1 Split GFRP-I-2 Split GFRP-I-3 Split GFRP-I-4 Split GFRP-I-5
9
1
4
1
10000
12000
Fig. 6 Axial force vs. strain plot of one and two layer CFRP rings
480
10
Axial force/layer (kN)
FRP characterization with split-disk tests
The ultimate force and stiffness data were averaged for sets of five specimens, and are reported in Table 2. The stiffness was found from the slope of the axial force versus strain plots between 10% and 40% of the ultimate force. It is seen that the stiffness obtained
Axial force/layer (kN)
of the FRP away from the laps. Strains in subsequent tests are monitored with only the two strain gauges SG-1 and SG-2 in line with the split.
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
Layers
Stiffness per layer (kN/microstrain) Average
SD*
Ultimate load per layer (kN) Average
SD*
–5
CFRP rings
1
6.6×10
5.3×10
5.20
0.70
2
6.8×10–4
8.3×10–5
6.40
0.90
CFRP coupons
1
6.5×10
–5
5.0×10
5.10
1.60
2
6.6×10–4
5.8×10–5
7.40
1.51
GFRP rings
1
6.3×10
1.0×10
7.80
1.30
2
6.0×10
–5
5.9×10
8.40
0.53
GFRP coupons
1
6.4×10–4
2.3×10–5
8.50
0.85
2
5.9×10
4.4×10
7.15
1.10
–4
–4
–4 –4
* SD = Standard Deviation
–4
–5
–5
from the force per layer is approximately the same for one and two layered rings, for both CFRP and GFRP. As the number of layers increases, the load-carrying capacity increases to some extent, with the ultimate force for two layered rings being more than double that of the one layered rings. Therefore, it appears that tests on one layered specimens, in the characterization of FRP, would give conservative results for multiple layered FRP applications. This confirms the conclusions reached by earlier researchers2,4,17,18.
based on ASTM D30391, between the strain values of 1000 and 3000 microstrains. The values of modulus, ultimate strain and tensile strength forthe different rings are reported in Table 3, in terms of the averages and standard deviations, obtained from 5 tests in each case. The ultimate strain has been taken as the maximum strain registered in the strain gauge at the rupture location. However, for the characterization of the elastic modulus, the average strains of SG-1 and SG-2 are considered. Comparison with the parameters obtained from coupon tests
(a)
(b)
Fig. 8 Typical failure pattern of CFRP (a) and GFRP (b) rings during testing
Nominal stress-strain characteristics were evaluated based on the average minimum thickness and width of the specimens. The measured load is converted to the nominal tensile stress by dividing half the applied load by the average cross-sectional area at the split, as in ASTM D 229013. The elastic modulus was computed
Uniaxial tests were conducted as per ASTM D 30391, using coupons of 25 mm width and lengths of 250 mm for CFRP and 300 mm for GFRP. The FRP specimens were tested with 2 mm thick CFRP tabs provided at the ends to prevent premature failure at the grips (Fig. 9(a)). Electrical strain gauges of 2 mm gauge length were mounted at mid-length in the axial direction on the coupons. The coupons were tested in a 250 kN servo controlled universal testing machine (Fig. 9(b)), with flat grips and a grip-pressure of 3.5 N/mm2. The load was applied at a rate of 2 mm/min till failure. Figure 10 compares the typical axial load per unit width per layer versus strain plots of one and two layered CFRP and GFRP split disk and coupon specimens, and relevant data obtained are compared in Table 3. The ultimate strains are found to be lower in the split disk tests than in the uniaxial coupon tests
Table 3 Average test results for ultimate strain, elastic modulus and tensile strength of split disk and uniaxial tests FRP thickness (mm) Type CFRP rings
Elastic modulus (GPa)
Ultimate strain
Tensile strength (MPa)
(%)
Layers Average#
SD*
Average
SD*
Average
SD*
Average
SD*
1
0.75
0.12
42.0
3.8
0.896
0.15
394.6
38.9
2
1.17
0.14
46.0
4.5
0.967
0.13
467.0
24.0
CFRP coupons
1
0.60
0.03
38.0
7.2
1.100
0.24
458.0
93.8
2
0.98
0.03
39.0
7.4
1.100
0.12
476.4
81.9
GFRP rings
1
1.39
0.14
25.0
5.7
1.298
0.27
276.5
31.7
2
2.28
0.04
28.0
4.9
1.484
0.23
277.0
22.6
1
1.07
0.07
25.5
0.6
1.500
0.20
315.4
5.6
2
1.88
0.05
25.5
5.0
1.300
0.28
338.8
41.8
GFRP coupons
* SD = Standard Deviation, # = average minimum thickness
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
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(a)
(b)
Fig. 9 (a) FRP coupons with tabs; (b) Testing of FRP coupons
Axial force/unit width/layer (N/mm)
400
CFRP I lay coupon CFRP II lay coupon CFRP I lay split CFRP II lay split
350 300 250 200 150 100 50 0
0
Axial force/unit width/layer (N/mm)
400
2000 4000 6000 8000 10000 12000 14000 16000 Strain (microstrain) Coup GFRP-I Coup GFRP-II Split GFRP-I Split GFRP-II
350 300 250
Compression tests on FRP confined concrete cylinders
200 150 100 50 0
0
2000 4000 6000 8000 10000 12000 14000 16000 Strain (microstrain)
Fig. 10 Axial load strain plots of one and two layered CFRP and GFRP rings and coupons
for CFRP; the ultimate strains for one and two layered CFRP are about 1.10% in the uniaxial tension tests whereas they are in the range of 0.90–1.00 % in the split disk specimens. In the case of GFRP, the rings and coupons exhibit ultimate strains of 1.30–1.50 %. The ratio between the ultimate strains obtained from the 482
ring tests to that obtained from the coupon tests is about 0.81 for one layer and 0.87 for two layers of CFRP, respectively, and about 0.86 and 1.14 for one and two layersof GFRP, respectively. Lam and Teng (2004) obtained the ratios of 0.68 for CFRP and 0.85 for GFRP rings. It, therefore, appears that the ratio between the ultimate strains obtained from the ring and coupon tests can vary significantly from one material to another, and a unique value cannot be used without verification. Further, the test results of GFRP exhibit more scatter than for CFRP as the former has woven fibres that lead to higher resin thickness and more initial deformation before the longitudinal fibres are loaded. The values of the elastic modulus for CFRP are marginally lower in the coupon tests than the split disk tests whereas they are similar in the GFRP. Table 3 shows that the standard deviations of the elastic modulus obtained from the ring test results are comparable to those found from the coupon tests, indicating that, from a statistical point of view, the quality of the split disk test data is similar to that of the coupon tests. The results confirm the observations of Lam and Teng4, Chen et al.15 and Tamzus et al.14, who noticed lower ultimate strain and tensile strength values in the ring tests than in the coupon tests. The curvature leads to bending of the FRP in the former test but the effect is material dependent, with the curvature of the FRP jacket having more detrimental effect in CFRP than for GFRP.
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
Cylinders of 150 mm in diameter and 300 mm in height were prepared from concrete with a design strength of 40 MPa, and tested under uniaxial compression, after wrapping with carbon and glass FRP sheets, using a wet layup process by impregnating a continuous fabric sheet with suitable epoxy resin. The FRP preparation is similar to that used for the preparation of the FRP coupons and rings. The lateral strains were measured using six TML BFLA strain gauges of 5 mm length mounted at midheight, with three of them on the lapped region and three away from the 150 mm laps. The specimens were tested in a servo-hydraulic MTS system of 2.5 MN capacity, under piston displacement control at a rate of 0.2 mm/min (Fig. 11). The axial stress versus lateral strain curves of the different FRP-confined concretes
specimens wrapped with one layer of GFRP. In the case of two layers of CFRP, the average maximum lateral strain for the confined specimen is in the order of 1.0% and it is 1.6% for the GFRP wraps. 70 60 Axial stress (MPa)
are shown in Figs. 12 to 15, with the strain obtained at locations away from the lap and at the overlap region. The hoop strain distribution is seento be non-uniform in the post-peak regime, which corresponds to formation of cracks and localization of strains. Consequently, the strain at failure varies significantly from one location to the other, with lower values in the overlap regions.
50 40 30 20 10 0
0
2000
4000 6000 8000 10000 12000 lateral strain (mm/mm) (a)
0
2000
4000 6000 8000 10000 12000 lateral strain (mm/mm) (b)
70
Axial stress (MPa)
60
Fig. 11 Test setup for uniaxial compression of wrapped cylinder Table 4 Results of compression tests on FRP confined concrete cylinders Type of wrap
FRP strain away from lap (%)
FRP strain at the lap (%)
Average
SD*
Average
SD*
One layer CFRP
0.627
0.22
0.416
0.15
Two layer CFRP
0.985
0.12
0.656
0.13
One layer GFRP
1.395
0.25
0.836
0.23
Two layer GFRP
1.567
0.19
0.650
0.21
* SD = Standard Deviation
The average ultimate strains in the FRP are given in Table 4, in terms of mean and standard deviations from three tests. The strains in the FRP have been averaged separately for the lap region and away from the lap region. The maximum lateral strain measured on CFRP confined concrete cylinders is found to be 0.63%, on average, when one layer is used,whereas it is 1.4% for
50 40 30 20 10 0
Fig. 12 Axial stress versus lateral strain data for a typical cylinder wrapped with one layer of CFRP, (a) away from the lap and (b) at the lap
The ultimate strain values observed in compression tests of FRP wrapped cylinders are known to be lower than those obtained from tests of flat coupons2,19-23. Consequently, Pessiki et al.21, and Lam and Teng23 proposedan FRP efficiency factor (ke) given as the ratio of the average strain in the FRP wrapsat rupture to the ultimate tensile strain obtained from the coupon tests. A similar factor has also been given in terms of the split disk test. The efficiency factor has been generally defined as: ke = kε1 kε2 (1)
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
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70
70
60
60
50 40 30 20
40 30 20
0
2000
4000 6000 8000 Lateral strain (mm/mm) (a)
0
10000 12000
70
70
60
60
50
Axial stress (MPa)
Axial stress (MPa)
0
50
10
10
40 30 20 10 0
0
2000 4000 6000 8000 10000 12000 14000 16000 18000
0
2000 4000 6000 8000 10000 12000 14000 16000 18000
Lateral strain (mm/mm) (a)
50 40 30 20 10
0
2000
4000 6000 8000 Lateral strain (mm/mm) (b)
10000 12000
Fig. 13 Axial stress versus lateral strain data for a typical cylinder wrapped with two layer of CFRP, (a) away from the lap and (b) at the lap
484
Applying Eq. (1) to the results obtained from the confined concrete and the coupon tests, it is seen that the efficiency factor is 0.56 and 0.89 for one and two layers of CFRP, respectively, considering the strains in the region away from the laps (Table 5). The values are comparable to those determined by Lam and Teng23. Similarly, values of 0.93 for one and 1.20 for two layers of GFRP, respectively, were obtained. All the efficiency factors are within the range of 0.57 to 1.22 obtained recently by Sadeghian and Fam24, and above the value of 0.55 given in the design procedure of ACI 440 2R0816. When the efficiency factors are determined (Eq. (1)) using the split-disk data, values of 1.08 and 1.15 are obtained for one and two layers of CFRP, respectively, considering the strains in the region away from the
Axial stress (MPa)
Axial stress (MPa)
where kε1 is the ratio between the average hoop strain in the FRP wrapped cylinder at rupture to the in-situ strain capacity of the FRP, which is the maximum strain in the FRP at rupture, and kε2 is the ratio between the insitu strain capacity and the material strain capacity of the FRP, given by the value obtained from the coupon or split-disk tests. The first factor kε1 accounts for the non-uniform strain distribution in the FRP jacket due to the heterogeneity of the deformations in concrete, and the second factor, kε2, accounts for the reduction in the ultimate strain capacity of the FRP when used as a wrap to confine concrete, which can be attributed to the local misalignment or waviness of fibers in the layup process leading to unequal stretching of the fibers and temperature effects, and the cumulative possibility of weaknesses in the FRP jacket21,22.
Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
0
Lateral strain (mm/mm) (b)
Fig. 14 Axial stress versus lateral strain data for a typical cylinder wrapped with one layer of GFRP, (a) away from the lap and (b) at the lap
Table 5 Efficiency factors for CFRP and GFRP wrapped specimens Average FRP hoop rupture strain (confined cylinder) (%)
Average FRP hoop rupture strain (split-disk) (%)
Ke (coupons)
Ke (split-disk)
outside overlapping zone
over the whole circumference
outside over-lapping zone
outside overlapping zone
outside overlapping zone
Type
Layers
CFRP
1
0.627
0.415
0.970
0.570
1.082
2
0.982
0.656
1.120
0.892
1.158
1
1.395
0.835
1.470
0.930
1.132
2
1.567
0.650
1.480
1.205
0.997
GFRP
in accordance with the conclusions of Mirmiran and Shahawy25, Lam and Teng4, and Tamuzs et al.14 that the ultimate strain in FRP determined in the split disk test is closer to the FRP rupture hoop strain in wrapped columns than that obtained from coupon tests. The split ring disk test with the proposed specimen preparation seems, therefore, to provide values that can be used for predictions of the behavior of FRP in confined concrete columns.
70
Axial stress (MPa)
60 50 40 30 20 10 0
Conclusions 0
2000 4000 6000 8000 10000 12000 14000 16000 18000
0
2000 4000 6000 8000 10000 12000 14000 16000 18000
Lateral strain (mm/mm) (a)
70
Axial stress (MPa)
60 50 40 30 20 10 0
Lateral strain (mm/mm) (b)
Fig. 15 Axial stress versus lateral strain data for a typical cylinder wrapped with two layer of GFRP, (a) away from the lap and (b) at the lap
laps (Table 5). Similarly, the efficiency factors are 0.99 and 1.13 for one and two layers of GFRP, respectively. Overall, it appears that the experimental efficiency factor for FRP based on the split-disk test is around 1.0 for both the CFRP and GFRP. The present data are
An experimental investigation was conducted to examine the ultimate strain and tensile load carrying capacity of FRP using a modified split disk test. Based on tests carried out on FRP (with carbon and glass fibres), the following conclusions are drawn: • The tensile behaviour of FRP can be evaluated with a simple split disk test setup with an appropriate specimen preparation methodology. • The ultimate FRP strain and tensile strength measured in the split disk test are lower than the values obtained from the flat coupon test, except for the case of two layer GFRP specimens, which can be attributed to the bending of the FRP during the test. The FRP parameters obtained in the split disk test are closer and more appropriate for applications with externally bonding, such as the confinement of concrete columns by FRP wrapping. • There is only a marginal difference in the elastic modulus between the data from the split disk and the coupon tests. • From the experimental observation on number of layers of FRP the load-carrying capacity increases to some extent, with the ultimate load for two Journal of Structural Engineering Vol. 43, No. 5, December 2016 - January 2017
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•
layered specimens being more than double that of the one layered specimen on both coupon and split-disk testing. Tests on one layered FRP specimens give conservative results for the design of multiple layered FRP applications. The experimental efficiency factor for FRP based on the split-disk test is around 1.0 for both the CFRP and GFRP.
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(Discussion on this article must reach the editor before March 31, 2017)
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