Behavior of geopolymeric recycled aggregate concrete-filled FRP tube (GRACFFT) columns under lateral cyclic loading

Behavior of geopolymeric recycled aggregate concrete-filled FRP tube (GRACFFT) columns under lateral cyclic loading

Engineering Structures 222 (2020) 111047 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/...

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Engineering Structures 222 (2020) 111047

Contents lists available at ScienceDirect

Engineering Structures journal homepage: www.elsevier.com/locate/engstruct

Behavior of geopolymeric recycled aggregate concrete-filled FRP tube (GRACFFT) columns under lateral cyclic loading Jingming Caia, Haoyu Haoa, Togay Ozbakkaloglub, Yamei Zhangc, Jinlong Pana,

T



a

Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing, China Ingram School of Engineering, Texas State University, San Marcos, TX, USA c School of Materials Science and Engineering, Jiangsu Key Laboratory of Construction Materials, Southeast University, Nanjing, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: FRP Geopolymer Recycled aggregate Seismic behavior

In order to improve the environmental sustainability of concrete-filled FRP (fibre-reinforced plastic) tube (CFFT) column, a new kind of composite column, geopolymeric recycled aggregate concrete-filled FRP tube (GRACFFT) column, was proposed in this paper. The hysteretic behaviors of GRACFFT columns were experimentally investigated. The influences of different parameters, i.e., fiber types, section form, and reinforcement type were discussed. Based on the strain analysis, the plastic hinge length and utilization factor of FRP fibers for each GRACFFT column were also investigated. According to the experimental results, it was found that the proposed GRACFFT column showed favorable seismic performance with both high carrying capacity and deformation ability. The proposed GRACFFT column made with carbon FRP tube showed the best seismic behaviors in terms of yield displacement, peak load, ultimate displacement, and cumulative energy consumption. In terms of crosssection, the seismic behaviors of the circular GRACFFT column were better than that of the square column. Also, it was found that the substitution of steel reinforcements with steel tube could effectively increase the seismic behaviors of GRACFFT columns.

1. Introduction As a common construction material, concrete has been widely used in the construction and repair of infrastructures. Ordinary Portland cement (OPC) is commonly used as binder material for concrete. However, the production of OPC is prone to consume a large amount of natural material and simultaneously releases a substantial quantity of greenhouse gases. Generally, the production of 1 ton OPC requires 1.5 tons of resource materials and releases 0.9 tons of CO2 into the atmosphere [1]. It has been estimated that the total emissions from the cement industry contributed about 8% of global CO2 emissions [2]. Besides, the production of concrete also requires aggregates. For the production of natural aggregates, however, the mining and quarrying processes are normally energy-intensive and could cause irreversible changes to the earth’s hydrological and surface conditions [3]. In the past three years, the shortage of natural aggregates in many parts of China leads to long-distance hauling and higher costs [4]. Growing awareness of sustainability and environmental issues in recent decades has been pushing the whole construction industry to seek alternative building materials. In recent years, a novel construction material, geopolymeric



recycled aggregate concrete (GRAC), has attracted increasing attention from both researchers and engineers [5–7]. For GRAC, the geopolymer paste and recycled aggregate are used as binder and aggregates, respectively [8]. With the replacement of both OPC and natural aggregates, GRAC could be much more eco-friendly than conventional concrete. As the binder of GRAC, geopolymer is an alternative cementitious material synthesized by combining source materials rich in silica and alumina with strong alkali solutions [9]. Compared with conventional cement, geopolymers have the following advantages. First, the aluminosilicate materials for geopolymer can be from different sources such as industry and construction wastes [10]. Second, unlike OPC which requires high-temperature clinkering for its production, geopolymer can be produced at ambient temperature [11]. Third, geopolymers are capable of developing its full strength within a much shorter curing duration (generally 7 days) than that of OPC [12]. The basic material properties of GRAC have been investigated by many researchers, including compressive strength [13–14], microstructures [15] and durability [16]. Generally, it was concluded that the compressive strength of GRAC was comparable with conventional concrete, while the brittleness and efflorescence were more serious and evident [17–18]. The external confinement method may be a possible solution

Corresponding author. E-mail address: [email protected] (J. Pan).

https://doi.org/10.1016/j.engstruct.2020.111047 Received 18 March 2020; Received in revised form 28 June 2020; Accepted 30 June 2020 0141-0296/ © 2020 Elsevier Ltd. All rights reserved.

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illustrate the nomenclature: (1) the first letter “B” denotes the type of fiber and there are four types of fibers, i.e., basalt (B), flax (F), carbon (C) and glass (G); (2) the second letter “C” denotes the section form, which is either circular (C) or square (S); (3)the third letter “9” denotes the fiber layer; (4) the fourth letter denotes reinforcement type, which is either steel bar (B) or steel tube (T). According to the former researches [32], the confinement ratioη is listed in Table 1, which is defined as follows:

for the structural application of GRAC. Shi et.al [19] proposed GRAC filled steel tubular columns and the uniaxial compressive behaviors were investigated. It was found that the GRAC filled steel tubular column has both high strength and ductility under compressive load. A broader application of GRAC in the field of composite structures as well as their mechanical behaviors under different loading conditions, however, is still essential and open for discussions. In order to further increase the application field of GRAC, the external confinement of fiber-reinforced polymer (FRP) may be one of the potential solutions. As the third largest modern structural material after steel and concrete, FRPs have been successfully applied in the field of external reinforcements for concrete columns [20], beams [21], beamcolumn joints [22] and shear walls [23]. As typical FRP composite members, the concrete-filled FRP tubes (CFFTs) have attracted increasing attention during recent years. The statics behaviors of CFFT columns have been extensively studied, it was concluded that CFFTs have both higher strength and ductility than conventional steel reinforced concrete (RC) members under axial compression [24], eccentrical loading [25] and flexural loading [26]. Ozbakkaloglu et.al [27–28] studied the seismic performances of both square and circular CFFT columns. It was concluded that the application FRP tube, especially for carbon FRP, as confinement jackets could effectively increase the deformability and ductility of high strength concrete. The feasibility of using FRP tube as stay-in-place formwork for high strength concrete columns was also investigated [29]. It was found that the FRP formwork could provide continuous confinement for the entire RC columns. Zaghi et.al [30] conducted the shaking table test for CFFT bridge columns, it was found that the accumulated dissipated energy and nominal plastic hinge length for CFFT column were about 1.6 times and 2 times higher than those of the conventional RC columns, respectively. Based on this research background, the geopolymeric recycled aggregate concrete-filled FRP tube (GRACFFT) column was proposed in this paper. As a novel composite column, the GRACFFT column is a combination of both GRAC and FRP tube. Compared with the conventional CFFT column, the replacement of core concrete with GRAC could further increase its environmental friendliness, which would be beneficial for the sustainability of the construction industry. Compared with steel reinforced GRAC, the external confinement of FRP tube could further increase the ductility and durability of GRAC, which would be beneficial for the serviceability of GRAC materials [31]. In this paper, the hysteretic behaviors GRACFFT columns were investigated experimentally. Based on the experimental results, the influences of different parameters, i.e., fiber types, section form, and reinforcement type, were discussed. The plastic hinge length for each GRACFFT column was also investigated based on the strain analysis.

η=

fl f co'

(1)

f co'

is the where fl is the maximum confining pressure of FRP jacket and compressive strength of unconfined GRAC. The parameter fl depends on the thickness and elastic modulus of FRP, which can be seen Ref. [32] . Even though different fiber types for FRP tubes were applied in this experiment, it can be seen that all the circular GRACFFT columns have similar confinement ratios as 0.23 or 0.24. The confinement stiffness Kl is also provided in Table 1, which is defined as follows:

Kl =

2Ef t f (2)

D

where Kl is the confinement stiffness of the FRP tube, fl is the confinement strength of the FRP tube, Ef is the elastic modulus of the FRP, t f is the total thickness of FRP fiber sheets, and D is the inner diameter of the FRP tube. It is evident that the specimen CC3-B has the highest confinement stiffness than other specimens. The axial load (P ) for all specimens are set as follows: (3)

P = 0.3P0

where P0 is the nominal axial load carrying capacity of GRACFFT column and can be calculate as follows [33]:

P0 = 0.85f co' (Ac − As ) + As f y

(4)

where Ac and AS are the cross-sectional area of GRAC and longitudinal rebar, respectively. Accordingly, f co' and f y are the compressive strength of GRAC and yield strength of steel rebar. The geometry of GRACFFT specimen is shown in Fig. 1, three types of section form for GRACFFT column were designed while the foundation beams for all specimens were identical. The cover thickness and stirrup spacing for both type 1 and type 2 were set as 25 mm and 950 mm, respectively. Two types of steel rebars with the diameter of 8 mm and 22 mm were used as the stirrup and longitudinal reinforcements, respectively. For type 3, there is an inner steel tube embedded in the GRAC. In order to keep an identical cross-section strength between steel tube and longitudinal reinforcement, the thickness of the steel tube for type 3 was designed as 6 mm.

2. Experimental program 2.2. Material properties 2.1. Test specimens GRAC material was made with fly ash, slag, river sand, recycled aggregate, and alkali activator. The mix design for GRAC is shown in Table 2. The geopolymer paste was made with solid precursors and alkali activator. The fly ash and slag were applied as the solid precursors while the macro and micro morphologies of both fly ash and

Six half-size GRACFFT columns were prepared and the details for each specimen are shown in Table 1. The clear height for all specimens was set as 2000 mm and the cross-section for the foundation beam was set as 500 mm × 600 mm. The specimen BC9-B was selected to Table 1 Specimen parameters and details. Specimen

Fiber types

Fiber layer

t (mm/ply)

Section type

η

K l (GPa)

ω (kN.m)

BC9-B FC26-B CC3-B GC5-B GS8-B GC5-T

Basalt Flax Carbon Glass Glass Glass

9 26 3 5 8 5

0.107 0.100 0.167 0.210 0.210 0.210

Type Type Type Type Type Type

0.24 0.23 0.23 0.24 0.38 0.24

0.59 0.40 0.78 0.53 0.47 0.53

146.76 224.06 444.45 162.29 146.52 238.49

1 1 1 1 2 3

Note: t is the nominal layer thickness; η is the confinement ratio; Kl is the confinement stiffness; ω is the energy dissipation 2

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Fig. 1. Geometry of GRACFFT column.

Table 2 The mix design for GRAC (kg/m3).

Table 3 Chemical composition of fly ash and slag (%).

Materials

Fly ash

Slag

River sand

Recycled aggregate

Alkali activator

Compositions

CaO

SiO2

Al2O3

Fe2O3

MgO

K2O

Na2O

P2O5

GRAC

200

200

680

985

300

Fly ash Slag

6.83 35.69

53.36 32.57

27.58 15.68

5.49 0.25

1.59 6.65

1.30 0.31

0.79 0.75

0.34 4.60

slag are shown in Fig. 2. The chemical composition for fly ash and slag are shown in Table 3. It was noticed that slag contains 35% calcium oxide (CaO), while fly ash mainly contains aluminosilicate materials, i.e., silicon dioxide (SiO2) and aluminum oxide (Al2O3). According to the previous researches, with the proper addition of both calcium oxide and aluminosilicate materials, it is feasible to develop high strength geopolymer paste under ambient curing conditions [34–35]. The alkali activator was made with both Na2SiO3 (38.7 wt% Na2SiO3, relative density 1.39 g/cm3) and NaOH solution. The NaOH solution with a constant concentration of 12 mol/L was prepared by dissolving NaOH flakes (98% purity, density 2.13 g/cm3) in pure water. The mass ratio between Na2SiO3 solution and NaOH solution was set as 1.5 by weight. Before mixed with solid precursors and aggregates, the Na2SiO3 and NaOH solutions were prepared and then mixed together with a cooling process for 24 h. The recycled coarse aggregates were provided by Shanghai Youhong New Material Co., Ltd (China) and the grading curve of the recycled coarse aggregate are showed in Fig. 3. The flow chart of manufacturing GRAC is shown in Fig. 4. The fly ash and slag were first mixed together for 1 min and then the rived sand, as well as recycled coarse aggregate, were mixed together for 2 min. Then, the alkali activator was poured into the solid content and mixed for another 2 min with the formation of GRAC paste. FRP material was made with fiber sheet and epoxy resin. The epoxy for FRP was purchased from Nanjing Mankart Technology co. LTD (China). The manufacturing processes and design methods for FRP jacket can be found in ACI 440.2R-17 [36].

100

Cumulative passing (%)

90 80 70 60 50 40 30 20 10

Recycled coarse aggregate

0 2.36

4.75

9.5

16

19

26.5

Sieve size (mm) Fig. 3. The grading curve of the recycled coarse aggregate.

Fig. 2. The macro- and micro- morphologies of slag and fly ash (a) macro-morphology of slag; (b) macro-morphology of fly ash; (c) micro-morphology of slag; (d) micro-morphology of fly ash; 3

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Fig. 4. The flow chart of manufacturing GRAC.

to 85% of its maximum value; (3) the composite column lost its integrity or load-carrying ability.

Table 4 Properties of fibers used in FRP tubes. Fibers

Basalt Carbon Flax Glass

Average weight (g/m2)

Ultimate tensile strength (MPa)

Elastic modulus (GPa)

Ultimate rupture strain (%)

300 300 150 450

1849 3200 650 1718

91.3 230 22.5 71.7

2.02 1.48 2.88 2.26

3. Test results and discussion 3.1. Failure modes and test observations With the increase of horizontal cyclic displacement, all the GRACFFT columns have similar failure processes. The first stage was accompanied by the development of microcracks on FRP tube. It was noticed that the micro-cracks were first appeared on the tensile side of the FRP tube. Then, these micro-cracks developed continuously with the sound of fiber fracture and rupture. During this stage, the inner GRAC was still effectively confined by FRP tube. With the further increase of horizontal displacement, the inner GRAC reached its compressive strength and began to be crushed. For carbon FRP tube (specimen CC-3-B), the debonding process between FRP and inner GRAC was observed accompanied by relatively stable failure processes. By contrast, for flax FRP tube (specimen FC-26-B), the catastrophic rupture of FRP tube was observed and the longitudinal bar yielded suddenly after the rupture of BFRP tube. The failure modes for all specimens are shown in Fig. 8, the rupture of FRP tubes were observed for all GRACFFT columns, especially in the column base area. Particularly, the buckling of steel rebar and spalling of GRAC were overserved for specimen FC26-B.

Table 5 Material properties for GRAC and steel. Parameter

Value

Average Average Average Average

55.8 MPa 412 MPa 410 MPa 215 MPa

compressive strength of GRAC yield strength of longitudinal bar yield strength of stirrup yield strength of steel tube

The material property tests for both GRAC and FRP were also conducted. A total of six GRAC cylinder specimens were prepared and tested after natural curing for 28 days following ASTM C39 [37]. The mechanical properties of fibers used in FRP tubes were tested according to ASTM D3039 [38] and the results are shown in Table 4. In order to determine the yield strength of the steel tubes and the steel reinforcements, tensile coupon tests for steel were also conducted according to ASTM E.8 M [39] with the CMT-5305 tensile testing system. The material properties of GRAC and steel are listed in Table 5. It can be seen that the average compressive strength of GRAC cylinder specimens was as high as 55.8 MPa.

3.2. Lateral load–displacement hysteretic curves The hysteresis curves for all GRACFFT columns are shown in Fig. 9. All the hysteresis curves are generally full and no evident pinching effect was observed, indicating the GRACFFT columns have favorable energy consumption capacity under cyclic loading. The lateral drift for all specimens was in the range of 6% to10%, which is comparable with normal CFFT columns under cyclic loading [28]. It was noticed that the hysteresis curves for all specimen are approximately symmetrical expect for specimen GC5-T. For specimen GC5-T, an inner steel tube was embedded in GRAC. There may be two reasons responsible for the dissymmetry hysteresis curves of GC5-T. First, since there is no shear stud welded on the steel tube, the bond-slip between steel tube and GRAC may be very severe under cyclic loading conditions. Thus, the rigidity of the column on both sides, i.e., tensile side and compressive side, may be different. Second, the local yielding or buckling of steel tube may also result in uneven rigidities on both sides, especially during the post-peak stage. It was also noticed that the stiffness, horizontal bearing capacity, and ultimate displacement of specimen CC3-B are significantly higher than other specimens. It may be attributed to the reason that CFRP has much higher confinement stiffness than other types of FRP, thus CFRP tube could provide more effective confinement to the inner GRAC [40].

2.3. Specimen preparation and test set-up The main processes for the preparation of GRACFFT specimens are shown in Fig. 5. The reinforcement cage and FRP tube were prepared separately, as shown in Fig. 5(a) and (b). After the epoxy of FRP tube became hardened, the FRP tube was embed in the reinforcement cage from top to bottom of the column and then fit together. The support bracket was also applied to ensure that both reinforcement cage and FRP tube could maintain its geometric position during the casting process, as shown in Fig. 5(c). Then, GRAC was carefully cast into the FRP tube as well as bottom beam. During the casting process of GRAC, an electrical vibrator was applied to increase the compaction of GRAC. All GRACFFT specimens were demolded after two days and then cured in the natural atmosphere for another 28 days, as shown in Fig. 5(d). The test setup for lateral cyclic loading is shown in Fig. 6. In order to prevent horizontal sway during the cyclic loading processes, the GRACFFT specimen was fixed on a strong ground with steel anchors. The vertical load was first applied with a hydraulic jack prior to the horizontal cyclic load. A hydraulic actuator, which was fixed on the strong reaction wall, was applied to exert the horizontal cyclic load. To record the hoop strain of the outer FRP tube, eight strain gauges of each side with a measurement range of 20,000 με were symmetrically pasted on the surface of FRP tube. The loading history curve for the horizontal cyclic load is shown in Fig. 7. The first stage was conducted with a displacement increment of 3.6 mm while the second stage had a displacement increment of 18 mm. The cyclic load process would be terminated if: (1) a sudden rupture or fracture occurred on the FRP tube; (2) the horizontal bearing capacity of the composite column decreased

3.3. Skeleton curves The skeleton curve was obtained by successively connecting the peak points on the hysteresis curves shown in Fig. 9, which is shown in Fig. 10. As can be seen in Fig. 10(a), the stiffness and bearing capacity of specimen CC3-B are much higher than other specimens. It may be attributed to the reason that CFRP tube has much higher confinement stiffness than other FRP tubes. As shown in Table 1, the confinement 4

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(a) reinforcement cage

b FRP tube

(c) casting GRAC

d

natural curing

Fig. 5. Specimen preparation processes.

for inner GRAC while the square FRP tube mainly provides confinement stress in the corner area [42]. The influence of reinforcement types, i.e., steel rebar (GC5-B) and steel tube (GC5-T), are shown in Fig. 10(c). It was noticed that the specimen GC5-T showed significantly higher bearing capacity during the reverse loading stage, while the loading carrying capacity was much lower during the forward loading stage. As has been discussed above, the bond-slip behavior as well as local buckling of steel tube may be responsible for the dissymmetry of skeleton curve for specimen GC5-T.

stiffness for CFRP tube is as high as 0.78 GPa. Thus, CFRP tube could provide more effective confinement to core GRAC than other FRP tubes, especially during the initial loading stage [41]. The skeleton curves for composite columns with different section forms, i.e., circular (GC5-B) and square (GS8-B), are shown in Fig. 10(b). Since the cross-section area for specimen GS8-B is larger than specimen GC5-B, the bearing capacity and initial stiffness for GS8B are higher than GC5-B. However, the post-peak ductility and ultimate displacement for GC5-B are higher than GS8-B, indicating the circular FRP tube could provide more uniform and effective confinement stress

(a) Setup illustration

(b) Photograph

Fig. 6. Test setup for lateral cyclic loading. 5

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Horizontal Displacement(mm)

60 40

36

20 0

influences of FRP types were negligible after the debonding between FRP and GRAC, and the inner GRAC would be the controlling factor during the post-peak stage. Comparing GC5-B with GS8-B, as shown in Fig. 11(b), it can be found that the initial rigidity of the square column is 75.7% higher than that of the circular column since the cross- section area for GS8-B is higher than GC5-B. However, it was also noticed that the rigidity deterioration rate of specimen GS8-B is faster than that of specimen GC5B, which is attributed to the stress concentration effect in the corner area of square GRACFFT column. Comparing GC5-B with GC5-T, as shown in Fig. 11(c), it can be seen that the initial rigidity of GC5-T is 17.7% higher than that of GC5-B. It may be attributed to the reason that the steel tube could provide more continuous and higher confinement stress for inner GRAC when compared with longitudinal reinforcements.

54

18

10.8

3.6

7.2 -20 -40

14.4

The second loading stage The first loading stage

-60 0

1

2

3

4

5

6

7

8

9

Cyclic number

10 11 12 13 14 15

3.5. Energy dissipation

Fig. 7. The loading history curve.

For the hysteresis curves shown in Fig. 9, the energy dissipation capacity during each stage was defined as the enclosed area of each hysteresis loop. The energy dissipation curves for all GRACFFT columns are shown in Fig. 12. The influences of fiber types on the energy dissipation behaviors are shown in Fig. 12(a). The energy dissipation capacities for all composite columns are very similar during the initial stage. After the horizontal displacement increased to 18 mm, it can be seen that the energy dissipation capacity for specimen CC3-B is much higher than other specimens. On one hand, the CFRP tube could provide more effective confinement than the other FRP tubes due to its higher confinement stiffness. On the other hand, the ultimate displacement for specimen CC3-B is also higher than other specimens, as shown in Fig. 10. Comparing GC5-B with GS8-B, as shown in Fig. 12(b), the energy dissipation capacity of the square column is higher than that of the circular column when the horizontal displacement was in the range of 18 mm to 108 mm. It is also because the cross-section area for specimen GS8-B is higher than that for GC5-B. However, the final cumulative energy dissipation for specimen GC5-B is 10.76% higher than that GS8-

3.4. Rigidity deterioration In order to evaluate the rigidity deterioration processes for GRACFFT columns, the rigidity deterioration coefficient (K ) was introduced and defined as follows:

K=

|+Fi| + |−Fi| |+Δi| + |−Δi |

(5)

where + Fi and − Fi are the peak load at each level, + Δi and − Δi are the corresponding displacement for + Fi and − Fi [43]. As can be seen in Fig. 11(a), the rigidity deterioration processes for specimen BC9-B, FC26-B, and GC5-B are almost identical. By contrast, the initial rigidity of specimen CC3-B is much higher than other specimens. Once again demonstrating that CFRP tube could provide more effective confinement to the inner GRAC. However, the rigidity of CC3B decreased faster than other specimens, and the final rigidity for all GRACFFT columns are almost identical. It is reasonable since the

Fig. 8. The failure modes for composite columns. 6

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150

-162

-108

-54

0

54

Lateral displacement (mm)

Lateral displacement (mm)

Lateral displacement (mm) 108

-162

162

-108

-54

0

54

108

162

150

100

Load (kN)

Load (kN)

Load (kN)

-50

0 -50

-100

-9

-6

-3

0

3

Lateral drift (%)

6

FC26-B -9

9

-6

(a)BC9-B -162 150

-108

-54

0

54

-162

162

-50

-3

0

3

Lateral drift (%)

6

CC3-B -9

9

-6

-108

-54

0

-3

0

3

Lateral drift (%)

6

9

(c)CC3-B

54

Lateral displacement (mm)

108

-162 100

162

-108

-54

0

54

-6

-3

0

3

108

162

50

100

50 0

50

Load (kN)

Load (kN)

Load (kN)

162

150

100

0 -50

-100 -50

-6

-3

0

3

6

Lateral drift (%)

9

0 -50

-100

-150

GC5-B -100 -9

108

-150

Lateral displacement (mm)

108

54

0

(b)FC26-B

Lateral displacement (mm)

0

-100

-100

BC9-B -150

-54

50

50

0

-108

100

100 50

-162

GC5-T

GS8-B

-200 -9

-6

(d)GC5-B

-3

0

3

Lateral drift (%)

6

-150 -9

9

(b)GS8-B

6

Lateral drift (%)

9

(c)GC5-T

Fig. 9. Hysteresis curves for GRACFFT columns.

3.6. Strain analysis As has been shown in Fig. 6, eight strain gauges were applied to recorded the strain development of FRP tube. The recorded hoop strain of FRP tubes for all GRACFFT columns are shown in Fig. 13. When the horizontal displacement was lower than 18 mm, the hoop strain at each section fluctuated around zero, indicating there are negligible interactions between FRP tube and inner GRAC. As the horizontal displacement increased to 36 mm, the hoop strain at each section of FRP tube increased significantly. It can be inferred that the inner GRAC expanded

200

200

150

150

150

100

100

100

50 0 -50

-100

BC9-B FC26-B CC3-B GC5-B

-150 -200 -200 -150 -100 -50

0

50

Load(kN)

200

Load(kN)

Load(kN)

B, as shown in Table 1. It is reasonable since the circular GRACFFT column has a better composite effect as well as higher ultimate displacement, as shown in Fig. 10. Comparing GC5-B with GC5-T, as shown in Fig. 12(c), it can be seen that the energy dissipation capacity of GC5-T is higher than of GC5-B during the whole loading stage. The final cumulative energy consumption for GC5-T is 46.95% higher than GC5-B, as shown in Table 1. It is because the inner steel tube could provide more effective confinement effect for GRAC, thus specimen GC5-T has higher bearing capacity and ductility especially during the post-peak stage.

50 0 -50

50

forward loading

0 -50

-100

-100 -150

GC5-B GS8-B

100 150 200 -200 -200 -150 -100 -50

0

50

100 150 200

Displacement(mm)

Displacement(mm)

(a) Fiber types

(b) Section form

-150

-200 -200 -150 -100 -50

0

50

100 150 200

Displacement(mm)

(c) Reinforcement type

Fig. 10. Skeleton curves for composite columns with different parameters. 7

GC5-B GC5-T

reverse loading

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12

8 6 4 2 0

0

50

100

150

200

Displacement(mm)

GC5-B GS8-B

8

Rigidity (kN/mm)

Rigidity (kN/mm)

10

10

Rigidity (kN/mm)

BC9-B FC26-B CC3-B GC5-B

6 4

GC5-B GC5-T

6

4

2

2 0

0

0

50

100

150

Displacement(mm)

(a) Fiber types

0

50

100

150

Displacement(mm)

(b) Section form

(c) Reinforcement type

Fig. 11. Skeleton curves for composite columns with different parameters. 200

400 350 300 250 200 150

BC9-B FC26-B CC3-B GC5-B

100 50 0

0

20 40 60 80 100 120 140 160 180 200

Displacement(mm)

(a) Fiber types

300

150

100

50

0

GC5-B GS8-B

0

20

40

60

80

100

Displacement(mm)

120

Energy dissipation(kNm)

450

Energy dissipation(kNm)

Energy dissipation(kNm)

500

140

(b) Section form

250 200 150 100 50 0

GC5-B GC5-T

0

20

40

60

80

100 120 140

Displacement(mm)

(c) Reinforcement type

Fig. 12. Energy Dissipation curves for specimens.

can be defined as follows:

gradually and was confined by FRP tube during this stage. Based on the recorded strain development of FRP tube, it is feasible to analyze the plastic hinge of GRACFFT column. The plastic hinge regions were defined as the column area where the flexural moments exceed its yielding capacity. The length of the plastic hinge region is an important design parameter that will influence the energy dissipation capacity of column under earthquake. According to research conducted by Ozbakkaloglu et.al [44], the plastic hinge length (H0 ) is defined as the distance from column base to the section where the hoop strain is one-third of the maximum hoop strain (εm ) of the column, as shown Fig. 14(a). The plastic hinge length for all GRACFFT columns are shown in Fig. 14(b). For those specimens with different fiber types, i.e., BC9-B, F26-B, CC3-B, and GC5-B, it was noticed that the specimen CC3-B has the lowest plastic hinge length. It is reasonable since the CFRP tube has a much higher tensile elastic modulus as well as confinement stiffness than other types of FRP tubes. However, the highest confinement stiffness may not be conducive for the progressive of plastic deformation and the main parts of GRACFFT column may still under its elastic stage. Comparing GC5-B with GS8-B, it was found that the section form has negligible influences on the plastic hinge length of GRACFFT columns. By contrast, the plastic hinge length for specimen GC5-T is about 33% lower than GC5-B, indicating the substitution of steel reinforcements with steel tube could have a negative effect on the plastic hinge length of the GRACFFT column. It may be also attributed to the bondslip behavior as well as local buckling of steel tube during the cyclic loading processes, which prohibited the development of plastic area of GRACFFT column. Based on the strain analysis, the utilization factor for FRP fibers (ξ )

ξ=

εu ε0

(6)

where εu is the recorded ultimate strain shown in Fig. 13 and ε0 is the ultimate rupture strain of FRP fibers shown in Table 4. The parameter ξ for GRACFFT column could reflect the composite effect between inner GRAC and FRP tube. A higher ξ value indicates that the hoop strain of FRP tube was closer to its ultimate rupture strain, thus more effective confinement was provided by FRP tube. The utilization factors of FRP fibers for different GRACFFT specimens are shown in Table 6. It was noticed that the ξ value for GC5-B is 89% higher than that for GS8-B, indicating that the circular GRACFFT specimen has better composite effect than square specimen. As has been discussed above, it may be attributed to the stress concentration effect of square GRACFFT column in the corner area. Thus, the FRP tube in the corner area would rupture during the early loading stage which is consistent with the experimental observation as shown in Fig. 8. For circular GRACFFT specimens, it was noticed that the ξ value for CC3-B is about two times higher than that for FC26-B, indicating that the carbon FRP has better composite effect with inner GRAC [45]. It is reasonable since carbon FRP tube has a much higher confinement stiffness than flax FRP tube, thus the carbon FRP tube could have more effective confinement effect with GRAC. 4. Conclusions In order to improve the environmental sustainability of CFFT column, the GRACFFT column was proposed in this paper. The 8

Engineering Structures 222 (2020) 111047

J. Cai, et al.

2 0

0

4 2 0

2000 4000 6000 8000 10000 12000

0

Strain(PH

2 0

0

2000 4000 6000 8000 10000 12000

Strain(PH

(c)CC3-B 3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm

6 4 2 0

0

2

Strain(PH

8

Section number

Section number

4

4

(b)FC26-B 3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm 126mm

6

6

0

2000 4000 6000 8000 10000 12000

(a)BC9-B 8

Section number

4

6

3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm 126mm 144mm 162mm 180mm

8

2000 4000 6000 8000 10000 12000

6 4 2 0

0

Strain(PH

(d)GC5-B

2000

4000

6000

Strain(PH

3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm 126mm

8

Section number

Section number

6

3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm 126mm 144mm

8

Section number

3.6mm 7.2mm 10.8mm 14.4mm 18mm 36mm 54mm 72mm 90mm 108mm 126mm

8

8000

0

2000

(b)GS8-B

4000

6000

Strain(PH

8000

10000

(c)GC5-T

Fig. 13. The recorded hoop strain of FRP tube.

hysteretic behaviors of GRACFFT columns were experimentally investigated and the influences of different parameters were discussed. Based on the experimental results, the key conclusions are summarized as below:

Table 6 The utilization factor of FRP fibers. Specimen

BC9-B

FC26-B

CC3-B

GC5-B

GS8-B

GC5-T

ξ

0.57

0.34

0.73

0.51

0.27

0.35

1. It is feasible to develop novel green building materials with industrial waste and recycled aggregate. Also, the proposed GRACFFT 60

Plastic hinge length(mm)

Plastic hinge length 50 40

45.2

45.3 37.5

30.9

30

22.5 20 10 0

BC9-B FC26-B CC3-B

GC5-B

Specimen

(a) the definition of plastic hinge length

45.6

GS8-B

GC5-T

(b) the plastic hinge length for each specimen

Fig. 14. The plastic hinge length. 9

Engineering Structures 222 (2020) 111047

J. Cai, et al.

column showed favorable good seismic performance in terms of carrying capacity and deformation ability. 2. 2 It was found that the GRACFFT column made with carbon FRP tube showed the best seismic behaviors in terms of yield displacement, peak load, ultimate displacement, and cumulative energy consumption. 3. The ultimate displacement, post-peak ductility as well as cumulative energy dissipation for square GRACFFT column are lower than that of the circular column, indicating the circular cross-section could have a better composite effect. 4. The substitution of steel reinforcements with steel tube could effectively increase the rigidity, ductility, and cumulative energy consumption of the GRACFFT column. However, the bond-slip behavior as well as early-stage local buckling of steel tube may have negative effects on the seismic behaviors of the composite columns.

[13] [14] [15]

[16]

[17] [18]

[19]

[20]

CRediT authorship contribution statement [21]

Jingming Cai: Methodology, Writing - original draft. Haoyu Hao: Investigation, Writing - review & editing. Togay Ozbakkaloglu: Funding acquisition, Supervision. Yamei Zhang: Resources, Supervision. Jinlong Pan: Conceptualization, Funding acquisition.

[22]

[23] [24]

Declaration of Competing Interest [25]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[26] [27] [28]

Acknowledgments

[29]

This work was financially supported by Research Fund for International Young Scientists of Natural Science Foundation of China (5161101076) and National Key R&D Program of China (2016YFC0701907).

[30] [31] [32]

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