Expansive concrete confined by CFRP under eccentric compression

Construction and Building Materials 208 (2019) 113–124

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Expansive concrete confined by CFRP under eccentric compression Qi Cao a, Xiaojun Li a, Jianpu Zhou a, Zhongguo John Ma b,⇑ a b

State Key Laboratory of Costal and Offshore Engineering, Dalian Univ. of Technology, Dalian 116024, China F.ASCE, Department of Civil and Environmental Engineering, University of Tennessee Knoxville, 313 John D. Tickle Building, Knoxville, TN 37996, USA

h i g h l i g h t s
Expansive concrete filled CFRP tube achieves an effective prestress level.
Ultimate load increases with the CFRP layers’ increase and eccentricity’s decrease.
Ultimate lateral deflection increases as the loading eccentricity increases.
CCECs show smaller hoop strains than CCUCs under the same conditions.
CFRP confined expansive concrete performed well relative to ordinary concrete.

a r t i c l e

i n f o

Article history: Received 12 October 2018 Received in revised form 25 January 2019 Accepted 20 February 2019

Keywords: CFRP Expansive concrete Eccentric compression CFRP layer Eccentricity

a b s t r a c t CFRP confined expansive concrete (CCEC) has shown higher inflection load, intercept load and ultimate load than CFRP confined unexpansive concrete (CCUC) counterparts under axial compression. The aim of this study is to investigate the compression behavior of CFRP confined expansive concrete subjected to eccentric load. Eighteen concrete element specimens have been prepared in this experiment considering the parameters including prestress level, CFRP layer and eccentricity of load. Prestress in concrete and CFRP at 28-day were calculated through the recorded hoop strain on CFRP. It can be observed from the experimental results that the ultimate load increases with the increase of the number of CFRP layers and decreases with the increase of the eccentricity. Experimental and discussion results indicate that CFRP confined expansive concrete show higher intercept loads (average 19.6%), inflection loads (average 20.4%), ultimate loads (average 13.7%) than the corresponding CFRP confined unexpansive concrete under the same testing scenarios. But due to the effect of prestress, relative to unexpansive specimens, the prestressed specimens have smaller hoop strains, lateral deflections and curvatures. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, research studies have shown that externally FRP confined concrete column has been proved to be an effective way to increase load capacity and ductility of the composite columns [1–8]. The core concrete of the columns was subjected to triaxial compressive stress under the axial compression due to the restraint of external FRP tubes [9]. Wu and Jiang [10] investigated the eccentric compression behavior of 36 FRP confined concrete short columns under different loading eccentricity and FRP stiffness. Strain hardening was demonstrated in the stress-strain curves as the eccentricity increased. In the same time, the ultimate strain under eccentric compression was 50% higher than that under axial compression. Zhang et al. [11] performed eccentric compression tests on the GFRP confined SRC (steel reinforced con⇑ Corresponding author. https://doi.org/10.1016/j.conbuildmat.2019.02.127 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

crete) columns. Test parameters consist of slenderness, eccentricity and concrete strength. It was found that slenderness ratio and eccentricity were the two main factors influencing the load capacity of the columns. As slenderness ratio and eccentricity increased, the load capacity decreased. Results also indicated that the loadcarrying capacity of the composite columns increased as the concrete strength increased. However, this effect was weakened as eccentricity increased [11]. Niu et al. [12] conducted eccentrically loading experiment on the PVC-CFRP confined concrete columns. Test results showed that the load carrying capacity decreased as the distance between CFRP strips and the eccentricity of load increased. Specimens with lower eccentricity failed by crushing of concrete and PVC tubes, while specimens with higher eccentricity failed by the yielding of steel and tensile rupture of PVC tubes. In both cases, CFRP strips ruptured for all tested specimens. Csuka and Kollar [13] proposed an analytical model for calculating the stress–strain curves of the FRP confined concrete columns under

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Nomenclature f il E t

eo

R Nmax umax

prestress on concrete (MPa); elastic modulus of CFRP (GPa); total thickness of CFRP (mm) initial strain of CFRP; radius of the specimen (mm); ultimate load (KN); ultimate lateral deflection (mm);

eccentric load. The maximum axial stress predicted by the model for axial compression and eccentric compression were approximately identical. Song et al. [14] established the stress–strain relationship of FRP confined concrete columns through experiment. Eccentric compression test on the FRP confined square columns was also conducted to verify the proposed analytical model. Test results showed that the increase of the ultimate load was proportional to the increase of the FRP strengthening ratio and was inversely proportional to the increase of eccentricity. Lin and Teng [15] proposed a three-dimensional finite-element analytical model for FRP-confined circular concrete columns under eccentric loading. Based on analytical results, it concluded that the non-uniform FRP confinement effect increased as loading eccentricity increased. Chellapandian et al. [16] developed a numerical model to predict the behavior of concrete element under axial and eccentric compression, the prediction results agreed well with test results. Wang et al. [17] studied the mechanical properties of FRP confined concrete with different wrapping schemes, constructed experimental and analytical interaction (P-M) diagrams to study the behavior of FRP confined concrete under eccentric load. In the existing studies of FRP confined concrete columns, one problem found in Mortazavi et al.’s [18] study is that the FRP cannot be fully utilized due to the early cracking of the concrete prior to FRP rupture which restricts the application of FRP. In order to make the FRP and core concrete work efficiently, researchers proposed to use mechanical tensioning FRP [19,20]. Test results showed that the cracking load and axial stress at inflection point were both improved due to the effect of prestress. However, this method requires professional facility as well as skilled labor, which brings a high cost to field application. To solve this, Cao and Ma [21,22] combined external CFRP and core expansive concrete to study the behavior of beams. With regard to beam members, by placing expansive concrete into the prefabricated FRP molds, prestress was formed by the expansion of concrete and restraint of FRP. The proposed structural beam performed superior in eliminating crack, controlling crack development and improving load-carrying capacity. Cao et al. [23] also found, in another study, that the expansive concrete beams reinforced by hybrid CFRP and steel bars performed better in cracking load, yielding load, ultimate load and ductility than the companion conventional concrete beams. For column specimens, researchers have attempted to cast expansive concrete into steel [24] or GFRP [25] tubes. Mortazavi et al. [18] found that, after adding the expansive mortar between existing columns and FRP, the ultimate capacity and ductility of the composite columns were improved. Vincent and Ozbakkaloglu [26] filled the AFRP tube with high-strength expansive concrete. It indicated that stress loss was not observed in the experimental stress–strain curves. Cao et al. [8] found that the CFRP confined expansive concrete columns achieved significantly higher inflection load and ultimate load than the CFRP confined conventional concrete columns under axial compression. Cao et al. [27] also demonstrated that the inflection and ultimate load of FRP confined self-stressing columns and FRP-confined selfstressing concrete-filled steel tube columns were improved

eh,rup N e u M 0 f co

ultimate hoop strain of CFRP; axial load (KN); eccentricity (mm); lateral deflection (mm); N(e + u) (KNm); cylinder compressive strength (MPa).

compared with unexpansive specimens. However, the mechanical properties of CFRP confined expansive concrete columns under eccentric compression have not been studied so far. 2. Research significance FRP confined concrete can significantly increase their strength and ductility. Extensive research investigations have been conducted on the compressive behavior of FRP confined under concentric loads [3–8,18–20] as well as eccentric loads [11,13,14,15,28,29] under passive confinement. However, studies of FRP confined expansive (self-stressing) concrete element have rarely been seen which generates active prestressing effect for concrete core. It is important for understanding of the compression behavior of realistic column members in building and bridge structures. This study is to implement the expansive concrete in the FRP confined concrete element and explore how the prestressing effects could affect compressive behavior of the composite element under eccentric load. Authors believe that the outputs of this study would be beneficial for the development of design guidelines and specifications for the FRP confined self-stressing concrete system that would facilitate their construction applications. 3. Experimental program 3.1. Specimen design In order to investigate the mechanical behavior of expansive concrete as well as conventional concrete under eccentric load, eighteen CFRP confined concrete elements were designed. The experimental matrix is listed in Table 1. Two kinds of concrete (expansive concrete, unexpansive concrete), three FRP layers (1layer, 2-layer, and 3-layer) and three load eccentricity (0 mm, 20 mm (0.788 in.), and 40 mm (1.576 in.)) were selected as test levels for three different test parameters. The inner diameter of the CFRP tube is 150 mm and the height is 300 mm. Among the eighteen specimens, nine were made for CFRP confined expansive concrete elements (CCECs) and nine were fabricated for CFRP confined unexpansive concrete elements (CCUCs). In order to measure the axial strain and hoop strain of the specimens, four groups of strain gauges were attached at the middle height of the CFRP tubes. The layout of the strain gauges is shown in Fig. 1. 3.2. Material properties The expansive concrete was made of lime-based expansive agent. The mixture proportions and cylindrical compressive strength at 28-day for the expansive concrete and the conventional concrete are shown in Table 2. The CFRP tube specimens, as shown in Fig. 2, were made by wet hand layup in the laboratory. In this test, the elastic modulus of CFRP is obtained by recording the stress and strain during tension test by taking points at intervals. Then, the tensile modulus and tensile strength were both calculated.

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Q. Cao et al. / Construction and Building Materials 208 (2019) 113–124 Table 1 Experimental matrix of CFRP-confined concrete elements. Specimen CCUC

Layers of FRP

Eccentric value(mm)

Specimen CCEC

Layers of FRP

Eccentric value(mm)

C-1-00 C-1-20 C-1-40 C-2-00 C-2-20 C-2-40 C-3-00 C-3-20 C-3-40

1 1 1 2 2 2 3 3 3

0 20 40 0 20 40 0 20 40

E-1-00 E-1-20 E-1-40 E-2-00 E-2-20 E-2-40 E-3-00 E-3-20 E-3-40

1 1 1 2 2 2 3 3 3

0 20 40 0 20 40 0 20 40

Notes: 1. CFRP confined expansive concrete (CCEC) CFRP confined unexpansive concrete (CCUC) 2. Take ‘‘C(E)-1-20” as an example, ‘‘C” represents conventional (unexpansive) concrete, ‘‘E” represents expansive concrete, number ‘‘1” represents the layer of CFRP tube, number ‘‘20” indicates the loading eccentricity of 20 mm. 1 mm = 0.0394 in.

Fig. 2. Prefabricated FRP tubes. Fig. 1. Layout of strain gauges.

The stress–strain curve, tensile coupon test as well as failure modes are shown in Fig. 3. The tested material properties are shown in Table 3 according to ASTM D3039 standard, and the material properties of fiber and resin provided by manufacturers are shown in Table 4.

3.3. Expansion strain monitoring In this study, the free expansion of 100 mm  100 mm  400 mm concrete prisms was measured by two LVDTs instrumented on both ends of the concrete prism specimens. The results indicated that the expansive concrete expands rapidly in the first two days. The expansion value is about 2200 microstrains. Then the speed of expansion started to decrease. By the fifth day or so, the expansion became stable and no longer increases. During 28 days, the expansion value was about 2675 microstrains. For unexpansive concrete, the 28-day shrinkage value is approximately 260 microstrains. For the CFRP confined expansive concrete elements (CCECs), prestress was formed by the expansion of concrete and confine-

ment of CFRP, as shown in Fig. 4. For the CFRP confined expansive concrete elements (CCECs), four hoop strain gauges outside the overlap region were chosen randomly to record the expansion value of the CFRP after the concrete was poured in. The recorded strain data during the 28-day curing period is shown in Fig. 5. It can be seen that, the expansion of the concrete reached the peak value after two days or so, and then it kept constant. It also shows from Fig. 5 that the expansion value of the concrete was inversely proportional to the CFRP layer. With the increase of the number of CFRP layers, the confinement stiffness of CFRP on concrete increases. Therefore, CFRP can inhibit the expansion of concrete more. The average expansion value of CFRP with one, two and three layers were 2588 le, 1723 le and 1516 le, respectively. The calculated prestress on the concrete was calculated by Eq. (1)

f il ¼

Eeo t R

ð1Þ

where the f il is the prestress on concrete, E is the elastic modulus of CFRP, t is the total thickness of CFRP, eo is the initial strain of CFRP and the R is the radius of elements.

Table 2 Mixture proportions and compressive strength. Type of concrete

Cement (kg/m3)

Expansive agent (kg/m3)

Fine aggregate (kg/m3)

Coarse aggregate (kg/m3)

Water (kg/m3)

Compressive strength f’co(MPa)

Conventional concrete Expansive concrete

450 450

0 45

636.5 591.5

995.5 995.5

220 220

29.90 32.53

Note:1 kg/m3 = 0.0624 lb/ft3, 1 MPa = 145.14 psi.

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(a)

(b)

4000 3500

stress(MPa)

3000 2500 2000 1500 1000 500 0 0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

strain

(c) Fig. 3. CFRP coupon tensile test (a) CFRP coupon specimen before test; (b) failure mode; (c) Stress–strain curve of CFRP.

Table 3 Material properties of CFRP obtained from flat coupon tests.

3.4. Test setup

Thickness (mm/layer)

Ultimate tensile strength (MPa)

Ultimate tensile strain (microstrain)

Elasticity modulus (GPa)

0.167

3536

17,253

205

The calculated prestress of the concrete and CFRP due to the expansion is listed in Table 5. Results exhibit that higher total thickness of CFRP leads to lower prestress in the CFRP but higher prestress in the concrete.

After the CFRP confined concrete specimens were cured for 28 days, compression tests were carried out on a 10000 kN (2248kip) capacity universal testing machine under displacement control at a rate of 0.2 mm/min (0.0079 in./min). The test setup of concentric and eccentric compression are shown in Fig. 6. The load, the axial strain, hoop strain, the axial deformation at 120 mm (4.728in.) height above bottom of specimen located both at the compressive side (strain gauges B side, as shown in Fig. 1) and the tensile side (strain gauges D side) were collected for all tested specimens. For the eccentric compression test, the

Table 4 Material properties of fiber and resin provided by manufacturers.

Fiber Resin

Thickness (mm/layer)

Ultimate tensile strength (MPa)

Ultimate tensile strain (microstrain)

Elasticity modulus (GPa)

0.167 –

3848 45

16,033 –

240 2.62

Note: 1 mm = 0.0394 in., 1 MPa = 145.14 psi, 1 GPa = 145.14 ksi.

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Confinement of CFRP on concrete expansion

Expansion of expansive concrete during curing period

Pretension stress

Pretension stress

Prestressing stress

CFRP

Concrete

Fig. 4. Schematic diagram of prestress before load was applied on expansive specimens.

3000

2500

Expansion strain (10-6)

Expansion strain (10-6)

2000

2000

1500

1000

C1-1 C1-2 C1-3

500

5

10

15

20

25

1000

C2-1 C2-2 C2-3 C2-4

500

0 0

1500

0

30

0

5

10

Time (days)

15

20

25

30

Time (days)

(b)

(a) 3000

Average expansion strain (10-6)

Expansion strain (10-6)

2000

1500

1000

C3-1 C3-2 C3-3 C3-4

500

0 0

5

10

15

20

Time (days)

(c)

25

30

2500

2000

1500

1000

C1-AVERAGE C2-AVERAGE C3-AVERAGE

500

0 0

5

10

15

20

Time (days)

(d)

Fig. 5. Strain-time curves of FRP confined concrete elements (a) 1-layer (b) 2-layer (c) 3-layer (d) average value.

25

30

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mid-height lateral deflections at the compressive side and the tensile side were also collected by linear variable differential transformers (LVDTs). 4. Results and discussions 4.1. Failure modes For the specimens under concentric compression, the specimens were compressed as the load increased in the axial direction. In the same time, the core concrete expanded in the hoop direction due to Poisson ratio effect which resulted in the increase of the strain of CFRP in the hoop direction. When the strain of CFRP reached the ultimate tensile strain, the CFRP ruptured and a loud

sound was heard simultaneously. Then the core concrete confined by CFRP crushed at the maximum failure load. The typical failure mode of expansive concrete specimens and unexpansive concrete specimens can be seen in Fig. 7. For the specimens under eccentric compression, the concrete near the eccentric side would be compressed most as load increased in the axial direction. The CFRP in the eccentric compression side was tensioned severely and the hoop strain increased until it reached the ultimate tensile strain. Finally, the CFRP ruptured and the specimen failed. The typical failure mode for eccentrically loaded CFRP confined expansive concrete and unexpansive concrete specimens are shown in Fig. 8. It also shows that the CFRP tube and concrete were tensioned apart at the tensile side. Most of specimens failed in CFRP rupture which occurred in the middle

Table 5 Prestress of CFRP-confined concrete at 28 days. Specimen

Average expansive strain of concrete and CFRP (microstrain)

Expansive prestress of FRP (MPa)

Expansive prestress of concrete (MPa)

E-1-00 (20,40) E-2-00 (20,40) E-3-00 (20,40)

2588 1723 1516

530.40 353.12 310.70

1.18 1.57 2.08

Note: 1 MPa = 145.14 psi.

Fig. 6. Test setup of (a) concentrically confined elements (b) eccentrically confined elements.

Fig. 7. Failure mode of concentrically loaded specimens (a) CCUCs (b) CCECs.

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2000

00 20 40

1800

1897

Ultimate load (kN)

1600 1405

1400 1200 1000

1151 1004 899

800 586

600

472

400

325

271

200 0 C-1

C-2

C-3

Specimens

(a) 00 20 40

2000

2023

Ultimate load (kN)

1600

1200

1488 1236

1168 991

800

721 471

426

400 Fig. 8. Failure mode of eccentrically loaded specimens (a) CCUCs (b) CCECs.

333

0 E-1

height of the specimens as the concrete at the bottom and above was restrained by the loading plates.

E-2

E-3

Specimens

(b) Fig. 9. Ultimate load comparison of (a) conventional concrete elements (b) expansive concrete elements.

4.2. Ultimate loads The ultimate loads Nmax of the specimens are listed in Table 6, and the comparison are shown in Fig. 9. It shows that, as expected, the ultimate load increases as the CFRP layer increases for both expansive concrete and conventional concrete specimens. The average ultimate load increases 37% and 36% for conventional concrete specimens and 31% and 24% for expansive specimens when the CFRP layer increases from one to two and two to three, respectively. Meanwhile, it shows a reduction trend as the eccentricity of

loading increases. The average decrease of ultimate load is 39%, 59% for CCUCs and 37%, 58% for CCECs as the eccentricity increases from 0 to 20 mm (0.788in.) and 20 mm (0.788in.) to 40 mm (1.576in.). This indicates that the relation between ultimate load decrease and eccentricity increase is disproportional and higher eccentricity tends to cause higher reduction in load carrying capacity for both CCECs and CCUCs compared with lower eccentricity at

Table 6 Test results. Specimen

Nmax (kN)

umax (mm)

eh,rup (microstrain)

Specimen

Nmax (kN)

umax (mm)

eh,rup(microstrain)

Nmax,E/Nmax,C

C-1-00 C-1-20 C-1-40 C-2-00 C-2-20 C-2-40 C-3-00 C-3-20 C-3-40 Average

1004 586 271 1405 899 325 1897 1151 472

\ 3.50 9.67 \ 6.38 13.44 \ 7.50 12.54

14,800 14,372 13,751 17,690 14,974 14,150 15,600 17,590 13,690 15,179

E-1-00 E-1-20 E-1-40 E-2-00 E-2-20 E-2-40 E-3-00 E-3-20 E-3-40

1168 721 333 1488 991 426 2023 1236 471

\ 3.79 7.65 \ 5.08 13.33 \ 8.06 12.60

15,178 13,678 11,628 11,383 12,643 15,073 16,546 11,746 14,246 13,569

1.16 1.23 1.22 1.05 1.10 1.31 1.07 1.07 1.00 1.14

Note: 1 kN = 224.8lbf, 1 mm = 0.0394 in.

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120000

14

100000

C-1-00 E-1-00 C-1-20 E-1-20 C-1-40 E-1-40

80000

10

Axial strain(10-6)

Lateral deflection (mm)

12

8

C-20 E-20 C-40 E-40

6

4

60000

40000

20000

0 0

1

2

2000

4000

6000

3

8000

10000 12000

14000

16000

-6

Hoop strain(10 )

CFRP layers

(a)

Fig. 10. Lateral deflection versus CFRP layers curves.

160000 140000

C-2-00 E-2-00 C-2-20 E-2-20 C-2-40 E-2-40

120000

Axial strain(10-6)

the same eccentricity reduction range (20 mm). Compared with CCUCs, CCECs show higher ultimate loads under the same conditions only except specimen E-3-40 and C-3-40 (471 kN (105.9 kip) v.s. 472 kN (106.1 kip)). In general, the average ultimate load of CCECs is 1.14 times of that of CCUCs at the same CFRP layer and loading eccentricity conditions. It exhibits the benefit of using expansive concrete as core concrete, which improves the ultimate capacities of CFRP confined expansive concrete specimens.

100000 80000 60000 40000

4.3. Ultimate lateral deflections

20000 0 0

(b) 160000 140000

C-3-00 E-3-00 C-3-20 E-3-20 C-3-40 E-3-40

120000 100000 80000 60000 40000 20000 0 0

4.4. Ultimate hoop strain in CFRP The ultimate hoop strains of CFRP eh,rup are also listed in Table 6. Please note that the ultimate strains of the CCECs have included the recorded expansion value given in Table 5. Results indicate that CFRP layer and load eccentricity do not show significant effect on the ultimate strains of tested specimens. The average ultimate strain in CFRP for CCUCs and CCECs are 15179 le and 13569 le respectively, which are 88% and 79% of the ultimate rupture strain obtained in the laboratory by the FRP tension tests. This is because

2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

Hoop strain(10-6)

Axial strain(10-6)

The ultimate lateral deflection is the maximum lateral deflection of the tension side of the eccentric specimens. The ultimate lateral deflections umax of tested specimens are listed in Table 6. It can be seen that the ultimate lateral deflection shows the same trend for both CCUCs and CCECs. The ultimate lateral deflection increases as the loading eccentricity increases at each CFRP layer, as expected. But different trends are found as the CFRP layer increases at each loading eccentricity as shown in Fig. 10. The lateral deflections show an upward trend as the CFRP layer increases at the eccentricity of 20 mm (0.788in.). However, at the eccentricity of 40 mm (1.576in.), the lateral deflections first increased and followed by a decrease as the CFRP layer increases. This demonstrates that, in general, both at low loading eccentricity (e = 20 mm) and high loading eccentricity (e = 40 mm), the ultimate deflection was improved by increasing CFRP layer. Furthermore, by comparison, it does not show any significant difference of the ultimate lateral deflections between CCECs and CCUCs. It should be noticed that the lateral deflection at the tensile side was recorded and taken as the ultimate lateral deflection of each specimen because the deformation of the concrete is governed by the compressive side of specimen in design methodology.

5000

10000

15000

20000

Hoop strain(10-6)

(c) Fig. 11. Axial strain-hoop strain curves: (a) one layer (b) two layers (c) three layers.

the crack development makes the CFRP stress uneven and the stress mode of the CFRP in the composite specimen is different from the unidirectional tensile form in the tensile test when the composite specimen is subjected to stress. Meanwhile, it is not

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1400

800

1200

700

Axial load (kN)

Axial load (kN)

C-1-20 E-1-20 C-1-40 E-1-40

600

1000 800

C-1-20 E-1-20 C-2-20 E-2-20 C-3-20 E-3-20

600 400 200

500 400 300 200 100

0

0 0

2

4

6

8

0

2

4

6

8

(a)

(a) 1000

500

C-2-20 E-2-20 C-2-40 E-2-40

800

300

C-1-40 E-1-40 C-2-40 E-2-40 C-3-40 E-3-40

200

100

Axial load (kN)

400

Axial load (kN)

10

Lateral deflection (mm)

Lateral deflection (mm)

600

400

200

0 0

0 0

2

4

6

8

10

12

2

4

6

8

10

12

Lateral deflection (mm)

(b)

(b)

1400

Fig. 12. Comparison of axial load-lateral deflection curves for different CFRP layers: (a) e = 20 mm (b) e = 40 mm.

1200

4.5. Axial strain-hoop strain curves The axial deformation at the 120 mm (4.728in.) distance in the middle of the specimen is used to calculate the axial strain of the CFRP confined concrete specimen. Meanwhile, the hoop strain was measured by strain gages attached to the surface of CFRP. The axial strain and hoop strain are averaged from two strain gauges on both side of CFRP for axial loaded specimens. For the eccentrically loaded specimen, the axial strain and hoop strain data at the most severely compressed side of specimens are taken for analysis. The axial strain-hoop strain relationship of all tested specimens are shown in Fig. 11. It can be seen that, from Fig. 11 (a)–(c), the hoop strain decreases as the eccentricity increases under the same CFRP layer and axial strain. This can be explained by the fact that an axial strain gradient existed due to the eccentric compression, i.e., the concrete at the compression side tends to

Axial load (kN)

1000

found that CCECs present higher ultimate strain in CFRP than those of CCUCs. This demonstrates that, CFRP are not more utilized in CCECs than in CCUCs under eccentric loading.

14

Lateral deflection (mm)

14

C-3-20 E-3-20 C-3-40 E-3-40

800 600 400 200 0 0

2

4

6

8

10

12

14

Lateral deflection (mm)

(c) Fig. 13. Comparison of axial load-lateral deflection curves for different eccentricities of load: (a) one layer (b) two layers (c) three layers.

expand to the less compressed region. The similar phenomena was also observed by Yu et al. [30]. Compared with CCUCs, CCECs

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show higher hoop strain under the same conditions, i.e., the same eccentricity, CFRP layer and axial strain. It is speculated that, due to

2000

4.6. Axial load-lateral deflection curves

1500

Load (KN)

the effect of prestress CFRP in CCECs plays a role earlier relative to CCUCs. Besides, for the eccentrically loaded specimens, it can be found, from Fig. 12, that the lateral deflections of CCUCs are higher than those of CCECs under the same axial load N and eccentricity e. Therefore, the moment M (M = N(e + u)) of CCUCs are higher than those of CCECs, which causes higher axial strain and hoop strain of CCUC specimens.

C-1-00 E-1-00 C-2-00 E-2-00 C-3-00 E-3-00

1000

500

(a) 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Axial displacement (mm)

For the eccentrically loaded specimens, the axial load-lateral deflection curves of the tested specimens are shown in Fig. 12. In general, the curves consist of two linear parts. The curve starts with a high slope curve and then the deflection increases slowly as the load increases. Then, when the load reaches the inflection load, the curves turned into the second branch with much smaller slope and the deflection increased sharply. It can be found that, from Fig. 12(a)–(b), the lateral deflection decreases as the layer of CFRP increases at the same loading eccentricity and axial load. It can also be inferred that, as the CFRP layer increases, the inflection load increases. Meanwhile, based on Fig. 13(a)–(c), it also illustrates that the lateral deflection increases as the eccentricity

1400 1400

1200 1200

1000

800

C-1-20 E-1-20 C-2-20 E-2-20 C-3-20 E-3-20

600

400

200

Load (kN)

Load (KN)

1000

-5

600

C-1-20 E-1-20 C-2-20 E-2-20 C-3-20 E-3-20

400 200

(b)

0

0 -10

800

0

5

10

0.0

15

0.2

0.4

0.6

0.8

6

Curvature (1/mm)x10

Axial displacement (mm)

(a) 500 500

400

Load (KN)

300

200

C-1-40 E-1-40 C-2-40 E-2-40 C-3-40 E-3-40

100

(c)

Load (kN)

400

300

C-1-40 E-1-40 C-2-40 E-2-40 C-3-40 E-3-40

200

100

0

0 -20

-15

-10

-5

0.0

0

5

10

15

20

Axial displacement (mm) Fig. 14. Comparison of axial load-displacement curves (a) e = 0 mm (b) e = 20 mm (c) e = 40 mm.

0.2

0.4

0.6

0.8

1.0

1.2

1.4

6

Curvature (1/mm)x10

(b) Fig. 15. Axial load-curvature curves: (a) e = 20 mm (b) e = 40 mm.

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Q. Cao et al. / Construction and Building Materials 208 (2019) 113–124 Table 7 The intercept and inflection loads. Specimen

Intercept load (kN)

Inflection load (kN)

Specimen

Intercept load (kN)

Inflection load (kN)

Specimen

Intercept load (kN)

Inflection load (kN)

C-1-20 E-1-20 C-1-40 E-1-40

475 620 210 230

490 625 250 290

C-2-20 E-2-20 C-2-40 E-2-40

615 765 225 300

657 788 263 338

C-3-20 E-3-20 C-3-40 E-3-40

750 825 280 315

770 870 300 375

Note: 1 kN = 224.8lbf.

Table 8 The curvatures of CFRP-confined concrete elements. Specimen

Load

Curvature (1/mm)106

Ratio of curvature (CCEC/CCUC)

Average ratio

C-1-20 E-1-20 C-2-20 E-2-20 C-3-20 E-3-20

At fixed load 600 kN

0.216 0.093 0.115 0.060 0.078 0.053

1 0.43 1 0.52 1 0.68

0.54

Note: 1 kN = 224.8lbf.

increases. Contrary to CFRP layer, load eccentricity shows the opposite influence on the inflection load. The inflection loads of CCECs are higher than those of CCUCs. It should be noted that the load-deflection curves of specimens with e = 40 mm (1.576 in.) even possess a descending branch. It is because the eccentricity was very high so that the moment M = N(e + u) reached the ultimate load capacity before the ultimate deformation capacity. And as the lateral deflection u continued to increase, the axial load N descended correspondingly. Last but not least, compared with CCUCs, CCECs show lower lateral deflections under all other same conditions which shows the benefit of using CFRP confined expansive concrete element members. 4.7. Axial load- displacement curves The axial load – axial displacement curves of the test specimens are shown in Fig. 14. As can be seen, generally, the axial displacement was increasing during the loading stage. It can be seen in Fig. 14(c), different from Fig. 14(a) and (b), the axial load increases as displacement increases, and then the load decreases as axial displacement increases at the second branch at high eccentricity of 40 mm. It can be found from Fig. 14(a)-(c) that, the axial displacement decreases as the layer of CFRP increases at the same loading eccentricity, type of concrete and axial load. It is because the confinement stiffness of CFRP increases with the increase of CFRP’s layer. At the same time, as can be seen from Fig. 14, the ultimate axial displacement increases with the increase of eccentricity and CFRP’s layer. Moreover, it does not show significant difference of ultimate axial displacements between CCUCs and CCECs. It should be mentioned that the axial strains were not documented due to inconsistent results. 4.8. Axial load- curvature curves The curvatures were calculated (the difference between the readings of vertical LVDTs at B and D sides (B and D sides are shown in Fig. 1) divided by the product of 120 mm (4.728 in.) and 250 mm (9.85 in.)) and the axial load-curvature curves are plotted in Fig. 15. As shown, the axial load-curvature curves show similar trends as the axial load-lateral deflection curves. The curvature of the specimens decreases as the CFRP layer increases and increases as the eccentricity increases. In Fig. 15, the intercept of the second ascending section is defined as the intercept load and the intersec-

tion between initial linear segment and the second ascending section is defined as the inflection load. In the same time, the intercept load and inflection load increase as CFRP layer increases and decrease as load eccentricity increases. The intercept loads and inflection loads of the specimens are listed in Table 7. For the intercept loads, when the layer of CFRP increases from one to two, and from two to three, the average intercept load increases 24.1% (384 kN (86.3kip) v.s. 476 kN (107.1kip)) and 13.9% (476 kN (107.1kip) v.s. 543 kN (122.0kip)), respectively. The intercept load decreases 61.5% when the eccentricity increases from 20 mm (0.788 in.) to 40 mm (1.576 in.) on average (675 kN (151.7kip) v.s. 260 kN (58.4kip)). And the average intercept load of CCECs is 1.20 times of that of CCUCs (509 kN (114.5kip) v.s. 426 kN (95.7kip)). For the inflection loads, when the layer of CFRP increases from one to two, and from two to three, the average inflection load increases 23.7% (414 kN (93.1kip) v.s. 512 kN (115.1kip)) and 13.1% (512 kN (115.1kip)v.s. 579 kN (130.2kip)), respectively. The inflection load decreases 56.7% when the eccentricity increases from 20 mm (0.788 in.) to 40 mm (1.576 in.) on average (700 kN (157.4kip) v.s. 303 kN (68.1kip)). And the average inflection load of CCECs is 1.20 times of that of CCUCs (548 kN (123.2kip) v.s. 455 kN (102.3kip)). Descending branches were also noticed in the curves of specimens with e = 40 mm (1.576 in.). As shown in Fig. 15, expansive concrete specimens CCECs performed better than conventional concrete ones CCUCs in curvature as lower curvature is evident for CCECs under same tested conditions. As the axial load-curvature curves demonstrate, the specimen with e = 40 mm (1.576 in.) have the descending trend after the turning point. Therefore, to make reasonable comparisons, the curvatures of specimens with e = 20 mm (0.788 in.) at fixed load of 600 kN (134.9 kip) are also listed in Table 8. It is inferred that, compared with CCUCs, the curvatures of CCECs decrease with average ratio of 0.54. Last but not least, it is worthwhile mentioning that the advantage of CCECs in lowering the curvature was weakened as the CFRP layer and eccentricity increases (0.43 (1-layer) v.s. 0.52 (2-layer) v.s. 0.68(3-layer)). 5. Conclusions Based on the experiment findings and discussion results in this study, the following conclusions can be drawn, (1) The strain monitoring experimental results on CFRP confined expansive concrete specimens show that prestress did exist in both the CFRP jackets and the expansive concrete.

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(2) Test results indicate that the ultimate load of tested specimens increase as the layer of CFRP increases, and decrease as load eccentricity increases. The average ultimate load capacity of expansive concrete specimen (CCECs) is 14% higher than that of the conventional concrete specimens (CCUCs) at the same conditions. (3) The ultimate lateral deflection increases as the loading eccentricity increases at each CFRP layer. The lateral deflections show an upward trend as the CFRP layer increases at the eccentricity of 20 mm. However, at the eccentricity of 40 mm, the lateral deflection first increases, followed by a decrease as the CFRP layer increases. (4) The hoop strains of the expansive concrete specimens (CCECs) are higher than those of the conventional concrete specimens (CCUCs) under the same tested conditions. Meanwhile, it is not found that CCECs present higher ultimate strain in CFRP than those of CCUCs. This demonstrates that, CFRP are not more utilized in CCECs than in CCUCs under eccentric loading. (5) Axial deformation results show that expansive concrete specimens CCECs performed better than conventional concrete specimen counterparts as lower curvature is evident for CCECs under same tested conditions. It also shows that this advantage is weakened as the CFRP layer and eccentricity increased. Conflict of interest None. Acknowledgements The financial support provided by National Natural Science Foundation of China (Under Grants 51208077 and 51421064), the Natural Science Foundation of Liaoning Province (Project No. 20170540168), and the Fundamental Research Funds for the Central Universities (Project No.DUT17JC02) are greatly appreciated. References [1] A. Mirmiran, M. Shahawy, Behavior of concrete columns confined by fiber composites, J. Struct. Eng., ASCE 123 (5) (1997) 583–590. [2] J.B. Mander, M.J.N. Priestley, R. Park, Theoretical stress-strain model for confined concrete, J. Struct. Eng.,-ASCE. 114 (8) (1988) 1804–1826. [3] L. Lam, J.G. Teng, Design-oriented stress–strain model for FRP-confined concrete, Constr. Build. Mater. 17 (6–7) (2003) 471–489. [4] V. Valdmanis, L. De Lorenzis, T. Rousakis, R. Tepfers, Behaviour and capacity of CFRP-confined concrete cylinders subjected to monotonic and cyclic axial compressive load, Struct. Concr. 8 (3) (2007) 187–200. [5] T. Yu, X.L. Fang, J.G. Teng, FRP-confined self-compacting concrete under axial compression, J. Mater. Civ. Eng. ASCE 26 (11) (2014) 04014082. [6] Z.C. Deng, J.L. Qu, The experimental studies on behavior of ultrahighperformance concrete confined by hybrid fiber-reinforced polymer tubes, Adv. Mater. Sci. Eng. (2015), https://doi.org/10.1155/2015/201289. 201289.

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