The influence of steel fiber on water permeability of concrete under sustained compressive load

Construction and Building Materials 242 (2020) 118058

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The influence of steel fiber on water permeability of concrete under sustained compressive load Dong Li ⇑, Shi Liu College of Civil Engineering, Liaoning Technical University, Fuxin 123000, China

h i g h l i g h t s
A self-designed water permeability device under compressive load was introduced.
The influence of steel fiber on water permeability of concrete was investigated.
The ultrasonic pulse velocity of concrete under compressive load was studied.
The correlation of water permeability and ultrasonic pulse velocity was explored.

a r t i c l e

i n f o

Article history: Received 23 July 2019 Received in revised form 24 November 2019 Accepted 2 January 2020

Keywords: Water permeability Steel fiber Concrete Compressive load Ultrasonic pulse velocity

a b s t r a c t In order to investigate the influence of steel fibers on the water permeability of concrete at serviceability stage, a self-designed device was adopted and the permeability test was conducted on hollow cylindrical specimens under a sustained compressive load. The combined effect of steel fiber and compressive load on water permeability of concrete was analyzed. Ultrasonic pulse velocity was also measured to verify the damage of the concrete caused by the compressive load. The results demonstrated that the water permeability of the concrete was affected by the compressive load, a significant increment in the permeability coefficient occurred when the applied load exceeded the threshold value. The addition of steel fiber demonstrated positive effects on permeability of concrete under compressive load. The threshold value corresponding to permeability properties of the concrete under compressive load was improved with the increasing of fiber content. The ultrasonic pulse velocity of the concrete under compressive load was also influenced by addition of steel fiber. The variation of the ultrasonic pulse velocity has a certain similarity with the variation of the permeability coefficient of the specimen under compressive load. Ultrasonic pulse velocity test may be an efficient method to evaluate the permeability of concrete at service load. Ó 2020 Elsevier Ltd. All rights reserved.

1. Introduction Permeability is one of the important factors for durability design of concrete structures [1–6]. Permeability of concrete is mainly affected by two aspects: interconnectivity of pores in the cement paste and micro-cracks in the concrete [7,8]. Traditional test methods for evaluating concrete permeability are mainly focused on unloaded specimens [9–12], and the obtained results only reflect the interconnected porosity of the concrete. However, real concrete structures usually bear different types of loads at serviceability stage [13], and the loads often cause the occurrence and development of micro-cracks in the matrix. The indicators based

⇑ Corresponding author. E-mail address: [email protected] (D. Li). https://doi.org/10.1016/j.conbuildmat.2020.118058 0950-0618/Ó 2020 Elsevier Ltd. All rights reserved.

on the specimens without any damage may not truly reflect the permeability properties of the concrete at service loads. Several investigations were conducted by the researchers to evaluate the influence of the compressive load on the permeability performance of the concrete. Banthia et al. [14] studied the effect of compressive load on water permeability of concrete at early age, the results showed that the permeability coefficient was influenced by the compressive load. Bao and Wang [15] verified the water absorption capacity of concrete under uniaxial load, the results demonstrated that the cumulative water content initially decreased and then increased with the increment of the compressive load. Ma et al. [16] studied the influence of compressive load on chloride penetration of recycled concrete and established the relationship between the chloride diffusivity and the damage caused by load. Sugiyama et al. [17] investigated the gas permeability of concrete under compressive load; the results illustrated

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D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Nomenclature NC SFRC40 Kw h r1 D

vt

SFRC20

plain concrete without any reinforcement steel fiber reinforced concrete with fiber dosage of 40 kg/m3 water permeability coefficient height of the specimen internal diameter of the specimen damage modulus ultrasonic pulse velocity of the specimen under compressive load steel fiber reinforced concrete with fiber dosage of 20 kg/m3

that the permeability increment of the concrete was obvious when the applied load was 76%–79% of the ultimate load bearing capacity. The addition of fibers can reduce the brittleness, enhance ductility and improve durability properties of the concrete [18–27]. However, the investigations of the effect of fibers on water permeability of concrete under sustained compressive load are still rare. The Ultrasonic Pulse Velocity (UPV) technique is one of the most widely used Nondestructive Testing (NDT) methods to evaluate the properties of cementitious materials [28–32]. Without external load, the ultrasonic pulse velocity of the concrete was not affected obviously with low steel fiber contents (<1% by volume) [33,34]. For plain concrete without any reinforcement, due to the progressive growth of cracks, the ultrasonic pulse velocity of the matrix decreased with the increasing of compressive stress [35]. The durability performance of the concrete at serviceability stage is also associated with the propagation of the internal cracks. Therefore, the UPV technique may be a workable method to verify the durability properties of the concrete. In this study, the influence of steel fiber with different dosage on water permeability of concrete was investigated under sustained compressive load. A self-designed experimental system was introduced to continuously measure the permeability property of the samples under the applied load. The coefficient values of water permeability of concrete with and without fiber reinforcement were analyzed. The influence of the compressive load on ultrasonic pulse velocity of the samples was also verified and the variations of the permeability coefficient and the ultrasonic pulse velocity of the concrete were studied. 2. Experiments 2.1. Materials The basic mix proportion was as follows: ordinary Portland cement (PO 42.5R), 390 kg/m3; fly ash, 155 kg/m3; water, 273 kg/m3; coarse aggregates with particle size of 5–20 mm, 822 kg/m3; fine aggregate with maximum particle size of 5 mm, 848 kg/m3; polycarboxylate super plasticizer, 5.5 kg/m3, waterbinder ratio, 0.5. The chemical composition of the cement and fly ash are illustrated in Table 1. In order to evaluate the influence of steel fiber on water permeability of concrete, hooked-end steel fibers with different fiber contents (20 kg/m3, 40 kg/m3, 60 kg/m3)

SFRC60 Q DP r2

v0

fu n

steel fiber reinforced concrete with fiber dosage of 60 kg/m3 rate of water flow water pressure external diameter of the specimen ultrasonic pulse velocity of the specimen without external load ultimate compressive stress stress level

were added into the concrete. Fig. 1 demonstrated the steel fiber adopted in this test. The properties of the fiber were illustrated in Table 2. 2.2. Samples preparation The distribution of fibers plays a significant role in the performance of fiber reinforced concrete. In order to ensure the uniform distribution of fibers, the mixing procedure was conducted according to CECS 13 [36]. First, the fine and coarse aggregates were homogenized by dry mixing for 1 min, and then the cement and fly ash were added (+1 min of dry mixing). The cementitious materials and aggregates were mixed for 2 min; the water and super plasticizer were poured during this procedure. After that, the fibers were slowly added into the mixture; at last, all the materials were mixed for 1 min. The whole mixing procedure took about 8 min. For each mixture, six cylindrical specimens (100 mm in diameter and 200 mm in height with a 50 mm in diameter hollow cylindrical core at the center) were prepared. The cylindrical specimens were made by coring from the cuboids (500 mm in length, 250 mm in width and 210 mm in height). After casting, the cuboids were covered with plastic sheets, and left in the casting room. The specimens were demoulded within one day after cast. Then they were cured in the curing room for 28 d, the cylindrical specimens were cored after 28 d. Three of the cylindrical samples were used for compressive strength test and the average value of the result was adopted as the reference of the applied load for the permeability test. The applied load was about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 of the ultimate load. The left three samples were prepared for permeability test, and the permeability coefficient of the concrete was the mean value of the three samples. In addition, six cubic samples (150 mm  150 mm  150 mm) were also prepared. Three of the cubic samples were studied for the compressive strength of the matrix [36,37], and the left were for the ultrasonic pulse velocity experiment. 2.3. Water permeability test The ends of the specimens were ground to obtain two smooth parallel surfaces and the silicon pads were fixed to the ends in order to avoid water leakage during experiment (see Fig. 2).

Table 1 Chemical composition of cement and fly ash.

a

Type

CaO

SiO2

Al2O3

Fe2O3

MgO

K2O

Na2O

SO3

LOIa

Cement Fly ash

61.13 3.20

21.45 55.20

5.24 26.30

2.89 8.33

2.08 1.11

0.81 2.43

0.77 1.00

2.50 0.57

3.13 1.86

Loss on ignition.

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D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Fig. 1. Steel fibers.

Fig. 2. Specimen and the silicon pads.

The specimen was placed in a specially designed cell such that water would permeate through the wall of the cylindrical sample and be collected from the inner hollow core. The schematic of the experimental device was illustrated in Fig. 3 and the permeability device was shown in Fig. 4. The schematic of the water permeability system for concrete was shown in Fig. 5. The permeability test system mainly included a counter-force frame, a screw jack, a load sensor, a permeability device and a data acquisition unit. The screw jack was adopted as the device of applying sustain compressive load. In order to monitor the applied load, a load sensor was placed between the screw jack and the permeability device. The collected water from the hollow core was drained out to a collection reservoir where its mass was measured continuously and accurately using a computer controlled weighing sensor. For hollow core cylindrical specimens, the water penetrates inward from the outer wall; the water permeability coefficient Kw can be obtained by applying Darcy’s law [38,39]

Kw ¼

Q r2 ln 2phDP r 1

ð1Þ

where Kw denotes the coefficient of water permeability, m/s; Q denotes the rate of water flow, m3/s; h denotes the height of the specimen, m; DP denotes the water pressure, MPa, in this study DP is 0.7 MPa; r1 denotes the internal diameter of the specimen, m; r2 denotes the external diameter of the sample, m.

Fig. 3. Schematic of the experimental device.

where, D means the damage modulus; v0 means the ultrasonic pulse velocity of the concrete without external load, m/s; vt means the ultrasonic pulse velocity of the concrete under different compressive load, m/s.

2.4. Ultrasonic pulse velocity test 3. Results and discussion The ultrasonic detector was adopted and the ultrasonic pulse velocity of cube samples was measured under different sustained compressive load, as shown in Fig. 6. The average value of three samples was adopted as the value of the corresponding type of concrete. Damage variable D in damage mechanics was introduced, the relationship between D and ultrasonic pulse velocity [40,41]

D¼1

v 2t v 20

ð2Þ

3.1. Compressive strength The compressive strength of each group is the mean value of three samples [36,37]. The values of compressive strength of the specimens at different ages are shown in Table 3. In order to verify the effect of fibers on compressive strength of concrete, the variation of the compressive strength of each mixture (the values of NC set as 1.0) is also given, as shown in Fig. 7.

Table 2 Properties of the steel fiber. Types

Length/mm

Diameter/mm

Aspect ratio

Tensile strength/MPa

Steel fibers

35

0.54

65

1345

4

D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Fig. 4. Water permeability device.

Fig. 6. Ultrasonic pulse velocity measurement of the concrete.

3.2. Permeability properties

Fig. 5. Water permeability test system under sustained compressive load.

From Table 3 and Fig. 7, it can be seen that the variation of values of each mixture is relatively small (<±5%). Take the results at the curing age of 28 d as an example, the values of NC, SFRC20, SFRC40 and SFRC60 are 36.4 MPa, 35.9 MPa, 37.9 MPa and 35.6 MPa, respectively. Compared to the compressive strength of NC, the values of the SFRC20 and SFRC 60 decreases by about 1% and 2%, respectively; the corresponding value of the SFRC40 increases by about 4%. By considering the allowable deviation of the test values for compression (±15%) [36,37], we can see that the addition of steel fibers does not have noticeable influence on compressive strength of the concrete, as confirmed by [23,42,43]. The main reason may as follows: the compressive stress does not tend to cause cracking directly in a plane perpendicular to the stress as happens for tension, so fibers are expected to have a negligible reinforcing effect in the direction of applied stress [44].

The results of water permeability for NC specimens under different stress level n (ratio to ultimate stress fu) are given in Fig. 8 and Table 4. From Fig. 8 and Table 4, it can be seen that the value of the permeability coefficient decreased slightly when the stress level increased to about 0.3 fu and then the permeability of the sample started to increase. This phenomenon has been emphasized by experimental results, as reported in [45,46]. Although the variation of the permeability coefficient was observed at the stress level of about 0.3 fu, it is obvious that the increment of the permeability was significantly at the stress level of about 0.6 fu. The permeability coefficient of NC samples, under 0.58 fu and 0.67 fu are 9.62E  14 m/s and 1.81E  13 m/s, respectively, compared to the value under 0.58 fu, the permeability coefficient value of NC sample under 0.67 fu increased by about 88%. While the test results of NC samples under the 0.67 fu–0.88 fu indicate that the compressive load give rise to a significant increment in the water permeability, compared to the value under 0.58 fu, the permeability coefficient value of NC sample under 0.88 fu increased by about 3569%. Based on the analysis above, we can see that the water permeability of plain concrete was affected by the applied compressive stress and a threshold stress level of approximately 0.6 fu was verified. The result is agreement with the findings by Hoseini [39]. In other words, at stress levels below the threshold value, there is a small change in water permeability, whereas at higher stress levels beyond this threshold, the water permeability of the concrete demonstrates significant increment [47]. The reasons may as follows: as the stress level is low, the microcrack extends slightly along the interfacial transition zone between the coarse aggregate and cement paste [48–50], in this stage, the effect of micro-cracks on the permeability coefficient of the concrete is not evident; with the increasing of compressive stress, the micro-cracks existing on the interfacial transition zone gradually extend and new cracks continuously appear on other interfacial transition zones. Some cracks on the interfacial transition zones gradually develop into the cement paste, in this stage, due to the cracks extended in the matrix, the obvious increment of the permeability coefficient of the samples is observed; at high levels of compressive stress, cracks in the cement paste develop rapidly and link with adjacent cracks on the interfacial transition zones. These cracks link up and compose several continuous cracks paralleling approximately to the direction of compressive stress, therefore the permeability of the concrete increases significantly.

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D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058 Table 3 Compressive strength of the specimens at different curing ages. Types

Fiber contents (kg/m3)

NC SFRC20 SFRC40 SFRC60

0 20 40 60

Compressive strength (MPa) 7d

28 d

60 d

24.6 23.8 23.9 24.1

36.4 35.9 37.9 35.6

40.1 39.4 40.6 38.9

Fig. 7. Variation of the compressive strength of each matrix at different curing ages.

to the permeability of the matrix is delayed until about 0.8 of the ultimate compressive stress. The permeability coefficients of SFRC60 samples under 0.09 fu and 0.79 fu are barely changed (1.3 2E  13 m/s and 1.33E  13 m/s, respectively). At higher stress level beyond 0.79 fu, the permeability coefficient increased noticeably. Compared to the value under 0.79 fu, the permeability coefficient of SFRC sample under 0.93 fu increased by about 1294%. The comparison of the coefficient values between NC specimens and steel fiber reinforced specimens with different fiber dosage (SFRC20, SFRC40, SFRC60) under sustained compressive stress is illustrated in Fig. 10. In order to compare the variation of the permeability coefficient, the permeability coefficients of the samples are normalized by dividing the corresponding initial value, respectively. The normalized permeability coefficients of samples under stress level of 0.6, 0.7, 0.8 and 0.9 are illustrated in Table 5. From Fig. 10, the following interesting information can be obtained:
When the fiber dosage is 20 kg/m3, the influence of steel fiber on permeability coefficient of the concrete is not obvious. It means that the threshold stress level of the concrete relevant to the permeability is not improved notably.
Compared to the permeability under the stress level of about 0.1, the permeability coefficient of NC increases by 83% under the stress level of about 0.7; while the permeability coefficient of SFRC 40 only increases by 39% under the stress level of about 0.7. It means that the permeability of the concrete is influenced remarkably when the fiber content is up to 40 kg/m3.
It demonstrates that water permeability of concrete was influenced by the addition of steel fibers under sustained compressive load and the threshold stress level corresponding to the significant variation of permeability was enhanced with the increment of fiber dosage. The reasons may as follows: with the increment of the compressive stress, the micro-cracks gradually extend from the interfacial transition zone to the cement paste, during the process, the steel fiber spanning across the cracks can transmit tensile stress and limit the propagation of cracks. The increment of the permeability coefficient of the samples is unremarkable; at high levels of compressive stress, in spite of the fibers on cracking control; cracks in the cement paste are interconnected with each other. The influence of fibers inhibition on permeability of concrete is weakened gradually. 3.3. Influence of compressive stress on ultrasonic pulse velocity of concrete

Fig. 8. Variation of permeability coefficients of NC specimen under different stress level.

The result of water permeability for SFRC60 specimens under different stress level n is given in Fig. 9. From Fig. 9, it can be seen that the variation of the permeability coefficient of SFRC60 at the stress level of about 0.3 fu is similar to that of plain concrete and the characteristic was also observed in softwood fiber reinforced concrete, as reported in [45]. While with the addition of steel fiber, the threshold stress level corresponding

The ultrasonic pulse velocity of the samples under different stress level is shown in Fig. 11. The initial ultrasonic pulse velocity of the samples is demonstrated in Table 6. From Fig. 11 and Table 6, it can be seen that: (1) With the increment of steel fiber content, the maximum change of the ultrasonic pulse velocity of the samples is only 2%. It means that the ultrasonic pulse velocity of the concrete is not significantly affected by addition of steel fiber.

Table 4 Permeability coefficients of NC specimen under different stress level (m/s). Type

NC

Stress level (ratio to ultimate compressive stress fu) 0.08

0.17

0.33

0.42

0.50

0.58

0.67

0.75

0.88

9.95E  14

9.69E  14

9.56E  14

9.72E  14

9.84E  14

9.62E  14

1.81E  13

5.62E  13

3.53E  12

6

D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Fig. 9. Permeability of SFRC60 under different stress level. Fig. 11. Variation of the ultrasonic pulse velocity of the samples under compressive load.

Table 6 Ultrasonic pulse velocity of the samples without load. Item

NC

SFRC20

SFRC40

SFRC60

Ultrasonic pulse velocity (m/s)

4250

4290

4330

4180

(3) With addition of steel fiber, the stress level corresponding to the remarkable variation of ultrasonic pulse velocity is enhanced; the decrease of the ultrasonic pulse velocity of SFRC60 is obvious when the stress level is greater than about 0.8.

Fig. 10. Comparison of permeability coefficients of different specimens under different compressive stress.

(2) The ultrasonic pulse velocity of the samples was influenced by the compressive load. For plain concrete NC, when the stress level is less than about 0.6, the ultrasonic pulse velocity of the sample does not change significantly; while, when the stress level is greater than about 0.6, the ultrasonic pulse velocity of the sample begins to decrease significantly. For example, the ultrasonic pulse velocity of the samples under stress level of 0.62 and 0.80 are 4250 m/s and 3540 m/s, respectively. Compared to the value under stress level of 0.62, the ultrasonic pulse velocity of the sample decreases by about 17% under stress level of 0.80.

Similar variation of the ultrasonic pulse velocity of the plain concrete under sustained load has been previously reported by Qasrawi et al. [35]. According to Qasrawi et al. the sharp decrease of the ultrasonic pulse velocity of the concrete was attributed to the formation and propagation of cracks inside the samples. In addition to the proposed effects by Qasrawi et al. the addition of steel fibers might have an effect on the reduction of ultrasonic pulse velocity as steel fibers act as randomly distributed reinforcement resulting in restriction of crack propagation. The damage factor D of the samples under different stress level is shown in Fig. 12. From Fig. 12, it can be seen that: 1. For plain concrete, the damage factor D of the samples varied remarkably when the stress level is up to 0.6. 2. The damage of the samples is restrained and the increment of the damage factor D of the sample is delayed with the increasing of steel fiber dosage. For example, the damage factors D of NC and SFRC60 under stress level of about 0.8 are 0.31 and

Table 5 Comparison of normalized permeability coefficients of samples under different stress level (m/s). Types

Stress level (ratio to ultimate compressive stress fu) 0.6

NC SFRC20 SFRC40 SFRC60

0.97 1.23 1.20 0.94

0.7 (0.58) (0.57) (0.59) (0.63)

1.83 1.68 1.39 0.96

0.8 (0.67) (0.67) (0.67) (0.71)

Notes: The value in () is the real stress level corresponding to the normalized permeability.

5.64 2.82 1.91 1.00

0.9 (0.75) (0.76) (0.78) (0.79)

35.49 39.35 22.23 13.92

(0.88) (0.86) (0.88) (0.93)

D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

7

The variations of the ultrasonic pulse velocity and permeability coefficients of the samples under compressive load are illustrated in Fig. 13. For comparison, the ultrasonic pulse velocity and permeability coefficients of the samples are normalized by dividing the corresponding initial value, respectively. From Fig. 13, it can be seen that:

Fig. 12. Variation of the damage factor D of the samples under compressive load.

0.05, respectively. Compared to NC, the damage factor D of the SFRC60 decreases by 84%. It means that the damage of steel fiber reinforced concrete is lower than that of the plain concrete without any reinforcement. The reasons may also attribute to the function of fibers on controlling cracking under sustained compressive load.

(1) The variations of the ultrasonic pulse velocity and the permeability coefficient of the specimens under sustained compressive load demonstrate certain similarities. Take NC specimen as an example, when the stress level is less than 0.6, there is no marked change in the ultrasonic pulse velocity and the permeability coefficient of the sample; while, when the stress level is greater than 0.6, the ultrasonic pulse velocity of the sample decreases significantly, and the permeability coefficient of the sample increases remarkably. (2) For steel fiber reinforced samples under compressive load, there also exists a good correlation between the variation of ultrasonic pulse velocity and the variation of permeability coefficient. (3) It may be a feasible way to verify the water permeability of the concrete with the variation of the ultrasonic pulse velocity of the concrete under compressive load. It is obvious that remarkable jump in the values of ultrasonic pulse velocity and the permeability coefficient occurs when the applied stress exceeds the threshold stress level. The variation of

Fig. 13. Relationship between ultrasonic pulse velocity and permeability of the samples under compressive load: (a) NC, (b) SFRC20, (c) SFRC40, (d) SFRC60.

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D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Fig. 14. Relationship between variation of ultrasonic pulse velocity and variation of permeability coefficient: (a) NC, (b) SFRC20, (c) SFRC40, (d) SFRC60.

ultrasonic pulse velocity is correlated with the variation of permeability coefficient over the threshold value, as shown in Fig. 14. From Fig. 14, it can be seen that the correlation coefficients R2 of three groups (NC, SFRC20 and SFRC60) are larger than 0.950, and the R2 value of only one group is about 0.800, which substantiates an exponential correlation between the variation of ultrasonic pulse velocity and the variation of permeability coefficient. The form of the exponential relationship is in agreement with the observations of Hoseini [39], while he did not consider the influence of macro fibers. In the real structures, the variation of the ultrasonic pulse velocity of the element may reflect the water permeability properties of the matrix. 4. Conclusion

2. Due to the effect of fibers on restricting the crack propagation of the cementitious materials, the addition of steel fiber demonstrated positive influence on water permeability of concrete under compressive load. 3. The variation of the ultrasonic pulse velocity of the concrete under compressive load was influenced by addition of steel fiber. The threshold value of stress level corresponding to ultrasonic pulse velocity of concrete was increased from about 0.6– 0.8 with the fiber dosage of 60 kg/m3. 4. The variations of the ultrasonic pulse velocity and the permeability coefficient of the concrete under compressive load showed similarities. The ultrasonic pulse velocity test may be introduced to evaluate the permeability of concrete at serviceability stage.

The following conclusions can be drawn according to the experimental and analytical investigation: CRediT authorship contribution statement 1. The water permeability coefficient of the concrete under sustained compressive load was affected by addition of steel fibers. The threshold value of compressive stress level corresponding to water permeability of concrete was enhanced with the increasing of fiber dosage.

Dong Li: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing - original draft, Writing - review & editing. Shi Liu: Investigation, Methodology, Writing - review & editing.

D. Li, S. Liu / Construction and Building Materials 242 (2020) 118058

Declaration of Competing Interest 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.

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Acknowledgement The authors gratefully acknowledge the National Natural Science Foundation of China (Grant: 51578109).

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