Experimental investigation of concrete fracture behavior with different loading rates based on acoustic emission

Construction and Building Materials 237 (2020) 117472

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Experimental investigation of concrete fracture behavior with different loading rates based on acoustic emission Chen Chen a, Xiangqian Fan b,⇑, Xudong Chen a a b

College of Civil and Transportation Engineering, Hohai University, Nanjing 210024, China Department of Materials and Structural Engineering, Nanjing Hydraulic Research Institute, Nanjing 210024, China

h i g h l i g h t s
Fracture toughness has rate effect.
AE event can describe the width of FPZ and size effect.
RA value increase with the increase of loading rate.
The failure forms under different loading rates can be analyzed by b value.

a r t i c l e

i n f o

Article history: Received 26 May 2019 Received in revised form 20 October 2019 Accepted 3 November 2019

Keywords: Concrete Fracture behavior Acoustic emission technology Fracture process zone

a b s t r a c t To study the effect of loading rate on the fracture behavior, three-point bending fracture tests of concrete with loading rates of 0.0001 mm/s, 0.001 mm/s, 0.01 mm/s and 0.1 mm/s were carried out, respectively. And acoustic emission (AE) technology was adopted for real-time monitoring. The results show that the unable toughness of concrete has obvious rate effect. Meanwhile, there are two obvious inflection points in the curve of cumulative AE hits and the cumulative ringing count with the change of time, one of which may represent the starting point of concrete boundary effect. The number of AE events can represent the crack width of concrete fracture. It is found that with the increase of loading rate, the crack width and ductility of concrete decrease, while the quantity of shear crack of concrete increases. By comparing the fracture energy of concrete with the AE cumulative energy, both of them increase with the increase of loading rate, indicating that the AE cumulative energy can represent the change of fracture energy under different loading rates. Finally, the failure forms under different loading rates are analyzed by the change of b value. Based on the above research, AE technology can be used to study the fracture failure under low loading rate. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction As an important engineering material, the mechanical properties of concrete have been widely studied. At present, most of the researches on the performance of concrete materials focuses on the static condition [1,2], but the concrete structures may be subjected to dynamic load sometimes, such as dynamic water pressure for the concrete dam, the impact of waves for the ocean platform, and the action of seismic load for the building structure [3,4]. Therefore, dynamic mechanical properties of concrete should be studied. Existing studies show that the compressive strength and tensile strength of concrete have rate sensitivity [5–8]. Bischoff and Perry [5] reviewed the dynamic uniaxial compressive strength ⇑ Corresponding author. E-mail address: [email protected] (X. Fan). https://doi.org/10.1016/j.conbuildmat.2019.117472 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

of plain concrete. Shi et al. [6] studied the dynamic flexural tensile fracture behavior of pervious pavement concrete under static preloading, and the result showed that as the loading rate increases under the same preloads, both the flexural load and the dynamic fracture toughness increase. Chen et al. [7] found that the dynamic strength and stress-strain curve of normal concrete at high temperature still experienced remarkable strain rate effects. Grote et al. [8] found that the bearing capacities of the concrete and mortar increase significantly with strain rate and hydrostatic pressure. For the concrete fracture, it also has the rate effect. Zhang et al. [9] studied the fracture properties of concrete with different sizes under different loading rates, and found the peak loads increase with an increase in the strain rate; the rate dependence of the peak load is stronger for larger specimens than for smaller ones. However, there are few researches about it. Therefore, it is

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

significant to study the concrete fracture under different loading rates. AE technology refers to the detection, recording and analysis of AE signals by means of sensitive sensors, and these signals are used to analyze AE sources quantitatively and qualitatively. It is a new dynamic non-destructive testing technology, which has been applied in civil engineering [10–12]. Nair and Cai [13] used AE to study the prestressed concrete bridge and steel bridge and estimate the damage by the AE data. Chen et al. [14] studied the fatigue crack growth of self-compacting concrete based on the AE technology. Meanwhile, AE technology is also widely used in the study of fiber-reinforced concretes. Li et al. [15] and Bhosale et al. [11] studied the AE characteristics of fiber-reinforced concrete with different fiber types and fiber volume fractions during flexural test, and found that with the increase of fiber volume fraction, the proportion of shear cracks in fiber concrete fracture increased. Xargay et al. [16] found that with the increase of temperature damage, AF value of self-compacted steel fiber concrete decreased while RA value increased. Furthermore, a large number of studies had shown that AE parameters are related to the crack propagation and fracture mechanism of concrete. Mihashi et al. [17] and Muralidhara et al. [18] used the AE events to estimate the fracture process zone (FPZ). Shah and Kishen [19] found that the width of fracture process zone and damage zone calculated by AE data doesn’t have effects on the concrete-concrete interface with different mix proportions. Han et al. [20] analyzed the damage mechanism and failure mode of concrete beams with different rubber content using AE technology. From the above literature review, majority of the previous work has concentrated on studying the fracture behavior of concrete, much less attention has been paid to the facture behavior of concrete under different loading rate. Furthermore, only a few studies in past have used the AE technique for understanding the facture behavior of concrete under different loading rate. Thus, a systematical investigation of the AE behavior of concrete with different loading rates is necessary. And the objectives of this study are (1) to characterize the FPZ of concrete under different loading rate using AE; (2) to evaluate the fracture failure mode under different loading rates using AE; (3) to analysis fracture failure process of concrete under different loading rates using AE. In conclusion, it should be pointed out that researchers have conducted critical studies on the AE behavior of concrete and the fracture rate effect, but the existing research work is independent. Few scholars put the two factors together to study, that is, there are few researches on the AE characteristics of concrete fracture under different loading rates. Therefore, the three-point bending fracture tests of concrete under different loading rates are done to study the rate effect of fracture parameters. Meanwhile, AE can used to monitor the concrete fracture process and analyze the change of acoustic emission characteristic parameters under different loading rates. Furthermore, some acoustic emission analysis methods, such as AF (the number of threshold crossings divided by the duration of each signal) versus RA (the duration of the initial rising part of the waveform, over its maximum amplitude) values, and b value are used to study the damage mechanism and failure mode of concrete at different loading rates.

2. Experimental program 2.1. Materials

Table 1 The mixture proportion (kg/m3). Water

Cement

Sand

Fly ash

Gravel

Water reducer

120

210

711

90

1250

1.64

the fineness modulus of 2.56, was used as fine aggregate. The grading curves of aggregates are shown in Fig. 1, according to JGJ 522006 [21]. Polycarboxylic acid superplasticizer was used to improve the work ability of concrete. The specimens with size of 400 mm  100 mm  100 mm were cast and the pre-cast vertical notch was created in the middle of the length with 40 mm using a 2 mm thick stainless-steel plate during the concrete casting. The mold was demoulded in the next day and watered for curing until 28 days. The compressive strength of concrete is 27.2 MPa. 2.2. Test setup 2.2.1. Fracture test The hydraulic servo testing machine MTS-810 is used to carry out the fracture test of three-point beams, according to the RILEM. The span S of specimen is 300 mm. In the test, the clip gauge is used to measure the value of crack mouth opening distance. The test loading control method was CMOD control, and the loading rates were 0.1 mm/s, 0.01 mm/s, 0.001 mm/s and 0.0001 mm/s, respectively. Four specimens were repeated in each working condition. 2.2.2. Acoustic emission The SAMOSTM detection system developed by American PAC company is used in this paper. Four acoustic emission probes are arranged on the specimen is shown in Fig. 2. Among them, the AE signal waveform output by AE is very complex, but these waveforms contain a lot of characteristic information of acoustic emission source, among which parameter analysis method is a commonly used way to identify and evaluate acoustic emission source signals. AE parameters are mainly divided into basic parameters and characteristic parameters. The characteristic parameters mainly refer to the information parameters extracted from the basic parameters of AE to describe the process and state changes. The characteristic parameters studied in this paper mainly include the number of hits, rising time, number of events, energy, duration, amplitude and signal intensity. The threshold value selected is 35 dB

100

Percent passing (by weight)

2

80

60

40

20

0 0.1

The mix proportion of concrete is shown in Table 1. In the mix proportion, Portland cement PO. 42.5 was used. The coarse aggregate was gravel, with maximum size of 19 mm. River sand, with

Fine agg.lower limit Fine agg.upper limit Sand Coarse agg.lower limit Coarse agg.upper limit Gravel 1

10

Size(mm) Fig. 1. The graving curve of aggregates.

100

3

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

Fig. 2. The schematic diagram of AE device.

3. Results and discussion 3.1. Fracture mechanical properties 3.1.1. P-CMOD curve The P-CMOD curves of concrete fracture under different loading rates are shown in Fig. 3. As shown in Fig. 4, it can be found that with the increase of loading rate, the peak load increases, indicating that the fracture stress has an obvious rate effect. According to the previous researches [5-8], the mechanical properties of concrete has rate effect, especially tensile properties, which means that the strength increases with the strain rate. Hillerborg et al.

Fig. 4. The relationship between the peak load and strain rate.

[22] proposed fictitious crack theory, which the peak load of fracture is determined by the uniaxial tensile strength. Therefore, because the strain rate effect, the uniaxial tensile strength increases, which results in the increases of peak load of concrete fracture, and it is similar to the conclusion of the research [23].

4.0 3.0

3.5

0.0001mm/s-1 0.0001mm/s-2 0.0001mm/s-3 0.0001mm/s-4

3.0

P (kN)

2.5

P (kN)

0.001-1 mm/s 0.001-2 mm/s 0.001-3 mm/s 0.001-4 mm/s

2.5

2.0

2.0 1.5

1.5 1.0 1.0 0.5

0.5 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.0

0.6

0.2

0.4

0.6

CMOD (mm)

(a) 0.0001mm/s

1.0

1.2

1.4

(b) 0.001mm/s

4.0

5

3.5

0.01mm/s-1 0.01mm/s-2 0.01mm/s-3 0.01mm/s-4

3.0

0.1mm/s-1 0.1mm/s-2 0.1mm/s-3 0.1mm/s-4

4

P (kN)

2.5

P (kN)

0.8

CMOD (mm)

2.0

3

2

1.5 1.0

1 0.5 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 0.0

0.2

0.4

0.6

0.8

1.0

CMOD (mm)

CMOD (mm)

(c) 0.01mm/s

(d) 0.1mm/s

Fig. 3. The P-CMOD curves under different loading rates.

1.2

1.4

4

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

where, K un IC is the unable fracture toughness, F max is the peak load, mis the concrete mass between the supports, g is the gravity acceleration, S is the span, b is the width of the specimen, h the height of the specimen, ac is the critical effective crack length. And ac can be calculated by the following equation.

ac ¼

 1=2 PECMODc ðh þ h0 Þarctan  0:1135  h0 p 32:6F max 2

ð3Þ

where h0 is the thickness of the clip gauge, CMODc is the critical value of CMOD, E is the elastic modulus, and the equation is shown as follows:

E¼

3.1.2. Fracture toughness According to Xu et al. [24,25], the unable fracture toughness can be calculated by following equation:

ac h

3.0

2.5

Accumulate counts Accumulate hits Load

60000

6000

3.0

48000

4800

50000

5000

2.5

40000

4000

4000

2.0

32000

1.5

24000

40000

1.5

30000

1.0

20000

0.5

10000

0

1000

2000

3000

4000

5000

0 6000

3000 2000

Accumulate counts Accumulate hits Load

1.0

1000

0.5

0

0.0

0

250

500

750

Time(s)

1000

35000

3500 3.0

3.0

30000

3000

Acumulate counts Accumulate hits Load

2.0

20000

Accumulate counts Accumulate hits Load

0.5

20

40

60

80

100

120

140

160

15000 10000

2500 2000 1500 1000

Load(kN)

25000

Accumulate hits

2.5

Accumulate counts

Load (kN)

2.5

0.0 0

800

0 1500

1250

1600

0

(b) 0.001mm/s

3.5

1.0

8000

2400

Time(s)

(a) 0.0001 mm/s

1.5

16000

3200

70000

5600

60000

4800

50000

4000

2.0 40000 1.5 30000 1.0

5000

500

0.5

0

0

0.0 0

2

4

6

8

10

Time (s)

Time(s)

(c) 0.01 mm/s

(d) 0.1 mm/s

12

3200 2400

20000

1600

10000

800

0

0

14

Fig. 6. The relationship among load, AE cumulative hits and counts: (a) 0.0001 mm/s; (b) 0.001 mm/s; (c) 0.01 mm/s; (d) 0.1 mm/s.

Accumulate hits

0.0

Fig. 6 shows the relationships among load, AE cumulative counts and hits under different loading rates. There are two obvious inflection points in the AE cumulative counts versus time curve under different loading rates. One of the first inflection point is corresponding to peak load, the reason for it maybe that the fracture

Acumulate counts

Load(kN)

2.0

ð2Þ

3.2. AE hits and counts

Accumulate counts

;a ¼

where a0 is the initial notch to depth ratio, t is the specimen thickness, C i is the initial compliance. In this paper, the value of C i can be obtained by the linear regression of the P-CMOD curve when the load achieved 20% of the maximum value [26]. As shown in Fig. 5, with the increase of loading rate, the unable toughness of concrete increases. And there is a logarithmic relationship between the unable fracture toughness and strain rate, which agrees with the conclusion of Bazant’s [27] and Hu’s studies [28].

Load(kN)

ð1 þ 2aÞð1  aÞ

3=2

ð1Þ

Accumulate hits

f ðaÞ ¼

f ðaÞ 2 bh   1:99  að1  aÞ 2:15  3:93a þ 2:7a2

Accumulate counts

K un IC ¼


 1=2 1:5 F max þ mg  102  103 Sac 2

ð4Þ

Accumulate hits

Fig. 5. The relationship between the unable toughness and strain rate.

   1 p a0 þ h0 3:70 þ 32:60tan2 tci 2 h þ h0

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

gf

GF

energy recommended by RILEM changes with size. Based on the work of Hu et al. [32–34], local fracture energy is introduced, which can be used to explain the size effect. As shown in Fig. 7, the local fracture energy g f at cohesive crack tip region referred in this paper is actually approximately adopted as the average of the distributed local fracture energies in the very limited stable crack growth region. Therefore, expressions relating specific fracture energy and are more convenient for calculations.

Transition fracture process zone

(

a

al*

5

x

Fig. 7. The schematic diagram of local fracture energy and boundary effect [34].

strength of concrete at this time reaches to peak, and then enter the post peak softening stage, the growth of AE cumulative hits and counts accelerate. Another inflection point is in the softening stage. The authors think appearance of this point is due to the boundary effect of concrete, which indicates crack reaching the transition fracture process zone, the schematic diagram is shown in Fig. 7. According to the previous research and analysis [29– 31], the fracture of concrete has obvious size effect, and fracture

gf ¼

  GF for x < W  a  al   for x  W  a  al GF Wax a

ð5Þ

l

8 h i al > < GF 1  2ðWa for ðW  aÞ > al Þ h i Gf ¼ > : GF Wa for ðW  aÞ  al 2a

ð6Þ

l

In which g f is the local fracture energy, Gf is the specific fracture energy, GF is the true facture energy, W is the overall depth of the beam, a is the initial notch depth and al is the transition ligament length. Karihaloo et al. [35,36] studied the fracture energy of concrete by the local fracture and boundary effect, and verified that the fracture energy is independent with size of concrete. Therefore, the authors think when the crack enter in the transition fracture

Fig. 8. The AE location of concrete fracture under different loading rates.

6

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

zone, the local fracture energy decreases gradually, then the AE signal intensity is weak, AE cumulative hits and counts change, and the rising trend slows down. 3.3. AE event Visual observation can only capture the displacement or strain that occur on the surface of the concrete beam, while AE technology can reveal the location of the main internal and external damage in the crack localization stage. In AE technology, threedimensional acoustic emission source location can be obtained by the principle of AE sensor arrival time difference. According to the AE-win software, the AE source location is shown in Fig. 8. It can be found that the location of AE sources is mainly concentrated on both sides of the fracture ligament, and there is no obvious quantitative relationship between the cumulative number of events and loading rate. According to previous researches [37,38], the change of AE source location can be used to evaluate the characteristics of FPZ of concrete. Alam et al. [39] used AE and DIC technology to monitor

FPZ of concrete beam, respectively, and found the fracture length evaluated by two methods have similar tendency. To study the width of FPZ, it is necessary to cumulate the AE events at each Xposition. Fig. 9 shows the AE cumulative events at each X-position under different loading rates. It can be seen that the peak value near the position of the precast crack, and both sides decreases. To obtain the width of FPZ, it’s necessary to assume that [40]: (1) Cumulative AE events at each X- position in the width of FPZ are greater than or equal to 20% of N max (N max is the peak value of cumulative AE events); (2) Outside the first region, the cumulative AE event at X position are less than 20% of N max , and the corresponding damage is relatively low. As shown in Fig. 9, the width of FPZ decreases gradually with the increase of loading rate. According to Behnia’s research [18], the smaller of the width of FPZ is, the higher the brittleness is and the smaller the ductility is. Therefore, the brittleness of concrete increases with the increase of loading rate, which agrees well

16 30

14

10

AE events

AE events

WFPZ=30mm

25

WFPZ=40mm

12

8

20

15

6 10

4 5

2

0

0 180

190

200

210

220

230

240

250

170

260

190

200

210

220

X-position (mm)

(a) 0.0001 mm/s

(b) 0.001 mm/s

50

230

240

35 30

40

WFPZ=20mm

WFPZ=20mm

25

30

AE events

AE events

180

X-position(mm)

20

20 15 10

10 5

0 160

170

180

190

200

210

220

230

240

0 170

180

190

200

210

X-position (mm)

X-position (mm)

(c) 0.01 mm/s

(d) 0.1 mm/s

220

230

240

Fig. 9. The distribution of cumulative AE events at each X position under different loading rates: (a) 0.0001 mm/s; (b) 0.001 mm/s; (c) 0.01 mm/s; (d) 0.1 mm/s.

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

7

with the conclusion described in the literature [23] about the failure state of concrete.

lative energy is studied. According to the JCI code [47], the fracture energy of concrete can be calculated as follows:

3.4. AE energy

Gf ¼

Shah et al. [19] studied the AE energy and found that fracture energy is correlated to AE cumulative energy. Based on this, the relationship among the loading rate, fracture energy and AE cumu-

W 1 ¼ 0:75

Table 2 The fracture energy and AE cumulative energy under different loading rate. Loading rate

Fracture energy (N/mm)

AE accumulate energy

0.001 mm/s-1 0.001 mm/s-2 0.001 mm/s-3 0.001 mm/s-4 0.01 mm/s-1 0.01 mm/s-2 0.01 mm/s-3 0.01 mm/s-4 0.1 mm/s-1 0.1 mm/s-2 0.1 mm/s-3 0.1 mm/s-4

137.5 246.1 194.1 165.5 185.3 158.8 217.1 216.4 224.7 219.7 189.7 234.1

12,467 23,491 19,149 16,573 19,625 13,728 21,671 23,864 21,034 27,267 18,356 24,412

0:75W 0 þ W 1 Alig   S m1 þ 2m2 g  CMODf L

ð5Þ

ð6Þ

where Gf is fracture energy, W 0 is the area enclosed by the P-CMOD curve, W 1 is the work done by weight of specimen, Alig is the area of ligament, m1 is specimen weight, m2 is the weight of jig on the specimen, CMODf is value of CMOD at failure. The value of CMODf is so small that be wiped out under the loading rate of 0.0001 mm/s. Therefore, the changes of fracture energy of concrete under different loading rate are studied, as shown in Table 2. It can be found that the fracture energy increases with the increases of loading rate, which agrees well with Zhang’s research [41]. Meanwhile, compared the AE cumulative energy under different loading rates, it can be found that the two have the similar change trend, that is, the AE cumulative energy increases with the increase of loading rate. Therefore, for the fracture of concrete with different loading rates, AE cumulative energy can characterize the change trend of fracture energy.

Fig. 10. The curves of AE RA value and AF under different loading rates: (a) 0.0001 mm/s; (b) 0.001 mm/s; (c) 0.01 mm/s; (d) 0.1 mm/s.

8

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

3.5. AF-RA

3.6. b value

The AF and RA can be used to distinguish the tensile crack and shear crack in the fracture process [20]. Generally speaking, bending failure is accompanied by tensile crack, which leads to transient or transient burst volume change in the material, and converts the released energy into compressional wave/expansion wave (P wave). Therefore, tensile crack failure would produce the P wave. On the other hand, shear crack failure would produce the shear waves (S wave). However, the propagation of P- wave is faster than S-wave. Thus, the S wave results in a longer rise time. Therefore, the RA value of shear failure is higher than that of bending failure, and the average frequency of bending failure is higher [40,42,43]. As shown in Fig. 10, the maximum value of RA increases. The main reason is that with the increase of loading rate, the failure time decreases, and the failure surface is flatter, which shows the obvious shear failure phenomenon, especially for the coarse aggregate shear break. And there are more than 50% tensile cracks in the process of fracture crack under different loading rate. Therefore, it still can be considered as type I fracture of concrete, and verified the accuracy of fictitious crack theory proposed by Hillerborg et al. [22], even if at the loading rate of 0.1 mm/s. Meanwhile, it also shows that with the increase of loading rate, the brittleness increases.

b value was first proposed by Gutenberg and Richter, and it is mainly used to measure the level of seismic activity in a certain region. The seismic wave activity is similar to the form of acoustic emission phenomenon, so the b value is used to measure the important parameter of concrete crack expansion. According to previous researches [44,45], the AE b value can be effectively calculated by the 1/20 of AE amplitude replace M. The main reason is that large AE events are less likely to occur than small ones, and the amplitude must be divided by a factor of 20 to obtain a b value comparable to the earthquake value. The formula is as follows:

logðNÞ ¼ a  bðAdB =20Þ

ð7Þ

where N is the number of AE hits with an amplitude higher than AdB , a is a constant, b is b value. The change of b value indicates the change from microcrack to macrocrack. Fig. 11 shows the b value of concrete under different loading rates. It can be found that the trend of b value is very concentrated at the beginning and then tends to disperse. Due to the relationship mentioned in the paper [45] between the increasing and decreasing of the b-value and the microcracks and macrocracks, the decreasing b-value trend suggests macrocracks have formed, while the increasing b-value trend implies microcracks growth. Therefore, crack propagation in concrete fracture process

Fig. 11. The relationship among b value, load and time under different loading rates.

C. Chen et al. / Construction and Building Materials 237 (2020) 117472

can be divided into three stages by the density of b-value [46]: (I) initiation, (II) coalescence and (III) nucleation. With the transition from the coalescence stage to the nucleation stage, the b value fluctuates greatly in the final stage, indicating that the formation of local macroscopic cracks leads to the ultimate failure. However, the three stages are indistinguishable under high loading rate. The loading rate can accelerate the crack propagation. The release of high-energy events leads to the rapid development of cracks under the high loading rate, the rapid fluctuation of b value, and the trend of b value is more dispersed. 4. Conclusion In this paper, the fracture characteristics of concrete under different loading rates were studied. At the same time, AE technology was used for monitoring, and AE parameters were used to reflect the fracture characteristics of concrete under different loading rates. The specific research conclusions are as follows: (1) With the increase of loading rate, the peak load increases and fracture toughness increases, which shows that fracture of concrete has rate effect. (2) In the process of fracture failure under different loading rates, there are two obvious inflection points in the curves of AE cumulative number of hits and ringing count versus the time. The first inflection point corresponds to the peak load, at which time the whole curve of concrete fracture enters the softening section. (3) The number of AE events can well represent the FPZ of concrete under different loading rate. And, the width of FPZ decreases with the increase of loading rate, the brittleness of concrete increases. (4) By studying the changes of AF and RA values, the proportion of the shear crack of concrete increases with the increase of loading rate, and the shear failure trend increases. (5) b value can well represent the three stage of crack propagation concrete under different loading rates, but it’s indistinguishable under high loading rate. Although this paper studied the AE properties of concrete fracture under different loading rate, the loading rate is not high enough. In future study, the higher loading rate showed been done by the drop-weight machine, and the AE used to represent the fracture properties of concrete under the medium and low strain rate. 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. Acknowledgments This work was supported by the National Key R&D Program of China (SN: 2016YFC0401907); National Natural Science Foundation of China (SN: 51739008, 51527811, 51879168, 51679150); the National Natural Science Foundation of Jiangsu Province (SN: BK20180051); and the Fund Project of NHRI (Y417015, Y419003). References [1] F. Xiao, B. Putman, S. Amirkhanian, Rheological characteristics investigation of high percentage RAP binders with WMA technology at various aging states, Constr. Build. Mater. 98 (6063) (2015) 315–324. [2] H. Naderpour, A.H. Rafiean, P. Fakharian, Compressive strength prediction of environmentally friendly concrete using artificial neural networks, J. Build. Eng. 16 (2018) 213–219.

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