# Study on Acoustic Emission Localization of Concrete Using Modified Velocity

Authors: Jiapeng Li; Lei Qin
Journal: Advances in Civil Engineering
 (2019)

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## Abstract

To improve the accuracy of the acoustic emission (AE) source localization, a new 3D AE source localization method was investigated which used a combination of the modified velocity and the 3D localization algorithm based on the exhaustive method. The wave speed has a significant effect on the AE location results. With the increase of distance, the AE signal seriously attenuated due to the anisotropy of concrete, and the measured velocity changed for various distances. The velocity-distance curves were obtained when employing different water-cement ratios (W/C) and aggregate sizes. However, the current AE location system adopted constant wave velocity. As a result, the error was tremendous. The accuracy of the localization before and after the modified velocity was compared. The 3D localization results showed that compared with the constant wave velocity, the position deviation of the modified velocity was smaller and the localization results with the modified velocity were more accurate.


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## Body

## 1. Introduction

Acoustic emission (AE) is a dynamic nondestructive testing method among all aspects of structural health monitoring (SHM) [1]. Compared to other nondestructive techniques, AE technique can determine the position of the cracks or monitor the entire structure without damaging the structure [2–4]. The advantages of AE method also include real-time capability, high sensitivity, and global monitoring capability [5–9]. In the past few decades, the AE technique had a rapid development. Some researchers focused on damage identification of concrete [10–12], whereas damage localization was the others’ objective [13–16].There were many researches about the AE source localization in concrete. Karaiskos et al. [17] investigated localization accuracy on healed concrete with a piezoelectric transducer. The localization accuracy could be improved when the cracks heal autonomously. In order to eliminate false or inaccurate information, Kurz et al. [18] proposed an improved algorithm based on the Akaike information criterion (AIC), and the accuracy of AE signals was improved. Aljets et al. [19] developed a certain sensor array to identify the position of acoustic emission source, and a new source location algorithm was adopted, which was beneficial to improving the localization accuracy. Ohtsu et al. [20–23] investigated the AE source of concrete with simplified moment tensor analysis, such as parameter-based method and simplified Green’s functions. Lu et al. [11] located the AE source by the 3D localization iteration program and Green’s function, which was beneficial to reducing the number of sensors. Cheng et al. [24] used wavelet analysis to research the localization of AE source, and the location of AE source could be certain by analyzing the AE parameters. However, the most traditional AE source localization methodology adopted constant wave velocity, and the attenuation of the AE signal was not taken into account. As a result, the localization accuracy was not accurate.In this research, the source localization accuracy was investigated by using the location algorithm based on the 3D enumeration method and combining the modified AE velocity of concrete. Firstly, the velocity test was performed. AE performance with different water-cement ratios and maximum aggregate sizes was studied. The wave propagation speed will have a little reduction due to the anisotropy of concrete. Then the relationships between AE signal attenuation and water-cement ratio and max aggregate size were obtained. However, the constant wave velocity was chosen in the current acoustic emission location system, and small variations in the wave speed had a greatly effect on source localization [10, 25, 26]. As a result, 3D localization algorithm of AE based on exhaustive method combining the modified AE velocity of concrete must be selected to ensure the source localization accuracy. There are parameter-based approach and signal-based approach in recording and analyzing AE signals [27–29]. AE parameter-based approach offers real-time monitoring, whereas AE signal-based approach usually offers analysis in a postprocessing environment. So, it is vital for AE to select an analysis type [30].


## 2. Velocity Test

### 2.1. Experimental Specimens

To determine the relationship between the inhomogeneity of concrete and AE signal attenuation, a velocity test was conducted. It was necessary to extend the acoustic wave propagation path as far as possible in concrete. All of the concrete beams were 150 × 150 × 450 mm. The mixture properties of concrete could be seen in Table1. The water-cement ratio was 0.40, 0.45, 0.5, and 0.55. The maximum aggregate size was 25 mm, 19 mm, 12.5 mm, and 4.75 mm, respectively. The curing age was 28 days.Table 1
Mixture proportion of the material.
W/C
0.40
0.45
0.50
0.55
Cement (kg)
5.28
4.63
4.19
3.71
Sand (kg)
6.34
6.95
7.33
8.16
Aggregate (kg)
10.56
10.65
10.68
10.39
Water (kg)
2.11
2.08
2.10
2.04


### 2.2. Experimental Procedure

Pencil lead break testing was implemented to determine the wave velocity. The position of sensor 1 was fixed when measuring the velocity using surface bonded WSa sensor. The testing instruments are all from Physical Acoustic Corporation (PAC). However, the position of sensor 2 moved with the AE source and the pencil was broken at the side of the sensor 1. The distance from the AE source to sensor 1 was 10, 20, 30, and 40 cm, respectively. Four points were tested for each distance. Figure1 shows the velocity measurement using the surface-bonded WSa sensor. The wave velocity was calculated using the onset time difference of the two sensors.Figure 1
Pencil lead break testing.
(a)
(b)


### 2.3. Results and Discussion

The relationships between AE signal and different aggregate sizes or different water-cement ratios were investigated. Pencil lead break testing was close to sensor 1. AE counts were seriously attenuated, and the rise time became longer. As is shown in Figure2, when the water-cement ratio is determined, with the increasing distance, the larger the particle size of the aggregate, the lower the amplitude and counts. For example, when the water-cement ratio was 0.55 and the maximum aggregate size was 25 mm, the amplitude and counts decreased 26.7% and 32.4%, respectively. The rise time increased 264.5%. When the maximum aggregate size is 4.75 mm, the amplitude and counts decreased 16.7% and 24.1%, respectively. The rise time increased 215%. When the maximum aggregate size is determined, with the increasing of the distance, the larger the W/C, the lower the amplitude and counts. For example, when the maximum aggregate size was 4.75 mm and the water-cement ratio was 0.4, the amplitude and counts decreased 26.7% and 32.4%, respectively. The rise time increased 264.5%. When the W/C is 0.5, the amplitude and counts decreased 16.7% and 24.1%, respectively. The rise time increased 215%.Figure 2
Counts and amplitude with different distances under different W/C and aggregate sizes. (a) W/C is 0.55 and maximum aggregate size is 25 mm; (b) W/C is 0.55 and maximum aggregate size is 4.75 mm; (c) W/C is 0.4 and maximum aggregate size is 25 mm; (d) W/C is 0.7 and maximum aggregate size is 4.75 mm.
(a)
(b)
(c)
(d)To investigate the AE velocity under different distances with different water-cement ratios or different aggregate sizes, on the basis of experimental data, the velocity-distance curves can be obtained by the time difference from the AE source to the sensors. The influences of W/C on AE velocity are shown in Figure3.Figure 3
AE velocity with different distances under different water-cement ratios and the same aggregate size. Maximum aggregate size is (a) 4.75 mm, (b) 12.5 mm, (c) 19 mm, and (d) 25 mm.
(a)
(b)
(c)
(d)At the same time, when the W/C is determined, the velocity-distance curves could be drawn by different aggregate sizes, as shown in Figure4.Figure 4
AE velocity with different distances under the same water-cement ratio and different aggregate sizes. Water-cement ratio is (a) 0.4, (b) 0.45, (c) 0.5, and (d) 0.55.
(a)
(b)
(c)
(d)When acoustic propagation distance is 100 mm and 300 mm, the histogram of AE wave velocity changing with the water-cement ratio and aggregate size was drawn and is shown in Figure5.Figure 5
Impact of W/C and aggregate particle size on AE wave velocity with different distances: (a) 100 mm and (b) 300 mm.
(a)
(b)It can be seen from Figures3 and 5, when the propagation distance is determined, the larger the water-cement ratio, the smaller the average wave velocity. For example, for 100 mm, when the maximum aggregate is 4.75 mm, the water-cement ratio is 0.4 and the average velocity is about 4750 m/s. However, the water-cement ratio is 0.55, the average velocity is about 4250 m/s, and the wave speed decreases by 10.5%. The same pattern was found in the concrete with other aggregate sizes.It also can be indicated from Figures4 and 5 that the larger the aggregate particle size, the smaller the corresponding wave speed at different distances. For example, for 300 mm, when the water-cement ratio is 0.4, the maximum aggregate is 4.75 mm and the average velocity is about 4385 m/s. However, the maximum aggregate is 25 mm, the average velocity is about 4000 m/s, and the wave speed decreases by 9.62%. The same pattern was found in the concrete with other water-cement ratios.The reasons are as follows. When the water-cement ratio is determined, the larger the particle size of the aggregate, the higher the directional alignment of calcium hydroxide crystal in the transition zone, thus weakening the surface area interfacial transition zone (ITZ), and more wool stoma hole will be produced due to it. On the one hand, this is equivalent to increasing the AE signal propagation path, and the velocity in the air is less than in the concrete, which leads to lower average velocity in concrete. On the other hand, the interface between the coarse aggregate and cement may be bigger due to the larger aggregate particle size, which increases the probability of reflection, refraction, and diffraction. As a result, average wave velocity is lower.


## 2.1. Experimental Specimens

To determine the relationship between the inhomogeneity of concrete and AE signal attenuation, a velocity test was conducted. It was necessary to extend the acoustic wave propagation path as far as possible in concrete. All of the concrete beams were 150 × 150 × 450 mm. The mixture properties of concrete could be seen in Table1. The water-cement ratio was 0.40, 0.45, 0.5, and 0.55. The maximum aggregate size was 25 mm, 19 mm, 12.5 mm, and 4.75 mm, respectively. The curing age was 28 days.Table 1
Mixture proportion of the material.
W/C
0.40
0.45
0.50
0.55
Cement (kg)
5.28
4.63
4.19
3.71
Sand (kg)
6.34
6.95
7.33
8.16
Aggregate (kg)
10.56
10.65
10.68
10.39
Water (kg)
2.11
2.08
2.10
2.04


## 2.2. Experimental Procedure

Pencil lead break testing was implemented to determine the wave velocity. The position of sensor 1 was fixed when measuring the velocity using surface bonded WSa sensor. The testing instruments are all from Physical Acoustic Corporation (PAC). However, the position of sensor 2 moved with the AE source and the pencil was broken at the side of the sensor 1. The distance from the AE source to sensor 1 was 10, 20, 30, and 40 cm, respectively. Four points were tested for each distance. Figure1 shows the velocity measurement using the surface-bonded WSa sensor. The wave velocity was calculated using the onset time difference of the two sensors.Figure 1
Pencil lead break testing.
(a)
(b)


## 2.3. Results and Discussion

The relationships between AE signal and different aggregate sizes or different water-cement ratios were investigated. Pencil lead break testing was close to sensor 1. AE counts were seriously attenuated, and the rise time became longer. As is shown in Figure2, when the water-cement ratio is determined, with the increasing distance, the larger the particle size of the aggregate, the lower the amplitude and counts. For example, when the water-cement ratio was 0.55 and the maximum aggregate size was 25 mm, the amplitude and counts decreased 26.7% and 32.4%, respectively. The rise time increased 264.5%. When the maximum aggregate size is 4.75 mm, the amplitude and counts decreased 16.7% and 24.1%, respectively. The rise time increased 215%. When the maximum aggregate size is determined, with the increasing of the distance, the larger the W/C, the lower the amplitude and counts. For example, when the maximum aggregate size was 4.75 mm and the water-cement ratio was 0.4, the amplitude and counts decreased 26.7% and 32.4%, respectively. The rise time increased 264.5%. When the W/C is 0.5, the amplitude and counts decreased 16.7% and 24.1%, respectively. The rise time increased 215%.Figure 2
Counts and amplitude with different distances under different W/C and aggregate sizes. (a) W/C is 0.55 and maximum aggregate size is 25 mm; (b) W/C is 0.55 and maximum aggregate size is 4.75 mm; (c) W/C is 0.4 and maximum aggregate size is 25 mm; (d) W/C is 0.7 and maximum aggregate size is 4.75 mm.
(a)
(b)
(c)
(d)To investigate the AE velocity under different distances with different water-cement ratios or different aggregate sizes, on the basis of experimental data, the velocity-distance curves can be obtained by the time difference from the AE source to the sensors. The influences of W/C on AE velocity are shown in Figure3.Figure 3
AE velocity with different distances under different water-cement ratios and the same aggregate size. Maximum aggregate size is (a) 4.75 mm, (b) 12.5 mm, (c) 19 mm, and (d) 25 mm.
(a)
(b)
(c)
(d)At the same time, when the W/C is determined, the velocity-distance curves could be drawn by different aggregate sizes, as shown in Figure4.Figure 4
AE velocity with different distances under the same water-cement ratio and different aggregate sizes. Water-cement ratio is (a) 0.4, (b) 0.45, (c) 0.5, and (d) 0.55.
(a)
(b)
(c)
(d)When acoustic propagation distance is 100 mm and 300 mm, the histogram of AE wave velocity changing with the water-cement ratio and aggregate size was drawn and is shown in Figure5.Figure 5
Impact of W/C and aggregate particle size on AE wave velocity with different distances: (a) 100 mm and (b) 300 mm.
(a)
(b)It can be seen from Figures3 and 5, when the propagation distance is determined, the larger the water-cement ratio, the smaller the average wave velocity. For example, for 100 mm, when the maximum aggregate is 4.75 mm, the water-cement ratio is 0.4 and the average velocity is about 4750 m/s. However, the water-cement ratio is 0.55, the average velocity is about 4250 m/s, and the wave speed decreases by 10.5%. The same pattern was found in the concrete with other aggregate sizes.It also can be indicated from Figures4 and 5 that the larger the aggregate particle size, the smaller the corresponding wave speed at different distances. For example, for 300 mm, when the water-cement ratio is 0.4, the maximum aggregate is 4.75 mm and the average velocity is about 4385 m/s. However, the maximum aggregate is 25 mm, the average velocity is about 4000 m/s, and the wave speed decreases by 9.62%. The same pattern was found in the concrete with other water-cement ratios.The reasons are as follows. When the water-cement ratio is determined, the larger the particle size of the aggregate, the higher the directional alignment of calcium hydroxide crystal in the transition zone, thus weakening the surface area interfacial transition zone (ITZ), and more wool stoma hole will be produced due to it. On the one hand, this is equivalent to increasing the AE signal propagation path, and the velocity in the air is less than in the concrete, which leads to lower average velocity in concrete. On the other hand, the interface between the coarse aggregate and cement may be bigger due to the larger aggregate particle size, which increases the probability of reflection, refraction, and diffraction. As a result, average wave velocity is lower.


## 3. 3D Localization Test

### 3.1. Experimental Specimens

The SAEU2S AE system (produced by PAC) was used for the 3D localization test. All of the concrete cubes were 300 × 300 × 300 mm. The mixture properties of concrete are listed in Table2. After 28 days of curing, the 3D localization test was conducted. Pencil lead break testing was implemented to improve the accuracy of 3D localization.Table 2
The ratio of mixture proportion of the material.
Group
W/C
Maximum aggregate (mm)
Mixture
1
0.40
4.75
1 : 1.2 : 2 : 0.4
2
0.45
12.5
1 : 1.5 : 2.3 : 0.45
3
0.50
19
1 : 1.75 : 2.55 : 0.5
4
0.55
25
1 : 2 : 2.8 : 0.55On the surface of concrete, 9 broken lead points were selected, and the broken lead points were arranged, as shown in Figure6. The sensors are arranged in a diamond shape. The sensors are coupled to the corresponding position through the vacuum grease. The tape is fixed to prevent the sensor from moving. The sensors coordinates are as follows: 1# (0, 0, 0), 2# (300, 0, 300), 3# (300, 300, 0), and 4# (0, 300, 300). Sensors are shown in Figure 7.Figure 6
Schematic diagram of the broken lead points.Figure 7
Schematic diagram of the sensor layout.


### 3.2. Experimental Procedure

Pencil lead break excitation was conducted near each pencil lead break point; some AE waveform parameters like its amplitude, rise time, and duration were stored and calculated in a 4-channel AE monitoring system. The software is AE system (PAC) which has the 3D localization function.


### 3.3. Results and Discussion

The 3D localization results of the concrete are shown in Figure8. In order to modify the AE wave velocity, the location algorithm based on the 3D enumeration method was performed in this paper. AE 3D localization software was adopted by using MATLAB. The wave velocity function obtained by wave velocity test was written to software. To achieve the accurate localization, a modified wave velocity is chosen.Figure 8
3D localization results before (left) and after (right) the wave speed modification: (a) 1#; (b) 2#; (c) 3#; (d) 4# (blue points are break points, and red points are locating points).
(a)
(b)
(c)
(d)The accuracy of the localization before and after the modified was compared, respectively. Different plane projections of localization results were drawn. The plane projections of No. 1 and No. 4 concrete cubes are shown due to the large number, as shown in Figures9 and 10.Figure 9
Different plane projections of localization results for sensor 1#. Blue points are break points, red points are locating points, and semicircles are deviation area. The localization results before (left) and after (right) the wave speed modification on (a)x-y plane projections, (b) y-z plane projections, and (c) x-z plane projections.
(a)
(b)
(c)Figure 10
Different plane projections of localization results for sensor 4#. Blue points are break points, red points are locating points, and semicircles are deviation area. The localization results before (left) and after (right) the wave speed modification on (a)x-y plane projections, (b) y-z plane projections, and (c) x-z plane projections.
(a)
(b)
(c)During the propagation of concrete materials, the attenuation of AE wave is more serious and the velocity will be greatly reduced. The longer the AE wave travels, the more severe the attenuation of the wave speed is. Different plane projections of localization results were compared. It can be seen from the different plane projections of localization results that the 3D localization results of AE source before and after the correction of wave speed have a great difference. The accuracy of the localization was poor with constant wave velocity. However, the accuracy of the localization was improved largely due to modified wave velocity. After the modification of AE wave speed, the localization results were closer to the actual broken lead point. All the localization results fall within the circle with the lead point as the center and the distance from the lead point to the furthest positioning point as the radius. By comparison, the radius of the position deviation before the modification of the wave speed is larger than the modified velocity. The locating points are more concentrated, and the dispersiveness of the localization results is larger. The position deviation of the modified velocity is smaller. The localization results with the modified velocity can be viewed more accurately.It illustrates that the 3D localization results deviates greatly from the real position if a constant wave velocity was selected. On the contrary, the wave velocity is corrected according to the propagation distance, which is beneficial to improving the localization accuracy of the AE sources.


## 3.1. Experimental Specimens

The SAEU2S AE system (produced by PAC) was used for the 3D localization test. All of the concrete cubes were 300 × 300 × 300 mm. The mixture properties of concrete are listed in Table2. After 28 days of curing, the 3D localization test was conducted. Pencil lead break testing was implemented to improve the accuracy of 3D localization.Table 2
The ratio of mixture proportion of the material.
Group
W/C
Maximum aggregate (mm)
Mixture
1
0.40
4.75
1 : 1.2 : 2 : 0.4
2
0.45
12.5
1 : 1.5 : 2.3 : 0.45
3
0.50
19
1 : 1.75 : 2.55 : 0.5
4
0.55
25
1 : 2 : 2.8 : 0.55On the surface of concrete, 9 broken lead points were selected, and the broken lead points were arranged, as shown in Figure6. The sensors are arranged in a diamond shape. The sensors are coupled to the corresponding position through the vacuum grease. The tape is fixed to prevent the sensor from moving. The sensors coordinates are as follows: 1# (0, 0, 0), 2# (300, 0, 300), 3# (300, 300, 0), and 4# (0, 300, 300). Sensors are shown in Figure 7.Figure 6
Schematic diagram of the broken lead points.Figure 7
Schematic diagram of the sensor layout.


## 3.2. Experimental Procedure

Pencil lead break excitation was conducted near each pencil lead break point; some AE waveform parameters like its amplitude, rise time, and duration were stored and calculated in a 4-channel AE monitoring system. The software is AE system (PAC) which has the 3D localization function.


## 3.3. Results and Discussion

The 3D localization results of the concrete are shown in Figure8. In order to modify the AE wave velocity, the location algorithm based on the 3D enumeration method was performed in this paper. AE 3D localization software was adopted by using MATLAB. The wave velocity function obtained by wave velocity test was written to software. To achieve the accurate localization, a modified wave velocity is chosen.Figure 8
3D localization results before (left) and after (right) the wave speed modification: (a) 1#; (b) 2#; (c) 3#; (d) 4# (blue points are break points, and red points are locating points).
(a)
(b)
(c)
(d)The accuracy of the localization before and after the modified was compared, respectively. Different plane projections of localization results were drawn. The plane projections of No. 1 and No. 4 concrete cubes are shown due to the large number, as shown in Figures9 and 10.Figure 9
Different plane projections of localization results for sensor 1#. Blue points are break points, red points are locating points, and semicircles are deviation area. The localization results before (left) and after (right) the wave speed modification on (a)x-y plane projections, (b) y-z plane projections, and (c) x-z plane projections.
(a)
(b)
(c)Figure 10
Different plane projections of localization results for sensor 4#. Blue points are break points, red points are locating points, and semicircles are deviation area. The localization results before (left) and after (right) the wave speed modification on (a)x-y plane projections, (b) y-z plane projections, and (c) x-z plane projections.
(a)
(b)
(c)During the propagation of concrete materials, the attenuation of AE wave is more serious and the velocity will be greatly reduced. The longer the AE wave travels, the more severe the attenuation of the wave speed is. Different plane projections of localization results were compared. It can be seen from the different plane projections of localization results that the 3D localization results of AE source before and after the correction of wave speed have a great difference. The accuracy of the localization was poor with constant wave velocity. However, the accuracy of the localization was improved largely due to modified wave velocity. After the modification of AE wave speed, the localization results were closer to the actual broken lead point. All the localization results fall within the circle with the lead point as the center and the distance from the lead point to the furthest positioning point as the radius. By comparison, the radius of the position deviation before the modification of the wave speed is larger than the modified velocity. The locating points are more concentrated, and the dispersiveness of the localization results is larger. The position deviation of the modified velocity is smaller. The localization results with the modified velocity can be viewed more accurately.It illustrates that the 3D localization results deviates greatly from the real position if a constant wave velocity was selected. On the contrary, the wave velocity is corrected according to the propagation distance, which is beneficial to improving the localization accuracy of the AE sources.


## 4. Conclusions

In this paper, velocity test is conducted to determine the impact on velocity with different W/C and aggregate size, and the localization results of before and after the wave speed modification are studied. The following conclusions can be drawn:(1)
With the increase of water-cement ratio (W/C) and aggregate size, the number of AE hits increased and the average wave velocity is attenuated obviously.(2)
The 3D localizations of the AE sources by modified velocity show higher accuracy.(3)
Instead of improving the accuracy of onset time, using a modified velocity according to the distance can greatly improve the localization accuracy. It shows a feasible and effective way to monitor and assess the health state of a concrete structure.



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*Source: 6890327-2019-05-21.xml*