An investigation on the aggregate-shape embedded piezoelectric sensor for civil infrastructure health monitoring

Construction and Building Materials 131 (2017) 57–65

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

An investigation on the aggregate-shape embedded piezoelectric sensor for civil infrastructure health monitoring Shanglin Song a, Yue Hou a,⇑, Meng Guo a, Linbing Wang b, Xinlong Tong a, Jiangfeng Wu a a b

National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China Joint USTB-Virginia Tech Lab on Multifunctional Materials, USTB, Beijing, Virginia Tech, Blacksburg, VA 24061, United States

h i g h l i g h t s
A new piezoelectric sensor for infrastructure health monitoring is designed.
3D printing technique is used for sensor packaging.
Sensor is designed in aggregate shape.

a r t i c l e

i n f o

Article history: Received 17 August 2016 Received in revised form 3 October 2016 Accepted 12 November 2016

Keywords: Piezoelectric sensor Aggregate-shape Embedded Infrastructure Health monitoring

a b s t r a c t In this paper, a new type of aggregate-shape embedded piezoelectric sensor for health monitoring of civil infrastructures was developed. The sensor was designed by using piezoelectric ceramic chip as the functional phase, and a new composite material as packaging phase. 3D printing technology was also used for packaging system design and sensor fabrication. The frequency independence, linearity, sensitivity, response rate, and service performance of the sensor were tested by frequency scanning and amplitude scanning. Experimental results show that within the vibration frequency range of common civil engineering structures, the new aggregate-shape embedded piezoelectric sensor has good mechanical and workability properties. Test results also show that the new embedded sensors have very good mechanicalelectrical coupling performance, which builds a solid foundation for further application in the civil engineering infrastructures. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, with the fast development of national economy and construction technology, many large-scale civil engineering infrastructures such as highways and bridges have been built in China. These infrastructures will endure erosion, aging, fatigue and other hostile environmental conditions during the long term service, which may cause a catastrophic accident in extreme cases [1]. Generally, before the destruction of civil engineering infrastructures, fracture, fatigue cracking and other diseases will gradually appear in the structures. It is therefore necessary to conduct long-term health monitoring on these structures by using sensor network. The sensors can be embedded into the structure during the construction process so as to monitor the service status of ⇑ Corresponding author. E-mail addresses: [email protected] (S. Song), [email protected] (Y. Hou), [email protected] (M. Guo), [email protected] (L. Wang), [email protected] (X. Tong), [email protected] (J. Wu). http://dx.doi.org/10.1016/j.conbuildmat.2016.11.050 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

infrastructures from the beginning of the construction, and further to ensure the safety of the civil engineering infrastructure [2,3]. Normally, the traditional monitoring sensors include accelerometers, displacement sensor, force sensor, resistance strain gauge [4,5]. There are also new sensor materials such as optical fiber, memory alloy and piezoelectric materials [6–8]. However, the commonly used sensors in civil engineering infrastructures have some shortcomings. For example, the service life of the sensor is much less than the life of the infrastructure [9]; sensors in the structure are easy to fail; sensor material and the concrete structure have poor compatibility; embedded sensors may change the stress distribution of the structure; sensor equipment is expensive [10,11]. In 2001, Li proposed a new type of piezoelectric composite material, which is very suitable for the health monitoring of concrete structure [12,13]. However, the piezoelectric material cannot be directly used as the sensors embedded in the structures [14]. A proper design of packaging the material using composite materials is needed to improve the structure compatibility and the mechanical performance [15]. Meanwhile, although

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there have been many researches in the civil infrastructure material characterizations [16,17], there still needs research on the field of smart health monitoring of civil infrastructures and develop a smart material with both perceptive function and drive functions. It should also have a good compatibility with the traditional infrastructure materials. The piezoelectric sensor has these advantages. In addition, it has fast response rate and good durability compared to the traditional sensors. The most prominent advantage of 3D printing technology is that the product with any shape can be directly generated using the computer graphics data. This doesn’t depend on the mechanical processing mold any longer. To implement the sensor into the civil infrastructure, it should be compatible with the infrastructure characteristics. In this way, an aggregate-shape sensor could satisfy such requirement, where there have been several researches in this field [18]. Therefore, it can shorten the producing time of production, reduce the cost and increase the precision. 3D printing technology can also help us to quickly realize the structure design of sensors with different size, different structures and different shapes. Based on the previous research in infrastructure health monitoring [19,20], in this paper, a new aggregate-shape embedded piezoelectric sensor was designed. Within the vibrations frequency range of civil engineering structure, the workability and service performance of the sensors were tested. First, through the experiment, the optimum mixing ratio of composite materials for packaging was obtained. Its material properties and mechanical performance were also tested; second, combined with the 3D printing technology, the packaging for piezoelectric materials using composite materials to manufacture the aggregate-shape sensors were designed. Different piezoelectric sensors are prepared and the efficiencies were tested; finally, the frequency independence, the linearity, the response rate and the service performance of the different sensors were tested. The experimental results verify the feasibility of the aggregate-shape embedded piezoelectric sensors for the application in monitoring civil engineering infrastructure.

2. Design and performance test 2.1. Mix design The packaging material has two functions: one is to protect the piezoelectric elements from external environment (including rainfall and frost) erosion so as to improve the sensor’s durability; another is to ensure the electrically and insulation performance of piezoelectric sensor. The packaging material of this research mainly consists of cement, epoxy resin, curing agent, diluent and anti-foam agent. By adjusting the ratio of filler, polymer and other agents, a composite material with good mechanical properties, excellent waterproof performance, and good corrosion resistance can be obtained. The design of mix proportion is shown in Table 1. Materials used in this test mainly includes cement, epoxy resin, curing agent, diluent and anti-foam agent. All of them are nontoxic and environmentally friendly products. The detailed information is shown in Table 2 and Table 3.

2.2. Mechanical properties test The compressive strength and flexural strength of the packaging composite materials of different mix ratio at different ages are tested, as shown in Table 4. The tested samples are prisms, and its size is 40 mm  40 mm  160 mm. The compressive strength and bending strength were tested according to the specification ‘‘IS0 679-2009 cement – test methods – determination of strength”. It is can be seen that with the cement increasing, the compressive strength of the composite materials at the same age presents an increasing first and then a decreasing trend later. The compressive strengths at 3d, 7d and 28d are highest when epoxy resin: cement = 3:2. This is due to that with the increasing in cement packing, the polymer material for cement particles decreases, the defects of epoxy composite structure begin to accumulate and the compacting property and compressive strength decrease. A higher content of rigid fillers results in greater flexural strength. When the cement content increases about four times, the flexural bending strength increases about twice. This trend applied to the curing time from 3 days to 28 days. With the curing time increasing, the flexural bending strength also increased. But the increase amplitude was not obvious from 3 days to 7 days. By contrast, the increase of flexural bending strength from 7 days to 28 days was significant. 2.3. Micro topography analysis The cement and epoxy resin composite material is a combination of inorganic material and organic material. The cement was used as the supporting filler, the epoxy resin and the additive were used as the bonding material. Polymerization reaction occurred and gradually a complete membrane structure formed. Fig. 1 shows the Scanning Electron Microscopy (SEM) images of the micro structure of the composites with different mix ratios (epoxy resin: cement). It is can be observed that, when the polymer-cement ratio is 3:1, there are many micro pores and cavities on the surface. When the polymer-cement ratio is 3:2, the solidification products of cement and epoxy resin are more dense, and there are much less pores and cavities, indicating a better performance; when the polymer-cement ratio are 3:3 and 3:4, with the increase of cement, epoxy resin content reduced, which results in the weakening of the interfacial bonding property of colloid material and filler [21,22]. It is can be concluded that the composite material has a good microstructure when the ratio of polymer to cement ratio is 3:2. This is due to that the polymer gradually forms into a continuous film, bonding with the cement, to build a network structure. The performance of the composite structure is improved, including the compressive strength, flexural strength, durability, flexibility. 2.4. Water absorption test In order to further validate the micro-morphology analysis, the water absorption test is carried out. Table 5 shows the water absorption testing results of the designed composite materials.

Table 1 Design of mix proportion of packaging composite materials. Mix ratio number

Cement

Epoxy resin

Curing agent

Diluent (%)

Anti-foam agent (%)

01 02 03 04

1 2 3 4

3 3 3 3

1.1 1.1 1.1 1.1

10 10 10 10

1 1 1 1

Note: the data in the table are the mass ratio.

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S. Song et al. / Construction and Building Materials 131 (2017) 57–65 Table 2 Manufacturer and type of packaging composite materials. Item

Cement

Epoxy resin

Curing agent

Diluent

Anti-foam agent

Manufacturer

Yutian county JiYu cement Co., Ltd. C42.5

Beijing Tonglian Hengxing Technology Co., Ltd. E51

Beijing Tonglian Hengxing Technology Co., Ltd. 6230

Beijing Tonglian Hengxing Technology Co., Ltd. TL-A100

Beijing Tonglian Hengxing Technology Co., Ltd. TL-X90

Type

Table 3 Mechanical properties of 42.5 ordinary portland cement. Strength grade

Compressive strength

42.5

Bending strength

3d

28d

3d

28d

=17

=42.5

=3.5

=6.5

Table 4 Mechanical properties of encapsulated composites. Mix proportion

Compressive strength/MPa

Bending strength/MPa

3d

7d

28d

3d

7d

28d

01 02 03 04

89.7 106.7 100.4 97.3

101.1 109.3 102.2 100.6

110.4 120.8 116.0 113.3

21.1 25.0 34.4 43.0

21.5 25.8 35.4 49.8

26.3 33.9 37.4 54.4

Mix proportion

Standard deviation of compressive strength/MPa

01 02 03 04

Standard deviation of Bending strength/MPa

3d

7d

28d

3d

7d

28d

3.37 1.12 1.33 2.45

0.81 1.24 1.30 2.78

2.27 2.25 1.44 2.03

0.78 0.13 0.74 1.60

1.18 0.77 1.84 1.19

0.29 0.34 2.44 0.09

Fig. 1. SEM images of microstructure of composite material at different mix ratios.

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Table 5 Water absorption ratio of designed composite material with different mix ratios. Mix ratio number

01

02

03

04

Water absorption rate/%

0.23

0.17

0.28

0.32

The devices of water absorption test include electronic scale, oven and water sink. First, the prepared specimen was kept in 100 ± 5 °C oven for 48 ± 0.5 h. And then it was weighed and recorded as m0; Second, the specimen was put in the water sink with the formed surface facing down, and two rebars were used to support the specimen. The specimen was totally immersed in water, and the height between water surface and specimen surface should be more than 20 mm. After 48 ± 0.5 h, it was taken out. A surface dry dishcloth was used to wipe the water on surface of specimen. The mass was weighed again as m1. At last, the water absorption ratio can be calculated according to the following Eq. (1). For each test, three parallel specimens were tested and the average was calculated.

Wx ¼

m1  m0 m0

ð1Þ

Water absorption rate is an important parameter to characterize the porosity of materials, and is also an important parameter to reflect the microstructure of materials. Table 3 shows that water absorption rate has a trend of first decreasing and then increasing when the polymer-cement ratio is 3:2. With the increase of cement filler, the internal defect of the composite gradually occurs [23,24]. Therefore, the water absorption rate shows a trend of first decreasing and then increasing. This also indicates that the test results of water absorption are consistent with the results of micro topography analysis. Based on the testing results, the ratio of polymer and cement is selected as 3:2 to obtain the packaging composite material with the best mechanical performance. 3. Design of packaging and fabrication 3.1. Fabrication of piezoelectric sheet The basic performances of piezoelectric element are shown in Table 6. In this study, the P5 series piezoelectric ceramic was used as the sensor material, which has high electromechanical coupling coefficient and piezoelectric constant [25]. Different sizes of piezoelectric plates were selected for testing (2 mm in diameter and thickness of 1.5 mm, 2 mm and 2.5 mm), to study the size effect of piezoelectric patch on the performance of the sensor. Copper substrates (t = 1 mm) were bonded to the upper and lower surface of the piezoelectric sheet. Fig. 2 shows the design of the piezoelectric sheet. At present, the resonant frequency of the piezoelectric sensor for nondestructive testing of concrete structures in engineering is

Table 6 Properties of piezoelectric elements. Thickness of piezoelectric film

t = 1.5 mm

t = 2 mm

t = 2.5 mm

d1 33/PCN Kp Ct (nF) Qm Fs (Hz) Fp (Hz)

385 0.665 2.4961 93.15 106,840 125,500

392 0.678 1.9867 87.25 103,680 122,400

425 0.712 1.7983 84.28 102,200 122,600

Note: d33 is the piezoelectric strain constant; Kp is the planar electromechanical coupling factor; Ct is the free capacitance; Qm is the Mechanical quality number; Fs is the resonant frequency; Fp is the anti-resonance frequency.

20–250 kHz. We therefore selected the piezoelectric element resonant frequency as about 100 kHz. ZJ-3 type quasi static d33 tester produced by Acoustics Institute of Chinese Academy of Sciences, as shown in Fig. 3(a) and (b), was used to test the piezoelectric constant and the impedance. The test frequency was 110 Hz. Each sample was tested 8 times and the average value was calculated. The results are shown in Table 7.

3.2. Design of packaging system and fabrication of sensor Nowadays, 3D printing technology becomes more and more popular, especially in the fields of industrial design, aerospace, medical, photography, art design, architecture, education, etc. [26,27]. In this study, 3D printing technology was used for a fast and standard building of the packaging material. A threedimensional model of the mold was established, which was later cut into thin slices. These thin layers were sent to the printer, stacked from bottom to top, and eventually formed a complete package system. Fig. 4 shows the principle of 3D printing technology in our experiment. 3D printing is a material-increasing manufacture technology. Based on the mechanism of 3D printing technique, a 3D model was firstly established by using CAD software. And then the slicing tool was used to cut the model into thin slices. At last, 3D printer was used to print this model layer by layer. During this process, the sensor molds with different size, different volume, and different structure can be designed and made. Aggregate properties including shape, angularity, etc. will significantly affect the mixture performance [28–31] while 3D printing technology has a high accuracy in aggregate-shape sensor manufacturing. The printing of middle interlayer gasket can effectively keep the piezoelectric element at a horizontal position, and ensure that the stress distributes uniformly at the cross section of sensor element. It can maximize the charge collection under loading. Fig. 5 shows the design and fabrication flow chart of piezoelectric sensor. The size of the cubic mold is 38 mm  38 mm  38 mm. For piezoelectric ceramic, when the stress of the piezoelectric element is perpendicular to the surface, it can achieve the best charge performance [32]. It is therefore necessary to ensure that the surface of the piezoelectric element is parallel to the compression surface of the packaging material. For this purpose, we designed the middle layer gasket with 1 mm thickness as shown in Fig. 5(c). The piezoelectric element was glued in the middle of the gasket with the groove of the same size, which ensures that the piezoelectric element is in a parallel position. The wire went through the hole and was connected to the piezoelectric element, which can prevent the signal from falling off. Two aggregate-shape packaging shapes were used in the test, i.e. cubic and cylinder, as shown in Fig. 5(e) and (f).

4. Tests on sensor performance The schematic diagram of the test for the performance evaluation of the sensor is shown in Fig. 6. The test instruments includes MTS control system, charge amplifier, oscilloscope and data processing system. By setting different loading modes, the different loadings were applied to the piezoelectric sensor by MTS. Under the loading, the surface of the piezoelectric element would produce a high impedance of the charge signal. Since the charge signal was weak, the charge amplifier was needed to detect the signal. The charge amplifier module size was 25.5  25.5  12 mm, which comprised a charge conversion circuit for low noise and high input impedance.

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(a) Copper substrate (b) Piezoelectric ceramic sensors (c) Combination of (a) and (b) Fig. 2. Design of piezoelectric sheet.

(b) Impedance analyzer

(a) d33 tester

Fig. 3. (a) d33 tester (b) Impedance analyzer.

Table 7 Measurement of piezoelectric strain constant of different sizes of piezoelectric elements. d33/PCN1 t = 1.5 mm t = 2 mm t = 2.5 mm

386 391 426

385 390 424

387 394 426

385 393 426

384 392 426

386 389 425

383 391 425

386 393 423

Average value

Variance

385 392 425

1.6 2.2 1.3

Fig. 4. Principle of 3D printing in sensor packaging.

The linearity of the sensor properties is important to ensure the accurate measurement of the sensor. The loading system of amplitude scanning was designed to test the linear performance of the sensor. The input load frequency was 15 Hz. The load input amplitude was set from 600 N to 3400 N, so as to simulate the pressure on the sensor from 0.42 MPa to 2.35 MPa. Fig. 7 shows

the relationship between the output voltage amplitude and the input load amplitude of the sensor with different thicknesses. As can be seen from Fig. 7, there is a significant linear relationship between the amplitude of the input load and the output voltage amplitude. All the correlation coefficients of the three sensors are more than 0.99, indicating a very strong linear correlation.

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Fig. 5. Design and fabrication flow chart of piezoelectric sensor.

Fig. 6. Schematic diagram of the performance test device of embedded piezoelectric sensor.

Fig. 7. Relationship between the output voltage amplitude and the input load amplitude.

Fig. 8. Relationship between input and output signal phase difference and amplitude of input load.

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Fig. 9. Piezoelectric response of the sensor under the condition of frequency scanning.

Fig. 10. Response of different sensor under frequency scanning.

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Fig. 8 shows the relationship between the phase difference of the input and output signals of the sensor with different thicknesses (t = 1.5 mm, 2 mm, and 2.5 mm). As can be seen, the difference between three sensors on phase difference is almost zero. The smaller the phase difference is, the faster the response of the sensor to the surface is, and the better the performance of the sensor is. Fig. 8 implies a strong potential of the application of sensors in actual engineering project. The frequency independence is one of the most important performance indicators of the sensor. For this purpose, a special loading system - frequency sweep loading, was designed. In this loading condition, the amplitude of the sinusoidal input load remained unchanged, the input load amplitude peak was 2700 N, and the equilibrium position was 2000 N. The frequency of the input load was changed from 0.01 Hz to 20 Hz, which was the common frequency range of the dynamic text of civil engineering structures [33]. Fig. 9 shows the relationship between the output voltage amplitude and frequency of the sensor. The load mode is amplitude sweep, and the input frequency of sine load is 15 Hz. The change range of load is from 600 N to 3400 N, and the average load is 2000 N. It is can be seen that the output voltage amplitude is almost constant within the whole simulation frequency range. When the piezoelectric film thickness is 1.5 mm, the output voltage is lowest among the three. When the piezoelectric film thickness is 2 mm thick, the maximum stable value of the output voltage is 2120 mv. When the piezoelectric film thickness is 2.5 mm, the output voltage decreases. Therefore, a 2 mm film is used for sensor packaging. Fig. 10 shows the relationship between the phase difference and the frequency of the input and output signals of the sensor under the frequency scanning and loading system. It is can be seen that the phase difference of the input and output signal of different sensors is almost zero. Test results show that the sensor can reflect the external load in time. The durability of the sensors is also needed to be evaluated. Fig. 11 shows the device for testing fatigue performance. Different fatigue loads are applied on the sensor. The fatigue load is applied with sine wave, the frequency is 15 Hz, equilibrium position is 3000 N, and the loading amplitude 1600 N. Fig. 12 shows the output voltage amplitude of different sensors in different fatigue load cycles.

Fig. 11. Fatigue test device.

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S. Song et al. / Construction and Building Materials 131 (2017) 57–65

Fig. 12. Output response of the sensor under different fatigue load cycles.

the piezoelectric response of the sensor does not decrease after 1.1 million loading cycles. The compressive strength of the sensor before and after the fatigue loading was also tested. Fig. 14(a) shows the test results of the compressive strength of the sensor before the fatigue load, the deformation of the sensor is respectively loaded to 98.5 MPa and 78.6 MPa. Fig. 14(b) shows the compressive strength test of the sensor after the 1.1 million fatigue loading cycles. We stopped loading when the sensor appeared cracks under 77.8 MPa load. Based on the test results, the cracks appeared in the middle and the specimen begins to fail when loading was added to 78.6 MPa. After 1,100,000 times of fatigue loading, the compressive strength of the specimen reached 77.8 MPa. It is can be observed that the compressive strength before the fatigue test is very close to the compressive strength after the fatigue test. The results show that after using the new composite material as packaging phase, the sensor can still has good piezoelectric output performance after significant fatigue loadings. This implies the reliability of the new aggregate-shape piezoelectric sensor. Also note that future study will focus on the compatibility with bituminous material properties [34,35], in order to be applied in the on-site various asphalt pavements [36].

5. Conclusions and recommendations

Fig. 13. Piezoelectric response of the sensor under fatigue load.

As seen from Fig. 12, the output voltage of the sensor remains almost constant under different fatigue load cycles, which indicates that the output signal of the sensor is not affected by fatigue load. Fig. 13 shows the piezoelectric (t = 2 mm) response of the sensor under fatigue load. The load mode was sine load, and frequency is 15 Hz. The balance position 3000 N, and load amplitude 1600 N. Loading number was 1100 thousand times. It is found that

In this paper, a new type of aggregate-shape embedded piezoelectric sensor for civil engineering infrastructure health monitoring was developed. The sensor was designed by using piezoelectric ceramic chip as the functional phase, and a new composite material as packaging phase. Combining with 3D printing technology, the packaging system and sensor fabrication were designed. The frequency independence, linearity, sensitivity, response rate, and service performance of the sensor were tested by frequency scanning and amplitude scanning tests. Experimental results show that within the vibration frequency range of common civil engineering structure, the new aggregate-shape embedded piezoelectric sensor has good mechanical and workability properties. Test results also show that the new embedded sensors have very good mechanical-electrical coupling performance. This builds a solid foundation for the application of aggregate-shape embedded piezoelectric sensor in the civil engineering infrastructure. In order to make this novel sensor more practical, more work is needed to be studied in the further, including effect of the frost, shape of real aggregate, implementation of these devices, differentiation between asphalt and concrete, coating sensors with mineral filler, wireless monitoring and data transfer and energy generation and self-monitoring.

Fig. 14. The effect of fatigue load on the compressive strength of the sensor.

S. Song et al. / Construction and Building Materials 131 (2017) 57–65

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