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Study on wireless sensing for monitoring the corrosion of reinforcement in concrete structures
Measurement 43 (2010) 375–380
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Study on wireless sensing for monitoring the corrosion of reinforcement in concrete structures Jin Wu *, Wencao Wu Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 13 July 2007 Received in revised form 26 October 2009 Accepted 5 December 2009 Available online 11 December 2009 Keywords: Reinforcement corrosion Wireless sensor Resonant frequency Encapsulation
a b s t r a c t The reinforcement corrosion has a serious impact on durability and safety of reinforced concrete structures. Based on radio frequency technology, the wireless sensor for the monitoring of reinforcement corrosion is investigated in this paper. The sensors are fabricated and experiments on the sensors are carried out. Experimental results show that it is feasible to determine whether the steel wire is broken or not by monitoring the resonant frequency of the circuit, and the sensor with spring switch designed in this paper can solve the problem of the resonant frequency missing of the sensor during the process of the steel wire corrosion. The encapsulation materials for the sensors are studied. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The failure of the structures caused by reinforcement corrosion is a worldwide problem. In 2000, the collapse of the North Carolina speedway pedestrian bridge in USA is one of the serious safety hazards caused by reinforcement corrosion [1]. Therefore, the monitoring of the corrosion of reinforcement in concrete structures is an important part of health monitoring of structures. The real time monitoring of the status of the reinforcement corrosion can provide very important information relating to the actual damage of the structures, and protect the structures from the serious safety hazards such as the collapse of the North Carolina speedway pedestrian bridge in USA. And it can provide important basis for the assessment of the durability of the structures, the prediction of remaining service life of existing concrete structures and the strengthening and repairing of the structures. Currently there are several sensors commercially available that are designed to aid in the early detection of corrosion in reinforced concrete [2]. For example, CMS system * Corresponding author. E-mail address: [email protected] (J. Wu). 0263-2241/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2009.12.003
used in the Hun-Min viaduct works in Shanghai [3], and anode-ladder used in the Hangzhou Bay Bridge [4]. Unfortunately, the sensors are expensive, and the need to wire each sensor together also adds to the installation costs. Bernhard and Heeltaps et al. of University of Illinois have investigated an embedded corrosion monitoring system in concrete bridge [5]. In the sensor, the ultrasonic technique is used to detect corrosion and the data is transmitted by broad band microwave antenna. But the use of ultrasonic technique increased the cost of this sensor system. Also, with the need for batteries, the sensors can work during the time period much short than the lifetime of the structures which they are designed to monitor. Engineers at the Johns Hopkins University have developed a smart sensor that is designed to provide data about corrosion status in the bridge deck [6]. This sensor can measure the concrete conductivity and the data of sensors is transmitted by wireless chip. University of Texas developed a kind of wireless sensor to monitor steel wire corrosion using radio frequency technology [7]. But the signal of this sensor is missing sometimes. The mechanism of the wireless sensors for the monitoring of reinforcement corrosion based on LC circuit is investigated in this paper. The wireless sensor for the monitoring
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of reinforcement corrosion is designed and the experiment is carried out. 2. Designs of sensors LC circuit is shown in Fig. 1, and the resonant frequency of the whole circuit is given by Thompson formula:
f ¼
1 pffiffiffiffiffiffiffiffiffi 2p L C
ð1Þ
where L is the total inductance of the circuit; C is the total capacitance of the circuit. According to the Eq. (1), the change of the inductance or the capacitance of the circuit will lead to the change of resonant frequency. As shown in Fig. 1a, a steel wire K is added to the circuit. When the steel wire is not broken, the total capacitance of the circuit is equal to the summation of the two capacitances (C = C1 + C2). When the steel wire is broken due to corrosion, the total capacitance of the circuit becomes C1. So the resonant frequency of the circuit depends on the state of the steel wire. Because the diameter of the steel wire is much smaller, the steel wire will break due to corrosion before appreciable corrosion damage has occurred in the reinforcement. As shown in Fig. 1b, a set of wires of different diameter are added to the circuit. The resonance frequency will change when each wire breaks due to corrosion. If the relationship between the diameter of the broken steel wire and the corrosion level of the reinforcement is obtained by experiments, the induced resonance frequency shift would determine the amount of corrosion of reinforcement in concrete when each wire breaks due to corrosion. Therefore the sensors with different diameters steel wires can be used to monitor a set of the discrete states of the reinforcement corrosion. The right part of Fig. 1 is LC circuit, which is sealed except the steel wire and embedded in the concrete near the
reinforcement to be monitored. The left part of Fig. 1 is the reader, which is fixed on the surface of the concrete structure. It is used to measure the resonant frequency of the sensor. When the reinforcement in the concrete is under corrosion, the steel wire is also corroded and will break, which causes a change in the resonant frequency of the sensor, and the signal of the reinforcement corrosion is transmitted from inside to outside of concrete. This technology allows the sensor to be free of an internal power source, which means that the sensors will be readable throughout the lifetime of the structure. So the sensor is wireless, passive and inexpensive. According to the mechanism of the resonant circuit, the resonant frequency of the circuit can be obtained through
voltage
Resonant frequency
frequency
Fig. 2. The amplitude frequency characteristic curve of the voltage on the inductance.
Fig. 3. Photo of the sensor.
R concrete Singal Generator
Steel wire Oscilloscope
Fig. 1. Circuit diagram for sensor.
Fig. 4. The sensor and reader.
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the phase frequency characteristic curve of the impedance or the amplitude frequency characteristic curve of the voltage on the inductance (Fig. 2). The resonant frequency of the circuit of the sensor is determined according to the dip in the voltage curve. There is a ‘‘transition zone” during the corrosion process of the steel wire. In that stage, the dip on the phase frequency characteristic curve or the amplitude frequency characteristic curve decreases with the increase of the resistance of the steel wire [7], and the resonant frequency cannot be measured. The signal missing occurs.
3.5
3. Fabrications of sensors A magnetic core is added into the coil in order to improve the coupling efficiency between the reader loop and the sensor coil. The outer diameter of the magnetic core is 44 mm and the thickness of the magnetic core is 20 mm. And it is very convenient to locate the coil of the reader on the top of the sensor due to the magnetism of the magnetic core. The inductors are fabricated by winding enamel-insulated copper magnet wire around the magnetic core. And the inductor with 20 turns of the wire
voltage/V
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Fig. 5. The amplitude frequency characteristic curve of the voltage of the sensors during the corrosion.
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was selected for the sensor coil. The capacitance has a large impact on the dip on the amplitude frequency characteristic curve of the voltage. The ceramic capacitors of 1000 pF are selected for experiments. The coil and capacitance are assembled inside a plastic box, which is later filled with encapsulating compound. Six sensors (B1–B6) were fabricated in the experiment (Fig. 3). The diameters of the steel wires in the sensors are 0.25 mm, 0.4 mm, 0.5 mm, 0.7 mm, 0.8 mm and 2.0 mm, respectively. The sensors with different wire diameters can be used to identify different corrosion thresholds. 3.1. Encapsulation of sensors Epoxy is the most widely used encapsulation material. But the cracking of the concrete is easily caused by the difference between thermal expansion coefficient of epoxy and that of concrete. Seventy percentage of silicon dioxide is added into the epoxy in this experiment to decrease the thermal expansion coefficient of epoxy. After the circuit is welded, the welded joint is sealed with insulated adhesive tape in order to avoid short circuit. The epoxy, benzene dimethylamine and silicon dioxide is mixed by 2:1:7, and the mixture is filled into the box. At last, the cover of the box is put on and the superfluous epoxy is extruded out from the hole in the box. After the epoxy is hardened, 703 silica latex is smeared on the wire and the cover of the box to protect the sensor further.
nant frequency is easily distinguishable, which minimizes the risk of the sensor output being interrupted by noise due to environmental factors. So the state of the steel wire can be determined through the measurement of the resonant frequency. In order to verify the information about the state of the steel wire, the specimens are split to see the inside and it is shown that all the steel were broken due to corrosion. From Fig. 5, it is found that the dips in the most curves are observed. But the dips in the curves of sensor B4 and B6 are not observed. In such cases, the resonant frequency of the sensor cannot be measured. It is because of the existence of the ‘‘transition zone”. The depth of the dip in the curve decreases with the increase of the resistance of steel wire, and the resonant frequency of the sensor is missing (Fig. 6). 4.2. Improvement of the sensor and experiment verification In order to solve the problem of signal missing during the ‘‘transition zone”, the steel wire is replaced with a spring switch (Fig. 7). The stainless steel sheet is fixed by steel wire, and the switch is closed when the steel wire does not break. Except the steel wire, the switch is sealed with epoxy and 703 silica latex. The steel wire is in the concrete, and it is not connected to the circuit. The switch is controlled by the stainless steel sheet, plastic stick and spring system. When the steel wire is broken, the stainless steel sheet turns straight. Because of the spring, the plastic stick and the electric material 1 will move upwards, and
3.2. Protection of circuit in accelerated corrosion experiment When the steel wire is electrified in accelerated corrosion experiment, if the circuit of the sensor is connected with the steel wire directly, it will be damaged. So the steel wire cannot be connected to the circuit directly. Instead, the circuit of the sensor and the steel wire are separated. When the steel wire is electrified, it is not connected with the sensor. And only when the resonant frequency is measured, these two parts are connected (Fig. 4).
3.5
voltage/V
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2
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1
4. Experiments and analysis of sensors The sealed sensors are cast in 150 mm 150 mm 80 mm concrete specimen. The distance between the surface of concrete and the sensor is 40 mm. The compressive strength of concrete is 20.7 MPa. The standard-curing of specimen lasts for 28 days. And then the accelerated corrosion experiment is carried out. The signal generator generates a frequency sweep in the radio frequency band. The amplitude of the voltage on the Oscilloscope under different frequency is recorded and the amplitude frequency characteristic curve of the voltage is obtained. The resonant frequency can be determined by the dip in the curve. 4.1. Experiment result and analysis The amplitude frequency characteristic curve of the voltage is shown in Fig. 5. From Fig. 5, it is found that the difference in initial resonant frequency and the final reso-
0.5
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0 0
200
400
600
800
1000 1200
1300
Fig. 6. The amplitude frequency characteristic curve of the voltage of the B4 sensor during the ‘‘transition zone”.
stainless steel sheet
steel wire
broken not broken spring plastic stick electric materiel 1
wire
electric materiel 2
Fig. 7. The switch.
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5. Conclusions
(1) The foundation for the sensors in this paper is radio frequency technology. This technology allows the sensors to be wireless. Also the technology allows them to be free of an internal power source, which means that the sensors will be readable throughout the lifetime of the structure. So the advantages of the sensor are wireless, passive and inexpensive and simple to manufacture. (2) The sensors are fabricated and experiments on the sensors are carried out. Experimental results show that it is feasible to determine whether the steel wire is broken or not by monitoring the resonant frequency of the circuit. The resonant frequency of the sensor may be missing due to the existence of the ‘‘transition zone”, but this problem can be solved by the use of the improved sensors with spring switch. (3) Seventy percentage of Silicon dioxide is added into the epoxy to decrease the difference between thermal expansion coefficients of epoxy and that of concrete. The mixture is used to encapsulate sensors. After the epoxy is hardened, the box is not removed to enhance the protection of the circuit. From the result of the experiment, it is found that the encapsulation cannot only protect the circuit, but also ensure the signal of the sensor to be normal.
Fig. 8. The photo of the sensor with spring switch.
the electric material 1 will be separated from electric material 2. Then the switch is open. Because the switch is sealed, it is not corroded, and the resistance of the switch will be constant. Then the ‘‘transition zone” does not exist. In order to ensure the movement of the stainless steel sheet, it is protected by foam and plastic sheet to prevent it from being filled with concrete. Three improved sensors (A1–A3) are designed and fabricated in the experiment (Fig. 8). Then the sensors are cast in concrete specimens. The amplitude frequency characteristic curves of the voltage of sensors are shown in Fig. 9. It is found that there are only the initial resonant frequency and the final resonant frequency. So the improvement of the sensor with spring switch can solve the problem of the ‘‘transition zone”. The specimens are split to see the inside, and it is shown that all the stainless steel sheets turn straight due to the fracture of the steel wire. 3.5
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(c) A3 sensor Fig. 9. The amplitude frequency characteristic curve of the voltage of the sensors with spring switch during the corrosion.
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(4) There is only one switch in this sensor, which can only indicate whether the steel wire breaks or not. And only two corrosion states of the reinforcement can be monitored. Whether or not more corrosion states of the reinforcement can be indicated when more switches with different diameters of steel wires are added into the sensor and how to obtain the relationship between the diameter of the steel wire and the loss of the weight of reinforcement need to be studied further.
Acknowledgments The work carried out in this paper was sponsored by National Natural Science Foundation of China (50879034) and Natural Science Foundation of Jiangsu Province (BK2006193).
References [1] S.C. Cederquist, Motor speedway bridge collapse caused by corrosion, Materials Performance 39 (7) (2000) 18–19. [2] Geng Our, Li Gao, Yuan Yangzhou, Applications of electrochemical detection techniques in concrete reinforcement corrosion, Concrete (2) (2005) 20–23 (in Chinese). [3] Shen Lei, Application of CMS rust supervision system in 2(nd) phase of humin viaduct works, China Municipal Engineering (2) (2004) 46–47 (in Chinese). [4] Zhang Baosheng, Gan Weizhong, Chen Tao, Strategies to ensure durability of concrete structure for Hangzhou Bay Bridge, China Civil Engineering Journal 39 (6) (2006) 72–77 (in Chinese). [5] J.T. Bernhard, K. Hietpas, E. George, An interdisciplinary effort to develop a wireless embedded sensor system to monitor and assess corrosion in the tendons of prestressed concrete girders, in: 2003 IEEE Topical Conference on Wireless Communication Technology, 2003, pp. 241–243. [6] B. Carkhuff, R. Cain, Corrosion sensors for concrete bridges, IEEE Instrumentation and Measurement Magazine 9 (6) (2003) 19–24. [7] Jarkko T. Simonen, Matthew M. Andringa, K.M. Grizzleet, et al. Wireless sensors for monitoring corrosion in reinforced concrete members, in: Proceedings of SPIE, Smart Structures and Materials 2004: Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, 2004, pp. 587–596.
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Study on wireless sensing for monitoring the corrosion of reinforcement in concrete structures Jin Wu *, Wencao Wu Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 13 July 2007 Received in revised form 26 October 2009 Accepted 5 December 2009 Available online 11 December 2009 Keywords: Reinforcement corrosion Wireless sensor Resonant frequency Encapsulation
a b s t r a c t The reinforcement corrosion has a serious impact on durability and safety of reinforced concrete structures. Based on radio frequency technology, the wireless sensor for the monitoring of reinforcement corrosion is investigated in this paper. The sensors are fabricated and experiments on the sensors are carried out. Experimental results show that it is feasible to determine whether the steel wire is broken or not by monitoring the resonant frequency of the circuit, and the sensor with spring switch designed in this paper can solve the problem of the resonant frequency missing of the sensor during the process of the steel wire corrosion. The encapsulation materials for the sensors are studied. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The failure of the structures caused by reinforcement corrosion is a worldwide problem. In 2000, the collapse of the North Carolina speedway pedestrian bridge in USA is one of the serious safety hazards caused by reinforcement corrosion [1]. Therefore, the monitoring of the corrosion of reinforcement in concrete structures is an important part of health monitoring of structures. The real time monitoring of the status of the reinforcement corrosion can provide very important information relating to the actual damage of the structures, and protect the structures from the serious safety hazards such as the collapse of the North Carolina speedway pedestrian bridge in USA. And it can provide important basis for the assessment of the durability of the structures, the prediction of remaining service life of existing concrete structures and the strengthening and repairing of the structures. Currently there are several sensors commercially available that are designed to aid in the early detection of corrosion in reinforced concrete [2]. For example, CMS system * Corresponding author. E-mail address: [email protected] (J. Wu). 0263-2241/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2009.12.003
used in the Hun-Min viaduct works in Shanghai [3], and anode-ladder used in the Hangzhou Bay Bridge [4]. Unfortunately, the sensors are expensive, and the need to wire each sensor together also adds to the installation costs. Bernhard and Heeltaps et al. of University of Illinois have investigated an embedded corrosion monitoring system in concrete bridge [5]. In the sensor, the ultrasonic technique is used to detect corrosion and the data is transmitted by broad band microwave antenna. But the use of ultrasonic technique increased the cost of this sensor system. Also, with the need for batteries, the sensors can work during the time period much short than the lifetime of the structures which they are designed to monitor. Engineers at the Johns Hopkins University have developed a smart sensor that is designed to provide data about corrosion status in the bridge deck [6]. This sensor can measure the concrete conductivity and the data of sensors is transmitted by wireless chip. University of Texas developed a kind of wireless sensor to monitor steel wire corrosion using radio frequency technology [7]. But the signal of this sensor is missing sometimes. The mechanism of the wireless sensors for the monitoring of reinforcement corrosion based on LC circuit is investigated in this paper. The wireless sensor for the monitoring
376
J. Wu, W. Wu / Measurement 43 (2010) 375–380
of reinforcement corrosion is designed and the experiment is carried out. 2. Designs of sensors LC circuit is shown in Fig. 1, and the resonant frequency of the whole circuit is given by Thompson formula:
f ¼
1 pffiffiffiffiffiffiffiffiffi 2p L C
ð1Þ
where L is the total inductance of the circuit; C is the total capacitance of the circuit. According to the Eq. (1), the change of the inductance or the capacitance of the circuit will lead to the change of resonant frequency. As shown in Fig. 1a, a steel wire K is added to the circuit. When the steel wire is not broken, the total capacitance of the circuit is equal to the summation of the two capacitances (C = C1 + C2). When the steel wire is broken due to corrosion, the total capacitance of the circuit becomes C1. So the resonant frequency of the circuit depends on the state of the steel wire. Because the diameter of the steel wire is much smaller, the steel wire will break due to corrosion before appreciable corrosion damage has occurred in the reinforcement. As shown in Fig. 1b, a set of wires of different diameter are added to the circuit. The resonance frequency will change when each wire breaks due to corrosion. If the relationship between the diameter of the broken steel wire and the corrosion level of the reinforcement is obtained by experiments, the induced resonance frequency shift would determine the amount of corrosion of reinforcement in concrete when each wire breaks due to corrosion. Therefore the sensors with different diameters steel wires can be used to monitor a set of the discrete states of the reinforcement corrosion. The right part of Fig. 1 is LC circuit, which is sealed except the steel wire and embedded in the concrete near the
reinforcement to be monitored. The left part of Fig. 1 is the reader, which is fixed on the surface of the concrete structure. It is used to measure the resonant frequency of the sensor. When the reinforcement in the concrete is under corrosion, the steel wire is also corroded and will break, which causes a change in the resonant frequency of the sensor, and the signal of the reinforcement corrosion is transmitted from inside to outside of concrete. This technology allows the sensor to be free of an internal power source, which means that the sensors will be readable throughout the lifetime of the structure. So the sensor is wireless, passive and inexpensive. According to the mechanism of the resonant circuit, the resonant frequency of the circuit can be obtained through
voltage
Resonant frequency
frequency
Fig. 2. The amplitude frequency characteristic curve of the voltage on the inductance.
Fig. 3. Photo of the sensor.
R concrete Singal Generator
Steel wire Oscilloscope
Fig. 1. Circuit diagram for sensor.
Fig. 4. The sensor and reader.
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J. Wu, W. Wu / Measurement 43 (2010) 375–380
the phase frequency characteristic curve of the impedance or the amplitude frequency characteristic curve of the voltage on the inductance (Fig. 2). The resonant frequency of the circuit of the sensor is determined according to the dip in the voltage curve. There is a ‘‘transition zone” during the corrosion process of the steel wire. In that stage, the dip on the phase frequency characteristic curve or the amplitude frequency characteristic curve decreases with the increase of the resistance of the steel wire [7], and the resonant frequency cannot be measured. The signal missing occurs.
3.5
3. Fabrications of sensors A magnetic core is added into the coil in order to improve the coupling efficiency between the reader loop and the sensor coil. The outer diameter of the magnetic core is 44 mm and the thickness of the magnetic core is 20 mm. And it is very convenient to locate the coil of the reader on the top of the sensor due to the magnetism of the magnetic core. The inductors are fabricated by winding enamel-insulated copper magnet wire around the magnetic core. And the inductor with 20 turns of the wire
voltage/V
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3 2.5
2.5
The first measurement The second measurement
2
The first measurement The second measurement
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The third measurement
The third measurement
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Fig. 5. The amplitude frequency characteristic curve of the voltage of the sensors during the corrosion.
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was selected for the sensor coil. The capacitance has a large impact on the dip on the amplitude frequency characteristic curve of the voltage. The ceramic capacitors of 1000 pF are selected for experiments. The coil and capacitance are assembled inside a plastic box, which is later filled with encapsulating compound. Six sensors (B1–B6) were fabricated in the experiment (Fig. 3). The diameters of the steel wires in the sensors are 0.25 mm, 0.4 mm, 0.5 mm, 0.7 mm, 0.8 mm and 2.0 mm, respectively. The sensors with different wire diameters can be used to identify different corrosion thresholds. 3.1. Encapsulation of sensors Epoxy is the most widely used encapsulation material. But the cracking of the concrete is easily caused by the difference between thermal expansion coefficient of epoxy and that of concrete. Seventy percentage of silicon dioxide is added into the epoxy in this experiment to decrease the thermal expansion coefficient of epoxy. After the circuit is welded, the welded joint is sealed with insulated adhesive tape in order to avoid short circuit. The epoxy, benzene dimethylamine and silicon dioxide is mixed by 2:1:7, and the mixture is filled into the box. At last, the cover of the box is put on and the superfluous epoxy is extruded out from the hole in the box. After the epoxy is hardened, 703 silica latex is smeared on the wire and the cover of the box to protect the sensor further.
nant frequency is easily distinguishable, which minimizes the risk of the sensor output being interrupted by noise due to environmental factors. So the state of the steel wire can be determined through the measurement of the resonant frequency. In order to verify the information about the state of the steel wire, the specimens are split to see the inside and it is shown that all the steel were broken due to corrosion. From Fig. 5, it is found that the dips in the most curves are observed. But the dips in the curves of sensor B4 and B6 are not observed. In such cases, the resonant frequency of the sensor cannot be measured. It is because of the existence of the ‘‘transition zone”. The depth of the dip in the curve decreases with the increase of the resistance of steel wire, and the resonant frequency of the sensor is missing (Fig. 6). 4.2. Improvement of the sensor and experiment verification In order to solve the problem of signal missing during the ‘‘transition zone”, the steel wire is replaced with a spring switch (Fig. 7). The stainless steel sheet is fixed by steel wire, and the switch is closed when the steel wire does not break. Except the steel wire, the switch is sealed with epoxy and 703 silica latex. The steel wire is in the concrete, and it is not connected to the circuit. The switch is controlled by the stainless steel sheet, plastic stick and spring system. When the steel wire is broken, the stainless steel sheet turns straight. Because of the spring, the plastic stick and the electric material 1 will move upwards, and
3.2. Protection of circuit in accelerated corrosion experiment When the steel wire is electrified in accelerated corrosion experiment, if the circuit of the sensor is connected with the steel wire directly, it will be damaged. So the steel wire cannot be connected to the circuit directly. Instead, the circuit of the sensor and the steel wire are separated. When the steel wire is electrified, it is not connected with the sensor. And only when the resonant frequency is measured, these two parts are connected (Fig. 4).
3.5
voltage/V
3 2.5 The first measurement
2
The second measurement The third measurement
1.5
The fourth measurement
1
4. Experiments and analysis of sensors The sealed sensors are cast in 150 mm 150 mm 80 mm concrete specimen. The distance between the surface of concrete and the sensor is 40 mm. The compressive strength of concrete is 20.7 MPa. The standard-curing of specimen lasts for 28 days. And then the accelerated corrosion experiment is carried out. The signal generator generates a frequency sweep in the radio frequency band. The amplitude of the voltage on the Oscilloscope under different frequency is recorded and the amplitude frequency characteristic curve of the voltage is obtained. The resonant frequency can be determined by the dip in the curve. 4.1. Experiment result and analysis The amplitude frequency characteristic curve of the voltage is shown in Fig. 5. From Fig. 5, it is found that the difference in initial resonant frequency and the final reso-
0.5
Frequency /kHz
0 0
200
400
600
800
1000 1200
1300
Fig. 6. The amplitude frequency characteristic curve of the voltage of the B4 sensor during the ‘‘transition zone”.
stainless steel sheet
steel wire
broken not broken spring plastic stick electric materiel 1
wire
electric materiel 2
Fig. 7. The switch.
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5. Conclusions
(1) The foundation for the sensors in this paper is radio frequency technology. This technology allows the sensors to be wireless. Also the technology allows them to be free of an internal power source, which means that the sensors will be readable throughout the lifetime of the structure. So the advantages of the sensor are wireless, passive and inexpensive and simple to manufacture. (2) The sensors are fabricated and experiments on the sensors are carried out. Experimental results show that it is feasible to determine whether the steel wire is broken or not by monitoring the resonant frequency of the circuit. The resonant frequency of the sensor may be missing due to the existence of the ‘‘transition zone”, but this problem can be solved by the use of the improved sensors with spring switch. (3) Seventy percentage of Silicon dioxide is added into the epoxy to decrease the difference between thermal expansion coefficients of epoxy and that of concrete. The mixture is used to encapsulate sensors. After the epoxy is hardened, the box is not removed to enhance the protection of the circuit. From the result of the experiment, it is found that the encapsulation cannot only protect the circuit, but also ensure the signal of the sensor to be normal.
Fig. 8. The photo of the sensor with spring switch.
the electric material 1 will be separated from electric material 2. Then the switch is open. Because the switch is sealed, it is not corroded, and the resistance of the switch will be constant. Then the ‘‘transition zone” does not exist. In order to ensure the movement of the stainless steel sheet, it is protected by foam and plastic sheet to prevent it from being filled with concrete. Three improved sensors (A1–A3) are designed and fabricated in the experiment (Fig. 8). Then the sensors are cast in concrete specimens. The amplitude frequency characteristic curves of the voltage of sensors are shown in Fig. 9. It is found that there are only the initial resonant frequency and the final resonant frequency. So the improvement of the sensor with spring switch can solve the problem of the ‘‘transition zone”. The specimens are split to see the inside, and it is shown that all the stainless steel sheets turn straight due to the fracture of the steel wire. 3.5
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(4) There is only one switch in this sensor, which can only indicate whether the steel wire breaks or not. And only two corrosion states of the reinforcement can be monitored. Whether or not more corrosion states of the reinforcement can be indicated when more switches with different diameters of steel wires are added into the sensor and how to obtain the relationship between the diameter of the steel wire and the loss of the weight of reinforcement need to be studied further.
Acknowledgments The work carried out in this paper was sponsored by National Natural Science Foundation of China (50879034) and Natural Science Foundation of Jiangsu Province (BK2006193).
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