- Email: [email protected]
Monitoring of corrosion in reinforced concrete structure using Bragg grating sensing
NDT&E International 44 (2011) 202–205
Contents lists available at ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Monitoring of corrosion in reinforced concrete structure using Bragg grating sensing Junqi Gaon, Jin Wu, Jun Li, Xinming Zhao Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2010 Received in revised form 27 November 2010 Accepted 29 November 2010 Available online 10 December 2010
Corrosion in reinforced concrete structures is a major problem that seriously affects the service life of the structures. In order to detect rebar corrosion, a fiber optic corrosion sensor (FOCS) made of one fiber Bragg grating (FBG) sensor and twin steel rebar elements was designed and packaged up with concrete. Subsequently, a series of experiments were carried out to verify its feasibility. A constant current accelerated corrosion test was performed on five fiber optic corrosion sensors and the relationship between reflected wavelength change from the grating and the weight loss rate of rebar was obtained by the gravimetric weight loss method. The experiment shows that it is feasible to monitor the degree of corrosion of reinforced steel in reinforced concrete structures using FOCS. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fiber optic corrosion sensor Corrosion monitoring FBG Constant current Accelerated corrosion test Rebar
1. Introduction Corrosion monitoring is one of the important measures for maintenance of a reinforced concrete structure. It can reveal the onset or progress of corrosion damage of reinforcement in the structures being monitored. This is particularly important in the early stages before the damage becomes significant. Corrosion monitoring is not only the basis for adopting appropriate prevention measures, but also provides essential data for prediction of the service life of structures. Among many techniques of corrosion monitoring, which are used in the laboratory and in the field, some have been widely used, such as half-cell potential, linear polarization resistance, polarization curves, and AC electrochemical impedance spectroscopy. Recently through the measurement of parameters relating to the corrosion reaction of reinforcement some corrosion sensors using optical fibers were presented [1–6]. Fuhr and Huston [1] present multiple-parameter sensing fiber optic sensors to detect corrosion in reinforced concrete roadways and bridges. Lo and Xiao [2] design a pre-strained Bragg grating as a corrosion transducer and a temperature sensor. Yang et al. [3] designed a device, which was made of a fiber grating, a steel spring and some mechanical parts and was packaged under pre-strained condition of the spring, to monitor metal materials corrosion. Benounis and Jaffrezic-Renault [5] present an optical fiber corrosion sensor, fabricated by thermal
n
Corresponding author. E-mail address: [email protected] (J. Gao).
0963-8695/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2010.11.011
deposition of an aluminum film onto an optical fiber core within the sensing region, and tested by optical and electrochemical potential methods. In order to confirm their accuracy, some techniques were generally accompanied by the gravimetric weight loss method [7,8]. Especially in the aviation industry similar approaches were developed to monitor corrosion weight loss rates [9,10]. Therefore we also couple the optical and gravimetric weight loss methods by correlating the parameters of the shift of the peak wavelength and the rebar weight loss rate. In this paper, a fiber optic corrosion sensor comprising one FBG sensor and twin steel rebar elements was proposed, and its principle was analyzed. Subsequently a series of experiments was carried out. In the end, the experimental results were given and discussed. 2. Fiber optic corrosion sensor design A fiber optic corrosion sensor proposed herein is made of one FBG sensor and twin rebar elements and packaged up with concrete as shown in Figs. 1 and 2. Twin rebars are screw-thread steel and their diameters are all 25 mm. The rebar length is 90 mm. First, rebar 1 is split into R1-1 and R1-2, and rebar 2 is split into R2-1 and R2-2. Second, a FBG sensor is set vertically to the twin rebar axis and bonded on their planed surface after R1-2 and R2-2 are placed side by side. Third, R1-1 and R2-1 are attached to R1-2 and R2-2, respectively. Last, the FBG sensor and twin rebar elements are packaged up with concrete as shown in Fig. 1(a). Under normal circumstances, the corrosion of the twin rebar would result in the
J. Gao et al. / NDT&E International 44 (2011) 202–205
shift of the peak wavelength of the grating because of their volume expansion. In order to compensate for the temperature effect, a fiber optic temperature sensor (FOTS) is proposed, which is made of one loose grating and twin stainless rebar elements and also packaged up with concrete. The layout of the new fiber optic temperature sensor is like FOCS. Because no corrosion and volume expansion generally occur in the stainless rebar, the fiber optic temperature sensor can detect the environmental temperature. Under real conditions, the FOTS will be installed in civil structures and located near FOCS. A schematic of reinforcement volume change during corrosion is shown in Fig. 1(b). The wavelength shift of the FBG affixed in FOCS rebar is given as
Dlb1 ¼ Ze þ gðTT0 Þ lb1 ð0,T0 Þ
ð1Þ
The wavelength shift of the reference FBG affixed in FOTS stainless rebar is given as
Dlb2 ¼ gðTT0 Þ lb2 ð0,T0 Þ
203
the temperature gage factor, and T T0 is equal to temperature variation DT. Because the two gratings are in the same circumstance and g is constant, the relationship between wavelength shift and strain is
Dlb ¼
Dlb1 Dlb2 ¼ Ze lb1 ð0,T0 Þ lb2 ð0,T0 Þ
ð3Þ
From Fig. 1(b), strain e can also be obtained as
e¼
DD0 DD ¼ D0 D0
ð4Þ
Accounting for Eqs. (3) and (4), the degree of corrosion r can be expressed as DV ðD0 þ DDÞ2 D0 2 DD 2 r¼ ¼ ¼ 1 þ 1 V0 D0 D0 2 2 Dlb 1 ð5Þ ¼ ð1þ eÞ2 1 ¼ 1 þ
Z
ð2Þ
where T0 is a reference temperature, lb1(0,T0) and lb2(0,T0) are the initial center wavelengths, Z is the strain gage factor, g is
Fig. 1. Scheme of corrosion sensor: (a) rebars without corrosion and (b) rebars volume expansion due to corrosion.
where D0 is the diameter of the rebar, D is the diameter after corrosion, DD is the change in diameter, and r is the percentage of steel volume loss. From Eqs. (4) and (5), the diameter change DD and the degree of corrosion r can be expressed by strain e. In order to prevent FBG from gliding from the adhering point or breaking off, the tensile strain measured by FBG sensor is not suitable to exceed 3000 me. Before these sensors designed were used to detect the corrosion degree of rebars in a real structure, the research work should be performed in five steps. (1) Elaboration of FOCS and FOTS—this experimental work was operated in our laboratory, and finally these sensors were packaged up with concrete. (2) Standardization of FOCS—the objective of this work is to relate reflected wavelength change from the grating with the weight loss rate of rebar by the accelerating corrosion test. (3) Installation of FOCS and FOTS—the FOCS and FOTS would be placed near the steel being monitored and cast in place using concrete as a real reinforced concrete structure was constructed. As illustrated in Fig. 3, sensors A and C are two FOCS, and sensors B and D are two FOTS. Because the twin rebar elements packaged in FOCS and the steel being monitored were in the same corrosion circumstance, they would corrode at the same time. Therefore, by analyzing the wavelength shift signal collected from FOCS and FOTS, the corrosion of the steel in a reinforced concrete structure can be detected readily. (4) Real-time monitoring—these sensors will be connected directly to an analyzer
Fig. 2. Photograph of fiber optic corrosion sensor.
204
J. Gao et al. / NDT&E International 44 (2011) 202–205
placed at a control room. So we can real-time obtain the reflected wavelength changes. (5) Data analysis and corrosion degree evaluation—the present residual strength and, by extrapolation, the remaining service life of the structure can then be estimated. This article mostly describes the elaboration and calibration of FOCS.
3. Experimental work 3.1. Preparation of test In this experiment, ten screw-thread steel elements are processed, whose diameters are all 25 mm. First, the protective oxide film of the steel rebar is ground off. Second, the weight of them is weighed by a balance. Some parameters of the ten steel rebar elements are listed in Table 1. All of the steel rebar elements were used for five fiber optic corrosion sensors, which were designed and constructed one month ahead of the test at the materials lab of Nanjing University of Aeronautics and Astronautics. 3.2. Accelerating corrosion test
Fig. 3. Typical layout of corrosion sensors in reinforced concrete structure.
Table 1 Parameters of fiber optic corrosion sensors. FOCS
Rebar code
Weight (g)
Weight loss (g)
FBG peak wavelength (nm)
A
A1–1 A1–2 A2–1 A2–2 B1–1 B1–2 B2–1 B2–2 C1–1 C1–2 C2–1 C2–2 D1–1 D1–2 D2–1 D2–2 E1–1 E1–2 E2–1 E2–2
157.20 165.81 158.07 167.26 161.23 161.94 157.83 166.69 160.06 160.99 162.20 163.21 164.62 157.01 164.71 157.71 167.19 157.73 166.45 157.74
13.92 9.54 14.22 15.12 27.87 13.79 26.76 15.93 10.65 7.03 7.22 4.21 6.01 4.38 2.20 4.21 4.98 17.97 8.67 20.21
1547.7
B
C
D
E
1547.800
1547.800
1549.871
1549.858
In order to ascertain the effectiveness of this fiber optic corrosion monitoring method, five fiber optic corrosion sensors were manufactured. The five FOCS are installed separately in five concrete specimens, which are all 170 mm 190 mm 85 mm and the cover thickness is 70 mm. A cathodic method of accelerating corrosion on a FOCS was assembled, as shown in Fig. 4. The FOCS is placed in a water basin containing salt saturated water (NaCl:H2O¼1:20). In order to speed up corrosion, an accelerating corrosion cathode is installed in the FOCS, and the twin rebar elements are connected to constant current flow. The peak wavelengths of the five FBG sensors used in this test are listed in Table 1. In order to compensate for the temperature effect, a FOTS is used in this test, whose peak wavelength is 1546.936 nm. The outputs of FOCS and FOTS are all monitored by an optical spectrum analyzer (AQ6317C), as shown in Fig. 4. At the end of the test, the fiber optic corrosion sensors were broken one by one, and pieces of concrete were removed. The twin rebars were immersed in inhibited hydrochloric acid for 15 min, washed with water, alcohol and acetone, weighed, and their weight loss was found, as shown in Table 1.
4. Results and discussion The test results of FOCS A–E are all shown in Fig. 5. The measurement results here and subsequently have been corrected for temperature compensation using the FOTS. The figure shows data for time from 1 to 126 days. In the original 40 days, because of the slight corrosion of rebar, drying shrinkage of concrete, and rust filling into the pore space in concrete, the wavelength shifts detected by FOCS A–E do not appear to increase obviously. After 40 days, the wavelength shifts detected by FOCS A, C, D, and E
Fig. 4. Typical sensor layout and measurement system.
J. Gao et al. / NDT&E International 44 (2011) 202–205
205
the correlation between reflected wavelength change and the weight loss rate of rebar in concrete, we can monitor the corrosion level of reinforcement steel in concrete structures by FOCS presented in this paper. Law et al. [7] used the linear polarization resistance technique to measure the loss of steel in concrete. A value of 25 mV has been adopted for corroding steel. The weight loss measurements were conducted after 1705 days for three chloride exposure specimens. A mean total weight loss for all six bars of 1.08 g was measured. The initial weight of each bar was between 65 and 75 g. The weight loss rate of rebar is 1.54% [7]. In our experiment, one of the weight loss rate of the rebar is 8.1%, which is the approximate equivalent in time to 24 years relative to 1.54%. It can be predicted that our FOCS has good long term stability even for more than 10 years.
5. Summary Fig. 5. Wavelength shifts of FOCS with time.
In order to detect rebar corrosion, a fiber optic corrosion sensor comprising one FBG sensor, twin steel rebar elements and some protectors was designed. One FBG sensor can be installed safely into twin rebars. A series of experiments were carried out in the lab. A constant current accelerated corrosion test was performed on five fiber optic corrosion sensors. In the original 40 days, the wavelength shifts detected by FOCS A–E do not appear to increase obviously. After 40 days, the wavelength shifts detected by FOCS A, C, D and E increase gradually with time. After 100 days, the wavelength shift detected by FOCS B also increases gradually with time. Furthermore, the relationship between reflected wavelength change from the grating and the weight loss rate of rebar was obtained by the gravimetric weight loss method. Using the relationship between reflected wavelength change and the weight loss rate, the FOCS can be used as a sensor for rebar corrosion in reinforced concrete structure.
Acknowledgments
Fig. 6. Relationship between reflected wavelength shift and weight loss rate.
increase gradually with time. After 100 days, the wavelength shift detected by FOCS B also increases gradually with time. From Fig. 5 it is shown that at this stage the volume expansion of rebar caused by corrosion occurs, which results in the shift of the peak wavelength of the grating. But it can also be seen that these average corrosion rates measured by FOCS are different. In addition, corrosion damage refers to the final stage of reinforcement undergoing corrosion attack. Consequently, the measured weight loss data for the twin rebar elements in a FOCS enable us to confirm the degree of corrosion. So the fiber optic corrosion sensors A–E were broken one by one, and the weight loss data are calculated after the test and also listed in Table 1. Using the weight loss of the rebar, the relationship between the reflected wavelength change from the grating and the weight loss rate of the rebar can be obtained. So it is feasible to monitor the corrosion level of reinforcement steel in concrete structures using the fiber optic corrosion sensor presented in this paper. The weight loss rate can be described as the ratio of weight loss for twin rebars to their weight before corrosion. Fig. 6 shows the relationship between reflected wavelength change from the grating and the weight loss rate of rebar in concrete. In Fig. 6, it can be seen that the larger the wavelength shift, the greater the weight loss rate of rebar in concrete. The fitting curve from the measurement data is shown as a parabolic curve. Using
This work is supported by grants from Natural Science Foundation of China (Grant no. 50879034) and the Traffic Science Research project of Jiangsu (Grant no. 07Y09). References [1] Fuhr PL, Huston DR. Corrosion detection in reinforced concrete roadways and bridges via embedded fiber optic sensors. Smart Materials and Structures 1998;7(2):217–28. [2] Lo YL, Xiao FY. Measurement of corrosion and temperature using a single-pitch Bragg grating fiber sensor. Journal of Intelligent Material Systems and Structures 1998;9(10):800–7. [3] Yang S, Cai HW, Geng JX, et al. Advanced fiber grating corrosion sensors for structural health monitoring, In: Ou JP, Li H, Duan ZD, editors. Proceedings of the second international conference on structural health monitoring of intelligent infrastructure. Shenzhen, China: 2006. p. 441–43. [4] Abderrahmane S, Himour A, Kherrat R, et al. An optical fibre corrosion sensor with an electroless deposit of Ni–P. Sensors and Actuators B: Chemical 2001; 75(1–2):1–4. [5] Benounis M, Jaffrezic-Renault N. Elaboration of an optical fibre corrosion sensor for aircraft applications. Sensors and Actuators B: chemical 2004; 100(1–2):1–8. [6] Cooper KR, Elster J, Jones M, Kelly RG. Optical fiber-based corrosion sensor systems for health monitoring. IEEE 2001:847–56. [7] Law DW, Cairns J, Millard SG, Bungey JH. Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements. NDT&E International 2004;37:381–8. [8] Batis G, Routoulas Th. Steel rebars corrosion investigation with strain gages. Cement & Concrete Composites 1999;21:163–171. [9] Dove MFA, Logan N, Mauger JP, Allan BD, Arndt RE, Hawk CW. Laboratory methodologies for propellant corrosion research. Journal of Propulsion and Power 1996;12(3):580–4. [10] Rosenberg SD, Gage ML, Homer GD, Franklin JE. Hydrocarbon-fuel/copper combustion chamber liner compatibility, corrosion prevention, and refurbishment. Journal of Propulsion and Power 1992;8(6):1200–7.
Contents lists available at ScienceDirect
NDT&E International journal homepage: www.elsevier.com/locate/ndteint
Monitoring of corrosion in reinforced concrete structure using Bragg grating sensing Junqi Gaon, Jin Wu, Jun Li, Xinming Zhao Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2010 Received in revised form 27 November 2010 Accepted 29 November 2010 Available online 10 December 2010
Corrosion in reinforced concrete structures is a major problem that seriously affects the service life of the structures. In order to detect rebar corrosion, a fiber optic corrosion sensor (FOCS) made of one fiber Bragg grating (FBG) sensor and twin steel rebar elements was designed and packaged up with concrete. Subsequently, a series of experiments were carried out to verify its feasibility. A constant current accelerated corrosion test was performed on five fiber optic corrosion sensors and the relationship between reflected wavelength change from the grating and the weight loss rate of rebar was obtained by the gravimetric weight loss method. The experiment shows that it is feasible to monitor the degree of corrosion of reinforced steel in reinforced concrete structures using FOCS. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Fiber optic corrosion sensor Corrosion monitoring FBG Constant current Accelerated corrosion test Rebar
1. Introduction Corrosion monitoring is one of the important measures for maintenance of a reinforced concrete structure. It can reveal the onset or progress of corrosion damage of reinforcement in the structures being monitored. This is particularly important in the early stages before the damage becomes significant. Corrosion monitoring is not only the basis for adopting appropriate prevention measures, but also provides essential data for prediction of the service life of structures. Among many techniques of corrosion monitoring, which are used in the laboratory and in the field, some have been widely used, such as half-cell potential, linear polarization resistance, polarization curves, and AC electrochemical impedance spectroscopy. Recently through the measurement of parameters relating to the corrosion reaction of reinforcement some corrosion sensors using optical fibers were presented [1–6]. Fuhr and Huston [1] present multiple-parameter sensing fiber optic sensors to detect corrosion in reinforced concrete roadways and bridges. Lo and Xiao [2] design a pre-strained Bragg grating as a corrosion transducer and a temperature sensor. Yang et al. [3] designed a device, which was made of a fiber grating, a steel spring and some mechanical parts and was packaged under pre-strained condition of the spring, to monitor metal materials corrosion. Benounis and Jaffrezic-Renault [5] present an optical fiber corrosion sensor, fabricated by thermal
n
Corresponding author. E-mail address: [email protected] (J. Gao).
0963-8695/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2010.11.011
deposition of an aluminum film onto an optical fiber core within the sensing region, and tested by optical and electrochemical potential methods. In order to confirm their accuracy, some techniques were generally accompanied by the gravimetric weight loss method [7,8]. Especially in the aviation industry similar approaches were developed to monitor corrosion weight loss rates [9,10]. Therefore we also couple the optical and gravimetric weight loss methods by correlating the parameters of the shift of the peak wavelength and the rebar weight loss rate. In this paper, a fiber optic corrosion sensor comprising one FBG sensor and twin steel rebar elements was proposed, and its principle was analyzed. Subsequently a series of experiments was carried out. In the end, the experimental results were given and discussed. 2. Fiber optic corrosion sensor design A fiber optic corrosion sensor proposed herein is made of one FBG sensor and twin rebar elements and packaged up with concrete as shown in Figs. 1 and 2. Twin rebars are screw-thread steel and their diameters are all 25 mm. The rebar length is 90 mm. First, rebar 1 is split into R1-1 and R1-2, and rebar 2 is split into R2-1 and R2-2. Second, a FBG sensor is set vertically to the twin rebar axis and bonded on their planed surface after R1-2 and R2-2 are placed side by side. Third, R1-1 and R2-1 are attached to R1-2 and R2-2, respectively. Last, the FBG sensor and twin rebar elements are packaged up with concrete as shown in Fig. 1(a). Under normal circumstances, the corrosion of the twin rebar would result in the
J. Gao et al. / NDT&E International 44 (2011) 202–205
shift of the peak wavelength of the grating because of their volume expansion. In order to compensate for the temperature effect, a fiber optic temperature sensor (FOTS) is proposed, which is made of one loose grating and twin stainless rebar elements and also packaged up with concrete. The layout of the new fiber optic temperature sensor is like FOCS. Because no corrosion and volume expansion generally occur in the stainless rebar, the fiber optic temperature sensor can detect the environmental temperature. Under real conditions, the FOTS will be installed in civil structures and located near FOCS. A schematic of reinforcement volume change during corrosion is shown in Fig. 1(b). The wavelength shift of the FBG affixed in FOCS rebar is given as
Dlb1 ¼ Ze þ gðTT0 Þ lb1 ð0,T0 Þ
ð1Þ
The wavelength shift of the reference FBG affixed in FOTS stainless rebar is given as
Dlb2 ¼ gðTT0 Þ lb2 ð0,T0 Þ
203
the temperature gage factor, and T T0 is equal to temperature variation DT. Because the two gratings are in the same circumstance and g is constant, the relationship between wavelength shift and strain is
Dlb ¼
Dlb1 Dlb2 ¼ Ze lb1 ð0,T0 Þ lb2 ð0,T0 Þ
ð3Þ
From Fig. 1(b), strain e can also be obtained as
e¼
DD0 DD ¼ D0 D0
ð4Þ
Accounting for Eqs. (3) and (4), the degree of corrosion r can be expressed as DV ðD0 þ DDÞ2 D0 2 DD 2 r¼ ¼ ¼ 1 þ 1 V0 D0 D0 2 2 Dlb 1 ð5Þ ¼ ð1þ eÞ2 1 ¼ 1 þ
Z
ð2Þ
where T0 is a reference temperature, lb1(0,T0) and lb2(0,T0) are the initial center wavelengths, Z is the strain gage factor, g is
Fig. 1. Scheme of corrosion sensor: (a) rebars without corrosion and (b) rebars volume expansion due to corrosion.
where D0 is the diameter of the rebar, D is the diameter after corrosion, DD is the change in diameter, and r is the percentage of steel volume loss. From Eqs. (4) and (5), the diameter change DD and the degree of corrosion r can be expressed by strain e. In order to prevent FBG from gliding from the adhering point or breaking off, the tensile strain measured by FBG sensor is not suitable to exceed 3000 me. Before these sensors designed were used to detect the corrosion degree of rebars in a real structure, the research work should be performed in five steps. (1) Elaboration of FOCS and FOTS—this experimental work was operated in our laboratory, and finally these sensors were packaged up with concrete. (2) Standardization of FOCS—the objective of this work is to relate reflected wavelength change from the grating with the weight loss rate of rebar by the accelerating corrosion test. (3) Installation of FOCS and FOTS—the FOCS and FOTS would be placed near the steel being monitored and cast in place using concrete as a real reinforced concrete structure was constructed. As illustrated in Fig. 3, sensors A and C are two FOCS, and sensors B and D are two FOTS. Because the twin rebar elements packaged in FOCS and the steel being monitored were in the same corrosion circumstance, they would corrode at the same time. Therefore, by analyzing the wavelength shift signal collected from FOCS and FOTS, the corrosion of the steel in a reinforced concrete structure can be detected readily. (4) Real-time monitoring—these sensors will be connected directly to an analyzer
Fig. 2. Photograph of fiber optic corrosion sensor.
204
J. Gao et al. / NDT&E International 44 (2011) 202–205
placed at a control room. So we can real-time obtain the reflected wavelength changes. (5) Data analysis and corrosion degree evaluation—the present residual strength and, by extrapolation, the remaining service life of the structure can then be estimated. This article mostly describes the elaboration and calibration of FOCS.
3. Experimental work 3.1. Preparation of test In this experiment, ten screw-thread steel elements are processed, whose diameters are all 25 mm. First, the protective oxide film of the steel rebar is ground off. Second, the weight of them is weighed by a balance. Some parameters of the ten steel rebar elements are listed in Table 1. All of the steel rebar elements were used for five fiber optic corrosion sensors, which were designed and constructed one month ahead of the test at the materials lab of Nanjing University of Aeronautics and Astronautics. 3.2. Accelerating corrosion test
Fig. 3. Typical layout of corrosion sensors in reinforced concrete structure.
Table 1 Parameters of fiber optic corrosion sensors. FOCS
Rebar code
Weight (g)
Weight loss (g)
FBG peak wavelength (nm)
A
A1–1 A1–2 A2–1 A2–2 B1–1 B1–2 B2–1 B2–2 C1–1 C1–2 C2–1 C2–2 D1–1 D1–2 D2–1 D2–2 E1–1 E1–2 E2–1 E2–2
157.20 165.81 158.07 167.26 161.23 161.94 157.83 166.69 160.06 160.99 162.20 163.21 164.62 157.01 164.71 157.71 167.19 157.73 166.45 157.74
13.92 9.54 14.22 15.12 27.87 13.79 26.76 15.93 10.65 7.03 7.22 4.21 6.01 4.38 2.20 4.21 4.98 17.97 8.67 20.21
1547.7
B
C
D
E
1547.800
1547.800
1549.871
1549.858
In order to ascertain the effectiveness of this fiber optic corrosion monitoring method, five fiber optic corrosion sensors were manufactured. The five FOCS are installed separately in five concrete specimens, which are all 170 mm 190 mm 85 mm and the cover thickness is 70 mm. A cathodic method of accelerating corrosion on a FOCS was assembled, as shown in Fig. 4. The FOCS is placed in a water basin containing salt saturated water (NaCl:H2O¼1:20). In order to speed up corrosion, an accelerating corrosion cathode is installed in the FOCS, and the twin rebar elements are connected to constant current flow. The peak wavelengths of the five FBG sensors used in this test are listed in Table 1. In order to compensate for the temperature effect, a FOTS is used in this test, whose peak wavelength is 1546.936 nm. The outputs of FOCS and FOTS are all monitored by an optical spectrum analyzer (AQ6317C), as shown in Fig. 4. At the end of the test, the fiber optic corrosion sensors were broken one by one, and pieces of concrete were removed. The twin rebars were immersed in inhibited hydrochloric acid for 15 min, washed with water, alcohol and acetone, weighed, and their weight loss was found, as shown in Table 1.
4. Results and discussion The test results of FOCS A–E are all shown in Fig. 5. The measurement results here and subsequently have been corrected for temperature compensation using the FOTS. The figure shows data for time from 1 to 126 days. In the original 40 days, because of the slight corrosion of rebar, drying shrinkage of concrete, and rust filling into the pore space in concrete, the wavelength shifts detected by FOCS A–E do not appear to increase obviously. After 40 days, the wavelength shifts detected by FOCS A, C, D, and E
Fig. 4. Typical sensor layout and measurement system.
J. Gao et al. / NDT&E International 44 (2011) 202–205
205
the correlation between reflected wavelength change and the weight loss rate of rebar in concrete, we can monitor the corrosion level of reinforcement steel in concrete structures by FOCS presented in this paper. Law et al. [7] used the linear polarization resistance technique to measure the loss of steel in concrete. A value of 25 mV has been adopted for corroding steel. The weight loss measurements were conducted after 1705 days for three chloride exposure specimens. A mean total weight loss for all six bars of 1.08 g was measured. The initial weight of each bar was between 65 and 75 g. The weight loss rate of rebar is 1.54% [7]. In our experiment, one of the weight loss rate of the rebar is 8.1%, which is the approximate equivalent in time to 24 years relative to 1.54%. It can be predicted that our FOCS has good long term stability even for more than 10 years.
5. Summary Fig. 5. Wavelength shifts of FOCS with time.
In order to detect rebar corrosion, a fiber optic corrosion sensor comprising one FBG sensor, twin steel rebar elements and some protectors was designed. One FBG sensor can be installed safely into twin rebars. A series of experiments were carried out in the lab. A constant current accelerated corrosion test was performed on five fiber optic corrosion sensors. In the original 40 days, the wavelength shifts detected by FOCS A–E do not appear to increase obviously. After 40 days, the wavelength shifts detected by FOCS A, C, D and E increase gradually with time. After 100 days, the wavelength shift detected by FOCS B also increases gradually with time. Furthermore, the relationship between reflected wavelength change from the grating and the weight loss rate of rebar was obtained by the gravimetric weight loss method. Using the relationship between reflected wavelength change and the weight loss rate, the FOCS can be used as a sensor for rebar corrosion in reinforced concrete structure.
Acknowledgments
Fig. 6. Relationship between reflected wavelength shift and weight loss rate.
increase gradually with time. After 100 days, the wavelength shift detected by FOCS B also increases gradually with time. From Fig. 5 it is shown that at this stage the volume expansion of rebar caused by corrosion occurs, which results in the shift of the peak wavelength of the grating. But it can also be seen that these average corrosion rates measured by FOCS are different. In addition, corrosion damage refers to the final stage of reinforcement undergoing corrosion attack. Consequently, the measured weight loss data for the twin rebar elements in a FOCS enable us to confirm the degree of corrosion. So the fiber optic corrosion sensors A–E were broken one by one, and the weight loss data are calculated after the test and also listed in Table 1. Using the weight loss of the rebar, the relationship between the reflected wavelength change from the grating and the weight loss rate of the rebar can be obtained. So it is feasible to monitor the corrosion level of reinforcement steel in concrete structures using the fiber optic corrosion sensor presented in this paper. The weight loss rate can be described as the ratio of weight loss for twin rebars to their weight before corrosion. Fig. 6 shows the relationship between reflected wavelength change from the grating and the weight loss rate of rebar in concrete. In Fig. 6, it can be seen that the larger the wavelength shift, the greater the weight loss rate of rebar in concrete. The fitting curve from the measurement data is shown as a parabolic curve. Using
This work is supported by grants from Natural Science Foundation of China (Grant no. 50879034) and the Traffic Science Research project of Jiangsu (Grant no. 07Y09). References [1] Fuhr PL, Huston DR. Corrosion detection in reinforced concrete roadways and bridges via embedded fiber optic sensors. Smart Materials and Structures 1998;7(2):217–28. [2] Lo YL, Xiao FY. Measurement of corrosion and temperature using a single-pitch Bragg grating fiber sensor. Journal of Intelligent Material Systems and Structures 1998;9(10):800–7. [3] Yang S, Cai HW, Geng JX, et al. Advanced fiber grating corrosion sensors for structural health monitoring, In: Ou JP, Li H, Duan ZD, editors. Proceedings of the second international conference on structural health monitoring of intelligent infrastructure. Shenzhen, China: 2006. p. 441–43. [4] Abderrahmane S, Himour A, Kherrat R, et al. An optical fibre corrosion sensor with an electroless deposit of Ni–P. Sensors and Actuators B: Chemical 2001; 75(1–2):1–4. [5] Benounis M, Jaffrezic-Renault N. Elaboration of an optical fibre corrosion sensor for aircraft applications. Sensors and Actuators B: chemical 2004; 100(1–2):1–8. [6] Cooper KR, Elster J, Jones M, Kelly RG. Optical fiber-based corrosion sensor systems for health monitoring. IEEE 2001:847–56. [7] Law DW, Cairns J, Millard SG, Bungey JH. Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements. NDT&E International 2004;37:381–8. [8] Batis G, Routoulas Th. Steel rebars corrosion investigation with strain gages. Cement & Concrete Composites 1999;21:163–171. [9] Dove MFA, Logan N, Mauger JP, Allan BD, Arndt RE, Hawk CW. Laboratory methodologies for propellant corrosion research. Journal of Propulsion and Power 1996;12(3):580–4. [10] Rosenberg SD, Gage ML, Homer GD, Franklin JE. Hydrocarbon-fuel/copper combustion chamber liner compatibility, corrosion prevention, and refurbishment. Journal of Propulsion and Power 1992;8(6):1200–7.