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Durability tests of a fiber optic corrosion sensor.

Wan KT, Leung CK - Sensors (Basel) (2012)

Bottom Line: Steel corrosion is a major cause of degradation in reinforced concrete structures, and there is a need to develop cost-effective methods to detect the initiation of corrosion in such structures.If the surrounding environment is corrosive, the film is corroded and the intensity of the reflected signal drops significantly.In this paper, the method of multiplexing several sensors by optical time domain reflectometer (OTDR) and optical splitter is introduced, together with the interpretation of OTDR results.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil Engineering, Chu Hai College of Higher Education, Riviera Garden, Tsuen Wan, Hong Kong, China. ktwan@chuhai.edu.hk

ABSTRACT
Steel corrosion is a major cause of degradation in reinforced concrete structures, and there is a need to develop cost-effective methods to detect the initiation of corrosion in such structures. This paper presents a low cost, easy to use fiber optic corrosion sensor for practical application. Thin iron film is deposited on the end surface of a cleaved optical fiber by sputtering. When light is sent into the fiber, most of it is reflected by the coating. If the surrounding environment is corrosive, the film is corroded and the intensity of the reflected signal drops significantly. In previous work, the sensing principle was verified by various experiments in laboratory and a packaging method was introduced. In this paper, the method of multiplexing several sensors by optical time domain reflectometer (OTDR) and optical splitter is introduced, together with the interpretation of OTDR results. The practical applicability of the proposed sensors is demonstrated in a three-year field trial with the sensors installed in an aggressive marine environment. The durability of the sensor against chemical degradation and physical degradation is also verified by accelerated life test and freeze-thaw cycling test, respectively.

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Related in: MedlinePlus

Summary of the number of days of the complete depletion of the coating in the field trial.
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f8-sensors-12-03656: Summary of the number of days of the complete depletion of the coating in the field trial.

Mentions: Figure 8 summarizes the number of days after installation for the last reading before the coating had been completely depleted. The trend generally agrees with the discussion in Section 3.1. The coating of sensors 1, 2, 3, 6, 7 and 8, which were installed near the seaward side surface, were completely depleted within 3 months, while sensors 4 and 9, which were installed at locations where the chloride concentration should be much lower than the seaward side, had their coatings completely depleted after 200 days. However, one can see variations in the trend for the two piers. Among sensors 1, 2 and 3, depletion of coating occurred earlier for a sensor closer to the sea. However, among sensors 6, 7 and 8, the coating was completely depleted for sensor 7 at an earlier time than sensor 6 (which is closer to the sea). In principle, the thin iron film should be depleted within a very short time if the chloride content is higher than certain threshold value. The rust stains at the seaward side surface indicated that the chloride content was already higher than the threshold value. The difference between the number of days required to completely deplete the coating was due to the variation of the distance between the sensor tip and the interface of the drilled hole. During sensor packaging, the sensor tip was embedded within 1 or 2 mm from the end of the protective rubber tube, but the actual distance might vary. Also, during the injection of the grout into the drilled hole, the packaged sensor might be pushed away from the interface. In addition, the packaged sensor might not be aligned perfectly perpendicular to the interface in a hole drilled at an angle. The actual time required to completely deplete the sensor coating depends on both the chloride content at the surrounding environment of the drilled hole and the travel distance to the sensor tip. From the results, it is likely that the fiber tips for sensors 1 and 1 and 7 are closer to the interface of the holes. Following the above arguments, the corrosion sensor does have a limitation in its accuracy and consistency. However, as corrosion is a very slow process, delaying the detection by even a few months is not a real concern. The important point is that one can clearly see the difference in results between sensors near the sea (sensors 1–3 and 6–8) and those on the opposite side (sensors 4 and 9) where the chloride content should be lower. Also, at the locations with little chloride content, where sensors 5 and 10 were placed, the sensors would not give false alarm due to their own deterioration under the concrete environment. Before closing, it should be mentioned that at least six strong tropical cyclones approached Hong Kong during the monitoring period, when business and schools were all closed and everybody stayed at home. The sensors are still able to provide signals under these conditions, showing the robustness of the packaging and installation techniques.


Durability tests of a fiber optic corrosion sensor.

Wan KT, Leung CK - Sensors (Basel) (2012)

Summary of the number of days of the complete depletion of the coating in the field trial.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376588&req=5

f8-sensors-12-03656: Summary of the number of days of the complete depletion of the coating in the field trial.
Mentions: Figure 8 summarizes the number of days after installation for the last reading before the coating had been completely depleted. The trend generally agrees with the discussion in Section 3.1. The coating of sensors 1, 2, 3, 6, 7 and 8, which were installed near the seaward side surface, were completely depleted within 3 months, while sensors 4 and 9, which were installed at locations where the chloride concentration should be much lower than the seaward side, had their coatings completely depleted after 200 days. However, one can see variations in the trend for the two piers. Among sensors 1, 2 and 3, depletion of coating occurred earlier for a sensor closer to the sea. However, among sensors 6, 7 and 8, the coating was completely depleted for sensor 7 at an earlier time than sensor 6 (which is closer to the sea). In principle, the thin iron film should be depleted within a very short time if the chloride content is higher than certain threshold value. The rust stains at the seaward side surface indicated that the chloride content was already higher than the threshold value. The difference between the number of days required to completely deplete the coating was due to the variation of the distance between the sensor tip and the interface of the drilled hole. During sensor packaging, the sensor tip was embedded within 1 or 2 mm from the end of the protective rubber tube, but the actual distance might vary. Also, during the injection of the grout into the drilled hole, the packaged sensor might be pushed away from the interface. In addition, the packaged sensor might not be aligned perfectly perpendicular to the interface in a hole drilled at an angle. The actual time required to completely deplete the sensor coating depends on both the chloride content at the surrounding environment of the drilled hole and the travel distance to the sensor tip. From the results, it is likely that the fiber tips for sensors 1 and 1 and 7 are closer to the interface of the holes. Following the above arguments, the corrosion sensor does have a limitation in its accuracy and consistency. However, as corrosion is a very slow process, delaying the detection by even a few months is not a real concern. The important point is that one can clearly see the difference in results between sensors near the sea (sensors 1–3 and 6–8) and those on the opposite side (sensors 4 and 9) where the chloride content should be lower. Also, at the locations with little chloride content, where sensors 5 and 10 were placed, the sensors would not give false alarm due to their own deterioration under the concrete environment. Before closing, it should be mentioned that at least six strong tropical cyclones approached Hong Kong during the monitoring period, when business and schools were all closed and everybody stayed at home. The sensors are still able to provide signals under these conditions, showing the robustness of the packaging and installation techniques.

Bottom Line: Steel corrosion is a major cause of degradation in reinforced concrete structures, and there is a need to develop cost-effective methods to detect the initiation of corrosion in such structures.If the surrounding environment is corrosive, the film is corroded and the intensity of the reflected signal drops significantly.In this paper, the method of multiplexing several sensors by optical time domain reflectometer (OTDR) and optical splitter is introduced, together with the interpretation of OTDR results.

View Article: PubMed Central - PubMed

Affiliation: Department of Civil Engineering, Chu Hai College of Higher Education, Riviera Garden, Tsuen Wan, Hong Kong, China. ktwan@chuhai.edu.hk

ABSTRACT
Steel corrosion is a major cause of degradation in reinforced concrete structures, and there is a need to develop cost-effective methods to detect the initiation of corrosion in such structures. This paper presents a low cost, easy to use fiber optic corrosion sensor for practical application. Thin iron film is deposited on the end surface of a cleaved optical fiber by sputtering. When light is sent into the fiber, most of it is reflected by the coating. If the surrounding environment is corrosive, the film is corroded and the intensity of the reflected signal drops significantly. In previous work, the sensing principle was verified by various experiments in laboratory and a packaging method was introduced. In this paper, the method of multiplexing several sensors by optical time domain reflectometer (OTDR) and optical splitter is introduced, together with the interpretation of OTDR results. The practical applicability of the proposed sensors is demonstrated in a three-year field trial with the sensors installed in an aggressive marine environment. The durability of the sensor against chemical degradation and physical degradation is also verified by accelerated life test and freeze-thaw cycling test, respectively.

Show MeSH
Related in: MedlinePlus