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An Insight into Fiber Optic Sensors to Monitor Reinforced Concrete Corrosion

Corrosion of reinforcement can affect durability and in-tegrity of reinforced concrete structures. Repair cost for a badly corroded structure can be very costly and time

consuming. Monitoring of important structures throughout its working life is normally needed to ensure its performance under expected applied loads. This usually involves investi-gation of cracking and damage due to applied loads, corrosion of steel, failure of bonds, excessive strain in concrete and re-inforcing bars, critical deflection etc. The conventional tech-niques such as strain gauges, vibrating wires strain trans-lucers, etc. have their own limitations. These sensors may not be functional and effective sensor throughout the life of structures. This being the position, the need of development of effective senor was being felt quite long. In recent years fi-ber optic based sensors have attracted considerable interest as non destructive structural health monitoring technique. Fiber optic based sensing technology has shown remarkable progress and accuracy level in structural health monitor-ing of civil structures especially in concrete structures. The technology of fiber optic sensors, initially developed for use in aerospace industry, is currently investigated for its appli-cability in civil engineering. Advances in the structural ap-plication of this technology will facilitate the use of built-in monitoring capability in reinforced concrete members, and consequently enable the production of smart structures. Refer Figure 1.

Ankita Adhikary

The Need

The durability and practicability of reinforced concrete structures is one of the primary concerns in the present day science. Corrosion of reinforcement steel (Refer Figure 2) is on of the main durability issues for concrete structures, especially in costal areas and in countries where de-icing salt is regularly used. Because, majority of modern physical in-frastructures are being constructed of reinforced concrete structures. These concrete structures can be degraded or deteriorated for various reasons such as low funding, popu-lation growth, tighter health and environmental needs, sub-standard installation, insufficient inspection and maintenance, and lack of uniformity in design, construction and operation practices usually have adverse effects on reinforced con-crete structures. There are various types of concrete degra-dation but steel corrosion is one primary cause for concrete deterioration. Naturally, inside concrete structures, steel is surrounded by the alkaline environment and also in a pas-sive condition with a negligible corrosion rate. Nevertheless when the concrete cover of the steel is being infiltrated by carbon dioxide and chlorides to get in touch with the steel, the corrosion rate also gets accelerated. To avoid such situ-ation, a proper, effective and accurate steel corrosion detec-tion method is highly required to be developed. Fiber optic based sensing method for detecting the onset of steel cor-

The sensors are extremely simple to deploy and can transmit their data wirelessly to the measure-ment hub, thus eliminating the need to install any wiring in the structure to be monitored

Figure 1 : Sensors for Corrosion Monitoring in RCC

CORROSION MONITORING: FIBER-OPTIC SENSORS

Image Courtesy : www.buildipedia.com

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can explain the transmission of light through optical fiber. Snell’s Law relates index of refraction and incoming and out-going angles for light passing from one material to another by the equation :

N1Sin 1 = N2 Sin 2. When a light travels from a fiber core that has higher

refractive index (e.g. 1.46) into the cladding with a lower re-fractive index (e.g. 1.40), the light wave is totally reflected back into the core as shown in Figure 4.

rosion inside reinforced concrete structure with particular application on concrete structures provides the engineering community an effective tool to check this process and save reinforced concrete structure from severe damage. The sensors are able to measure several parameters, which are critical to evaluate the present and future risk of rebar cor-rosion in concrete. In particular the corrosion current and the concrete humidity are measured at several depths be-tween the concrete surface and the rebar depth, to analyze the progression of the corrosion front as well as evaluate the performance of hydrophobic coatings or waterproof mem-branes.

Figure 3: A Typical Sensor (Image Courtesy : www.cosasco.com)

Figure 4: Light Transmission in Optical Fiber

The Sensor

A smart structure is capable of accessing damage and warning of impending weakness in structural integrity. Fi-ber optic sensors offers many advantages over conventional electronic system when applied to civil structures as the over-all size of most civil structures poses many problems which may possibly be overcome by using fiber optic sensors. Fiber optic sensors are light weight, non hindering, can be configured to arbitrary shapes, low power utilization, flexibility, immune to electric and electromagnetic interference. The most impor-tant feature of fiber optic sensors is that it serves both as sensing element and signal transmission medium, allowing the electronic instrumentation to be located remotely from the measurement site. Thus the application of these sensors may lead to designing an implementation of safer building and structures. Refer Figure 3 for a typical sensor.

Figure 2 : Concrete Corrosion a major threat to durability

Light Transmission Nature of Optical Fiber

Snell’s Law and the concept of total internal reflection

Desired Characteristics of Fibers

The characteristics desired for the ideal optical fibre as intrinsic or extrinsic strain or temperature sensors in civil structure applications would include the following: (a) reli-able; (b) the ability to provide localised or field measurements; (c) adequate sensitivity and dynamic range; (d) insensitive to thermal fluctuations; (e) immune to power interruption; (f) able to multiplex; (g) easy to mass produce; and (h) durability for the life of the structure. Moreover, the fibre must be able to withstand a high alkaline content environment as well as the physical abuse caused by installation in concrete.

Working Philosophy

In the fibre-optic sensors, external perturbations such as strain, pressure or temperature variations induce changes in the phase, intensity, or wavelength of light waves propagat-ing through the optical fibres. The changes in one or more of the properties of light can then be related to the parameter being measured. Optical fibres are geometrically versatile and can be configured to arbitrary shapes. The smart struc-ture concept takes advantage of the geometric adaptability of optical fibres. In this technology, fibre-optic sensors are embedded within the structural material or bonded on the structure surface for the purpose of real-time damage as-sessment. The most attractive feature of fibre-optic sensors is their inherent ability to serve as both the sensing element and the signal transmission medium, allowing the electronic instrumentation to be located remotely from the measure-ment site. This is especially useful for remote monitoring of the condition of bridges. Moreover, the advantages of using embedded fibre-optic sensors in composite materials are dimensional and material compatibility. The fibres do not de-grade during curing, they do not corrode, and bond strongly to the matrix. Incorporation of the fibres during the process-ing stage also offers the opportunity to monitor the condition of structural elements during fabrication.

The success of fibre-optic sensor technology in the con-dition monitoring of composite materials led to a limited number of research and development activities in the civil engineering discipline. A number of researchers realised that this emerging field of technology could have an impact on the condition monitoring of civil structures, so that dura-bility, safety and efficiency of the infrastructure system could

CORROSION MONITORING: FIBER-OPTIC SENSORS

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ponent to observe strain concentration in homogenous and isotropic host. Narendran, Shukla and Letcher attached op-tical fibers sensors to a cantilever beam for strain measure-ment. J.K. Chhabra et al. embedded optical fiber sensors in concrete beam to monitor the overall health of structures. Rossi and Lee Maon embedded optical fibers sensors in re-inforced concrete beams to detect cracks under the action of applied loads. Wolff and Meisseler developed fiber optic sen-sor for monitoring of pre-stressing force and crack forma-tion in Schiessbergstrasse Bridge in Germany. Xianglim Gu, et al. developed embedded optical fiber sensors for crack detection in reinforce concrete structure. An optical time domain reflectometer (OTDR) was used for the operation of sensors signal. Pandey used fiber optic sensors for monitor-ing of structural defects.

Some Application Examples & The Material Insight

In recent years, multiplexed grating sensors have been installed along a pre-stressed girder in the Taylor Bridge to monitor strain responses of different positions during ser-vice. Refer Figure 8 which shows the installation process of the grating sensors on the girder surface.

Figure 5 : Localised Sensor

Figure 6: Typical arrangement of multiplexed sensor system

Figure 7 : Typical Arrangement of Distributed sensor systems

be improved. Proper application of fibre-optic sensors to the concrete structure requires an understanding of certain fun-damental methodologies pertaining to sensor mechanisms as well as sensor multiplexing strategies.

Types of Sensors:-

Currently, three different types of fibre-optic sensor ar-rangements have been developed in real-life applications; they include localised, multiplexed and distributed sensor systems.a. Localised Sensor: Refer Figure 5 for typical localized

sensor systems

Figure 8 : Installation process of the grating sensors on the girder surface

Figure 9: Schematic illustration of strain measurements from the Beddington Trail Bridge sensor array

Researcher’s have demonstrated the static strain mea-surements from the Beddington Trail Bridge sensor array as illustrated in Figure 9. The sensor array was made by multi-plexed grating sensors in one singlemode optical fibre. The sensors could also be used for monitoring curing of the con-crete. Since the girders have been post-tensioned, the strain measurements during the process revealed the conditions of stress relaxation in the tendons from the combined effects of destressing, concrete shrinkage and creep, the dead loading of the bridge deck, and the post-tensioning applied across the two spans. The sensors, however remained in the bridge as smart sensor for the health monitoring of the bridge.

b. Multiplexed Sensor System : Refer Figure 6 for typical multiplexed sensor systems.

c. Distributed Sensor System : Refer Figure 7for a typical distributed sensor systems.

Available Literatures & history

The use of fiber optic technology in the telecommunica-tion industry has started as early as in the mid 1960’s. Since then the success of fiber optic technology in the field of com-munication engineering as well as various other fields has recently lead to research and development activities of this technology in Civil Engineering also. Mathews and Sirkis used embedded optical fiber sensor as a single phase com-

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Figure 10 illustrates the structure of a typical optical fi-bre made for strain and temperature monitoring. In general, the optical fibre consists of a silica glass core surrounded by a glass cladding material, which has a slightly less refractive index than the core in order to produce a total reflection in the core region. Since the fibre core and cladding are made of the same material, the fibre core and cladding are com-monly referred as a ‘core’ region in most theoretical and nu-merical studies. The outer shell of the optical fibre is made of a rubber-like material, which is use to protect the core from being flawed by abrasion, particularly in the presence of moisture.

Conclusion

Corrosion is one of the leading concerns in reinforced concrete structures and often limits their durability. It is therefore advantageous to complement the current moni-toring strategies with a direct measurement of the corrosion progression. Fibre-optic sensors exhibit considerable poten-tial for real-life application particularly for monitoring corro-sion in reinforced concrete structures. Due to the physical size of the optical fibre, sensors are relatively small com-pared with infrastructure elements, embedding the sensors into the structure does not degrade the mechanical and geo-metrical properties of the structure. Smart composites have been widely adopted in most high technology engineering applications such as a smart wing in aerospace and aircraft industry. However, this innovative concept is not yet adopted in all civil engineering applications. In the future, the devel-opment of smart materials and structures will pay a major role in all engineering disciplines. The optical fibre sensors will be one of the most structural health monitoring devices in achieving this goal. The use of a smart composite as an externally bonded reinforcement as well as real-time struc-tural health monitoring device can greatly improve durability and substantially improve the safety of the structure.

Reference

- Mathews, C.T. and Sirkis, J.S., “ The interaction mechanics of inter ferrometric optical fiber sensors embedded in monolithic struc-tures,” Fiber optic smart structures and Skins III, Vol. 1370, Society of photo optical instrumentation, San Jose, CA, 1990 Pg. 142-153.

- Narendran, N., Shukla, A., and Letcher, S., “Optical Fiber Interferro-metric strain sensor using a single fiber,” Concrete. Nov/Dec (1992). Pg. 33-36

- Chhabra, J.K., Shah, M.L., Kumar, A., and Aggarwal, A.K., “Embed-ded fiber optic sensors for reinforced concrete structures”. Com-munication in instrumentation, Vol. 6, No. 1, April-June 1998, Pg. 40-46

- Rossi, P. and Lee Maon, F., “New method of detecting cracks in con-crete using fiber optics”, RILEM Mater. Struc. 22(1989), Pg.437-442.

- Wolff, R., and Meisseler, “Application with optical fiber sensor sys-tems for monitoring pre-stressed concrete structures,” Structures Congress Abstracts, ASCE, New York,1990, Pg.94-95.

- Xianglin G. W., Zhongyuchen and Ansari, F., “Embedded Fiber op-tic crack sensors for reinforced concrete structures, ACI Structural Journal, May-June 2000, Pg.468-476.

- Pandey, N.K., “ Fiber optic pressure sensor for monitoring of struc-tural defects.” Optica applicata, Vol. XXXVII, No. 1-2, 2007.

- http://www.smartec.ch/content/download/680/5037/file/c199.pdf- http://myweb.polyu.edu.hk/~mmktlau/MCR%20950.pdf- http://www.aensiweb.com/old/anas/2013/179-185.pdf- https://elearning.just.edu.jo/jjce/issues/paper.php?p=8.pdf- http://ijret.org/Volumes/V03/I08/IJRET_110308010.pdf- http://cdn.intechopen.com/pdfs-wm/9123.pdf w

Figure 11: Corrosion sensor for concrete applications

Concrete Corrosion Detecting Sensor

Researcher Lo and Xiao and Panova et al. have devel-oped a single pitch Bragg grating corrosion and pH-sensitive sensors, respectively. A thin copper shell is coated onto a pre-strained optical fibre at the grating region to form a cor-rosion sensor (Refer Figure 11). The principle of the sensor is that environment corrosion would change the thickness of the coating, and eventually cause the changes of residual strain inside the grating region. A parameter, which corre-sponds to the corrosion rate, can be measured according to the reflected wavelength change from the grating. This sensor is able to measure the curing condition of the concrete and may be also used to measure the penetration rate of the chemical substances in the concrete, which is situated in a highly aggressive environment.

Figure 12 : A Typical Corrosion Detecting Sensor System (Image Courtesy : www.smartec.com)

A typical Corrosion detecting sensor system available in market is composed of four main elements:

- The sensors, measuring corrosion current, resistivity and temperature

- Data loggers to record the date and transmit it wirelessly- Central Unit, receiving and concentrating the data from

the data loggers and transmitting or storing it for further analysis.

- Software for data management and publishing, including web interfaceA Typical system is presented schematically in Figure 12.

Figure 10 : The structure of a typical optical fibre

CORROSION MONITORING: FIBER-OPTIC SENSORS