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Tilted Fiber Bragg Grating Sensors for Reinforcement Corrosion Measurement in Marine Concrete Structure Md. Rajibul Islam, Marya Bagherifaez, M. Mahmood Ali, Graduate Student Member, IEEE, Hwa Kian Chai, Kok-Sing Lim, Member, IEEE, and Harith Ahmad

Abstract— In this paper, the tilted fiber Bragg grating is proposed as a corrosion sensor and experimentally demonstrated for measuring and evaluating corrosion of steel rebars in wet conditions. To expedite the corrosion process, the impressed current technique was used to achieve a 28.9% mass loss in the rebar for a total time of 24 h. It is observed that the cladding resonant wavelengths red-shift during the corrosion process and the wavelength shifts are greater for higher order resonances. The proposed technique can be used for monitoring steel corrosion. Index Terms— Fiber optic sensor, impressed current technique, steel rebar corrosion measurement, tilted fiber Bragg grating (TFBG).

I. I NTRODUCTION

C

ORROSION is one of the major factors undermining engineering structures, such as bridges and pipelines, as well as mobile transport structures such as marine ships or aircraft which normally operate in a highly corrosive environment. The risk of corrosion damage increases with the age of the structure, particularly when the original design service life is lapsed. Therefore, the detection of steel corrosion at the early stages is very important to ensure safety and prevention of structural damage. In addition, an extensive monitoring system for corrosion can help estimate the remaining service life of the structures. Several techniques have been used to measure corrosion in marine concrete structures. Half-cell potential measurement is one of the techniques that is often used for evaluating the corrosion activity and its impact on the steel reinforcement in concrete structures. However, the technique is limited to estimating the corrosion rate but incapable of interpreting the

Manuscript received March 30, 2015; revised June 17, 2015; accepted June 18, 2015. This work was supported by the University of Malaya for providing the PPP Project under Grant PG006-2014A, HIR Project under Grant UM.C/625/1/HIR/MOHE/ENG/54, and eScience Project under Grant SF008-2014. The Associate Editor coordinating the review process was Dr. George Xiao. M. R. Islam, M. M. Ali, K.-S. Lim, and H. Ahmad are with the Photonics Research Centre, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). M. Bagherifaez and H. K. Chai are with the Department of Civil Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIM.2015.2459511

level of corrosion in wet conditions [1]. According to [2], monitoring pipeline cathodic protection (CP) systems is a technically complex field. In many cases, condition monitoring requirements are specified by the regular authorities. Since CP systems are expected to operate in demanding environmental conditions over long periods, the requirements for reliability of the associated hardware are high. Regular monitoring of the equipment is therefore an important aspect of any CP program. A PCB-based wireless passive sensor (91-mm long) has been proposed for detection of corrosion initiation and long-term monitoring [3]. Though a single interrogation system with an interrogation coil is required to monitor/detect corrosion, such a system is both labor and time intensive and it may not be a cost-effective solution for multiplepoint detection. Corrosion monitoring is a sophisticated procedure that involves a number of important considerations and different strategies in the implementation. For instance, the nature of corrosion may be uniform or localized, the rate of corrosion varies substantially at different locations, and there is no single measurement technique that is applicable to all corrosion conditions [4]. Besides, there are other important criteria for advanced fiber-optic sensing such as the potential for multiplexing and multitasking. It is a cost-effective system to achieve data acquisition for multiple sensors using a single centralized opto-electronic detection system [5]. Other than opto-electronic detection, several techniques have been proposed; for instance, the polarization resistance technique has been used for monitoring the corrosion level based on the chloride threshold [6]. Corrosion level was measured by the electrochemical impedance of the corroding metal and can be evaluated by measurements of naturally generated by-corrosion processes, voltage, and current fluctuations (electrochemical noise) based on [7]. However, this method is applicable only to uniform corrosion processes. Some other commonly used methods, for instance break examination, require destructive testing to be performed on the structure but it is not suitable for in situ monitoring of corrosion [8]. The Cement Seawater Battery Energy Harvester technique has been used for marine corrosion monitoring [9]. Electrochemical techniques were used to monitor the corrosion performance of the reinforcing bar [10]. In the last decade, the development of optical fiber sensors for structural health monitoring has attracted a great deal

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of attention due to the advantages of small and compact sensors that provide the convenience of remote and continuous operation with the target subject. Optical techniques have formed the foundation for numerous chemical sensors. In fact, they are the oldest and most robust devices for chemical sensing. A corrosion process usually includes chemical or electrochemical transformations that can be detected by optical sensors. A number of optical sensor configurations have been proposed for corrosion monitoring. These include fiber Bragg gratings (FBGs) [11], long-period gratings (LPGs) [12], metal cladding optical fiber [13], [14], point optical fiber sensor [15], and twin-fiber technique [16]. The change in SRI can be presented in the form of variation in output laser intensity or spectrum. The sensitivity can be enhanced by removing the cladding or tapering the fiber to enhance the evanescent field of the device to allow greater interaction between the core mode and ambient medium [17]–[20]. However, the reduction in the fiber diameter weakens the fiber mechanical strength and the fiber devices may not be suitable for the applications in harsh environments. LPGs are sensitive to the SRI caused by the coupling of radiation after the propagating mode of the core to the forward-propagating modes of cladding [21]. The most prominent drawback of LPGs is the high sensitivity to other parameters such as temperature and strain, which can cause undesirable effects as an SRI sensor. A remote refractive index difference meter was developed based on the measurement of the refractive angle change that is nearly proportional to the salinity, using a position-sensitive detector in an optical setup, but cross sensitivity of temperature and radio interference (RI) introduced measurement errors in RI sensing [22]. Recently, the TFBGs have attracted extensive attention to numerous industrial sensing applications, such as temperature-independent vibration measurement [23], strain measurement [24], concentration sensor [25], magnetic field sensing [26], and twist/torsion measurement [27]. The tilted periodic modulation in the fiber core triggers the couplings between core and cladding modes [28], which contribute to a generation of multiple dips in the transmission spectrum which can be easily achieved by tilting the grating plane or phasemask during the grating inscription process. The excited cladding modes have stronger interaction with the ambient medium of the fiber and cladding removal is not required to achieve that. This makes it an attractive sensor for RI [29]–[31]. In this paper, the use of a TFBG for corrosion monitoring is reported. The monitoring of corrosion in rebar accelerated by the impressed current technique in NaCl solution is performed based on the variations of multiple cladding resonance wavelengths, and the results of SRI change during the corrosion process. For this purpose, a new data processing method for the transmission spectrum of TFBG is presented. II. T HEORIES OF ACCELERATED C ORROSION M ONITORING VIA TFBG In the calculation of mass loss of steel rebar during the corrosion process. We use Faraday’s law to estimate the mass

of rust formed per unit surface area of steel bar based on the applied current and time period [32] given by MIt (1) ZF where W is the mass of steel loss (g), I is the corrosion current (A), t denotes time (s), F is the Faraday’s constant (96 487 C/mol), Z is the number of electrons transferred (Fe (2)), and M is the molar mass (55.847 g/mol for iron). According to ASTM G1 standard practice, the gravimetric test can be used to determine the real mass of rust per unit surface area [33] on steel rebar extracted from the NaCl solution after it is subjected to the accelerated corrosion test W =

(Wi − W f ) (2) π DL where the real mass of rust is Mac per unit surface area of the steel rebar (g/cm2 ), initial mass of the rebar before corrosion (g) is Wi , mass after corrosion (g) is W f for a specified time period of induced corrosion (t), D is the rebar diameter (cm), and length of the rebar sample (cm) is L. The percentage mass loss (ρ) that determines the degree of induced corrosion can be calculated as (Wi − W f ) ρ= × 100. (3) Wi Mac =

Assume that the theoretical and real mass of rust are the same (Iapp = Icorr ), based on the relations in (1) and (2), the corrosion current density (Icorr ) is given by (Wi − W f ) . (4) π DLW t In our experiment, the total mass loss of steel obtained was 75 g, after applying the impressed current technique for corrosion acceleration using a constant current (I ) of 3 A for 24 h (86 400 s). The percentage of corrosion was obtained as 28.9% using (3) in total, averaging at 1.2%/h. The corroded reinforcing rebars were extracted and cleaned as per the ASTM G1-90 standard [33] toward the end of corrosion phase, before being weighed to determine the mass loss. Bragg wavelength λBragg and cladding resonance wavelengths λiclad of the proposed TFBG are achieved by the following phase matching situation [25]: Icorr =

λBragg = 2Neff (core) ∗ / cos(θ ) λclad = (Neff (core) +

i Neff (clad)

(5) ∗ / cos(θ )

(6)

where the effective indices of the i th cladding mode and the i (clad) and Neff (core), respectively. fiber core mode are Neff θ is the tilt angle and  is the grating pitch. The effective refractive index of each corresponding cladding mode is closely related to the refractive index difference between cladding mode and the surrounding environment. As SRI increases, the resonant wavelengths are shifted toward the red end of the spectrum and the cladding mode suffers greater attenuation due to the increasing coupling between the cladding modes and the surrounding medium. The cladding modes will be vanishing from the fiber when i (clad) approach and are equal to the effective indices Neff

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SRI. This can be seen from the gradually diminishing resonant wavelengths from shorter to longer wavelengths in the transmission spectrum [27]. Iron hydroxide (Fe(OH)2 ), iron trihydroxide (Fe(OH)3), goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ -FeOOH), feroxyhite (δ-FeOOH), hematite (α-Fe2O3), maghemite (γ -Fe2 O3 ), and magnetite (Fe3 O4 ) are the nine leading corrosion products of iron [34]. In the electrochemical reaction, the oxides turn into hydroxides or oxyhydroxides in a solution, where α-FeOOH and β-FeOOH are the most abundant and stable corrosion products. The mixture of these oxides, hydroxides and oxyhydroxides produce rusts of the steel rebar and the variation of refractive index of the solution is produced by the collective properties of several corrosion substances.

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Fig. 1. Measured transmission spectrum of the TFBG with the coverage of cladding modes and fundamental mode at different corrosion states (X represents lower order cladding resonance whereas Y represents higher order cladding resonance).

III. ACCELERATED C ORROSION P ROCESS AND O PTICAL S ETUP The impressed current technique, also known as the galvanostatic method, is commonly used to accelerate corrosion process in durability tests for steel reinforcement. The process requires the supply of a constant current to the steel rebar embedded in concrete to induce corrosion in a short time span. The amount of induced corrosion can be theoretically determined using Faraday’s law based on the applied current and time or gravimetric test on the extracted corroded rebars to determine the percentage of real mass loss. The current density of a corresponding corrosion process can be determined by the actual quantity of steel loss. In our experiment, a dc power source along with an electrolyte and a counter electrode was used to conduct the impressed current technique for inducing reinforcement corrosion. One steel bar (anode) was connected to the positive terminal of the dc power source, whereas the negative is with the cathode, which is another rebar. Both rebars were immersed in NaCl solution. By nature, the current would be impressed from the cathode to the anode through the electrolyte (NaCl solution) [35], [36]. The TFBG was inscribed in a hydrogen-loaded photosensitive fiber (Nufern FUD-2300) by a 193-nm ArF excimer laser via a phase mask. The phase mask is horizontally tilted at an angle of ∼2° in the fiber plane to produce the tilted grating structure in the fiber. In this paper, the effective length of TFBG was 15 mm and the Bragg wavelength was 1552.6 nm. The measured transmission spectrum is presented in Fig. 1 and spectral change within the wavelength band of 1495–1555 nm was observed for SRI variations. The coupling of core and cladding modes in the grating can be observed from the multiple dips in transmission spectrum. The materials used for this experiment were 190 g of NaCl salt, 12.5 L of water, two steel rebars of 260 g each with 10-mm diameter and 440-mm length. A constant current 0.3 A was applied while the voltage was 9.4 ± 0.4 V. The distance between the anode and the cathode rebars was 180 mm. The concentration of NaCl in the solution was 1.5 wt%. The TFBGs were calibrated using salinity measurement. The immersion liquids used for the salinity test were NaCl–water

Fig. 2. (a) RI results of NaCl solution obtained by prism coupler. (b) Output response of cladding resonances X and Y to NaCl solution with different refractive indices.

solutions providing a range of RI from 1.31 to 1.33 as shown in Fig. 2. The refractive indices of the solutions were measured by a prism coupler at 1550 nm [see Fig. 2(a)]. As presented in Fig. 2(b), it is observed that both low-order resonance X and high-order resonance Y exhibit linear response against RI variation of NaCl solution. It is apparent that high-order resonance has greater sensitivity than low-order resonance. The idea of this experiment is straightforward, which is to measure corrosion rate by analyzing the spectral response of cladding resonances of the TFBG to the change in SRI during corrosion acceleration process. Fig. 3 shows the

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Fig. 4. Transmission spectrum of cladding resonances of the TFBG at different corrosion times. Fig. 3. Schematic of a tilted FBG sensor to the rebar and impressed current technique setup for corrosion test.

experimental setup for monitoring corrosion rate by TFBG. The ASE source (from EDFA) was employed for lighting the TFBG throughout the corrosion process in the experiment. The circulator is used to guide the reflection wave from the TFBG to an optical spectrum analyzer for analysis and recorded at a resolution of 3 pm. The entire experiment and data recording were conducted in an air-conditioned laboratory in which room temperature was kept at 24 ± 1°C. This is to prevent the variation of temperature that may potentially influence the corrosion cell formation process. In the accelerated corrosion test, two steel rebars were immersed into a container filled with a water solution containing 1.5 wt% of NaCl. A TFBG sensor was attached on the rebar that is electrically attached to anode where a constant current of 3 A was induced to the system by dc power supply. The TFBG was tangle-attached loosely on the steel rebar to avoid any possible strain applied on the TFBG during the accelerated corrosion process. The transmission spectra were routinely recorded every 30 min by a laptop using a LabVIEW program during the accelerated corrosion process by the impressed current technique. After 24 h, the weight of the cathode rebar has been reduced from 260 to 184.9 g which is equivalent to 28.9% mass loss. IV. R ESULTS AND D ISCUSSION During the corrosion process, an electrochemical reaction between the steel rebar and the NaCl solution was accelerated by the impressed current technique. The passive film on the rebar surface collapsed after the electrochemical-assisted process was initiated. As a result, the created rust deposited on the anode rebar and decreased the pH level of the solution during the process. The imposed acidic environment accelerates the corrosion process of the rebars. Fig. 4 shows the variations i (clad) in in the effective indices of the cladding mode Neff response to the corrosion. With the increasing SRI, more cladding resonances were affected and transmission curve was smoothened starting from the short-wavelength region. However, the ghost wavelength and Bragg wavelength were not affected by the process. The wavelength in every cladding

Fig. 5. Chosen transmission spectra for analyzing the variation of responses through low-order (A) to high-order (P) cladding resonance.

resonance shifts linearly corresponding to the SRI variation of the solution. This shift can be correlated with the mass loss through calibration in this investigation. Faraday’s law has been used to calculate mass loss based on time and the response of wavelength shift has been recorded based on time. Therefore, wavelength shift and mass loss are correlated based on time that capable to interpret actual corrosion rate. Fig. 5 shows the overlaid transmission spectra of the TFBG during the corrosion process. The variation of cladding resonances in the wavelength range of 1513.7–1536.7 nm in the transmission spectrum is considered in this investigation as shown in Fig. 5. This wavelength region is chosen because of the lower noise level, and the noise can be further eliminated by using a moving average filter. Based on the principle of TFBG, the effective refractive indices of the associated resonances influence the amplitude of each dip and the spectral position in the transmission spectrum for a known angle of tilt and grating pitch [37]. It is noticeable that the wavelength shifts of high- and low-order modes are different. Highorder cladding resonances exhibit greater wavelength shifts than low-order cladding resonances (close to the fundamental mode) (see Fig. 7). That indicates that the excited high-order cladding modes are extremely sensitive to the outer surrounding environment. It is obvious that each cladding resonance shifts and effortlessly measurable concerning the spacing variances of the resonances as presented in Fig. 6. To compute the

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Fig. 6. Measured dynamic response of low- and high-order cladding mode of the TFBG corrosion sensor.

Fig. 7.

Wavelength sensitivity of different cladding resonances.

variations of wavelength shift from long-wavelength-region to short-wavelength-region, each dip has been labeled as A to P from low-order mode to high-order mode as shown in Fig. 5. High- and low-order cladding resonances show wavelength shifts of 36.7 and 7.3 pm/h, respectively. Each single resonance and the total spacing of all cladding resonances in the considered wavelength range can be used to measure corrosion level. The resonance shift will continue to that extent that the effective index of the corresponding cladding mode approaches and is equal to the SRI. The variations of wavelength shifts from high- to low-order cladding resonances can be considered for the measurement of the corrosion level. Fig. 7 presents corrosion sensitivity of low- to high-order cladding resonances of the TFBG. The corrosion sensitivity increases from low- to high-order cladding resonances. Further analysis is conducted with this calibrated TFBG with the intention of investigating the variations of refractive index in the NaCl solution during the impressed current technique. Fig. 8 shows the response of the total wavelength spacing of the chosen cladding resonance based on the percentage of corrosion rate increment. Besides, it provides accurate information about corrosion and it is unaffected by temperature. This allows simultaneous detection of corrosion and temperature. Fig. 9 shows the steel rebar attached with TFBG before and after the application of the impressed current process. Fig. 10 presents the state of the steel rebar as observed at different stages in the experiment. The quality of the images has been affected by the higher turbidity and water reflection effects of the solution.

Fig. 8.

Variations of the wavelength spacing against corrosion over time.

Fig. 9.

28.9% corroded rebar after 24 h of impressed current technique.

The experiment of corrosion measurement was carried out in a controlled environment with a constant temperature. In our observation, there was no wavelength shift for Bragg wavelength throughout the entire accelerated corrosion process. Considering the insensitivity of Bragg wavelength to RI changes, this indicates that there was no variation of temperature and strain to the TFBG during the process. However in the practical condition where the effects of temperature

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Fig. 10.

State of steel rebar at different times during the corrosion process.

and strain cannot be avoided, an additional FBG can be incorporated to the sensing system to discriminate temperature and strain from the measurement of TFBG. It is known that the temperature sensitivities of all cladding resonance wavelengths and Bragg wavelength [38], [39] of the TFBG are similar, and the elimination of the temperature from the corrosion reading can easily be done with a simple deduction. On the other hand, the sensitivities of the cladding resonance wavelengths and Bragg wavelength to strain are different. Nonetheless, the elimination of the strain element can also be achieved provided that the strain characteristics data of TFBG are given. For accurate measurement, system calibration for the sensor with standard solutions is necessary [40]. V. C ONCLUSION As a conclusion, the TFBG has been used for in situ monitoring of the corrosion in steel rebars immersed in NaCl solution. Impressed current technique is used to accelerate the corrosion process for this study. During the corrosion process, it is observed that the higher order cladding resonances exhibited relatively higher sensitivities to the variation of corrosion compared with the lower order cladding resonance existing near the Bragg wavelength. The highest sensitivity of 36.7 pm/h and the lowest sensitivity of 7.3 pm/h have been obtained for high-order as well as low-order cladding modes, respectively, considering a wavelength range of 1513.7–1536.7 nm in the transmission spectrum. The wavelength shifts and the spacing variations between cladding resonances can be used for in situ monitoring of the corrosion and damage of steel reinforcement in marine concrete structures. R EFERENCES [1] H. E. Sørensen and T. Frølund, “Monitoring of reinforcement corrosion in marine concrete structures by the galvanostatic pulse method,” in Proc. Int. IABSE Conf. Concrete Marine Environ., Hanoi, Vietnam, 2002, pp. 213–220. [2] P. R. Roberge and R. W. Revie, Corrosion Inspection and Monitoring. New York, NY, USA: Wiley, 2007, ch. 2, pp. 190–219. [3] K. Perveen, G. E. Bridges, S. Bhadra, and D. J. Thomson, “Corrosion potential sensor for remote monitoring of civil structure based on printed circuit board sensor,” IEEE Trans. Instrum. Meas., vol. 63, no. 10, pp. 2422–2431, Oct. 2014. [4] P. K. S. Babu, A. Mathiazhagan, and C. G. Nandakumar, “Corrosion health monitoring system for steel ship structures,” Int. J. Environ. Sci. Develop., vol. 5, no. 5, pp. 491–495, 2014.

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Md. Rajibul Islam was born in Bangladesh. He received the B.C.A. degree from Indira Gandhi National Open University, New Delhi, India, in 2004, and the M.Sc. degree in information technology from Multimedia University, Melaka, Malaysia, in 2010. He is currently pursuing the Ph.D. degree in photonics with the Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia. His current research interests include photonics, fiber Bragg grating sensors, acousto-optic sensors, fiber optic vibration sensors, and structural health monitoring.

Marya Bagherifaez received the B.E. and M.E. degrees from the Yerevan University of Architecture and Construction, Yerevan, Armenia, in 2011. She is currently a Research Assistant with the Department of Civil Engineering, University of Malaya, Kuala Lumpur, Malaysia. Her current research interests include structural health monitoring using nondestructive methods, in particular, fiber optic sensing method and acoustic emission technique.

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M. Mahmood Ali (GSM’14) received the B.S. degree in electronics engineering from the University College of Engineering and Technology, The Islamia University of Bahawaplur, Pakistan, in 2009, and the M.S. degree in electronics engineering from the Ghulam Ishaq Khan Institute of Engineering Sciences and Technology, Topi, Pakistan, in 2012. He is currently pursuing the Ph.D. degree in photonic engineering under the Bright Spark Fellowship with the Photonics Research Centre, University of Malaya, Kuala Lumpur, Malaysia. His current research interests include fiber Bragg grating sensors and their applications, fiber lasers, nonlinear fiber optics, advanced digital signal and image processing, wave propagation in bi-isotropic media, advance electromagnetic field theory, and microwave engineering. Mr. Ali is a member of the Pakistan Engineering Council, and the National Academy of Young Scientists in Pakistan.

Hwa Kian Chai received the Ph.D. degree in civil engineering from Osaka University, Osaka, Japan, and the M.Sc. degree in concrete materials from the University of Malaya, Kuala Lumpur, Malaysia. He was a Research Engineer with the Research Institute of Technology, Tobishima Corporation, Chiba, Japan, prior to serving in the current position. He is currently a Senior Lecturer with the Department of Civil Engineering, University of Malaya. His current research interests include nondestructive evaluation, structural health monitoring, and repair and strengthening of structures.

Kok-Sing Lim (M’13) received the B.E. degree from the Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia, in 2008, and the Ph.D. degree from the Department of Physics, Photonics Research Centre, in 2012. He is currently a Senior Lecturer with the Photonics Research Centre, University of Malaya. His current research interests include optics of microfiber resonators, fiber Bragg grating sensors, and fiber lasers.

Harith Ahmad received the Ph.D. degree in laser technology from the University of Wales Swansea, Wales, U.K., in 1983. He is currently a Professor with the Department of Physics, University of Malaya, Kuala Lumpur, Malaysia, where he is also the Director of the Photonics Research Centre. Prof. Dr. Ahmad is a fellow of the Malaysian Academic of Science.