Reinforcement steel in concrete structures is initially protected by a passive film formed due to the alkaline concrete pore solution, which contains hydroxyl ions released from cement hydration. This protective film may be destroyed due to neutralization of the concrete pore solution or penetration of aggressive chemicals such as chloride from de-icing salt or marine environments. Breakdown of passive film causes initiation of corrosion that is followed by corrosion propagation, and over time results in cracking of concrete cover, reduction of mechanical properties of steel bar, bond loss between steel and concrete, as well as reduction of carrying capacity of concrete structural members and systems. According to a NACE report released in 2002, the annual direct corrosion cost for replacement and maintenance of highway bridges in the U.S. was estimated $13.6 billion ^[1]. The indirect cost due to traffic delay or loss of productivity during the replacement or maintenance period is ten times higher than the direct cost. Moreover, steel corrosion in concrete structures is also a potential threat for human life if structures collapse when the key members are subjected to corrosion attack.

A lot of methods or techniques have been proposed or developed to monitor corrosion of reinforcement steel in concrete structures, which can be broadly divided into two categories: electrochemical and non-electrochemical. Electrochemical methods include half-cell potential measurements, electrochemical noise, electrochemical impedance, linear polarization resistance and solid embeddable reference electrodes. Non-electrochemical methods use techniques such as acoustic emission, ultrasonic wave, piezoelectric transducer, ground penetration radar, and optical fiber. Use of optic fiber to monitor steel corrosion in concrete structures have many advantages compared with conventional methods, such as small size, immunity to electromagnetic interference, high precision and sensitivity, capability of multiplexing in large sensor networks.

In our previous study, an optical fiber corrosion sensor was proposed based on the sensing principle of long period grating ^[2], and its mechanism and sensitivity was characterized in 3.5 wt.% NaCl solution ^[3]. However, the effectiveness of this corrosion sensor to monitor corrosion evolution of reinforcement steel has not been investigated yet. In this study, the corrosion sensor was employed to monitor corrosion behavior of steel bar in 3.5 wt.% NaCl solution, and its correlation with corrosion evolution of steel bar and its effective service life were studied experimentally. This fiber optic corrosion sensor was coated with an inner layer of silver and an outer layer of Fe-C coating. The inner silver was sputtered coated with a thickness of 800 nm, and the outer Fe-C coating was electroplated with a thickness of 20 um. A small piece of steel bar was cut, prepared and tested in 3.5 wt.% NaCl solution, together with the fiber optic corrosion sensor. The corrosion behavior of both the outer Fe-C layer and the steel bar was investigated with electrochemical impedance spectroscopy (EIS), and the change in the light spectrum was recorded with an optical sensing interrogator. EIS results showed that the corrosion behavior of steel bar remained stable throughout the whole test period, while the corrosion resistance of Fe-C layer gradually decreased with time. Due to the limitation of interaction range between the coating layer and the light propagating in the fiber as well as the coating thickness, this fiber optic corrosion sensor can only monitor the corrosion of steel bar for 18 hours in 3.5 wt.% NaCl solution.

1. P.H. Koch, N.G. Brongers, Y.P. Thompson, “Corrosion Costs and Preventive Strategies in the United States”, NACE International Houston, TX, USA, 2002 Publication No. FHWA-RD-01–156.
2. Chen, F. Tang, Y. Bao, Y. Tang, G. Chen, “A Fe-C Coated Long-Period Fiber Grating Sensor for Corrosion-Induced Mass Loss Measurement”, Optics Letters, Vol. 41, pp 2306-2309, 2016.
3. Chen, F. Tang, Y. Tang, M. J. O’Keefe, G. Chen, “Mechanism and Sensitivity of Fe-C Coated Long Period Fiber Grating Sensors for Steel Corrosion Monitoring of RC Structures”, Corrosion Science, Vol. 127, pp 70-81, 2017.