Study On The Structural Corrosion Progress Monitoring By Using Fiber-optic Sensors

Corrosion of steel is one of the most important factors in the structural durability studies. Especially, the corrosion progress could be a kernel factor to be considered for extending the serving period of the structure. Now, the main techniques for corrosion progress monitoring are based on the electrochemical techniques and evaluation of the steel mass loss rate. The electrochemical technique is used for corrosion progress evaluation by measuring an impedance change in a specific area on the surface of the reinforced concrete structures. On the other hand, the steel mass loss rate method is to measure the loss of the corroded steel. However, these methods could not meet the demond on the on-line corrosion progress monitoring of a structure.In this dissertation, a low coherent fiber-optic strain sensor was designed to monitor the corrosion-induced expansion strain in the steel reinforced concrete structures. And then the fiber-optic Bragg gratings (FBGs) were employed to detect a crack-induced dynamic strain and the photonic crystal fibers (PCF)-based sensors were designed to analyse the environmental factors that made the corrosion happen. The major achievements were outlined as followings:1. A low coherent fiber-optic strain sensor was introduced to monitor the corrosion progress in the steel-reinforced concrete structures. In practice, the sensing fiber was wound on the surface of a steel rod in two different ways:one was directly wound the sensing fiber on the steel rod and the interface of steel with cerment must be destroyed by the sensor itself; the other was making about 2 mm-thick cement mortar cushion layer a coated on the steel rebar and formed a solidification pillar first, then the sensing fiber was wound on it. This could protect the interface of steel with cerment continuity as it should be. The laboratory accerelated electrochemical experiments demonstrated that the fiber-optic strain sensor system could evaluate the corrosion progress by measuring the corrosion induced strain. The sensor configurated in the Michelson interferometer could approach 3 ?m absolute resolution in strain measurements. The experimental results proved that the sensor can monitor corrosion progress. The analysis demonstrated that the continuity of the interface of the steel with concrete played a crucial role in corrosion development. This presented a new way for long time monitoring of corrosion progress in the concrete structures.2. The low coherent fiber-optic strain sensor was employed to monitor the corrosion progress in the prestressed concrete cylinder pipe (PCCP) in situ of XXX power supply plant. The configured sensing fiber could approach a minimum detectable average strain 0.025 ?? when a sensing fiber length chosen as 50 m. After half years observation, the results showed that the sensor could discriminate the corrosion expansion progress of PCCP. A rapid expansion strain was monitored as the speed of 53.04 ??/month. These results provide a solid practical data for the safety management of the sea-water supply pipelines, thereby it was important for the safety production.3. An adaptive source, comprising by a reflective semiconductor optical amplifier (RSOA) source that forms a linear cavity with a FBG sensor and a demodulator based on an unbalance Michelson interferometer, were designed to detect the dynamic strain induced by a crack happened in a structure. A set of experiments comparing the RSOA-FBG fiber cavity laser and the EDFA-based ring laser was carried out to demonstrate the adaptivity and the multiplexing. The results showed that the adaptivity of the RSOA-FBG fiber cavity laser could be used to detect a high frequencies vibration and reached to the megahertz range. For a FBG sensor with a spectral width of 0.2 nm, the minimum detectable strain is 0.3 ?? and the detectable frequency was larger than 10 MHz.4. PCF for refractive index measurements were disgned to measure an environmental factor, which could induce a corrosion happening in the steel-reinforced concrete structures. The PCF was designed in two configurations:optical fiber coupler and a long period grating (LPG) sensor, respectively. The coupler could approach a resolution of 3.33×10-5 RIU for measuring refractive index of liquids; while, the PCF in LPG confugrations bears a sensivity to 18173.97 nm/RIU. The numerical simulation and experimental results showed that the sensors combined a high sensitivity with a wide measuring range together.Based on both the labtorary study and the practical application, this dissertation demonstrated that a low coherent fiber-optic strain sensor could be used to monitor the corrosion progress in the steel-reinforced concrete structures and proposed a corrosion mechanism:the interface of the steel with concrete played a crucial role in corrosion development. The author also made efforts on the dynamic strain induced by a crack happened in a structure and the corrosion environmental factor.