Study On Bridges Safety Monitoring Technology Based On OTDR

The degradation of concrete structure's safety is always accompanied by the formation of cracks. Hence, the health condition of a concrete bridge can be assessed by monitoring its internal cracks. Optical fiber can both be a transmitter and a sensor, which makes it easy to form fiber optic sensing nets to accomplish crack distributed monitoring, especially where the crack locations are not known in a-priori. The present investigation is aim to develop a novel fiber optic sensor which can real-time detect and subsequent monitor cracks accurately and comprehensively so as to keep the bridge in safe condition. By studying how cracks occur, develop and distribute in concrete, analyzing the transmission and loss characteristics of optical fibers, a distributed fiber concrete crack sensing system based on back-Rayleigh scattering theory and OTDR technology is given. In order to validate the feasibility of the system in project use, a crack deformer which can simulate crack's development is designed and with it, an experimental crack monitor system is composed. Test result shows that the sensing characteristics of the system can satisfy practical project use the most when the angle between fiber and crack is 60°, which provides a reliable basis for the diagonal layout design of distributed optical fiber. Test result also indicates that test waves with different wavelength will have different influences on the sensing characteristics of the system, on the basic of which, a combined test method that uses two different test waves is presented. The proposed method can collect the information of cracks'position, width and direction with only one route of sensing fiber. By analyzing test data, a fitting Semi-empirical formula which can properly describe the relation between optical power loss and crack width is finally obtained. The proposed Exponential function Semi-empirical formula is proved to be more accurate than power function.