The Application Of Distributed Optical Fiber Sensing System To Crack Detecting In Concrete Bridges

Cracks are taken as one of the most common diseases of concrete bridges, which are also mostly worthy of being attached great importance to. The occurrence and development of cracks, relating closely to the mechanical behaviors and operational state of bridge structures, embody the health of them directly. As important ways and aspects of both damage-identifying and health-diagnostics of bridges, Capturing and supervising of cracks provide significant base to bridge-structure-state monitoring and evaluation, and at the meantime guide the behavior of bridge maintenance and decision-making for management. In 21'st century, bridge engineering is heading for a form with greater span and more complicated structure, and meanwhile, intelligentization is becoming a rising trend in this field. In this background, optical fiber sensing skill is more and more applied to bridge health diagnostics, and has become the frontier of research in this field.This research firstly introduces the development and the status of the application of optical fiber sensors to structural health diagnostics. Then based on this, a method in which distributed optical fiber sensing net is introduced to identify cracks in concrete bridges is put forward in view of the practical application to bridge health diagnostics.In regard to the method itself, the basic concepts and theory of power loss due to micro-bending in optical fibers and the skill of Optical Time Domain Reflectometry (OTDR) are dissertated, meanwhile the mechanism that micro-cracks cause micro-curvature in optical fibers is analyzed, based on which, taking into account the applicability in practical engineering course, a few forms of surface-stuck distributed optical fiber sensing net, the corresponding fitting techniques and the mode of the whole sensing system are brought forward.To further study the theory of optical fiber crack-sensing and facilitate thecombination progress of the theory with the engineering application, a great deal ofexperimentation is carried out within the momentous research project—"Intelligentized Bridge" ,which is supported by the National Railway Ministry, and plenty of valuable results are obtained:For the sake of validating the ability of the system to detect cracks and comparing the sensitivity of different types of optical fibers, differently shaped sensing nets are attached to different part of 3 simply supported PC beam models separately, and breaking tests are carried out. The result indicate that the system established is able to take reaction to cracks with naked optical fibers as the sensor, which are, for its high sensitivity, fit for the net to detect micro cracks.In order to explore the basic properties of the surface-stuck distributed optical fiber sensing net in sensing cracks in concrete structures, sensing nets with different angles between cracks and optical fibers are applied to 9 simply supported RC test module slabs, and breaking tests are carried out. With the test data coming out, the crack-sensing-characteristics of the systems with different testing-light-wavelengths, different angles between cracks and optical fibers and different thickness of the adhesive colophony are studied. The principles, in which parameters of the sensing system are chosen, are put forward, and practical values based on which semi-experiential relational functions and curves are gained from the test data, are recommended.Aiming at the application to actual bridge structures, optical fiber crack-sensing net are designed and applied to a 3-span PC continuous beam module bridge, according to the characteristics of the mechanical behavior and the target of diagnostics. Multi-loading-point test is carried out on the module, and the result turns out: that the optical fiber crack-sensing system established in this research is capable of in-timely detecting the occurring of tiny cracks in actual PC bridge structures during the initial stage, and it also comes up with the exact location where the cracks are taken place, while identifying the degree to which the cracks are. With the cracks developing further, the system captures all the cause.Theoretical and experimental research indicate that, with its unique advantages, this type of optical fiber crack-sensing system meets the demand of health diagnostics in concrete bridge structures and is worthy of further study and application to actual structures.