Experimental Study On Monitoring Of Steel Corrosion Cracking In Concrete Based On Distributed Optical Fiber

Reinforced concrete is the most widely used composite material in the field of civil engineering in the world.Reinforced concrete bridge structures are often in a complex corrosive environment.Whether it is concrete or steel bar corrosion will deteriorate the performance of the steel-concrete structure.In the increasingly severe corrosion of steel bars,not only the bonding force of the steel bars themselves is continuously reduced,but also the overall stability of the reinforced concrete members is continuously reduced.When the corrosion of steel bars is severe,it can cause sudden structural damage and cause major safety accidents,causing casualties and economic losses.Therefore,it is necessary to monitor the corrosion of the steel bars in the concrete to understand the service status of the structure in time.In this paper,OFDR(Optical Frequency Domain Reflectometer)distributed optical fiber is used to monitor the corrosion of steel bars in concrete under chloride ion environment.The research content mainly includes the following aspects:Summarize the existing steel corrosion monitoring technology,analyze the advantages and disadvantages of electrochemical methods and non-electrochemical methods,whether it is electrochemical method or non-electrochemical method,it is difficult to achieve the corrosion of steel corrosion in time and space Continuous monitoring.Electrochemical analysis reveals the mechanism of steel bar corrosion,and discusses the causes of uneven steel bar corrosion and its effect on the structure.Combining Faraday’s law and elastic mechanics theory,the theoretical calculation formulas of rust rate,rust expansion force and strain are derived.The principle of OFDR distributed optical fiber sensing is analyzed,and an OFDR distributed optical fiber corrosion sensor is designed based on this principle.The transmission coefficient of the distributed optical fiber during spiral winding is determined through the sensor calibration test for different optical fiber winding pitch.In this paper,18 concrete(mortar)cylindrical specimens with different diameter steel bars and different protective layer thicknesses were fabricated.The optical fiber was wound on the concrete(mortar)column specimens with different pitch spirals for accelerated corrosion tests.In the accelerated corrosion test,the strain generated on the surface of the concrete(mortar)caused by the corrosion of the steel bar is obtained through the OFDR sensing fiber.The feasibility of OFDR distributed optical fiber monitoring of steel corrosion cracking was verified through experimental research.The test results show that the corrosion of steel bars leads to cracking of the protective layer of concrete(mortar),which in turn causes peaks in the OFDR strain spectrum.As the corrosion loss of the steel bar increases,the peak strain increases.Under the same corrosion condition,the larger the steel bar diameter(or the smaller the protective layer),the higher the crack cracking speed and the larger the crack width.The smaller the winding distance of the optical fiber,the more data is monitored,and the lower the blind spot,but the integrity in time and space is reduced.The spiral-wound optical fiber is based on OFDR technology to monitor the cracking of concrete(mortar)caused by steel corrosion.Reflecting the stage of the concrete surface just cracking,the overall monitoring effect of the test piece with a diameter of less than 5cm when the fiber winding distance is appropriate is better.