A Long-term Monitoring Method Of Spiral Distributed Fiber Optics For Corrosion In Pre-stressed Anchorage Systems

Pre-stressed anchor cable,as an efficient,convenient,and economical underground anchoring component,is widely used in engineering in China.With the increasing service life,corrosion has become the main reason for damaging the performance of pre-stressed anchor cables,and the number of cases of failure of pre-stressed anchor cables is gradually increasing.Currently,there are two types of corrosion monitoring methods for pre-stressed anchor cables: electrochemical and non-electrochemical methods.However,these methods are susceptible to interference and have poor durability.The distributed optical fiber sensing method based on Raman scattering has the advantages of good durability,strong anti-interference performance,and long-term stability.Therefore,this paper proposes a long-term corrosion monitoring method based on spiral distributed optical fiber and designs a sensor for measuring corrosion of pre-stressed anchor cables.The corrosion monitoring sensor has the characteristics of adjustable range and sensitivity,the ability to locate cracks,high measurement accuracy,and accurate and reliable monitoring results.The main work is as follows:(1)The design of spiral distributed optical fiber sensor.Based on the principle of Raman scattering distributed optical fiber and Bragg grating fiber optic grating sensing,a spiral distributed optical fiber corrosion sensor is designed.The mathematical expression of fiber optic strain and mass loss rate is derived by Faraday’s law,and the influence of fiber optic winding angle parameters on the sensor performance is discussed.The research results show that the fiber optic winding angle should be between 30° and 60° to balance the sensitivity,corrosion range and measurement length of the sensor.(2)COMSOL finite element simulation of spiral distributed optical fiber sensor.Combined with COMSOL finite element software,a simulated model of reinforced concrete components is established to analyze the variation of steel bar layer thickness with corrosion time and the influence of fiber optic winding angle on its sensing performance.In addition,the corrosion law and cracking mode of reinforced concrete components are studied.The research results show that cracks in reinforced concrete components first appear on the upper and lower surfaces,and then extend to the side of the component;the thickness of the corrosion layer along the circumferential direction of the steel bar appears uneven;the optimal fiber optic winding angle is between 30° and 60°to balance the sensitivity and corrosion range of the sensor.(3)Corrosion experiment of reinforcing bars and pre-stressed anchors based on spiral distributed sensors.By constructing an electrochemical accelerated corrosion test platform,the mathematical model of corrosion-fiber optic strain of spiral distributed optical fiber sensors before concrete cracking was studied,and crack localization was also investigated.The research results show that the sensor can locate the crack with a maximum positioning error of 2cm,and the smaller the fiber optic winding angle,the higher the sensitivity to the crack.A mathematical model of the first-stage fiber optic strain and corrosion mass loss rate during corrosion is established.Its model is a linear model,which is ε=72ρ+4.78、ε=109ρ-4.98、ε=182ρ-11.77 andε=166ρ+2.23,respectively.The mathematical model of fiber optic strain and corrosion mass loss rate during corrosion of pre-stressed anchors is also a linear model,and the sensitivity of fiber optic strain is 130με/1%.