The Long-Period Fiber Grating Sensing Technology For Corrosion Monitoring Of Underground Structures

In recent years,the corrosion of steel reinforced concrete has become one of the most concerned issues in durability.Due to the various geotechnical environment arounding underground reinforced concrete(RC)structures,The corrosion problem is more prominent compared with civil buildings,which threatens the durability and safety of most buildings and structures underground directly.The electronic sensor is the common technique used for corrosion monitoring of steel bars,which is for the post-corrosion stage of reinforced concrete structures.Also the traditional sensors is not suitbale for a long-term monitoring,due to the underground corrosive environment.In this paper,we proposed long-period fiber grating sensors as for steel bars corrosion monitoring,because of the excellent performance in corrosion resistance,sensitivity and distributing purpose.The simulations of transmission spectrum and model analysis are carried out using simulation software,and the parameters of the grating and the photonic crystal structure,which displaying the best sensing performance,are obtained.The main research results are as follows:(1)According to the mechanism of concrete carbonation and chloride erosion,the target medium and critical value of long-period fiber grating sensor for steel bar corrosion monitoring were obtained.For concrete carbonation,pH = 9 and pH = 15 pH = 9 and pH = 15 are the values of the complete destruction and initiation of the passivation film,respectively;for chloride ion erosion,the critical value of Cl—/OH— is given by the fitting function.(2)Simulation and analysis of LPG spectra characteristics were carried out by software OptiGrating.The temperature,strain and refractive index of LPG were calculated respectively.The inscription parameters of LPG were obtained by simulating the sensing of LPG for CO2.The LPG humidity sensor is experimented as well,and the error between the experimental data and the simulated data is negligible.(3)Using the optical finite element software Lumerical MODE Solutions to analyze the photonic crystal fiber long-period grating,we find the geometric parameters of IGPCF which shows the highest sensitivity,the power overlap and sensitivity coefficient of model H5-2.8-7.0 are over 60 times higher than the fundamental core mode and much more accurate corrosion information can be acquired.