Preliminary Study On Non-Destructive Inspection Of Early-Stage Rebar Corrosion Using Polarimetric Ground Penetrating Radar

With the development of country infrastructure construction,the durability problem of building structure has gradually become serious.Rebar corrosion is the main reason for the deterioration of the durability of concrete structures.The corroded rebar in reinforced concrete structures causes delamination,spalling,and cracking,which affects the life cycle of the structure and also causes a lot of economic losses.Therefore,its detection and evaluation are a key technology for infrastructure health diagnosis.However,the current rebar corrosion detection is mostly based on electrochemical methods,which are limited by damage,susceptibility to external interference,poor adaptability to harsh environments,and low detection efficiency,only a rough estimate of the degree of steel corrosion can be achieved,so it is difficult to achieve early corrosion detection.This paper proposes a new method for non-destructive testing and inspection of rebar corrosion by polarimetric ground penetrating radar（GPR）,which can measure radar signal reflected from the embedded rebar at different corrosion stages recorded in four polarimetric channels.First,the finite element method is used to simulate the corrosion form of rebar and the law of concrete crack propagation under uniform corrosion conditions,and the corrosion damage model diagrams at different stages are obtained.Furthermore,image matching technology is used to give the material corresponding dielectric constant and conductivity values according to the color attributes to establish electromagnetic simulation models of reinforced concrete at different corrosion stages.The electromagnetic echo signals of polarization channels such as HH,VV,and VH at different corrosion stages are obtained through electromagnetic simulation by the finite difference time domain method,and the electromagnetic scattering mechanism and time-varying law of rebar during the corrosion process are studied.Secondly,optimize the design of the radar antenna for corrosion detection,and complete the establishment of the polarimetric GPR system.The radar system adopts a hybrid polarization data acquisition mode,that is,a circular polarization antenna transmits electromagnetic signals,and a linear polarization antenna receives electromagnetic signals,avoiding repeated switching of the antenna switch in acquisition full-polarization data and improving the efficiency of data acquisition.In order to obtain the true polarization scattering characteristics,an indoor polarization calibration experiment was carried out using the grid trihedral angle as the calibration part to eliminate the imbalance and crosstalk of the radar polarization channel.Then carry out radar accelerated corrosion of reinforced monitoring tests.The concrete test block was half immersed in sodium chloride solution,and the corrosion of rebar was accelerated by applying current.At the same time,the polarimetric GPR was used to collect radar data during the accelerated corrosion process,get an electromagnetic scattering of rebar with different degrees of corrosion.After preprocessing the radar signals of different polarization channels,the radar profile before and after the corrosion and the relative reflection amplitude values of the rebars are analyzed.The reverse time migration imaging algorithm is used to locate and image the corroded rebar,and the polarization scattering characteristics of the rebar at different times are extracted from the migrated multi-polarization channel scattering matrix with the help of the H-Alpha polarization decomposition.Finally,this study proposes a qualitative assessment method of steel corrosion degree based on the relative amplitude ratio of polarimetric GPR,extracts early-stage rebar corrosion features through polarization decomposition,and establishes early corrosion evaluation criteria.Integrating migration imaging and polarization decomposition to realize color-coded imaging of corroded rebar is conducive to the identification of corroded areas.The research results lay the foundation for in-situ detection and evaluation of rebar corrosion at an early stage,promote the development of health diagnosis technology for reinforced concrete structures,and ensure the safety of important infrastructures in service.