Local Reinforcement Corrosion Monitoring Of RC Structures Based On Fractal Characteristics Of Piezoelectric Guided Waves

The phenomenon of steel corrosion can be commonly found in offshore reinforced concrete(RC)structures due to high humidity and high chloride content,how to monitor the early production and development as well as entire destructive process of steel corrosion is of great significance.However,the steel corrosion in RC structures has the characteristics of locality and development along the interface between concrete and rebar,bringing great challenges to the evaluation of steel corrosion degree(especially the corrosion area)and positioning.In this paper,theoretical analysis,numerical calculation and experiment are used to study the local steel corrosion monitoring and evaluation technologies of RC structures,aiming at proposing a monitoring and evaluation method of local steel corrosion based on piezoelectric ultrasonic guided waves.The research contents are described as following:(1)Establish a steel corrosion monitoring system based on piezoceramic guided waves and investigate the reasonable selection method for guided wave excitation and reception and corresponding devices.Piezoceramic element(Lead Zirconate Titanate,PZT)are used as primary elements for actuating and receiving guided waves,and a monitoring system is formed by combining the PZT elements and corresponding setup.(2)Establish dispersion equations of guided waves for a layered structure of rebar and surrounding concrete,further analyze the frequency dispersion and multimodal characteristics of the guided waves with different frequencies in the given waveguide,obtain dispersion curves of ultrasonic guided waves under different corrosion conditions,and investigate the change laws of the dispersion curves.(3)Propose an evaluation index of steel corrosion and establish a corresponding evaluation system based on the echo energy value and fractal dimension characteristic value of sensing signals containing corrosion damage information.The proposed method can increase the interfacial damage identification precision.(4)Verify the effectiveness of the proposed dispersion curves and algorithm by the accelerated corrosion test and damage identification test based on guided wave motion method.The correctness of the dispersion curves are experimentally validated,then time of flight(TOF)method is used for positioning the damage,and the damage level is evaluated by the proposed damage index.The research results show that:(1)The obtained frequency dispersion curves and basic features of guided waves in RC structures under different corrosion conditions are correct.The extracted energy and fractal characteristics of the sensing signals are closely related to the corrosion length and thickness and the fitting relationship between the corrosion degree(length and thickness)and the basic characteristics of the sensing signal is established.(2)Steel corrosion has certain influences on both the energy and fractal characteristics of echo signal of longitudinal ultrasonic guided waves.The influences have been validated by the proposed damage index and damage evaluation system based on the energy and fractal characteristics of sensing signals.The larger the corrosion length and the thicker the corrosion layer,the smaller the energy value of echo signal and the smaller the fractal characteristic value,and the larger the corrosion damage index value proposed.