Corrosion Monitoring Of Rebar Embedded In Concrete Using Guided Ultrasonic Waves

The corrosion of steel bar will lead to the deterioration of the bearing capacity of concrete structures.The steel bars were embedded in concrete,thus their corrosion is difficult to be detected timely,which may result in structural failure,casualties and huge economic losses.The guided ultrasonic wave(GUW)based structural health monitoring methods can detect a large area where the wave propagating using only one probe point,which is especially suitable for the monitoring the minor structural damage in a wide range,and the structure covered by the other materials.GUW based methods have lots of advantages,such as wide monitoring range,high sensitivity to small damage,simple equipment and low cost.Therefore,it is an effective mean to monitor the corrosion of steel bar in concrete.In this work,the propagation characteristics of guided waves and monitoring methods of corrosion in steel bar not or embedded in concrete are studied mainly.The detailed contents are as follows:(1)For the steel bar not embedded in concrete,the time of flight(TOF)of the main packet of the received signal was chosen as the monitoring indicator of uniform corrosion.To improve the sensitivity of the TOF based method,the optimum TOF monitoring frequency is determined through theory analysis and numerical calculation.The proposed method is verified with the electrochemical corrosion experiments,and the selection principle of monitoring frequency is determined for uniform corrosion.(2)The local damage of the steel bar caused the reflection and scattering of guided waves,which lead to the change of guided waveform.Therefore,the amplitude of signal,RMS(Root Mean Square)of first wave packet,and the total energy ratio of first wave packet are chosen as the monitoring indicators.Among of them,the total energy ratio of first wave packet is reference free,thus it is more suitable for the practical applications.Moreover,combining the signal characteristics in time and frequency domains,the uniform and local corrosions of the steel bar can be distinguished.(3)For the steel bars embedded in concretes,the relationship between the attenuation characteristics and the displacement wave structure of guided waves is determined by analyzing a two-layer composite model.The low attenuation wave modes in high frequency range are obtained in the reinforced concrete.(4)The electrochemical accelerating corrosion was carried out for the reinforced concrete.The whole corrosion process can be divided into 3 stages.It was found that at the last stage of the corrosion,the steel bar and the concrete debonded completely,and the physical state of the embedded bar in the concrete was similar to that of the steel bar without covering the concrete.In the corrosion process,the amplitude,TOF and frequency spectrum of the guided wave signal changed significantly,and corresponded to the 3 stages of concrete corrosion.Therefore,the early stage of corrosion for the reinforced concrete can be identified according to the characteristics of monitoring signals.