Piezoelectric Transducer And Ultrasonic Technology For Monitoring Corrosion Of Reinforced Concrete

Nowadays,reinforced concrete structures are widely used in the field of architectural engineering.As main composition of infrastructure,reinforced concrete structures should safely endure the designed working condition within the duration of service,meet various index of the required design,and have sufficient durability.However,,due to effects from self defect of the structure or the severe environment,these civil engineering often can’t achieve the normal life in their service period.There are many examples of structural failure due to corrosion of steel bars.The safety and reliability of reinforced concrete structure directly threaten the safety of people’s lives and property.Therefore,it is of great significance to conduct real-time online health monitoring for structures.In recent years,intelligent sensing system and data acquisition and analysis system are widely applied in the field of nondestructive monitoring.Piezoelectric transducer has many advantages,such as driving and sensing performance,good linearity,low energy consumption,low cost and flexible installation.Based on above background,piezoelectric transducer and ultrasonic technology are employedto monitor the corrosion of reinforced bar.The research content and conclusion are summarized as follows:(1)Based on guided wave theory,three propagation modes of ultrasonic guided wave in the steel bar were studied,that is,longitudinal mode(L),torsion mode(T),and bending mode(F).The relationship between frequency and modal velocity is calculated,and the dispersion phenomenon of guided waves is also analyzed.The results show that when the frequency is greater than 200 kHz,the mode of guided waves increases,and the mode of guided waves is the least at 80-150 kHz.(2)The structure of the piezoelectric transducer was designed,and annular piezoelectric transducer,1-3 piezoelectric composite transducer,and disk piezoelectric transducer were prepared.The test results show that the resonant frequency of the annular transducer in series is near 100 kHz,and its performance is stable,impedance value and the signal-to-noise ratio is large.In the modal wave velocity test,the deviation of L wave velocity and F wave velocity with theoretical value is only 1.2% and 1.3%.(3)The ultrasonic and mechanical monitoring of the mechanical damage and corrosion damage of the rebar were conducted.The results show that the amplitudes of the L-wave and the F-wave rapidly decrease when the damage depth increases by 1 mm for the mechanical damage of the single and two-point rebars.The amplitude of L and F wave echo decreases with the increase of the depth of damage.The echo amplitude of damage first increases and then decreases as the depth of damage increases.At two points of mechanical damage,the acoustic wave is affected by multiple points of damage resulting in increased reflection echoes,and time domain waveforms are more complex than single points of mechanical damage.(4)Corrosion damage test results show that under the condition of uniform corrosion,with the increase of corrosion rate,there is no reflection echo in the time domain waveform,and the amplitude of head wave reduces comparing with the original status.As the corrosion rate increases,the L wave speed decreases slightly,and the F wave speed decreases to 2800 m/s.Under single corrosion conditions,the amplitudes of L and F waves gradually decrease with the increase of corrosion time.When the corrosion reaches a certain level,corresponding reflection echoes appear and are affected by the diameter and depth of the corrosion pits.Spot corrosion has less effect on wave speed.The effects of single point of rust damage and two points of rust damage on acoustic waves are similar to those of mechanical damage.The difference is that the point of rust corrosion is relatively less.(5)Electrochemical workstations were used to test the impedance spectrum of reinforced concrete structures.The results show that the steel bars are in a passivated state,the double-layer transmission resistance increases,the impedance spectrum is a straight line in the low frequency region of,and a semicircle in the high frequency region.When the passivation film breaks,the diffusion circuit-related components appear in the effective circuit,and a 45° straight line appears in the low-frequency area.With continuous corrosion,the transfer resistance of charge decreases due to the concrete cracking,and the equivalent circuit components change,and two semicircular arcs appear.The monitoring results of transducer show that the amplitude of acoustic wave reduces by 10 times after the steel bar was wrapped by concrete.When the corrosion rate exceeds 5%,the influence of the corrosion layer on the acoustic wave is enhanced.A single point corrosion only appears when the corrosion reaches a certain level,and the time domain waveform of two-point corrosion is more complex than other types of damage,and there are more couplings phenomena aroudn the peak location.(6)Wavelet transform was used to transform the time domain waveforms into time-frequency domain spectra.The results show that the main energy envelope of uniform corrosion and single point corrosion is concentrated in the first wave of L,and the energy value of the back echo is small,and the main energy envelope is dispersed on the time axis in three parts and the reflected echo occupies most of the energy in the two points corrosion.According to the energy value,the first wave frequency range of uniform corrosion and single point corrosion is 80-200 kHz,and the first wave frequency range of two points is 80-300 kHz.