| As the core prestressed component of bridge structure,rebar will be corroded to varying degrees with the increase of service life,which leads to the decrease of bearing capacity of bridge structure.The rebar is usually wrapped in concrete,and its damage cannot be directly identified.Therefore,selecting a reasonable and effective non-destructive testing method to identify the corrosion damage in the in-service rebar can provide a guarantee for the safety of the bridge.In this thesis,theoretical analysis,finite element simulation and experimental research are combined to carry out the ultrasonic identification of the corrosion degree of bare rebar and the corrosion stage of rebar in concrete,which provides technical support for the non-destructive detection of corrosion damage in rebar.The following aspects are mainly studied :(1)Ultrasonic simulation of corroded rebar was carried out.The finite element models of different corrosion degrees were established by using finite element software.The guided wave waveform signals were extracted and analyzed,and the variation of signal amplitude and travel time with corrosion degree was obtained.Based on this,a method for identifying the corrosion of rebar based on the change of damage index of guided wave characteristics is proposed.The variation of each damage index value with the degree of corrosion is analyzed,and the influence of different frequencies on its variation is discussed.(2)The ultrasonic test of corroded rebar was carried out.The guided wave propagation test of corroded rebar was carried out by using a single sinusoidal signal with a frequency of 50 k Hz,80 k Hz,100 k Hz and a broadband signal with a frequency of 0.1k Hz-1MHz as the excitation source,and then the variation of each damage index with the corrosion degree of rebar was analyzed.The spectrum amplitude difference,which is most sensitive to the corrosion degree of rebar,is selected to analyze the influence of frequency size and frequency type on its variation law.The damage indexes at different single frequencies have similar rules,and the variation of the spectral amplitude difference with the corrosion degree in the test is consistent with the finite element.However,under the broadband excitation signal,the sensitivity of the spectral amplitude difference to the corrosion degree is weaker than that of the single frequency excitation signal.(3)The propagation characteristics of guided wave of rebar in concrete were studied.Through the numerical simulation of the guided wave of the rebar in concrete,the variation law of the time domain characteristics of the rebar in concrete is analyzed.Aiming at the influence of different outsourcing thickness on guided wave propagation,the variation curve of signal time domain amplitude with outsourcing thickness is established.According to the simulation and test results of the guided wave of the rebar in concrete,the maximum group velocity of the guided wave in the rebar in concrete is calculated by extracting the arrival time of the first wave packet in the time domain feature,and it is compared with the theoretical dispersion curve to verify the correctness of the finite element simulation and the effectiveness of the test.(4)The ultrasonic test of rebar in concrete after corrosion was carried out.Ultrasonic guided waves were used to measure the rebar in concrete at different corrosion times,and the variation of time domain and spectral characteristics of guided wave signals with corrosion time was analyzed.By establishing the change curve of damage index and corrosion time that characterizes the change of time domain characteristics,the results show that only the spectral amplitude difference is effective for identifying the corrosion stage of rebar in concrete,and the critical time of different corrosion stages can be obtained.(5)The test results were verified based on the corrosion model of rebar in concrete.By expounding the existing corrosion cracking model of reinforced concrete,the initial cracking time inside the concrete and the cracking time on the surface of the protective layer are calculated.The theoretical cracking time is compared with the critical time(cracking time test value)of different corrosion stages obtained from the ultrasonic test results.The results show that there is a good correspondence between the two,which indicates the effectiveness of the ultrasonic test results. |
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