| Under the effects of long-term natural environment and vehicle load,the asphalt pavement can produce complex damage and failure modes,which would reduce service performance and service life of the pavement.Therefore,it is of great importance to study the cracking characteristics and failure modes of asphalt pavement in order to improve pavement performance and optimize design of pavement structure.In this research,acoustic emission(AE)technology was used to evaluate the acoustic wave propagation,acoustic wave velocity,semi-circular bending(SCB),freeze-thaw splitting,and three-point bending of asphalt mixtures.On this basis,the damage,cracking and failure characteristics of asphalt mixtures with different gradations and specimen shapes under different test conditions were analyzed by using various monitoring and evaluation methods such as high-speed camera recording method,digital image correlation(DIC)method and acoustic emission simulation method.The main research work of this paper is as follows:(1)Based on the "active excitation function" and "piezoelectric effect" of acoustic emission sensors,the acoustic wave propagation of three asphalt pavement materials(coarse aggregate,asphalt and asphalt mixture)was tested by automatic sensing test(AST)method.Based on the test results,the time domain and frequency domain characteristics of acoustic waves of asphalt pavement materials at different temperatures were analyzed,and the mechanism of acoustic wave propagation was discussed.(2)A wave velocity measurement method based on arbitrary wave(AW)method was proposed and designed.By comparing and analyzing the initial wave velocity discreteness of each wave velocity measurement method and the timefrequency domain characteristics of the corresponding acoustic emission and noise signals,it is concluded that the AW method,as an acoustic wave velocity measurement method,has the advantages of small attenuation and discreteness,no signal crosstalk,fixed and continuous waveform,and easy to distinguish from noise signals.(3)The semi-circular bending(SCB)method was used for loading test of prefabricated crack asphalt mixture with different gradations at different loading rates.Acoustic emission signals were collected and high-speed cameras were taken during the whole process.Crack propagation characteristics of asphalt mixture were analyzed based on acoustic emission parameters and high-speed images.The acoustic emission peak frequency distribution and peak frequency band characteristics of asphalt mixture with different loading rates and different nominal maximum particle sizes were studied.The failure modes of prefabricated crack asphalt mixture were identified and classified by RA-AF correlation analysis method based on acoustic emission parameters.(4)The RFPA acoustic emission simulation software was used to establish the two-dimensional model of semi-circular asphalt mixture with five different prefabricated crack angles.Damage distribution zone(DDZ)was defined based on acoustic emission event distribution to characterize the damage and microcrack generation of asphalt mixture.The characteristics of acoustic emission parameters of crack propagation in asphalt mixture with different prefabricated crack angles were analyzed.Meanwhile,the shape,angle and radius of fracture core zone(FCZ),fracture process zone(FPZ)and damage distribution zone(DDZ)of asphalt mixture with different prefabricated crack angles were further studied.(5)Multiple freeze-thaw cycle tests were used for asphalt mixtures with different gradations,and splitting loading and acoustic emission monitoring were carried out for asphalt mixtures before and after freeze-thaw cycle.The splitting tensile strength(RT1)and freeze-thaw splitting tensile strength ratio(TSR)of asphalt mixtures with different gradations were studied.Based on acoustic emission b value and historic index(HI value),the water stability and tensile strength characteristics of asphalt mixture under different gradations and different number of freezing-thawing cycles were analyzed.Among them,the changing trend of acoustic emission b value can be used to characterize the internal damage,cracking and failure characteristics of asphalt mixture before and after freeze-thaw cycle.Acoustic emission HI value can be used as an important index to measure the water damage resistance of asphalt mixture,and the characteristic key points extracted from the change trend of acoustic emission HI value can be used for the characterization of splitting crack transition and prediction of splitting tensile strength limit of asphalt mixture.(6)The three-point bending test,DIC method and acoustic emission method were used to test the asphalt mixture beams with different gradation under different loading rates.At the same time,a three-dimensional finite element model of asphalt mixture beam was established to simulate the acoustic emission source location and damage distribution.Based on DIC results,the full-field strain distribution and the location and number of principal strain points during the loading process of asphalt mixture beams were analyzed.The MATLAB software was used for wavelet packet decomposition and energy distribution calculation of acoustic emission signals at characteristic points in the loading process of asphalt mixture,so as to obtain the dominant frequency distribution range of acoustic emission at each stage of asphalt mixture bending fracture.Based on the results of simulated acoustic emission source location,the damage distribution,main fracture development trend and fracture surface of asphalt mixture beam in the whole bending fracture process were obtained.Finally,the advantages and limitations of various methods in the evaluation of the bending fracture characteristics of asphalt mixture beams were compared and analyzed.The results of this study are helpful for practitioners to better understand the damage,microcrack development and macroscopic cracking process of asphalt mixture,and also provide new research ideas and analysis methods for the road performance evaluation of asphalt mixtures. |
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