# Acoustic Emission Wave Velocity Measurement of Asphalt Mixture by Arbitrary Wave Method
## 6 Conclusions  
A new method, the AW method, was designed for measuring the AE wave velocity of asphalt mixtures, and the AE wave velocity of an asphalt mixture beam was measured by the AW method and traditional AE wave velocity test methods (the PLB method and the AST method), respectively. By comparing and analyzing the wave velocities measured by the three methods and the corresponding AE signals, the main conclusions are as follows.  
By comparing the initial AE wave velocity obtained by three wave velocity measurement methods, it can be concluded that the dispersion of the initial AE wave velocity (3100-3200 m/s) measured by the AW method is the smallest, which is more suitable for the AE wave velocity measurement of asphalt mixtures.  
Comparing the time-domain signals obtained by three wave velocity measurement methods, it can be concluded that the PLB method and the AST method obtain a burst AE signal, while the AW method obtains a continuous AE signal. Signal attenuation was in the order of PLB method > AST method > AW method. The AE waveforms of the PLB method are different at different lead-breaking positions, while the AE waveforms of the AST method and the AW method are the same at different excitation positions.  
For the PLB method and the AST method, the frequency spectrum of the noise signal presents the characteristics of a low frequency and a large amplitude, while the frequency spectrum of the AE signal presents the characteristics of a large frequency and a low amplitude. For the AW method, the frequency spectrum of the noise signal is characterized by a low frequency and a small amplitude, while the frequency spectrum of the AE signal is characterized by a high frequency and a large amplitude. The peak amplitude and frequency distribution range of the noise signal was in the order of PLB method > AST method > AW method. The frequency distribution range of the AE signal was in the order of AW method > PLB method > AST method.  
The AW method was first proposed and designed to measure the AE wave velocity of an asphalt mixture in this study. Through the wave velocity test and in comparison with the existing wave velocity measurement methods, it is concluded that the AW method is superior in wave velocity measurement of asphalt mixtures. This is a successful attempt. At the same time, this method also provides a new idea for wave velocity measurement of other heterogeneous composites. In future research, this method will be used to explore the AE wave velocity and propagation mechanism of different gradations and types of asphalt mixture. At the same time, this method will also be used in the calculation of AE source locations.  
Conceptualization, J.L. and L.W.; methodology, J.L.; validation, J.L., Z.Y. and W.W.; formal analysis, J.L.; investigation, J.L., H.L. and K.Z.; resources, J.L. and L.W.; data curation, J.L. and K.Z.; writing--original draft preparation, J.L. and H.L.; writing--review and editing, L.W., W.W. and Z.Y.; visualization, J.L.; supervision, L.W. and J.L.; project administration, L.W.; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Beijing major science and technology projects, grant number Z191100008019002. The authors declare no conflict of interest.