# Acoustic Emission Wave Velocity Measurement of Asphalt Mixture by Arbitrary Wave Method
###### Abstract  
The wave velocity of acoustic emission (AE) can reflect the properties of materials, the types of AE sources and the propagation characteristics of AE in materials. At the same time, the wave velocity of AE is also an important parameter in source location calculation by the time-difference method. In this paper, a new AE wave velocity measurement method, the arbitrary wave (AW) method, is proposed and designed to measure the AE wave velocity of an asphalt mixture. This method is compared with the pencil lead break (PLB) method and the automatic sensor test (AST) method. Through comparison and analysis, as a new wave velocity measurement method of AE, the AW method shows the following advantages: A continuous AE signal with small attenuation, no crosstalk and a fixed waveform can be obtained by the AW method, which is more advantageous to distinguish the first arrival time of the acoustic wave and calculate the wave velocity of AE more accurately; the AE signal measured by the AW method has the characteristics of a high frequency and large amplitude, which is easy to distinguish from the noise signal with the characteristics of a low frequency and small amplitude; and the dispersion of the AE wave velocity measured by the AW method is smaller, which is more suitable for the measurement of the AE wave velocity of an asphalt mixture. s +
Footnote †: journal:used to analyze and evaluate the low-temperature performance of field asphalt pavement samples [7; 8]. [PERSON] et al. [9] studied the dynamic evolution process of low-temperature cracking of asphalt mixtures by using AE technology and the discrete element simulation method. In terms of fatigue damage, AE technology was used to monitor the self-healing ability of asphalt mixtures [10]. At the same time, AE b-value [11], k-clustering [12] and fractal theory [13] were used to explore the fatigue damage process and failure characteristics of asphalt mixture. [PERSON] et al. [14] established a correlation between the fracture damage of an asphalt mixture under a freeze-thaw cycle and its AE signal. In terms of aging, AE technology was used to evaluate the road performance of asphalt mixtures with different aging levels [15; 16]. Recently, the characteristics of moisture damage of asphalt mixtures have attracted researchers' attention. AE technology was used to monitor the internal damage, crack type, fracture mode and other characteristics of a steel slag permeable asphalt mixture under compression and splitting [17; 18; 19; 20]. Acoustic waves are usually collected by AE or an ultrasonic instrument. Under the action of external factors, the AE signal will be generated and propagated inside the material. The AE wave velocity can be obtained by calculating the distance between acoustic sensors and the time difference of acoustic waves received by the sensors. Due to the differences in composition, homogeneity and property of materials, different materials have their own AE wave velocity range. [PERSON] et al. [21; 22] monitored the propagation characteristics (wave velocity and attenuation) of AE in asphalt mixture. Due to the complex composition of asphalt mixtures, the transmission path and speed of AE in asphalt mixtures are very different. AE wave velocity can not only reflect the internal damage and structural characteristics of the material, but also plays an important role in the calculation of a source location. However, the current research on the AE wave velocity mainly focuses on rock. The wave velocity in different directions is anisotropic, which can be used to study the damage development and mechanism of marble [23]. In addition, the existence of natural fractures and bedding reduced the wave velocity and mechanical strength of the rock [24]. Under different temperatures and water environments, there is a correlation between the wave velocity and hydraulic properties (permeability and diffusivity) [25; 26].