沥青混凝土超声波检测的衰减特征与影响因素研究

为有效获得沥青混凝土的超声波检测结果，基于压电换能器模拟和沥青砂浆Burgers黏弹性模型，建立三相介质的沥青混凝土有限元模型，在20~100 kHz频段范围内，比较9种入射频率下沥青混凝土中超声波细观的衰减特征及宏观的振幅变化，分析3种集料粒径（13.2，19.0，26.5 mm）及不同温度（-10℃~60℃）对沥青混凝土中超声波衰减的影响；针对数值模拟结果，使用超声横波成像仪MIRA-A1040进行室内试验验证，研究9种入射频率下实测振幅和断面重构图的衰减规律。研究结果表明：当温度为20℃时，在频段20~100 kHz内，入射频率每升高10 kHz，反射波振幅线性降低约3 dB；随着入射频率的升高，沥青砂浆中声压等值线的规则性、连续性及试件断面重构图的辨识度均大致呈线性降低趋势；当入射频率和温度不变时，集料粒径引起的散射衰减会引起振幅小幅度的降低；当集料粒径不变时，影响振幅降低速率的主要因素是温度，本质为沥青砂浆黏弹性引起的吸收衰减；以入射频率为30 kHz的沥青混凝土AC-13为例，温度每升高10℃，振幅约线性降低5 dB；2种衰减综合决定了反射波振幅的数值；沥青混凝土检测的入射频率不宜超过60 kHz，温度不宜超过40℃；用于沥青路面面层的检测时，建议入射频率范围为20~60 kHz；较低的入射频率与温度有利于沥青混凝土的超声波检测。

Attenuation Characteristics and Influencing Factors of Ultrasonic Testing of Asphalt Concrete

To effectively obtain the ultrasonic testing results of asphalt concrete, a three-phase-media asphalt concrete mesoscopic model was established based on a simulation of piezoelectric transducers and the Burgers viscoelastic model. In the frequency range of 20-100 kHz, the microscopic characteristics of ultrasonic attenuation and the macroscopic amplitude changes in asphalt concrete were compared at nine incident frequencies. Then, the influences of three aggregate sizes (13.2 mm, 19.0 mm, and 26.5 mm) and different temperatures (-10℃-60℃) on ultrasonic attenuation were analyzed. For the numeral simulation, the ultrasonic transverse-wave imaging equipment MIRA-A1040 was utilized for laboratory testing and verification; then, the changes in the measured amplitude and the reconstructed sections were studied. The research results show that at a temperature of 20℃, the amplitude decreases linearly by approximately 3 dB for every 10 kHz increase in incident frequency within the frequency range of 20-100 kHz. As the incident frequency increases, the regularity and continuity of the acoustic pressure isolines in the asphalt mortar, and the distinguishing degree of the reconstructed sections all decrease linearly. For a constant incident frequency and temperature, the scattering attenuation caused by aggregate size results in a small reduction in amplitude. When the aggregate size is unchanged, the reduction rate of the amplitude is mainly affected by temperature, which essentially means the absorption attenuation caused by the viscoelastic asphalt mortar. Taking asphalt concrete AC-13 with an incident frequency of 30 kHz as an example, the amplitude decreases linearly by approximately 5 dB for every 10℃ increase in temperature. These two types of attenuation determine the reflected wave amplitude together. The incident frequency and temperature should not exceed 60 kHz and 40℃, respectively, for asphalt concrete. The recommended incident frequency range for detecting the surface layer in asphalt pavement is 20-60 kHz. The low incident frequency and temperature are favorable for ultrasonic testing of asphalt concrete.