Determination of the softening point and temperature interval of asphalt mixture for the healing process based on the contactless nonlinear ultrasonic method

Abstract

The softening point and gradient healing have significant engineering significance for the self-healing of asphalt mixtures. This study introduces a novel approach utilizing a contactless ultrasonic system in conjunction with the Second Harmonic Generation (SHG) technique to determine the softening point and monitor the gradient healing process of asphalt mixtures. Three distinct fracture patterns resulting from temperature variations are accurately characterized through simulation using the Discrete Element Method (DEM). The ultrasonic nonlinear parameter β, reflecting different crack interfaces, proves to be a highly effective alternative for characterizing asphalt fractures, outperforming traditional methods such as bearing force and wave velocity. In experimental analysis, variations in the healing index highlight the significant influence of asphalt mixture fracture patterns on healing efficiency. The observed decrease in ultrasound amplitude signifies the transition of the asphalt binder from a solid to a liquefied state, accompanied by an increase in the nonlinear parameter indicating enhanced plasticity. The softening point is precisely determined when the Rayleigh surface wave disappears. The cumulative healing process is visualized through the continuous decrease in the nonlinear parameter. To further enhance the understanding of the asphalt self-healing process, the internal temperature gradient field is characterized using the proposed energy difference method. A correlation between the nonlinear parameter and the depth of the temperature interval is established, providing a non-destructive method for evaluating the distribution of the temperature interval. This innovative approach significantly accelerates the development of non-destructive testing in the field of asphalt self-healing.