Characterization of optimal healing efficiency based on a new nonlinear ultrasonic method for the process of gradient heating healing in asphalt mixture

In this research, we proposed a new nonlinear ultrasonic method, second harmonic generation (SHG) technique, to investigate the gradient heating-healing properties and determine the optimal healing efficiency in asphalt mixture based on the principle of ultrasonic Rayleigh surface wave. The energy approach is used to characterize the released energy based on the laws of Newton's convection. The released energy can be used to determine the optimum healing period according to law of conservation of energy. Three healing indices, absorbed energy, effective healing depth and nonlinear ultrasonic parameter, are respectively defined to characterize the healing efficiency of asphalt mixture for different healing periods. The healing gradient direction is verified by the X-ray computed tomography (CT) scanning images and corresponding porosity calculation. The results indicate the existence of an optimal healing period and healing efficiency in the healing process of asphalt mixture, and the calculation method proposed here is suitable to predict the optimal healing efficiency. While the nonlinear parameter is considerably prominent due to its sensitivity and high resolution for each healing period. Finally, through the correlation analysis of three healing indices, it is found that the nonlinear ultrasonic parameter is an ideal alternative of two other healing indices in terms of characterizing the gradient heating-healing properties of asphalt mixture specimens.