Surface cracks directly influence the integrity of asphalt pavement structures, the durability of the pavement, and driving safety. Assessment of cracking distress is of vital importance for pavement maintenance and rehabilitation. Compared with other methods, the spectral analysis of surface waves (SASW) method, a seismic wave-based nondestructive method, has advantages in estimating the deterioration of modulus and quantitatively evaluating the depth and severity of surface cracks by means of the Rayleigh wave propagation characteristic. The objectives of this paper are to monitor long-term attenuation characteristics of in-situ modulus of a semi-rigid asphalt pavement, to determine when the surface-opening cracks will occur as associated to the degree of modulus reduction, and to assess the depth of surface-opening crack through the dispersion characteristics of Rayleigh wave propagation using the SASW method. First, a general trend of modulus deterioration of asphalt layer was developed in an accelerated pavement testing (APT). Then the factors affecting the dispersion characteristics of the Rayleigh wave were determined through theoretical derivation. Finally, a series of experimental tests on a pavement segment with well-controlled surface-opening cracks was performed to explore how a surface-opening crack in asphalt pavement would vertically influence the propagation of Rayleigh waves at different crack widths. It was found that the general trend of modulus deterioration could be divided into four stages, and the surface-opening cracks occurred in the fourth stage with 40–50% modulus reduction rate. In addition, the relationship between crack depth and the shortest wavelength in the Rayleigh wave dispersion curve was developed according to different crack widths.
