## 2 Research on Anti-Skid Mechanisms of Asphalt Pavement
### Factors Affecting the Skid Resistance of Asphalt Pavements
#### 2.2.2 Influence of Gradation Type of Mix on Skid Resistance of Asphalt Pavements  
Asphalt mixture is composed of coarse aggregate, fine aggregate, filler, and asphalt material in accordance with a fixed proportion of a visco-elastic composite material. A reasonable mixture grading allows for the aggregate particles to have a beneficial embedded role and internal friction resistance. This, in turn, ensures that the asphalt pavement can maintain its design strength under the action of vehicular loads. The grading type of the mixture has a significant impact on the structure between coarse and fine aggregate particles, which is also known as macrotexture (horizontal wavelength of 0.5-50 mm, amplitude of 0.1-20 mm). The formation of macrotexture is primarily influenced by the following factors [33]: the size, shape, and distribution of coarse and fine aggregates; the nominal maximum particle size of the aggregates; and the use of pavement construction in the construction methods are the main factors that affect the grading type of the mix. Different gradation types of mixes can have different effects on the skid resistance of pavements. For example, [PERSON] et al. [34] demonstrated that the proportion of coarse aggregate (>9.5 mm) in asphalt mixtures influences the complexity of the pavement texture and the magnitude of the surface fractal dimension. Consequently, investigating the relationship between the proportion of coarse aggregate and the skid resistance of pavements has emerged as a novel research area. [PERSON] et al. [35] studied the relationship between the concave-convex distribution characteristics of the pavement texture and the gradation, andYun et al. [36] constructed a relationship function between the two, and initially determined the law between the initial skid resistance performance of the pavement and the gradation of the mixture, but the experimental results need more outdoor tests to verify the stability of the functional equation and should be further studied to identify the link between the type of pavement gradation and the law of the attenuation of the skid resistance performance. Compared with previous studies, [PERSON] et al. [37] carried out research on the connection between 1D-textured, 2D-textured, and gradation-type surfaces, respectively, but the established model between texture and gradation had a limited range of applicability and multiple limitations, and the streamlined design of the model limited the subsequent development and extension of the model to a certain extent. The grading type of the mix and the pavement texture have different purposes in the characterisation of the skid resistance of asphalt pavements, and in general, identifying the relationship between the grading type and the pavement macro- and microtexture can help the construction of a comprehensive skid resistance grading design model. As the macro-/microtexture of asphalt pavement shows a good positive correlation with the void ratio [38], pavement with a good macrotexture can improve the safety of vehicles travelling at high speeds as well as the anti-skid performance of asphalt pavement during rainy and snowy weather [39; 40]. [PERSON] et al. [41] conducted accelerated loading tests indoors on three gradation types, namely AC-13C, SMA-13, and GAC-13C, and analysed them using the empirical modal decomposition model (EMD) with the Hilbert-Huang transform (HHT). The results showed that the skidding resistance of the SMA-13 asphalt mixtures had the lowest decay rate. The macroscopic tectonic depth and pendulum value after 1 million axial loads were the largest in the SMA-13 mixes, which indicates that they have good skid resistance. [PERSON] et al.