# A Review of Long-Term Skid Resistance of Asphalt Pavement
## 3 Skid Resistance Mechanism and Influencing Factors of
### Effect of Asphalt Quality on Ski Resistance of Asphalt Pavement  
Bitumen is a kind of mineral oil which is separated, purified, and concentrated from residual oil, and it has a certain degree of adhesion and viscosity [74]. Bitumen plays the role of bonding aggregates in the mixture so that the aggregates can be effectively combined, and good adhesion and water stabilization can ensure that the skid resistance of the pavement will not be reduced due to the spalling of the aggregates and the occurrence of various water damages [75]. In addition, the impact of the amount of asphalt on the skid resistance of asphalt concrete pavement is also obvious. When the asphalt content of AP is less, it cannot wrap aggregates well and aggregates are prone to loosen, resulting in a decline in the skid resistance of the road surface [76]. If the amount of asphalt is too large, there will be a large amount of free asphalt in the mixture, resulting in an \"oil flooding\" phenomenon [77]. This leads to the occurrence of coarse aggregate relative sliding, reducing the stability of the mixture, while the free asphalt generated by oil flooding will fill the surface micro-textures of the coarse aggregate on the pavement, reducing the skid resistance of the pavement [78]. And on rainy days, if the rainwater on the surface cannot be discharged in a timely manner, it will affect the safety of the vehicle. However, while low asphalt content may lead to aggregate loss, the relationship between asphalt content and skid resistance is more complex. Low asphalt content may initially expose more aggregate, increasing macro-texture and skid resistance, while long-term aggregate polishing can reduce friction. Conversely, high asphalt content may lead to excessive coverage of aggregates, reducing macro-texture. Therefore, the asphalt viscosity and asphalt dosage on AP skid resistance has a greater impact, and through the degree of influence of the friction coefficient of asphalt viscosity in quantitative analysis, to improve the AP skid resistance, a reasonable selection of asphalt type is of great significance. In calculating the adhesion component of the coefficient of friction, the estimated value of cohesion is  
\[c=\eta\dot{\gamma} \tag{13}\]  
where \(c\) is the adhesion between asphalt and tire (MPa);  
\(\eta\) is the viscosity coefficient of asphalt (Pa\(\cdot\)s);  
\(\dot{\gamma}\) is the shear rate (1/s). From the bending shear stress theory, the shear strain rate is converted to the shear strain rate of the asphalt membrane to obtain  
\[\dot{\gamma}=\frac{8P\lambda^{2}v}{\pi^{3}}\frac{(1+v_{a})}{E_{a}\delta_{a}} \tag{14}\]  
\(\delta_{a}\) is the thickness of surface asphalt mortar layer (um);  
\(\lambda\) includes parameters of the asphalt surface layer relative to the base layer (1/m), determined by Equation (15):  
\[\lambda=\left[\frac{4k_{b}(1+v_{a})}{E_{a}h_{a}^{3}}\right]^{\frac{1}{2}} \tag{15}\]  
\(k_{b}\) is the foundation reaction factor at the top of the subgrade (MPa/m);\(\
u_{a}\) is [PERSON]'s ratio of AP;  
\(E_{a}\) is the modulus of elasticity of AP (MPa);  
\(h_{a}\) is the thickness of the asphalt surface layer (m). It was found that the range of variation in the viscosity coefficient of asphalt was related to the type of asphalt, the test temperature, and the degree of aging [79; 80]. Newly paved AP has a layer of asphalt film wrapped around the aggregate surface. When the AP is flooded with oil, the asphalt film also exists on the road surface. The friction coefficient component of the asphalt film is  
\[\mu_{a1}=\frac{\eta\dot{\gamma}\sqrt{1-k^{2}}}{kp_{a}} \tag{16}\]  
where \(\mu_{a1}\) is the friction coefficient component of asphalt film;  
\(p_{a}\) is the pressure acting on AP (MPa). The friction coefficient of the asphalt film increases with the increase in the viscosity coefficient of asphalt, increases with the increase in the loading frequency, and decreases with the increase in the loading stress. The value and the influence trend are basically in line with daily engineering experience [81; 82].