# Fatigue Performance and Model of Polyacrylonitrile Fiber Reinforced Asphalt Mixture
## 3 Specimen Preparation and Test Procedure
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\
\begin{tabular}{c} **Fiber** \\ **Content** \\ **(\%)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Asphal** \\ **Content** \\ **(\%)** \\ \end{tabular} } & \multicolumn{3}{c}{**Volumetric Parameters**} & \multicolumn{3}{c}{**Fatigue**} \\  & & & **Air Void** & **VMA** & **VFA** & **Range (\(\mu\varepsilon\))** & **Coefficients 1** & \(R^{2}\) **(\%)** \\ \hline \multirow{4}{*}{0.15} & 4.4 & 7.2 & 16.5 & 56.4 & 600–1500 & 0.021 & 2.236 & 99.4 \\  & 4.9 & 6.9 & 17.9 & 61.5 & 600–1500 & 0.002 & 2.517 & 92.9 \\  & 5.4 & 7.5 & 16.6 & 54.9 & 600–1500 & 0.016 & 2.405 & 95.6 \\  & 4.5 & 6.8 & 17.9 & 62.1 & 600–2500 & 1.034 & 1.530 & 96.8 \\  & 5 & 7.1 & 18.0 & 60.6 & 600–2500 & 0.323 & 1.861 & 98.7 \\  & 5.5 & 6.5 & 18.7 & 65.3 & 600–2500 & 0.636 & 1.760 & 93.5 \\  & 4.6 & 6.9 & 18.4 & 62.5 & 600–2500 & 0.833 & 1.736 & 97.3 \\  & 5.1 & 6.7 & 18.8 & 64.3 & 600–2500 & 0.271 & 1.805 & 95.4 \\  & 5.6 & 7.4 & 18.0 & 59 & 600–2500 & 0.790 & 1.879 & 98.1 \\  & 4.8 & 7.2 & 17.0 & 57.7 & 600–2500 & 0.814 & 1.901 & 93.9 \\  & 5.3 & 6.9 & 17.5 & 60.5 & 600–2500 & 1.952 & 1.648 & 95.3 \\  & 5.8 & 6.4 & 18.5 & 65.4 & 600–2500 & 1.039 & 1.812 & 96.2 \\ \hline \hline \end{tabular} \({}^{1}\) Fatigue coefficients, \(k_{1}\) and \(k_{2}\), are determined by Equation (1).  
\end{table} 
Table 5: Test results of volumetric parameters and fatigue coefficients. In Figures 6b and 7, the reinforced asphalt mixture obviously has an optimum fiber content for the fatigue life. When the fiber content is less than the optimum level, the fatigue life increases monotonically as the fiber content increases. Once above the optimum level, the fatigue life actually falls as fiber content increases. The potential reasons are:  
* As seen from Table 4, the optimum asphalt content of 0.3% fiber content in the asphalt mixture increased by nearly 8% compared to that without fiber. The total surface area of aggregates is basically equal for the same gradation aggregates. This means that the thickness of the asphalt film around aggregates increases by nearly 8%, which is beneficial to the fatigue life of the asphalt mixture;
* On the one hand, the physical and chemical reactions occur on the interface between the asphalt and fiber; on the other hand, the asphalt mortar with a higher viscosity is formed after the addition of fibers. Therefore, the thickness of the structural asphalt film on the aggregate surface and the toughness of the asphalt, as well as the adhesive force to the aggregate, has been increased, which will help to improve the adhesive capacity between the aggregate and asphalt;
* A large number of fibers distributed randomly in three dimensions and evenly formed net-like structures in the mixture, which inhibited the appearance and propagation of cracks, as shown in Figure 8. The healing of cracks of asphalt pavement by themselves is enhanced and the  
Figure 6: Fatigue properties. (**a**) Different asphalt contents at 0.3% fiber content; (**b**) different fiber contents at the optimum asphalt content. Figure 7: Effect of fiber content on fatigue life at the same strain level. occurrence of cracks is postponed. The reinforced role is similar to the traditional mortar mixed with short straws;
* With the mixture fiber content increasing, the aforesaid reinforced action will be more frequent and the fatigue life will be improved; and
* It also can be seen from Figure 8c, when the amount of fiber is excessive (above the optimum content), the redundant \"free fibers\" will appear in the mixture. Those \"free fibers\" even agglomerate somewhere. These \"free fibers\" and agglomerated fibers will weaken the cohesion between the asphalt and aggregate. It leads to a decrease of the fatigue life of the mixture.