# Influence of Different Fibers on Performance of Bitumen Binders and Thin-Overlay Bitumen Mixtures
## 3 Results and Discussion
### Performance Analysis of Fiber Bitumen Mixture
#### 3.3.2 The Semi-Circular Bending Test  
The SCB test was employed to explore how various fiber types and their respective concentrations influence the crack resistance and toughness of the mixture. The progression of cracking is illustrated in Figure 10, while the results from the tests are presented in Figures 11 and 12. Introducing fiber into the bitumen mixture notably enhances both its \(G_{f}\) and \(K_{\text{IC}}\) properties. At a 2% concentration, the \(G_{f}\) values for B-2, P-2, and G-2 saw remarkable increases of 51%, 37%, and 64%, respectively, with glass fiber demonstrating the most impressive results. At a fiber concentration of 4%, the \(G_{f}\) for B-4, P-4, and G-4 rose by 67%, 78%, and just 11%, respectively. The lifting effect of polypropylene fiber is the best, and basalt fiber has a downward trend. The trend of \(K_{\text{IC}}\) is similar to that of \(G_{f}\). The \(K_{\text{IC}}\) improvement for P-2 is not as good as that for B-2 and G-2. At a fiber concentration of 4%, the \(K_{\text{IC}}\) of P-4 increased significantly, while the \(K_{\text{IC}}\) of B-2 and G-2 decreased. The value of B-2 is even slightly lower than that of the undoped fiber mixture, which may be caused by test deviation. The incorporation of fibers effectively enhances the crack resistance and toughness of the mixture and enhances its ability to resist crack propagation. The reason is that, in addition to the'reinforcement' effect of fiber, the fiber adsorbs the light components in the bitumen, weakens the lubrication effect of the bitumen, and then improves the interface strength between the bitumen and the aggregate [63; 64]. The ability of the mixture to withstand the moving load of the vehicle is enhanced. It is worth noting that too high a dosage may also have adverse effects. The _FI_ of the bitumen mixtures rises as the fiber content increases. At 2% dosage, the _FI_ values for B-2, P-2, and G-2 showed rises of 49.86%, 43.39%, and 5.44%, respectively. Both basalt and polypropylene fibers exhibited substantial benefits. At 4% dosage, the _FI_ for B-4, P-4, and G-4 experienced increases of 76.34%, 84.32%, and 28.32%, respectively, with polypropylene fiber demonstrating the most significant improvement. It is worth noting that the changing trend for _FI_ is not always synchronized with _G_/ and _K_IC. _FI_ is not only based on _G_/, but also integrates the peak change rate of the stress-strain curve  
Figure 11: Some results of SCB test. Figure 10: Cracking process of SCB specimen. after fracture. This index serves as a thorough representation of the material's toughness and its ability to absorb energy. It offers a fresh lens through which to assess the fiber modifications in bitumen mixtures, enabling a more holistic evaluation of how fibers impact the performance of these mixtures. The inclusion of fiber markedly enhances the _TI_ of the bitumen blend, while also improving the material's resistance to cracking and overall durability. At the dosage of 2%, the _TI_ of the three fiber mixtures was improved compared with that before incorporation, and the difference was small, but the polypropylene fiber still showed the best improvement effect. At the dosage of 4%, the _TI_ for both P-4 and G-4 saw increases of 63% and 15%, respectively. Conversely, the _TI_ for B-4 dipped by 5%, yet it remained above the baseline value of the undoped fiber. Polypropylene fiber significantly enhances the overall toughness of the mixture. Incorporating fiber into the bitumen blend can certainly boost its resistance to cracking and improve its toughness. Nonetheless, it is essential to consider the appropriate fiber dosage with great care. For fibers with high tensile strength but weak bonding ability with bitumen, excessive content may lead to curling and agglomeration of fibers in the mixture. This diminishes the strengthening benefits of fiber, potentially impacting the bond between bitumen and aggregate, and ultimately compromising the overall structural integrity of the mixture.