# Influence of Different Fibers on Performance of Bitumen Binders and Thin-Overlay Bitumen Mixtures
## 3 Results and Discussion
### Performance Analysis of Fiber Bitumen Mixture  
#### 3.3.1 Indirect Tensile Testing  
Through IDT, the influence of different fiber types and content on the crack resistance of the mixture was explored. The test results are shown in Figures 7-9. The fiber content is positively correlated with the \(R_{\text{T}}\) of the mixture, and the gain effect of different types of fibers is different. At a fiber concentration of 2%, the \(R_{\text{T}}\) for samples B-2, P-2, and G-2 rose by 6.55%, 14.1%, and 7.96%, respectively. When the fiber content was elevated to 4%, the \(R_{\text{T}}\) values for B-4, P-4, and G-4 experienced increases of 8.13%, 29.14%, and 10.53%, respectively. The improvement effect of polypropylene fiber is particularly significant. This is because the stability of the mixture skeleton is enhanced after the fiber is incorporated. Fiber can absorb bitumen, which boosts the structural bitumen ratio and strengthens the adhesion between bitumen and aggregate [61]. As the amount of fiber rises, this benefit becomes more pronounced. Especially when the  
Figure 7: Some results of IDT. content of polypropylene fiber reaches 4%, the anti-deformation effect of the mixture is significantly improved, and the increase in \(R_{\rm T}\) is about three times that of the other two fibers. Polypropylene fiber has good dispersion performance and can build a more complex network structure in the mixed material, thereby significantly improving the overall effect. It can be speculated that if the fiber content exceeds 4%, its dispersibility may be affected. The force applied to the specimen cannot be effectively dispersed, thereby weakening some of the crack resistance. The benefits of adding fiber may show a decreasing trend. Therefore, in practical applications, ensuring dispersion is one of the keys to maximizing the benefits of fiber modification. Incorporating fiber into the mixture can boost both the \(G_{if}\) and \(G_{if}\), ultimately strengthening its resistance to cracking and the progression of cracks. When the fiber content was 2%, the \(G_{\overline{g}}\) of B-2, P-2, and G-2 increased by 16.29%, 11.14%, and 4.57%, respectively, and the \(G_{\overline{g}}\) increased by 8.5%, 4.56%, and 2.28%, respectively. At a fiber concentration of 4%, the \(G_{\overline{g}}\) of B-4, P-4, and G-4 increased by 6.29%, 14%, and 14.29%, respectively, and the \(G_{\overline{g}}\) increased by 10.54%, 47.29%, and 8.58%, respectively. Basalt fiber shows the most significant enhancement at lower quantities, whereas polypropylene fiber provides the most balanced overall improvement. This can be attributed to the fact that basalt fiber boasts superior tensile strength, despite its less effective dispersion. As a result, the agglomeration is not significant  
Figure 8: Stress–horizontal strain curves before and after normalization. Figure 9: Toughness index. at low dosages, and the improvement effect is better. At high dosages, the agglomeration phenomenon is aggravated and the performance is decreased. The incorporation of polypropylene fiber into bitumen proves to be more efficient thanks to its outstanding ability to disperse and absorb oil. This significantly enhances the bonding characteristics at the interface of the mixture, particularly when used in higher concentrations. Upon examining the stress-horizontal strain curve, it is evident that the horizontal strain that aligns with the maximum stress of various specimens typically hovers around 0.5 mm. In order to carry out quantitative analysis and unified evaluation, a unified standard was adopted when calculating the toughness index of the mixture. After the stress reaches the peak value, when the stress decreases to 50% of the peak value, the corresponding normalized horizontal deformation is \(X_{X/\chi p}\), and the area under the normalized curve is \(A_{X/\chi p}\). The toughness of the mixture was assessed by calculating the proportion of the area beneath the 50% normalized curve, following the peak, to the normalized horizontal strain of the section.