# Influence of Different Fibers on Performance of Bitumen Binders and Thin-Overlay Bitumen Mixtures
## 3 Results and Discussion
### Performance Analysis of Fiber Bitumen Mortar
#### 3.2.2 The Force-Ductility Method  
The 5 \({}^{\circ}\)C force-ductility test was conducted to examine how fibers affect the mechanical characteristics and low-temperature performance of bitumen. The findings of the test are presented in Figures 5 and 6.  
Figure 4: DSR test results.  
Incorporating fiber enhances the viscosity of SBS bitumen but may negatively impact its tensile characteristics at low temperatures. The pre-peak gradient of the force-ductility curve serves as an indicator of the bitumen's viscosity. Compared with the typical curve, the pre-peak slope of the measured curve of the composite-modified bitumen increases, the curve fluctuates significantly after the peak, the tension decreases rapidly, and the creep stage disappears. The findings indicate that adding fiber enhances the viscosity of bitumen, while its brittleness increases in colder temperatures. As a result, the maximum tensile force of fiber-reinforced bitumen sees an increase, although its ductility diminishes. Throughout the testing, internal damage develops within the bitumen. Certain fibers at the fracture surface of the specimen experience a sequence of 'curved-straight-broken,' while others are pulled out from the bitumen itself [59]. This intricate interaction results in noticeable fluctuations within the curve. After the peak tension, the polymer inside the bitumen cannot be changed from disordered to ordered, and the sample is destroyed, resulting in the disappearance of the SBS bitumen strength 'not falling but rising' stage, and the curve shows a cliff-like decline.  
The inclusion of fiber enhances the mechanical characteristics of SBS bitumen at low temperatures while also diminishing its elasticity. Specifically, this results in a notable increase in the \(E\) of SBS bitumen, alongside a gradual reduction in its \(S\). When the fiber content was 2%, the \(E\) of B-2, P-2, and G-2 increased by 138%, 72%, and 192% respectively, while the S decreased by 45%, 70%, and 80% respectively. The above phenomena indicate that bitumen needs to do more work in the yield stage, but its overall tensile work decreases  
Figure 5: Fiber-modified bitumen force–ductility curve.  
Figure 6: Yield strain energy and toughness area (tensile work).  
significantly with the decrease in ductility. The reason is that after adding fibers, the unbroken fibers in the tensile process can connect the broken ends of the specimen, and the structural bitumen increases, so the fracture of the specimen needs to be stronger [60]. As the fiber concentration rose to 4%, the enhancements in \(E\) for B-4, P-4, and G-4 were striking, climbing by 69%, 256%, and 98%, respectively, when juxtaposed with SBS bitumen. Notably, only polypropylene fiber played a continuous role in the improvement. The \(S\) of B-4, P-4, and G-4 decreased by 63%, 72%, and 89%, respectively, compared with SBS bitumen. Compared with S at 2% content, the decrease for P-4 was the smallest, only 7%. The fiber dispersion is not good, and the agglomeration is easily destroyed. Under low dosage, agglomeration appears less, and its disadvantage is not significant. With the increase in fiber content, agglomeration hinders the formation of network structure, and the disadvantages become more prominent. It shows an increase in brittleness in the test. And the reduction in free bitumen will increase the viscosity. Under the two effects, the breaking process is greatly shortened, and the tensile force in a short time is significantly improved. The dispersion of basalt fiber and glass fiber is poor, and the content increases, and the agglomeration has a significant effect on it. This causes \(E\) to decrease. As the fiber content rises from 2% to 4%, polypropylene fibers exhibit better integration with the bitumen due to their greater oil absorption capability, resulting in the lowest rate of change in \(S\) for SBS bitumen.