## 4 Results and Discussion
### Step Load Mode  
Figure 6 presents the step load mode results, offering valuable insights into the deformation resistance and overall performance of unmodified and modified bitumen samples under sustained loading conditions. The unmodified sample exhibits a sharp increase in shear strain over time, highlighting its vulnerability to deformation when subjected to constant stress. This behavior indicates that the base bitumen lacks sufficient elasticity to recover from or withstand continuous loading, making it more prone to permanent deformation in high-load or high-traffic environments where stress is sustained.  
In contrast, bitumen samples modified with styrene-butadiene-styrene (SBS) at 4% and 5% concentrations demonstrate a marked reduction in the rate of shear strain increase over time. The 5% SBS sample shows the greatest deformation resistance among the SBS group. This suggests that SBS effectively enhances the elastic properties of bitumen, enabling it to withstand prolonged loading without significant deformation. This improvement likely stems from the ability of SBS to form a more stable polymer network within the bitumen matrix, reinforcing its resistance to shear forces. Consequently, SBS-modified bitumen is especially suited for applications demanding high durability and resilience under sustained loads, as it maintains structural integrity more effectively.  
Bitumen samples modified with animal bone ashes (AB) at 4%, 5%, and 6% concentrations also exhibit a slower increase in shear strain over time. The 6% AB sample shows the highest resistance to deformation within the AB group. This indicates that higher AB concentrations contribute to greater structural stability, likely by reinforcing the bitumen matrix and enhancing elasticity. This improvement in performance makes AB-modified bitumen well-suited for applications where materials encounter repetitive or sustained loads, as it resists permanent deformation more effectively than the unmodified sample.  
Waste cooking oil (WCO)-modified samples at 3%, 4%, and 5% concentrations demonstrate moderate improvements in shear strain resistance compared to the unmodified bitumen. However, these enhancements are less pronounced than those observed with SBS and AB modifications. The best performance among WCO samples occurs at the 5% concentration, though it still falls short of the resistance achieved by SBS and AB. This is likely due to WCO's influence on flexibility rather than rigidity, which introduces a level of elasticity without substantially reinforcing the bitumen structure. Therefore, WCO-modified bitumen may be more appropriate for applications requiring moderate flexibility instead of maximum stiffness, as it allows some deformation without offering the robustness provided by SBS and AB.  
In conclusion, the step load mode results in Figure 6 demonstrate that SBS and AB modifications significantly improve deformation resistance under sustained loading, with optimal performance observed in the 5% SBS and 6% AB samples. These additives enhance bitumen's elasticity and structural integrity, likely due to stable network formations that resist shear forces. By contrast, WCO modifications offer moderate flexibility but do not match the stiffness and deformation resistance achieved with SBS and AB. These findings suggest that SBS and AB are optimal for applications that require high resilience under constant loading, while WCO is a viable option where moderate flexibility without extensive rigidity is beneficial.