## 5 Conclusions  
In this research, the effect of three different additives--styrene-butadiene-styrene (SBS), animal bone ash (AB), and waste cooking oil (WCO)--on the rheological and fatigue properties of bitumen at different loading and temperatures was measured methodically. The use of frequency sweep, amplitude sweep, LAS, and VECD analyses enabled a holistic view of how each modifier impacts the binder's elasticity, stiffness, and performance durability.  
The findings establish that SBS is the best additive, especially at 5%. It had the best elasticity, resistance to deformation, and fatigue life under all the test conditions. SBS-modified binders had consistent rheological properties and maintained pseudo-stiffness levels during fatigue testing, showing optimal long-term performance for severe conditions like hot temperature and heavy-traffic pavement applications.  
The 6% animal bone ash substantially enhanced the stiffness and structural integrity of the binder, but with an intense stiffening action that diminished relaxation tendency and enhanced the risk of thermal or cycle load-induced cracking, particularly at low temperatures. These results indicate that optimizing AB dosage must balance strength and flexibility.  
Waste cooking oil had the maximum softening effect, enhancing binder flexibility and lowering stiffness at low temperatures. Of the WCO-modified samples, 4% had the most balanced performance. Although it did not outperform SBS or AB for fatigue resistance or high-temperature properties, its environmental benefits and capacity to improve workability qualify it for low to moderate traffic applications where flexibility is of higher concern than structural rigidity.  
Overall, SBS is the most suitable additive for improving the performance of applications, AB is only helpful for structural strength but needs control of dosage, and WCO assists with sustainable pavement design with added flexibility and efficiency of mixing. These results prove that specific additive choices may be applied to designing bituminous binders for various climate and traffic conditions.  
Recommendations:
* SBS (5%) should be prioritized for pavements exposed to heavy traffic and dynamic loading.
* AB (4-5%) can be considered in moderate to warm climates where stiffness is needed but thermal cracking risk is manageable.
* WCO (4%) serves as a viable eco-friendly alternative for low-load applications requiring flexibility, particularly in temperate regions.  
Future work should explore the potential synergistic effects of combining these additives, as well as their long-term aging performance and environmental lifecycle impacts. This would further support the development of durable, cost-effective, and environmentally responsible pavement solutions.  
Conceptualization, A.A.; Methodology, A.A. and S.T.N.N.; Software, A.A.; Validation, A.A.; Formal analysis, A.A. and S.T.N.N.; Investigation, A.A. and S.T.N.N.; Resources, A.A. and S.T.N.N.; Data curation, S.T.N.N.; Writing--original draft, A.A. and S.T.N.N.; Writing--review & editing, A.A.; Visualization, S.T.N.N.; Supervision, A.A. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available in article.  
**Conflicts of Interest:** The authors declare no conflict of interest.