# Oil Sludge as a Rejuvenator for Aged Bitumen: Structural and Thermal Effect
## 3 Results
### Effect of Oil Sludge on the Rheological Properties of Bitumen  
The authors [34] note that the aging process is accompanied by the loss of volatile substances and the migration of oily components from the bitumen into the aggregate, as well as slow, reversible hardening upon heating, which is manifested in a change in the viscosity of the bitumen over time. For a deeper understanding of the structural changes in bitumen, dynamic shear rheometry (DSR) was employed as a key method to assess its viscoelastic behavior. These methods are widely used to analyze the properties of bitumen materials, since each of them provides unique information on molecular structure and rheological characteristics. Dynamic shear rheometry (DSR) is an important method for the analysis of the mechanical properties of bitumen binders, allowing the evaluation of the characteristics of bitumen in practical applications [35]. The softening point is traditionally regarded as a temperature threshold at which the binder transitions from a high-viscosity to a more flowable, low-viscosity state, indirectly reflecting its thermal behavior during practical application [36]. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Indicators**} & \multirow{2}{*}{**Aged Bitumen**} & \multicolumn{3}{c}{**Amount of Oil Sludge, \%**} \\ \cline{3-5}  & & **1** & **3** & **5** \\ \hline Penetration, 0.1 mm at 25 \({}^{\circ}\)C & 60 & 63 & 67 & 71 \\ Softening point, \({}^{\circ}\)C & 55 & 60 & 68 & 72 \\ \hline \hline \end{tabular} Note: All values represent the average of three replicate measurements.  
\end{table} 
Table 2: Effect of oil sludge application on the physical properties of bitumen. The introduction of oil sludge (Table 3) into aged bitumen leads to a decrease in the complex modulus (\(G^{*}\)) and phase angle (\(\delta\)), indicating the restoration of the material's elasticity and flexibility (Figure 1). According to the study by [PERSON] et al. [37], a decrease in the \(G^{*}/sin\)\(\delta\) value indicates lower energy loss under repeated loading and improved fatigue properties of the material. Thus, the use of oil sludge helps improve the fatigue properties of bitumen, which is similar to the effect of rejuvenators that restore the molecular structure of bitumen. According to the Superpave performance grading specifications (AASHTO M320: Standard Specification for Performance-Graded Asphalt Binder), the rutting factor (\(G^{*}/sin\)\(\delta\)) of RITFOT-aged binders must not be lower than 2.20 kPa at the designated high temperature to ensure sufficient resistance to permanent deformation. In this study, the aged bitumen exhibited a \(G^{*}/sin\)\(\delta\) value of 655 Pa at 66.5 \({}^{\circ}\)C, indicating excessive stiffness due to oxidative aging. Upon modification with 1-5% oil sludge, \(G^{*}/sin\)\(\delta\) values decreased to 597.5, 584.2, and 584.5 Pa, respectively, at elevated test temperatures (71.4-71.6 \({}^{\circ}\)C). Although these values remain above the Superpave threshold, their gradual decline confirms the partial restoration of rheological balance and improved high-temperature workability. These  
Figure 1: Variation in rheological parameters (**(a)**\(G\), **(b)**\(\delta\), and **(c)**\(G/sin\)\(\delta\)) with oil sludge content in aged bitumen. \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**DSR Results** & \(\delta\) [\({}^{\circ}\)] & **\(G^{*}\) [Pa]** & **\(G^{*}/sin\)\(\delta\) [Pa]** & **T [\({}^{\circ}\)C]** \\ \hline Aged bitumen & 84.70 \(\pm\) 0.1 & 652.2 \(\pm\) 0.5 & 655 \(\pm\) 0.5 & 66.5 \(\pm\) 0.3 \\
1\% oil sludge & 84.55 \(\pm\) 0.1 & 594.8 \(\pm\) 0.8 & 597.5 \(\pm\) 1.0 & 71.6 \(\pm\) 0.5 \\
3\% oil sludge & 84.52 \(\pm\) 0.1 & 581.5 \(\pm\) 0.8 & 584.2 \(\pm\) 0.8 & 71.6 \(\pm\) 0.3 \\
5\% oil sludge & 84.12 \(\pm\) 0.1 & 582.1 \(\pm\) 0.9 & 584.5 \(\pm\) 1.0 & 71.4 \(\pm\) 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: DSR results for bitumen samples.