# Low Temperature Resistance Increase for Bitumen by Compounding with Tar
## 3 Results and Discussion
### Chemical Structure of Bitumens
#### 3.3.1 Group Chemical Composition  
Currently, an explanation of the physical, mechanical and rheological characteristics of bitumens based on their group chemical composition is generally accepted among road specialists [1; 2; 3]. According to the concept of a group chemical composition, the entire complex chemical structure of bitumens is divided into two groups: asphaltenes and maltenes; maltenes, in turn, are divided into resins and oils. Asphaltenes make up the highest molecular weight part of bitumens: black solids are not dissolved in light alkanes (n-pentane, n-hexane, n-heptane, petroleum ether, etc. ), they are dissolved in carbon disulfide and chloroform. Asphaltenes are aggregates of flat condensed naphthenic and aromatic molecules (usually 4-6 layers), which are parallel above one another with the inclusion of heterocycles mainly with sulfur and nitrogen, they also contain lateral substituents in the form of limit aliphatic chains and naphthenic rings [1; 35; 36; 37; 38]. Oils are the lightest fractions of bitumen. The oils are a viscous yellow liquid on their external view. They contain paraffinic and naphthenic compounds and aromatic (monocyclic, bicyclic and polycyclic) hydrocarbons. Resins occupy an intermediate position between asphaltenes and oils according to their degree of condensation. They are solid or paste-like substances of reddish-brown color. Compared to oils, resins contain more condensed hydrocarbons and heteroatoms. The viscosity of the resins is substantially greater than that of the oils. From the above brief characteristics of asphaltenes, resins and oils, one can understand that their content ratios determine the degree ofplasticity of bitumen at low temperatures and, therefore, their low temperature stability. External views of asphaltenes, resins and oils of the compounded bitumen of optimal composition are shown in Figure 7. Figure 8 shows the group chemical compositions of unaged samples of the virgin bitumen, tar and the compounded bitumen. As can be observed, virgin blown bitumen contains 17.4% of asphaltenes, 26% of resins and 56.6% of oils; tar is characterized by significantly lower content of asphaltenes (4.1%) and relatively large contents of resins (33.9%) and oils (62%). When tar is added to neat bitumen in the amount of 20% by weight, the compounded bitumen is obtained, which has the following group chemical composition: asphaltenes--13.4%; resins--23.7%; oils--62.9%. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Tar Content, \%** & 0 & 5 & 10 & 15 & 20 & 25 & 27 & 30 \\ \hline
**Measurement** & & & & & & & & & \\
**Temperature, \({}^{\circ}\)C** & 45.0 & 44.5 & 43.0 & 40.5 & 40.0 & 38.0 & 37.8 & 36.6 \\ \hline \end{tabular}
\end{table} 
Table 7: Bitumen softening point at different tar contents. Figure 7: External view of asphaltenes, resins and oils of the compounded bitumen. The above variations in the group chemical composition of the bitumen after compounding compared with the virgin bitumen point to the chemical reactions proceeding between molecular of the bitumen and the tar during compounding. It should be noted that the contents of resins in the virgin bitumen and the tar were equal to 26% and 33.9%, respectively. After compounding with tar, the content of resins was 23.7%. As the process of compounding was performed at a high temperature (120 \({}^{\circ}\)C), at intensive mixing (450-500 revolutions per minute) and at a duration of 30-40 min for the compounding, one can suppose that there is an interaction between the molecules of the bitumen and the tar, and the component composition variation occurs as the result of it and part of resins transfers into oils. Thus, optimal compounding of the bitumen with the tar (20% by weight) changes significantly its group chemical composition: asphaltenes and resins decreased by 23% and 8.8%, respectively, and oils increased by 11.1%, i.e., the content of solid and high-viscosity components of the bitumen significantly decreased, and the content of its low-viscosity components, on the contrary, increased.