# Low Temperature Resistance Increase for Bitumen by Compounding with Tar
## 3 Results and Discussion  
### Bitumen Stiffness  
In accordance with the requirements of the standard of Kazakhstan ST RK 1373-2013, as well as in many countries of the world, the grade of a road bitumen is determined under the penetration value of 25 \({}^{\circ}\)C, which evaluates indirectly the consistency (viscosity) of bitumen. Table 5 represents the penetration values of the compounded bitumen at different tar contents. As it is observed, bitumen penetration continuously decreased with tar content increase. It was to be expected, as bitumen viscosity decreased with tar content \begin{table}
\begin{tabular}{c c c} \hline \hline \(\delta\) (\({}^{13}\)C), & **Notation of Atoms** & **Functional Group** \\ ppm & & Atoms of carbon of methyl groups connected \\
7–17 & C\({}_{\text{pm}}\) & with a methylene group. \\
17–25 & C\({}_{\text{pa}}\) & Atoms of carbon of methyl groups connected \\
25–50 & C\({}_{\text{af}}\) & Quaternary aliphatic atoms of carbon. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Variation limits and interpretation of chemical shifts of \({}^{13}\)C nuclei. increase. Thus, it was impossible to determine the optimal tar content in the compounded bitumen according to penetration variation, as it required the use of another, more modern test method. Currently, in road science and practice, one of the generally accepted and reliable methods for the evaluation of deformability of both virgin and modified bitumen at low temperatures is testing them on a bending beam rheometer (BBR). Meanwhile, bitumen with less stiffness is considered more crack-resistant at low temperatures. The stiffness values of the tested virgin and compounded bitumens with a load duration of 60 s at the temperatures of \(-24\)\({}^{\circ}\)C, \(-30\)\({}^{\circ}\)C and \(-36\)\({}^{\circ}\)C are shown in Figure 2. As it can be observed, the compounding effect is stronger with the decrease in temperature and at the tar content of 20% and more; the stiffness of the compounded bitumen remains constant at all the low temperatures considered. Therefore, tar content equal to 20% by weight is considered as the optimal one when compounding the selected road viscous bitumen with penetration grade 100-130. The graphs showing the m-value (relaxation rate) of the bitumens at different tar contents and the considered low temperatures are represented in Figure 3. In spite of some scatter, it is clearly observed that m-value decreased with tar content increase at all the considered temperatures. However, even at tar content 30% at all the considered temperatures, the m-value is more than 0.3, which is the minimum allowable value according to Superpave. It follows that the selection of optimum tar content in the bitumen according to the stiffness is the justified one. Figures 4-6 represent the graphs showing a variation in stiffness of the virgin bitumen and the compounded one (20% of the tar) in time at the temperatures considered. It can be observed that during the entire time period considered (240 s) at all test temperatures, the stiffness of the compounded bitumen of the optimal composition is obviously lower (from 18% to 34%) than that one of the neat bitumen. Figure 2: Bitumen stiffness at various tar contents and different temperatures. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Tar** & 0 & 5 & 10 & 15 & 20 & 25 & 27 & 30 \\
**Content, \%** & & & & & & & \\ \hline
**Penetration at 25 \({}^{\circ}\)C,** & & & & & & & \\
**0.1 mm** & 116 & 118 & 128 & 160 & 170 & 182 & 203 & 216 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Bitumen penetration at different tar contents. Figure 4: Stiffness variation of the virgin bitumen and the compounded one (20% of the tar) in time at the temperature of \(-24\)\({}^{\circ}\)C.  
Figure 5: Stiffness variation of the virgin bitumen and the compounded one (20% of the tar) in time at the temperature of \(-30\)\({}^{\circ}\)C.  
Figure 3: Them-value of the bitumen at various tar contents and different temperatures.