## 3 Results and Discussions  
### Conventional Asphalt Binder Properties  
Figure 8 shows the effect of MWCNTs on the penetration and softening point value of modified and unmodified bitumen. Penetration value reflects the stiffness and hardening of asphalt binder at moderate temperature. Lower penetration value indicates that binder has become stiff. In Figure 8 it can be seen that, by increasing CNTs in bitumen, the penetration value decreased, which is indicative of a decrease in fluency and increase in the stiffness of bitumen. When 0.5% (by mass of bitumen) CNTs was added, penetration significantly decreased by 11%. By increasing the CNT dosage to 1.5% and 3%, 25% and 28% decreases in penetration value were observed, respectively. Increasing the CNT dosage beyond 1% did not have a significant effect on the penetration value.  
The softening point test is commonly used as a standard test for describing an approximate limit between viscous and viscoelastic bitumen behavior, and it reflects the deformation resistance of bitumen at high temperature. A dosage of 1.5% CNTs (by mass of bitumen) increased the softening point by about 7 \({}^{\circ}\)C as compared to base binder. When about 3% by mass of CNTs was added, it resulted in an increase in softening point of bitumen of about 8.4 \({}^{\circ}\)C. This increase in softening point and decrease in penetration value may be attributed to the large surface energy, high Young's modulus and presence of interaction forces between the CNTs, which make binder stiff [40; 41].  
Figure 8: Effect on penetration value and softening point value with addition of CNTs.  
PI was used to study the effect of the addition of CNTs in the asphalt binder on its temperature susceptibility. The temperature susceptibility evaluation is described below, as illustrated by [PERSON] et al. [42], using following equations.  
\[PI=\ \frac{20-500A}{1+50A}\ \text{where}\ A=\ \frac{log(800)-log(Pen\ at\ T)}{SP-25} \tag{1}\]  
where \(T\) is the temperature at which the penetration test is performed (25 \({}^{\circ}\)C), \(Pen\) at \(T\) is the penetration at the required temperature \(T\) (0.1 mm), \(SP\) is the softening point of asphalt (\({}^{\circ}\)C), and \(Pen\) at \(SP\) is the penetration at the softening point and assumed as 800 (0.1 mm) [43].  
Higher value of \(PI\) indicates lower temperature susceptibility. In addition, it is known that most asphalt binders have \(PI\) values between \(-2\) and \(+2\). Binders are considered as highly temperature susceptible if their \(PI\) values go below \(-2\) and they will be more brittle at lower temperatures and experience transverse cracking in cold weather regions [44].  
The \(PI\) results for all the used CNTs percentages, as shown in Table 5 and Figure 9, are within the normal \(PI\) ranges (from \(-2\) to \(+2\)). With the addition of CNTs, the \(PI\) value of bitumen increased, which means the bitumen temperature susceptibility decreased.  
Figure 10 shows that decrease in ductility was observed with addition of CNTs. A 21% decrease in ductility value was observed when CNT content of up to 1.5% was added in bitumen. With 3% addition of CNTs, ductility value reduced by 26% as compared to neat binder. This reduction in ductility value was due to increase in stiffness of bitumen with the addition of CNTs, which accelerated the bitumen fracture process when subjected to tensile stress [11].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & **0\% CNTs** & **0.5\% CNTs** & **1\% CNTs** & **1.5\% CNTs** & **3\% CNTs** \\ \hline PI & \(-0.7692\) & \(-0.66\) & 0.0187 & 0.060 & 0.065 \\ A & 0.04584 & 0.0442 & 0.03758 & 0.0378 & 0.0374 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Penetration Index for modified and unmodified bitumen.  
Figure 9: Effect on Penetration Index and Thermal Susceptibility with addition of CNTs.