# Rheological Performance of Bio-Char Modified Asphal with Different Particle Sizes
## 5 Results and Discussions
### Bending Beam Rheometer Test  
#### 5.4.1 Low-Temperature Cracking Resistance Analysis  
The BBR test was conducted for all the asphalt binders, to evaluate their low temperature cracking resistance. Based on the Superpave TM specification, \(-18\)\({}^{\circ}\)C is recommended for the test temperature.  
Figure 5: Rutting indexes (\(\mathrm{G}^{\star}/\mathrm{sin}\mathrm{\delta}\)) of different types of asphalt binders.  
\begin{table}
\begin{tabular}{l c c c c c c c c} \hline  & **Control Asphal** & **4\% GS** & **2\% BL** & **4\% BL** & **8\% BL** & **2\% BS** & **4\% BS** & **8\% BS** \\ \hline \(\mathrm{T}_{\mathrm{o}}\)(\(\mathrm{G}^{\star}/\mathrm{sin}\mathrm{\delta}\)) & 63.56 & 63.87 & 63.90 & 65.13 & 67.07 & 65.76 & 66.48 & 68.03 \\ \(\mathrm{T}_{\mathrm{R}}\)(\(\mathrm{G}^{\star}/\mathrm{sin}\mathrm{\delta}\)) & 64.61 & 65.18 & 66.62 & 66.34 & 68.21 & 65.91 & 66.22 & 68.09 \\ \(\mathrm{T}\)(\(\mathrm{G}^{\star}/\mathrm{sin}\mathrm{\delta}\)) & 63.56 & 63.87 & 63.90 & 65.13 & 67.07 & 65.76 & 66.48 & 68.03 \\ \hline \end{tabular}
\end{table}
Table 3: The high critical temperatures of control, graphite modified, and bio-char modified asphalts.  
And the specification requirement for the PAV-aged sample is the creep stiffness of less than 300 MPa and an m-value of higher than 0.3. The test results of creep stiffness and m-value for control, graphite modified, and bio-char modified asphalts are shown in Table 4.  
The stiffness of 2% BL, 4% BL, 8% BL, 2% BS, 4% BS, and 8% BS was higher by \(-\)2 MPa, 64 MPa, 66 MPa, 34 MPa, 83 MPa, and 105 MPa, respectively, compared to the control asphalt. Though bio-char modified asphalts had higher creep stiffness than the control asphalt binder, they all satisfied the specification requirement of less than 300 MPa. The m-values of 4% BL, 8% BL, and 8% BS were 0.29, 0.28, and 0.28, respectively, which could not satisfy the requirements of the specification. As the bio-char content increased, the low-temperature properties of bio-char modified asphalts decreased. Therefore, there should have been a limit on the bio-char content of less than 4%. The reason is that stiff particles made the asphalt binder stiffer and more brittle, at low temperatures. This led to the smaller deformation capacity or compliance, according to the weak low-temperature properties. It could also be found that the bio-char particle size had an effect on the low-temperature performances, and the bio-char modified asphalts, with smaller-sized particles, had better low temperature cracking resistance than those with larger-sized particles. The m-values of 2% BS and 4% BS were 0.33% and 2.4% higher than those of 2% BL and 4% BL, respectively. The PAV-aged samples were used in the BBR test. This might have been caused by the more uniform distribution and interaction with the smaller stiff particles. Alternatively, the better anti-aging properties of bio-char modified asphalts, with smaller-sized particles, might have made the asphalt binder softer, and, thus, led to better cracking resistance at low temperatures. The m-values of 4% BS and 4% GS were both 0.30. This illustrated that bio-char modified asphalts had very similar low-temperature properties as that of the graphite asphalt binder, for the same particle sizes.