## 5 Results and Discussion
### Rutting Results  
Wheel tracking studies were performed to accurately characterise the effect of GGBFS addition on permanent deformation resistance in DG hot mix asphalt. As a result, this study offers a useful comparison and review of rutting and fatigue efficiency with filler inclusion. The tests were carried out in accordance with Austroads standard AG:PT/T231 [26] and the results are presented in Tables 11 and 12.  
The rut depths of the control mixture (1.5% HL) and Mixture 1 (1.5% GGBFS) are similar, as shown in Figure 7 and in the table. The rut depth in both mixtures is 3.70 mm.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Mix** & **Average Rut Depth (mm)** & **Standard Deviation** \\ \hline Control (1.5\% HL) & 3.73 & 0.55 \\ \hline
1.5\% GGBFS & 3.70 & 0.36 \\ \hline
3\% GGBFS & 2.70 & 0.10 \\ \hline
5\% GGBFS & 3.13 & 0.25 \\ \hline \hline \end{tabular}
\end{table}
Table 11: Wheel tracking test results.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mixture** & **Rutting Depth at 10,000 (mm)** & **Rutting Depth at 4000 (mm)** & **Tangential Slope** & **SSTR** \\ \hline Control (1.5\% HL) & 3.76 & 2.76 & \(1.67\times 10^{-4}\) & \(1.67\times 10^{-2}\) \\ \hline
1.5\% GGBFS & 3.68 & 2.65 & \(1.72\times 10^{-4}\) & \(1.72\times 10^{-2}\) \\ \hline
3\% GGBFS & 2.71 & 1.94 & \(1.27\times 10^{-4}\) & \(1.27\times 10^{-2}\) \\ \hline
5\% GGBFS & 3.15 & 2.26 & \(1.47\times 10^{-4}\) & \(1.47\times 10^{-2}\) \\ \hline \hline \end{tabular}
\end{table}
Table 12: Rutting Test Results.  
When 3% GGBFS was added to the asphalt mixture, the rutting depth decreased significantly. The rut depth of Mixture 2 (3% GGBFS) was 2.70 mm, a decrease of 27% compared to the control mixture (1.5% HL). This is because the fillers produce asphalt with greater packing and density as the fine filler particles act as micro-rollers to reduce air voids and increase compaction [9; 11]. In addition, the hardness of the bitumen is improved because a higher filler content results in more particles being absorbed and a higher viscosity bitumen-filler mastic. This increased viscosity improves the mixture's stability and resistance to permanent deformation, but reduces its fatigue life.  
This pattern, however, continued up to a filler concentration of 3%, after which the performance against permanent deformation began to regress. The rutting depth of Mixture 3 (5% GGBFS) was 3.15 mm, which is 16% higher compared to Mixture 2 (3% GGBFS). This is consistent with [PERSON], [PERSON], and [PERSON] [8], who found that when the filler amount is too high, the bitumen cannot coat the filler and aggregates adequately.  
This phenomenon is caused by two main factors. Increasing the amount of filler increases the total surface area, which must be coated with the same amount of bitumen as specified in the mix design. Secondly, as the amount of filler particles consumed increases, the bitumen's viscosity increases, making it more difficult for the bitumen to migrate through the mix and provide a dense and consistent coating.  
These two factors result in a poorly performing mixture with reduced stability and resistance to permanent deformation. These findings are consistent with [PERSON] and [PERSON] [31], who looked at the impact of fly ash as a filler at concentrations of 1-5%. The same patterns emerged; increasing the filler content up to 3% improved rutting efficiency before it deteriorated with further filler. The GGBFS was expected to perform similarly to fly ash. Figure 8 shows the highest rutting depth.  
Figure 7: Rutting depths of different asphalt mixtures.