## 3 Results and Discussion
### Rheological Properties  
The effect of filler packing on mastic was studied in terms of master curves, as shown in Figures 5 and 6. The results indicated that filler acts as a stiffening agent to increase the stiffening effect of the composite. The penetration grade 40/60 has considerably lower stiffness as compared to all mastics. This effect is more apparent at high temperatures, as shown in Figure 5. The dynamic shear modulus of mastic is dependent on the moduli of both bitumen and filler. In these results of relatively high filler compositional volume (29%) mastics, the effective volume concentrations were 44, 54, 49, and 47% for limestone, ordinary Portland cement (OPC), gritstone, and granite, respectively. These values correspond to 33, 45, 40, and 38% Rigden voids, respectively. The granite and gritstone fillers have similar effective volume and packing, so their behaviour is identical. The phase angle is sensitive to the compliance of the machine, so it may not be able to provide reliable information for very stiff binders. For this reason, shift factor plots were made to evaluate the fitting of master curves (Figure 7).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Limestone** & \multicolumn{2}{c}{**Granite**} \\ \hline
**Mineral** & **\%** & **Mineral** & **\%** \\ \hline Calcite & 72.8 & Quartz & 68.8 \\ Dolomite & 20.2 & Calcite & 23.7 \\ Quartz & 4.7 & Vandendriesscheite & 5.1 \\ Colusite & 1.4 & Cuprite & 2.4 \\ \hline \hline \end{tabular}
\end{table}
Table 4: XRD analysis for mineralogical composition.  
The addition of ordinary Portland cement (OPC) has a considerable effect on the mastic stiffening potential of mastics (Figure 5). Similar behaviour is observed from phase angle master curves (Figure 6). The stiffness of OPC is attributed to its effective volume in this mastic. OPC mastics have more Rigden voids in the same volume and subsequently more fine particles. The limestone filler has a lower effective volume as compared to gritstone and granite. It is expected to have a less stiffening effect in comparison with the two fillers. However, limestone has a similar stiffening effect. A possible explanation is that limestone is a basic filler. It is thus expected to increase the stiffening as per Winniford when compared to gritstone and granite (acidic fillers) [43]. The acidic fillers result in lower viscosity, while basic filler results in a higher viscosity of mastic. The theory that basic fillers have more stiffening potential explains the behaviour seen in the phase angle master curves.  
Figure 5: Complex modulus master curves.  
Figure 6: Phase angle master curves of mastics.  
It is concluded that the fractional void theory of [PERSON] does not provide a full picture of bitumen-filler interaction. Several studies support this observation, such as [PERSON], [PERSON] and [PERSON], and [PERSON] and [PERSON] [16; 19; 44]. The discussion of this study is consistent with various works about the different reinforcement effect of different types of fillers in the mastic. Additionally, the stiffening also depends on the type of bitumen [9; 16; 45]. We recommend studying the effect of packing of fillers in terms of its role in oxidative ageing through bitumen-aggregate interaction.  
In the next discussion, we have measured the stiffening effect of two fillers with increasing filler content based on rheology tests. Then, we selected the micromechanics models with the use of \(K_{\text{E}}\) and maximum packing fraction (\(\phi_{\text{m}}\)) to predict the stiffening effect of mastics.