## 4 Discussion
### Performance of Stabilised Improved Marginal Materials  
The improved granular materials produced FBB materials with higher modulus values than the equivalent unimproved modulus values (Figure 8). On average, the Weipa I FBB modulus was 23% higher than the equivalent Weipa FBB modulus, while the average Stonemaster I FBB modulus was 18% higher than the equivalent Stonemaster FBB modulus. The greater improvement associated with the Weipa granular material reflects the fact that it was initially more marginal than the Stonemaster material and the improvement in the granular materials properties (Table 5) was therefore greater.  
Furthermore, the average increase in the FBB modulus associated with the Stonemaster granular material was comparable for both the soaked and saturated conditions, at 19% and 17%, respectively. In contrast, the increase in the FBB modulus produced with the Weipa granular material was greater in the saturated condition (31%) than in the cured condition (14%). This reflects the high percentage of reactive fine material in the unimproved Weipa granular material and the significant reduction in PI and fines ratio achieved by its gradation improvement, compared to the lesser improvement achieved in the Stonemaster granular material. This emphasises the importance of understanding the basis on which a material is considered to be marginal, the extent of the deviation from the specification requirement for standard material properties, and how an improving treatment will affect the marginal properties.  
Figure 7: Marginal material FBB modulus compared to reference FBB modulus.  
Figure 9 compares the improved FBB modulus values to the reference (Ayreshire) and Bluerock FBB modulus values, at all curing times, in both the cured and saturated conditions. The Weipa I FBB modulus values were, on average, 99% of the equivalent Ayreshire FBB values. This is up from 81% for the unimproved Weipa FBB. In fact, the Weipa I modulus was consistently the highest of all the materials except for the reference material. The Stonemaster I FBB averaged 80% of the Ayreshire FBB modulus values, up from 67% before improvement. Despite being effectively a standard material and not worthy of improvement, the FBB produced with the Bluerock material consistently had the lowest modulus of all the FBB materials. This would not have been predicted based on comparison of the various material gradations (Figures 2 and 3) and the material properties (Tables 1 and 5). This likely reflects the importance of having enough fine particles in the granular material to be available for mixing with the foamed bitumen and to subsequently spot weld the coarse aggregate particles together. This also indicates that plasticity in the fine aggregate particles is not necessarily detrimental to FBB performance because the FBB with the lowest modulus also had the lowest weighted PI and the lowest weighted shrinkage of all the granular materials considered. This is consistent with research reported elsewhere, where the adhesive bitumen in freshly recycled asphalt was found to bind-up the fine particles in an existing granular material, with additional fines required from a sandy loam to allow suitable FBB to be produced [21].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Property** & **Method** & **Limits** & **Stonemaster** & **Stonemaster I** & **Weipa** & **Weipa I** \\ \hline Liquid limit (\%) & Q104A & 25 max & 22 & 18 & 36 & 28 \\ Plasticity index (\%) & Q105 & 10 max & 7 & 5 & 12 & 9 \\ Weighted Plasticity (\%) & Q105 & 150 max & **175** & 95 & 342 & 230 \\ Linear shrinkage (\%) & Q106 & 6 max & 5 & 3 & 5 & 4 \\ Weighted shrinkage (\%) & Q106 & 85 max & **115** & 53 & **140** & 96 \\ Fines ratio (\%) & Q103A & 0.30–0.55 & 0.40 & 0.44 & **0.57** & 0.42 \\ \hline \hline \end{tabular} Red text denotes properties that fall outside the limits.  
\end{table}
Table 5: Improved granular material properties.  
Figure 8: Effect of improvement on FBB modulus.