# Waste Silt as Filler in Hot Mix Asphalt: A Laboratory Characterization
## 4 Results
### Hot Mix Asphalt Results  
#### 4.2.1 Optimal Binder Content and Air Voids  
The control mix design (HMA-C) obtained from the previous analysis had an OBC of 5.5% on the mass of aggregates. However, the samples produced with untreated (HMA-SN)  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Stress Creep 0.1 kPa**} & \multicolumn{2}{c}{**Stress Creep 3.2 kPa**} \\ \cline{2-5}
**Sample** & **Percent Recovery (\%)** & **Jnr (\%)** & **Percent Recovery (\%)** & **Jnr (\%)** \\ \hline M-C & 1.440 & 1.740 & 0.000 & 1.880 \\ M-SC & 0.294 & 1.570 & 0.000 & 1.699 \\ M-SN & 0.000 & 2.740 & 0.000 & 2.896 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Average recoverable strain and non-recoverable compliance at the two stress levels.  
Figure 3: The results of the MSCR tests at 60 \({}^{\circ}\)C for all mastic samples.  
or calcined silt (HMA-SC) could require a different amount of binder depending on the possible different absorption properties of the powders. Thus, the optimal binder content of the experimental HMAs was calculated.  
For each mixture, five different binder contents (5.0-6.0%) were selected, resulting in 15 Marshall samples for each HMA type. The samples were produced according to the EN 12697-34 standard by applying 75 blows to each side during the sample compaction. Samples were immersed in water for 30 min at 60 \({}^{\circ}\)C and were tested using the Marshall stability test. The average data for the Marshall stability and air voids content (Va, EN 12697-8) are summarized in Table 3 and presented in Figure 4.  
The maximum Marshall stability of 17.20 kN was reported for the control sample (HMA-C). The mixtures produced with thermally treated silt followed closely behind with a maximum stability equal to 16.68 kN. The lowest Marshall stability value was obtained when untreated silt was used as filler in the mixtures. The performance of calcined silt was better in terms of stability when compared with untreated silt. A similar trend was observed when thermally treated and untreated bentonite clays were used as fillers in HMA mixtures [20]. However, the returned value for the optimal binder content was 5.5% for both mixtures. Thus, the substitution of the limestone filler with untreated or calcined silt did not affect the optimal binder content value. Moreover, the reported stability values were higher than the 11 kN suggest by the Italian technical specifications for this type of bituminous mixture.  
The air voids content calculated for samples produced with the optimal binder content is also shown in Table 3. The highest compaction rate was related to the control sample.  
Figure 4: Marshall stability for the HMA samples.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Bitumen (\%)** & **Stability (kN)** & **Va (\%)** \\ \hline HMA-C & 5.5 & 17.20 & 3.27 \\ \hline HMA-SN1 & 5.00 & 11.53 \(\pm\) 0.12 & 4.03 \(\pm\) 0.03 \\ HMA-SN2 & 5.25 & 12.51 \(\pm\) 0.29 & 4.01 \(\pm\) 0.04 \\ HMA-SN3 & 5.50 & 13.25 \(\pm\) 0.37 & 3.89 \(\pm\) 0.07 \\ HMA-SN4 & 5.75 & 11.93 \(\pm\) 0.15 & 3.88 \(\pm\) 0.05 \\ HMA-SN5 & 6.00 & 11.67 \(\pm\) 0.30 & 3.81 \(\pm\) 0.03 \\ \hline HMA-SC1 & 5.00 & 12.99 \(\pm\) 0.22 & 3.76 \(\pm\) 0.05 \\ HMA-SC2 & 5.25 & 15.47 \(\pm\) 0.14 & 3.82 \(\pm\) 0.03 \\ HMA-SC3 & 5.50 & 16.68 \(\pm\) 0.19 & 3.91 \(\pm\) 0.08 \\ HMA-SC4 & 5.75 & 16.11 \(\pm\) 0.25 & 3.95 \(\pm\) 0.01 \\ HMA-SC5 & 6.00 & 15.27 \(\pm\) 0.10 & 3.93 \(\pm\) 0.07 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Marshall stability and air voids content for HMA mixtures.  
The addition of waste silt to asphalt mixtures increased the air voids content regardless of the type of silt used. However, all values were within the range of 3-4.5% suggested by the Italian technical specifications taken as a reference.