## Materials
### NHSS-AC Samples Preparation  
Nano hydrophobic silane silica modified asphalt samples with NHSS (1, 3, 5% by mass of base asphalt) were prepared to determine the optimal NHSS content. Due to the high specific surface of NHSS, pre-mixing of the heated NHSS and asphalt binder is necessary for easier mixing. Firstly, base asphalt and prepared NHSS was heated to 165 \({}^{\circ}\)C in an oven (2008, Xinxing Test Instruments Co. Ltd, Changchun, China) and preserved for 1h. After pre-mixing, a high-speed mixer (2008, Xinxing Test Instruments Co. Ltd., Changchun, China) was adopted to make the NHSS mixed more uniformly with base asphalt by maintaining a low speed of 1000 r/min for 5 min and proceeding to a higher speed of 3000 r/min for 15 min afterwards. The property tests of NHSS modified asphalt samples were conducted to determine the optimal NHSS content. The results are shown in Table 4.  
The results show that the addition of NHSS modifiers improves some properties of the base asphalt. The softening point value, 5 \({}^{\circ}\)C ductility value, PI value, and 135 \({}^{\circ}\)C apparent viscosity value of 3% NHSS-asphalt are greater than 1% NHSS-asphalt and base asphalt, and the softening point value, 135 \({}^{\circ}\)C apparent viscosity of 5% NHSS-asphalt are greater than 3% NHSS-asphalt. Thus, considering that the more the additive is added, the greater the cost is, and the improvement of the overall performance, 3% NHSS was subjectively identified as the optimum content for base asphalt.  
The optimum bitumen content of NHSS modified asphalt concrete (NHSS-AC) was determined by Marshall test according to Chinese specifications for design of highway asphalt pavement (JTG D50-2006). Four asphalt mixes were used--4.5%, 5.0%, 5.5%, and 6.0%--and the NHSS-AC were prepared using the standard Marshall mix design procedure with 75 blows on each side of the cylindrical specimens. The Marshall stability (MS), flow (FL), volume of air voids (VV), and voids filled with asphalt (VFA) values of the mixtures with different asphalt content were obtained to determine the optimum asphalt content (OBC) of NHSS-AC. The optimum modified asphalt content was found to be 5.7% for the control mixture. In this study, all NHSS-AC samples were made with 5.7% modified asphalt content in order to maintain consistency throughout the research. Moreover, base asphalt concrete (AC) was prepared for the same tests as a comparative, and the optimum asphalt content was found to be 5.01% for the control mixture. The reason why the OBC of NHSS-AC is higher than AC  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Technical** & \multicolumn{3}{c}{**Penetration (mm)**} & **Softening** & **10 \({}^{\circ}\)C** & **5 \({}^{\circ}\)C ductility** & **Penetration** & **135 \({}^{\circ}\)C Apparent** \\
**Parameters** & **15 \({}^{\circ}\)C** & **25 \({}^{\circ}\)C** & **30 \({}^{\circ}\)C** & **Point (\({}^{\circ}\)C)** & **Ductility (cm)** & **(cm)** & **Index** & **Viscosity (mpa-s)** \\ \hline base asphalt & 27.5 & 81.3 & 132.6 & 44.2 & 45.7 & 6.3 & \(-0.87\) & 485.3 \\
1% NHSS-asphalt & 26.8 & 71.5 & 113.9 & 47 & 20.9 & 6.5 & \(-0.32\) & 721.3 \\
3% NHSS-asphalt & 28.4 & 71.2 & 106.9 & 48.6 & 17 & 6.6 & 0.24 & 2219.6 \\
5% NHSS-asphalt & 24.0 & 68.4 & 105.9 & 50 & 12.8 & 6.1 & \(-0.52\) & 7455.7 \\ \hline \end{tabular}
\end{table}
Table 4: Property parameters of modified asphalt with different NHSS.  
may be that some of the NHSS has chemically reacted with the asphalt, and the other part of the NHSS and the asphalt are simply physically blended, the physically blended NHSS acts as a mineral powder, so the OBC of NHSS-AC is higher than AC.