## 2 Materials and Method
### Asphalt Specimen Preparation  
The asphalt mix design was based on the Marshall Mix design method to incorporate the use of natural and BOF slag aggregates. The optimum asphalt contents in various asphalt mixtures (dense, SMA, and PA) were determined in accordance to the Asphalt Institute method, MS-2 [30]. Three samples for each mixture was made at 4.5%, 5.0%, 5.5%, 6.0% and 6.5% asphalt by dry weight for a total of 15 samples. In addition, the viscosity blending chart was utilized to determine the binding and compaction temperature of the asphalt mixtures. The 15 samples were then heated to 120 \({}^{\circ}\)C to simulate the asphalt mix temperature during asphalt paving. In this study, the viscosity blending chart was employed to determine the optimum blend and compaction temperature of the new asphalt binder. Asphalt samples were also prepared for Marshall Mix testing at a blend temperature of 120 \({}^{\circ}\)C and a compaction temperature of 115 \({}^{\circ}\)C.  
The purpose of using the Marshall Mix design is to ensure that the proportion distribution of each volume of aggregate in the asphalt mixture can comply with the standards specifications of designed gradation. Marshall Mix design mainly measures different property parameters of asphalt content, such as asphalt mixture stability value, flow value, unit weight, air void (VA), VFA (voids filled with asphalt), and VMA (voids in the mineral aggregate), to determine the optimum asphalt content for all mixtures. The BOF slag was used to substitute portions of the coarse aggregate, while virgin fine aggregate was applied. The percentages of the BOF slag used were 0%, 20%, 40% and 60% by volume of coarse aggregate, which corresponded to 0%, 12%, 27% and 42% of total aggregate by volume, correspondingly. This study adopted AC-20 grade asphalt cement as the binder and mixed BOF slag with dense SMA and PA-graded, respectively; as well, the additive volume of coarse aggregate was set in the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test**} & **Results** & **Criteria** & **Specification** \\ \hline Specific gravity & 1.037 & – & AASHTO T228 [22] \\ Softening point (\({}^{\circ}\)C) & 57 & 50 & AASHTO T36 [23] \\ Ductility (25 \({}^{\circ}\)C, 5 cm/min, cm) & 100+ & 50 & AASHTO T51 [24] \\ Flash point (\({}^{\circ}\)C) & 230+ & 232 & AASHTO T48 [25] \\ Viscosity (60 \({}^{\circ}\)C, poise) & 1665 & 2000 \(\pm\) 400 & AASHTO M226 [26] \\ Solubility (\%) & 99.5 & 99 & AASHTO T44 [27] \\ Mix temperature (170 \(\pm\) 20 \(\mathrm{\SIUnitSymbolMicro}\), \({}^{\circ}\)C) & 155 & – & ASTM D2493 [28] \\ Compaction temperature (280 \(\pm\) 30 \(\mathrm{\SIUnitSymbolMicro}\), \({}^{\circ}\)C) & 143 & – & ASTM D2493 \\ Penetration (25 \({}^{\circ}\)C, 100 g, 5 s 0.1 mm) & 68.7 & 60 & AASHTO M20 [29] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The asphalt binder properties. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Test**} & **BOF slag** &
\begin{tabular}{c} **Natural Coarse** \\ **Aggregate** \\ \end{tabular} & **Criteria** & **Specification** \\ \hline Apparent specific gravity & 3.17 & 2.60 & – & ASTM C127-07 [18] \\ Water absorption (\%) & 2.60 & 1.69 & 3 & ASTM C127-07 \\ Los Angeles abrasion test (\%) & 11.30 & 28.89 & 40 & AASHTO T96 [19] \\ Fracture (\(\leq\)3\%) & 2.64 & 22.24 & – & ASTM D5821-01 [20] \\ Surface (\(\geq\)3\%) & 96.30 & 78.80 & – & ASTM D5821-01 \\ Flat and elongated ratio (\(\mathrm{\SIUnitSymbolMicro}\), \(\mathrm{\SIUnitSymbolMicro}\), \(\mathrm{\SIUnitSymbolMicro}\)) & 0.91 & 6.88 & 7 & ASTM D4791-10 [21] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The basic oxygen furnace slag (BOF) slag and natural aggregate properties. following ratios: 20%, 40% and 60%, and the test results of sieve analysis are shown in Tables 3-5. The nominal maximum size for each aggregate is 19 mm.