## 3 Test Results and Discussions
### BOF Substitute Proportional Influence on Specim Cooling  
Figure 3 shows the temperature variations at different BOF slag substitution ratios in the specimens. Since the specimen surfaces were in contact with the air, the temperature dropped quickly. All of the specimens have a maximum cooling gradient in the initial high-temperature status. As seen in Figure 3a, different amounts of BOF slag in dense-graded mixtures have a starting temperature of around 105-112 \({}^{\circ}\)C. The temperatures at the surface decreased quickly, but stabilized after 80-90 min. At the end of the measurements at 180 min, the mixtures contained 20% of BOF slag had reached a stable condition at a temperature of 67 \({}^{\circ}\)C. However, for BOF slag at 40% and 60%, the corresponding temperatures measured at 180 min were 69 \({}^{\circ}\)C and 74 \({}^{\circ}\)C, respectively.  
Since the specimen is placed in a confined indoor space and the control room temperature is about 25 \({}^{\circ}\)C, the atmospheric temperature has only a slight effect on the cooling tendency among specimens. All of the specimens' temperatures at 0 minutes are different.  
As seen in of Figure 3b,c, after SMA and PA-graded mixtures are cooled for 120 min, the temperature tended to moderate, indicating that the heat-releasing effect of SMA and PA-graded mixtures tended to moderate, with heat preservation effectiveness. Additionally, the cooling rate of SMA and PA-graded mixtures decreased with the increased substitute proportion of BOF slag, which is similar to the cooling effect of adding BOF slag in dense-graded mixtures, and the cooling rate decreased significantly. Similar replacement proportions of 0%, 20%, 40% and 60% had the temperature when compared at 180 min; where the temperatures of SMA gradation were 66 \({}^{\circ}\)C, 69 \({}^{\circ}\)C,70 \({}^{\circ}\)C and 73 \({}^{\circ}\)C, respectively; the temperatures of the PA gradation were 68 \({}^{\circ}\)C, 69 \({}^{\circ}\)C, 70 \({}^{\circ}\)C and 74 \({}^{\circ}\)C, correspondingly.  
Figure 3: Temperature gradient variation diagram.  
After the compaction of asphalt specimens, the cooling temperature increased with the increased proportion of BOF slag, which means that besides having lipophilic qualities, BOF slag also has a heat preservation effect to maintain the temperature between aggregates, and facilitates the stable binding of asphalt with aggregate under sufficient temperature. The asphalt mixture used a high-proportion BOF aggregate; it needs to extend the heat to be released completely. The asphalt mixture can have highly stable properties. Furthermore, SMA and PA-graded mixtures are designed with a high proportion of coarse aggregate, and the aggregate skeleton is in direct contact between coarse aggregates to form the interlocking mechanism, as well as adding the high-proportion BOF as the coarse aggregate. Thus, the temperature can be preserved at the inter-contact status, and the cooling rate would be slow as well.  
The purpose of this study was to estimate the cooling variable for each graded mixture and different BOF slag substitutions. Based on this, all of the specimens were placed in a confined space at room temperature and were maintained at 25 \({}^{\circ}\)C. The effects of different graded mixtures and slag substitutions on the cooling effect for each specimen are much higher than the atmospheric temperature.