## 3 Test Results and Discussions
### Void Distribution of the Asphalt Mixture  
This study used infrared thermal imaging photos to prove the void distribution of the specimen by adding 40% BOF aggregate into three asphalt mixture gradations, for example, when comparing the cooling time at the 60 th minute. As shown in Figure 8a, the dense mixture surface temperature image shows the center temperature of the specimen to be 91.7 \({}^{\circ}\)C; the temperature at the right side of the specimen is 94.8 \({}^{\circ}\)C. Since the heat exchange process would be transferred to the area with more voids, it is determined that the right side of the specimen may have more voids.  
Based on measuring of the surface and internal temperatures from the sides of the dense mixture, as shown in Figure 8b, it is found that the surface temperature of the specimen is about 92.4 \({}^{\circ}\)C, and its internal temperature cooling rate is fast. The yellow area is about 60-70 \({}^{\circ}\)C, which shows that after compacting the pavement in the cooling phase, the internal heat energy of asphalt mixture is  
Figure 8: Dense-graded mixture thermal image (\({}^{\circ}\)C).  
continuously transferred upward; therefore, the surface temperature of pavement was higher than the internal one. In measuring the test results, as expected, for dense mixture, there was a significant difference between the internal and surface temperatures. Furthermore, from the thermal image, it can be determined that the distribution of voids of dense mixture is not uniform and the thermal energy temperature is inconsistent. This indicates that the non-uniform distribution of the voids on dense mixture caused the cooling process to decrease the loading resistance.  
From Figure 9a, it can be found that the center temperature of the SMA specimen was 85.78 \({}^{\circ}\)C; those temperatures around the sides without any significant difference were about 85.16-85.98 \({}^{\circ}\)C, meaning that the internal void of the SMA mixture was more uniform than that of the dense mixture. The characteristic of the asphalt binder exhibits viscous-plastic behavior at a higher temperature. Thus, the film of the asphalt binder can be filled with voids during the high-temperature mixing process, and it would have smaller voids and heat energy that might not be easily dissipated. Observed from the side thermal image, as shown in Figure 9b, the SMA mixture heat can be transferred from the internal space up to the surface of the specimen. Since its internal voids are more uniform, its internal heat energy (yellow area) area occupied a larger range (60-80 \({}^{\circ}\)C); this showed that, in the cooling phase, the uniform void distribution can maintain the temperature with a slower heat dissipation rate than that of the dense mixture.  
As shown in Figure 10a, when the center temperature of the PA mixture is 88.59 \({}^{\circ}\)C, temperatures at the sides without any significant difference are about 89.12-87.83 \({}^{\circ}\)C. A similar trend is also observed for the SMA mixture. In Figure 10b, the internal heat energy (yellow area) area on the PA specimen is more significant (60-80 \({}^{\circ}\)C), which means that after adding BOF slag into the PA mixture, it increased the heat preservation effect of BOF slag with a slower cooling rate, promoting the bonding capacity for the PA mixture.  
Figure 10: PA-graded mixture thermal image (\({}^{\circ}\)C).  
Figure 9: SMA-graded mixture thermal image (\({}^{\circ}\)C).