## 3 Results and Discussion
### Effect of Steel Fibers and Steel Slag on the Heating and Cooling Characteristics of Asphalt Mixture  
The average surface temperature variations of different asphalt mixtures during induction heating and cooling were shown in Figure 7. As shown in Figure 7a, the average surface temperature of the samples increased linearly with heating time. The heating speed can be obtained by fitting line to get the slope of each curve. The induction heating speeds of the three mixtures (SS, SF, and SS + SF) were 0.15, 1.11 and 1.25 \({}^{\circ}\)C/s respectively. Steel slag containing a limited amount of metal oxides can be induction heated. Due to the limited amount of steel slag replacing mineral aggregates (only size 9.5-13.2 mm), the induction heating speed of SS mixture was quite low. In SS mixture, steel slag was the heat generating source and heat transferred from steel slag to mortar and other basalt aggregate was slow, leading to a more uneven heating in asphalt mixture. The steel fibers distributed quite uniformly in mortar of asphalt mixture, forming loop coil which can produce closed current. So, steel fibers can be heated very quickly under electromagnetic induction heating and SF showed a high heating speed. The heating speed of asphalt mixture containing both steel fibers and steel slag was the highest because of the co-heating effect of steel slag and steel fibers. The heating speed of SS + SF  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mixture Type** & **Thermal Conductivity** & **Thermal Diffusivity** & **Volumetric Heat Capacity** \\  & **(W/(m-K))** & **(mm2/s)** & **(MJ/(m\({}^{3}\)-K))** \\ \hline BA & 1.521 & 0.717 & 2.121 \\ SF & 1.676 & 0.834 & 2.009 \\ SS & 1.434 & 0.586 & 2.447 \\ SF + SS & 1.529 & 0.692 & 2.210 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Thermal parameters of four kinds of asphalt mixtures.  
mixture was exactly the sum of the heating speeds of SS and SF, indicating that the composite of steel fibers and steel slag can enhance induction heating speed of asphalt mixture. It is expected that the induction heating speed of asphalt mixture can be further improved by increasing the proportion of steel slag replacing mineral aggregates.  
Crack healing occurs both during induction heating and cooling after heating. But the healing speed will gradually decrease due to the decrease of the temperature. Therefore, it is necessary to analyze the cooling process of different types of asphalt mixtures after induction heating. The heating of SS was quite slow and uneven, so the cooling curve of SS was not discussed in Figure 7b. The cooling curves of other two kinds of mixtures were shown in Figure 7b. The samples were heated to 80 \({}^{\circ}\)C (average top surface temperature) and then placed in a constant temperature room to monitor the cooling process with an infrared camera. As shown in the cooling lines of the two asphalt mixtures, the temperature of SF decreased faster compared with SF + SS. The difference of cooling performance is mainly due to the performance of materials, because the initial temperature and the environment temperature are the same. It has been demonstrated from the thermal constants test that the thermal  
Figure 7: Temperature-versus-time curves of asphalt mixtures with induction heating. (**a**) During heating; (**b**) During cooling.  
conductivity and thermal diffusion of SF are the highest, and the volumetric heat capacity of SS is highest. At the surface of the specimen heat transfers both to the lower part of the sample and to the air, thus the temperature decreased more rapidly than the inner positions. SF mixture lost heat more quickly as the material had higher thermal conductivity and thermal diffusion, which explained the difference of cooling rates of the two different kinds of mixtures.