# Influence of the Microwave Heating Time on the Self-Healing Properties of Asphal Mixtures
## 3 Results and Discussion
### Influence of Heating Time on the Thermal Behaviour of Asphalt Mixtures with Fibres
Furthermore, in view of the results presented in Table 2, it can be concluded that asphalt mixtures with higher contents of fibres reached, in general, the highest surface temperatures. Nevertheless, in the case of the samples with 8% of fibres, they did not register the highest surface temperature values. This was attributed to the bad distribution of the fibres inside the asphalt mixtures with 8% of fibres, which generates located heating in certain zones of the samples with presence of fibre clusters, see Figures 4d and 5d, respectively. Then, the located heating is distributed to the rest of the sample, as it can be observed in the sample with 2% of fibres, see Figure 4a. Thus, samples with 6% (Figure 4c), and 4% (Figure 4b) of fibres reached the highest surface temperatures (Table 2) for the heating time of 40 s, since they present a better distribution of the fibres in the asphalt mixture matrix. This result can also be seen in the CT-Scan images shown in Figure 5a-d.  
Additionally, Table 3 presents the results of bulk density and air voids content of the asphalt mixtures with different fibres content. As it can be observed, bulk density slightly decreased with the increase of fibres, while the air voids content increased with the increase of the amount of fibres added to the mixture. Therefore, the heat distribution in the samples with a high content of air voids is not the optimum (see Figure 4d), which is attributed to the heat dissipation through the air voids inside these asphalt mixtures. Conversely, regarding the cooling results shown in Table 2, it can be observed that in general the longest cooling times were recorded after the heating times of 20 s and 30 s. In contrast, the shortest cooling times were registered after 10 s and 40 s of heating. Additionally, these results are related with the temperatures reached by the samples. Thus, samples without fibres reached lower temperatures and consequently needed a shorter time to cool down, while samples with 6% and 8% of fibres reached the highest surface temperatures and took longer times to cool down. Finally, it can be deduced from Table 2 that, from a practical point of view, the best solution is to heat the samples with 4% or 6% of fibres during 40 s. This is because these samples presented the highest surface temperature increases and, also, they last 54 min and 63 min to cool down, respectively. These times are in the range of the cooling times obtained with a heating of 10 s. Besides, regarding  
Figure 4: Thermographic images of asphalt mixtures with different fibre content after 40 s of microwave heating: mixtures with (**a**) 2%; (**b**) 4%; (**c**) 6%; (**d**) 8% of steel wool fibres. Figure 5: CT-scan 3D reconstructions of fibre spatial distribution inside asphalt mixture samples with different fibre contents: (**a**) 2%; (**b**) 4%; (**c**) 6%; (**d**) 8% of steel wool fibres. Bounding box dimensions are 25 \(\times\) 25 \(\times\) 12 mm, approximately. the application of these asphalt mixtures in road pavements, they could be microwave heated during 40 s, reaching surface temperatures over 85 \({}^{\circ}\)C, implying that the traffic should be stopped during approximately 1 h to heal the cracks and return to the service temperature.