# Influence of the Microwave Heating Time on the Self-Healing Properties of Asphal Mixtures
## 3 Results and Discussion
### Influence of Fibres and Heating Time on the Asphalt Self-Healing Properties  
Figure 6a-d shows the results of the healing levels reached by the asphalt mixtures with fibres depending on the number of healing cycles, using microwave heating at 4 different times: 10 s, 20 s, 30 s and 40 s. In these Figures, it can be observed that microwave heating helps to the partial resistance recovery of cracked asphalt mixtures with fibres. In addition, it can be observed that the healing levels reached by the mixtures increased with the increase of the microwave heating time. Thus, the highest healing levels reached by the samples are the obtained using 40 s as the heating time, followed by those obtained using 30 s, 20 s and 10 s, respectively (see Figure 6). This was produced because the longer the heating time, the higher the temperature reached by the samples (see Table 2), which allowed to reach higher healing levels. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Fibres Content** & \multirow{2}{*}{**Bulk Density (g/cm\({}^{3}\))**} & \multirow{2}{*}{**Air Voids Content (\%)**} \\
**(\% vol. bitumen)** & & \\ \hline
**2\%** & 2.33 \(\pm\) 0.02 & 7.45 \(\pm\) 0.98 \\
**4\%** & 2.32 \(\pm\) 0.02 & 8.70 \(\pm\) 0.98 \\
**6\%** & 2.31 \(\pm\) 0.02 & 9.64 \(\pm\) 0.92 \\
**8\%** & 2.28 \(\pm\) 0.03 & 11.43 \(\pm\) 0.97 \\ \hline
**Ref. 0\%** & 2.35 \(\pm\) 0.02 & 6.70 \(\pm\) 0.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bulk density and air void content of asphalt mixtures (Average \(\pm\) Stand. Deviation). Figure 6: Healing levels of asphalt mixtures with different content of fibres, depending on the number of healing cycles, heated in a microwave at 4 different times: (**a**) 10 s; (**b**) 20 s; (**c**) 30 s and (**d**) 40 s.  
Additionally, it can be seen that in the cases of short heating times (10 and 20 s), the number of healing cycles has not a significant influence on the healing level of the samples. For example, in the case of 20 s heating times (see Figure 6b), samples with 6% of fibres reached healing levels of 38% and 29% after the first and tenth cycles, respectively. In contrast, with longer heating times (30 and 40 s), it can be observed that the healing levels reduced with the increase of the number of healing cycles. In the case of samples with 4% of fibres heated 40 s (see Figure 6d), healing levels of 93% and 49% are reached after the first and tenth healing cycles, respectively. The variation can be attributed to the high temperatures reached by the asphalt samples when heated for long times (see Table 2). This behaviour was also observed in previous researches [17] that demonstrated that microwave heating can change the internal structure of asphalt mixtures, modifying the aggregates position inside the samples and thus changing the air void content distribution of the mixtures. In this way, the mechanical resistance of asphalt mixtures and consequently their healing levels are reduced after each cycle. In addition, using these heating times (30 and 40 s), the bitumen can also be damaged in each cycle (see Figure 7). Furthermore, in cases of long heating times it can be observed that healing levels also depend on the fibres content added to the asphalt mixtures. In general, mixtures with 6% and 2% of fibres registered the highest healing levels in the case of 30 s heating (see Figure 6c), followed by mixtures with 4% and 8% of fibres, respectively. Therefore, higher healing levels are not necessary attributed to higher mechanical resistance and higher fibres contents, as can be deduced from the obtained results of the maximum flexural force (force initially resisted by the samples before submitted to cracking-healing cycles) in Figure 8a. This result can be due to the temperature reached by the samples when heated during 30 s (see Table 2). Thus, the samples that reached high temperatures (6% of fibres) presented high healing levels (60% on average).