# 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  
Table 2 shows the average results of the heating-cooling test of asphalt mixtures. The values presented in this Table are: T\({}_{\text{max}}\), maximum surface temperature reached by the samples; and CT, cooling time until the samples reached the temperature before heating (20 \({}^{\circ}\)C). Regarding to heating of asphalt mixtures, it can be observed that the surface temperature of the samples (T\({}_{\text{max}}\) in Table 2) increased with the increase of the microwave heating time, and that this temperature was higher in samples with higher fibres contents. Thus, samples heated during 40 s registered the highest temperatures, followed by samples heated during 30 s, 20 s and 10 s, respectively. Additionally, it can be seen that asphalt mixtures with 6% of fibres registered the highest surface temperatures, followed by mixtures with 4%, 8%, 2% and 0%, in that order (reaching surface temperatures of 98.86 \({}^{\circ}\)C, 87.41 \({}^{\circ}\)C, 78.71 \({}^{\circ}\)C, 59.76 \({}^{\circ}\)C and 50.46 \({}^{\circ}\)C, respectively) when they were heated during 40 s. Also, a similar trend was observed with the other three heating times. Moreover, using short heating times (10 and 20 s), asphalt mixtures with different contents of fibres presented similar temperature increases (see Table 2). For example, samples with 0% and 8% of fibres registered temperature increases of 7.91 \({}^{\circ}\)C and 10.33 \({}^{\circ}\)C respectively, when they were heated during 10 s. However, in the case of using long heating times (30 and 40 s), the asphalt mixtures with different contents of fibres registered big differences regarding to their temperature increase, with the same microwave heating time. Specifically, samples with 2% and 6% of fibres presented an increase on their surface temperature of 39.76 \({}^{\circ}\)C and 78.86 \({}^{\circ}\)C respectively, when they were heated during 40 s. This reveals that 10 s and 20 s are too short heating times to induce a significant heating of the fibres that can distribute the temperature through the asphalt mixture and melt the bitumen, resulting in those similar temperature increases in the samples regardless their fibre content. Therefore, it does not seem likely that asphalt mixtures can heal their cracks using heating times of 10 s and 20 s, due to the fact that the bitumen did not reach the temperature threshold required to reduce its viscosity and flow through the cracks opened in the pavement, sealing it [21]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**Microwave Heating Time (s)**} \\ \cline{2-9}
**Fibres Content** & \multicolumn{2}{c}{**10**} & \multicolumn{2}{c}{**20**} & \multicolumn{2}{c}{**30**} & \multicolumn{2}{c}{**40**} \\ \cline{2-9}
**(\% vol. Bitumen)** & \multicolumn{2}{c}{**T\({}_{\text{max}}\)**} & \multicolumn{1}{c}{**CT**} & \multicolumn{1}{c}{**T\({}_{\text{max}}\)**} & \multicolumn{1}{c}{**CT**} & \multicolumn{1}{c}{**T\({}_{\text{max}}\)**} & \multicolumn{1}{c}{**CT**} & \multicolumn{1}{c}{**T\({}_{\text{max}}\)**} & \multicolumn{1}{c}{**CT**} \\  & **(\({}^{\circ}\)C)** & **(min)** & **(\({}^{\circ}\)C)** & **(min)** & **(\({}^{\circ}\)C)** & **(min)** & **(\({}^{\circ}\)C)** & **(min)** \\ \hline
**2\%** & 28.11 & 55 & 40.76 & 60 & 47.33 & 63 & 59.76 & 56 \\
**4\%** & 28.13 & 55 & 45.56 & 81 & 64.90 & 85 & 87.41 & 54 \\
**6\%** & 29.19 & 63 & 55.71 & 90 & 65.30 & 87 & 98.86 & 63 \\
**8\%** & 30.33 & 65 & 47.19 & 88 & 61.93 & 81 & 78.71 & 92 \\ \hline
**Ref. 0\%** & 27.91 & 48 & 40.08 & 22 & 45.26 & 52 & 50.46 & 44 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average results of the heating-cooling test on asphalt mixtures. Figure 3: Schematic diagram of the measurement of the crack size before and after microwave heating.