# Influence of the Microwave Heating Time on the Self-Healing Properties of Asphal Mixtures
## 4 Conclusions  
This paper has studied the effect of the microwave heating time on the self-healing properties of asphalt mixtures containing metallic fibres. To do this, an experimental study including heating-cooling tests and cracking-healing cyclic tests was carried out on asphalt mixtures with four different contents  
Figure 11: (**a**) Box plot representation of the crack size before healing (heating time = 0 s) and after healing of asphalt mixture samples at different heating times; (**b**) Optical images of a cracked asphalt sample before and after heating by microwave radiation at 40 s (Image dimensions are \(8\times 10\) mm, approximately). Figure 10: (**a**) Frequency histogram and statistical parameters of the crack initial size data; and (**b**) Normal probability-probability plot for the crack sizes. of steel wool fibres, evaluating the (1) thermal behaviour under microwave heating; (2) self-healing properties at different heating times; and (3) crack-sizes in the asphalt mixtures before and after healing. The main conclusions of this article are summarised as follows:  
* It was proved that the longer the microwave heating time and the higher the amount of fibres, the higher the reached temperatures. However, the influence of the metallic fibres amount on the asphalt mixtures heating occurs in the case of long times (30 s and 40 s). * Additionally, it was found that asphalt mixtures with 6% of fibres reached the highest surface temperatures followed by mixtures with 4% of fibres, while mixtures with 8% of fibres presented difficulties during heating due to the bad distribution of the fibres, proved by CT scans analysis. * In addition, cooling times after heating the asphalt mixtures during 40 s are in the range of those obtained with a heating of 10 s (between 54 min and 63 min). Consequently, the best solution is to heat the samples with 4% or 6% of fibres during 40 s, thus achieving that the application of these asphalt mixtures in road pavements should imply that they could be microwave heated during 40 s, implying that the traffic should be stopped for approximately 1 h to heal the cracks and return to the service temperature. * Moreover, it was found that the healing level of the asphalt mixtures increased with the increase of the microwave heating time, due to the higher temperatures reached by the samples when heated longer times. However, using long heating times (30 s and 40 s), the mechanical resistance of asphalt mixtures and consequently their healing levels were reduced after each healing cycle. * It was also found that the temperature achieved as a result of the heating time is the most influential variable on the self-healing properties of asphalt mixtures, while the fibres amount did not have a significant influence on the healing levels of asphalt mixtures. * In addition, based in the results of this and previous research, it was concluded that the optimum microwave heating time is 40 s, since the highest healing levels are reached without an early damage of the bitumen. * Finally, the probability analysis of the morphological study of the cracks revealed that the presence of fibres did not have influence on the obtained crack-size. Additionally, in view of the crack-sizes obtained with the different microwave heating times, it was demonstrated that the cracks can be sealed using long heating times (30 s and 40 s), consequently achieving the self-healing of asphalt mixtures via microwave heating. The results contained in this paper are part of a research about self-healing asphalt mixtures including metallic waste materials that is being developed in Chile. The second part of the study, currently under development, is the evaluation of the effect of adding different metallic waste to asphalt mixtures, e.g., steel shavings, analysing their self-healing properties under microwave radiation.