Effect of metallic waste addition on the electrical, thermophysical and microwave crack-healing properties of asphalt mixtures

This paper aims to evaluate the effect of metallic waste addition on the electrical, thermophysical and microwave crack-healing properties of asphalt mixtures. With this purpose, asphalt mixtures with two different types of metallic waste, steel wool fibres and steel shavings, added in four different contents, were tested. Electrical and thermophysical properties of asphalt mixture specimens with, and without, metallic waste were measured. The spatial distribution of the metallic waste inside the asphalt mixture samples was evaluated by using X-ray computed tomography. In addition, crack-healing properties of asphalt samples using microwave radiation heating were assessed at two different healing times, 30 s and 40 s. To quantify the efficiency of the healing process, five healing cycles were carried out for each asphalt sample. The main results showed that asphalt mixtures with shavings presented lower air void contents than mixtures with fibres. Moreover, fibres produced an increase in the electrical conductivity of the mixtures because long fibres in the mixtures form electrically conductive channels. In contrast, shavings did not have significant effect on the electrical properties of the mixtures. Likewise, it was proven that metallic waste reduced the thermal conductivity and the specific heat capacity of asphalt mixtures. Conversely, shavings decreased the thermal diffusivity of asphalt mixtures regardless of their content. Overall, it was found that the healing level reached by the asphalt mixtures tested by microwave radiation depends on the healing time and the type and content of metallic waste used. CT-scans results proved that the spatial distribution of metallic waste inside the asphalt mixture samples was not uniform and played an important role in the asphalt self-healing properties using microwave radiation heating.