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.

Microwave heating is regarded as a promising technique to promote crack-healing of asphalt mixtures reinforced with steel wool fibres. In addition to serving as a heat source when subject to microwave radiation, steel wool fibres are expected to affect the physical and mechanical properties of the asphalt pavements. However, it is not clear what this effect is, and what is the optimum fibre content that can provide effective crack-healing without having a negative impact on other relevant mixture properties. This paper reports a study of the steel wool fibres spatial distribution and their influence on the physical and mechanical properties of asphalt mixtures. For this purpose, five different dense asphalt mixtures, with the same aggregates gradation and bitumen content, but with five different percentages of steel wool fibres were manufactured. Then, their mechanical properties such as particle loss resistance in dry and wet conditions, and stiffness modulus and cracking resistance in Mode I of fracture at four different temperatures were evaluated. Samples of these mixtures were examined using Scanning Electron Microscopy and analysed using X-ray micro computed tomography to study the condition and distribution of fibres within the bitumen matrix. Microscopy results showed that fibres can be damaged during the mixing and compaction processes. A larger variability in the local distribution of fibres for mixtures incorporating a higher fibre content was observed in the tomography analysis, with presence of fibre clusters more than double of the average fibre content of the mixture. Although addition of fibres appears to reduce the bulk density of mixtures, according to tomography analysis differences in average porosity between samples were not statistically significant. Finally, it was confirmed that regardless of test temperature, steel wool fibres did not have a relevant influence on the improvement of particle loss resistance, stiffness modulus and cracking resistance of asphalt mixtures.