3.2. Influence of Aggregate Nature and Content of Steel Wool Fibers

For both aggregate types, the heating rate increases with the content of ferromagnetic additives. Asphalt mixtures with 2% steel wool fibers only reached 100 °C at the maximum current level, whereas mixtures with 4% content achieved this temperature even at medium current levels.

Moreover, the difference in heating rates between 2% and 4% fiber content becomes more significant at higher current intensities. The difference in temperature increase between the two fiber contents is smaller at current intensity 1, but becomes more substantial at intensity levels 2 and 3.

Thus, it can be concluded that both fiber content and current intensity are key factors in the induction heating of asphalt mixtures.

3.3. Influence of Frequency and Coil Shape

One of the key phenomena in induction heating is the “skin effect”, where alternating current (AC) tends to concentrate near the surface of a conductive material rather than penetrating uniformly. This occurs due to the generation of eddy currents, which create opposing magnetic fields that limit current flow to the material’s outer layers. The depth at which current density significantly decreases is called the “skin depth”, and it depends on the material’s conductivity, magnetic permeability, and induction frequency. In theory, higher frequencies or more conductive materials result in a shallower skin depth, making induction heating highly efficient for surface heating.

When heating with a single-turn coil, the steel slag coarse aggregate specimens showed similar surface and depth temperature gradients, regardless of induction frequency. However, for asphalt mixtures with porphyry coarse aggregate, the temperature gradient decreased with increasing frequency.

With a double-turn coil, the surface temperature gradient for asphalt mixtures with steel slag aggregate remained nearly constant across frequencies, while the depth gradient increased by about 10 °C as frequency increased. In contrast, for porphyry coarse aggregate, both surface and depth gradients decreased with frequency, resulting in the highest heating homogeneity observed in this study for the mixture with porphyry coarse aggregate and 4% fiber content.

Finally, with a centered two-turn coil, more heterogeneous heating was observed for asphalt mixtures with porphyry coarse aggregate, with greater surface gradients as the frequency increased. No significant differences in temperature gradients were noted for mixtures with steel slag aggregate.

Thus, it can be concluded that the frequency does not significantly affect heating homogeneity in asphalt mixtures with steel slag coarse aggregate. However, for mixtures with porphyry coarse aggregate, heating becomes more homogeneous with increasing frequency, especially in depth distribution. This behavior, contrary to expectations, suggests that asphalt mixtures do not behave purely as conductive materials, and assumptions based on conductive material behavior may lead to incorrect conclusions when studying self-healing asphalt.

Regarding coil shape, more homogeneous heating was achieved with the double-turn coil compared to the single-turn coil. However, when comparing the double-turn coil to the centered two-turn coil, better heat distribution was observed with the former. This is likely due to the accelerated surface heating with the centered two-turn coil, which generates higher temperatures near the coils and increases both the surface and depth temperature gradients, with a more pronounced effect at the surface. To prevent surface damage and ensure the internal bitumen flow, it is essential that heating is as homogeneous as possible, as high temperature gradients could harm the surface bitumen, limiting the healing effect.