According to WPB, a new scientific study has demonstrated that incorporating steel slag powder into asphalt mixtures can significantly improve their ability to repair cracking damage when exposed to microwave heating. The research, published on October 10, 2026 achieved a healing index of 84.7% under optimized laboratory conditions. However, repeated heating also accelerated asphalt binder aging, highlighting a critical challenge for the long-term durability of self-healing pavement technologies.
The study, conducted by a research team led by Dongmei Zhang and colleagues, investigated the combined effects of microwave heating on crack recovery and asphalt aging. Researchers used steel slag powder obtained from electric arc furnace (EAF) steelmaking to replace part of the conventional mineral filler in an AC-13 asphalt mixture. The objective was to improve the material's response to microwave energy while determining how repeated heating and healing cycles influence its mechanical and chemical properties.
Steel slag is particularly suitable for microwave-assisted heating because it contains iron oxides and other compounds capable of absorbing electromagnetic energy and converting it into heat. Conventional asphalt mixtures generally absorb microwave energy less efficiently, limiting the effectiveness of externally stimulated healing. Introducing steel slag powder can increase heating efficiency and help raise the temperature of the bituminous binder around damaged areas.
The technology works by temporarily heating the asphalt mixture to improve binder mobility. As the binder softens, it can flow into small cracks and reconnect damaged interfaces, allowing part of the material's mechanical performance to recover. Unlike fully autonomous self-healing systems, this approach requires an external microwave energy source to activate the repair process.
Using semicircular bending (SCB) tests and an orthogonal experimental design, the researchers evaluated different combinations of steel slag content, microwave exposure, healing temperature, and resting time. The recommended laboratory conditions involved replacing 60% of the mineral filler with steel slag powder, applying microwave heating for 120 seconds, reaching a healing temperature of 80°C, and allowing a four-hour healing period. Under these conditions, the mixture achieved a healing index of 84.7%.
The 60% figure refers specifically to the proportion of mineral filler replaced by steel slag powder, not the recycled content of the entire asphalt mixture. Similarly, the 84.7% healing index is a laboratory performance indicator and should not be interpreted as proof that 84.7% of visible road cracks would disappear or that pavement service life would increase by the same proportion.
A major contribution of the study was its examination of repeated damage and repair. Researchers subjected the asphalt mixtures to successive fracture, microwave-healing, and re-fracture cycles to determine whether the material could retain its healing capability after multiple treatments. The steel slag-modified mixture maintained a healing index of 61.3% after nine cycles, with a slower decline in performance than the conventional asphalt mixture used for comparison.
Although these results demonstrate the potential for repeated recovery, they also reveal an important limitation. Microwave heating promotes the movement and redistribution of the asphalt binder, but repeated exposure to elevated temperatures can accelerate thermal-oxidative aging. The same process that helps repair cracks can gradually alter the binder's composition and reduce its flexibility.
To investigate this effect, the research team combined chemical, rheological, and microscopic characterization methods, including SARA fraction analysis, dynamic shear rheometer (DSR) testing, Fourier-transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). These techniques provided evidence of changes in binder composition, mechanical behavior, and microscopic structure following repeated heating.
The findings showed that microwave exposure accelerated the transition from lighter asphalt components toward heavier fractions. Carbonyl and sulfoxide indices, commonly associated with asphalt oxidation, increased during the treatment cycles. The researchers also observed higher complex modulus and rutting factor values, together with changes in the binder's microscopic structures and increased surface roughness.
These developments indicate progressive binder stiffening. Although higher stiffness can influence resistance to permanent deformation, excessive aging may reduce flexibility and increase susceptibility to cracking, particularly under repeated loading or low-temperature conditions. This creates a trade-off between restoring existing damage and preserving the material's ability to resist future deterioration.
Based on the experimental results, the researchers identified five healing cycles as a suggested operational threshold for balancing recovery performance against accumulated aging under the investigated conditions. This recommendation is specific to the laboratory materials and treatment procedure and does not establish a universal limit for microwave repair operations on actual roads.
For the asphalt and bitumen industries, the research combines two important technological approaches: the reuse of industrial steelmaking byproducts and the development of repair methods that could reduce the need for premature pavement replacement. If successfully adapted for practical applications, microwave-assisted healing could potentially support preventive maintenance by treating early-stage cracks before more extensive structural damage develops.
However, commercial implementation would require further investigation into field heating equipment, energy consumption, treatment uniformity, environmental exposure, traffic loading, and the behavior of different bitumen grades and asphalt mixtures. The environmental benefits of using recycled steel slag would also need to be assessed alongside the energy required for repeated microwave treatments.
The results remain limited to controlled laboratory testing. No long-term field performance, verified reduction in road maintenance costs, or measurable extension of pavement service life has been established by this study.
The research nevertheless provides valuable evidence that steel slag powder can improve microwave-induced asphalt healing while also identifying the durability limits associated with repeated thermal treatment. Its significance lies not only in achieving an 84.7% healing index, but also in demonstrating why controlling binder aging will be essential if self-healing pavement technology is to progress toward practical road maintenance applications.
By WPB
Self-Healing Asphalt, Steel Slag Asphalt, Microwave Heating, Asphalt Healing Index, Bitumen Aging, Asphalt Crack Repair, Electric Arc Furnace Slag, Asphalt Recycling, Asphalt Durability, Thermal Oxidative Aging, Pavement Technology, Sustainable Road Construction, Asphalt Binder, Microwave Self-Healing, Construction and Building Materials
