Comprehensive Review of Thermally Induced Self-Healing Behavior in Asphalt Mixtures and the Role of Steel Slag

Abstract

Asphalt pavements face escalating challenges from traffic loading, climate change, and material degradation, necessitating innovative maintenance solutions. Thermally induced self-healing technologies, leveraging the viscoelastic properties of asphalt binders, can autonomously repair microcracks through targeted thermal activation. This review explored thermally induced self-healing in asphalt mixtures, with a focus on leveraging steel slag as a functional aggregate to enhance sustainability and durability. Two thermal-activation methods, electromagnetic induction and microwave heating, were critically analyzed, highlighting their distinct advantages in heating efficiency, depth, and uniformity. Steel slag offers dual benefits: improving mechanical interlock and skid resistance in mixtures while facilitating efficient heat generation via electromagnetic induction or microwave heating. However, challenges such as hydration-induced expansion, heterogeneous slag composition, and energy-intensive heating processes impede widespread adoption. Pretreatment methods, including natural aging, carbonation, and surface modifications, are essential to mitigate volumetric instability and optimize slag performance. Key factors influencing healing efficacy, including binder properties, operational parameters (e.g., microwave power, frequency), and environmental trade-offs, were systematically evaluated. Future research directions emphasized standardized pretreatment protocols, hybrid heating technologies for uniform temperature distribution, and smart-infrastructure integration for predictive maintenance.

1. Introduction

Asphalt pavements play a critical role in contemporary transportation systems, forming the essential foundation for roads, highways, and airport runways across the globe. The extensive adoption of asphalt concrete as a paving material, accounting for over 90% of newly constructed roads, highlights its vital importance in the transportation sector, attributed to its favorable characteristics such as exceptional waterproofing, strong adhesion, and high resistance to erosion.

Nevertheless, the escalating demands of modern vehicular traffic, characterized by increased axle loads and higher traffic volumes, combined with the adverse effects of climate change, have placed significant stress on asphalt pavements [1]. In regions subjected to extreme weather conditions, such as harsh winters in northern areas and heavy rainfall in southern regions, asphalt surfaces are susceptible to various forms of distress [2,3]. Permanent deformation, known as rutting, can occur under the strain of heavy traffic at elevated temperatures. Additionally, cracking can arise from a multitude of factors, including thermal stress, fatigue, and moisture-related damage [4]. Ultraviolet (UV) radiation and other factors also accelerate the aging of asphalt materials, leading to a reduction in their strength, thereby causing pavement damage during the service life of the pavement [5]. These issues not only diminish the longevity of the pavement but also compromise road safety and escalate maintenance expenditures. The advent of self-healing technologies presents an innovative approach to addressing these challenges [6].

Thermally induced self-healing represents a cutting-edge technology with the potential to transform asphalt pavement maintenance practices [7]. By applying thermal energy to the pavement, the asphalt binder can be softened, allowing it to flow and effectively seal cracks and other forms of damage. This mechanism relies on the viscoelastic properties of asphalt, which enable it to regain mechanical strength upon heating [8]. Two primary methods facilitate thermally induced self-healing in asphalt mixtures: electromagnetic induction heating and microwave heating [9,10]. Electromagnetic induction heating generates a magnetic field through a coil carrying an alternating current, which induces heat in functional fillers, such as steel slag, positioned within the magnetic field. This approach provides rapid heating and allows for targeted treatment of specific pavement areas. Conversely, microwave heating interacts directly with dipole molecules in the material, causing them to rotate and generate heat. This internal heating mechanism is notably more efficient than traditional external heat transfer methods and can achieve a more uniform temperature distribution within the asphalt mixture.

In parallel, the urgent need to reduce dependence on natural aggregates and manage industrial waste has led to increased interest in alternative materials that align with circular economy principles [11,12]. Steel slag, a byproduct of steel manufacturing, has emerged as a viable option for various applications due to its widespread availability [13]. With global steel production exceeding 1.9 billion metric tons annually, approximately 15–20% of this volume is produced as slag [14]. The incorporation of steel slag in thermally induced self-healing systems offers numerous benefits. Steel slag aggregates are characterized by high density, angular shape, and a rough surface texture, which enhance interlocking, skid resistance, and structural stability in asphalt mixtures [15,16]. Furthermore, the alkaline properties of steel slag improve adhesion with asphalt binders, thereby reducing moisture susceptibility [17,18]. Chemically, steel slag is rich in Fe₂O₃ and CaO, making it a functional material with promising applications in thermally induced self-healing systems [19]. Traditional asphalt pavements are vulnerable to microcrack propagation due to cyclic loading and thermal stresses, significantly shortening their service life [20]. Self-healing technologies, facilitated by electromagnetic induction or microwave heating, offer a groundbreaking solution by enabling autonomous damage repair. The ferromagnetic properties of steel slag and its dielectric characteristics promote rapid, energy-efficient heating, positioning it as a dual-purpose material that effectively addresses both performance and sustainability challenges [21].

This review conducted a systematic literature search across the Scopus, Web of Science, and Engineering Village databases, focusing on publications from 2010 to 2024. The search strategy employed targeted keyword combinations, including “thermally induced” OR “heat-triggered” (to capture thermal activation mechanisms), “self-heal” OR “crack repair” (to address healing behaviors), “asphalt” OR “bitumen” (as binder-related terms), “steel slag” OR “industrial byproduct” (to identify functional aggregates), “electromagnetic induction” OR “microwave heating” (for thermal methods), and “energy efficienc” OR “heating uniformity” (to evaluate performance metrics). This approach ensured balanced coverage of material properties, healing mechanisms, and sustainability considerations. This review aims to provide a thorough and detailed examination of the thermally induced self-healing behavior of asphalt mixtures, with a specific emphasis on the role of steel slag. This review elucidates the fundamental mechanisms underlying thermally induced self-healing in asphalt mixtures, including the role of steel slag in enhancing the healing process. Additionally, it evaluates the various factors that influence the self-healing efficiency of asphalt mixtures, such as material properties, heating methods, and operational conditions [22]. It identifies the challenges and limitations associated with the use of steel slag in asphalt mixtures and thermally induced self-healing technologies and proposes potential solutions to overcome them. Furthermore, this review provides insights into future research directions in this field, with a view to promoting the development of more sustainable and efficient asphalt pavement solutions. The exploration of thermally induced self-healing behavior in asphalt mixtures, particularly when integrated with steel slag, represents a significant advancement in the quest for durable and resilient transportation infrastructure. Self-healing capability not only enhances the longevity of pavements but also reduces the frequency and cost of maintenance interventions, thereby contributing to overall sustainability.

2. Fundamentals of Thermally Induced Self-Healing in Asphalt Mixtures

2.1. Mechanisms of Self-Healing

The self-healing phenomenon in asphalt materials is inspired by the intrinsic ability of viscoelastic materials to restore mechanical integrity at crack interfaces when subjected to elevated temperatures, a process fundamentally driven by thermally activated molecular mobility and energy dissipation [23]. Unlike passive repair methods, thermally induced self-healing leverages the inherent properties of asphalt binder, such as its temperature-dependent viscosity and surface energy, to autonomously repair microcracks, offering a proactive strategy to extend pavement service life [24].

2.1.1. Key Stages of the Self-Healing Process

The self-healing process in asphalt can be systematically divided into three interconnected stages, each influenced by thermal activation and the viscoelastic behavior of the binder:

• Crack surface wetting and contact: At elevated temperatures (typically ≥ 50 °C), the surface energy gradient drives the initial wetting of crack surfaces by the softened asphalt binder [25]. As the binder’s viscosity decreases (following the Walther equation), it flows to fill microcavities and re-establish adhesive contact across crack faces. This stage is critical for overcoming the surface tension that maintains crack openness, with the binder’s polar functional groups (e.g., carboxylic acids) facilitating physical adsorption onto aggregate surfaces [26].
• Stress-driven crack closure: As the asphalt mixture is heated, the reduction in binder stiffness increases its ductility, allowing mechanical stresses (residual or induced) to drive crack closure [27]. In this phase, the viscoelastic binder undergoes creep deformation, gradually reducing the crack width through cohesive flow. Research using dynamic shear rheometry (DSR) has shown that the rate of crack closure is directly proportional to the temperature-induced decrease in complex shear modulus (|G*|), with a 50% reduction in |G*| at 80 °C compared to 25 °C [28].
• Molecular diffusion and mechanical recovery: The final stage involves molecular diffusion across the re-contacted crack interface, where asphaltenes and maltenes interpenetrate to restore cohesive strength [29]. At optimal healing temperatures (80–100 °C), the increased kinetic energy of binder molecules enhances interdiffusion, with small molecular weight maltenes (e.g., saturates and aromatics) migrating faster than asphaltenes, leading to a gradual recovery of mechanical properties [30]. Fourier transform infrared spectroscopy (FTIR) has confirmed the re-establishment of van der Waals interactions and hydrogen bonding across healed interfaces, with tensile strength recovery reaching 60–80% after 24 h of thermal treatment [31].

2.1.2. Theoretical Frameworks for Self-Healing

Three primary theories provide mechanistic insights into thermally induced self-healing, each addressing different aspects of the process from molecular to macroscale:

• Molecular diffusion theory: Rooted in polymer science, this theory posits that crack healing involves sequential stages of surface rearrangement, wetting, molecular diffusion, and randomization of polymer chains [32]. In asphalt, the complex mixture of high-molecular-weight asphaltenes and low-viscosity maltenes creates a heterogeneous diffusion environment. Maltenes, acting as a “solvent” for asphaltenes, facilitate the flow of binder across cracks, with diffusion kinetics governed by the Stokes–Einstein equation [33]. Experimental studies using fluorescent tracers have shown that diffusion rates increase exponentially with temperature, with a 30% acceleration observed when heating from 60 °C to 90 °C [34].
• Crack surface energy theory: From a fracture mechanics perspective, self-healing is defined as a reduction in crack surface energy (Γ), which is the energy required to create new crack surfaces [35]. The healing process minimizes Γ through the wetting and bonding of crack faces, described by the Young–Dupré equation for adhesive work. Schapery [36] extended this concept to viscoelastic materials, deriving the relationship. This theory highlights the role of temperature in reducing Γ_h by enhancing binder-aggregate adhesion, with studies showing a 15–20% decrease in surface energy for heated asphalt mixtures compared to unheated controls [37]. Furthermore, Luo et al. [38] conducted a comprehensive mechanical analysis of the asphalt healing process and developed a theoretical model to describe the healing behavior of asphalt mixtures. As illustrated in Figure 1, the internal actual stress and interfacial attraction are the two primary driving forces behind the self-healing of asphalt.
• Capillary flow theory: García [39] proposed a model where capillary pressure drives binder flow into cracks at elevated temperatures, assuming asphalt behaves as a Newtonian fluid. The capillary pressure (P_c) is provided by the Young–Laplace equation. As temperature increases, the contact angle decreases (improving wetting), and binder viscosity drops, enhancing P_c and flow rate. This theory is particularly applicable to microcracks (<2 mm width), where capillary forces dominate over gravitational or mechanical stresses. Experimental validation using microfluidic models has shown that cracks narrower than 500 μm are fully healed within 30 min at 90 °C, with flow rates increasing by 50% compared to room temperature [40].

2.1.3. Temperature and Time Dependencies

The efficiency of thermally induced self-healing is strongly governed by temperature and healing duration, with optimal conditions defined by the binder’s viscoelastic response:

• Temperature thresholds: Dense-graded asphalt mixtures begin self-healing at 50 °C, with healing rates increasing monotonically up to 100 °C, where the binder transitions into a Newtonian fluid state [41]. Below 50 °C, viscosity remains too high for effective flow, while above 100 °C, excessive thermal aging may degrade binder properties, reducing long-term durability [42].
• Healing kinetics: The healing process follows a biphasic pattern: an initial rapid strength recovery (60–70% within 1 h) due to crack closure, followed by a slower diffusion-driven phase (additional 10–20% recovery over 24 h) [43]. Time-temperature superposition principles have been used to model this behavior, showing that a 10 °C increase in healing temperature is equivalent to a three-fold increase in healing time [44].
• Optimal Timing: Induction heating is most effective when applied at 30–70% of the pavement’s fatigue life, corresponding to the stage of microcrack propagation before macrocrack formation [45]. Premature activation (e.g., <20% fatigue life) may waste energy, while delayed activation (>80% fatigue life) may fail to repair critical damage, leading to structural failure [46].

2.2. Heating Methods for Enhanced Self-Healing

Two primary thermal-activation methods have been developed to induce self-healing in asphalt mixtures, each leveraging distinct energy-transfer mechanisms to achieve efficient crack repair:

2.2.1. Electromagnetic Induction Heating

Electromagnetic induction heating has gained traction in pavement engineering due to its contactless operation, rapid temperature rise, and scalability, analogous to household induction cooking technology. The process involves generating an alternating magnetic field through a coil, which induces eddy currents and hysteresis losses in conductive or ferromagnetic fillers within the asphalt mixture, converting electrical energy into thermal energy [47]. Functional fillers for this technique typically include high-conductivity materials like steel fibers, carbon fibers, or iron-based aggregates, which efficiently couple with the magnetic field to generate heat. Figure 2 shows the self-healing process of the asphalt mixture by electromagnetic induction heating [48].

The self-healing process via induction heating relies on three key mechanisms:

• Eddy current and hysteresis heating: Conductive fillers (e.g., steel fibers) generate heat through eddy current losses, while ferromagnetic components (e.g., magnetite) produce heat via hysteresis effects during magnetic field oscillations [49]. This targeted heating softens the asphalt binder, promoting crack closure and molecular diffusion.
• Void content optimization: Incorporation of dense, angular fillers reduces air voids, enhancing mechanical interlock and reducing pathways for moisture ingress. Studies indicate that each 4% increase in void content decreases repair efficiency by 10%, highlighting the importance of compact microstructure for effective healing [50].
• Thermal property modification: Fillers with low thermal conductivity and high heat-storage capacity prolong the duration of effective healing by retaining heat within the mixture. Microporous fillers, for example, interrupt heat transfer, maintaining optimal temperatures (50–100 °C) for extended periods [51].

Particle size and composition of functional fillers significantly influence heating performance. Larger particles (9.5–16 mm) with higher metallic content exhibit superior heating depth due to reduced thermal conductivity (1.275 vs. 1.382 W/(m·K) for smaller particles), while finer fillers enhance surface heating uniformity [34]. However, excessive filler agglomeration can lead to localized overheating, necessitating careful gradation design.

2.2.2. Microwave Heating

Microwave heating distinguishes itself through volumetric energy absorption, where electromagnetic waves (0.3–300 GHz) interact with polar molecules (e.g., water, dipolar functional groups in asphalt) or lossy materials, inducing molecular rotation and friction that generates heat internally [52,53]. This method offers faster heating rates and more uniform temperature distribution compared to conventional conductive heating, with efficiency governed by the material’s dielectric loss factor (ε″) and magnetic loss tangent (tanδ) [54,55]. Materials with higher loss factors, such as steel slag, are preferred for thermally induced self-healing applications [56].

Key advantages of microwave heating for self-healing include:

• Dielectric heating efficiency: Materials with high ε″, such as aggregates containing metal oxides (e.g., Fe₂O₃, CaO), absorb microwaves more effectively, converting electromagnetic energy to heat at rates proportional to their loss properties [57]. This allows targeted activation of filler-rich zones, accelerating binder softening and crack repair.
• Porous structure enhancement: Porous fillers enhance microwave reflections and energy dissipation, creating a more uniform thermal environment. For instance, fillers with rough, interconnected pore structures improve heat distribution, reducing thermal gradients within the mixture [58].
• Moisture mitigation: While water can degrade healing by vaporizing at 100 °C, fillers with low water absorption restrict outward diffusion, maintaining stable thermal conditions for binder flow. Dense-graded mixtures, in particular, show better freeze–thaw resistance during microwave healing due to reduced void connectivity [59].

Performance optimization involves balancing filler content and particle size. Mixtures with ≥40% high-loss fillers demonstrate significantly increased microwave absorption, with temperature rises exceeding 20 °C within 2 min of exposure [60]. However, excessive filler content can reduce workability and increase energy consumption. Larger aggregates (4.75–19 mm) enhance microwave penetration depth, making them suitable for deep crack repair, while smaller particles (0–2.36 mm) improve surface heating efficiency due to higher specific surface area [61]. Experimental studies using co-precipitation methods to enhance iron oxide content in steel slags have shown up to 30% faster heating rates compared to conventional aggregates, highlighting the role of compositional design in optimizing microwave response [62].

In summary, thermal-activation methods offer transformative pathways for enhancing self-healing in asphalt mixtures, with electromagnetic induction and microwave heating providing distinct advantages for different repair scenarios.

Table 1. Comparison between both thermal-activation methods.

Heating Method	Mechanism	Advantages	Limitations	Optimal Applications
Electromagnetic Induction	Eddy currents/hysteresis in conductive/ ferromagnetic fillers	Deep heating (5–10 cm), rapid temperature rise	Localized overheating, filler agglomeration	Subsurface crack repair, thick pavement layers
Microwave Heating	Dielectric/magnetic losses in polar/lossy materials	Uniform volumetric heating, surface/ mid-depth repair	Dependence on filler dielectric properties	Surface/mid-depth cracks, thin overlays

lists the comparative analysis between electromagnetic induction heating and microwave heating. In addition, both methods require careful selection of functional fillers to balance heating efficiency, mechanical performance, and long-term durability.

2.3. Limitations of Self-Healing Methods

While thermally induced self-healing holds significant promise, several challenges must be addressed for practical implementation:

Binder aging: Prolonged exposure to high temperatures (≥120 °C) accelerates oxidative aging, reducing binder ductility and future healing potential. Thermal cycling studies show that repeated heating (≥5 cycles) can decrease healing efficiency by 15–20% due to asphaltene aggregation [63].

Moisture interference: Water within cracks vaporizes at 100 °C, creating steam pressure that disrupts binder flow and induces delamination. Pre-heating, drying, or hydrophobic surface treatments are essential to mitigate this effect [64].

Crack size limitations: Self-healing is most effective for microcracks (<2 mm), as larger cracks (≥5 mm) require excessive binder flow that exceeds the asphalt mixture’s capacity, leading to incomplete repair [65].

Thermally induced self-healing in asphalt mixtures represents a novel approach in pavement maintenance, leveraging the viscoelastic properties of asphalt binder and controlled thermal activation to restore mechanical integrity. The process is governed by three core stages, wetting, closure, and diffusion, supported by theoretical frameworks that link molecular mobility, surface energy, and capillary forces to healing efficiency. While temperature and time are critical parameters, the choice of heating method (induction vs. microwave) and careful management of binder aging and moisture effects are essential for practical success.

3. Steel Slag as a Functional Aggregate for Enhanced Thermal Self-Healing

3.1. Physical and Chemical Properties of Steel Slag Aggregates

Steel slag, a byproduct of steel production, exhibits distinct physical and chemical properties that distinguish it from natural aggregates, making it a promising substitute in asphalt mixtures. These properties directly influence its mechanical performance, adhesion with asphalt binder, and suitability for sustainable pavement applications. Figure 3 illustrates the key physical property comparisons between steel slag and natural aggregates, highlighting its superior density and roughness while noting the trade-off in water absorption [66]. In addition to differences in fundamental physical properties, the variations in asphalt-aggregate affinity between steel slag and natural materials also significantly influence their application in asphalt mixtures. As highlighted in the study [67], the affinity of steel slag with asphalt is positioned as better than greywacke, basalt, and granite aggregates, but worse than limestone.

Steel slag particles feature a rugged, angular surface with pronounced roughness and a porous structure, often exhibiting honeycomb-like textures with large interconnected pores [68]. This morphology enhances inter-particle interlock within asphalt mixtures, improving resistance to permanent deformation under traffic loads. With a density of 2.95–3.5 g/cm³, steel slag is denser than most natural aggregates (2.5–2.7 g/cm³), a trait attributed to its rich content of heavy metal oxides (Fe₂O₃, MnO, MgO) [69]. This high density contributes to increased structural stiffness and load-bearing capacity, reducing rutting potential in high-traffic pavements. Due to the presence of heavy metals in steel slag, careful consideration must be provided to the leaching risks of heavy metals during its application. Current research indicates that while steel slag alone exhibits elevated leaching risks, its combination with asphalt mixtures significantly reduces heavy metal mobility, rendering it safe for use in asphalt pavements under standard environmental conditions [70,71,72].

The non-uniform distribution of mineral phases and their varying micro-hardness values within the steel slag also contribute to its high polishing value, an important factor for assessing the skid resistance of pavement materials [73]. Steel slag demonstrates excellent resistance to abrasion and crushing, with Los Angeles abrasion values (11–13.2%) and crushing values (12.1–14.2%) significantly lower than natural aggregates (15–30% and 9.5–20%, respectively) [74]. These properties make it suitable for heavy-duty applications, such as port terminals and airport runways, where mechanical stresses are extreme.

Despite its advantages, steel slag has a higher water absorption rate (0.9–2.5%) compared to natural aggregates (0.2–1.1%), primarily due to its porous microstructure [75]. This characteristic necessitates careful consideration in moisture-prone environments, as increased water retention may enhance susceptibility to freeze–thaw cycles and asphalt-binder stripping.

The chemical composition of steel slag is highly variable, influenced by ore sources, steelmaking processes (e.g., basic oxygen furnace, electric arc furnace), and post-production treatments [76]. The primary constituents of steel slag typically include CaO, SiO₂, Al₂O₃, Fe₂O₃, MgO, and MnO. Additionally, steel slag may contain trace amounts of other oxides and sulfides, such as TiO₂, V₂O₅, CaS, and FeS [77]. Table 2 summarizes typical chemical compositions from global studies, with CaO (27–55%) and Fe₂O₃ (5–35%) as the dominant components, followed by SiO₂ (11–28%), Al₂O₃ (1–13%), MgO (1–11%), and MnO (0.4–5%).

The high CaO content imparts strong alkalinity (pH 12–13) to steel slag, promoting chemical bonding with asphalt binder through acid–base interactions. This enhances adhesive strength at the aggregate–binder interface, reducing moisture-induced stripping and improving cohesive failure resistance [78]. Fourier transform infrared spectroscopy (FTIR) has confirmed the formation of calcium carboxylate bonds between the slag’s alkaline surface and the binder’s acidic functional groups [79].

The aforementioned physical and mechanical properties of steel slag enable it to positively influence pavement structures and road stability when used as an aggregate substitute while also posing potential challenges [80]. The specific positive impacts include:

• Enhanced Mechanical Performance: When replacing coarse aggregates (particularly BOFS and EAFS), steel slag significantly improves the anti-rutting, fatigue resistance, and permanent deformation resistance of asphalt mixtures [81,82]. Its angular particles and rough surface enhance mechanical interlock among aggregates.
• Superior Skid Resistance: Steel slag’s wear resistance, high polishing resistance, and irregular surface morphology substantially improve pavement skid resistance (e.g., higher British Pendulum Number (BPN) and Mean Texture Depth (MTD) compared to conventional aggregates), making it suitable for high-traffic roads [83,84,85].
• Temperature Adaptability: Steel slag mixtures exhibit higher dynamic modulus across a broad temperature range (−10–54 °C), extending their applicability to diverse climatic conditions [74].

Key Challenges in Application:

• Dosage Sensitivity: Coarse aggregate replacement yields optimal results, but substituting fine aggregates or fillers increases moisture susceptibility [86]. High dosages (>30%) may elevate bulk density and air voids, necessitating adjustments in binder content and raising costs [87].
• Volumetric Stability Risks: Free calcium/magnesium oxides in steel slag pose hydration-induced expansion risks, requiring pretreatment (e.g., carbonation) to mitigate swelling [88].

In summary, the unique physical and chemical characteristics of steel slag make it an attractive alternative to natural aggregates in asphalt mixtures. Its high density, durability, and favorable adhesion properties contribute to enhanced pavement performance. However, careful consideration of its water absorption characteristics is necessary to ensure optimal performance in various applications [89]. The ongoing research and development in this area will further elucidate the potential of steel slag in promoting sustainable infrastructure solutions.

Table 2. Chemical composition of steel slag.

Reference	Source	CaO (%)	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	MgO (%)	MnO (%)
[90]	Jiangsu, China	39.53	17.62	8.62	16.75	7.81	2.85
[91]	Rugao, China	37.56	17.66	7.64	19.07	5.61	2.32
[92]	Benxi, China	42.49	16.05	6.81	23.33	5.34	2.18
[93]	Nanjing, China	45.87	19.76	2.51	14.10	5.47	2.35
[94]	Shanxi, China	45.50	13.50	2.31	24.30	5.80	2.24
[95]	Wuhan, China	37.88	28.05	9.60	9.53	8.70	3.02
[96]	Binzhou, China	41.59	13.30	4.48	24.19	5.90	−
[97]	Tangshan, China	44.24	12.00	4.05	29.74	4.51	−
[98]	Zhanjiang, China	42.92	11.63	9.46	23.36	5.35	−
[98]	Xinyu, China	41.35	11.04	1.35	25.43	8.62	−
[99]	Korea	35.59	14.60	3.52	33.82	5.06	3.70
[100]	Canada	46.5	23.3	4.02	7.71	11	1.20
[100]	Canada	28	18.7	12.6	18	7.84	0.39
[101]	Netherlands	41.55	11.47	2.24	31.35	3.78	4.78
[102]	Netherlands	40.87	15.91	2.35	25.21	5.21	5.00
[103]	Brazil	36.80	14.60	3.70	32.20	5.50	3.70
[103]	Brazil	27.40	19.00	6.50	33.50	5.60	4.00
[104]	Italy	49.00	23.00	10.80	5.20	8.50	1.0
[105]	Finland	54.59	12.99	1.15	21.35	1.86	−
[105]	Finland	55.40	15.19	2.15	16.06	1.64	−

Table 2. Chemical composition of steel slag.

Reference	Source	CaO (%)	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	MgO (%)	MnO (%)
[90]	Jiangsu, China	39.53	17.62	8.62	16.75	7.81	2.85
[91]	Rugao, China	37.56	17.66	7.64	19.07	5.61	2.32
[92]	Benxi, China	42.49	16.05	6.81	23.33	5.34	2.18
[93]	Nanjing, China	45.87	19.76	2.51	14.10	5.47	2.35
[94]	Shanxi, China	45.50	13.50	2.31	24.30	5.80	2.24
[95]	Wuhan, China	37.88	28.05	9.60	9.53	8.70	3.02
[96]	Binzhou, China	41.59	13.30	4.48	24.19	5.90	−
[97]	Tangshan, China	44.24	12.00	4.05	29.74	4.51	−
[98]	Zhanjiang, China	42.92	11.63	9.46	23.36	5.35	−
[98]	Xinyu, China	41.35	11.04	1.35	25.43	8.62	−
[99]	Korea	35.59	14.60	3.52	33.82	5.06	3.70
[100]	Canada	46.5	23.3	4.02	7.71	11	1.20
[100]	Canada	28	18.7	12.6	18	7.84	0.39
[101]	Netherlands	41.55	11.47	2.24	31.35	3.78	4.78
[102]	Netherlands	40.87	15.91	2.35	25.21	5.21	5.00
[103]	Brazil	36.80	14.60	3.70	32.20	5.50	3.70
[103]	Brazil	27.40	19.00	6.50	33.50	5.60	4.00
[104]	Italy	49.00	23.00	10.80	5.20	8.50	1.0
[105]	Finland	54.59	12.99	1.15	21.35	1.86	−
[105]	Finland	55.40	15.19	2.15	16.06	1.64	−

3.2. Thermal and Electromagnetic Characteristics

The thermal and electromagnetic properties of steel slag are critical for its role in enhancing the performance of asphalt mixtures, particularly in thermally induced self-healing applications. These properties govern heat transfer dynamics, energy absorption, and compatibility with activation methods like electromagnetic induction and microwave heating [106].

Steel slag exhibits distinct thermal behavior that differentiates it from natural aggregates:

• Low thermal conductivity and diffusivity: Due to its porous microstructure with interconnected micropores, steel slag has lower thermal conductivity (1.2–1.5 W/(m·K)) and diffusivity (0.4–0.6 × 10-6 m²/s) compared to natural aggregates like limestone (2.0–2.5 W/(m·K)) and basalt (1.5–2.0 W/(m·K)) [107,108]. The pores disrupt heat-transfer pathways, slowing down heat dissipation and enhancing heat-storage capacity. This feature is advantageous during pavement construction, as it prolongs the temperature window for compaction, especially in cold climates, by maintaining workable temperatures for 10–15% longer than conventional mixtures [109]. Steel slag–asphalt mixtures, leveraging these properties (thermal conductivity, mechanical strength, and self-healing capabilities), exhibit particularly superior performance under specific road types and traffic conditions. Examples include urban roads, highways, sun-exposed parking lots, and bridge pavements in tropical or subtropical regions. This is attributed to the region’s relatively higher ambient temperatures, which enable faster pavement temperature elevation compared to colder regions, thereby enhancing the efficiency of self-healing mechanisms.
• Enhanced heat storage: The high density of steel slag (2.95–3.5 g/cm³) combined with its porous structure provides it a specific heat capacity of 0.7–0.9 kJ/(kg·K), higher than natural aggregates [110]. This allows steel slag–modified mixtures to retain heat for extended periods, reducing energy consumption during transportation and enabling more efficient thermal activation of self-healing mechanisms [58].

Wan et al. [111] demonstrated that asphalt mixtures containing steel slag exhibited 20–30% higher healing efficiency than basalt-based mixtures, attributed to the slag’s ability to maintain optimal healing temperatures (50–100 °C) for longer durations. The microscale pores act as thermal insulators, minimizing heat loss and ensuring uniform temperature distribution across the mixture.

Steel slag’s electromagnetic characteristics make it an ideal functional filler for energy-efficient heating:

• Dielectric properties: Steel slag has a high dielectric constant and loss factor across microwave frequencies, significantly higher than limestone and basalt [112]. The polar metal oxides within the slag interact with microwave fields, causing dipole rotation and friction that generate heat internally. This volumetric heating mechanism ensures uniform temperature rise, with mixtures containing 40% steel slag achieving a 15 °C temperature increase within 2 min of microwave exposure [60].
• Magnetic permeability: The complex magnetic permeability of steel slag is dominated by its ferromagnetic components, such as magnetite (Fe₃O₄) and metallic iron. The real part ranges from 1.05–1.09 at low frequencies, enabling efficient energy coupling in induction heating systems [113]. Iron-rich fractions in steel slag generate eddy currents under alternating magnetic fields, converting electrical energy to heat via resistive losses. In contrast, Fe₃O₄ relies on hysteresis losses, making it effective for magnetic induction even at micron scales [114].
• Particle size and content effects: Larger steel slag particles (9.5–16 mm) with higher iron content (25–30%) enhance induction heating depth (5–8 cm) due to reduced skin depth, while finer particles (0–2.36 mm) improve microwave absorption efficiency by increasing specific surface area [115]. Optimal steel slag content for balanced heating efficiency and workability is 30–50%, beyond which agglomeration may reduce uniformity [116].

In summary, steel slag’s unique thermal and electromagnetic characteristics position it as a transformative material for asphalt mixtures, particularly in applications requiring efficient thermal activation of self-healing. Its low thermal conductivity, high heat storage, and superior electromagnetic response enable rapid, uniform heating, reducing energy use, and enhancing repair efficiency.

3.3. Pretreatment of Steel Slag for Improved Performance

The volumetric stability of SSAMs is critical for preventing pavement distresses such as cracking, heaving, and structural degradation, which are primarily driven by the hydration of reactive oxides in steel slag [117,118]. The primary cause of volume expansion in SSAMs is the hydration of free lime (f-CaO) and free magnesia (f-MgO) in steel slag:

$$\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}\right)+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{a}\left(\mathrm{O}\mathrm{H}\right)}_2(\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{i}\mathrm{n}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{e}:98\%)$$

(1)

$$\mathrm{M}\mathrm{g}\mathrm{O}\left(\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}\right)+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}\mathrm{g}\left(\mathrm{O}\mathrm{H}\right)}_2(\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e} \mathrm{i}\mathrm{n}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{s}\mathrm{e}:148\%)$$

(2)

These reactions generate internal stresses due to the significant volume expansion of hydration products, leading to crack initiation and aggregate disbanding. Research has clearly shown that there is a positive correlation between the expansion rate and the steel slag content in the mixture [119]. Finer steel slag particles, in particular, have a greater potential for expansion. This is because they possess a larger specific surface area, which allows for more contact with moisture, and they also exhibit higher reactivity [120].

Experimental results have consistently confirmed the direct relationship between steel slag content and volumetric expansion. As depicted in Figure 4, under water-immersion conditions, beam specimens of SSAMs show localized swelling and partial aggregate disbanding [121]. The severity of this damage intensifies as the proportion of steel slag increases. Mixtures with more than 50% steel slag by weight have significantly higher expansion rates compared to those with lower slag content [122]. Particle-size analysis has revealed that fine steel slag (with a particle size less than 2.36 mm) has notably higher expansion rates than coarse steel slag (particle size greater than 4.75 mm). This can be attributed to the accumulation of unstable materials in fine particles and their increased specific surface area [123]. Interestingly, when the proportion of fine steel slag reaches 54%, a decrease in the expansion rate is observed. This is likely due to the increased asphalt content, which reduces the contact between steel slag and water.

Due to the tendency of steel slag to expand upon contact with water, it cannot be directly used in road construction without proper pretreatment to ensure its volume stability [124]. The expansion ratio of asphalt mixtures varies with different steel slag contents [125,126,127]. As the steel slag content rises, the amounts of free lime and free magnesia in the steel slag aggregate also increase, thereby escalating the risk of volume expansion. Currently, three primary methods are employed to modify steel slag: natural aging and carbonation modification, inorganic modification, and organic modification, as summarized in

Table 3. Comparative analysis of pretreatment methods.

Method	Mechanism	Advantages	Limitations	Optimal Application
Natural Aging	Gradual hydration/ carbonation	Low cost, environmental compatibility	Long treatment time (6–12 months)	Large-scale storage facilities
Carbonation Modification	CO2-induced calcium carbonate formation	Rapid stabilization, CO2 utilization	High-energy autoclave requirements	Industrial sites with CO2 waste streams
Inorganic Modification	Pozzolanic reaction/acid etching	Improved mechanical properties, cost-effective	Potential alkalinity changes	Heavy-duty pavements requiring high strength
Organic Modification	Hydrophobic surface coating	Excellent moisture resistance, binder adhesion	Higher treatment cost, potential aging sensitivity	Moisture-prone environments (coastal, high-rain areas)

.

Natural aging and carbonation modification aim to promote hydration and oxidation within steel slag to mitigate expansion [128,129]. Natural aging involves storing steel slag outdoors, allowing it to react with atmospheric components. Usually, this process takes over 6 months to effectively stabilize the volume, and research suggests that a 12-month aging period leads to optimal performance [130,131]. To expedite this process, elevated-temperature water bath treatments, such as at 60 °C, have been used. These can double the dissolution efficiency compared to room-temperature conditions [132]. Carbonation modification, on the other hand, makes use of CO₂ gas to react with the oxides in steel slag [133]. Sealing steel slag in a high-pressure autoclave with CO₂ can enhance volume stability by forming calcium carbonate (CaCO₃). Reported carbonation conversion rates can reach up to 68.3% [134]. This method not only improves the material properties but also provides economic benefits by utilizing the CO₂ emissions generated during steel production.

Inorganic modification focuses on the surface treatment of steel slag with inorganic materials to reduce its expansion [135]. For example, adding cement and silica powder can significantly lower the water absorption rate [136]. Other studies have indicated that using quarry dust and cement can effectively address the volume expansion of steel slag aggregates [137]. Additionally, treating steel slag with acidic solutions, such as hydrochloric or phosphoric acid, can substantially decrease its water absorption and enhance its compressive strength [138].

Organic modification uses organic materials for surface treatment. This includes organosilicon compounds and hydrophobic agents [139,140]. These treatments can significantly reduce the water-absorption rate of steel slag, thereby improving its performance in asphalt mixtures [141]. For example, silicone emulsions have been proven to effectively lower the water absorption of steel slag, and silicon resin modifications can reduce expansion and enhance the overall stability of the material [142].

Pretreatment indirectly enhances thermally induced self-healing by:

• Stabilizing microstructure: Reduced expansion ensures that thermal cycles do not induce secondary damage, maintaining the integrity of the aggregate–binder interface during heating [143].
• Optimizing moisture balance: Hydrophobic treatments (organic/inorganic) prevent water-induced disruptions to microwave/induction heating, ensuring consistent temperature distribution and binder flow [144].
• Enhancing thermal compatibility: Carbonated or aged slag exhibits more stable thermal expansion coefficients, reducing thermal stress during healing cycles [145].

In conclusion, pretreatment of steel slag is indispensable for unlocking its potential in asphalt mixtures, addressing volumetric instability while enhancing mechanical and thermal performance. Natural aging, carbonation, and organic/inorganic modifications each offer unique benefits, with selection dependent on project requirements (e.g., climate, traffic load, sustainability goals). Future research should focus on integrating pretreatment with self-healing system design, ensuring synergistic improvements in durability and energy efficiency for next-generation pavements.

4. Critical Factors Influencing Self-Healing Efficacy

4.1. Material Parameters

The self-healing capabilities of asphalt mixtures are profoundly influenced by various material parameters, particularly the type of asphalt binder utilized. The composition and properties of the binder are crucial in determining the mixture’s capacity for self-repair following cracking [146]. Different asphalt binders exhibit diverse behaviors regarding viscosity, fluidity, and adhesion, all of which directly affect self-healing performance.

For example, modified asphalt binders, such as those incorporating styrene-butadiene-styrene (SBS), typically outperform conventional binders [147]. This enhancement is attributed to their superior elasticity and improved resistance to aging, which helps maintain fluidity and facilitates healing under stress. Research indicates that asphalt mixtures containing SBS-modified binders can achieve significantly higher healing rates compared to those using standard 70# asphalt under identical healing conditions [148]. This underscores the importance of binder characteristics, including viscosity and aging resistance, in promoting effective self-healing.

In addition, the gradation and type of asphalt mixture also play a vital role in its self-healing effectiveness. Different gradation types, such as AC-13 and SMA-13, feature varying aggregate sizes and distributions, which influence the availability of binder to fill cracks [149]. Studies have shown that AC-13 mixtures generally demonstrate superior self-healing performance compared to SMA-13 mixtures [150]. The coarser aggregate structure of SMA-13 requires a larger volume of binder to effectively seal cracks, potentially diminishing its self-healing efficiency.

In summary, the performance of asphalt binders and the specific design of the mixture are critical factors affecting the self-healing properties of asphalt mixtures. Key considerations include binder viscosity, the fluidity of aged asphalt, and the adhesion between the asphalt and aggregate surfaces at the crack interfaces. Understanding these parameters is essential for optimizing the composition and design of asphalt mixtures to enhance their self-healing capabilities.

4.2. Operational Conditions

Operational conditions significantly impact the self-healing performance of asphalt mixtures, particularly concerning the application of thermal activation methods, such as microwave heating. The effectiveness of these methods is influenced by several factors, including microwave power levels and frequencies, which dictate how heat is distributed throughout the asphalt mixture [151].

When microwave energy is applied to asphalt mixtures, the interaction between the microwave radiation and the binder generates heat through dielectric heating. The efficiency of this process is largely contingent upon the power level employed. Higher power levels lead to a more substantial temperature increase within the asphalt, enhancing the mobility of the binder and facilitating self-healing. Research has demonstrated that significant improvements in self-healing performance can be achieved at elevated microwave power levels, as increased temperatures promote the flow of the binder into cracks, resulting in better sealing and cohesion [54].

Moreover, the frequency of microwave radiation also plays a crucial role in the heating characteristics of asphalt mixtures. Different frequencies can affect both the rate of temperature rise and the depth of microwave penetration. For instance, higher frequencies may enable rapid heating but can result in shallower penetration, while lower frequencies may penetrate deeper but heat the material more gradually [48]. This trade-off indicates that optimizing both power and frequency is vital for achieving optimal self-healing outcomes.

In practical applications, it is essential to balance these operational parameters to ensure that the asphalt mixture reaches an optimal healing temperature without risking damage to the material [152]. By adjusting microwave power and frequency, engineers can enhance the self-healing capacity of asphalt mixtures, thereby improving their longevity and performance under various environmental conditions. A thorough understanding of these operational conditions is crucial for developing more effective maintenance strategies and designing asphalt mixtures that can withstand the stresses imposed by traffic and environmental factors.

4.3. Economic and Environmental Trade-Offs

The self-healing capabilities of asphalt mixtures are influenced by a range of factors, including heating duration, moisture content, and the extent of damage incurred. Understanding these factors is essential for optimizing the self-healing process while considering both economic and environmental implications.

The self-healing capabilities of asphalt mixtures are influenced by a range of factors, including heating duration, moisture content, and the extent of damage incurred. Understanding these factors is essential for optimizing the self-healing process while considering both economic and environmental implications [153]. However, it is crucial to identify an optimal heating duration, as excessive heating can degrade the binder’s properties and adversely affect its performance [154]. Prolonged exposure to high temperatures can alter the chemical structure of the asphalt, resulting in reduced elasticity and increased brittleness. Therefore, a balance must be established between sufficient heating for effective healing and the preservation of the binder’s mechanical properties.

Moisture content is another significant factor that influences the self-healing process. An optimal moisture range can enhance heating efficiency and improve self-healing performance. However, excessive moisture can have detrimental effects [155]. High water content may necessitate increased energy for vaporization, diverting heat away from the healing process. Additionally, moisture can create barriers at the interface between the asphalt and aggregates, hindering adhesion and flow. Therefore, managing moisture content is essential to maximize self-healing efficacy while minimizing energy consumption.

The extent of damage sustained by the asphalt mixture is a critical consideration as well. As the severity of cracking increases, the capacity for self-healing can diminish significantly [156]. Larger cracks, or those accompanied by structural damage to aggregates, can severely restrict the asphalt’s ability to flow and seal, thereby reducing the effectiveness of self-healing mechanisms. This highlights the necessity for early intervention and maintenance to prevent extensive damage that compromises the healing potential.

In conclusion, optimizing the self-healing performance of asphalt mixtures involves a careful evaluation of various factors, including the type of asphalt binder, mixture gradation, microwave power and frequency, heating duration, moisture content, and the level of damage. Establishing ideal conditions—such as selecting specific asphalt grades, appropriate gradation, and controlling heating durations and moisture levels—can significantly enhance the self-healing capabilities of asphalt mixtures. However, it is imperative to consider potential trade-offs to avoid excessive aging, high energy consumption, and considerable damage, all of which can undermine the self-healing process.

5. Challenges and Future Directions

5.1. Challenges of Thermally Induced Self-Healing

The integration of steel slag into asphalt mixtures and thermally induced self-healing systems represents a significant leap toward sustainable pavement engineering. However, several technical, environmental, and operational challenges must be addressed to realize their full potential, alongside opportunities for transformative innovation.

The primary obstacle to widespread steel slag adoption is the hydration-induced expansion from free lime (f-CaO) and magnesia (f-MgO), which can cause progressive cracking and structural degradation [157]. While natural aging, carbonation, and chemical treatments mitigate expansion, no universal pretreatment standards exist, leading to variability in slag quality. Additionally, long-term field data on pretreated slag performance (e.g., over 10 years of service) are scarce, limiting confidence in its durability across diverse climates and traffic loads.

Steel slag chemistry varies significantly based on steelmaking processes (basic oxygen furnace vs. electric arc furnace) and ore sources, with CaO content ranging from 27–55% and Fe₂O₃ ranging from 5–35% (Table 1). This heterogeneity complicates mix design consistency, as slag from different sources exhibits divergent mechanical (e.g., crushing strength) and electromagnetic (e.g., dielectric loss) properties. Tailored mix designs for each slag type increase costs and technical complexity, discouraging standardized applications.

Another critical challenge lies in the optimization of self-healing techniques.

• Electromagnetic induction: Uneven temperature distribution due to steel slag agglomeration causes localized overheating (ΔT > 15 °C in clusters), risking binder oxidation and insufficient deep-layer repair [158]. The dependency on ferromagnetic components (e.g., Fe3O4) also limits its effectiveness with low-iron slag variants.
• Microwave heating: While offering uniform surface heating, prolonged exposure (≥10 min at 100 °C) accelerates asphalt aging, reducing binder ductility by 15–20% after five cycles. High energy consumption (0.5–1 kWh/m2 per healing session) from non-renewable sources contradicts sustainability goals, particularly for large-scale pavement networks.

From an environmental perspective, the potential leaching of heavy metals (e.g., chromium, vanadium) from steel slag remains a contentious issue. While encapsulation in asphalt reduces leaching risks, prolonged exposure to rainwater and acidic conditions may still release trace contaminants. Robust lifecycle assessments (LCAs) and long-term leaching studies are essential to ensure compliance with environmental regulations and public health standards.

5.2. Future Directions for Innovation

To address the technical and sustainability challenges outlined, future research and development should focus on three interrelated areas of innovation, each grounded in empirical evidence and aligned with practical engineering needs.

5.2.1. Advanced Pretreatment and Material Design for Predictable Performance

Standardized pretreatment remains a critical bottleneck for steel slag utilization. Future efforts should prioritize the development of adaptive pretreatment protocols that leverage real-time material characterization. For example, integrating X-ray diffraction (XRD) and thermogravimetric analysis (TGA) into industrial processing lines can enable precise quantification of f-CaO/f-MgO content, allowing automated adjustment of aging durations or carbonation parameters to achieve target expansion rates. Nanoscale surface modification offers another avenue, with silica-based coatings demonstrating potential to reduce water absorption by 40% while preserving the slag’s electromagnetic properties. These coatings, applied via sol–gel processes, create a hydrophobic barrier that mitigates moisture-induced expansion without compromising the rough surface texture essential for binder adhesion. Additionally, developing regional slag databases, cataloging chemical composition, mechanical properties, and optimal pretreatment methods for slag from different steel plants, would facilitate data-driven mix design, reducing reliance on trial-and-error approaches.

5.2.2. Optimized Self-Healing Systems for Energy Efficiency and Uniformity

Overcoming the limitations of current heating methods requires a shift toward hybrid technologies and material innovation. Induction-microwave hybrid systems, for instance, can combine the deep penetration of induction (5–10 cm) with the uniform surface heating of microwaves, minimizing hotspots and improving healing efficiency for multi-layered pavements. Phase-change materials (PCMs) such as fatty acids (e.g., lauric acid) embedded within steel slag pores can store thermal energy during peak heating, maintaining therapeutic temperatures for 1.5–2 h post-activation, thereby reducing energy consumption.

5.2.3. Smart-Infrastructure Integration and Data-Driven Optimization

The next generation of SSAMs should embrace digital transformation through IoT-enabled monitoring. Wireless strain sensors (e.g., fiber Bragg gratings) embedded within pavements can detect microcracks, triggering automated heating cycles via preinstalled induction coils or microwave emitters. Furthermore, functionalizing steel slag aggregates with self-sensing capabilities—for instance, by embedding piezoelectric quartz within slag particles—enables continuous monitoring of internal stress states. This real-time feedback mechanism supports predictive maintenance strategies, thereby extending pavement service life.

6. Conclusions

Thermally induced self-healing in asphalt mixtures, particularly when integrated with steel slag, represents a transformative paradigm in pavement engineering, addressing critical challenges of durability, maintenance costs, and sustainability. This review synthesized the fundamental mechanisms of self-healing, highlighting the pivotal role of steel slag as a multifunctional aggregate that enhances both mechanical performance and thermal-activation efficiency. By leveraging the viscoelastic properties of asphalt binder and the unique electromagnetic and thermal characteristics of steel slag, this technology offers a proactive strategy to repair microcracks.

Steel slag, as a byproduct of steel production, distinguishes itself through high density, angular morphology, and rich iron oxide content, enabling efficient heat generation via electromagnetic induction or microwave heating while improving interlock and skid resistance in asphalt mixtures. Its alkaline surface promotes strong adhesion with asphalt binder, mitigating moisture-induced damage, though its inherent porosity and reactive oxides (f-CaO, f-MgO) necessitate pretreatment, such as natural aging, carbonation, or organic modification, to ensure volumetric stability. These treatments not only address expansion risks but also optimize thermal compatibility and moisture resistance, which is critical for consistent self-healing performance.

Despite its promise, several challenges persist. Heterogeneity in steel slag chemistry and mechanical properties complicates standardized mix design, while localized overheating in induction systems and binder aging in microwave treatments remain technical hurdles. Environmental concerns regarding heavy metal leaching and the energy intensity of pretreatment and heating processes demand rigorous lifecycle assessments and innovation in energy-efficient technologies.

Looking forward, the integration of advanced material science, smart infrastructure, and data-driven engineering holds immense potential. Standardized pretreatment protocols enabled by real-time material characterization, hybrid heating systems that combine induction and microwave technologies for uniform temperature distribution, and IoT-enabled monitoring for predictive self-healing activation represent key frontiers. By addressing these opportunities, thermally induced self-healing with steel slag can evolve from a promising concept to a mainstream solution, fostering the development of addresilient, sustainable transportation infrastructure.