Steel slag is a waste product obtained through the separation of molten steel from impurities in steel-making furnaces during metallurgical manufacturing. Although the wide utilization of slags is rather recent, the use of iron wastes dates back to the origin of the first metallurgical materials and melting production processes. Among the earliest reports of slag utilization, one is from the Greek physician Aristotele in the field of medicine; the Roman Empire, more than 2000 years ago, firstly utilized slag from crude iron-making forges to build road bases [1]. During the following centuries, several examples exist concerning slag utilization: cannon ball production in Germany at the end of the 1500s, masonry works developed in the 1700s across various European countries, the first road building in England in the early 1800s, and even military road construction during World War I. The amount of steel slag by-products derived from steel-making increased significantly with the rapid development of the steel industry in the last century, resulting in growing needs for effective recycling, also in civil engineering ^[2][3][4][5][2,3,4,5] and road constructions ^[6][7][8][9][6,7,8,9].

The literature reports that worldwide steel production in 2005 was about 1100 million tons, with approximately 21 million tons of steel slag obtained, for instance, each year in the United States ^[10][11] (commercial value exceeding 140 million dollars ^[11][12]). It is estimated that 15–40% of those slags are initially stockpiled in steel plants before being sent to slag disposal sites ^[12][13]. Generally, it is possible to categorize four main slag types, based on the corresponding furnace used: Blast Furnace Slag (BFS), Basic Oxygen Furnace Slag (BOFS), Electric Arc Furnace Slag (EAFS), and Ladle Furnace Slag (LFS), which is also called refining slag ^[13][14]. For instance, the United States produces 14 million tons of BFS annually ^[14][15] (220–370 kg per ton of iron produced) ^[15][16], although EAF steel-making is becoming competitive and is beginning to dominate the American steel industry (55% of total steel production in 2006) ^[16][17].

China’s crude steel output was 808.1 million tons in 2016, and steel slag production exceeded 100 million tons ^[17][18]. As examples for Europe, the United Kingdom stockpiles about 4 million tons of slags (98% are reused as aggregates in concrete and asphalt mixtures in 2002) ^[18][19], continuing to provide 1 million tons each year. Additionally, 97% of BOF slags are recycled in the same way in Germany ^[19][20]. The total annual BOF steel production in Spain is 16 million tons, while 4.5 million tons of BFS, 1.5 million tons of EAFS, and 0.33 million tons of LFS are produced each year in that country ^[20][21]. French EAF steel production is about 12 million tons (with 100–200 kg of BOFS per ton of steel, depending on the raw material). In Italy, most of the slags produced every year are EAFS (3 million tons) ^[21][22].

As expected, the huge amounts of steel by-products lead to an urgent need for recycling, which could also be a promising way to save natural resources ^[22][23] since the availability of raw mineral materials is decreasing rapidly ^[23][24]. Conversion and reutilization of wastes can be a successful practice to tackle awareness of the environment, pollution reduction, and cost control. In this regard, a recent study demonstrates that the incorporation of EAF by-products and RA (50%) in warm wearing courses could lead to quantifiable environmental benefits, referred to as the “cradle-to-gate” approach, which are mostly related to the preservation of raw materials rather than the reduction in paving temperatures ^[24][25]. Therefore, slag’s reuse in road construction nowadays constitutes a promising way to accomplish the above-mentioned aims ^[25][26].

In this regard, metallurgical wastes are reported to be marginally utilized as aggregates in bases, sub-bases ^[26][27][28][27,28,29], and subgrades ^[29][30]. As examples, Rhode et al. (2003) [9] studied the effects of steel slag replacing traditional dense graded crushed sound rocks in unbound low-volume roads, similarly to Akbarnejad et al. (2012) ^[30][31], who evaluated the possibility of sub-base stabilization with slags to enhance pavement strength and reduce maintenance needs. More recently, Yildirim and Prezzi (2020) ^[31][32] performed soil stabilization using steel slags; through unconfined strength tests, they demonstrated that EAF by-products were able to hinder the swelling behavior of in-situ clayey soils thanks to their enhanced strength-gain characteristics. On the other hand, it was demonstrated that a porous asphalt mixture containing steel slag aggregate could be used as a foundation for high-quality pavement layers ^[32][33].

Despite the variety of possible steel slag applications in roads, the recycling of metallurgical by-products in asphalt mixtures is prevalent. In this sense, the rough texture, high angularity, and superior mechanical properties of steel slags suggest the partial or full substitution of natural aggregates in bituminous mixtures to improve their mechanical and functional properties ^[33][34][34,35]. The early studies concerning steel slag in asphalt mixtures date back to the beginning of the 1970s ^[35][36]; since then, studies have been conducted worldwide to characterize steel slag-based materials for both high traffic ^[36][37] and secondary roads ^[37][38]. In recent years, steel slag has been tested to assess its suitability with non-conventional materials such as foamed ^[38][39], polymer-modified (elastomeric or plastomeric), and additive-modified asphalt mixes (warm and cold) ^[39][40][41][40,41,42], thin bituminous overlays (LFS with porous asphalt) ^[42][43], or in combination with additional recycled materials as Reclaimed Asphalt (RA) ^[43][44][44,45] and Asphalt Rubber (AR) ^[45][46][46,47].

2. Steel Slags in Asphalt Pavements

2.1. Laboratory Studies

2.1.1. Mix Design

The most frequently used mix design procedure for asphalt mixtures with steel slag is the well-known Marshall method. In order to evaluate the effect of aggregate substitution, experimental studies involving steel slag are usually performed through a comparison with traditional asphalt mixtures, constituted by natural aggregates only (such as limestone, granite, or basalt). It is generally observed that the addition of steel slags increases the Optimum Bitumen Content (OBC) as a result of the porous surface of slags. Typical higher porosity, clearly demonstrated by SEM analysis ^[47][48][137,138], is determined by the steel-making production processes originating the slags when microscopic air bubbles are trapped in the melted mass during the rapid cooling procedures to which the oxidized and superficial liquid phase are subjected ^[49][50][139,140]. Such porosity could also be detected at the filler-scale, as reported by Tao et al. (2019) ^[51][105], who found diffused micro-pores with a size order of 0.5–5 μm while analyzing slag fillers through SEM. In particular, higher OBC can be quantified in the presence of electric arc furnace ^[47][52][53][76,137,141] and basic oxygen furnace slags ^[54][142]. Generally, this is found regardless of the bitumen type (both traditional and polymer modified binders) ^[52][76], or it is found, together, with the partial addition of recycled concrete aggregate from construction and demolition (C&D) procedures ^[55][143]. Other studies, with the total substitution of natural aggregates, attributed the highest optimum bitumen content to the high void space of the mixture ^[56][144]. However, other studies did not record significant OBC differences, despite the presence of EAFS ^[57][145], probably because of a mix design with lower content of the slag fine fraction. As far as mix design parameters are concerned, the Marshall Quotient (MQ) is reported to increase significantly with the progressive substitution of natural aggregates by slags ^{[34][47][55][57][58][59][60][61][62]}[35,75,137,143,145,146,147,148,149], depending on binder characteristics ^[52][57][76,145]. This is due to the higher Marshall stability and lower Marshall flow ^{[23][55][57][63][64]}[24,82,87,143,145] that are generally collected in the cases of lithic matrixes characterized by aggregates with higher sharpness, angularity, and internal friction angles that guarantee a better aggregate interlock ^[47][137]. Similar trends are also reported with the introduction of steel slag in stone mastic asphalt (SMA) mixtures ^[58][65][75,150]. However, some studies showed quite similar, or even higher, Marshall flows in the case of asphalt mixes with steel slags ^[47][66][83,137]; this could be due to non-negligible variations in the volumetric proportions of mixtures. In this sense, voids in mineral aggregate (VMA) and void content (V) values are sometimes in contrast when also using EAFS ^[23][24], BOFS ^[67][151], or LFS ^[68][67]: VMA and V increase or decrease depending on the aggregate gradations and bitumen contents ^[69][70][152,153]. With respect to the Marshall mix design, Luan et al. (2022) ^[71][154] proposed a modified method for considering steel slag (iron and steel types) within the bituminous mixture, including the concept of a nominal asphalt–aggregate ratio for both dense-graded and SMA mixes. Fewer studies involved the use of the Superpave mix design ^[72][155] procedure through the use of a shear gyratory compactor ^[73][74][75][156,157,158]. In general, an increase in OBC was, again, observed when adding steel slags to asphalt mixtures (for instance, the absorbed bitumen of an EAFS mixture was found to be 0.3–0.4% higher with respect to a control mixture ^[76][159]). The mix design Bailey method ^[77][160] was selected by some authors ^[78][161], showing that SSA mixes, having significant differences in the specific gravity between aggregate fractions, can be produced with similar VMA with respect to reference mixtures. Overall, the literature review seems to demonstrate that, to carefully develop a correct mix design, the granulometric distribution, as well as the bitumen content, must be chosen according to the volumetric proportions of the constituent materials; this is particularly true when natural aggregates are replaced by porous heavy steel slags (a proper volumetric design should lead to similar VMA and residual voids).

2.1.2. Indirect Tensile Strength

Although the test is not directly simulative of a field response, the Indirect Tensile Strength (ITS) is a widely utilized parameter in bituminous mixture characterization and acceptance (EN 12697-23 ^[79][162]) since there is vast experience correlating ITS and field behavior ^[47][80][137,163]. ITS values are reported to be influenced by several factors, such as the aggregate surface textures and the void content of the mixture ^[62][65][149,150], as well as the bituminous film thickness (providing high adhesive forces and binder-particle interlocking abilities) and binder properties ^[68][67]. Thus, better cohesive strength, and subsequently higher ITS, can be achieved with the utilization of highly abrasive, rough textured, highly angular aggregates such as steel slag. In this sense, Ahmedzade and Sengoz (2009) ^[62][149] found higher ITS results with mixtures containing only steel slag coarse aggregates (with respect to limestone-based control mixes) in agreement with Hosseinzadeh et al. (2016) ^[34][35], Behnood and Ameri (2012) ^[65][150] (replacing crushed stones with EAFS), and Lin et al. (2015) ^[67][151]. However, Ameri et al. (2013) ^[47][137] found higher tensile strengths for hot mix asphalt specimens prepared with limestone only, and they explained this result on the basis of the different aggregate–binder affinity (lower EAF slag alkalinity, i.e., the ratio between CaO and SiO₂).

2.1.3. Stiffness Characteristics

A wide variety of literature determines the stiffness modulus through the non-destructive Indirect Tensile Stiffness Modulus (ITSM) test (EN 12697-26 ^[81][164]); although regulations do not generally specify any acceptance limit for bituminous mixtures, the stiffness parameter is considered a key indicator for analyzing the performance of mixes for asphalt layers in terms of structural bearing capacity ^[61][62][148,149]. Commonly, the progressive steel slag introduction, as a replacement for natural aggregate, leads to a higher stiffness modulus as a function of the bitumen type (unmodified or SBS polymer modified) and test temperature ^{[62][82][83][84][85]}[77,149,165,166,167]. The test temperature seems to directly affect the stiffness increase in the case of steel slag mixes, with it being more significant at higher temperature ^[52][61][76,148]. Similar trends have been reported with the simultaneous inclusion of recycled concrete aggregate, and they have mainly been attributed to the high angularity of such aggregates ^[55][143]. In the case of stone mastic asphalt, better mechanical properties can be ascribed to both the integration of the steel aggregates in the lithic skeleton and the high filler–bitumen ratio of asphalt mastic ^[86][72]. Such a stiffening effect is documented when including steel slags in both the coarse ^[43][44] and fine ^[51][105] fractions. Other studies used the resilient modulus to characterize the asphalt surface, base, and subgrade layers of flexible pavements, providing a more representative description of their mechanical properties ^[47][137]. ASTM D7369-11 ^[87][168] can be used for the determination of the resilient modulus of asphalt mixes by repeated-load indirect tension tests, which are conducted through repetitive applications of compressive loads in a haversine waveform. Such a stiffness modulus is expressed as the ratio of the repeated axial deviator stress to the recoverable axial strain ^[88][169]. Again, many studies report that resilient moduli are sensibly increased in steel slag mixtures with respect to the control ones ^[89][170]. An increase in the resilient modulus is also mentioned in the case of recycled concrete fine aggregates, steel slag coarse aggregates ^[55][143], or stone mastic asphalts ^[65][150]. Evaluating the temperature effect on the resilient modulus, Jain et al. (2015) ^[90][171] analyzed ACBFS-based and conventional mixes, demonstrating that resilient modulus increases were not affected by a test temperature in the range between 25 and 45 °C; comparable findings were also found by Ameri et al. (2013) ^[47][137], Lin et al. (2015) ^[67][151], and Hainin et al. (2014) ^[89][170] using laboratory or field specimens. On the other hand, some studies ^[91][92][172,173] documented an increase in resilient moduli, until a slag replacement was used, of up to 70%. A sudden decrease in the resilient modulus was found for higher slag content, probably, because of poor interlocking between slag aggregate particles. Increasing stiffness trends were also detected with stress or strain-controlled laboratory dynamic tests (e.g., with sinusoidal loads) developed at different temperatures and frequency ranges in Four Point Bending—4PB ^[84][166], compression ^[91][172], or tension–compression ^[93][174] configurations. For instance, Abulkhair et al. (2020) ^[94][175] tested EAFS mixtures using axial compression haversine loads and observed that steel slag-based materials exhibited higher complex stiffness moduli with respect to conventional asphalt concretes; moreover, they concluded that this finding highlights a beneficial aspect that is useful to reduce asphalt layer thicknesses during pavement design. A steel slag filler was employed by Awed et al. (2020) ^[95][176] on dense-graded asphalt mixtures and led to similar stiffness behaviors, with respect to control mixes, under continuous sinusoidal loadings. Stiffness characteristics can also be investigated through an assessment of the rheological properties of mastics, analyzing the material responses at a lower scale ^{[36][96][97][98][99]}[37,177,178,179,180]. In this sense, Pasetto et al. (2016) ^[100][181] evaluated the stiffness complex modulus of EAFS mastics and control mastics (limestone-based) with frequency sweep tests performed through a dynamic shear rheometer, recording increasing stiffness with the total replacement of limestone filler with slags, particularly in the presence of a warm chemical additive (probably because of enhanced bitumen–particle interactions). Accordingly, Wang et al. (2011) ^[101][182] found a noticeably higher stiffness modulus for slag mastic, again highlighting the role of bitumen type (addition/modification) and, hence, the importance of binder–filler interactions. According to Chen et al. (2022) ^[102][183] and Zhang et al. (2022) ^[103][184], stable bonding interaction between asphalt and steel slag powder can be achieved thanks to its high alkalinity, as well as rich convex and corrugated morphologies. In other words, steel slag seems to increase the concentration of each asphalt component on the mineral surface; this is often found even with water erosion (peak concentrations of resin and asphaltene are higher using steel slag) ^[104][86]. From this perspective, consolidated literature links such findings with the above-mentioned superior physical characteristics of slags that are able to enhance the bitumen–aggregate affinity and binder adhesiveness ^[51][105]. EAF steel slags in different fractions were also tested by Preti et al. (2019) ^[83][165] in terms of stiffness characteristics: at mixture-scale, open-graded asphalt mixes did not seem to be particularly influenced by the slag presence; analogously, slags neither improved nor worsened the overall behavior of dense-graded mixtures. Actually, the effectiveness of slag introduction must always be evaluated depending on its specific composition affected by the raw material origin, production, treating, and seasoning processes ^[95][176].

2.1.4. Rutting Potential

Rutting resistance of asphalt pavements is defined as the resistance to permanent deformation under traffic loading, occurring predominantly at elevated temperatures/low traffic speed and dependent on the magnitude of the loads and the relative strength of the pavement layers. Surface rutting performance is reported to be mainly connected to the layer resistance to plastic flow, rather than to densification and volume changes ^[105][185]. The susceptibility of bituminous mixtures to permanent deformation can be analyzed with several methods, i.e., dynamic, cyclic, or repeated stress tests that try to simulate the loading action of vehicles. Typically, the inclusion of steel slag in asphalt mixture structures involves a clear improvement in permanent deformation resistance ^{[48][106][107][108][109]}[138,186,187,188,189], with creep modulus (i.e., the ratio between applied stress and final cumulative axial strain) increments up to about 50% ^[61][148] or clear increased flow number (i.e., the cycle at which a tertiary strain flow starts) ^[94][175] in the case of EAFS-integrated asphalt mixtures. The literature also mentions lower creep rates (i.e., the slope of the quasi-constant part of the creep curve) using BOFS by-products ^[110][190], as the steel slag coarse portion in SMA mixtures ^[65][111][150,191], or with the simultaneous inclusion of recycled concrete aggregate ^[55][112][143,192]. Analogously, other studies found similar results through the Repeated Load Axial Test (RLAT), according to BS DD 226 reference standard, with creep modulus increases of 40% in the case of EAFS ^[52][76] and 49% when testing SMA mixtures ^[58][75]. Similarly, Pasetto et al. (2016) ^[41][42] observed that the presence of steel slag, in partial substitution for limestone, did not compromise permanent deformation resistance. On the contrary, Ameri et al. (2013) ^[47][137] recorded a decrease in rutting resistances in hot asphalt mixes when steel slag was used within the fine fraction, whereas higher performance was found if only the coarse fraction was replaced. The permanent deformation behavior of bituminous mixtures can be also determined, with the wheel tracking devices performing simulative tests (EN 12697-22 ^[113][193]) able to correlate laboratory results with in-service pavement performance through rutting depth analysis. Once again, steel slag materials (for instance, EAFS and ACBFS) generally showed reduced rut depths ^[90][171] for different types of mixtures (dense-graded, porous asphalt, SMA). Hence, high angularity, rough texture, and polyhedral shapes of steel slags seem to guarantee a better performance, in terms of rutting resistance, since they promote high internal frictions and aggregate interlocks, as well as ensure better densification ^[114][194]. Among different steel slag types, EAF asphalt mixture (100% of slag) was found to exhibit higher rutting resistance with respect to conventional HMA (Hot Mix Asphalt) and BOF asphalt concrete with 100% slag ^[115][195]. As anticipated, important information regarding bituminous mixture performance can also be derived from the rheological analysis at mastic and mortar-scale: elastic and plastic material behaviors, affecting recoverable and permanent deformations, are generally examined through the Multiple Stress Creep Recovery (MSCR) test, according to the EN 16659 ^[116][196] standard, where creep–recovery cycles at high temperatures are provided. Comparing limestone and EAFS mastics, Pasetto et al. (2016) ^[100][181] and Alnadish et al. (2021) ^[117][197] verified a significant anti-rutting effect when fillers or coarse EAFS were combined with chemical warm-modified binders. Other studies ^[101][118][182,198] presented a multiple regression analysis through a stepwise procedure to consider the effect of filler properties on the mastic and rutting potential of mixtures (with the utilization of steel slags); they mainly showed a poor correlation between mixture rutting potential and filler properties when the data were grouped for all binder types and aggregate gradations (thus, they demonstrated that asphalt binder type and aggregate gradation dominate the mechanical properties of asphalt mixtures). Analogous results were found at the mastic-scale by Pasetto et al. (2023) ^[119][199]. The anti-rutting effect, due to the presence of steel slag fractions, was demonstrated by Preti et al. (2019) ^[83][165] and Pasetto et al. (2023) ^[120][200], who included fine or coarse EAFS in different asphalt mixes.

2.1.5. Fatigue Resistance

The cracking of asphalt pavements related to fatigue phenomena is an unavoidable problem, caused by the repetitive tensile stresses induced by traffic ^[121][201], which leads to progressive crack propagation and causes the gradual weakening of the structures. The complete fracture of the road pavement represents the final step of fatigue cracking: a distress typically occurring at low and mid-range in-service temperatures. Fatigue resistance is reported to be mainly influenced by the bitumen content and its rheological properties, as well as by the compaction degree of the mixtures. In order to evaluate the fatigue performance of bituminous mixes, various experimental protocols have been developed. According to EN 12697-24 ^[122][202], asphalt mixtures can be tested by applying sinusoidal, or other controlled dynamic loading, with different configurations (for example, the Four Point Bending—4PB—and Indirect Tensile Fatigue—ITF—configurations). Regardless of the stress or strain-controlled mode, the fatigue life could be clearly represented by the physical failure of the specimens when cracks evolve until complete failure; otherwise, many studies consider the classical approach based on a 50% reduction in the initial stiffness of the mixtures ^[123][124][203,204]. More advanced fatigue energy approaches can also be used ^[125][126][205,206]. The literature about mixture fatigue analysis is vast, and it also concerns steel slag asphalt mixes, for which ITF results are widely available. In this perspective, Pasetto and Baldo (2012) ^[58][75] highlighted that the mixes with slags presented higher fatigue resistance than those with natural aggregates. Moreover, enhanced fatigue performance has also been demonstrated to belong to the slag inclusion in SMA. Similar results have been found by Aziz et al. (2020) ^[36][37], Pasetto and Baldo (2010, 2011) ^[52][61][76,148], Arabani and Azarhoosh (2012) ^[55][143], Qazizadeh et al. (2018) ^[127][207], and Groenniger and Wistuba (2017) ^[128][208] when testing very different materials, including EAFS and LFS. These findings seem to be ascribable to the rough surface and high angularity of steel slag, regardless of the utilized fraction. Similar results were obtained with three-point bending tests (3PBT) by Xue et al. (2006) ^[110][190], who showed good fatigue resistances using three aggregate types with similar gradation. In the four-point bending configuration, Pasetto and Baldo (2013) ^[129][80] detected good fatigue performance of different mixes containing EAFS. The study also highlighted the importance of binder type (and the role of polymer modification), according to Kavussi and Qazizadeh (2014) ^[23][24], who recorded fatigue life improvement with EAFS mixes, along with a higher initial stiffness. Evaluating the influence of steel slag types, BOF asphalt mixture (100% of slag) was found to exhibit superior fatigue life performance with respect to conventional hot mix asphalt and EAF asphalt concrete with 100% slag ^[115][195]. Since it is widely recognized that a crack’s behavior is mainly related to the bituminous mastic properties, as well as to the bitumen–filler interaction, the literature also mentions an experimental analysis on the fatigue performance of mastics. Bahia et al. (2010) ^[118][198] reported laboratory protocols to consider the variability of fatigue-representative parameters G*·sinδ, depending on the filler type, even if they observed no consistent patterns in the effect of the filler source nor in the slag-based mastic cases. Few literature studies reported slightly lower fatigue resistance when using steel slags, at least in comparative terms, with respect to mixtures prepared with natural aggregates. For instance, under specific experimental conditions, some authors reported worse fatigue performance when using EAFS ^[94][175] or BOFS ^[127][207], instead of limestone or gabbro aggregate, within dense-graded hot mix asphalts. This finding could be ascribed to the greater stiffness documented for many slag-based asphalt mixes that could lead to increased brittleness ^[130][209]. As an example, Tao et al. (2019) ^[51][105], trying to evaluate the influence of slag filler in hot mix asphalt, found slightly lower cycles to fatigue failure that were caused by a higher complex shear modulus of the asphalt mortar. Again, the lower fatigue performance of warm mixtures containing EAF steel slag was shown by Pasetto et al. (2017) ^[84][166], and it was reasonably related to the higher stiffening effect and embrittlement provided by the manufactured fine fraction.

2.1.6. Low-Temperature Behavior

Generally, bitumen characteristics are considered to be the most significant factors influencing the low-temperature cracking performance of bituminous mixtures (ductility, stiffness, penetration, etc.) ^{[131][132][133]}[210,211,212]. However, some correlations between the aggregate characteristics and low-temperature cracking can also be found since the lithic part affects the internal friction and adhesive bond. Low-temperature performance assessments in asphalt mixes can be performed according to the EN 12697-46 ^[134][213] standard. However, tests of indirect tensile strength at low temperatures (e.g., 0 °C) are often conducted to study resistance to cracking and analyze stress–strain curves ^[48][135][138,214]. Based on the few existing studies, mixtures prepared with steel slags seem to show increased critical values with respect to reference mixtures, demonstrating better resistance to low-temperature cracking and good failure behavior (e.g., comparable values substituting gabbro aggregates with ladle furnace slags, in representative mixtures of base course and surface asphalt layers, or enhanced benefits with electric arc furnace slags included in stone mastic asphalt mixtures) ^{[48][128][136]}[138,208,215]. Similarly, the low-temperature cracking resistance of steel slag asphalt mixtures was found to be greater than that of conventional asphalt mixes (containing basalt aggregates) through three-point bending tests (the edge angle of basalt was greater with respect to steel slag, and this caused serious stress concentrations, making the specimens easy to crack) ^[24][25]. On the other hand, Pasetto and Baldo (2015) ^[135][214] obtained a lower thermal cracking resistance while investigating asphalt mixtures for road base courses made with reclaimed asphalt pavement and steel slags, depending on the RA and steel slag contents (up to 70% in weight as limestone substitution), as well as the aging conditions of samples. Suitable bituminous mixtures can be produced using both EAF steel slags and RA (up to 50%), in combination with the warm technology (by synthetic wax additive), without compromising the fracture characteristics of the asphalt concrete ^[137][216]. Here, the limited number of studies addressing this specific aspect, along with the variability of findings, are highlighted. Thus, further research is needed to depict a more consistent picture of the low-temperature properties of asphalt mixtures containing steel slags.

2.1.7. Durability

The assessment of asphalt mixture durability is generally referred to with multiple aspects. The most analyzed issue concerns moisture susceptibility, even if other relevant studies about the effects of aging and raveling resistance can be mentioned. As far as moisture resistance is concerned, the threatening water presence in asphalt mixtures is believed to represent one of the main reasons for flexible pavement failures, leading to structural and functional degradations ^[138][139][217,218]: these phenomena, caused by adhesive (aggregate–mastic bonding) and cohesive (bitumen film bonding) failure ^[140][219], are widely influenced by the properties of aggregates, binders, aggregate–binder interfaces, and mixing temperatures ^[141][220]. The moisture susceptibility of asphalt mixtures is generally evaluated through the determination of the typical parameters (that may not be representative of the fundamental material characteristics) of unconditioned and water-conditioned samples prepared with the same method: a common procedure is Indirect Tensile Strength Ratio (ITSR) analysis, i.e., the ratio of the indirect tensile strength of water-conditioned specimens to the indirect tensile strength of unconditioned samples ^[142][143][221,222]. Furthermore, ITSR can be substituted or integrated by the analysis of other parameters, such as the retained stability (Marshall Stability Ratio, MSR), the Resilient Modulus Ratio (RMR), and Fracture Energy Ratio (FER), or by energy-based parameters, such as the Dissipated Creep Strain Energy limits (DCSEf) ^[144][145][223,224]. In order to determine water damage, rut depth can also be used: EN 12697-22 ^[113][193] reports the procedure to perform wheel tracking tests in a saturated environment, and AASHTO T324-14 ^[146][225] designates the development of wet rutting and stripping tests with a Hamburg Wheel Tracking Device—HWTD. Obviously, the conditioning procedures are crucial for moisture damage responses. The widespread ASTM D4867M-09 ^[147][226] method considers moisture conditioning through partially saturated specimens soaked in distilled water (60.0 ± 1.0 °C) or with samples subjected to a single freeze–thaw cycle. In terms of ITSR results, steel slag mixtures are generally reported to be characterized by higher water resistance than the corresponding traditional mixes ^{[31][62][148]}[32,149,227], usually presenting satisfactory ITSR values (higher than 0.7) in the case of EAFS ^[57][145], BOFS ^[110][149][190,228], and ACBFS ^[90][171]. Sometimes, appreciable moisture resistance improvements were recorded when slags were combined with warm modified bitumen ^[47][141][137,220] or warm asphalt containing RA ^[150][229]. Consistent conclusions can be obtained from other analyzed parameters, mainly MSR, RMR, and FER ^[141][220], as well as DCSEf ^[54][142] and rut depth ^[110][190]. These findings can surely be attributed to the rougher slag surface that improves the adhesion ability of the asphalt binder ^[151][230]; indeed, it is worth noting that the integration of slags in the lithic skeleton involves the need for a proper mix design ^[152][64]. This could often require a higher binder content ^[58][75] to obtain a proper thickness of the bitumen film covering the aggregate grains that can prevent moisture damage due to possible water penetration into the porous structure of steel slags ^[57][145]. In this sense, stripping could be significantly mitigated ^[58][149][75,228]. In general, the inclusion of coarse slag fractions is suggested to improve the water resistance of asphalt mixtures thanks to enhanced aggregate surface characteristics, above all, in the presence of polymer-modified bitumen ^[153][65]. As far as open-graded asphalt mixtures are concerned, the complete replacement of natural aggregates with BOFS has been demonstrated to be a very effective technique to optimize water resistance; this is mainly due to the significant role of the adhesion mechanisms developed, which are a direct consequence of an excellent bitumen–aggregate affinity, within both alkaline and non-alkaline environments ^[154][231]. Concerning aging resistance, durability must be correlated with the adhesion properties linked to the aging processes affecting the bitumen; in this sense, Bell and Sosnovske (1994) ^[155][232] found that the higher the adhesion, the lower the effects of aging. On the other hand, Oluwasola et al. (2015) ^[57][145] verified that aged EAFS mixes exhibited higher properties if compared to aged granite-based mixes in terms of resilient modulus.

2.1.8. Thermal Properties

A further factor addressed in the literature is related to the thermal state of the in-service road pavement containing slags ^[156][233] since it is known that the surface and inner temperatures can strongly affect the mixtures’ mechanical properties, long-term serviceability ^[157][234], as well as specific sustainability aspects connected to the heating of the environment ^[158][235]. In this sense, recent studies tried to assess the thermal responses of various bituminous mixtures manufactured with different steel slags. As an example, Luo et al. (2022) ^[159][236] modeled the behavior of asphalt mixes containing steel slags, coupling electromagnetic and heat transfer processes; they found that microwave heating performance was enhanced with the increase in slag content up to a certain limit (excessive slag contents led to overheating in most areas of the asphalt mixture). Jiao et al. (2020) ^[160][237] proposed an experimental study to investigate the thermal conductivity of open-graded polymer-modified asphalt mixes prepared with steel slags (up to the complete replacement of the natural aggregate). Through laboratory infrared tests and 3D image modeling, they were able to optimize the overall conductivity of the mixture using a threshold value of steel slag content equal to 60%. Analogously, some researchers proposed a laboratory investigation to study several asphalt mixtures with different steel slag contents and found that, compared with traditional aggregates, slags had a higher thermal conductivity thanks to their higher density, composition (Fe₂O₃), and particle size (the most effective particle sizes were 9.5–13.2 and 1.18–2.36 mm) ^[160][237]. On the other hand, some authors reported slightly lower thermal conductivity values in the case of the EAF slag-based asphalt mix ^[161][238]; other literature findings seem to confirm this trend (typical conductivity values for standard paving materials range from 1.2 to 2.2 W/m·°C and 1.3 to 1.7 W/m·°C for steel slag-based mixes) ^{[162][163][164]}[239,240,241]. However, such concerns should not compromise the overall conductivity of a pavement ^[161][238]. For instance, a coupled analysis of the microscopic and thermal properties of an open-grade asphalt mixture containing BOF slag demonstrated that the optimum amount of steel slag should not exceed 60% of the aggregate volume of 3–5 mm (6.6% of the total mixture) in order to have a beneficial effect on the thermal conductivity (linked to a reduced iron oxide content). At 60% content, the highest thermal coefficient (equal to 1.746 W/m·°C) was recorded for the mixture ^[165][242].

2.2. Field Applications

Despite the significant costs and time, field applications are crucial for assessing the effective pavement behavior and verifying the consistency with laboratory studies. However, given that the inclusion of slags in road constructions is a relatively recent practice, little literature exists about long-term field monitoring. In this sense, field pendulum tests (EN 13036-4 ^[166][243] show that coarse steel slag aggregates definitely improve the surface skid resistance ^{[42][48][149][167][168]}[43,138,228,244,245]. In particular, Xue et al. (2006) ^[110][190] performed skid and abrasion tests on a 2-km-long field trial, subjected to a traffic volume of approximately 3 × 10⁷ standard axels, a design vehicle speed of 110 km/h, with annual rainfall near to 2000 mm, and average temperatures from −10 to 40 °C. They found excellent performance in terms of roughness, without cracking, rutting, or stripping phenomena in the first 2 years of service-life for steel slag pavements. Similar findings were reported by Emery (1984) ^[35][36], who performed skid resistance tests on 17 road sections constructed with limestone, steel slag, and ACBFS aggregates. These flexible pavements were subjected to high-traffic volumes and seemed to show adequate friction values; however, the real-scale observations also demonstrated relevant weather influence on slag pavements’ behaviors. Stock et al. (1996) ^[169][246] conducted summer skidding surveys on 69 sites, providing relationships between skid values and traffic densities, as well as recording better skid resistance for slag pavements. They also observed that the steel slag surfaces were able to retain their skid resistance over the service periods, exhibiting more durable characteristics, at least, with respect to those of corresponding sections made with natural aggregates. Furthermore, in order to compare steel slag and basalt stone mastic asphalt, Wu et al. (2007) ^[48][138] analyzed 2 test sections in China, 400 and 2000 m long, subjected to an annual traffic volume up to 1·10⁷ standard axels, under severe rainfall (more than 2000 mm) and temperature (from 0 to 70 °C) conditions. Through visual inspections over 2 years of service, they found no significant distresses (rutting, bleeding, cracking, or stripping) on steel slag paved surfaces; they also recorded excellent performances for steel slag SMA pavements in terms of surface texture depth, abrasion, and friction coefficient BPN (British Pendulum Number) measured through field tests planned every 6 months (55 BPN and 0.8 mm mean texture depth after 2 years of service life). Sofilić et al. (2010) ^[170][247] detected negligible issues in the paving operations constructing field trials with EAFS aggregates. They also observed surface course mixtures exhibiting excellent rutting and skid resistances in the case of EAFS asphalt mixes. On the other hand, the field suitability of asphalt mixtures including BOFS was assessed by Lin et al. (2015) ^[67][151]: they investigated the service quality of test roads, after 2 years with open traffic, through evenness and skid resistance measurements. BOFS asphalt concrete showed higher capabilities in maintaining the pavement evenness with respect to conventional mixtures (after 2 years of service life, BOFS pavement exhibited 50% higher performance than the conventional one subjected to the same traffic and environmental conditions). In addition, BPN reduction after 2 years was sensibly lower on BOFS asphalt concrete, with respect to the traditional one, thanks to the hardness and roughness characteristics of BOFS aggregates (after 2 years, the initial BPN of 77, registered after the construction phases for both pavements, decreased to 63 for the BOFS asphalt surface and to 49 for the conventional one). Finally, field analysis, in terms of rutting, showed shallower rut depths with BOFS (after 15 months of service, a rapid increase was observed in rut depth for the conventional pavement—close to 10 mm after 24 months, instead of 4 mm in the case of BOFS). The BOFS ability to enhance skid resistance has also recently been demonstrated by Li et al. (2020) ^[46][47], who related such findings, mainly, to the physical and mechanical properties of the slag (high density, angular shape, irregular texture), reporting a certain relationship between the skid durability over time and the final grading of the mixture. A further field experience was reported by Wang and Zhang (2021) ^[151][230], who dealt with high-grade highway surface layers made with asphalt concretes including steel slags (10 cm of thickness). They stated that, after 2 years of service of the tested highway, the road surface was still smooth, without visible cracks or ruts, and characterized by a good skid resistance and excellent overall performance.