Recycled Aggregates in Pavements

A foundational strategy for achieving more sustainable pavements involves the incorporation of recycled aggregates, which serve as a substitute for natural aggregates, particularly in the composition of pavement layers. Recycled aggregates are typically derived from construction and demolition (C&D) waste, including crushed concrete and masonry from decommissioned structures, as well as recycled concrete pavement itself (recycled concrete aggregate, RCA). Following the implementation of the requisite processing procedures (e.g., the elimination of contaminants, the reduction of size through crushing, and the sorting of materials based on size), these materials can be utilized in a variety of pavement applications. These include road base or subbase layers, asphalt mixtures, and new concrete for rigid pavements (Pereira & Vieira, 2022). The motivation for this study is evident: natural aggregate extraction has been demonstrated to have environmental costs, including habitat disruption and the energy required for quarrying and transport. Notably, aggregates constitute the most substantial component of pavement materials by volume, accounting for over 90% by weight in asphalt mixtures and approximately 70% in concrete (Tiza et al., 2022; Zhao, Webber, et al., 2022). The reuse of aggregates from waste has been demonstrated to reduce the demand for new extraction and divert material from landfills, thereby promoting a circular economy in the construction industry.

Recycled Concrete Aggregate (RCA)

One of the most prevalent recycled aggregates is RCA, which is derived from crushed concrete elements. RCA frequently contains not only the original aggregate but also remnants of hydrated cement paste attached to the particles. Notwithstanding, a substantial body of research has demonstrated that RCA can exhibit mechanical properties analogous to those of natural aggregates under specific applications. For instance, when RCA is utilized in unbound granular base or subbase layers, standard laboratory tests (e.g., resilient modulus and shear strength) indicate that properly graded RCA can meet the stiffness and strength requirements similar to conventional base materials (Jayakody et al., 2019).

 Jayakody et al. (2019) demonstrated that compacted RCA specimens exhibit steadily increasing stiffness under repeated load, especially at optimal moisture and density conditions. This phenomenon is partly attributable to the angular shape and residual cement, which has the capacity to rehydrate and bind particles. A substantial body of field evidence supports the hypothesis that RCA that has undergone adequate processing can function effectively as a road base. For instance, the Illinois Tollway in the USA reported success using 100% RCA in certain base layers, achieving load-bearing capacity that is equal to or greater than that of traditional aggregate bases (Gillen, 2013).

In the context of concrete pavements, RCA has the potential to substitute for a proportion of virgin coarse aggregate. Researchers advise exercising caution in high percentages due to the possibility of slight reductions in strength or workability; however, moderate replacement has been demonstrated to be feasible. The replacement of 25–40% of the natural coarse aggregate with RCA in concrete mixes has been shown to yield compressive strengths that are within 5–15% of the control (sometimes nearly equal to the control by 28 days) (Kox et al., 2019; Marathe et al., 2021; Shmlls et al., 2022). For instance, a study revealed that substituting 40% of the coarse aggregate with RCA resulted in compressive strengths greater than 29 megapascals (MPa) after 28 days, indicating the material’s potential for enhanced durability in pavement concretes (Kox et al., 2019). Higher replacement levels (e.g., 50% or more) can be utilized with meticulous mix design; a study accomplished 50 MPa with 50% RCA by adjusting the mix and ensuring adequate curing (Mikhailenko et al., 2020). The primary concerns associated with RCA in concrete pertain to its elevated water absorption, attributable to the presence of attached old mortar, and the potential reduction in strength if the source concrete exhibits substandard quality. The incorporation of design modifications, such as the utilization of elevated initial water levels or the incorporation of water-reducing admixtures, has been demonstrated to enhance workability. Adherence to suitable replacement limits is instrumental in preserving strength and durability. In essence, RCA has been recognized as a “globally accepted option in the sustainable construction industry” under the condition that its implementation is accompanied by the verification of material properties and the establishment of suitable percentage limits (P. Wang et al., 2023). Many national standards, including those in effect in Europe and certain regions of North America, permit the use of RCA in concrete for specific applications, such as base layers and surface concrete in secondary roads, contingent upon the fulfillment of performance criteria (Federal Highway Administration, 2022).

In the field of asphalt mixtures, recycled concrete aggregate has been examined as a potential alternative aggregate. The incorporation of RCA in asphalt, whether as part of the fine or coarse aggregate fraction, has the potential to present challenges if the RCA exhibits residual porous mortar characteristics. This condition may lead to an increase in asphalt absorption, resulting in the mixture becoming dry or susceptible to stripping. Research has indicated that the substitution of natural aggregate with RCA, up to a proportion of 30–60% in asphalt mix, is a feasible proposition. However, this substitution may necessitate an augmentation in the asphalt binder content to account for the absorption phenomenon (Bastidas-Martínez et al., 2022; Jayakody et al., 2019; Polo-Mendoza et al., 2022; P. Wang et al., 2023). Furthermore, Marshall stability of hot-mix asphalt has been observed to increase with RCA usage, which is attributed to the angular, rough-textured nature of RCA improving interlock (Bastidas-Martínez et al., 2022). However, the optimal binder content also rises because RCA absorbs more binder than stone (Polo-Mendoza et al., 2022). An increase in optimum asphalt content by 2–18% (relative) was observed when 15–45% of aggregate was replaced by RCA in warm-mix asphalt, resulting in a slight reduction in net environmental gain due to the increased bitumen requirement (Polo-Mendoza et al., 2022). However, there are notable environmental and cost benefits associated with its use. The implementation of RCA in asphalt has been shown to conserve virgin aggregate and frequently results in reduced material costs, particularly when the RCA source is local, thereby eliminating the need for natural aggregate hauling.

Other Recycled Aggregates

In addition to RCA, other C&D waste management strategies have been explored in the context of pavement maintenance. One example of a suitable aggregate base is a blend of crushed brick or masonry from demolished buildings. Brick is generally softer than rock; therefore, it is typically utilized in smaller-scale applications or in areas with lower traffic volume. A study by Xiao et al. (2022) evaluated building demolition waste (BDW) as a whole, which is a mix of crushed concrete, brick, etc., as unbound granular fill for subbase. The study demonstrated that with proper gradation control, even heterogeneous C&D aggregate could achieve suitable modulus and strength for subbase layers. It was observed that BDW aggregate exhibited a higher proportion of fine material post-crushing, a consequence of the potential degradation of masonry materials. This observation suggests a potential enhancement in water absorption and the necessity for enhanced drainage measures to circumvent freeze-thaw cycles, as previously documented by Xiao et al. (2022). The model developed for predicting the elasticity of BDW-based subbase has facilitated a more profound comprehension of the manner in which particle breakage during the process of compaction influences performance (Xiao et al., 2022). Many transportation agencies permit the incorporation of recycled aggregate in base layers up to 100%, provided it meets the requisite grading and strength specifications. For instance, California’s Caltrans allows 100% reclaimed aggregate base, including crushed concrete.

A particular challenge with recycled aggregates pertains to ensuring consistency and implementing effective quality control measures. The composition, hardness, and contamination of aggregates can vary significantly depending on their respective sources. Aggregates may be comprised of concrete, brick, or asphalt pieces, among other materials. Additionally, these aggregates can be contaminated with elements such as wood, drywall, or glass. Consequently, the processes of processing and screening are of paramount importance. However, the environmental benefits of recycled aggregate are substantial. According to certain estimations, the utilization of recycled aggregates in road base can result in a 30–50% reduction in greenhouse gas emissions when compared with the alternative of landfilling the existing material and quarrying new stone. This is due to the avoidance of landfill emissions and the reduction of haul distances for new rock. Furthermore, it has been demonstrated to result in a reduction in the consumption of natural aggregate, a phenomenon that has been observed in regions where such aggregate is becoming increasingly scarce. In the Netherlands and Japan, the recycling rate for construction waste has consistently exceeded 80%, with a significant portion of these materials being utilized in the construction of new roads. This approach effectively demonstrates the comprehensive utilization of materials within a closed-loop system.

In summary, the utilization of recycled aggregates, particularly those derived from RCA, has been demonstrated to be a viable solution in various pavement applications, including the construction of bases, the incorporation of asphalt, and the production of new concrete. This viability is typically observed at partial replacement levels. The mechanical performance of these materials is comparable to that of conventional materials when usage is optimized, as evidenced by replacement percentages and mix adjustments. Furthermore, the environmental benefits of using these materials are evident in their ability to conserve resources. The utilization of C&D waste in pavements is expected to rise in tandem with the intensification of sustainability goals. However, to ensure reliable performance, there is a need for consistent specifications, processing methods, and potentially novel testing methods, such as the monitoring of aggregate abrasion and absorption properties.

Reclaimed Asphalt Pavement (RAP)

Reclaimed Asphalt Pavement (RAP) has emerged as a predominant recycled material within the asphalt industry. RAP is generated by milling or removing existing asphalt layers (during rehabilitation) or by full-depth removal of old pavements. The mixture consists of aged asphalt binder and aggregates that were originally present in the pavement (Arshad et al., 2017). Rather than perceiving this material as waste, it can be reintegrated into novel asphalt mixtures, thereby diminishing the necessity for virgin bitumen and aggregates. The utilization of RAP (reclaimed asphalt pavement) exemplifies the fundamental principle of sustainability: the reuse of materials in a high-value application, thereby achieving cost savings and a reduction in environmental impact. In many jurisdictions, RAP has been routinely utilized for decades at lower percentages (10–30% of the mix), and there is a push to increase these percentages to 50% or more with proper technology (such as rejuvenators or WMA) (Alwetaishi et al., 2019).

Environmental and Economic Benefits

The primary incentive to use RAP is the conservation of resources. RAP contains valuable components, including well-graded aggregates and bitumen. Consequently, substituting a portion of virgin materials with RAP results in direct savings. For instance, the incorporation of every ton of RAP in a new mixture has been shown to result in the conservation of approximately 1 ton of new aggregate and 20 kg (or more) of bitumen (depending on the RAP binder content and effectiveness) (Dughaishi et al., 2022). This phenomenon leads to cost savings, particularly in regions where aggregates or asphalt cement are costly or in short supply. In Oman, as Dughaishi et al. (2022) report, RAP is regarded as an economically viable option due to the scarcity of quality virgin aggregate in certain regions, which necessitates the importation of asphalt binder. Consequently, the recycling of existing pavement emerges as a strategy to ensure material supply and reduce costs. From an environmental perspective, the incorporation of RAP has been shown to reduce the demand for new asphalt binder, which is refined from crude oil and is a significant source of emissions. Additionally, it has been demonstrated that RAP avoids the energy use and emissions associated with the quarrying and hauling of new aggregates. Life-cycle assessment studies consistently find lower energy consumption and greenhouse gas footprints for pavements with RAP. A case study of LCA in the Gulf region revealed that the incorporation of 15% RAP in a WMA blend resulted in “markedly diminished environmental repercussions across all indicators” in comparison to a virgin-materials HMA over the course of a 30-year life cycle (Taha et al., 1999). Furthermore, case studies conducted in the United States and Europe have demonstrated that the incorporation of 20–30% RAP mixes can reduce total life-cycle CO₂ emissions of an asphalt layer by approximately 10–20%, while concurrently decreasing life-cycle costs due to reduced raw material requirements.

Mechanical Performance

A significant concern pertains to the impact of RAP on the performance of novel asphalt mixtures. When RAP is incorporated, the aged asphalt binder it contains undergoes a degree of blending with the fresh binder in the mixture. As a result of the oxidation that occurs during years of service, aged binders exhibit increased rigidity and fragility. Consequently, elevated RAP content has the potential to enhance the overall stiffness of the mixture and augment high-temperature rutting resistance. However, this increase in stiffness may be accompanied by a reduction in low-temperature flexibility and fatigue life, if not adequately mitigated (Tarsi et al., 2020). A multitude of studies have observed this equilibrium. The objective of this study is to examine the impact of RAP on the resistance of rutting, defined as the deformation of materials under load, with a focus on stiffer mixes that exhibit enhanced resistance to rutting. For instance, a study by Boriack et al. (2014) utilized up to 40% RAP, revealing a reduction in rut depth during wheel tracking tests when compared to 0% RAP mixes. This finding corroborates the enhanced rutting performance observed (Dughaishi et al., 2022). Conversely, the impact of this phenomenon on cracking is a subject of varied opinions in the extant literature. A recent study has indicated that the incorporation of moderate RAP (e.g., 20–30%) into mixes can yield a fatigue performance that is comparable to that of virgin mixes, particularly when the fresh binder exhibits a slight degree of softness or when a rejuvenator is utilized. However, other studies have reported that elevated RAP contents (>40%) without any binder grade adjustment or rejuvenation can result in diminished fracture energy and, consequently, augmented susceptibility to thermal and fatigue cracking (Dughaishi et al., 2022). Dughaishi et al. (2022) conducted a review of the extant literature on the effects of RAP on cracking. The authors noted contradictory findings on the subject. Some researchers found decreased thermal cracking resistance at high RAP, while others saw no dramatic loss in fatigue life up to a point (Dughaishi et al., 2022). This finding suggests that factors such as binder grade selection, the incorporation of rejuvenators (additives that restore aged binder ductility), and mix design specifics exert a substantial influence on outcomes.

To address these issues, specifications frequently impose limitations on the content of RAP, unless modifications are made to the binder. For instance, the U.S. asphalt pavement guidelines established by agencies such as the Department of Transportation (DOT) permit the incorporation of up to 15–20% RAP without altering the binder grade, 20–30% RAP with the utilization of a softer binder grade, and greater than 30% RAP exclusively through approved rejuvenation methods or for the application as base layers (Dughaishi et al., 2022). Rejuvenating agents encompass a variety of products, including waste engine oil residues, bio-oils derived from plants or cooking oil, and specialized softening agents. These agents are incorporated into the mixture with the objective of softening the RAP binder. These have demonstrated efficacy in enhancing the blend between RAP and virgin binder, thereby restoring a degree of flexibility (J. Wang et al., 2022). For instance, the combination of a small percentage of waste engine oil with RAP has been found to improve low-temperature performance and fatigue life of high-RAP mixtures (J. Wang et al., 2022). A number of studies have been conducted that explore the potential benefits of integrating RAP with WMA processes. Intriguingly, the findings suggest that WMA can serve as an enabler for enhanced RAP usage. The lower production temperature of WMA ensures that the RAP binder does not undergo excessive aging during the mixing process. Furthermore, the incorporation of WMA additives has been shown to enhance workability, thereby facilitating the compaction of mixes comprising stiff RAP material (Milad et al., 2022). A study cited in the Oman review found that approximately 85% RAP was successfully recycled in-plant using a warm-mix process, achieving comparable properties to a virgin HMA mix (Dughaishi et al., 2022). This high level of reuse, however, likely involved meticulously designed blending charts and rejuvenation processes.

Another potential application of RAP in sustainable pavements is its use in stabilized base or subbase layers. The utilization of RAP aggregates as unbound or cement-bound base material presents a viable alternative to the conventional application of RAP in surface asphalt. A multitude of studies have demonstrated that RAP aggregates, when utilized in isolation, exhibit diminished dry density and strength when compared to conventional base gravel. However, when these aggregates are integrated with a binding agent, such as cement or fly ash, or when they are blended with virgin aggregates, they demonstrate the capacity to perform satisfactorily. For instance, a laboratory study on RAP-aggregate blends for base found that adding 50% virgin aggregate and a small percentage of cement to RAP resulted in a base material with enhanced density and strength. Conversely, 100% RAP aggregate (unbound) exhibited certain limitations in terms of stability (Dughaishi et al., 2022). Many agencies permit the utilization of RAP in base layers, provided it is blended with a proportion of crushed stone to enhance angular interlock and strength.

Current Usage and Trends

RAP is the most recycled product per ton in the United States. According to the results of annual asphalt industry surveys, over 80 million tons of asphalt pavement are reclaimed each year, and a significant percentage is reused in new pavements, making it a notable recycling success story (Williams & Willis, 2020). Analogous trends have been observed in Europe and Asia, though usage rates exhibit variation by country. By 2020, some U.S. states, including Missouri and Texas, had adopted the practice of incorporating approximately 30% RAP (Reclaimed Aggregate Pavement) in surface mixes. Additionally, trial sections containing 50% RAP have demonstrated satisfactory performance when subjected to the application of rejuvenators. Recent advancements in the field have been witnessed in the Gulf region, as evidenced by the incorporation of RAP in highway projects by the United Arab Emirates (Dughaishi et al., 2022). For instance, approximately 15% of the layers in a newly constructed road have been found to contain RAP. Additionally, there has been a notable surge in regional interest in formalizing guidelines for RAP usage, a practice that has historically been limited.

Implementation Challenges

 While RAP offers clear benefits, challenges include ensuring uniform quality of RAP (given its potential to originate from disparate sources with varying gradations and binder properties), stockpile management (to prevent contamination or excessive aging in stockpiles), and mix design complexity (blending charts for binder, etc.). Another salient issue pertains to the feasibility of recycling asphalt on multiple occasions. Research on multi-recycled RAP indicates that asphalt can be recycled multiple times; however, the binder becomes progressively stiffer with each cycle. Each cycle likely requires additional rejuvenation to maintain the material’s viability (Dughaishi et al., 2022). Another challenge that must be addressed is the convincing of stakeholders regarding the performance of RAP mixes. Some road owners fear that high RAP mixes may be less durable. Nevertheless, the gradual emergence of successful pilot projects and a substantial corpus of supportive research have begun to modify this perception. In summary, RAP is a cornerstone of sustainable pavement engineering. When used with proper engineering controls, it yields pavements that are cost-effective and environmentally friendly without sacrificing performance.

Warm-Mix Asphalt (WMA)

Warm-Mix Asphalt (WMA) is a term used to describe a range of technologies and additives that facilitate the production and placement of asphalt mixtures at temperatures that are substantially lower than those required for traditional hot-mix asphalt. Conventional HMA is typically produced at temperatures ranging from 150 to 180°C to ensure the asphalt binder maintains sufficient fluidity for coating aggregates and the mixture remains workable for compaction purposes (Thives & Ghisi, 2017). WMA technologies have been shown to reduce production temperatures by approximately 10–40°C (with typical WMA production temperatures ranging from 110–140°C) while maintaining comparable coating and workability (Milad et al., 2022). The introduction of WMA was largely driven by the desire to reduce energy consumption and emissions in asphalt paving, which aligns perfectly with sustainability goals (Milad et al., 2022).

Types of WMA Technologies

The methods employed by WMA can be categorized into three distinct classifications. The following three categories of additives are employed: (1) Foaming processes, in which small amounts of water are introduced (either directly or via hydrophilic additives like zeolites) to create foam in the hot binder, thereby temporarily expanding its volume and reducing viscosity at lower temperatures; (2) Organic additives (waxes or fatty acid amides), which melt at around 90–120°C and act as viscosity reducers at that range; and (3) Chemical additives (surfactants, emulsifiers), which improve coating and workability through chemical modification of binder properties (Milad et al., 2022). The efficacy of these technologies has been demonstrated through various means. For instance, Sasobit (a synthetic wax) and Aspha-Min (a zeolite) were among the early WMA additives, and each can enable mix production about 20–30°C lower than HMA (Milad et al., 2022). Modern proprietary chemical additives (e.g., Evotherm™ and Rediset™) have also gained widespread adoption due to their ease of use and additional benefits, such as moisture resistance.

Environmental Benefits

The most immediate benefit of WMA is the reduction in fuel/energy consumption at the asphalt plant, since the mixing temperature is lower. This phenomenon directly correlates with a reduction in greenhouse gas emissions and air pollution. Research has documented energy savings ranging from 20 to 35% when comparing WMA to HMA production. Milad et al. (2022) summarized that WMA can reduce energy use by anywhere from 20% up to 75%, depending on factors like the specific technology and the extent to which the temperature is lowered. As less fuel is burned, CO₂ emissions are correspondingly reduced. Additionally, emissions of asphalt fumes, volatile organic compounds (VOCs), and other hazardous pollutants are significantly curtailed at the plant and paving site (Milad et al., 2022). For instance, a field trial conducted in the United States revealed that the adoption of WMA led to a substantial reduction in visible smoke, with measured decreases in CO₂ and carbon monoxide reaching approximately 30%. A reduction in mixing temperature corresponds to a decrease in paving temperature, thereby offering workers a reduction in exposure to heat and fumes. This, in turn, results in an enhancement of health and safety conditions on site (Milad et al., 2022). The aforementioned factors contribute to the classification of WMA as a more sustainable, “green” asphalt technology.

Of particular significance is the fact that WMA’s environmental advantages extend to facilitating other sustainable practices: As previously mentioned, WMA has been demonstrated to effectively accommodate higher RAP contents (Milad et al., 2022) and can enhance the compatibility of other additives, such as crumb rubber or plastics, by reducing the thermal degradation they might experience at HMA temperatures (some polymers or bio-additives may volatize or break down at very high heat). By increasing the use of RAP or other recycled materials, WMA indirectly enhances sustainability benefits through a synergistic effect.

Mechanical Performance and Durability

It is reasonable to hypothesize that decreasing the production temperature could pose a risk of inadequate drying of aggregates or incomplete binder coating. Historically, these factors have given rise to concerns regarding moisture damage (stripping) and reduced strength. However, extensive research and field performance data have demonstrated that when designed appropriately, WMA can equal HMA performance in most aspects (Milad et al., 2022). A substantial body of research has been dedicated to the comparison of WMA and HMA mixes in terms of rutting, fatigue, and low-temperature cracking. The results of these studies have generally shown comparable outcomes (Fakhri & Ahmadi, 2017; Fakhri & Hosseini, 2017; Padula et al., 2019; Saberi.K et al., 2017; Vargas-Nordcbeck & Timm, 2012). In certain instances, WMA mixes exhibit marginally diminished stiffness at the onset (a consequence of diminished binder aging during manufacturing and augmented retained volatiles in the binder). Nevertheless, this can result in enhanced cracking resistance without substantial repercussions on rutting performance in temperate climates.

It is imperative to note that moisture susceptibility is a pivotal parameter that must be closely monitored. Due to the reduced heating of WMA aggregates, there is a possibility of enhanced moisture retention. Concurrently, the viscosity of the binder exhibits an increase at lower temperatures, which may result in a diminished degree of initial bonding. Preliminary research has indicated that certain WMA blends exhibit a reduced Tensile Strength Ratio (TSR), a metric used to assess moisture damage resistance, in comparison to HMA. Fortunately, there are extant solutions to this problem. Numerous WMA additives include anti-strip components, and the mixture can incorporate traditional anti-stripping agents (e.g., liquid amines or hydrated lime) to counteract this phenomenon (Cheng et al., 2011; Mohd Hasan et al., 2015). Modern WMA products frequently promote enhanced adhesion properties. Evotherm, a chemical WMA additive, contains adhesion promoters that, in many cases, improve moisture resistance compared to HMA with no anti-strip. In a particular study, WMA treated with a specific surfactant exhibited higher TSR in comparison to the control HMA (Milad et al., 2022). Furthermore, minor adjustments to the mixture, such as ensuring aggregates are adequately dry and employing gradations that are less susceptible to moisture retention, can alleviate these issues. It has become increasingly prevalent for WMA mixes to align with the moisture damage criteria established for HMA in specifications, often necessitating the incorporation of a modest amount of anti-strip if required (Milad et al., 2022).

Another salient issue is compaction: The use of WMA has been demonstrated to enhance the process of field compaction. This is primarily due to the fact that the mixture remains workable for a longer duration as it undergoes a more gradual cooling process from a lower initial temperature. Additionally, the incorporation of additives into the mixture serves to reduce the viscosity of the binder, thereby enhancing its overall effectiveness. Contractors have frequently reported that they achieve target densities more easily with WMA than HMA (Goh et al., 2007). Proper compaction is imperative for the longevity of pavements. The correlation between density and parameters such as low permeability and high fatigue life underscores the significance of adequate compaction. In conditions of extreme cold or when dealing with thin lifts, there have been instances where WMA cools down too rapidly. However, the prevailing opinion suggests that WMA prolongs the paving season and facilitates extended hauls or thicker lifts due to its enhanced workability (Milad et al., 2022).

Global Adoption and Experience

Since its introduction in the early 2000s, WMA has seen widespread acceptance. Europe was an early adopter of this technology, with several technologies having been developed there. In contrast, the U.S. conducted a large-scale implementation effort in the late 2000s. By 2013, approximately 30% of asphalt production in the U.S. was WMA, and this figure has increased since then, as evidenced by data from the NAPA survey. In a similar vein, countries such as China and India have initiated the incorporation of WMA in highway development projects with the objective of reducing air pollution and fuel consumption. Numerous highway agencies have modified their specifications to permit the utilization of WMA as a substitute for HMA, with minimal alterations to existing procedures. The performance track record is positive. Numerous field trials demonstrate that WMA pavements exhibit comparable distress development (e.g., rutting, cracking) over time to HMA.

It is also noteworthy that there exist technologies for producing half-warm and cold mix asphalt, which utilize emulsions or foamed bitumen at ambient temperatures, yet these are typically employed for base layers or roads with low traffic volume. WMA, which exhibits comparable performance to HMA, has emerged as the predominant solution for mainstream surface asphalt layers on heavily trafficked roads (Environmental Protection Agency, 2022).

Challenges

The challenges posed by WMA are relatively minor and have largely been surmounted. These challenges primarily entail ensuring that contractors receive adequate training in the proper use of WMA, which is often a straightforward process involving the addition of an additive or water. Another key challenge is ensuring that plant equipment can effectively handle the introduction of additives or foaming apparatus. Finally, verifying that mixes meet all criteria at lower temperatures is essential, and this is typically the case for the majority of mixes. Economic considerations encompass the cost of additives or equipment in relation to the savings in fuel. In many cases, these economic factors offset each other, leading to financial savings, particularly during periods of elevated fuel prices (Milad et al., 2022). Furthermore, the augmented production of coolers enables paving in cooler weather without compromising density, thereby extending the construction season. This phenomenon offers an indirect economic benefit by allowing for greater flexibility in scheduling.

In summary, WMA has been demonstrated to be a mature, widely validated green technology in pavement engineering. By reducing the necessary heat, there is a considerable decrease in emissions and energy consumption, thereby contributing to sustainability objectives, such as a diminished carbon footprint in road construction (Milad et al., 2022). Furthermore, it functions as an enabler for other sustainable practices, including higher RAP and improved worker health. The future of WMA research entails the exploration of increasingly environmentally friendly additives, such as bio-based WMA additives, to substitute for the prevailing chemical agents. The review by Milad et al. (2022) recommends exploring “renewable, environmentally friendly, and cost-effective materials such as biomaterials as alternatives to conventional WMA additives.” This suggests that the next generation of WMA could integrate with bio-binders or other green modifiers, merging two sustainability approaches for compounded benefits.

Bio-Binders and Bio-Asphalt

The substantial reliance on petroleum asphalt binder in pavements (which constitutes approximately 4–6% of hot mix asphalt by weight; Alsolieman et al., 2021) has prompted exploration into bio-binders or bio-asphalt as sustainable alternatives. Bio-binders are materials derived from renewable biological sources (e.g., plants, algae, animal waste) that have the capacity to either supplant a portion of petroleum bitumen or function as a modifier to enhance or soften asphalt binder. The objective is twofold: first, to curtail the consumption of non-renewable bitumen (thereby reducing the carbon footprint and reliance on crude oil); and second, to identify binders that boast a superior environmental profile during production and subsequent paving (e.g., reduced emissions, potentially more biodegradable components).

Sources and Types of Bio-Binders

A plethora of bio-resources have been thoroughly investigated for their potential application as binders. Among the most salient of these are:

• Vegetable oils and their derivatives are a subject of interest in this study. Used cooking oil (WCO), soybean oil, corn oil, palm oil, and the like are often chemically modified (e.g., transesterification) to yield a bio-oil that can blend with asphalt (Ramadhansyah et al., 2020). These oils exhibit insufficient viscosity in their native state; however, through the processes of air-blowing or polymerization, they can acquire a more bitumen-like consistency.
• Tall oil and resinous byproducts resulting from the wood pulping process (from the paper industry) have been utilized in commercial rejuvenators and, on occasion, as partial binders.
• Lignin is a solid powder that is derived from woody biomass. This biomass is a byproduct of biofuel or paper production. Lignin has been demonstrated to function as a partial binder modifier, exhibiting a complex aromatic polymeric structure that can enhance stiffness and act as an antioxidant, thereby retarding the aging process of asphalt (Xue et al., 2021).
• The process of bio-asphalt production entails the pyrolysis of biomass, yielding bio-fluxes. Illustrative examples of biomass sources include pyrolyzed swine manure, waste wood, and agricultural waste. The resultant product is a black, tar-like bio-oil. Subsequent refinement of this oil can occur to remove excess water and adjust viscosity, thereby producing a “bio-binder” that has the potential to replace a portion of asphalt cement (Zhang et al., 2022).
• A discussion of natural asphalt substitutes is warranted. Natural asphalts, exemplified by Trinidad Lake asphalt and Gilsonite, are not bio-based due to their natural occurrence as fossil bitumen. However, it is noteworthy that certain formulations employ a combination of natural bitumen and bio-oils to yield hybrid binders.

Zhang et al. (2022) provide a comprehensive review of bio-binders, noting that bio-oils can be obtained via processes like pyrolysis, and the source materials range widely—wood biomass, waste cooking oil, and even animal manure have all been converted to bio-binders. The following definitions are provided for the aforementioned terms: bio-oil, defined as the raw product from biomass liquefaction; bio-binder, defined as the processed bio-oil that is ready to be used in asphalt, often after undergoing treatment such as oxidation or polymer modification; and bio-asphalt, defined as the final binder that results from blending bio-binder with petroleum asphalt, or sometimes fully replacing it (Zhang et al., 2022).

Environmental Perspective

The appeal of bio-binders is rooted in their capacity for renewable production. It is evident that plants absorb CO₂ during their growth cycle. Consequently, the utilization of a binder derived from plants can contribute to a carbon-neutral cycle, with the exception of the energy required for processing. Furthermore, a significant proportion of bio-binders employ waste products (e.g., WCO or animal manure) in their manufacturing processes, a practice that confers a distinct advantage in terms of sustainability by circumventing the potential environmental consequences associated with improper waste disposal (Zhang et al., 2022). The production of certain bio-binders can be conducted at temperatures lower than those employed in the refining of bitumen, a process that has the potential to result in significant energy savings. The substitution of bio-binders for a proportion of petroleum asphalt results in an immediate decrease in the demand for bitumen, a product of crude refining that is characterized by its substantial energy consumption and emission intensity. Life-cycle studies have indicated that partial substitution of asphalt with a bio-binder (e.g., 20% from vegetable oil) can reduce the carbon footprint of the binder component by a similar proportion, though the overall percentage reduction is smaller (since binder is a minor component of the total mix mass). Nevertheless, even the most modest contributions can contribute to the paving of millions of tons of asphalt.

Mechanical Performance

The pivotal inquiry concerns the capacity of bio-binders to perform at a level comparable to that of asphalt. A predominant finding is that bio-asphalt binders exhibit a tendency to be more malleable at elevated service temperatures and demonstrate reduced temperature sensitivity. Zhang et al. (2022) provided a summary of the findings, concluding that, in comparison to conventional asphalt, bio-modified binders generally exhibit “lower high-temperature performance, as well as higher low-temperature performance and aging resistance.” In summary, the incorporation of bio-oil has been demonstrated to exert a dual effect on the physical properties of binder. Specifically, it has been observed to curtail the rigidity of the binder at elevated temperatures, a phenomenon that can potentially exacerbate rutting. Concurrently, bio-oil has been shown to enhance the flexibility of the binder at low temperatures, thereby mitigating the risk of thermal cracking. Moreover, the addition of bio-oil has been shown to diminish the propensity of the binder to undergo oxidative aging. This protective effect can be attributed, in part, to the antioxidant properties of certain bio-oils or to their ability to merely dilute asphaltenes. (Zhang et al., 2022). For instance, a bio-asphalt composed of swine manure oil was observed to decrease the dynamic shear modulus at elevated temperatures, resulting in a softer and less resistant material to rutting. However, the fracture temperature was notably enhanced in bending beam rheometer tests, indicating enhanced performance in cold weather conditions. In a similar manner, the utilization of waste cooking oil as a rejuvenator has been demonstrated to restore ductility to aged binders. However, it should be noted that excessive amounts of this rejuvenator can result in undesirable outcomes, such as bleeding or the formation of soft mixes (Ramadhansyah et al., 2020). Consequently, numerous bio-binders are being regarded as partial replacements or modifiers rather than complete substitutes, with the objective of achieving a balanced equilibrium in properties.

However, exceptions to this general rule have been observed. Specifically, the addition of certain bio-based additives, such as chemically processed vegetable oils, has been shown to enhance the stiffness and viscosity of the binder (e.g., lignin has been observed to increase stiffness and viscosity) (Xue et al., 2021). It has been demonstrated that these substances can enhance the performance of high-temperature applications; however, their utilization in isolation has been observed to potentially compromise the integrity of low-temperature cracking mechanisms. A creative approach by Xue et al. (2021) combined a stiff bio-additive (lignin) with a soft bio-additive (waste engine oil, which is petrogenic but considered a “waste-derived” modifier) to balance out effects. The findings revealed that lignin led to an augmentation in viscosity and an enhancement in elasticity, while waste oil exhibited a reduction in viscosity and an improvement in performance at low temperatures. The amalgamation of these two substances resulted in a modified binder that exhibited commendable overall properties (Xue et al., 2021).

Mixture Performance

Preliminary investigations have indicated encouraging outcomes for asphalt mixtures incorporating bio-binders in short-term assessments. Rutting: It has been observed that certain combinations of bio-binders result in slightly elevated rut depths in wheel tracking when compared to conventional mixtures. This phenomenon is particularly evident when the bio-binder is rendered excessively soft. However, when only a portion of the mixture is bio (for example, 5–20% replacement of bitumen), rutting performance frequently remains within the acceptable range (Zhang et al., 2022). Cracking: The utilization of bio-modified mixes frequently results in enhanced fatigue life, attributable to the augmented ductility of the binder phase. Additionally, the phenomenon of aging resistance implies that, over the course of years, the binder may not harden as rapidly, which could result in a protracted lifespan for the pavement. Indeed, a number of studies have noted that bio-binders have the capacity to act as rejuvenators for RAP, thereby combining two sustainable practices: the use of bio-oil to refresh aged RAP binder. For instance, a critical review of waste cooking oil as a modifier to enhance binder properties revealed its effectiveness in restoring low-temperature performance for high-RAP content mixes (Ramadhansyah et al., 2020).

Notwithstanding the encouraging laboratory findings, as of 2022, there remains a paucity of long-term field data concerning bio-binders. The majority of these implementations have been in the form of trial sections. Questions persist regarding the long-term behavior of bio-binders, including their potential for biodegradation, moisture susceptibility, and compatibility with existing paving practices and recycling streams. It is noteworthy that certain bio-binders may exhibit heightened sensitivity to water, as evidenced by the observation that a bio-oil may not adhere to aggregates with the same efficacy as bitumen. In such instances, the incorporation of anti-stripping agents may be a requisite measure.

Challenges and Considerations

The variability of biomass sources poses a significant challenge. Petroleum asphalt is a relatively consistent material, with specifications that define certain ranges for parameters such as penetration and softening point. In contrast, bio materials may exhibit variability from batch to batch. The chemical composition of bio-oil from wood, algae, and waste cooking oil differs significantly (a mixture of fatty acids, esters, aldehydes, etc. in varying proportions) (Zhang et al., 2022). This indicates that bio-binders may exhibit variable behavior, complicating their standardization. Concurrent efforts are being made to establish grading systems for bio-binders or bio-modified binders that align with Superpave PG grading. Some have proposed the inclusion of extended parameters to account for the distinct behavior exhibited by bio-binders.

Another salient issue pertains to the blending and compatibility of these bio-oils with asphalt. It has been observed that certain bio-oils exhibit a propensity to blend readily with asphalt, while others demonstrate a lack of such compatibility, which can result in phase separation during the blending process. For instance, the addition of unmodified vegetable oil to asphalt can result in separation, as its polarity is lower than that of the asphalt. This separation can be prevented by ensuring proper reaction or emulsification of the vegetable oil. Techniques such as partial oxidative blending or the addition of compatibilizer polymers have been demonstrated to be effective in this regard (Zhang et al., 2022).

From an economic perspective, the cost-effectiveness of bio-binders is notable, particularly those derived from waste materials such as used cooking oil. The utilization of waste cooking oil in paving not only addresses the issue of waste disposal but also provides a cost-effective rejuvenator (Ramadhansyah et al., 2020). The market dynamics of other bio-based products, such as specialized crop oils and lignin, are subject to fluctuations. Lignin, for instance, is produced in significant quantities by the pulp industry and is relatively inexpensive. However, its processing into a form suitable for use as an asphalt additive (i.e., a fine powder or liquid derivative) incurs additional costs.

Notable Successes

The Netherlands recently conducted a trial involving the implementation of a fully bio-based asphalt in a bicycle road. This trial utilized lignin to substitute for a substantial proportion of bitumen, reaching up to 50% binder replacement. According to the reports, the performance of this asphalt proved to be satisfactory for its application, which exhibited low stress. In France, the Eco² asphalt initiative employed a blend of RAP and a bio-based rejuvenator to formulate asphalt with a substantially diminished carbon footprint. This blend exhibited comparable performance to conventional mixes over the course of several years of monitoring. These examples suggest that, with meticulous engineering, bio-binders can be practically implemented.

According to Zhang et al. (2022), future research endeavors should prioritize the maximization of bio-binder content in asphalt while maintaining performance standards, thereby extending the boundaries of current capabilities. Additionally, there is a call for further exploration into the chemical interplay between bio-binder and petroleum asphalt at the molecular level, with the objective of ensuring optimal compatibility. This research is accompanied by the proposal of establishing unified standards for bio-asphalt technology (Zhang et al., 2022). The identification of bio-binders with the potential to substitute for asphalt in the future is a subject of interest for research (Zhang et al., 2022). While the aspiration of a fully bio-derived asphalt binder that meets all performance requirements remains somewhat idealistic, incremental progress (e.g., 20% replacement, then 30%, etc.) is being made in a steady and consistent manner.

In summary, bio-binders offer a promising route to make pavements more sustainable by tapping renewable resources and recycling waste into binder material. These additives have been shown to enhance low-temperature performance and mitigate aging, though they frequently result in a compromise in high-temperature stiffness. To address this challenge, current methodologies involve the incorporation of these additives in conjunction with conventional asphalt (or with modifiers) to achieve a balanced property profile (Zhang et al., 2022). Continued research and development is necessary to refine bio-binder formulations, ensure consistency, and validate long-term field performance. However, the concept aligns well with global sustainability goals, such as reducing fossil fuel use and emissions. As these technologies continue to develop, the implementation of partial bio-asphalt pavements may become prevalent, particularly in regions with a plentiful supply of bio-feedstocks.

Industrial Byproducts (Fly Ash, Slag, and Others)

The utilization of industrial byproducts in construction materials has been a persistent practice, with pavement engineering being a notable example. The utilization of byproducts such as fly ash (from coal combustion), slag (from iron and steel production), and silica fume, red mud, phosphogypsum, and others has the potential to generate sustainability benefits. These benefits include the diversion of waste from landfills and the reduction in the necessity for energy-intensive virgin materials. In this study, the primary focus is on the most prevalent byproducts in pavements, namely fly ash and slag. Additionally, the utilization of steel slag aggregates and foundry sand is examined.

Fly Ash in Cementitious Stabilization and Concrete

Fly ash, a pozzolanic powder, is collected from the flue gases of coal-fired power plants. In pavement construction, its primary application is as a supplementary cementitious material (SCM) in concrete or cement-stabilized layers. Class F fly ash, characterized by its low calcium content derived from the combustion of bituminous coal, and Class C fly ash, distinguished by its high calcium content originating from sub-bituminous coal or lignite, have demonstrated the capacity to substitute for Portland cement in concrete mixtures to a certain extent. This practice has been thoroughly researched and is well-established; typically, 15–30% of cement by mass can be replaced by fly ash in pavement-quality concrete without compromising strength (Taylor, 2019). Indeed, fly ash has been shown to enhance workability and the later-age strength and durability of concrete, while concomitantly reducing heat of hydration (a beneficial effect in mass pours). For rigid pavements, the utilization of Class F fly ash is frequently employed to mitigate alkali-silica reaction and enhance long-term strength gain. In certain instances, Class C, which possesses inherent cementitious properties, can even permit higher replacement levels.

The sustainability impact of replacing cement with fly ash is significant. According to M.G. et al. (2022), replacing every ton of cement with fly ash avoids approximately one ton of CO₂ emissions. This is due to the fact that cement production is carbon-intensive. Consequently, high-volume fly ash concrete (HVFAC) is regarded as a sustainable material. For instance, a high-volume fly ash concrete (with 50% cement replacement) was utilized in a demonstration pavement in Minneapolis. This material exhibited adequate strength development and reduced permeability; however, it demonstrated slower early strength gain. This can be managed by mix design or curing practices. Life-cycle assessments have been utilized to corroborate the findings, demonstrating that such combinations exhibit a significantly reduced embodied energy and emissions.

Additionally, fly ash has found application in the stabilization of soil or bases. The integration of fly ash with soft soils has been demonstrated to enhance their bearing capacity. Specifically, Class C fly ash has been observed to undergo hydration and cementation processes, resulting in the consolidation of soils. In certain countries, fly ash is dispersed and amalgamated into clay subgrades with the objective of fortifying subgrade support for highway infrastructure. Furthermore, the utilization of fly ash as a filler in asphalt has been a subject of investigation, including the incorporation of Municipal Solid Waste Incineration (MSWI) fly ash. Zhao, Ge, et al. (2022) conducted a study in which they examined MSWI fly ash as a potential mineral filler in asphalt mixtures. The investigation revealed that the substitution of conventional filler with fly ash exhibited minimal impact on low-temperature properties. However, it was observed that fly ash significantly enhanced high-temperature performance, a phenomenon that can be attributed to the increased stiffness provided by fly ash as a filler. Specifically, as fly ash content increased, the asphalt mixture exhibited enhanced stiffness and rut resistance, though its low-temperature fracture tolerance exhibited a slight decrease, indicative of a typical trade-off. Additionally, it was noted that the use of excessive fly ash without implementing anti-stripping measures may result in a deterioration of moisture stability (Zhao, Ge, et al., 2022). However, the incorporation of fly ash in asphalt or concrete has been demonstrated to effectively mitigate the necessity for extracting additional mineral filler or cement. This approach is regarded as an eco-efficient utilization of a byproduct that would otherwise be considered waste.

Blast Furnace Slag and Steel Slag

Slag is another significant byproduct of this process. Ground-granulated blast furnace slag (GGBFS), a byproduct of iron production, has been utilized in a manner analogous to fly ash as a cement replacement. It possesses latent hydraulic properties and, upon activation (typically by the alkalinity of cement or other activators), can undergo hydration and enhance its strength. Slag cement (GGBFS) has been demonstrated to substitute for 30–50% of Portland cement or more in concrete applications. Slag has long been utilized in Europe for the construction of durable pavements, where its use is well-documented. The primary benefits of slag include enhanced long-term strength and reduced permeability. However, it should be noted that the early strength gain of slag is often slower in comparison to other materials, such as fly ash. Additionally, under certain conditions, there is a possibility of increased risk of scaling, unless proper curing procedures are followed. The substitution of cement with slag has been demonstrated to significantly reduce the carbon footprint of concrete pavements. This is due to the fact that slag is a repurposed industrial waste that only requires grinding, not calcination. GGBFS has been shown to impart a lighter color to concrete, a property that can prove beneficial for high-albedo “cool pavements.”

Steel slag, a byproduct of steelmaking processes, characteristically manifests in a crystalline structure. This material can be processed to yield aggregate of varying sizes. The material under consideration is characterized by its high density and hardness, which renders it particularly well-suited for use as an aggregate, especially in applications where polish resistance and high stability are paramount, such as in the production of heavy-duty asphalt or skid-resistant stone mastic asphalt. However, unprocessed steel slag contains free lime and periclase (MgO), which have the potential to hydrate and expand over time. The utilization of steel slag without undergoing aging or treatment can result in a series of undesirable consequences, including expansion and disintegration of pavements. This phenomenon has been documented in previous studies (Huang et al., 2022). To ensure the safe utilization of steel slag, methodologies such as weathering (subjecting slag to the elements for an extended period to facilitate the hydration of the lime/MgO prior to its application) or chemical treatment have been employed. Huang et al. (2022) conducted a study to examine the efficacy of treating steel slag with oxalic acid. The primary objective of this study was to neutralize the free lime present in the steel slag and thereby enhance its stability in asphalt. The application of acid treatment led to a substantial reduction in the propensity of the slag to undergo swelling. Additionally, it enhanced the water resistance of the slag aggregate within the asphalt mixture. Consequently, the performance of the slag-asphalt mixture was rendered comparable to that of a conventional aggregate mix (Huang et al., 2022). The treated slag asphalt demonstrated commendable stiffness and fatigue performance, and notably, exhibited no substantial expansion.

From a sustainability perspective, the utilization of steel slag aggregate represents a strategy for conserving natural aggregate and repurposing a byproduct that would otherwise be discarded. Indeed, Japan and several European nations employ steel slag in asphalt after subjecting it to proper aging (as reported by the Nippon Slag Association, which notes a high rate of slag utilization in roadways). The high density of steel slag, which results in higher transport costs per volume, is a minor drawback. However, its strength has the potential to allow for thinner layers.

Foundry Sand

Waste Foundry Sand (WFS) is derived from metal casting industries and is a high-quality silica sand that has been utilized with binders for molds. Following a series of uses, the device is disposed of. The WFS technique has been applied in both asphalt and concrete contexts. In essence, it functions as a fine aggregate substitute. According to the extant research, it appears that a maximum of 20% of the fine aggregate in concrete may be substituted with foundry sand without compromising its strength. A study revealed that up to 20% WFS had no substantial impact on the compressive or flexural strength of concrete, though it did result in a slight increase in water demand (Dash et al., 2016). The gradual replacement of 30% of the original material with a fine replacement gradually reduces strength and necessitates the addition of water or admixtures. This is due to the potential for WFS to contain residual binder, such as bentonite or chemicals. However, positive effects such as increased abrasion resistance have been observed in WFS concrete, potentially attributable to the presence of hard fine particles (Dash et al., 2016). In the context of asphalt, WFS has the capacity to function as a mineral filler. Its efficacy has been demonstrated in the context of hot mix asphalt (HMA) applications, with certain highway departments specifying its incorporation into the binder-stabilized aggregate (B-SA) or mineral filler (MF) categories. The advantages of this approach are manifold, including the reduction of virgin sand required, the mitigation of landfill waste from WFS, and the potential for cost savings.

Red Mud, Phosphogypsum, etc.

The utilization of alternative byproducts, which are less prevalent, is confined to specific applications. Red mud, a byproduct of alumina production, is highly caustic and has limited application. However, experimental studies have demonstrated the potential for stabilization with activators for use as a road base material. Phosphogypsum, a byproduct of the fertilizer industry, has found application in road base construction in areas where natural gypsum or calcium sources are beneficial, particularly in rural road construction where cost-effectiveness is a priority. These materials are not commonly used due to challenges in their handling, such as concerns regarding radioactivity in phosphogypsum.

Geopolymer and Full Replacement of Cement

An innovative application of fly ash and slag involves the use of geopolymer concrete, which entirely replaces Portland cement with an alkaline-activated binder derived from fly ash and slag. Research has demonstrated that geopolymer concrete exhibits a strength comparable to that of conventional concrete (Tayeh et al., 2022). In the context of pavement construction, the utilization of geopolymer concretes has emerged as a promising avenue for mitigating cement-related CO₂ emissions. Recent studies have indicated that the performance of fly ash/slag geopolymer concrete can be comparable to that of traditional concrete in structural applications (Almutairi et al., 2021). However, further field validation is necessary to ascertain its behavior under various exposure conditions, such as freeze-thaw cycles or sustained high temperatures, and to determine its long-term properties. Nonetheless, geopolymer technology offers a promising avenue for the utilization of industrial byproducts in the creation of a novel binder, exhibiting a substantially diminished carbon footprint—reportedly achieving up to 80% CO₂ reduction relative to OPC concrete, contingent upon the incorporation of ambient cured geopolymer in the fabrication of pavement.

Summary of Benefits

The incorporation of industrial byproducts in pavement construction offers a multifaceted approach to sustainability. The primary benefit of resource efficiency is the conservation of natural materials, such as cement and aggregate, for every ton of byproduct utilized. The secondary benefit is the reduction of waste, which prevents the accumulation of large volumes of industrial waste and enables its productive use. The tertiary benefit is often performance gains, as many byproducts offer durability benefits, such as fly ash reducing permeability and slag improving later strength. The utilization of fly ash and slag as construction materials can result in economic advantages due to their cost-effectiveness in comparison to cement, particularly when sourced from local materials. In certain instances, contractors may receive financial incentives for the disposal of specific waste materials.

Challenges

It is important to note that variability can be observed in each byproduct. For instance, the quality of fly ash is known to vary according to the coal source, and the properties of slag are contingent on the specific conditions of the furnace. Quality control and, on occasion, preliminary processing (i.e., grinding, slag aging, and the filtration of fly ash impurities) are requisite procedures. The availability of certain byproducts is constrained to specific regions. For instance, areas lacking coal power may experience a lack of fly ash, though the global transition away from coal is progressing. Consequently, the future supply of fly ash may decrease, thereby posing a sustainability challenge for fly ash concrete. The process may encounter regulatory impediments, such as the classification of materials as hazardous. For instance, some MSWI ashes contain heavy metals that necessitate encapsulation within the pavement to prevent leaching.

In practice, the utilization of fly ash and slag in concrete has been a well-established standard in many regions, with these materials being a component of durable concrete specifications. The utilization of steel slag and foundry sand in asphalt is not as prevalent as other methods, but it is increasing as advancements in processing techniques are made. In the future, as the pavement industry endeavors to achieve carbon neutrality, it will be imperative to maximize the incorporation of such byproducts. One could envision a future in which pavement structures are constructed with all layers containing recycled or industrial byproduct content. For example, a base could be stabilized with fly ash, a concrete pavement could contain 50% slag cement, or an asphalt surface could utilize WMA, RAP, and steel slag aggregate. These materials would combine to significantly reduce the embodied energy of the road.

Other Green Technologies and Innovations

Beyond the major categories previously mentioned, there are several other innovative materials and technologies that contribute to sustainability in pavements. These include the use of recycled rubber, recycled plastics, waste glass, and even more novel ideas like photocatalytic additives or carbon-sequestering concrete.

Crumb Rubber Modified Asphalt (CRMA)

The utilization of ground tire rubber from waste tires in asphalt has been a subject of interest since the 1960s. This approach effectively addresses the disposal issue of millions of scrap tires. The incorporation of crumb rubber can be achieved through two distinct methodologies: the “wet process,” which involves the dissolution of rubber particles into a molten asphalt binder, thereby yielding a rubberized binder, and the “dry process,” which entails the incorporation of rubber as an aggregate substitute in the mixture. Rubberized asphalt has been shown to offer several advantages in performance, including enhanced resistance to rutting and reduced reflective cracking due to the elastic nature of the rubber. A recent review by Bilema et al. (2023) concluded that “CR-modified asphalts had superior performance and longer service life” compared to traditional mixes. Specifically, crumb rubber has been shown to increase the viscosity and stiffness of binder at high temperatures, thereby reducing the incidence of rutting. Additionally, it has been demonstrated to enhance the elasticity of the material, which can lead to an improvement in fatigue life. However, the presence of rubber can also complicate binder management due to its high viscosity. If the rubber particles are not adequately digested, they have the potential to induce workability concerns. A further consideration pertains to the environment. The mixing of rubber generally necessitates elevated temperatures (particularly in the wet process, which reaches approximately 190°C). This can result in an increase in emissions at the manufacturing facility unless measures are implemented to mitigate the issue. However, certain waste management strategies have been employed to address this concern in the context of rubber mixes. However, a comprehensive life-cycle assessment indicates that rubber asphalt generally offers a more favorable environmental profile, primarily due to its ability to extend the service life of pavements, thereby reducing the frequency of rehabilitation. Additionally, the utilization of rubberized asphalt has been shown to address critical tire waste issues, with each lane-kilometer of such asphalt capable of replacing thousands of tires. Numerous US states (e.g., Arizona, California) and countries (e.g., China, Italy) have utilized rubber-modified asphalt extensively. From a sustainability perspective, rubber asphalt has been shown to reduce road noise significantly, thereby contributing to a more favorable environmental outcome in urban settings, as it reduces the necessity for noise barriers.

Recycled Plastic in Asphalt

Recently, there has been a surge of interest in incorporating waste plastics (e.g., packaging, bottles, bags) into asphalt mixtures as a modifier or partial aggregate replacement. A number of companies have developed plastic additive pellets made from mixed recycled plastics that can be added to asphalt mix. The concept is appealing in light of the global plastic waste issue. Research has demonstrated that specific plastics, particularly those of a harder consistency, such as HDPE, or those that mimic the behavior of fibers, can enhance asphalt’s resistance to rutting and augment its stiffness (J. Wang et al., 2022). For instance, shredded waste plastic (e.g., polyethylene) can act as a polymer modifier, thereby increasing the mix’s stability (Xu et al., 2021). A study referenced by J. Wang et al. (2022) observed that plastic-modified asphalt exhibited effective high-temperature performance; however, low-temperature performance could be compromised if the plastic renders the binder excessively rigid. Ensuring adequate plastic dispersal is paramount to avert potential complications, such as particle clustering or melting issues. Additionally, concerns have been raised regarding microplastics. In the event that plastic does not fully bind into the asphalt matrix, it is possible that it could fragment and release contaminants. Current evidence suggests that when properly mixed, the plastic is well encapsulated by bitumen; however, long-term environmental studies are required to ascertain the full extent of its environmental impact. It has been reported that certain municipalities (e.g., in India, the UK, and the Netherlands) have initiated a pilot program involving roads that utilize waste plastic-modified asphalt. These municipalities have documented positive outcomes, indicating that this innovative approach may offer a viable solution for addressing the challenges associated with waste management and infrastructure development. In a notable development, India has mandated the incorporation of plastic waste in road construction for specific projects, signifying a strategic approach to sustainable development.

Waste Glass

Crushed waste glass (cullet) has been demonstrated to function as a substitute for aggregate in asphalt and as a partial fine aggregate in concrete. Glass is a 100% recyclable material; however, not all glass is recycled due to contamination or economic factors. Consequently, its use in road construction is a viable option. Glass in asphalt, also referred to as “glasphalt,” has been employed in certain instances as a substitute for a portion of the aggregate, typically amounting to 10–15% of the fine aggregate. This incorporation of glass introduces angularity and, notably, has been observed to enhance skid resistance due to its high hardness. However, excessive use of glass can result in stripping of the surface, which can compromise the adhesion properties. Additionally, glass is susceptible to breakage, which can lead to raveling issues. Consequently, usage is typically restricted, and the incorporation of anti-stripping agents is advised. Concretely, finely ground glass can exhibit pozzolanic properties (akin to fly ash) if ground sufficiently fine (i.e., to a powder consistency). The incorporation of coarser glass as sand in concrete has been demonstrated to be susceptible to alkali-silica reaction, a phenomenon that can be mitigated by maintaining a low cement content or by other measures. This susceptibility arises from the inherent reactivity of glass, which is rich in silica. However, certain studies have utilized up to 20% glass sand in pavement concrete, yielding satisfactory outcomes, particularly when supplementary cementitious materials are incorporated to mitigate ASR.

Photocatalytic Pavements

An innovation that has the potential to enhance sustainability during the utilization phase is the incorporation of titanium dioxide (TiO₂) into pavement surfaces (typically concrete or, on occasion, asphalt coatings). This addition serves as a photocatalyst, which means it can facilitate the breakdown of air pollutants, such as NOx, released by vehicles. While this approach does not directly contribute to the conservation of materials or energy in construction, it does play a crucial role in enhancing environmental sustainability by promoting improved air quality. A number of trials in Europe of TiO₂-containing “smog-eating” pavements have demonstrated reductions in NO2 concentrations near roads by 20–30% under ideal sunlight conditions. The cost and long-term efficacy of this approach are currently under evaluation. The primary focus at this stage is on sustainability in terms of environmental and health impact, rather than on resource conservation.

Carbon-Sequestering Materials

Researchers are exploring cement alternatives that have the capacity to absorb CO₂ during the curing process. Examples include magnesium-based cements and carbonated aggregates. For instance, one company produces artificial aggregates by mixing CO₂ with steel slag fines, thereby creating a carbonate aggregate – a process that utilizes waste CO₂ and waste slag. These aggregates can subsequently be utilized in pavement layers, thereby effectively sequestering carbon in the pavement. Although these technologies are nascent and not yet prevalent, they signify the subsequent domain of sustainable materials.

Cool Pavements

Lighter-colored pavement or special coatings (not a different material, often just a pigment or binder choice) that reflect more sunlight can reduce the urban heat island effect and thus indirectly conserve energy (less cooling is required in nearby buildings, etc.). The utilization of slag, lighter aggregates, or concrete in lieu of asphalt, in addition to the incorporation of reflective pigments, are methodologies employed to achieve the objective of “cool pavements.” This is a design choice that has sustainability implications in the context of climate adaptation. Figure 1 provides a synopsis of the pavement layers associated with each waste material.