Scientometric and Systematic Review with SWOT Analysis of the Application and Performance of Synthetic and Composite Textile Waste-Derived Materials in Flexible Pavements

Abstract

The dramatic increase in the volume of postconsumer textile waste poses not only a major environmental problem but also an untapped opportunity for the development of sustainable infrastructure through the use of synthetic and composite textile waste-derived materials (SCTWDMs) in the field of asphalt pavement engineering, contributing to the achievement of the United Nations Sustainable Development Goals (SDGs 9, 11, 12, and 13). This systematic review was performed according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines. A systematic search of the literature in the field of SCTWDMs in asphalt pavement engineering was performed between 2010 and 2025 using the Web of Science and Scopus databases. A total of 65 studies were identified and analysed according to the inclusion and exclusion criteria of the current review. The quality of the studies and the risk of bias were assessed according to the transparency of the methods and the reporting of the results. The triangulated methodological framework consisted of bibliometric analysis, systematic review, and SWOT analysis. The bibliometric analysis was carried out via VOSviewer software version 1.6.20. The results of this study indicate an increase in the number of publications in SCTWDMs; however, there is fragmentation in the field. This denotes poor interrelationships among themes, insufficient collaboration across research streams, and scattered networks of keyword associations, suggesting a lack of a coherent research framework for SCTWDM research. The results of this study indicate that SCTWDMs generally improve the rheological properties, cracking resistance, and mechanical characteristics of asphalt mixtures. However, variability in fibre properties, optimisation of dosage, and limited field validation remain major challenges in SCTWDMs. The SWOT analysis also highlights important technical, institutional, and standardisation barriers, as well as opportunities for further development in sustainable pavement technologies. Despite this, the body of evidence is limited by heterogeneity in study design and a lack of long-term results. The review is not preregistered, but all the methodological procedures are transparently described. In conclusion, this body of evidence offers a strategic direction for further research, policy development, and industry practice, highlighting the importance of linking laboratory results to applications to position SCTWDMs as a viable option within the global sustainability agenda.

1. Introduction

The world’s textile industry is a significant example of a source of resource waste and pollution, emitting approximately 10% of carbon emissions from global industries and creating large amounts of uncontrolled postconsumer waste [1,2]. The current trend of embracing the circular economy approach is not yet matched by sufficient valorisation of textile wastes, where the vast majority are either landfilled or incinerated [1,3,4]. This highlights the inadequacy of using waste resources for engineering purposes as an important system limitation. Synthetic and composite textile waste-derived materials (SCTWDMs), which include waste denim, used face masks, and fibre from tyres, have been growing at an alarming rate due to changes in consumer behaviour, rapid industrial growth, and pandemic-related issues [5,6]. On the one hand, these materials constitute an environmental threat; however, on the other hand, they represent an enormous potential source of valuable materials that exhibit excellent tensile properties, resilience, and chemical resistance.

With respect to pavement engineering, this becomes a promising area. However, the literature on SCTWDMs is characterised by epistemic fragmentation and methodological limitations. Most existing studies tend to conduct tests at laboratory scales via diverse experimental designs without the use of integrative models that can link material performance with the evolution and implementation of the existing research. These drawbacks result in limited comparability of research and knowledge, weak knowledge integration, and inadequate transfer of findings into application areas. Waste denim, which is mostly made up of high-strength cotton–polyester mixtures, adds more than 2 million tonnes to the world’s postconsumer textile waste each year [5,6]. Because of its complicated material composition and low recyclability, a major portion of this garbage ends up in landfills or incineration facilities [6].

Similarly, the COVID-19 pandemic has led to an exponential increase in the use of synthetic face masks; estimates indicate that at its height, over 129 billion disposable masks were used monthly worldwide, the majority of which are constructed from nonbiodegradable thermoplastic polypropylene [7,8]. These masks are currently a new and enduring type of plastic pollution [8]. Furthermore, nearly 3 billion end-of-life tyres are disposed of each year due to waste tyre textiles, which are usually made of nylon, polyester, and aramid fibres [9,10]. Textile fibres make up approximately 10% to 15% of the tyre weight, which translates to 10–15 million tonnes of fibrous material worldwide [11,12]. Interestingly, these same materials have properties such as excellent tensile strength, hydrophobicity, and resistance to mechanical and chemical deterioration, which are highly valuable in pavement engineering [13,14,15]. The mechanical properties and durability of pavement composites can be improved owing to their inherent resilience and structural integrity [4,15]. Thus, in an attempt to resolve these challenges, this study presents an innovative triangulation approach of analysis based on systems thinking, consisting of scientometric analysis; systematic reviews with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria; and qualitative strength, weakness, opportunity, and threat (SWOT) analysis. Scientometric analysis helps in understanding the intellectual landscape and development of this area of study. Systematic reviews enable critical integration of evidence on material performance.

Finally, SWOT analysis helps put this evidence into a wider context of strategy, institution, and technology. In the case of adopting innovation in materials in a circular economy framework, this paper contributes to promoting the development of sustainable infrastructure systems and fulfils goals related to Sustainable Development Goals (SDGs) 9, 11, 12, and 13. This paper sets a logical platform for the prioritisation of future research efforts and applications by engineers and policymakers that seek to turn textile waste into an infrastructure asset. The results are intended to lead policy frameworks for more resilient and environmentally friendly transportation networks, impact material selection procedures, inform future research, and underscore the strategic role of pavement engineering in advancing infrastructure sustainability while addressing global textile waste challenges.

2. Theoretical Background

2.1. SCTWDM Generation and Application in Pavement Engineering

There has been a significant rise in the production of SCTWDMs owing to the growing demand for textiles, personal protective equipment, and industrial items globally [16,17,18]. Although various measures have been undertaken to recycle waste materials, the recycling process is limited despite the efforts put forth by policymakers [18,19]. This can be witnessed in the case of 26 billion kilograms of textile waste generated worldwide in 2022 [20,21], with less than 20% recycled, a trend also observed in the European Union and the United States, where recycling rates are often less than 25% [22,23]. Another problem posed by the waste of textile materials includes carbon-intensive manufacturing practices and persistence in nature [20,24]. Synthetic textiles such as polypropylene, polyester, and polyamide are oil-based, have high energy requirements (160 kWh/kg for producing polyamide), and take long periods to degrade [25,26]. Moreover, the waste of textiles, such as discarded face masks, denim, and textile fibres from tyres, creates problems during the recycling process because of their heterogeneous nature [5,27]. However, from the standpoint of materials engineering, such waste streams possess inherent qualities that make them well suited for use in road construction, including their excellent tensile strength, durability, and stability against physical and chemical degradation [5,10,27,28]. Fibres made from textile waste sourced from tyres, synthetic plastics, and mixed fabrics can be introduced into asphalt mixtures to increase their performance and durability [29,30]. Therefore, the use of SCTWDMs in asphalt pavements is a viable avenue for functional valorisation, converting waste into engineering material [31,32,33]. This process follows the concept of the circular economy by minimising the dependence on natural resources and increasing their efficient use [34,35]. In this context, the incorporation of SCTWDMs into pavements offers a platform for introducing various waste streams into large-scale construction activities.

2.2. SCTWDM Properties and Their Importance for Pavement Material Performance

Pavement engineering increasingly uses nonbiodegradable SCTWDMs, particularly from waste face masks, denim, and tyre textiles, because of their advantageous mechanical and environmental properties [36,37,38]. The thermoplastic polymers that make up these fibres, which include nylon, polyester, polypropylene, and polyethylene, provide high tensile strength, low water absorption, and resistance to thermal, chemical, and microbiological deterioration, making them ideal for use as reinforcing agents in flexible pavement systems [39,40].

Table 1. Relative characteristics of SCTWDMs and their applicability in pavement [44,45,46,47,48].

Property	Waste Denim Fibre	Waste Face Mask Fibre	Waste Tyre Textile Fibre
Textile Fibre Classification	Composite	Synthetic/composite	Synthetic/composite
Main Composition	Cotton and polyester blend	Polypropylene and polyethylene	Nylon, polyester, and aramid
Diameter (μm)	-	1–20	3.6–5.8
Length (mm)	5–20	-	4.8 to 12.1
Young’s Modulus (GPa)	5.5–12.6	-	3–7
Tensile Strength (MPa)	287 to 597	100 to 300	400 to 800
Specific Gravity	1.5 to 1.6	Approx. 0.91	1.14 to 1.38
Elongation (%)	7 to 8	-	-
Thermal Stability/Melting Point	Depends on the coating	160 to 170C	400 to 500C
Moisture Absorption	7–9%	Very low	Low
Degradation Period	10 to 30 years	>450 years	>50 years
Reinforcement Function	Asphalt binder retention and crack bridging	Asphalt binder retention, crack control	Rutting reduction, fatigue resistance, and load transfer

presents the characteristics of SCTWDMs and their applicability in pavement. The hydrophobic surfaces and microscale diameters of waste face mask fibres, which are derived from melt-blown and spun-bonded polypropylene layers, improve the stiffness, postcracking ductility, and crack bridging of asphalt mixtures [41,42]. Because of their rough texture and capacity to hold bitumen, denim waste fibres and cotton–polyester blends improve tensile resistance and bonding within the asphalt matrix [43,44]. By keeping persistent SCTWDMs out of landfills and lowering the demand for virgin synthetic additives, their use not only increases the service life of asphalt pavements but also has a substantial environmental impact. As a result, these fibres offer a novel combination of pavement performance optimisation and waste valorisation, supporting circular economy principles and sustainable development aims.

3. Methodology

3.1. Framework for Integrated Research

The integration of nonbiodegradable SCTWDMs, such as waste denim, tyre textile fibres, and disposable face masks, into asphalt pavement engineering is examined in this paper via an extensive and multidisciplinary methodological framework. To guarantee analytical rigour, transparency, and reproducibility, the methodology combines scientometric analysis with a systematic literature review (SLR) [49,50]. The SLR offers a structured procedure for finding, evaluating, and synthesising peer-reviewed papers pertinent to SCTWDM valorisation in asphalt pavement applications by adhering to and in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria [50,51] as can be seen in Supplementary File S1.

Scientometric analysis is used to objectively map the worldwide research environment to supplement qualitative synthesis. To identify significant authors, top universities, and changing research paths, this entails analysing publication trends, coauthorship patterns, keyword co-occurrences, and theme clusters. This data-driven method sheds light on the growing hotspots and intellectual framework in the field of sustainable pavement solutions that use SCTWDMs. On the basis of the scientometric findings, the evaluation also includes SWOT analysis to assess the advantages for the environment, technological possibilities, and consequences for the circular economy of using SCTWDMs in asphalt binder and mixtures. This comprehensive method identifies knowledge gaps, technological barriers, and strategic directions for further study, in addition to outlining the current state of the field. In the end, the framework is in line with more general objectives of sustainability, climate action, and resource circularity, which influence the creation of green infrastructure policies as well as scientific advances.

3.2. Study Motivation, Research Objectives, and Questions

The worldwide movement towards sustainability, the integration of the circular economy, and resource-efficient infrastructure have had considerable impacts on how engineers and researchers choose materials for pavement construction in recent years [52,53]. Nonbiodegradable SCTWDMs, such as worn face masks, discarded denim, and waste tyre textile fibres, stand out among the various solid waste streams receiving attention because of their vast bulk, low rates of degradation, and underutilised engineering potential [53]. These SCTWDMs, which have historically been viewed as environmental hazards, have important qualities, such as high tensile strength, thermal stability, and structural resilience [5,6,21], which make them excellent choices for use as reinforcing agents in asphalt mixtures. Our knowledge of how these various SCTWDMs are used in asphalt pavement applications is still fragmented, despite encouraging individual studies. Currently, there is no comprehensive analysis that combines the technical performance results of these materials in pavement contexts with worldwide research trends. Furthermore, it is imperative to evaluate how the upcycling of such materials can support more general sustainability objectives, such as material circularity, low-carbon infrastructure, and the development of green roads, as countries are under increasing pressure to cut emissions associated with construction and their reliance on landfills.

A systematic literature review is used to critically synthesise technical findings and environmental assessments, whereas scientometric analysis is used to map the evolution and intellectual structure of global research efforts [50,54] to address the fragmented understanding of textile waste applications in pavement engineering. In addition to showing the institutional and geographic distribution of scholarly work, this comprehensive approach assesses the contribution of SCTWDMs to pavement performance, highlights their drawbacks, and identifies areas for further improvement. The review is based on four main goals: to evaluate the wider implications for sustainability, integrate the circular economy, and pertinent policy frameworks; to identify the key materials, performance outcomes, and technical advancements reported; to identify critical knowledge and application gaps; and to examine the evolution of global research on the use of SCTWDMs in asphalt pavements.

Table 2. Study objectives and questions.

	Objectives	Questions
1	To conduct a scientometric study to map the academic landscape, research evolution, and global collaboration patterns in the field of using SCTWDMs in pavement construction.	RQ1: What are the most significant publishing trends, keywords, leading nations, and growing research clusters in the field of SCTWDMs for asphalt pavement engineering?
2	To conduct a systematic assessment of incorporation methods and mechanical performance outcomes of various SCTWDMs, such as waste face masks, denim, and tyre textile fibres used in asphalt pavement.	RQ2: What effects have different kinds of SCTWDMs obtained from textile waste had on structural durability, crack resistance, mixture performance, and binder modification in asphalt pavements?
3	To assess the environmental, economic, and circular economy consequences of using these SCTWDMs in asphalt binder and mixture infrastructure, as well as their strategic potential for sustainable asphalt pavement construction.	RQ3: What are the environmental benefits, technical constraints, and strategic opportunities connected with upcycling SCTWDMs in pavement construction, and how can these findings be used to improve sustainability in the industry?

presents a set of targeted research objectives and questions that were prepared for this study to guarantee analytical relevance and coherence.

3.3. Literature Data Retrieval and Bibliometric Data Sources

A thorough literature search was conducted to find peer-reviewed research pertinent to the valorisation of nonbiodegradable SCTWDMs in asphalt pavement construction to provide a solid basis for this study. Waste face masks, shredded denim, tyre textile fibres, and other composite textiles were given priority in this process because they are emerging as viable sustainable substitutes for enhancing binder and mixture qualities, following the objectives of the circular economy. Two relevant academic databases, Scopus and Web of Science, were used as primary data sources on the basis of their extensive coverage, intellectual reliability, and proven application in bibliometric and systematic research [50,55]. The inclusion and depth of the dataset were further improved by conducting additional manual searches on Google Scholar and ResearchGate. This allowed for the discovery of early-access papers and perhaps pertinent grey literature that were not yet indexed in the core databases [55]. To guarantee the reliability and consistency of the information collected, the use of the core dataset, which would later be analysed via scientometric methodology, was strictly limited to the Scopus and Web of Science databases. The possibility of bias in selection and indexing within those additional sources was acknowledged and considered while conducting the research process.

The literature search stage entailed the implementation of a well-structured and rigorous evaluation of current research outputs on the utilisation of SCTWDMs for use in flexible pavement technologies. A data extraction procedure was carefully crafted to align with the overall goals of this scientometric and systematic study. This study employed a reliable and replicable process of data gathering via major scientific databases such as Scopus and Web of Science, involving a judicious blend of controlled terms and free text keywords. The construction of the search string was methodically performed to reflect the multidimensional nature of the topic, including terms such as “synthetic textile waste,” “composite textile waste,” “textile fibres,” “waste tire textile fibres (WTTF),” “recycled polyester,” “polypropylene fibres,” “fibre-reinforced asphalt,” “asphalt mixtures,” “bituminous mixtures,” “flexible pavement,” “bitumen modification,” “mechanical performance,” “durability,” “rutting resistance,” “fatigue performance,” “sustainability,” and “environmental impact.” The time range considered for the literature review included articles published between 2010 and 2025; in this way, the latest breakthroughs and innovations in the valorisation of textile waste in the field of asphalt pavement engineering were considered.

3.3.1. Bibliographical Data Sources

An extensive analysis of reliable bibliographic databases was conducted to justify the scientometric and systematic aspects of this research into the use of SCTWDMs in flexible pavement engineering. The purpose of this stage was to define publication trends, intellectual structure, collaboration networks, and emerging knowledge gaps. To increase the scientific transparency and reproducibility of this research, the review protocol was developed according to the PRISMA framework, which is used to conduct high-level systematic evidence synthesis and analysis. This procedure ensured a clear and transparent process of selection, assessment, and inclusion in line with best practices of conducting high-quality reviews. Considering the fast-growing trend in sustainable pavement material research, it is crucial to choose databases that have the following characteristics: high-level coverage, indexing, and reliability. Therefore, the Web of Science and Scopus databases were chosen because of their reputation and suitability for such types of scientific analysis. To increase the level of comprehensiveness and avoid database bias, a supplementary search was performed via Google Scholar and ResearchGate. The use of these additional databases allowed for a number of additional publications on the topic to be found, including articles in early access, conference papers, and grey literature. As a result, all these sources helped to access a broad range of high-quality research materials concerning the application, technical characteristics, reinforcing properties, and ecological effects of the subject materials used in asphalt pavements.

3.3.2. Data Search Phrases

To accomplish the aims of this study, systematic keyword constructions were developed specifically for each chosen database. The keywords were formulated on the basis of the objectives of the present study, which were aimed at finding articles related to the utilisation and evaluation of the performance of SCTWDMs in flexible pavements. Specific terms such as “synthetic textile waste,” “composite textile waste,” “textile fibres,” “waste tire textile fibres,” “recycled polyester,” “polypropylene fibres,” “fibre-reinforced asphalt,” “asphalt mixtures,” “bituminous mixtures,” “flexible pavement,” “bitumen modification,” “binder,” “performance,” “durability,” “application,” and “environmental impact” were used in the search strings. A detailed description of the structure of the keyword string, along with its corresponding database results, is shown in

Table 3. Outline of article exclusion process.

Databases	Search String Formulation	Articles
Web of Science	TS = (“synthetic textile waste” OR “composite textile waste” OR “textile fibres” OR “waste tire textile fibres” OR “recycled polyester” OR “polypropylene fibres” OR “nylon fibres”) AND TS = (“pavement” OR “asphalt” OR “road construction” OR “hot mix asphalt” OR “cold mix asphalt” OR “bituminous mixture” OR “asphalt pavement” OR “flexible pavement” OR “pavement materials”)	87
Scopus	(TITLE-ABS-KEY (“synthetic textile waste” OR “composite textile waste” OR “textile fibres” OR “waste tire textile fibres” OR “recycled polyester” OR “polypropylene fibres” OR “nylon fibres”) AND (“pavement” OR “asphalt” OR “road construction” OR “hot mix asphalt” OR “cold mix asphalt” OR “bituminous mixture” OR “asphalt pavement” OR “flexible pavement” OR “pavement materials”))	96

. As a result of this procedure, a total of 183 records, including peer-reviewed articles, reviews, conference papers, and other scholarly materials, were identified.

The Boolean searches made possible by using logical operators (AND/OR) aided in obtaining a clear selection of papers on the utilisation, properties, environmental impacts, and recyclability of materials produced from SCTWDMs for flexible pavement engineering applications. Both the Web of Science and Scopus databases were used to ensure a thorough and representative pool of papers published between 2010 and 2025. As a supplementary measure to increase the comprehensiveness of the review, backwards snowballing was used, i.e., the systematic review of reference lists obtained from the initial set of selected papers to identify more relevant papers.

3.3.3. Filtering

The first process of filtration entailed the elimination of duplicates generated from various database sources. Afterward, a series of filters was used to refine the quality and relevance of the data collected. The search was narrowed down to publications within relevant disciplines, which included the following: engineering, environmental sciences, materials sciences, and sustainability. Irrelevant topics were omitted. To capture current trends within the topic, studies dated from 2010 to 2025 were selected. In addition, the document type was confined to scholarly journals and conference papers only; therefore, any other non-scholarly material was ignored. Non-English publications were excluded because of language barriers.

3.3.4. Eligibility

After the screening phase, a more refined eligibility process was conducted to ensure the consistency of the study results with the purpose of conducting a systematic review on SCTWDMs utilised in flexible pavement systems. The selected studies were carefully analysed on the basis of their relevance, validity, and importance for advancing the research topic. Primarily, preference was given to those studies that included original data obtained through experimental methods or analysis techniques and focused on a performance assessment of SCTWDMs incorporated in asphalt mixtures. Mechanical performance, durability features, sustainability aspects, and material incorporation techniques were considered most important when evaluating the selected studies. This selection method ensured the inclusion of high-quality and valuable research articles in the systematic review.

3.3.5. Inclusion and Exclusion Criteria

The database was analysed in a systematic manner by means of an inclusive/exclusive criterion-based methodology, which was instrumental in increasing the level of analysis quality and thematic validity. Following the database search procedure, a quantitative evaluation technique was used to statistically justify the choice of articles to be examined, which was referred to as the significance ratio (S). The significance ratio formula is based on four variables: the number of articles found for each year (e), the number of keywords chosen (b), the number of keyword combinations used (k), and the number of articles retrieved for each year (y).

$${\mathrm{S}}^{\mathrm{e}}={\displaystyle \frac{{\sum }_{\mathrm{i}}^{\mathrm{b}}\frac{{\mathrm{k}}_{\mathrm{i}}}{\mathrm{b}}}{{\mathrm{y}}^{\mathrm{a}}}}$$

To improve the selection procedure even further, a secondary decision criterion was utilised to establish inclusion eligibility in line with specific relevance criteria:

$$\mathrm{f}\left({\mathrm{k}}_{\mathrm{i}}\right)=\left\{\begin{array}{cc}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{d}\mathrm{e}\mathrm{d}& {\mathrm{S}}^{\mathrm{e}}<{\displaystyle \frac{{\mathrm{k}}_{\mathrm{i}}}{{\mathrm{y}}^{\mathrm{a}}}}\\ \mathrm{e}\mathrm{x}\mathrm{c}\mathrm{l}\mathrm{u}\mathrm{d}\mathrm{e}\mathrm{d}& \mathrm{o}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{w}\mathrm{i}\mathrm{s}\mathrm{e}\end{array}\right.$$

The combination of both criteria guaranteed the selection of those papers that clearly reflected thematic relevance and contributed to the utilisation of SCTWDMs in flexible pavements.

Table 4. Summary of the article screening and exclusion process.

Elimination Criteria	Database				Articles Excluded	Articles Included
	Scopus	Web of Science	Google Scholar	Research Gate
Collected data	96	87	11	7	0	201
Duplicated document	31	29	2	1	63	138
Non-English languages	4	2	0	0	6	132
Only abstract	3	4	1	1	9	123
Retracted articles	1	1	0	0	2	121
No access to full-text	4	5	1	0	10	111
Corrigendum	5	3	0	0	8	103
Dissertation	3	1	1	0	5	98
Editorial paper	4	3	0	0	7	91
Title and abstract analysis	9	5	0	0	14	77
Detailed analysis	7	5	0	0	12	65

shows that the summary of the article screening and exclusion process technique was divided into distinct steps. Initially, 201 scientific papers were identified. Among them, 136 were excluded on the basis of duplication, and other elimination criteria employed in the study are shown in Table 4. After the elimination process, only 65 papers were relevant to the present review. These studies were then categorised according to their thematic relevance to the integration of SCTWDMs in flexible pavement. With respect to data management, bibliography files and full texts of each paper were managed through Microsoft Excel, while OpenRefine, version 3.8.7, was used for cleaning to eliminate residual noise, fix inconsistencies, standardise entries, and perform a deduplication process. Figure 1 presents a schematic representation of the PRISMA methodological framework adopted in this study. The following three components were used for the data analysis: (i) bibliometric analysis: to identify new themes and areas of cooperative research, this quantitative evaluation focused on historical publishing trends, the geographic distribution of contributions, authorship networks, and keyword co-occurrence patterns; (ii) systematic review: to analyse the experimental designs, material characterisation, mechanical performance outcomes, environmental consequences, and noted limitations across the chosen research, a qualitative synthesis was conducted; and (iii) SWOT analysis: this analysis was conducted through evidence-based synthesis, wherein each element (strengths, weaknesses, opportunities, and threats) was generated on the basis of recurring themes observed in the PRISMA review process and backed up by the results of the bibliometric analysis. Strengths and weaknesses are considered intrinsic qualities of the topic, whereas opportunities and threats are considered extrinsic properties such as policy, standardisation, scalability, and industry adoption. Each element of the SWOT analysis was included only if it was mentioned in more than one study or if there was bibliometric clustering.

For this study, a single comprehensive analysis was used to present studies involving a variety of SCTWDMs, including face mask fibres, denim waste, and tyre textile waste, which are used in pavement construction. All pertinent SCTWDMs and categories were included in a single search string that was used throughout the bibliometric and systematic review process, which served as the basis for this choice. The integration of these wastes was justified by their common application domain in asphalt pavement engineering and thematic alignment with sustainable infrastructure development, despite their different origins and polymer compositions. Additionally, the conceptual coherence of the evaluated works was strengthened by bibliometric clustering and co-occurrence analyses, confirming the application of a single analytical framework to evaluate their contributions to sustainable pavement technologies.

4. Results and Discussion

4.1. Bibliometric Analysis

A well-known quantitative research method for evaluating the composition, development, and research output of a particular scientific field is bibliometric analysis, which makes use of bibliographic metadata [49,56]. This approach, which frequently uses bibliometric maps as visual aids, provides information about the connections among publications, authors, institutions, and nations [49]. Identifying research trends, keywords, significant contributions, and new directions within a topic is crucial [50]. The bibliometric mapping program VOSviewer version 1.6.20, which is widely used and freely accessible, was employed in this study because of its ability to create and visualise coauthorship networks, keyword co-occurrence, and citation-based grouping [56,57]. The software’s capacity to generate easily comprehensible maps makes it especially appropriate for investigating the conceptual terrain and thematic development of pavement engineering’s upcycling of fibrous materials obtained from textile waste.

4.1.1. Distribution of Publications by Year

Between 2010 and 2025, there was a slow but noticeable increase in the number of publications concerning the use of nonbiodegradable SCTWDMs in the construction of asphalt pavement. The increasing global focus on circular economy principles and the urgent environmental issues related to SCTWDM accumulation are reflected in this evolution. The annual publication trends of the articles by year from 2010 to 2025 are shown in Figure 2. In the year 2011, the exploratory stage of research in this specialised field began to receive scholarly attention. The growing scholarly and commercial interest in recycled materials, alternative materials for use in asphalt pavement, and sustainable pavement technologies drove a steady growth in publications throughout the first ten years (2010–2020). Despite a brief drop in 2020, which was probably caused by the disruption of the COVID-19 pandemic across a number of scientific fields, research production drastically increased starting in 2021. This return coincides with increased regulatory pressure to reuse waste materials, greater support for green infrastructure, and more general trends toward ecologically friendly building methods. A noticeable trend is the increase in publications in 2024 and early 2025, which points to a continuous increase in momentum and shows that research in this area is shifting from a specialised investigative focus to a strategic pillar in the development of sustainable asphalt pavement structures.

4.1.2. Publication Type

A rapidly expanding study environment is reflected in the evolution of scholarly work concerning the upcycling of SCTWDMs in pavement construction. Figure 3 illustrates the significant rise in publication volume and type from 2010 to 2025, highlighting a heightened worldwide consciousness of the contribution of textile waste to the advancement of sustainable infrastructure and circular economy principles in pavement engineering. The publication range is dominated by peer-reviewed journal papers, which make up 38 out of 65 documents, accounting for 58% of the total publications. The laboratory performance, mechanical and thermal characterisation, durability evaluations, and environmental effects of incorporating waste fibres such as denim, face masks, tyre textile fibres, and composite reinforcements into asphalt binders and mixtures are the main topics of these contributions. The field’s technical maturity and growing focus on long-term material performance and scientific validation are reflected in the preponderance of journal articles. Conference proceedings, which make up the second-largest category, with 14 documents accounting for 22% of the total publications, are early venues for sharing new developments, fibre modification methods, and pilot-scale applications that have not yet been published in full-length journal publications. These outputs frequently concentrate on material feasibility studies, comparative binder evaluations, and waste valorisation techniques specific to a certain region, providing insights into evolving research priorities and future goals. Critical summaries of recent developments are given by conference reviews of seven documents, accounting for 11%, and review articles of four documents, accounting for 6%, the latter of which offer organised summaries of performance trends, technical obstacles, and policy–research integration pathways for the reuse of SCTWDMs in asphalt pavement infrastructure. Despite their small number, review articles play crucial roles in identifying research gaps, promoting interdisciplinary cooperation, and coordinating research efforts with global sustainability objectives such as SDG 13 (Climate Action), SDG 12 (Responsible Consumption and Production), and SDG 9 (Industry, Innovation, and Infrastructure). Finally, the book chapter comprises a small portion with two documents, 3% of which frequently offer context-specific narratives on national-level textile waste management policies, standardisation concerns, or regulatory developments, especially in the Global South, where institutional frameworks for reuse practices are still developing. In summary, the prevalence of multiple publications indicates a developing ecosystem that supports scientific inquiry, policy involvement, and industrial collaboration, even as the preponderance of peer-reviewed journal articles emphasises the sector’s depth of research and methodological rigour. This varied output shows how upcycling textile waste is becoming more widely acknowledged as a feasible route to robust, low-carbon pavement infrastructure.

4.1.3. National and Geographical Distributions

To comprehend regional objectives and cooperative potential in sustainable pavement innovation, it is crucial to map the global distribution of research on the upcycling of SCTWDMs in asphalt pavement applications. To determine which nations and organisations were actively involved in this developing field, a qualitative content analysis of the publications that were retrieved was conducted. Contextual clues found in papers, such as institutional affiliations and regional case studies, were employed for classification. Figure 4 shows the various country publications on the upcycling of SCTWDMs in asphalt pavement. The findings reveal that research interest in this area is gaining momentum globally, with substantial contributions from countries such as China, the United States, Malaysia, Portugal, India, the United Kingdom, Pakistan, and Australia. These nations are at the forefront of integrating SCTWDMs into pavement technologies, reflecting their commitment to circular economy strategies and carbon footprint reduction. European countries and several Southeast Asian nations also show increasing scholarly engagement, although with comparatively fewer outputs. This geographic mapping plays a key role in locating possible centres for interdisciplinary cooperation and information sharing. However, it also highlights regional differences, especially in developing and low-income nations, where financing and infrastructure for research are still scarce. Therefore, to secure equitable worldwide advancement in circular construction practices and to encourage inclusive growth in sustainable asphalt technologies, strengthening international cooperation and capacity-building activities is essential.

4.1.4. Contribution to Continental Research

A more comprehensive grasp of regional priorities, technological capabilities, and strategic orientations toward sustainable pavement building can be gained by assessing research output at the continental level [58]. The geographical distribution of publications concerning the use of SCTWDMs that are not biodegradable in asphalt and pavement engineering is depicted in Figure 5. There is a pronounced geographical difference that is influenced by various environmental regulations, financial needs, and research facilities. With the highest percentage of publications, Asia is at the forefront of global research on the application of SCTWDMs in asphalt pavement. The main forces behind this are China, Malaysia, and India, where extensive laboratory-scale research, circular economy projects, and creative methods for fibre-modified asphalt mixtures are accelerating. Notable contributions also come from other Asian nations, such as Pakistan, Indonesia, Sri Lanka, Iraq, and Iran. The United States has led North American research, with a focus on sustainability assessments, life cycle assessments, and performance-based design. This indicates a significant preference for using SCTWDMs in green infrastructure frameworks. To integrate SCTWDM technology into pavement construction methods, Europe is a key player in standardisation, policy development, and multistakeholder partnerships. Germany, Spain, and the United Kingdom have all made significant contributions. New research initiatives in South America and Africa, particularly from South Africa, Egypt, and Nigeria, demonstrate a rising understanding of the economic and environmental advantages of valuing SCTWDMs. These contributions, however, continue to be restricted in terms of both quantity and cooperative scope. To scale sustainable material breakthroughs in developing countries, it is imperative to invest in laboratory capacities, strengthen regional research networks, and promote cross-continental relationships.

Overall, the global distribution pattern highlights highly industrialised countries’ leadership while also highlighting the Global South’s unrealised potential. To fully utilise textile waste upcycling in pavement engineering, it is imperative to improve capacity building, promote collaboration between South China and North China, and match research goals with local infrastructure requirements.

4.1.5. Keyword Co-Occurrence Network

Keywords are essential for synthesising a study domain’s intellectual structure since they provide information about rising trends and thematic focal areas. Co-occurrence analysis reveals how scholars think about integrating SCTWDMs in pavement engineering using the most commonly used keywords. The field’s dynamic evolution of knowledge is likewise captured by this method. By applying a minimum criterion of three occurrences to the bibliometric mapping via VOSviewer, 35 out of 583 unique keywords were found. The prominent concepts influencing the research landscape are represented by the top 35 most commonly used terms, which are displayed in

Table 5. Most frequently searched terms associated with the study field.

S/N	Keywords	Occurrence	Total Strength
1	Waste fibres	50	105
2	Fibres	49	103
3	Asphalt	47	100
4	Mixtures	46	98
5	Stone mastic asphalts	44	95
6	Binders	41	92
7	Asphalt concrete	38	89
8	Sustainable development	35	87
9	Recycling	34	84
10	Asphalt	33	82
11	Textile industry	31	79
12	Asphalt mixtures	28	73
13	Textile fibres	27	69
14	Reinforcement	24	62
15	Pavements	23	60
16	Rutting resistance	22	58
17	Additives	20	54
18	Asphalt pavements	19	51
19	Rheological properties	17	48
20	Asphalt binders	16	46
21	Concrete aggregate	15	43
22	Partial replacement	13	39
23	Engineering properties	13	36
24	Bituminous mixture	12	32
25	Response surface methodology	11	29
26	Mastic asphalt	10	26
27	Temperature	9	25
28	Rheology	8	21
30	Concrete mixtures	7	19
31	Reinforced plastic	6	16
32	Property	5	12
33	Fibre	4	11
34	Performance	3	9
35	Mechanical performance	3	7

. The term “occurrence” is defined as the number of times a keyword occurs in the analysed dataset, which represents its occurrence in the literature, and the total link strength (TLS) is defined as the sum of all weighted links of a keyword in the bibliometric network, thus representing its influence on the theme structure.

To determine how these keywords relate to one another, a keyword co-occurrence network was created, as shown in Figure 6. On the basis of link strength, the network’s eight separate clusters are each represented by a distinctive colour, signifying thematic cohesiveness. The thickness of the connecting lines indicates the overall link strength between keywords or how frequently they co-occur in the same publication, whereas the bubble size indicates the frequency of each keyword’s appearance. The most frequent and closely related terms are “waste fibres,” “fibres,” “asphalt,” “mixtures,” “binders,” “sustainable development,” and “recycling.” Additionally, the network reveals understudied connections, such as weak connections between fibre-reinforced composites, and long-term performance metrics, such as rutting resistance and fatigue. Additionally, the words “fibre”, “asphalt binder”, and “asphalt mixtures” are used sparingly, suggesting that the majority of studies are still laboratory-based and that modifiers and additives are commonly used in asphalt binder and mixtures.

4.1.6. Network Visualisation of Publication Sources

VOSviewer was used to conduct a bibliographic coupling analysis at the source level to determine the main diffusion routes of research pertaining to the upcycling of SCTWDMs in pavement construction. Using a minimum of two articles per source, eight core journals were identified as key publication outlets between 2010 and 2025. The top seven publication sources are shown in

Table 6. Journals with a minimum of four publications in the relevant field of study.

S/N	Journal	Documents	Citations	Total Link Strength
1	Construction and Building Materials	7	61	36
2	Journal of Cleaner Production	6	57	23
3	Case Studies in Construction Materials	5	51	19
4	Materials	5	47	13
5	Archives in Civil Engineering	4	34	10
6	International Journal of Civil Engineering	4	21	7
7	Buildings	3	16	4

, along with the citation counts and total link strength for each. Construction and Building Materials (seven publications, 61 citations), Journal of Cleaner Production (six publications, 57 citations), Case Studies in Construction Materials (five publications, 51 citations), Materials (five publications, 47 citations), Archives in Civil Engineering (four publications, 34 citations), International Journal of Civil Engineering (four publications, 21 citations), and Buildings (three publications, 16 citations) are among them. The value of SCTWDMs in pavement infrastructure may be broadly disseminated through these journals, which are well known for encouraging research in sustainability, materials innovation, circular economy methods, and green construction technologies.

The degree of linkage between these sources is depicted in the bibliographic network visualisation in Figure 7, where thicker links imply more bibliographic coupling and larger nodes indicate higher publishing output. In addition, clustering analysis revealed four unique theme groups that were color-coded in green, red, blue, and yellow to indicate convergence in the research emphasis. This mapping underscores the multidisciplinary nature of the field and demonstrates how a network of leading journals is coalescing around the shared vision of transforming SCTWDMs into viable construction materials, thereby advancing sustainability and circular resource management in asphalt pavement engineering.

4.1.7. Distribution of Publications by Country

Mapping the geographic distribution of research output provides important information about leadership and global involvement in the field of upcycling SCTWDMs for pavement construction. An analysis of this type not only reveals the leading contributors but also highlights areas for technology transfer and international cooperation. Fourteen countries contributed to the study domain on the basis of the bibliometric data that were retrieved. The top three providers were China, the United States, Malaysia, Portugal, India, the United Kingdom, Pakistan, and Australia. An increasing but nationally focused research interest was highlighted by the remaining nations, each of which produced one to four papers. The top five publishing nations are shown in

Table 7. Key nations with a minimum of two publications.

S/N	Country	Documents	Citations	Total Link Strength
1	China	7	57	189
2	United States	6	51	176
3	Malaysia	6	48	163
4	Portugal	5	43	158
5	India	5	39	146
6	United Kingdom	4	34	135
7	Pakistan	3	29	123
8	Australia	3	24	116
9	Iran	2	21	108
10	Iraq	2	19	98
11	Spain	2	13	84
12	Sri Lanka	2	11	73
13	United Arab Emirates	2	9	67
14	Indonesia	2	8	54

, along with the citation counts and TLSs for each. The total weight of international collaboration, mostly through coauthored articles, is represented by the TLS. A nation’s active participation in international research partnerships is indicated by a higher TLS. Understanding the degree of cross-border scientific interaction and the global connection of research depends on this measure. Every node (bubble) in the network map represents a nation, and research collaboration is indicated by the thickness of the connecting lines. Figure 8 shows the network visualisation of bibliographic coupling among key countries. On the basis of the results, strong bilateral ties were noted from the collaboration network, especially between China, the United States, Australia, Malaysia, and the United Kingdom, which reflects these countries’ crucial role in promoting cross-border and multidisciplinary research on sustainable pavement techniques.

The results underline the necessity of growing international collaborations, especially with underdeveloped countries, where cooperative, sustainable material advances could successfully address the twin problems of waste management and inadequate road infrastructure. Additionally, Indonesia, Sri Lanka, and Portugal show minimal international coauthorship relationships, and on the other hand, the United Arab Emirates seem to be somewhat isolated in the cooperation network, although interest has increased through recent publications. The statistical and visual assessments of the participating countries will be useful to researchers as they build scientific alliances, collaborate, and exchange innovative approaches and ideas. Research on SCTWDMs in asphalt pavement might be advanced by scholars from nations that wish to work with experts in the subject and benefit from their knowledge.

5. Application and Upcycling of SCTWDMs in Asphalt Pavement

5.1. Synthesis of Hybrid SCTWDMs for Upcycling Applications in Asphalt Pavement

The increasing global need for resilient pavement infrastructure continues to put enormous strain on finite natural resources while also considerably contributing to greenhouse gas emissions. Owing to their high embodied carbon content and resource depletion, conventional asphalt mixtures mostly rely on energy-intensive ingredients such as virgin aggregates and asphalt binder, which worsen environmental deterioration. A practical and sustainable substitute in this regard is the upcycling of nonbiodegradable SCTWDMs, such as face mask fibres, denim shreds, and tyre textile strands. When utilised as reinforcements or modifiers in asphalt pavements, these fibrous materials, which are made from postconsumer and industrial textile waste, not only keep persistent waste from landfills but also improve performance. This change is in line with the ideals of global sustainability and the circular economy, transforming environmental disasters into opportunities for creative, resource-efficient, low-carbon paving solutions.

A recent study by Al-Sabaeei et al. [59] assessed the possibility of employing discarded denim fibres, a textile byproduct produced during denim cutting operations, as a bitumen modifier. To evaluate the performance of the WDF, routine physical tests and dynamic shear rheometer (DSR) studies were performed at 0.5%, 1%, 1.5%, and 2% by weight of binder. According to these findings, the inclusion of WDF increased the binder softening point while decreasing penetration, ductility, and temperature sensitivity. Improved rheological characteristics and elastic behaviour within the 40–70 °C range were also shown by DSR tests. The binder that was altered with 2% WDF showed the greatest resistance to rutting, indicating that the material has the potential to be a sustainable additive for improving binder performance and lowering the build-up of textile waste. The RSM 2D and 3D plots the influence of WDF and NS on the penetration, softening point, and penetration index responses. In a similar study, Al-Sabaeei et al. [60] evaluated the efficacy of response surface methodology (RSM) and machine learning (ML) techniques in predicting important rheological parameters and examined the possibility of integrating nanosilica (NS) and waste denim fibre (WDF) in asphalt binder modification. The complex modulus (G)**, phase angle (δ), and rutting parameter (G/sinδ) were modelled before and after short-term ageing via NS (0–6%), WDF (0–6%), and testing temperatures (40–76 °C) as input variables. The RSM models showed good predictive power, with R2 values above 0.97 for every response, indicating that temperature, WDF, and NS had a major impact. Gaussian process regression (GPR), which achieved R2 = 0.99 and RMSE ≈ 1%, outperforms other machine learning algorithms among those studied, demonstrating excellent agreement between the anticipated and experimental outcomes. The research demonstrated the precision and promise of machine learning, namely, GPR, in predicting the rheological behaviour of NS/WDF-modified binders.

Furthermore, in a different study, Al-Sabaeei et al. [43] investigated the individual and combined effects of NS and WDF on the rheological behaviour and rutting resistance of asphalt binders. The study showed that NS synergistically improved the performance of WDF-modified binders, especially in terms of resistance to high-temperature deformation, according to RTFOT, temperature sweep, and MSCR tests. According to the MSCR data, the use of NS and WDF significantly improved the elastic recovery and decreased the nonrecoverable creep compliance. Superior rutting resistance was confirmed when the binder’s performance grade increased from PG 64S to PGV** at 4% content each. Owing to the fibre clustering that served as elastic reinforcement, the WDF made a greater contribution to elasticity. These results highlight the great potential of NS-WDF composites as reliable and efficient modifiers to enhance the performance of binders in flexible pavements.

In a different study, Al-Sabaeei et al. [61] examined the influence of WDF and NS on the durability of asphalt enhanced with S-TPO and assessed the RSM for predictive modelling. Compared with the separate modifiers, the composite WDF/NS combination more successfully enhanced the penetration, softening point, and temperature susceptibility. Modifier interactions were found to be significant by the RSM models (R2 > 0.74), with the best results obtained at 5.1% WDF and 2.6% NS. Particle agglomeration was cited as the cause of a slight performance reduction at higher dosages. To create high-performance sustainable binders, this study emphasises the potential of RSM and textile waste composites. Furthermore, Al-Sabaeei et al. [62] used the RSM for multiobjective optimisation, modelling, and experimental design, as they examined bitumen modification utilising waste denim fibre (WDF) and tyre pyrolysis oil (TPO). The influences on penetration, softening point, and temperature susceptibility were assessed after the asphalt had been modified with up to 15% TPO and 6% WDF. According to the results, both modifiers had a considerable effect on binder consistency, and the RSM models had high prediction accuracy (R2 = 0.982, 0.973, and 0.947, respectively). The potential of WDF to enhance the performance of TPO-modified binders was confirmed when the ideal formulation was found to be 8.4% TPO and 6% WDF. This study reaffirms the importance of modelling tools and textile waste in the creation of sustainable bituminous materials. Moreover, Al-Sabaeei et al. [63] assessed the predictive capabilities of response surface methodology (RSM) and various machine learning (ML) models for predicting the MSCR-based rutting parameters of shear strain, accumulated shear strain, nonrecoverable creep compliance (Jnr), and percentage recovery (%R) for base, nanosilica (NS), waste denim fibre (WDF), and NS/WDF composite asphalt binders. All MSCR replies had statistically significant NS and WDF contents according to the RSM analysis (R2 > 0.80). Among the ML models, decision tree regression (DTR) had greater accuracy for Jnr and %R (R2 = 0.99, RMSE < 1%), whereas extreme gradient boosting (XGB) was the best at predicting shear strain and accumulated strain. The prediction accuracy of the ML models was generally greater than that of the RSM, indicating their promise for reliable, data-driven modelling of intricate viscoelastic behaviour in modified asphalt mixtures.

Alonso-Troyano et al. [64] investigated the addition of recycled cotton textile fibres to stone mastic asphalt (SMA 11 surf 35/50). The ideal fibre concentration to reduce binder drainage and preserve structural integrity was found to be 0.3%. According to the laboratory results, there were significant gains in resistance to permanent deformation and moisture resistance (ITSR = 96.30%). Despite the WTSAIR value (0.12 mm/1000 cycles) being marginally below the T2 traffic requirements, it was recommended that the binder be gently adjusted to fulfil the requirements. This study highlights how the use of local textile waste in asphalt applications offers both technical and environmental advantages. To verify long-term durability, more investigations into fatigue performance, ageing resistance, and real-world validation are needed. Additionally, Pais et al. [65] investigated the use of textile fibres recovered from tyre recycling as functional additives in AC14 asphalt mixtures. Two binder grades (50/70 and 35/50 pen) were used to evaluate the mechanical behaviour of the fibre-modified mixtures via tests for fatigue, dynamic modulus, permanent deformation, and indirect tensile strength. The usage of tyre-derived fibres is difficult, according to the findings, but when a minimum binder content of 6.5% is added, the combinations perform noticeably better, especially in terms of fatigue and rutting resistance. According to the study, these fibres have conditional value in pavement applications as long as mix design modifications, specifically binder dosages, are optimised to ensure long-term performance. In a study conducted by Valdés-Vidal et al. [66], the use of waste textile fibre (WTF) as an additive in asphalt mixtures was assessed. WTF, which is frequently burned or landfilled, was recycled and added at weight percentages of 2%, 5%, and 8%. The stiffness modulus, moisture susceptibility, resistance to rutting (Hamburg Wheel Tracking), stripping, and cracking were tested mechanically. The findings showed that all dosages produced a >29% decrease in permanent deformation, whereas 2% and 5% WTF significantly improved critical performance measures. These results highlight the environmental benefits and technological viability of including WTF in asphalt mixtures, which is consistent with the ideas of the circular economy. Figure 9 shows the influence of the WTF content on the (a) moisture susceptibility and (b) rutting resistance of the asphalt mixtures.

A study conducted by Valdés-Vidal et al. [48] investigated the viability of using fibre material derived from WTF in place of commercial cellulose fibres in SMA mixtures as a sustainable substitute. In this study, WTF was added at replacement levels of 0%, 50%, 75%, and 100%. The stiffness modulus, moisture sensitivity, rutting resistance, and cracking resistance of the resulting mixtures were assessed. The results showed that the design and performance of the SMA mixture were preserved when 100% of the cellulose was substituted with WTF, and the mechanical behaviour was similar to that of the reference mixture. Figure 10 shows the effects of incorporating WTF content into SMA mixtures on (a) the stiffness modulus, (b) moisture damage, and (c) permanent deformation.

To alleviate the environmental burden associated with textile fibres from end-of-life tyre disposal, Calabi-Floody et al. [67] investigated the repurposing of textile fibres from end-of-life tyres as additives in asphalt binders (AB14 and AB24). Following binder modification at 0.1%, 0.3%, and 0.5% textile fibres, the fibres were characterised via SEM, polarised microscopy, and thermogravimetric analysis. According to experimental assessments using softening point, penetration, FRAASS, MSCR, LAS, SEM, polarised microscopy, and TGA analyses, 0.5% waste tyre textile fibres considerably decreased permanent deformation by up to 40% while preserving resistance to thermal cracking. With no negative effects on low-temperature performance, the ability of waste tyre textile fibres to improve the rutting and fatigue performance of asphalt binders while providing a low-percentage, sustainable additive solution was validated. Additionally, Bocci and Prosperi [68] examined a strategy to improve mechanical performance and promote waste valorisation by incorporating recycled textile fibres from end-of-life tyres (ELTs) into hot mix asphalt (HMA). In this study, semicircular bending (SCB), three-point bending (3PB), indirect tensile strength (ITS), the indirect tensile stiffness modulus (ITSM), and indirect tensile fatigue (ITF) were used to compare the fibre-modified and control mixes after the fibres were characterised via microscopy and FTIR spectroscopy. A substantial increase in fatigue resistance was noted, even though the inclusion of ELT fibres did not considerably improve the strength or stiffness characteristics. The capacity of fibres to bridge microcracks and slow the spread of cracks is responsible for this improvement, suggesting that they could be used as useful reinforcements in fatigue-critical layers of pavement. Additionally, Rodríguez et al. [69] evaluated the environmental impact of the Peruvian textile industry by evaluating the usage of leftover cotton fibres as an alternative to commercial stabilising fibres in SMA20 combinations. The study contrasted SMA mixtures with equal proportions of recovered textile fibres with a reference mixture that contained 0.6% asphalt and 0.30% commercial fibre. With a TSR of 95% (compared with 82%) and somewhat better rutting resistance, the results showed that 0.20% residual cotton fibre provided superior performance and satisfied the volumetric and binder drainage criteria. Additionally, the fibre improved the dynamic modulus at higher temperatures, indicating that it could be a technically feasible and sustainable substitute for traditional fibres in the production of SMAs. Furthermore, Pais et al. [70] investigated the viability of reinforcing traditional asphalt mixtures using textile fibres reclaimed from ground end-of-life tyres. Indirect tensile testing was used to assess nine distinct mixtures with variable asphalt and a fibre content of 1.5 by mixture weight, utilising both 50/70 and 35/50 pen binders on laboratory-compacted slabs. The results show that asphalt mixtures can be efficiently reinforced with textile fibres generated from tyres, increasing structural responsiveness without sacrificing constructability. This study supports the objective of the circular economy by highlighting the potential of these fibres as sustainable substitutes for conventional reinforcements. Similarly, Oner and Ozdas [71] investigated the viability of using textile waste fibres in place of traditional stabilising fibres in stone mastic asphalt (SMA) mixtures, which are popular owing to their exceptional resistance to high traffic loads. The high binder concentration and gap-graded structure of SMAs make bitumen drain-down a major design difficulty. This study used Schellenberg drainage and Marshall stability tests on mixes made with different proportions of cellulose fibres and textile waste. The results showed that waste fibres from textiles functioned similarly to conventional cellulose fibres and successfully managed binder drain-down. This study highlights the feasibility of using textile waste as a substitute reinforcement in SMAs, encouraging the reuse of sustainable materials without sacrificing performance. Additionally, Chen et al. [72] investigated the possibility of enhancing asphalt binder performance and ageing resistance by modifying recycled kapok fibre (KF) via a liquid-phase ball-milling process with waste engine oil (WEO). The resulting oil-grinded kapok fibre (OKF) had improved physical characteristics, such as a notable increase in pore volume (62.5%) and specific surface area (74.3%), as well as an 11.5% increase in crystallinity. OKF has shown potential for improving low-temperature performance and greatly increasing resistance to ageing when added to asphalt. Although more research is needed to confirm long-term durability and moisture susceptibility, this work shows how well mechanical surface activation transforms hydrophilic textile fibres into high-performance asphalt modifiers.

In a study conducted by Mohammed and Ismael [73], the use of polypropylene (PP) fibres was assessed as a modifier in warm-mix asphalt (WMA) to address moisture susceptibility problems. According to the findings, as the amount of PP fibres increases from 2 to 6% of the binder mass, the moisture resistance is improved through increases in the tensile strength ratio (TSR) and index of retained strength (IRS). An increase of 19.5% in the TSR value was observed, suggesting a reduction in moisture damage. Takaikaew et al. [74] analysed the use of polyolefin and aramid fibre reinforcement in HMA via AC60/70, natural rubber, and polymer-modified binders. The use of a small percentage of fibres (0.05%) in the HMA resulted in considerable improvement in mechanical behaviour, regardless of the binder, as witnessed by a rise in the Marshall stability value by approximately 17%, resilient modulus by 31%, ITS by 11%, and ITSM by 33%. Additionally, improvements in the rutting behaviour and fatigue strength were noted. Although the PMA binder showed better initial performance, reinforcement with fibres improved all the mixtures considerably, suggesting significant prospects for improving durability. Additionally, Ahmed et al. [75] studied the influence of the fibre content on the fatigue behaviour of thin asphalt overlain over granular base materials by means of four-point bending beams. The findings revealed that the optimum fibre content for good fatigue performance is 0.3% cellulose fibres. This fibre results in better fatigue behaviour than the control sample and the samples with other percentages of fibres used. The study findings show that thin beams incorporating 0.3% cellulose and textile fibres exhibited similar fatigue behaviours.

Valdes-Vidal et al. [76] examined the fatigue performance of SMA and HMA mixtures that had been altered using two granular forms of a new addition based on textile waste tyre fibre (TWTF). In SMA, one format completely replaces the cellulose stabiliser, whereas in HMA, one format is utilised. The four-point bending beam (4PB) test was used to assess fatigue resistance. According to the results, the fatigue lives of both the HMA mixtures and the SMA mixtures with TWTF-based additives considerably increased, with durability increases of up to 129% and 112%, respectively, over those of the reference mixes. The longer service life of pavements containing the TWTF-modified mixtures was further validated by mechanistic investigation. Additionally, Calabi-Floody et al. [77] assessed the possibility of reusing polymer fibres from waste to improve a mixture’s mechanical performance. This study evaluated the effects of waste tyre polymer fibres on asphalt mixtures throughout a temperature range of −10 °C to 50 °C using two binder grades (CA-24 and CA-14) and waste tyre polymer fibre levels of 0.1%, 0.3%, and 0.5%. Mechanical tests that measured the indirect tensile strength, stiffness modulus, resistance to thermal cracking, and rutting resistance revealed that 0.3% to 0.5% of waste tyre polymer fibres greatly enhanced performance, especially at high temperatures, with the rutting resistance increasing by more than 30%. The results validate PFELT’s significance as a sustainable solution to tyre waste and its efficacy as a reinforcing addition in HMA. In addition, Saltan et al. [78] investigated the influence of yarn waste fibres and the interaction of diethanolamide (CDEA) with bituminous binders on bituminous binder performance. For conventional binder qualities, 5% CDEA was determined to be ideal; nevertheless, it affects the moisture resistance and indirect tensile strength (ITS). To counteract this, 0.1%, 0.2%, and 0.3% yarn waste fibres were added. Initial experiments using unaltered binders revealed that 0.1% yarn fibre enhanced moisture resistance and ITS. The fibres did not, however, improve moisture sensitivity when added to mixes containing 5% CDEA, and the tensile strength ratio (TSR) values decreased below the specified range. According to the findings of this study, yarn waste fibres can improve the performance of basic binders, but they do not work as well with binders enhanced with CDEA, which calls for more material optimisation.

In a recent study, Nadupuru et al. [79] investigated how to improve the structural integrity and functionality of asphalt concrete by including synthetic fibres, particularly discarded denim, polyester, and polypropylene fibres. To guarantee even fibre dispersion throughout the bitumen matrix, a customised mixing technique was created. The fibre dosages ranged from 5% to 15% by weight of binder, and the optimum fibre and binder contents were calculated via the Marshall technique. Fibre addition increased the mixture density and mechanical strength, according to laboratory tests such as Marshall stability and volumetric studies. While additional research is needed to address long-term durability and moisture resistance, this study highlights the potential of employing waste and synthetic fibres as reinforcement ingredients in asphalt concrete. Extending upon this approach, Jin et al. [80] evaluated the long-term performance of asphalt mixtures that included sustainable modifiers such as tyre fabric fibres and ground tyre rubber (GTR). Viscoelastic experiments verified better mechanical durability than control mixtures did, and a comprehensive field test in Michigan showed that adding tyre fibres greatly increased resistance to rutting and cracking. Notably, noise levels were lowered by 2–3 dB, and overlays patched with fibre and GTR did not exhibit any obvious cracking after two years of use. According to the findings of this study, combining GTR with tyre fibres offers a more durable, eco-friendly, and silent substitute for conventional mixtures. In addition to these long-term and high-temperature performance improvements, Slebi-Acevedo et al. [81] examined the impact of synthetic textile fibres made of polyolefin-aramid (POA) and polyacrylonitrile (PAN) on the performance of asphalt mortar at low temperatures. Fibre-reinforced asphalt mixture (FRAM) samples with 0.1–0.3 weight percent fibres were assessed at 15 °C, 0 °C, and −15 °C, utilising toughness, fracture energy, postcracking energy, and indirect tensile strength as the main criteria. The results demonstrated better ductility and crack resistance, with 0.3% POA or PAN fibres showing a considerable increase in strength and fracture energy, especially at −15 °C. By addressing a research gap in the field of fibre reinforcement, this work highlights the potential of synthetic fibres to increase the low-temperature performance of asphalt at the binder scale. In addition to this understanding at the molecular scale, Shi et al. [82] used molecular dynamics (MD) simulations to study the behaviour of interfacial adhesion between asphalt and polyacrylonitrile (PAN) fibres, which is a crucial component affecting asphalt mixture performance. To evaluate the impacts of the PAN composition, asphalt type, temperature, ageing, and exposure to saltwater, several PAN/asphalt models were created. The findings demonstrated that PAN containing 95% acrylonitrile and 5% methyl acrylate had better adherence to asphalt, especially to its heavier constituents. While high temperatures increased the interfacial energy, the adhesion improved with age and was reduced when the samples were exposed to salt. Contact angle measurements were used to experimentally validate the modelling results, providing molecular-level knowledge that supports the ability of PAN to improve the performance of asphalt mixtures.

Badeli et al. [83] examined the effects of aramid pulp fibre (APF) on GB20 asphalt mixtures under harsh cold climate conditions, such as 300 freeze–thaw cycles. Through thermomechanical tests (dynamic modulus tests, fatigue tests, and TSRSTs), it was found that APF increases the durability, fatigue performance, and low-temperature crack resistance and thermal stress resistance of asphalt mixtures. The fibre-reinforced asphalt mixtures exhibited superior stiffness consistency in response to cyclic environmental loads, which demonstrates high applicability for cold climate regions. In another study, Xing et al. [84] investigated the effects of aramid fibre length and surface treatment with KH-570 silane on the properties of asphalt binders. The fibre length positively impacts the viscosity and reinforcement capabilities of asphalt binders. This is because an increase in the length of the fibres makes them more effective at reinforcing the binder. In addition, surface treatment helps improve the interaction between fibres and asphalt binders. Additionally, Sadeghi and Sabouri [85] investigated the efficiency of the application of different artificial fibres (polypropylene, A-glass, para-aramid, and polyester) for cold recycled mixtures based on emulsified asphalt in terms of modified I-FIT and IDEAL-CT criteria. The fibres increased the dry strength and cracking resistance but increased the moisture sensitivity. The maximum benefits from the use of fibres are provided at smaller fibre lengths and at low concentrations. The combination of fibres and cement maintains the mechanical properties and moisture stability while significantly increasing the resistance to cracks in comparison with mixtures with a greater content of asphalt.

Furthermore, to complement this chemically based research, Liu et al. [86] assessed the viability of replacing cellulose fibres in asphalt mixtures with cotton straw fibres, an agricultural byproduct from the manufacturing of cotton. A custom-developed planetary blade crusher was used to mechanically grind the cotton straw to create reinforcing fibres. According to laboratory studies, adding these fibres improved the resistance of asphalt mixtures to moisture, their flexibility at low temperatures, and their stability at high temperatures. By providing a low-cost, energy-efficient, and environmentally friendly alternative to traditional cellulose fibres in pavement engineering, this study highlights the economic and environmental benefits of using cotton straw waste. Morova et al. [87] explored the use of polyparaphenylene terephthalamide (PTF) fibres to improve the performance of hot mix asphalt (HMA). Marshall stability and indirect tensile strength (IDT) tests were used to assess the mechanical characteristics and moisture sensitivity for a range of fibre dosages (0.25–2.00% by weight of mixture) utilising limestone aggregate. According to the findings, the maximum Marshall stability was obtained with 0.25% PTF. The ideal fibre percentage was found to be effective throughout binder changes, according to additional investigations conducted at different bitumen levels (4.15%, 4.65%, and 5.15%). Furthermore, the enhanced performance at 0.25% fibre content was validated by IDT experiments. PTF is a viable fibre additive in HMA, according to the study’s findings, providing mechanical improvements without changing the ideal bitumen demand. Additionally, El-Desouky et al. [88] investigated the reuse of discarded polyester fibres in asphalt mixtures to improve pavement performance and solve disposal issues in the environment. The study assessed how Marshall stability and rutting resistance were affected by fibre length (1 cm and 2 cm) and dosage (0.2–0.65% by total mixture weight). According to experimental findings, uniformly distributed polyester fibres enhance the mechanical qualities of asphalt concrete overall, especially its ability to support loads and withstand permanent deformation. This study emphasises how recycling polyester waste can improve the structural performance of asphalt pavements while promoting sustainability. To address the issue of fibre reinforcement, Zhao et al. [89] investigated the utilisation of chemically modified fibres derived from recycled cotton stalks in microsurfacing applications via SBR-modified emulsified asphalt. Fibres with low hygroscopicity and high asphalt absorption were shown to have the greatest impact on road performance and construction behaviour. The most encouraging findings were obtained with CH₃COOH-modified cotton fibres, which increased the shear resistance by 41.13%, decreased the rutting depth by 10.07%, and reduced abrasion by 9.13%. They were shown to be more appropriate for seasonal freezing conditions and to have improved long-term durability. This study encourages pavement engineers to repurpose agricultural debris sustainably.

Cellulose acetate fibre (CAF) obtained from waste cigarette filters was analysed by Yan et al. [90] as a modifier in porous asphalt (PA-13) and binders. According to these findings, CAF has beneficial effects on high-temperature rheology, with an effective content of CAF as a binder of 1%, which increases the fatigue life by 4.3% and improves low-temperature behaviour; however, an increase in the content results in poor performance of CAF. In addition, the performance of CAF is better than that of lignin fibre in terms of fatigue life and less temperature dependence. With respect to the PA-13 mixtures, an increase in the content of CAF improved rutting, moisture, and stripping resistance until it reached 5%, whereas fatigue and low temperatures were best achieved with 3% CAF. In another study, He et al. [4] created a combination of fibre asphalt mixtures made of calcium sulphate whiskers (CSWs) and basalt fibres (BFs) with micrometre and millimetre sizes, respectively, through response surface methodology. The mixture composition producing the best overall pavement performance included 5% CSW and 6 mm BF with a BF content of 0.32%. CSWs should be added to the asphalt mixture via wet mixing, whereas BF should be added via dry mixing. This example shows that pavement testing at the mixture level is required since it is more representative of performance than testing at the mastic level.

Table 8. Summary of hybrid SCTWDM waste upcycling applications in asphalt pavement engineering.

S/N	Ref.	Textile Waste	Application	Mixture Composition	Tests	Findings	Comments
1	[64]	Waste cotton	Stone mastic asphalt	0.3% textile fibre in SMA 11 surf 35/50	Air spaces, bulk and maximum density, water sensitivity, and WTSAIR (rutting)	Enhanced moisture resistance, decreased binder drainage, and increased structural stability; for T2 traffic, there is a small WTSAIR deficit (0.12 mm/1000 cycles)	Lacks field validation, fatigue, and ageing testing; additional binder optimisation required for traffic class compatibility
2	[89]	Waste cotton stalk fibres	Asphalt binder	Emulsified asphalt treated with SBR and chemically modified cotton stalk fibres (such as CH3COOH)	Shear resistance, water demand, bitumen–aggregate ratio, rutting depth, and abrasion loss	Compared to unmodified fibres, CH3COOH-modified fibres substantially increased adhesion and durability; they also had better rutting, abrasion, and shear resistance.	More studies are required on the comparison of the performance with traditional cellulose or polymer fibres; long-term field validation under real traffic and freeze–thaw cycles
3	[72]	Waste kapok fibre (KF) and oil-grinded kapok fibre	Asphalt binder ageing resistance	Oil-grinded kapok fibre (OKF) is added to the asphalt binder using waste engine oil (WEO)	Viscosity, nanoindentation, FTIR, BET, XRD, SEM, and accelerated ageing	Crystallinity, surface area, and pore volume were all enhanced by oil-grinding; OKFA demonstrated improved resistance to ageing and possible enhancement of low-temperature performance	Studies on moisture susceptibility, field validation, and long-term performance data under actual traffic situations are needed
4	[77]	Waste tyre polymer fibre	Additive in asphalt mixtures	The fibres were added at 0.1%, 0.3%, and 0.5% to asphalt mixtures with CA-24 and CA-14 binders	Marshall method, ITS, TGA, SEM, microscopy, stiffness modulus, thermal cracking, and rutting	0.3–0.5% fibre increased thermal cracking resistance, rigidity, and rutting resistance, particularly at high temperatures	Further optimisation is required to address the impacts of waste tyre polymer fibre aggregates on mixture performance and scalability should be examined
5	[80]	Tyre fabric fibres and ground tyre rubber	Asphalt mixtures	Tyre fabric fibres and ground tyre rubber (GTR) were added to asphalt mixtures	Viscoelastic testing, field performance (two-year service), rutting and cracking tests, and noise evaluation	Mixtures with ground tyre rubber + fibres outperformed traditional mixtures in terms of rutting and cracking resistance, noise reduction of 2–3 dB, and no cracking after 2 years	More studies are needed on the composite-modified pavements’ long-term performance in a range of climates, and in terms of economic viability and recyclable nature
6	[87]	Polyparaphenylene terephthalamide textile fibre (PTF)	Asphalt mixture	PTF fibre at 0.25%, 0.50%, 0.75%, 1.00%, 1.50%, and 2.00%, at optimum bitumen content	Marshall stability, moisture sensitivity, and IDT tests	Maximum stability and optimal performance at 0.25% PTF; steady bitumen demand; enhanced IDT strength and moisture resistance at low PTF content	Lack of long-term field validation and performance under a variety of traffic and climate situations
7	[88]	Polyester waste fibre	Asphalt mixtures	Polyester fibres at 0.2%, 0.35%, 0.50%, and 0.65% by total weight (lengths: 1 cm, 2 cm)	Marshall stability and rutting resistance	Improved rutting resistance and Marshall stability across the majority of fibre lengths and doses	Future research should examine the effect of the fibre sizes, as well as how these sizes affect other mixture types and mechanical property tests
8	[86]	Cotton straw fibre	Asphalt mixture	Cotton straw fibre was mechanically ground and used as a replacement for cellulose fibre	Moisture resistance, low temperature cracking, and high temperature stability	Enhanced water stability, low temperature stability, and high temperature stability; encourages waste reduction and environmental preservation	Validation of field performance under environmental and traffic loads and long-term ageing behaviour is recommended
9	[82]	Polyacrylonitrile (PAN) fibre	Asphalt mixtures	Asphalt was mixed with PAN containing different molecular configurations using MD modelling	Contact angle test with molecular dynamics (MD) simulation	The optimum adherence was displayed by PAN with 95% AN/5% MA; adhesion was improved by heavier asphalt components; PAN–asphalt bonding was strengthened with age; PAN was more resistant to water damage than salt corrosion	Additional experimental verification in actual mixes; applying MD insights to macroscopic behaviours
10	[85]	Polypropylene, A-glass, para-aramid, and polyester	Emulsified cold recycled asphalt mixes	Different fibre contents and lengths, both with and without cement	Dry/wet ITS, modified I-FIT, and DEAL-CT	Fibres increased moisture sensitivity but not strength and resistance to cracking; they worked best at low content and short length; fibres with cement enhanced overall performance	No life cost analysis study; fibres increased vulnerability to moisture; there is a need for durability to balance optimisation
11	[79]	Waste fibres of polyester, polypropylene, and denim	Asphalt binder and mixtures	Asphalt mixtures with control binder and fibre-modified binder (5–15% fibre by weight of binder)	Volumetric properties, Marshall stability, optimum binder and fibre contents	Dispersion of fibres uniformly enhanced internal structural tightness; the Marshall method was used to determine OBC and OFC; fibres improved performance	Long-term durability, fatigue life, moisture susceptibility, and a proper performance–cost balance must all be evaluated
12	[66]	Textile fibre from tyre recycling	Asphalt mixtures	2%, 5%, and 8% textile fibre additives added to reference mixtures	Moisture susceptibility, stiffness modulus, rutting, stripping, and cracking resistance	Textile fibre additives of 2% and 5% increased resistance to cracking, rutting, moisture, and stiffness; permanent deformation was decreased by more than 29% at all contents	More research is needed to determine long-term ageing, field performance, and economic viability
13	[81]	Polyacrylonitrile (PAN) and polyolefin-aramid (POA) fibres	Asphalt mixtures	0.1–0.3% weight POA or PAN fibre-reinforced asphalt mortar	Fracture energy, postcracking energy, indirect tensile strength, and toughness (at 15 °C, 0 °C, and −15 °C)	Low-temperature strength and fracture energy were markedly increased by 0.3% fibre content; improved energy absorption and crack resistance were also observed	Insufficient investigation into the processes of fibre–matrix interaction at varying temperatures and fibre impacts at the mixture scale
14	[76]	Textile fibre from tyre waste	Reinforcement in HMA and SMA mixtures	The HMA and SMA were enhanced with two textile fibre-based additive granular forms	Four-point bending beam and mechanistic analysis	Compared to reference mixtures, textile fibre-modified mixes demonstrated improved fatigue resistance and longer pavement life, with durability rising by 129% (HMA) and 112% (SMA)	Lack of technoeconomic analysis, leachability testing, and field research
15	[78]	Yarn waste fibre	Asphalt binder	0.1–0.3% yarn waste fibres and 5% Cocamide Diethanolamide (CDEA)	Moisture sensitivity and indirect tensile strength	TSR values remained below specification; 0.1% yarn fibres increased ITS and moisture resistance in untreated binders, but did not improve ITS or moisture resistance in binders modified with CDEA	More investigation is required on the interactions between fibre and additives, as well as the best approach to treat the fibre to make it compatible with chemical modifiers
16	[70]	Textile fibre from tyre recycling	Reinforcement in asphalt mixtures	Nine different types of asphalt with 0 and 1.5% fibre and different binder proportions (50/70, 35/50 pen)	Slab-compacted specimens’ ITS	Tyre-derived recycled textile fibres improved the asphalt mixture; the mechanical feasibility was confirmed by establishing valid configurations for various binder grades	Lacks evaluation of long-term durability, moisture susceptibility, and real traffic performance
17	[68]	Textile fibre from tyre recycling	Asphalt mixtures	By mix weight, the asphalt concrete had a 0.3% fibre content	FTIR, semicircular bending (SCB), three-point bending (3PB), ITS, ITSM, ITF, and microscopy	ELT fibres significantly increased fatigue resistance while having little effect on strength or stiffness; this is probably because they bridged microcracks and prevented the spread of macrocracks	Durability and field validation in various traffic and environmental scenarios have not yet been evaluated
18	[84]	Aramid fibres treated with KH-570	Asphalt binder	Fibre lengths vary; silane-treated versus untreated fibres	SEM, viscosity, FTIR, DSR, contact angle, and BBR	Long fibres performed similarly at lower dosages; silane treatment improved fibre–binder adhesion; longer fibres improved viscosity and reinforcement	Long-term durability was not evaluated; mixture performance and field validation are absent; analysis is restricted to the binder level
19	[74]	Aramid and polyolefin fibres	Asphalt mixtures	0.05% fibre; AC60/70, with natural rubber, and polymer-modified binders	Fatigue, rutting, creep, Marshall stability, ITS, robust modulus, and ITSM	Notable gains include increased rutting and fatigue resistance in all binders, stability, modulus, ITS, and ITSM	Long-term ageing and field performance were not evaluated, and the mechanisms behind fibre–binder interactions were not thoroughly investigated
20	[67]	Textile fibre from tyre recycling	Asphalt binder modifier	Two types of binder, AB14 and AB24, were modified by 0.1%, 0.3%, and 0.5%; textile fibre by asphalt binder weight	Softening point, penetration, FRAASS, MSCR, LAS, SEM, polarised microscopy, and TGA	Permanent deformation was decreased by up to 40% with 0.5% TFELT while maintaining low-temperature cracking resistance; enhanced resistance to rutting and fatigue was observed at all ageing stages	Field tests are needed; scalability assessment and long-term durability research are required for complete incorporation into binder design
21	[65]	Textile fibre from tyre recycling	Asphalt mixtures	Two types of binders (35/50 pen and 50/70 pen) modified with textile fibre	Dynamic modulus, fatigue, permanent deformation, and indirect tensile strength	Employing fibre presented difficulties, but a binder percentage of at least 6.5% greatly enhanced mixture performance, particularly in terms of fatigue and rutting resistance; provides conditional worth in pavement applications	Additional study is required to optimise the mechanisms of fibre–binder interactions, long-term durability, and fibre dispersion
22	[69]	Waste cotton fibre	Replacement of commercial fibre in stone mastic asphalt	Fibre content of 0.20 and 0.30% in SMA20 with 6% binder (commercial versus textile)	Volumetric properties, dynamic modulus, rutting resistance, TSR, and binder drainage	0.20% textile fibre satisfied design requirements, enhanced high-temperature stiffness, and attained a TSR of 95% (compared to 82%); it also marginally increased rutting resistance (2.82 mm versus 2.46 mm)	Leachability, life cycle cost, and technological economic analyses are needed
23	[71]	Textile fibre	Additives in stone mastic asphalt	SMA blends containing 0.3% to 0.5% of cellulose fibre and textile waste	Schellenberg bitumen drain-down and Marshall stability	Textile waste fibres performed similarly to cellulose fibres in terms of stability and structure, and they successfully decreased bitumen drain-down	More research is required on long-term field performance, fatigue life, and moisture susceptibility
24	[48]	Textile fibre from tyre waste	Additives in stone mastic asphalt	SMA mixture with cellulose fibre substituted with 0%, 50%, 75%, and 100% textile fibre	Rutting resistance, moisture sensitivity, stiffness modulus, and cracking resistance	The performance of 100% WTTF replacement was comparable to that of the control mix; the mix’s mechanical properties were maintained, indicating the viability of textile fibre as a sustainable cellulose alternative	There is no evaluation of ageing performance or long-term durability; more field testing is necessary
25	[73]	Polypropylene fibres	Asphalt binder and warm mix asphalt	PP by binder weight: 2%, 4%, and 6%	Marshall properties, binder properties, TSR, and IRS	The fibre enhanced moisture resistance: optimal asphalt content rose to 11.3% with higher PP dosage, TSR climbed to 19.5%, and IRS also improved	Binder demand may increase costs and complicate mix composition; field performance and long-term durability were not evaluated
26	[59]	Denim fibre	Asphalt binder modifier	0.5%, 1%, 1.5%, and 2% WDF by asphalt binder weight	Ductility, penetration, softening point, rutting parameter, temperature susceptibility, and rheological properties	Reduced ductility, temperature sensitivity, and penetration; elevated rutting resistance and softening point; 2% denim fibre was the optimal content	Field validation, economic feasibility analysis, and long-term performance data (fatigue, moisture damage) are lacking
27	[60]	Denim fibre and nanosilica	Rheological property prediction modelling in binder modification	The RSM/ML model’s inputs include temperature (40–76 °C), denim fibre (0–6%), and nanosilica (0–6%).	RSM and ML methods are used to compare the complex modulus (G*), phase angle (δ), and rutting parameter (G*/sinδ) before and after short-term ageing	Strong correlations (R2 > 0.97) were found in RSM models; GPR performed better than other machine learning models, with R2 = 0.99 and ~1% RMSE; binder rheology was strongly impacted by WDF and NS; both before and after ageing, binder behaviour was reliably predicted by ML models	Limited ML/RSM model field validation; no scalability analysis, fatigue modelling, or long-term ageing prediction
28	[43]	Denim fibre and nanosilica	Modification of the binder for improved resistance to rutting	4% denim fibre and 4% nanosilica by asphalt binder weight	MSCR, RTFOT, and temperature sweep	NS improved the WDF-modified binders’ conventional qualities and resistance to rutting; a lower Jnr and a higher recovery percentage were seen in MSCR; between PG 64S and PGV, PG improved; WDF provided more flexibility because of the fibre clustering	More experiments on leachability and mechanical performance are needed
29	[61]	Nanosilica (NS) and waste denim fibre (WDF) in sludge tyre pyrolysis oil (S-TPO)	Enhancement of S-TPO-modified asphalt binders’ consistency	S-TPO binder with 5.1% denim fibre and 2.6% nanosilica	RSM modelling, penetration, softening point, and penetration index	WDF and NS greatly increased the viscosity of S-TPO-modified asphalt; binder consistency could be predicted with RSM models (R2 > 0.74); the impact of the composite WDF/NS was higher than that of the individual modifiers	There is no field applicability study and only a limited evaluation of mechanical performance (rheology, fatigue, and moisture resistance)
30	[62]	Waste denim fibre (WDF) and tyre pyrolysis oil (TPO)	RSM-based optimisation for binder modification	WDF: 0–6% and TPO: 0–15% by bitumen weight	RSM, softening point, penetration, and temperature susceptibility	Binder properties were enhanced by TPO and WDF; high accuracy was demonstrated by RSM models (R2 > 0.94); the best binder performance was attained at 6% WDF and 8.4% TPO	More mechanical performance testing and technoeconomic analysis are needed
31	[63]	WDF (waste denim fibre) combined with nanosilica (NS)	Predictive modelling for rutting parameters based on MSCR	0.2%, 2.4%, and 6% of NS and/or WDF by base binder weight for (individual and composite blend)	Accumulated strain, shear strain, Jnr, %R, RSM, and ML (XGB, DTR)	The significance of NS and WDF on MSCR parameters was validated by RSM models (R2 > 0.80); for strain predictions, ML models performed better than RSM: XGB was the best; DTR obtained R2 = 0.99 and RMSE < 1% for Jnr and %R.	No field validation; the model’s generalisability and long-term performance in real-world scenarios were not examined
32	[90]	Cellulose acetate fibre made from cigarette filters	PA-13 porous asphalt and asphalt binder	Binders modified with CAF (1–5%); PA-13 mixes with different CAF contents	Tensile strength ratio, rutting, fatigue study, moisture/stripping resistance, and rheological testing	1% CAF increased fatigue resistance and provided good low-temperature performance; PA-13 had the best fatigue and low-temperature resistance at 3% CAF, with maximum rutting and moisture resistance at 5% CAF; performance shows dose-dependent parabolic trends	Limited field validation, performance trade-offs at greater CAF contents, and no evaluation of long-term ageing and durability
33	[75]	Textile and cellulose fibres	Granular base with thin asphalt surface	Cellulose fibre content in asphalt beams: 0.2–0.5%; control beams	Fatigue test for four-point bending beams with thicknesses of 50 mm and 30 mm	Textile and cellulose fibres performed similarly in narrow beams; specimen thickness greatly affects fatigue response; 0.3% cellulose fibre provided the optimal fatigue resistance	Laboratory beam arrangement might not accurately reflect in-service pavement behaviour due to limited field validation
34	[4]	Basalt fibre (BF) and calcium sulphate whiskers (CSWs)	Asphalt mixture with hybrid fibres	Wet CSW + dry BF mixing; 5% CSW, 6 mm BF length, 0.32% BF content	Pavement performance tests, mix design validation, and response surface optimisation	The hybrid approach enhanced overall pavement performance; mixture-level testing is more realistic than mastic-level testing; optimal performance was attained at CSW (5%) and BF (0.32%, 6 mm)	Limited studies on ageing and limited mechanistic durability under traffic conditions
35	[83]	Aramid pulp fibre (APF)	GB20 asphalt mixture for cold climates	GB20 mixture reinforced with APF (freeze–thaw exposure up to 300 cycles)	Fatigue, TSRST, complex modulus, and freeze–thaw durability	Improved stiffness stability under cyclic loading; increased durability, fatigue life, and resistance to thermal cracking under freeze–thaw cycles	Lack of technoeconomic analysis and long-term field validation under actual traffic and environmental circumstances

shows a summary of hybrid SCTWDM upcycling applications in asphalt pavement engineering.

5.2. Synthesis of Synthetic and Composite Medical Textile Waste for Upcycling Applications in Asphalt Pavement Engineering

The spike in face mask use during the COVID-19 pandemic created a serious waste management problem. As a sustainable alternative, a study by Goli and Sadeghi [91] investigated the inclusion of waste face masks (FMs) in asphalt mixtures due to their polymeric content, specifically polypropylene. Using different FM contents (two fibre lengths: 8 mm and 12 mm), performance assessments were carried out for Marshall stability, robust modulus, indirect tensile strength, moisture susceptibility, rutting resistance, and fatigue behaviour. The results showed that adding FM fibres significantly improved the mechanical performance of the asphalt, with 12 mm fibres producing better gains than 8 mm fibres. Improved fatigue life, stiffness, and resistance to rutting were noted, indicating that FM fibres may be used as reinforcing agents in HMA. To supplement this mixture-level analysis, Lin et al. [92] investigated the viability of putting waste FMs into asphalt as a sustainable modifier. The study was carried out thorough analyses of FM-modified asphalt binders, looking at phase separation, ageing properties, rheological behaviour, rotational viscosity, and modification methods. The results showed that, particularly at a 5% inclusion rate, adding FMs improved Marshall stability, rutting resistance, and high-temperature performance. However, reduced low-temperature flexibility, ageing resistance, and storage stability are disadvantages. The physical characteristics of the FM and asphalt blend were validated via spectroscopic examination. Adapting this approach to the mixture scale, Ayyadurai et al. [93] investigated the possibility of adding discarded saline tubes (STs) and shredding single-use face masks (FMs) to asphalt mixtures as environmentally friendly additives. Fillers such as metakaolin and M-sand were employed in addition to different FM (1–2%) and ST (0.5–2.5%) proportions to partially replace the coarse aggregate and asphalt binder. While SEM examination confirmed the homogeneous dispersion of FMs and STs in the asphalt matrix, standard bitumen and aggregate tests were used to evaluate the material qualities. The Marshall stability test, which measures performance, revealed that the best results were obtained when 1.25% FM and 1% ST (1.25FM1ST) were combined. This improved the stability and structural performance. This study offers a practical method for turning medical plastic waste into useful materials for road construction. In a related study, Zhu et al. [94] investigated the addition of waste mask fibres (MFs) to stone mastic asphalt (SMA). The SMA samples produced with 0.3%, 0.5%, 0.7%, and 1.0% MF were compared with a control mixture containing 0.3% cellulose fibre (CF). Marshall stability, flow, robust modulus, dynamic creep, moisture susceptibility, and binder drain-off tests are among the extensive laboratory analyses that have been carried out. The findings showed that MF improved the stiffness, stability, resistance to permanent deformation, and moisture resistance of the SMAs. Both the 0.3% and 1.0% MF mixes satisfied industry requirements, but because of their balance of availability, performance, and regulatory compliance, 0.3% MF was found to be the ideal dosage. In SMA applications, the results provide credence to the sustained replacement of CF with MF. Adding to these findings, Wang et al. [95] investigated the use of shredded face mask (SFM) fibres as a sustainable component in hot mix asphalt (HMA) to increase rutting resistance. Because SFM fibres behave semiliquidly at asphalt mixing temperatures (115.5–160 °C), they work well as binding agents during paving and solidify when cooled to improve cohesiveness and stiffness. The study revealed that using up to 1.5% SFM fibres considerably decreased the rut depth, from 3.0 mm to 0.93 mm, on the basis of asphalt pavement analyser (APA) testing. In addition to improving the high-temperature performance and structural durability of extremely stressed pavement sections, these results imply that SFM-modified asphalt offers a practical way to recycle plastic debris associated with COVID-19. Massarra et al. [96] conducted an economic analysis of the use of modified HMAs made from waste masks via life cycle cost analysis (LCCA). Although the initial costs of construction are similar for smaller projects (less than 500 m in length or less than six lanes), they are marginally higher for large-scale projects. However, the proposed method is economically beneficial because it provides maintenance cost savings of approximately 29% throughout the 40-year lifetime due to its resistance to rutting and stiffness. Expanding on this research from the perspective of material interaction, Zhao et al. [97] looked into the possibility of using waste disposable medical masks (DMMs), which are mostly made of polypropylene, as sustainable asphalt modifiers for building roads. A wide range of experiments, including fluorescence microscopy, TGA, FTIR, SEM, and infrared spectroscopy, were used to examine the chemical makeup, morphology, and processes of interaction between the DMM and asphalt. DMMs were found to improve both high- and low-temperature performance by increasing the complex modulus, rotational viscosity, and tensile elongation while decreasing the phase angle in base asphalt. The alteration mechanism was found to be mostly physical blending, with strong compatibility between DMMs and asphalt and no effect on the thermal stability. DMMs considerably improved the resilience of asphalt mixtures to cracking, rutting, and moisture susceptibility when added as a fibre. Additionally, Nciri and Kim [42] used a microstructural and thermal stability approach to study the application of sterilised shredded mask residues (SMRs), predominantly consisting of polypropylene, as sustainable asphalt modifiers. The modified binders were extensively studied via TLC-FID, FT-IR, SEM, TGA, DSC, and rheological testing after SMRs were added to virgin AP-5 asphalt at different weight percentages (3%, 6%, and 9%). The findings showed that SMRs function as inert fillers, improving the rut resistance and thermal stability without undergoing a chemical reaction with the asphalt. The 3% SMR mixture showed the best microstructural compatibility, with decreased ductility and penetration and increased viscosity and softening point. Figure 11 shows the effects of the SMR content on the unaged AP-5 asphalt binder (a) penetration–viscosity number and (b) rutting factor.

In an assessment of waste mask integration into asphalt on the basis of performance by Yalcin et al. [98], the use of disposable face masks (DFMs) as a bitumen modifier was examined. DFMs were applied to virgin asphalt binder in different proportions (1–5%) after being finely shredded. Standard 3% styrene–butadiene–styrene (SBS) modification was used to compare their effects. The performance properties were evaluated via rheological, physical, and chemical tests. According to the results, introducing DFMs improved the high-temperature resistance by increasing the viscosity and softening point while lowering the penetration values. Additionally, binders containing more than 2% DFM performed better than 3% SBS, particularly in terms of maintaining elasticity despite temperature and stress variations. Wang et al. [99] investigated how compatibiliser composition affects the performance of asphalt modified with disposable medical masks in an effort to improve pavement sustainability and address waste caused by pandemics. The asphalt that had been changed with mask fibre was mixed with compatibilisers that had different proportions of aromatic and saturated ingredients. Thermal and rheological analyses, such as FTIR, DSC, DSR, and fluorescence microscopy, were performed. According to the results, compatibilisers with a greater aromatic content improved mask fibre dispersion, lowered the glass transition temperature, and improved performance at low and high temperatures. Together with better storage stability, the modified asphalt had a higher viscosity and softening point. This study provides a workable plan for recycling face mask trash in road construction by showing that a compatibiliser composition is essential for optimising mask fibre-modified asphalt properties.

Furthermore, Zhang et al. [100] investigated the recycling of waste cooking oil (WCO) and waste mask fibres (MFs) for use in HMA applications. Several asphalt binders modified by MF + WCO were created and assessed. According to the findings, MFs marginally reduced the low-temperature cracking resistance while increasing the high-temperature rutting resistance. However, when WCO was added to compensate for this loss, the MF5% + WCO3% blend performed best. FTIR analysis verified that the alteration was physical rather than chemical. The mixture complied with the requirements even though its moisture stability was somewhat diminished. Overall, the MF + WCO blend enhanced asphalt performance while simultaneously advancing environmental sustainability through lower carbon emissions and efficient waste management. Additionally, Putra et al. [101] further investigated the application of medical mask waste fibres (MMWFs) as asphalt modifiers to enhance rheological characteristics and reduce waste associated with pandemics. To perform laboratory testing, 1%, 2%, or 3% MMWF content by weight of asphalt was added. Unaged (initial), short-term aged (RTFOT), and long-term aged (PAV) conditions were used for the dynamic shear rheometer (DSR) experiments. The results demonstrated that the MMWFs greatly increased the stiffness modulus (E*) of the asphalt, reaching 253% with 3% fibre, suggesting improved resistance to rutting. However, increased stiffness at low temperatures raises the possibility of early fatigue cracking.

Musa et al. [102] also considered adding shredded face masks (SFMs) to Nigeria’s natural bitumen (NOB) for road construction. Even though Nigeria has a large supply of NOB, its unfavourable physical properties, high penetration, low softening point, and high viscosity make it unsuitable for paving roads in tropical climates. SFMs were mixed with NOB at different concentrations (1.5% to 9% by mass), which resulted in better physical characteristics. Per the AC-10 grade requirements, the modified binder demonstrated improved stiffness, a higher softening point, and decreased penetration. Furthermore, the modified NOB showed enhanced workability due to decreased viscosity while maintaining thermal stability. In a separate study, Putra et al. [103] examined how waste mask fibres, which are mostly made of polypropylene with nonwoven outer and inner layers and a melt-blown middle layer, affect the rheological performance of asphalt binders. Mask fibres were incorporated at different percentages (0%, 1%, 2%, and 3% by asphalt weight) into base asphalt with a penetration grade of 60/70. The addition of mask waste increased the viscosity, softening point, and flash point while decreasing penetration, ductility, specific gravity, and heat loss. These modifications lead to a noticeable improvement in the hardness of the asphalt, which translates into better resistance to rutting. The decreased elasticity and ductility, however, raise questions regarding possible brittleness and decreased resilience to breaking in fatigued or low-temperature environments. High-temperature stability is aided by the physical hardening effect, but flexibility is sacrificed in the process.

Buruiana et al. [104] examined the use of discarded surgical masks as novel modifiers in bituminous asphalt base course mixtures. The mask composition of ethylene propylene diene monomer (EPDM) rubber modified with polypropylene was verified via FTIR analysis. Morphological and mechanical characterisation were used to evaluate the effects of adding two dosage levels of shredded surgical masks to AB 31.5 bituminous base mixes. The homogeneous mask fibre dispersion and interfacial bonding of the asphalt matrix were demonstrated via SEM-EDX analysis. The addition of surgical masks improved the mechanical stability of the base course while maintaining adequate workability, according to performance tests that looked at Marshall stability, flow velocity, solid–liquid ratio, apparent density, and water absorption. Similarly, Kumar and Bhatia [105] investigated the inclusion of PPE trash based on polypropylene to reduce the environmental burden of plastic waste associated with COVID-19. The main objective of this study was to assess the mechanical performance of asphalt mixtures that contained different amounts of PPE waste (10–20%) as a partial bitumen substitute. To evaluate the structural performance, Marshall stability and flow tests were performed. The best blend, according to the results, had a composition of 6.5% and 10% PPE waste, improving stability without sacrificing flow. A better performance under loading was shown by the inclusion of PPE waste, which also improved the binder’s basic characteristics.

To lessen landfill pressure and environmental pollution, this study shows that it is feasible to turn excessive plastic waste into a useful building material. To further understand their viscoelastic impact on binder performance, Özdemir et al. [106] carried out a thorough rheological characterisation of bitumen treated with 1–3% waste face masks (WFMs). Frequency sweep tests were performed at different loading frequencies and temperatures via a dynamic shear rheometer (DSR). Four modelling frameworks, the sigmoidal model (SM), the Christensen–Anderson (CA) model, the Christensen–Anderson–Marasteanu (CAM) model, and the mechanistic Huet–Sayegh model (HSM), were used to analyse the rheological master curves of the modified binders. The results revealed notable improvements in rutting resistance across all WFM dosages, with modified binders outperforming unmodified bitumen at both low and high loading rates. On this empirical basis, Ding et al. [107] used quantum chemistry (QC) and molecular dynamics (MD) simulations to explore the physicochemical interactions in mask-modified asphalt (MMA) with the goal of optimising processing parameters. To minimise the mixing free energy and Flory–Huggins parameters, 170 °C was determined to be the ideal mixing temperature. After 72 h of heating, laboratory validation revealed a steady softening point difference (ΔTR&B) and separation ratio (RS). Multiple binding types, including perpendicular, parallel, toroidal, and spherical, were identified via fluorescence microscopy and spectroscopy. van der Waals forces were the predominant interactions, with electrostatic forces from polycyclic aromatic hydrocarbons serving as a supplement. This study is one of the first to optimise MMA design by combining laboratory analysis and computational chemistry, providing a solid theoretical foundation for enhancing performance and recycling medical mask waste in asphalt pavement applications.

In a related work, Li et al. [108] examined the thermal storage stability and high-temperature behaviour of asphalt treated with discarded masks (DMs), which are mainly made of polypropylene, by combining MD models and laboratory testing. According to the experimental data, DM modifiers, mostly polypropylene, improved the rutting resistance, softening point, and viscosity. However, higher dosages (1–4%) caused notable segregation during thermal storage, with softening point differences increasing from 2.2 °C to 17.1 °C. The physical co-blending was confirmed by FTIR analysis. According to the MD simulations, the DMs decreased the binding energy between the asphalt and modifier while increasing the cohesive energy density and decreasing the free volume. In asphalt applications, the combined macroscopic and molecular-level results revealed that a 1% DM concentration provides a sustainable reuse option for pandemic-related mask waste by striking a balance between enhanced high-temperature characteristics and acceptable thermal stability. Furthermore, Mansor et al. [109] used a comparative method to assess how various COVID-19-related personal protection equipment (PPE) wastes, such as nitrile butadiene rubber (NBR) gloves and disposable face masks (DFMs), affect the characteristics of stone mastic asphalt (SMA-14) and PEN 60/70 bitumen. Both dry and wet mixing techniques were used to shred and integrate sanitised garbage. According to the results, DFMs decreased the penetration and softening points, whereas NBR increased both. The national standards used 3% NBR and 6% DFM, which significantly increased the unconfined compressive strength and decreased binder drain-down. These PPE-derived fibres may improve pavement longevity and performance, according to previous studies. Moreover, Lan et al. [110] extended this line of research by addressing the inherent compatibility concerns of waste polypropylene in asphalt binders. Waste polypropylene has the potential to be reused in bitumen; however, its addition frequently results in phase segregation and inadequate resistance to cracking. This study investigated the combined use of maleic anhydride-grafted polypropylene (PP-g-MAH) and styrene–butadiene rubber (SBR) as compatibilisers to overcome these constraints. The modifiers greatly improved the low-temperature crack resistance and encouraged the uniform dispersion of WPP in the asphalt matrix. In addition to improving the mechanical and rheological qualities of modified bitumen, the suggested blend offers a practical way to reduce the environmental impact of discarded face masks. Li et al. [111] also studied the reuse of used face masks in the production of SBS-modified bitumen via PDA-PEI-coated fibres for better fibre–binder compatibility. The findings reveal that E-FMF can greatly improve the rheological properties and resistance to deformation, as shown by the higher complex modulus (12.27%), elastic modulus (16.39%), and loss modulus (13.35%) at 46 °C. It significantly improved the creep recovery and reduced the permanent deformation (Jnr), which means that it was less susceptible to rut formation.

A study was conducted by Mirjalili and Khabiri [112] on the interaction between waste mask fabric scraps and nanocarbon-modified filler in asphalt mixtures. The study revealed an optimum point at a fibre dosage of 0.3% and a fibre length of 12 mm, which produced noticeable increases in tensile strength (33%), moisture resistance (17%), and fatigue deformation reduction (70%). The performance was shown to have a dose-dependent peak with a decline beyond the optimum value. Statistical analysis confirmed the significance of these enhancements, and regression models were fitted via Design-Expert software version 12.0.3. Although there is potential benefit in terms of enhanced mechanical and durability characteristics, laboratory-based verification is challenging. The reuse of disposable medical protective clothing (DMPC) in asphalt modification was examined by Zhang et al. [113], who used a multiscale strategy to investigate the effect of DMPC on the macroscopic and microscopic characteristics of asphalt. The results showed that DMPC promotes asphalt consistency by reducing intermolecular space and increasing internal friction, which improves high-temperature rheological behaviour while somewhat impairing low-temperature flexibility. DMPC creates a solid 3D network within asphalt, which increases the thermal stability and compatibility. Additionally, DMPC absorbs light asphalt components, which improves its anti-cracking performance. Thermal breakdown of DMPC produces stable gases, which reduce hazardous emissions. The best results were obtained with a DMPC dosage of 3 wt%. This study shows that DMPC may be successfully recycled into asphalt, improving road properties while also promoting environmental sustainability and circular waste management.

Furthermore, Ali et al. [114] investigated the possibility of using disposable face masks made of polypropylene (PP) as a sustainable modifier for asphalt pavement. According to the study, adding different amounts of PP fibres to HMA (3–15% by binder weight) improves its mechanical performance. Compared with those of the control binders, improvements in Marshall stability, stiffness, rutting, and cracking resistance were noticeable at 3% PP, which produced the best results. In addition to enhancing pavement performance, waste PP provides a sustainable way to reduce environmental contamination caused by inappropriate mask disposal. Additionally, Putra et al. [115] studied the effect of waste mask fibres on porous asphalt mixtures at different dosages (from 0 to 3%). The results clearly indicate that a fibre dosage of 1% is ideal, providing increased stiffness, indirect tensile strength, and ravel resistance, as well as permanent deformation resistance. The mechanical properties, such as Marshall properties, ITSM, and ITS, were improved through fatigue and creep tests. Furthermore, Toro et al. [116] conducted a multiscale performance assessment that included both laboratory testing and ANSYS-based simulations to determine the effects of the PP fibre content on the rutting resistance and mixture stability. As a binding agent, amounts of 0.5%, 1.0%, and 1.5% PP were added to the asphalt mixtures. According to laboratory experiments, a 0.5% PP content marginally increased mixture stability, whereas a greater PP content increased the number of air spaces and decreased stability. The rutting performance and elastic modulus were assessed via simulations using ANSYS. The rut depth was significantly reduced from 4.87 mm (unmodified) to 1.03 mm (1.5% PP) in the Hamburg Wheel Tracking simulation. According to previous studies, recycled polypropylene fibres can successfully increase the resistance of asphalt to rutting, promoting longevity and environmental sustainability. In an expanded study, Abbas Bangash et al. [117] investigated the synergistic effects of mixing baby diapers (BDs) and FMs at particular dosages (up to 1.5% by aggregate weight for FMs and 4% by bitumen weight for BDs), which led to significant improvements in durability and mechanical properties. With a 39% increase in Marshall stability, an 18% increase in indirect tensile strength, a 27% decrease in deformation, a 53% increase in rutting resistance, and a 33% increase in the resilient modulus, the modified mix containing 4% BD and 1.5% FM demonstrated improved bonding. The method improves pavement performance while demonstrating a notable waste reduction of 36% for BDs and 61% for FMs. This study demonstrates a viable, environmentally acceptable substitute for sustainable road building that makes use of synthetic textile waste from the postpandemic period.

Cheng et al. [118] investigated the use of melt-blown cloth (MBC) from waste masks as a modifier in base asphalt to improve road performance and encourage environmentally friendly garbage disposal. Through integrated weighting analysis, grey correlation, and physical property testing, the best modification was found to be 3% MBC at 170 °C and 4000 rpm for 15 min. While marginally decreasing penetration and ductility, the amended asphalt demonstrated enhanced resistance to rutting at high temperatures through a better softening point, viscosity, and elastic recovery. Without any chemical reactions, physical changes were verified via chemical analysis. In contrast to the base and SBS-modified asphalt mixtures, the MBC-modified mixture demonstrated better moisture resistance and stability at high temperatures but somewhat worse performance at low temperatures. On the basis of this foundation, Cheng et al. [119] expanded the investigation to include morphological and rheological evaluations of MBF-modified asphalt. The preparation procedure (170 °C, 4000 r/min, 15 min) was optimised, and the physical, rheological, and morphological characteristics were assessed. MBF modification is a physical process that does not result in network structures or agglomerations. The MBF marginally decreased the low-temperature crack resistance while improving the high-temperature stability and moisture resistance. A 3% MBF addition produced the most evenly distributed improvement among the assessed homes, followed by 5% and 1%, according to radar chart analysis. The results show that MBF has the potential to be a sustainable asphalt modifier that supports resource efficiency in pavement engineering and is in line with waste reduction objectives.

Table 9. Summary of the application of medical waste SCTWDMs in flexible pavement engineering.

S/N	Ref.	Textile Waste	Application	Mixture Composition	Tests	Findings	Comments
1	[91]	Face masks	Asphalt mixtures	Asphalt mixtures containing four distinct weight percentages of shredded face mask fibres (8 mm and 12 mm)	Marshall test, resilient modulus, indirect tensile strength, moisture damage, rutting, and fatigue	Mask fibres enhanced mechanical performance, with 12 mm fibres outperforming 8 mm; improved resistance to wear, rutting, and moisture	Further investigation is necessary to comprehend the physical and chemical interactions between asphalt binders and fibre types
2	[105]	Polypropylene gown	Asphalt mixtures	10%, 12.5%, 15%, 17.5%, and 20% PPE waste added to mixture (by weight of binder)	Bitumen property, Marshall stability, and flow value tests	Optimum blend with 10% PPE waste and 6.5% bitumen content; better Marshall stability; improved binder properties	Leaching behaviour, field performance, ageing behaviours, and environmental safety evaluations have not yet been investigated
3	[104]	Surgical mask	Bituminous asphalt base course	Asphalt binder with 0.1 and 0.2%	Marshall stability, FTIR, SEM-EDX, flow, apparent density, water absorption, and solid–liquid ratio	Better Marshall stability, sufficient density, and flow; improved interfacial bonding as demonstrated by SEM-EDX; environmental advantages through the recycling of COVID-19 waste	Evaluation of long-term performance, field durability, and dosage optimisation for various climates is necessary
4	[98]	Polypropylene-based face mask	Asphalt binder	1%, 2%, 3%, 4%, and 5% waste mask-modified pure bitumen in comparison to 3% SBS-modified binder	Rheological (elasticity, temperature sensitivity), penetration, softening point, and viscosity	2% waste mask content enhanced softening point and viscosity, decreased penetration, and demonstrated superior elastic behaviour and thermal stability compared to a 3% SBS-modified binder	Further research is needed to fully comprehend the interfacial adhesion mechanisms and microstructural alterations between fibres and asphalt
5	[42]	Polypropylene-based sterilised shredded mask waste	Asphalt binder	Unmodified AP-5 asphalt that has SMR contents by weight of 3%, 6%, and 9%	Viscosity, ductility, softening point, penetration, FT-IR, SEM, TGA, DSC, and DSR	SMR improved resistance to rutting and thermal stability—3% SMR reduced ductility and penetration; increased viscosity and softening point	Evaluation is restricted to the binder scale; field performance and mix-level behaviour under actual loads and weathering were not investigated
6	[114]	Polypropylene waste face mask	Asphalt binder and mixtures	3%, 6%, 9%, 12%, and 15% PP (by binder weight) added to asphalt	Marshall stability, rutting, and cracking test at 4% optimal binder content	Improved durability, rigidity, and stability; better resistance to rutting and cracking than the control mix; 3% PP was found to be the optimal content	Long-term performance data under real-world traffic conditions, field validation, and studies on moisture susceptibility are needed
7	[97]	Medical mask	Asphalt binder and mixtures	Base asphalt plus polypropylene-based DMM modifier; asphalt blends containing DMM fibres	Fluorescence microscopy, cracking, moisture susceptibility, rutting resistance, FTIR, TGA, SEM, and rheology	MM enhanced the material’s resistance to rutting, cracking, dampness, and high and low temperatures; it also had good compatibility	More investigation into the surface energy, adhesion characteristics, and a LCA of the modified asphalt binders is recommended
8	[107]	Medical mask fibre	Asphalt binder	Masks with fine filaments (content: 4.0%)	Fluorescence microscopy, QC modelling, ΔTR&B, RS, MD simulation, and multiband spectroscopy	The ideal mixing temperature is 170 °C, as confirmed by MD and laboratory testing; various binding modes (perpendicular, parallel, toroidal, and spherical) were discovered; dominance of the van der Waals interaction; PAHs in asphalt cause electrostatic interaction	Mechanical performance under traffic loading, long-term durability, and field-scale application have not yet been confirmed
9	[108]	Discarded polypropylene masks	Asphalt binder	Mask fibre with 1%, 2%, 3%, and 4% by binder weight	Rotational viscosity, softening point, DSR, FT-IR, and MD simulation	Enhanced rutting resistance, viscosity, and softening point; segregation rose as DM content rose (ΔSP: 2.2 °C to 17.1 °C); the optimal equilibrium was provided by 1% DM; physical change was validated by FT-IR; MD 10 revealed lower FFV and higher CED	Additional research on the composite-modified pavements’ recyclable nature, economic feasibility, and long-term performance in various climates is required
10	[92]	Polypropylene from face mask waste	Asphalt binder and mixtures	5% shredded face mask (FM) fibre plus asphalt binder	Rutting, moisture susceptibility, low-temperature cracking, ageing tests, rheology, FTIR, fluorescence microscopy, LAS test, Marshall stability, and rotational viscosity	FM improved anticreep and high-temperature performance; 5% FM showed the best compatibility; it also enhanced Marshall stability and rutting resistance in blends; it had detrimental impacts on storage stability, ageing, and low-temperature performance	Limited studies on cost–benefit analysis (CBA) or full life cycle assessments (LCAs) to measure the environmental impact or viability of employing waste fibres are required
11	[106]	Waste face mask fibre	Asphalt binder	WFM with 1%, 1.5%, 2%, 2.5%, and 3% by binder weight	Rheological analysis, master curve analysis, frequency sweep	Improved viscoelastic performance at low and high loading rates; increased rutting resistance at all WFM levels; a good model fit for analytical and mechanistic rheological behaviour	Evaluation of fatigue, cracking performance, and long-term ageing (RTFO/PAV) is needed; field validation is still lacking
12	[100]	Waste face mask fibre and cooking oil	Asphalt binder and mixtures	Waste face mask fibre (5%), cooking oil (3%)	Physical testing, FTIR, moisture stability, rheological analysis, and evaluation of environmental benefits	MF increased resistance to rutting but decreased resistance to low-temperature cracking; cooking oil restored low-temperature performance	Additional research is required to understand the chemical and physical interactions between fibre and asphalt binders
13	[99]	Face mask fibre	Asphalt binder	Face mask fibre with compatibilisers (A, B, C, D, and E) with different aromatic/saturated concentrations and control asphalt	Softening point, penetration, Brookfield viscosity, DSR, DSC, FTIR, fluorescence microscopy, and storage stability	Higher aromatic content compatibilisers increased viscosity, thermal performance, storage stability, Tg reduction, and face mask dispersion	More research on the compatibiliser content effects on modified asphalt’s rheology, thermal stability, and compatibility is needed
14	[109]	Nitrile butadiene rubber (NBR) gloves and disposable face masks (DFMs)	Asphalt binder and SMA-14 mixture	NBR (3%, 6%, and 9%) and DFM (3%, 6%, and 9%) by binder weight	Penetration, UCS, softening point, and binder drain-down (dry and wet processes)	DFM reduced penetration and the softening point, while NBR enhanced both; optimal: reduced drain-down and improved UCS with 3% NBR and 6% DFM; both fibres complied with PWD requirements	Assessments of fatigue behaviour, long-term durability, and environmental impact in field settings have not yet been investigated
15	[93]	Face masks and saline tubes	Asphalt mixture	FM (1–2%) and ST (0.5–2.5%)	Bitumen testing, Marshall stability, and SEM	Optimal performance at 1.25% FM + 1% ST; enhanced structural performance and Marshall stability; SEM demonstrated uniform dispersion	The morphological characteristics and surface energy of the face mask-modified binders should be further examined
16	[101]	Face mask fibres	Asphalt binder	1%, 2%, and 3% face mask fibre by weight of asphalt binder	Rheology in normal, RTFOT, and PAV settings	Better resistance to rutting at high temperatures; increased stiffness at low temperatures may raise the risk of fatigue cracking; stiffness modulus (E*) improved by up to 253%	Further research is needed to fully comprehend the interfacial adhesion mechanisms and microstructural alterations between fibres and asphalt
17	[94]	Face mask fibres	Stone mastic asphalt	SMA with 0.3% cellulose fibre (CF); control SMA with 0.3%, 0.5%, 0.7%, and 1.0% MF (by weight of mix)	Volumetric evaluation, Marshall flow and stability, dynamic creep, moisture susceptibility, resilient modulus, and drain-off of the binder	Stability, modulus, rutting, and moisture resistance were all enhanced by MF; 0.3% MF outperformed and satisfied industry standards	Validation is required through lab-scale fatigue and freeze–thaw cycles, and there is no chemical reaction; data on fatigue and ageing effects are lacking
18	[95]	Face mask	Asphalt mixtures	FMs (0% to 1.5%) by mixture weight	Rutting test using asphalt pavement analysers	FMs improved rutting resistance by acting as a semiliquid binder during mixing and solidifying after cooling; at 1.5% FM, the rutting depth decreased from 3.0 mm to 0.93 mm	Additional research using cost–benefit analysis (CBA) or full LCA is recommended
19	[102]	Shredded face mask fibres	Asphalt binder modifier	SFMs (1.5%, 3%, 4.5%, 6%, 7.5%, and 9% by binder weight)	Penetration, softening point, viscosity, dynamic shear rheometer	SFM-modified NOB met the AC-10 bitumen grade with its enhanced stiffness, greater softening point, and decreased penetration; viscosity decreased, making workability better with no difference in thermal stability	Lack of rheological characterisation (DSR, MSCR); long-term ageing and durability performance not yet evaluated
20	[115]	Waste mask fibre	Porous asphalt mixtures	0–3% fibre at optimum binder content	Marshall properties, fatigue, creep, ITS, ITSM, and the Cantabro test	Optimal at 1%, sufficient fatigue and creep performance was observed with enhanced stiffness, ITS, ravelling resistance, and deformation resistance	Long-term ageing and field validation are lacking in this laboratory-based work, and more research is needed to understand performance at higher stress levels
21	[103]	Face mask	Asphalt binder modifier	Mask fibres at 0%, 1%, 2%, and 3% (by binder weight)	Viscosity, flash point, softening point, ductility, and penetration	Increased viscosity, hardness, and rutting resistance; decreased ductility and cracking resistance due to reduced elasticity	No evaluation of long-term cracking performance under fatigue and low-temperature circumstances, and there is no DSR-based rheological evaluation
22	[110]	Waste polypropylene (WPP) masks	Asphalt binder modifier	WPP masks plus SBR and PP-g-MAH (compatibilisers)	Low-temperature cracking, dispersion behaviour, and morphology	SBR with PP-g-MAH enhanced low-temperature crack resistance and better WPP dispersion in bitumen; recycling lessens the environmental impact	There is little research that addresses phase segregation and cracking in WPP-modified asphalt with reactive compatibilisers
23	[116]	Waste polypropylene masks	Asphalt binder and mixture modifier	0.5%, 1.0%, and 1.5% PP by asphalt binder weight	Marshall stability, void analysis, Hamburg Wheel Tracking, elastic modulus (using ANSYS)	A 0.5% PP addition marginally increased mix stability, and voids and decreased Marshall stability were caused by higher PP; additionally, as PP content increased, rutting increased from 4.87 to 1.03 mm	Limited experimental data on fatigue and moisture resistance; no long-term performance evaluation in field settings
24	[113]	Disposable medical protective clothing (DMPC)	Asphalt binder modifier	1–5 wt% DMPC by asphalt binder weight	Rheological tests (high and low temp), microstructure, compatibility, decomposition	DMPC improved high-temp rheology and reduced light components; it formed a 3D network in the binder and promoted physical bonding, enhances anti-cracking, and optimised performance at 3 wt%	Limited understanding of long-term field performance and low-temperature rheological degradation
25	[117]	Baby diapers (BDs) and face masks (FMs)	Asphalt binder and aggregate	Bitumen contains 4% shredded BDs, while aggregates have 0%, 0.5%, 1%, and 1.5% shredded FMs	Stability, rutting, resilient modulus, permanent deformation, and ITS	There was a 1.5% FM improvement in the aggregate characteristics; 1.5% FM and 4% BD combined increased the mixture stability, ITS, deformation, rut resistance, and resilient modulus	No evaluation of long-term ageing or moisture damage; no data on field performance and technoeconomic analysis
26	[118]	Melt-blown mask cloth	Asphalt binder	3% MBC, 170 °C, 4000 rpm, 15 min	Elastic recovery, ductility, softening point, and PG grading	Improved high-temperature stability, recovery rate, and viscosity; marginally worse low-temperature performance	More research is needed on microstructural evolution and fatigue behaviour
27	[111]	PDA-PEI-coated discarded face mask fibres (DFMFs)	SBS modified asphalt binder	2% DFMF in SBS binder	Multiple stress creep recovery (MSCR), creep recovery (R), and rheological tests	Superior rutting resistance was demonstrated by coated fibres, which also increased creep recovery, decreased permanent deformation, and improved rheological performance	Performance is dependent on the consistency of the coating process; there is little long-term ageing evaluation and no field validation
28	[112]	Waste mask fabric scraps (WMFS) plus nanocarbon modified filling (NCMF)	Asphalt mixtures	Fibre lengths 8–18 mm; WMFS/NCMF 0.3–0.5%	Marshall stability, ITS, statistical analysis, predictive modelling, moisture susceptibility	Tensile strength, moisture resistance, and fatigue deformation were all enhanced by the optimal mix of 0.3% and 12 mm in length, but the performance decreased	Performance is dependent on fibre dosage and dispersion; long-term ageing and durability are not evaluated; field validation is limited
29	[96]	Waste face masks made of polypropylene	Asphalt mixtures	Comparing mask-modified asphalt with traditional HMA (content not stated)	Life cycle cost analysis (LCCA) and cost estimation for collection, processing, and application	For pavement sections less than 500 m and fewer than six lanes, the initial construction costs are similar; larger parts (>500 m, 5–6 lanes) have slightly higher prices; nonetheless, LCCA showed a 29% decrease in maintenance costs during a 40-year design life, indicating a long-term financial benefit	Absence of mechanical performance validation and specific mix design characteristics; the robustness of sustainability assessments is limited in the absence of environmental effect quantification (e.g., LCA)
30	[119]	Melt-blown fabric (MBF) from discarded face masks	Asphalt binder	0%, 1%, 3%, and 5% MBFs by binder weight	Physical properties, morphological examination, rheological testing, and radar chart evaluation	High-temperature, stability, and moisture resistance were optimally enhanced by 3% MBF; low-temperature crack resistance decreased; physical alteration without agglomeration was achieved	Inadequate performance at low temperatures; more investigation is required to strengthen crack resistance and validate in a permanent field

shows a summary of medical waste SCTWDM upcycling applications in flexible pavement engineering.

6. Interaction Mechanisms of Some SCTWDMs with Asphalt Binders

The use of waste-derived additives to chemically modify asphalt has gained attention because of the increased interest in sustainable pavement materials. A study conducted by Nciri and Kim [42] used thin-layer chromatography–flame ionisation detection (TLC-FID), and the impact of shredded mask residues (SMRs) on the SARA fractions of saturates, aromatics, resins, and asphaltenes of basic AP-5 asphalt was examined in this context. Figure 12 shows the effects of SMR content on unaged asphalt binder SARA constituents. Despite the addition of SMRs, the saturates remained constant (~3.47 ± 0.46 to 3.57 ± 0.52 weight percent), indicating their nonpolar, chemically inert character. Aromatics, which solvate asphaltenes to preserve colloidal stability, slightly decreased with 9 percent of SMR by weight from 59.52 ± 1.36 weight percent to 57.97 ± 1.28 weight percent, indicating minor molecular interactions. Notably, the resin fraction increased from 16.30 (1.76 to 18.87 (0.75) weight percent), which is in line with the high resin content of SMRs. This increase suggests better asphaltene dispersion, possibly greater binder adherence, and increased moisture resistance because resins function as peptising agents [120]. Additionally, the asphaltene content decreased marginally from 20.67 ± 3.35 to 19.60 ± 1.56 weight percent, most likely as a result of resin enrichment-induced colloidal rebalancing. These results suggest that the asphalt matrix has undergone chemical rearrangement, which could lead to better binder performance and durability. This study emphasises the potential of SMRs as a useful, environmentally friendly asphalt modification additive.

Nciri and Kim [42] also evaluated the chemical interactions between SMRs and asphalt binder via Fourier transform infrared (FTIR) spectroscopy. A comparison of the FTIR spectra for different mask layers and the influence of the SMR content on the unaged asphalt binder is shown in Figure 13. The results revealed that the virgin binder spectra presented distinctive peaks at 3105–3656 cm⁻¹ (O–H/N–H), 2800–3000 cm⁻¹ (C–H stretching of aliphatic hydrocarbons), and a small carbonyl shoulder at 1742 cm⁻¹. Other peaks between 1450–1500 cm⁻¹ and 1030–1100 cm⁻¹ suggested alkyl groups and sulfoxides, respectively, whereas aromatic C=C stretches were observed close to 1576 cm⁻¹ and 1540 cm⁻¹. Additionally, signals at 742, 808, and 861 cm⁻¹ confirmed the presence of substituted aromatic compounds. The FT-IR spectra of the SMR layers revealed that polypropylene and polyamide were the predominant features. Near 2950, 2917, and 2876 cm⁻¹, the outer hydrophobic layer displayed –CH₃, –CH₂, and –CH stretches. The peaks for nitriles and carbonyls (2159, 2032, and 1742 cm⁻¹) indicated a complex network of polymers. The inner layer displayed C–O–C ether bands (996–1167 cm⁻¹), which are indicative of soft, flexible materials, whereas the middle layer, which was probably polyamide-based, presented high N–H absorption at approximately 1538–1579 cm⁻¹. The FT-IR spectra of the modified binders showed no signs of significant chemical interactions, even after the addition of SMRs. Accordingly, SMRs preserve the chemical integrity of asphalt while possibly improving its mechanical qualities through physical reinforcement. They essentially function as inert fillers rather than reactive modifiers.

FTIR was used by Buruiana et al. [104] to chemically characterise shredded surgical masks of ethylene propylene diene monomer (EPDM) rubber that had been polypropylene-modified (EPDM-PP) to determine whether they were compatible with bituminous materials. Broad absorption bands were observed in the FTIR spectra between 1161 cm and 1454 cm⁻¹ and at 2914 cm⁻¹. These bands corresponded to CH₂ asymmetric stretching, which is a hallmark of polypropylene (PP). The presence of additional bands at 1643 cm⁻¹ (C=C stretching for EPDM), 827 cm⁻¹ (perhaps indicating cross-linking between EPDM and PP), 670 cm⁻¹ (Si–O–Mg in talc filler), and 448 cm⁻¹ (S–S bond in cross-linked EPDM-PP) verified that the mask material was mainly composed of PP-blended ethylene propylene diene monomer (EPDM) rubber. The presence and interaction of EPDM-PP with the asphalt matrix were confirmed by the modified asphalt FTIR spectra, which displayed consistent peak positions when EPDM-PP was added to the bitumen at different contents (0.1% and 0.3% by weight).

Zhang et al. [113] used FTIR to assess the chemical interaction between recycled waste disposable medical protective gear (DMPC) pieces and asphalt binder. In contrast to those of the basic binder, the FTIR spectra of the modified asphalt did not reveal any new absorption peaks, suggesting that there was no chemical reaction between the asphalt and the DMPC modifier. The main absorption bands found were linked to polypropylene, notably the symmetric and CH₂ asymmetric stretching vibrations at 2847 and 2914 cm⁻¹, respectively. This finding indicates that polypropylene is the main chemical component of DMPC. This finding implies that physical blending, as opposed to chemical bonding, is how DMPC is incorporated into asphalt. Without changing its molecular structure, the polypropylene from the DMPC helps to create a polymeric network inside the asphalt matrix, improving its rheological and elastic properties.

Nciri and Kim [42] further examined the morphological and microstructural alterations in virgin binder due to the integration of SMRs at varying contents. A distinct evolution in surface morphology was shown by SEM investigation at 3000× magnification. The control reference, the unaltered asphalt (Figure 14A), had a consistent and smooth microtexture. Although small surface undulations that resembled soft ripples started to appear at 3% SMR (Figure 14B), the asphalt remained mostly cohesive and had no discernible SMR particles. The surface was more complex, including mild crinkling and enhanced tortuosity, after 6% of SMR by weight was added (Figure 14C). More visible microstructural damage was indicated by textural aberrations, despite the absence of distinct SMR fragments. The surface of the asphalt changed significantly at 9% wt SMR (Figure 14D). There were visible cavities, cracks, and embedded SMR fragments, which suggested a crucial area for structural accommodation. This degree of alteration implies that an excessive amount of SMRs could negatively impact the structural stability and internal cohesiveness of the asphalt matrix. The results show that although moderate SMR additions can be incorporated into the asphalt structure with little disturbance, greater concentrations result in notable microstructural changes that may affect long-term performance.

SEM and EDX spectroscopy were used by Landi to assess the microstructural development of hot mix asphalt treated with EPDM-PP made from shredded surgical masks. SEM revealed that the typical AB 31.5 asphalt mixture had a homogeneous, smooth microstructure. On the other hand, Samples 1 and 2, which were treated with EPDM-PP, showed better interfacial compactness between the particles and the binder. The surface porosity was greater in Sample 1, which had less EPDM-PP. The microstructure of Sample 2 (0.3% EPDM-PP) was denser, with crushed silica, sand, and limestone filler deeply buried in the matrix of the polymers. There were fewer voids and less surface roughness in the matrix. Mask-derived fibres were observed spanning the asphalt components in Sample 2 at higher magnification, indicating possible reinforcing effects and improved structural integrity. The EDX analysis of Sample 2 verified the presence of silicon, calcium, carbon, and oxygen, which were mostly attributed to the aggregate and surgical mask components. While the presence of calcium suggested good compatibility with the bitumen, the high carbon level suggested the addition of polypropylene fibres.

7. Strategic SWOT Analysis of SCTWDMs in the Pavement Industry

To assess the potential of SCTWDMs in asphalt pavement applications holistically, a strategic SWOT analysis was carried out. The intrinsic technological and material advantages, including enhanced mechanical performance and environmental advantages, as well as internal drawbacks, such as processing difficulties and SCTWDM morphological variability, are captured by this analytical framework. It also takes into consideration external opportunities brought about by changes in policy, sustainability requirements, and incentives for the circular economy, as well as risks brought about by erratic supply chains, unclear regulations, and a lack of field-scale validation. The conclusions of this analysis not only offer a tactical direction for the effective application of these waste-derived materials but also guide future research directions to hasten their assimilation into standard pavement engineering procedures.

7.1. Strength

• Potential for mechanical reinforcement: Both synthetic (such as polyester, aramid, and PAN) and composite textile fibres (such as tyre cords, glass fibre-reinforced textiles, and multilayer face masks) have high elastic moduli, thermal stabilities, and tensile strengths. These characteristics, when evenly distributed, greatly increase the tensile strength, fatigue life, fracture bridging, and resistance to rutting of asphalt mixtures [64,75].
• Increased service life and durability: Fibres increase durability under cyclic loads, postpone reflective cracking, and lessen permanent deformation. Additionally, composite fibres such as tyre cords improve the interfacial adhesion of asphalt, resulting in pavements that are more resilient to changes in traffic and weather [64,90].
• Circularity and resource efficiency: Including textile waste promotes closed-loop material cycles by keeping nonbiodegradable materials out of landfills and incinerators. It preserves natural resources and promotes sustainable consumption by lowering the need for virgin materials such as mineral filler or polymer modifiers [121,122,123].
• Benefits to the environment and carbon: By reducing energy consumption and greenhouse gas emissions during production and disposal, bitumen or mineral additives can be replaced with SCTWDMs, thereby reducing the carbon footprint. Climate-smart infrastructure solutions are in line with this [124,125,126].
• Underutilised and abundant material streams: One abundant but underutilised waste stream is SCTWDMs from fast fashion and tyre production [122,127]. A sustainable outlet is produced by repurposing this waste into high-value pavement.

7.2. Weakness

• Variability and unpredictability: In long-term field applications, variations in SCTWDM composition, geometry, pollutants, and degradation levels among waste sources (e.g., industrial aramid textiles versus COVID-19 masks) may result in unpredictable mechanical performance [7,92].
• Problems with compatibility and processing: SCTWDMs compatible with hot bitumen frequently need to be shredded, decontaminated, and surface-treated. Fibre agglomeration caused by poor dispersion can have a detrimental effect on mix homogeneity, compaction, and workability [92,94].
• The absence of standardised requirements: Widespread use is restricted, and uncertainty is created by the lack of internationally recognised standards for dosage, gradation, and SCTWDM-modified asphalt design; moreover, the empirical data lack repeatability across pavement types and climates and are frequently region-specific [128,129].
• Possible brittleness at low temperatures: Although fibres increase stability at high temperatures, some (such as rigid synthetics) can negatively impact flexibility at low temperatures and increase the risk of thermal cracking if they are not appropriately balanced with softer binders or additives [42,93].
• Operational and economic barriers: At asphalt mixing facilities, extra equipment or adjustments could be needed to guarantee consistent fibre dispersion, and for public agencies or contractors, pretreatment and quality control of waste fibres may result in higher short-term expenses [130,131].

7.3. Opportunities

• Sustainable pavement technology development: Global trends towards net-zero construction and green infrastructure are in line with asphalt modified with SCTWDMs. When combined with additional recycled materials (such as glass, crumb rubber, and plastics), high-performance, multiwaste composite pavements can be produced [131,132].
• Production of intelligent and sustainable materials: The use of tailored SCTWDMs integrated with sensors, self-healing chemicals, or nanoparticles can be investigated to develop smart pavements that can control temperature, mitigate damage, and monitor damage in real time [131,133].
• Collaborative hybrid fibre systems: It may be possible to customise asphalt behaviour over a wider temperature and loading range by combining fibres (for example, polyester + aramid or glass + PAN), which would maximise both stiffness and flexibility [134].
• Funding, regulation, and policy assistance: The adoption of SCTWDM-reinforced pavements may be encouraged by growing policy pressure to minimise synthetic and plastic waste, as well as green public procurement methods. Such technologies are very appropriate for climate-resilient infrastructure projects that are funded by governments and multilateral agencies [134].
• Integration with frameworks for performance-based design: To increase confidence and acceptance, textile waste fibres can be designed and verified for specific functional purposes (rutting, cracking, and moisture resistance) as the pavement industry transitions from empirical to mechanistic–empirical and performance-based design methodologies [135].
• Diversification of the market: Where durability and fatigue resistance are essential, fibre-reinforced mixes can be used for urban overlays, bridge decks, high-traffic highways, and airport runways, and as a result, eco-asphalt goods can now reach new markets [136,137].

7.4. Threats

• Environmental and regulatory issues: Microplastics, dyes, heavy metals, or volatile substances may be present in textile waste, especially from electronic trash or personal protective equipment. These could present occupational and environmental risks during production or pavement deterioration if strict quality control is not implemented [9,124,126].
• Knowledge and technology gaps: Industry trust is hampered by a lack of life cycle assessment studies and long-term field data. Broader deployment is hampered by uncertainty regarding microstructural deterioration, long-term ageing, and behaviour in harsh environments [5,16].
• Cultural barriers and market resistance: Because of risk aversion, a lack of training, or conservatism in specifications and codes, pavement stakeholders (such as road agencies, consultants, and contractors) may be reluctant to use unconventional materials [6,44,92].
• In other industries, waste from synthetic textiles can be used to make cement composites, insulation, fuel, or recycled textiles. For high-quality waste streams, these industries might be more competitive than the paving sector is [64,131].
• Reliance on streams of volatile waste: Long-term availability and supply chain resilience may be impacted by fluctuations in the supply of certain textile wastes (such as face masks and industrial aramid scraps) caused by world events or industrial trends [42,126].

According to the SWOT analysis, SCTWDMs have considerable potential to promote sustainable innovation in pavement engineering. The potential of these materials to improve pavement performance, lessen environmental burdens, and conform to the principles of the circular economy is highlighted by the strengths and opportunities that have been identified. These advantages are being bolstered by progressive environmental policies and advancements in fibre processing technologies. Nevertheless, significant obstacles still exist, mainly with respect to market acceptability, regulatory compliance, and variations in SCTWDM supply and quality. A coordinated effort, including thorough scientific study, the development of standardised testing and performance methods, and extensive demonstration projects, is needed to overcome these obstacles. The extensive use and long-term sustainability benefits of these waste-derived materials in asphalt pavement systems can be achieved only by means of such integrative approaches.

8. Economic and Environmental Performance Assessment of the SCTWDMs in Asphalt Pavement

An inventive approach to sustainable road infrastructure is the incorporation of SCTWDMs, such as disposable face masks and end-of-life tyre (ELT) fibres, into asphalt mixtures. This approach also addresses the growing environmental waste and performance issues associated with conventional pavement materials. The economic and environmental rationale for this technique has been extended by recent research. The life cycle cost effects of adding shredded face masks to HMAs were assessed by Massarra et al. [96]. According to their analysis, the long-term maintenance costs over a 40-year design life could be lowered by up to 29%, even though the initial construction costs for short sections (less than 500 m) remain comparatively stable. This was explained by the improved mechanical performance of the polymeric construction of face masks, especially in terms of stiffness and resistance to rutting, which are two important factors that affect pavement longevity.

Landi et al. [138] conducted a thorough life cycle assessment (LCA) in Italy, and standard, cellulose fibre-reinforced, and ELT fibre-reinforced HMAs were evaluated throughout a 30-year pavement lifespan from an environmental perspective. Because it improved the fatigue resistance by 70%, the ELT fibre-reinforced version was found to be the most environmentally friendly choice, with a 25% decrease in environmental impact compared to standard HMA. For several environmental indicators, such as cumulative energy demand (CED) and global warming potential (GWP), this extended service life resulted in fewer rehabilitation cycles and fewer cumulative impacts. The manufacturing and transportation stages of bitumen were shown to be the main sources of emissions, whereas the energy used for mixing and laying was essentially insignificant. Additionally, Zhang et al. [100] strengthened the environmental case by providing a life cycle emissions perspective on the reuse of COVID-19 disposable masks in HMA. Conventional disposal techniques, such as incineration, release 2.3 kg of CO₂ for every kilogram of mask, whereas landfilling results in significant encroachment on land resources. During the pandemic, the amount of mask trash produced worldwide peaked at 490,201 tonnes per month, which translates to approximately 1 million cubic metres of land occupation and over 1.1 million tonnes of CO₂ emissions per month. Zhang et al. reported that 941,397 face masks (3.77 tonnes) could be recycled into 1 km of asphalt pavement via a modified HMA blend (MF5% + WCO3%), which would save 0.77 m³ of land resources per kilometre paved and reduce CO₂ emissions by 8.66 tonnes.

Table 10. Economic and environmental assessments of SCTWDMs in asphalt pavement.

Mixture Type	Source	Assessment	Comment
Conventional mixture [138]	Natural	Highest impact on CED, GWP, and ReCiPe endpoints	Traditional mix with minimal mechanical and environmental advantages
ELT fibre-reinforced mixture [138]	Waste tyre	Approximately 25% reduction in CED and GWP, with superior ReCiPe midpoint and endpoint indicators	The optimal balance between sustainability and performance; longer pavement life
Waste face mask and cooking oil modified mix [100]	Polypropylene masks plus waste cooking oil	Approximately 8.66 tonnes of CO2 are saved per 1 km, along with 0.77 m3 of land resources	Strong combined benefit: waste valorisation and emission reduction
COVID-19 mask, modified asphalt [96]	Waste from face masks made of polypropylene	29% maintenance cost savings over 40-year life cycle; initial cost similar for <500 m sections; favourable beyond 6-lane, 500 m+ projects	Uses LCCA to demonstrate long-term cost-efficiency and promotes a change in policy for road construction towards a circular economy

shows several economic and environmental assessments of SCTWDMs in asphalt pavement.

Reusing synthetic textile waste, especially face masks and ELT fibres, in asphalt pavements is technically feasible, environmentally friendly, and economically appealing, as demonstrated by the convergence of LCA, LCCA, and laboratory-based performance evaluation throughout this research. A compelling case for integrating the circular economy into the road construction sector is supported by this multifaceted body of research. Standardised processing techniques, strong performance-based requirements, and pilot demonstration projects in various geographical and climatic contexts are necessary to realise this vision at scale. Translating laboratory innovations into standard infrastructure practices requires closing these gaps.

Circular Economy Potential and Contribution of SCTWDMs to the United Nations Sustainable Development Goals (SDGs)

The integration of SCTWDMs into asphalt pavement aligns with core principles of the circular economy by transforming nonbiodegradable, end-of-life SCTWDMs into value-added construction materials. As governments and industries shift toward zero-waste and carbon-neutral goals, textile waste utilisation in asphalt presents a scalable, eco-innovative solution that supports SDG 9 (Industry, Innovation, and Infrastructure), 11 (Sustainable Cities and Communities), 12 (Responsible Consumption and Production), and 13 (Climate Action) are the most notable goals that this innovation supports [139] (Figure 15). By supporting low-carbon technologies and alternative material use in pavement engineering, postconsumer textile fibres such as shredded face masks, textiles generated from tyres, and synthetic composites improve sustainable infrastructure and material innovation in line with SDG 9. It also lessens reliance on resources derived from virgin petroleum, boosts innovation in binder modification and reinforcement, and fortifies regional supply networks. By preventing the accumulation of solid waste and keeping nonbiodegradable synthetic textiles out of landfills or incinerated, these materials help achieve SDG 11 by creating more resilient and ecologically friendly transport infrastructure. Reusing textile waste also represents the core of SDG 12, which promotes resource efficiency, material circularity, and a reduction in the environmental imprint of road buildings. Environmentally speaking, using processed textile waste in place of bitumen or aggregates lowers energy use and emissions related to the extraction of raw materials and waste disposal, supporting SDG 13 (climate action) by reducing greenhouse gas emissions and improving the environmental performance of asphalt pavements. This approach considerably lowers the carbon emissions linked to waste incineration and asphalt production. The incorporation of synthetic fibres into road construction instead of their disposal in landfills improves pavement performance and lowers the overall carbon footprint, both of which significantly advance climate action objectives. The practice as a whole advances pavement engineering technology innovation while being in line with global sustainability concerns.

9. Conclusions

This paper provides a comprehensive summary of the state of research, technological developments, and sustainability issues related to the incorporation of SCTWDMs, such as waste face masks, tyre cords, polyolefins, aramids, and denim, into asphalt pavement systems. The findings provide several crucial insights:

• In terms of the mechanisms of SCTWDM action, the polymers mainly serve as microreinforcing agents, ensuring the proper transfer of stresses in the binder–aggregate system. However, differences in fibre structure, dispersion efficiency, and stability lead to considerable performance variations, especially at low temperatures, indicating the absence of uniform criteria for optimising their properties.
• The scientometric data indicate very rapid development of a scattered knowledge base, which is expressed in its thematic fragmentation, lack of interclustering communication, and weak technology transition from laboratory-scale innovations to practical implementations.
• On the basis of the principles of sustainability, the application of SCTWDMs can be considered an approach that can help achieve goals associated with the circular economy by diverting problematic textile waste streams and minimising dependency on virgin plasticisers. Nevertheless, the state-of-the-art status of SCTWDMs at present faces several limitations, which include (i) a lack of field validation; (ii) a lack of standards for design and testing; and (iii) material variability.
• SCTWDMs exhibit consistent performance enhancements, such as fatigue life, tensile strength, rutting resistance, and moisture resistance, which are contingent on the material composition and amount.
• The optimal fibre content lies between 0.2% and 0.5% by binder/mix mass, as exceeding this limit poses dispersion efficiency and workability problems.
• Micromechanical reinforcement remains the leading reinforcement technique, improving stress transfer, crack bridging, and binder–aggregate bonding, with limited instances of chemical alteration.
• Large variations can be seen among research methods in terms of fibre length (approximately 6–20 mm), dosage, and test methods. Although promising results have been achieved in the laboratory and environmental advantages (reduction in CO₂ emissions of 5% to 25%) can be derived from these materials, current research is still hindered by insufficient field validation, standardisation, and inadequate performance evaluations.

10. Research Gaps and Prospects

Despite a growing body of literature on the use of SCTWDMs in asphalt pavements, several critical research gaps remain, limiting their widespread implementation and standardisation. Bridging these gaps is critical for furthering the level of knowledge and realising the material’s transformational promise for sustainable infrastructure.

10.1. Research Gaps

• Fibre types and property standardisation: Most studies use SCTWDMs, which are readily available locally and have a wide range of physical, chemical, and thermal characteristics. Comparative analysis and repeatability are more difficult because of the lack of classification schemes and standardised fibre processing procedures. The creation of global standards for fibre pretreatment, sizing, and characterisation is urgently needed.
• Insufficient knowledge of the mechanisms of fibre–binder interactions: The molecular and microstructural interactions between various fibre types and bitumen matrices are still poorly understood, even though macroscale performance enhancements (such as rutting resistance and tensile strength) are well reported. It is still unclear how fibre chemistry, surface morphology, and thermal stability affect binder modification.
• Economic viability and supply chain integration: The logistics and scalability of locating, gathering, processing, and incorporating textile waste fibres into the commercial manufacturing of asphalt are not well studied. There is a lack of a technoeconomic analysis framework, especially when comparing costs with those of traditional modifiers.
• Limitations of the policy and regulatory framework: Most regions still lack design specifications and regulatory acceptance for asphalt modified with textile waste. Adoption is hampered by the absence of market incentives, supportive legislation, or inclusion in national road construction standards.
• Insufficient long-term durability and ageing studies: The majority of performance reviews are laboratory-based and have a short lifespan. The behaviour of SCTWDM-modified pavements under actual ageing conditions, including oxidation, UV exposure, freeze–thaw cycles, and traffic stress, is not well understood. Accelerated ageing techniques and longitudinal field experiments are needed.
• Trade-offs between the environment and LCA: Despite being marketed as environmentally friendly, SCTWDMs currently rarely have thorough LCAs that measure their energy usage, carbon impact, and possibilities for waste diversion. Furthermore, little research has been conducted on the trade-offs associated with end-of-life pavement recycling or microplastic leaching.

10.2. Prospects for Further Research

The following research priorities are suggested to fill these gaps and unleash the sustainable innovation potential of SCTWFs in asphalt pavement:

• Creation of Material Libraries and Fibre Taxonomy: Create a global classification database of waste textile fibres used in asphalt, along with standardised procedures for performance benchmarking, pretreatment, and characterisation.
• Digital Twin and AI-Driven Optimisation: Utilise machine learning and digital construction technologies to model fibre dispersion, forecast performance results, and optimise mix designs for a range of road classes and loading scenarios.
• Policy Involvement and Certification Routes: Work together with industry associations, environmental authorities, and road agencies to create certification standards, performance-based requirements, and roadmaps for the circular economy.
• Multiscale Characterisation Techniques: Examine fibre–binder interactions at the nanoscale, microscale, and macroscale sizes via sophisticated spectroscopy, microscopy, and rheological instruments. To predict compatibility and improve formulations, molecular dynamics simulations should be used.
• Long-term Field Trials and Monitoring: To assess the in-service performance of SCTWF-modified pavements under various traffic and climate situations, instrumented test sections and pilot demonstration projects should be conducted.
• Technoeconomic and Integrated Life Cycle Assessment (LCA) Models: Integrate economic and environmental performance into comprehensive models to evaluate feasibility. These ought to take into account supply logistics, end-of-life effects, emissions reductions, and energy inputs.