Framework for Development of Best Practices for Low-Volume Road Asphalt Pavements—A Roadmap to Increase Recycling

Abstract

The overall goal of this study is to synthesize the existing literature on mix design approaches and to develop recommendations for the best practices for the design of asphalt mixtures specific to LVRs. The synthesis of best practices encompasses material characterization, performance evaluation techniques, and recommendations for construction and maintenance practices. This review suggests the need for further laboratory and field testing to enhance performance measures, explore sustainable materials and construction practices, and develop standardized specifications for the diverse needs of low-volume road networks. The recommended changes (or guidelines) include, but are not limited to, updated recycled asphalt pavement (RAP) percentages as per current law requirements, the addition of performance tests (IDEAL-CT and IDEAL-RT), RAP content, design methodology, volumetrics, and design gyrations. The review suggests the need for further laboratory and field testing, including performance testing, long-term performance assessments in various conditions, and improved methodologies for evaluating testing parameters. These enhancements aim to ensure more reliable performance predictions and the better implementation of LVR technologies. Overall, this study will help agencies and the paving industry to understand the updates made to the current LVR specifications and evaluate the mix design considerations for low-volume roads.

1. Introduction

In transportation engineering, it has become increasingly imperative to focus on optimizing infrastructure design and materials, especially when addressing the unique challenges posed by low-volume roads (LVRs) that experience 300,000 equivalent single-axle loads (ESALs) or lower throughout their service lives [1]. LVRs play a vital role in providing connectivity and access to rural and less densely populated areas [2]. One critical aspect of ensuring the longevity, functionality, and cost-effectiveness of LVRs lies in the formulation of appropriate mix design specifications tailored to their specific requirements [3]. The significance of LVRs cannot be overstated, particularly in rural and remote areas where they serve as lifelines for transportation, commerce, and social interaction [4]. However, the design and construction of pavements for low-volume roadways present distinct issues when compared to high-volume highways and urban thoroughfares [5]. Factors such as limited traffic loads, varying environmental conditions, and the availability of local materials necessitate a nuanced approach to mix design that accounts for these specific conditions. Low-volume roads serve as crucial connectors for rural areas, including local communities and agricultural activities, and carry a low traffic volume as compared to urban roads and highways.

The design and maintenance of LVRs require different considerations compared to high-volume roads (HVRs), as they experience lower stress levels but often face budget constraints and material availability issues. Hence, the incorporation of a pavement material that is not only sustainable but also economical can serve this purpose. The increasing need for sustainable infrastructure has driven interest in the use of recycled asphalt pavement (RAP) material, which consists of reprocessed pavement materials containing asphalt and aggregates in road construction [6]. The use of RAP in fresh asphalt mixtures can reduce the consumption of virgin materials, limits waste accumulation in landfills, and lowers the environmental impacts associated with asphalt production and construction [7]. The typical RAP content in HVRs lies within the range of 10% to 15% to ensure that the pavement can withstand high stress and frequent loading cycles, but it requires more stringent performance criteria due to heavy and constant traffic loads [8]. The issue of high variability and significant asphalt binder aging is a matter of great concern in RAP used for LVRs [9]. According to Li et al. [10], the inclusion of RAP in LVRs results in increased variability in test outcomes, particularly with decreasing temperatures. In 2011, the Federal Highway Administration (FHWA) categorized asphalt concrete that contains more than 25% RAP by total weight of the mix as high-content RAP mixtures [11]. Moreover, research conducted by McDaniel et al. [12] suggests that adjustments to the asphalt binder grade are necessary for specific geographic areas when the RAP contents exceed 15% in LVRs. Similarly, Al-Qadi et al. [13] demonstrated significant changes in asphalt binder properties when RAP binder contents exceed 20% in LVRs. In addition, Lee et al. [14] found, through testing the asphalt binder in a dynamic shear rheometer, that increasing the RAP binder leads to increased binder stiffness and brittle behavior. The volumetric properties suggested by the conventional mix design procedure may not yield optimal performance, as a significant portion of the aggregates consists of RAP, which is coated with a layer of aged asphalt binder [15]. Variations in RAP applications and specifications can arise from different regulatory frameworks, engineering guidelines, and environmental considerations. Weather conditions such as temperature extremes, moisture variations, and freezing–thawing directly influence the performance of RAP; hence, it is necessary to tailor the specifications to account for climatic impacts.

The choice of materials, including aggregates, binders, and additives, significantly influences the performance and durability of LVRs. Studies suggest that locally available materials should be prioritized to enhance sustainability and reduce construction costs. LVRs must meet specific performance criteria concerning strength, stability, and durability. Various mix design methodologies, such as the Marshall method and Superpave, have been proposed for LVRs. However, a limited consensus exists regarding the most suitable methodology for different conditions and applications. As with low-volume roads, roads with fewer vehicles per day and low cumulative equivalent single-axle loads (ESALs) in the design period have durability issues that are more significant than stability issues. After the introduction of the Superpave system in recent years, the durability issue has become even more prevalent. Most studies indicate that the asphalt content has been reduced compared to the asphalt used before Superpave was introduced. As a result, the durability of mixes is currently the primary concern in developing a mix design system for low-volume roads. It is essential that the material display adequate durability to resist the effects of loads and the environment, as well as to prevent high maintenance costs. These mixes must, however, also be stable enough to resist excessive deformation during construction since low-volume pavements are usually constructed with conventional paving and rolling equipment. Additionally, mixes for low-volume roads should be compactable with standard construction equipment to ensure proper density. Therefore, the ideal mix for low-volume pavement must be easy to lay down and compact, durable, and strong enough to be used by construction workers and vehicles.

Different agencies across the nation employ similar yet different specifications for low-volume roads, mainly in material selection, mix design and gradation requirements, mix design methods, the use of recycled materials, acceptance testing criteria, and pavement preservation treatments. The existing literature emphasizes the importance of achieving a balance between performance, cost-effectiveness, and sustainability in mix design. The literature synthesis also reveals a growing emphasis on incorporating recycled materials such as reclaimed asphalt pavement (RAP) and reclaimed asphalt shingles (RASs) to mitigate environmental impact and conserve natural resources. The recommended changes (or guidelines) include, but are not limited to, updated RAP percentages as per the current law requirements, the addition of performance tests (IDEAL-CT and IDEAL-RT), criteria for minimum quality control sampling and test frequency, and volumetric requirements (e.g., Voids Filled with Asphalt) for high-RAP HMA mixtures.

The development of mix design specifications for low-volume roads remains a critical but understudied aspect of pavement engineering. This study undertakes a comprehensive literature review to evaluate state-of-the-art research related to low-volume road networks. By synthesizing existing knowledge from a wide range of sources, this study provides a holistic understanding of the opportunities and challenges associated with low-volume road pavements. Moreover, the variability in material properties, traffic conditions, and environmental factors necessitates a systematic approach to LVR mix design that accounts for these nuances while maximizing performance and sustainability. Addressing this gap is essential to unlock the full potential of RAP in low-volume road construction, enabling transportation agencies and practitioners to make informed decisions and achieve the dual objectives of cost-effectiveness and environmental sustainability. Optimized LVR mix design parameters (or acceptance criteria) may provide long-term crack-resistant, cost-effective, and longer-lasting pavements for parking lots, residential areas, scenic byways, and other applications in low-traffic areas. Overall, this study will help DOT personnel and contractors understand the updates made to the current low-volume road specifications, maintain uniformity in quality control sampling frequency, and evaluate the mix design considerations for low-volume roads compared to medium- or high-volume roads across the United States.

2. Goals and Objectives

This paper aims to recommend the best practices and specifications for the design of LVR asphalt mixtures. These recommendations are made as a result of a comprehensive review of the existing literature and interviews with subject matter experts. This study aimed to review existing specifications and recommend best practices for the design of asphalt mixtures for low-volume roads (LVRs). The specific objectives of this study included the following:

• Review the available literature pertaining to asphalt mix design specifications for state highway agencies (SHAs). This review focuses on asphalt mix design requirements and acceptance criteria for different bituminous materials, such as HMA for LVRs. Composition, gradation, performance tests, sampling frequency, and effects of significant parameters for different SHAs are examples of data collected and contrasted.
• Interview subject matter experts across the United States to discuss procedures for designing asphalt mixtures for LVRs and determine differences in the mixes for LVRs and other types. The topics covered by these interviews include current practices for LVR mix design methods, field-testing acceptance criteria for LVRs, quality assurance and quality control requirements, and overall observations for LVRs.
• Compare and evaluate important parameters to identify the best practices of mix design approaches, such as traffic volume, material selection, and the use of recycled materials, mix design methods, and quality control testing for LVR requirements.

3. Comparison of State Highway Agencies’ LVRS Specifications

The review of state highway agencies’ LVR specifications aimed to collect, catalog, and summarize information related to existing mix design requirements. The comprehensive literature synthesis consisted of several mix designs and associated parameters, including the definition of LVRs, the use of recycled materials, including allowed RAP content, gradation limits, design gyrations (N_design), mix design methodologies, and laboratory testing for the performance evaluation of mixtures. Based on the literature synthesis, the parameters in the mix design requirements were compared to identify the best practices and different areas in the existing protocols. The comparison of these parameters among SHAs is summarized in the following subsections.

3.1. Definition of LVRs

In the last decade, low-volume roadways have become significantly important as these roads serve transportation needs and boost a region’s socioeconomic standing. Low-volume roads serve as a fundamental component of the infrastructure, providing essential transportation for daily activities and playing a crucial role in sustaining the economic life of rural communities in the United States [4]. A committee on LVRs defined LVRs as those with fewer than 500 vehicles per day (vpds) at the 1975 International Conference on Low-Volume Roads in Boise, Idaho [16]. There are, however, differences in state definitions of LVRs. Various states have defined low-volume road (LVR) traffic based on vehicles per day (vpds) and equivalent standard axle loads (ESALs). Based on the literature, there are many definitions of LVRs, but it is suggested that the definition be consistent with Superpave and AASHTO, which is a less than 0.3 million design ESALs for 20 years of service life. There are about 20 states which define LVRs with roads of 400 vpds in line with the AASHTO 2001 and FHWA 2009 specifications. Six states, including Alabama, Iowa, Kansas, Connecticut, Oregon, and New Jersey, have adopted a Superpave definition for low-volume roads < 0.3 million design ESALs for 20 years of service life. Only three states (Arkansas, Georgia, and Massachusetts) have implemented the definition of LVRs per AASHTO 2019—2nd edition. The remaining 18 states define LVRs with various AADT, ADT, and vpd combinations. Figure 1 summarizes the definition of LVRs adopted by various SHAs.

3.2. Recycled Materials and RAP Content

While the use of RAP is well established for high-traffic roadways, its application in LVRs presents unique challenges and opportunities. The presence of RAP affects the gradation, binder content, and stiffness of an asphalt mixture. Therefore, mix design considers the properties of the RAP material and adjusts the virgin aggregate gradation, binder content, and additive usage accordingly to achieve the desired performance. The drawbacks associated with a high RAP content in asphalt mixtures, such as low cracking and moisture resistance, restrict the use of a high RAP content for low-volume roads owing to the aged binder [17]. The stiffness provided by the aged binder present in the RAP material can be compensated by adding a rejuvenator to the asphalt mixtures. Although mixtures with rejuvenators meet design specifications, they may initially appear dry and under-lubricated after mixing. To improve their visual appearance, field personnel may add extra virgin asphalt [18,19]. The use of softer binders and/or recycling agents can prove beneficial, even in high-RAP mixtures [20]. However, the degree of blending between aged and virgin asphalt in recycled mixtures also significantly affects the performance of asphalt mixtures, especially the fatigue and low-temperature cracking resistance [21]. The blending efficiency is affected by the degree of coating as well as the interaction between the degree of coating and the asphalt binder [22]. The common assumption in RAP mix design is the full blending of an aged RAP binder and virgin binder, which may not be true as many recycled mixtures are only partially blended; therefore, more precise design approaches by considering complex blending dynamics should be practiced [18]. The maximum limit for RAP content produced in hot mix asphalt batch facilities is usually accepted to be 50%, which is limited by both plant heat capacity and gaseous hydrocarbon emissions [23]. Most of the states (37 out of 50) allow for the use of RAP lower than 50%, while a few states (about 13 out of 50) allow for more than 50% (up to 70% or 100% as well in some cases). Figure 2 summarizes the allowed RAP content in LVRs adopted by various SHAs.

3.3. Design Gyrations (N_Design)

It was observed that the Superpave mix design approach, which most states have adopted, provides a guideline for the number of gyrations used during the mix design process for HMA [24]. The North Dakota DOT has found that 75 gyrations may be excessive for LVRs, potentially reducing the durability of the asphalt mix. A few other states (such as New England) follow similar adjustments for LVRs, where specific numbers of gyrations (fewer gyrations) are set based on local traffic conditions and environmental factors to increase asphalt binder content, enhancing durability against environmental stresses rather than traffic load. For LVRs, there are about 14 states which have adopted design gyrations of 50. Seven states, including Alaska, Delaware, Kentucky, Mississippi, Ohio, Tennessee, and Indiana, have adopted 75 design gyrations for LVRs for a 20-year service life. A few states, such as Arizona, Illinois, New York, Pennsylvania, Wisconsin, Georgia, Texas, and Oklahoma, have adopted a range for design gyrations, e.g., 50–60 or 50–75 [25]. The remaining 21 states have adopted 60 to 65 design gyrations based on their traffic and climate conditions. Figure 3 summarizes the design gyrations adopted by various SHAs.

Table 1. Recommendations for low-volume roads.

Parameter	Recommendation
Design gyrations	50–65
RAP content	26% to 50%
Design methodology	Superpave mix design
Gradation limits	4.75 to 37.5 NMAS
Performance criteria	IDEAL-CT and IDEAL-RT
Volumetrics	VFAa 70–80%

provides a summary of the N_design levels for different traffic volumes for various SHAs.

3.4. Gradation Limits

The different DOT road specifications recommend a wide range of nominal maximum aggregate sizes ranging from 4.75 mm to 37.5 mm. Ohio DOT (ODOT) increments the lower limits for Sieve no. 8 from 42% to 47% and 27% to 32% for Sieve no. 16 in the case of LVRs [26]. Also, the percent passing requirements for sieve no. 50 is reduced from 10% to 8% [27]. Many state agencies allow for the use of recycled materials in their preferred base course aggregate. Accordingly, Minnesota, as well as five other states, including Indiana, Iowa, Michigan, Wisconsin, and Wyoming, require aggregate gradation blends to be at most 1.5 inches in size. Five states require a minimum crushed content in terms of percentage passing a specific sieve size: Indiana, Montana, South Dakota, and Wyoming all require a no. 4 sieve (4.75 mm), while Iowa only requires a 3/8-inch sieve [28]. Minnesota and Michigan specify a percentage of the maximum aggregate size. The DOTs of Indiana, Iowa, Michigan, and Wisconsin specifically permit the use of recycled asphalt pavement, concrete pavement, or industrial byproducts in some combination. Minnesota favors virgin aggregate in these applications, and Wyoming DOT accepts only virgin or natural materials.

3.5. Design Methodologies

The Marshall mix design and Superpave mix design have been used by different DOTs across the country for low-volume roads. Prowell and Haddock [29] recommend using lower laboratory compaction levels than those traditionally used for low-volume roads in Virginia using the Marshall method of mix design. Cross [30] discovered that the level of compaction using the gyratory compactor was greater than the 50-blow Marshall compaction effort. Kansas DOT has compared both the Marshall and Superpave mixtures for low-volume roads. The Superpave mix consists of a PG 58–22 binder with a nominal maximum size of 19 mm using three different aggregates. The Marshall mix design involves five different asphalt contents compacted with 50 blows per face for each blend. This study concluded that the estimated asphalt content was lower when using the Superpave mix design for low-volume roads than the Marshall method [16]. One the other hand, Vitillo et al. [31] compared the new Superpave mixtures’ composition (gradations and binder content) with that of the proven Marshall mixtures developed for low-volume roads in New Jersey. This study contradicted the previous ones and found that the Superpave design for low-volume roads had a higher optimum asphalt binder content compared to the Marshall mix design.

3.6. Volumetrics

3.6.1. Voids Filled with Asphalt (VFAs)

DOTs across the country have given different volumetrics criteria concerning low-volume roads. For example, Kansas DOT recommended voids filled with asphalt (VFAs) in the range of 75% to 80%. Indiana DOT recommended the PG 64-22 binder for low-volume roads corresponding to less than 0.3 M ESALs of traffic, whereas the VFA criteria were set to 70–80%. The Minnesota DOT suggests voids filled with asphalt in the range of 72–80% for low-volume roads compared to 69–80% in high-volume roads [32]. The Pennsylvania DOT developed guidelines for low-volume roadways using warm mix asphalt and recommended 73–80% VFAs for 9.5 mm mix, 77% for 19 mm mix, and 69–81% for 25 mm mix.

3.6.2. Voids in Mineral Aggregates (VMAs)

Voids in Mineral Aggregates (VMAs), explained as a volumetric property and a function of compaction effort, have traditionally been the primary issue in meeting the mix requirements. Cross [30] suggested that the VMA requirements can be reduced by 0.5% to 1% while maintaining the performance of low-volume pavements. [33] investigated LVR mix design criteria in the New England region, specifically Connecticut, and recommended a design VMA of 16% based on the results of performance tests for producing durable and stable LVR mixes. The target VMAs for low-volume roads are opposed to 13.5% at design gyrations for Minnesota. The Pennsylvania DOT has suggested minimum VMA values of 13% for modified high-RAP mixes. AASHTO has a minimum VMA requirement of 16% for 4.75 mm mixes, and the Superpave criteria have a maximum VMA of 18%. Williams determined the critical VMAs value from the relationship with dust content to be 16%, and this matches the AASHTO and Superpave criteria. The Kansas DOT recommended minimum voids in mineral aggregates (VMAs) of 13%.

3.7. Laboratory Tests for Performance Evaluation

3.7.1. Asphalt Pavement Analyzer (APA)

The New England Transportation Consortium in Connecticut recommended the asphalt pavement analyzer (APA) as proof testing apparatus for cores from high-, moderate-, and low-volume roads to determine their rut depths obtained at 4000 cycles [33]. Similarly, the APA was recommended by North Dakota DOT for performance testing [25]. A maximum rut depth of 8 mm after 8000 cycles was suggested by the state agency of Ohio using the asphalt pavement analyzer.

3.7.2. Hamburg Wheel Tracking Test (HWTT)

The HWTT is majorly used by different DOTs such as California, Colorado, Georgia, Iowa, Illinois, Louisiana, Maine, Massachusetts, Montana, Oklahoma, Texas, Utah, and Washington to evaluate rutting resistance corresponding to a different number of passes; however, rutting failure is generally considered as 12.5 mm rut depth.

3.7.3. Indirect Tension Asphalt Cracking Test (IDEAL-CT)

As per NCHRP Project 20–44, the IDEAL-CT and IDEAL-RT are a coherent framework for the development of pavement materials [34]. This study recommended that the IDEAL-CT be used for the production of QC/QA testing [35]. The IDEAL-CT is being adopted by 14 DOTs as their cracking test, and the IDEAL-RT is being balloted in ASTM WK71466 as their rutting test. The Ohio DOT recommended the minimum CT_Index value for IDEAL-CT as 120.

3.7.4. Indirect Tension Asphalt Rutting Test (IDEAL-RT)

As per NCHRP Project 20–44(16): Implementation of IDEAL Cracking Test for Asphalt Mix Design QC/QA, it is recommended as an alternative rutting test for balanced mix design rutting performance evaluation and production QC/QA testing. HWTT rutting parameters are commonly used by many DOTs, and associated acceptance criteria are well established. To develop a relationship between the IDEAL-RT and the HWTT, 23 dense-graded mixtures were evaluated. Based on the relationship and the existing HWTT acceptance criteria, the research team calculated the RT_Index values corresponding to the HWTT rutting criteria for mixtures with PG64-XX, PG70-XX, and PG76-XX. For mixtures with PG64-XX (or lower) and PG70-XX (or higher) with 95 percent confidence, RT_Index is 60, and for mixtures with PG76-XX (or higher) with 98 percent confidence, RT_Index is 75. In addition to these, several other tests such as the modified Lottman test, Texas overlay tester, asphalt binder cracking device, and semi-circular bend test have also been used by different agencies [27].

4. Interviews with Subject Matter Experts (SMEs)

Ten experts from different states took part in videoconference interviews to discuss the processes involved in designing asphalt mixtures for low-volume roads (LVRs) and to highlight the key differences between mixes designed for LVRs and those used for medium- and high-volume roads. The experts were selected from different sectors such as academia, were research scientists, and from different counties to cover a broader domain. The findings from the interviews offered a good understanding of SHA needs and helped identify best practices for asphalt mix design approaches to meet the current LVR specifications. The topics covered by these interviews included current practices for LVRs mix design and gradation requirements, material selection criteria, performance testing criteria, field-testing acceptance criteria for LVRs, quality assurance, quality control requirements, and overall observations or challenges for LVRs.

The following sections present the interview questionnaire that was sent to different highway agencies in the US. The topics covered by this interview questionnaire include current practices for LVRs mix design methods, field-testing acceptance criteria for LVRs, quality assurance and quality control requirements, and overall observations for LVRs. The main objectives of the survey were to (a) collect information pertaining to LVRs mix design specifications from different highway agencies in the US and (b) recommend draft revisions, if necessary, to the current LVRs mix design specifications. The interviews were conducted via a phone call or a videoconference and took around 30–45 min to complete. The main questions were the following: (a) what is the definition of “low-volume roads” in your state? (e.g., AADT/ADT/ESALs); (b) what are the typical traffic patterns or usage characteristics observed on LVRs?; (c) what are the differences, if any, between designing asphalt mixtures for LVRs and heavy traffic roadways?; (d) are there special gradation requirements for LVR mixtures?; (e) specific design requirements (design gyrations/blow, air voids, min. binder content, others); (f) what is the maximum allowable RAP/RAS percentage in LVR mixtures?; (g) what performance testing do you employ, if any, as part of the mix design process of LVR mixtures?; (h) what binder grade is typically used for designing LVR mixtures?; and (i) what are the main challenges or issues associated with maintaining LVRs and monitoring their performance in your state?

The summary of the findings and lessons learned from these interviews established an understating of state highway agencies’ needs and provided recommendations on best practices for mix design specifications of LVR asphalt mixtures. The interview findings on significant parameters and overall lessons learned from them are discussed as follows:

• The ESAL thresholds for LVRs range from 10,000 to 50,000 ESALs per year, but these figures vary and can go up to 100,000 ESALs to 300,000 ESALs per year. The classification of a road as low-volume depends on various factors, including local standards, the purpose of the road, and the available infrastructure.
• A typical LVR has about 12 to 15% of trucks of various sizes (e.g., semi-tractors, dump trucks, agricultural vehicles, or trucks carrying agricultural equipment and other heavier/larger vehicles). By analyzing the current traffic patterns and characteristics, the experts suggested developing effective strategies for the design, maintenance, and management of LVRs to ensure that these roads meet the needs of their users while promoting safety and efficiency.
• A few states (e.g., New York, Ohio) have already started to use more cold mixes compared to hot mixes as the cold mixes have a “little” richer asphalt binder, which might be useful in the growth of mixture design for LVRs. The life of low-volume pavements is relatively short, at around 15–20 years. Several local agencies mostly consider about 10 years of service life for LVRs.
• The gradation and mix design requirements for LVRs involve careful consideration of local materials, aggregate gradation, binder selection, mix design methods, thickness design, and adequate drainage considerations (or coefficient).
• The selection of binder grade is closely tied to the desired performance characteristics of the pavement, including rutting resistance, fatigue resistance, and cracking resistance. The experts stressed the importance of considering the local climate when selecting the binder grade for low-volume mixes.
• The use of rejuvenators in the mix design of LVRs is seen as a valuable strategy to address aging issues, enhance binder properties, and improve the overall performance and sustainability of asphalt pavements. Careful consideration of rejuvenator selection, dosage, and compatibility within the mix design process is crucial for achieving optimal results in LVR construction.
• The experts recommended field testing, such as density tests (e.g., nuclear density gauge testing or sand cone testing), at regular intervals across the project site to identify areas of inadequate compaction and verify the in-place density is meeting the specified requirements for LVRs. Field testing and acceptance criteria for the mix designs of LVRs encompass a comprehensive range of assessments, including compaction, smoothness (e.g., accepted IRI-160 inch/mile, baseline IRI-280 inch/mile), density, distress evaluation, skid resistance, moisture susceptibility, and long-term performance monitoring.

5. Recommendations

The motivation for this study was related to providing an understanding of optimized asphalt mix design specifications for LVRs to help construct long-term crack-resistant, cost-effective, and longer-lasting pavements. A comprehensive literature review was conducted to evaluate potential parameters of asphalt mix design for LVRs that can be improved in current specifications by reviewing definitions of LVRs adopted by various states, applications in various states, factors affecting asphalt mix design such as gradation requirements, RAP content, mix design methodologies, volumetrics, and case studies employed by state highway agencies (SHAs). Additionally, a set of questions was prepared to discuss the procedures for designing asphalt mixtures for LVRs and to determine the main differences between designing asphalt mixes for LVRs and those designed for medium/high-volume roads. Based on the literature review and interviews, the following recommendations were drawn.

5.1. Design Gyrations

Based on the data analysis from all states, the emphasis has been on using lower gyration. Hence, the design gyrations for low-volume roads can be selected as 50–65, as compared to 75 design gyrations in typical high-volume roads, which may result in reduced durability of the asphalt mixtures owing to over-compaction.

5.2. RAP Content

Based on the literature review and interview with subject matter experts, a RAP content of 26% to 50% can be recommended for the construction of low-volume roads.

5.3. Design Methodology

Departments of Transportation (DOTs) nationwide have employed both the Marshall mix design and the Superpave mix design for low-volume roads. However, the Superpave mix design can be recommended based on compaction level (50 design gyrations) and lower asphalt binder content (5.1–5.7).

5.4. Gradation Limits

Based on the literature review, the same nominal maximum aggregate sizes (NMASs) used for high-volume roads ranging from 4.75 mm to 37.5 mm can be used for low-volume roads.

5.5. Performance Testing

Experimental trials in the laboratory must be conducted to validate and refine the proposed mix design specifications for additional recommended tests (indirect tensile asphalt cracking test (IDEAL-CT) and indirect tensile asphalt rutting test (IDEAL-RT)) to increase the percentage of RAP for LVRs. The criteria for IDEAL-CT and IDEAL-RT could be verified and evaluated through laboratory testing and evaluation of field performance.

5.6. Volumetrics

The range of volume filled with asphalt (VFA) in low-volume roads can be recommended to span from 70% to 80% as per data obtained from different departments of transportation.

5.7. Binder Grade

The selection of binder grade for designing low-volume mixes involves a nuanced consideration of factors such as climate, traffic loading, flexibility, ease of construction, and long-term performance (e.g., PG64-22, 64-10, 76-10–low-volume mixes; PG76-28 modified polymer–HVRs mixes).

Overall, SHAs’ specifications for asphalt mixtures on LVRs reflect a commitment to promoting performance-based design principles, ensuring the longevity and reliability of road infrastructure, and addressing the unique challenges associated with low-volume road networks. The ongoing efforts focus on refining current specifications, adopting innovative technologies, and promoting best practices to enhance the performance and longevity of low-volume road pavements across the nation. The overall recommendations are summarized in Table 1.

The recommendations presented in this article are the outcomes from a rigorous review of the existing literature, as thoroughly discussed within the manuscript, particularly in the context of low-volume roads. This study not only addresses the various aspects of testing methodologies but also provides a comprehensive overview of current practices adopted by multiple agencies across the United States. Accordingly, the proposed suggestions are not only theoretically sound but also demonstrate strong potential for implementation in real-world engineering scenarios, making them both practical and applicable within the industry.

6. Conclusions

This study aims to conduct a comprehensive analysis of existing mix design methodologies and formulate best practice guidelines related to the design of asphalt mixtures for low-volume roads. Overall, the current state of the practice regarding the design of asphalt mixtures for LVRs reflects a multifaceted approach, considering the requirements and constraints of these roadways, and the important findings from this literature synthesis include the following:

• The incorporation of reclaimed asphalt pavement (RAP) material should be prioritized in the mix design of low-volume roads, resulting in enhanced sustainability and cost efficiency.
• The durability and cracking resistance of asphalt mixtures can be enhanced by optimizing asphalt content and adjusting the gyrations between 50 and 65 by preventing over-compaction.
• The mix design for low-volume roads should integrate performance-based specifications that prioritize durability, flexibility, and cost-effectiveness. Performance tests like IDEAL-CT and IDEAL-RT can be employed to define general specifications for low-volume roads.
• Long-Term Impacts on Sustainable Practices in Pavements: The current study presents a comprehensive literature review concerning existing specifications of different state highway agencies regarding low-volume roads. The synthesis of research on low-volume roads holds vast potential in future studies for improving durability, sustainability, and cost-effectiveness. This study will serve as a catalyst to use higher percentages of RAP in materials and hence take a big step towards achieving sustainable practices.