Thinner pavements vs. longer lifespans
By Dr. Jhony Habbouche, Ph.D., P.E.
Asphalt has been the backbone of our roads since the early 1900s, but as traffic volume, axle loads and tire pressures have surged over the decades, the demands on this material have increased rapidly.
The flexible pavement engineering community hasn’t just kept pace – they’ve led the charge. By embracing cutting-edge technologies in asphalt binder production, advanced testing methods, and state-of-the-art construction equipment, they’ve revolutionized how we build our roads.
The challenge has always been about striking the perfect balance: creating asphalt mixtures that are stiff enough to resist rutting and shoving while being flexible enough to prevent any type of cracking. The game changer? Modified asphalt binders. These innovations have given transportation agencies the tools to design asphalt mixtures that stand up to the toughest conditions, offering not just durability but also longevity.
Modifying asphalt binders is far from a new idea, it’s a practice that’s been gaining traction over the past several decades. For the last 50 years, engineers have enhanced asphalt binders with a range of modifiers, from polymers and ground tire rubber to chemical agents like acids and even recycled engine oils, all in pursuit of achieving the perfect mix of properties.
For instance, SBS (styrene-butadiene-styrene) and SBR (styrene-butadiene rubber) have become staples in asphalt mixtures, known for their elasticity and recyclability. Polymer-modified asphalt (PMA) binders, enhanced with SBS, SBR or other engineered modifiers, consistently outperform nonmodified neat binders across all temperatures.
In 2005, the Asphalt Institute published IS-215 and ER-215, “Quantifying the Effects of PMA for Reducing Pavement Distress,” comparing the performance of neat HMA mixtures to that of companion sections built with one or more layers of PMA mixtures. The results showed how a typical maintenance and rehabilitation schedule for unmodified pavements could be extended with the use of PMA in just the wearing course or both the wearing course and base layers.
In 2018, the National Asphalt Pavement Association (NAPA) conducted a survey to assess the adoption of PMA binders in dense-graded asphalt mixtures used for pavement structural layers. The results revealed that 96 percent of the 47 responding agencies reported using PMA binders, each employing a range of asphalt binder specifications.
Introducing HP asphalt binders – a spectrum of definitions
PMA binders have traditionally been limited to a polymer content of around 3.5 percent by total weight of binder due to potential production issues such as binder pump clogging and reduced mixture workability. While PMA binders have consistently demonstrated improved long-term performance, recent research suggests that asphalt binders with higher polymer content (exceeding 3-3.5 percent) could provide additional benefits, particularly for flexible pavements exposed to heavy and slow-moving traffic loads.
A breakthrough in polymer technology now allows for its use at levels of 7-8 percent by total weight of binder. This innovation effectively doubles to triples the polymer content in the binder, leading to significantly enhanced elasticity. The resulting high polymer-modified (HP) binders are showing promise in addressing and mitigating several pavement failure modes of concern to transportation agencies.
In 2018, while I was part of the research team at the University of Nevada Reno (UNR), we conducted a critical review of HP binders and mixtures for the Florida Department of Transportation (DOT). The review concentrated on three key areas: laboratory evaluation of HP binders and mixtures, performance assessment of pavement sections using HP mixtures and techniques for assessing the structural capacity of these mixtures in pavement design.
The laboratory studies reviewed demonstrated that increasing polymer content consistently improves the performance of asphalt binders and mixtures, particularly in resisting oxidative aging and various forms of cracking. Several field projects across the United States, Canada, South America, Europe and Australia have been constructed to evaluate the performance of HP mixtures. These projects have demonstrated the versatility of HP mixtures, which have been successfully applied in a range of applications – from full-depth asphalt concrete layers to thin asphalt overlays under heavy traffic conditions, including intersections with slow braking loads. Importantly, no significant construction-related issues were reported concerning mixing temperatures or in-place compaction, with standard construction practices and equipment being used. The early field performance of these projects was encouraging, although information on long-term performance remained limited at that time.
A key question that emerged during this research was: “How are HP binders defined?” Specifically, “Is there a formal definition for HP binders and mixtures, and if so, is it related to a specific polymer content threshold?”
During my tenure at the Virginia Transportation Research Council (VTRC), the research division of Virginia DOT, I led a study on the laboratory and field performance evaluation of pavement sections with HP overlays. As part of this study, we aimed to document the state of practice for HP binders and mixtures across North America, with a particular focus on the United States. Among the agencies surveyed, 21 reported involvements in or construction of pilot projects using HP mixtures. When asked to define “HP binders” according to their specifications, most agencies emphasized specific binder rheology parameters and performance characteristics rather than polymer content, reflecting a more performance-oriented approach. The special provisions and specifications highlighted in the study underscore the importance of high elasticity and recovery in HP binders to ensure superior performance under heavy, stress-concentrated, or slow-moving traffic conditions. Despite these advancements, a significant gap remains: “How should agencies with no prior experience with HP binders establish a definition for them?” This underscores the ongoing challenge of formalizing a universal definition for HP binders and mixtures across different states.
Revisiting pavement design using innovative technologies
The legacy of AASHTO 1993 in modern pavement design
Despite the advancements in paving materials, many state DOTs – including Florida, Ohio and Alabama – continue to rely on the American Association of State Highway and Transportation Officials (AASHTO) Guide for designing new and rehabilitated highway pavements. The origins of this guide trace back to the AASHO Road Test conducted from 1958 to 1960 near Ottawa, Illinois, which evaluated pavement deterioration under traffic loads. Since then, several versions of the guide have been released, with the AASHTO 1993 Guide still widely used by transportation agencies today.
A key component of the AASHTO 1993 Guide is the calculation of the structural number for a given pavement layer, which is influenced by the “structural layer coefficient.” These coefficients are typically derived from factors such as material stiffness and compressive or tensile strength. However, the coefficients commonly used in pavement design are based on limited data from the original AASHO Road Test, which involved a single type of subgrade, gravel base, and asphalt mixture. Notably, these coefficients do not account for advanced paving materials like PMA and HP mixtures.
Recognizing this limitation, several agencies have considered recalibration of the structural layer coefficients for some paving materials by considering various factors, including the engineering properties of the materials, underlying support, position within the pavement structure and stress state. While previous studies have highlighted the benefits of HP asphalt binders and mixtures, there remains a significant gap in understanding their structural value as expressed through the structural layer coefficient in the AASHTO 1993 Guide.
Recalibration of HP structural layer coefficient
Several potential methods can be considered to recalibrate the structural layer coefficient for HP mixtures. This article explores four key approaches among many others, drawing on three databases and previous research efforts to elaborate on these methods.
Method I
Service life concept based on performance measures
In 2009, a section incorporating HP mixtures was constructed at the National Center for Asphalt Technology (NCAT) test track to thoroughly investigate the in-situ performance of these paving materials alongside a control section using typical or traditional PMA mixtures. This initiative was among the first in the United States to assess the performance of HP materials in real-world conditions. The HP section, labeled N7-HP, and the PMA control section, labeled S9-PMA, were built with the same type of binder (HP or PMA) across all asphalt layers – surface, intermediate, and base. This approach differs from typical state DOT practices, where HP mixtures might be confined to a specific layer within the pavement structure.
The HP section featured thinner intermediate and base layers than the PMA section, resulting in an overall HP asphalt layer thickness of 5.75 inches compared to 7.00 inches for the PMA asphalt layer. Both sections did not exhibit any signs of cracking (after being subjected to up to 8.9 million Equivalent Single Axle Loads [ESALs] at the time this analysis was performed). Rut depths for both sections remained under 0.25 inches, demonstrating strong resistance to rutting. Up to 3.5 million ESALs, the rutting performance of both sections was nearly identical. Based on these results, the structural coefficient for the HP mixture was estimated at 0.54, indicating that a 5.75-inch HP pavement layer could deliver the same rutting-based service life as a 7.00-inch PMA layer.
Method II
The use of non-destructive testing
Falling Weight Deflectometer (FWD) testing of the S9-PMA and N7-HP sections was conducted regularly to assess pavement response under various loads. Analysis revealed that the HP asphalt layer exhibited a 21.6 percent increase in structural capacity compared to the PMA layer. This suggests an estimated structural coefficient of 0.54 for HP mixtures, compared to the typical PMA coefficient of 0.44.
Method III
Serviceability index approach
This method estimates the loss in pavement serviceability over time and refers to the AASHTO 1993 Guide for recalculating structural numbers. After 8.9 million ESALs, the HP pavement section showed a terminal serviceability value of 3.9, compared to 3.1 for the PMA section. This analysis revealed that the structural layer coefficient for HP mixtures was 29.2 percent higher than that for PMA mixtures, suggesting a structural coefficient of 0.57 for HP mixes.
Method IV
The use of a mechanistic-based framework
The first three methods for determining the structural coefficient of HP mixtures are all rooted in the AASHTO 1993 Guide, with only slight variations in the analysis. As a result, it is expected that these approaches will yield similar coefficients.
The fourth method uses a mechanistic-based framework to estimate critical tensile strains at the bottom of the asphalt layer, which enables the calculation of equivalent fatigue life. It also evaluates compressive strains within the asphalt layers, base, and subgrade to estimate the pavement’s rutting performance life.
This method requires input parameters for the evaluated binder, such as rheological properties. It also requires input parameters for the evaluated mixture, including aggregate gradation, volumetric properties, dynamic modulus, and various performance characteristics. These parameters may also include fatigue-related data from conventional tests like bending beam fatigue at various temperatures and strains, direct tension cyclic fatigue, and other similar tests. In addition, rutting-related data from advanced tests, such as confined flow number at various temperatures and stress sweep rutting may also be incorporated.
The 3D-Move Analysis model was employed for computational analyses, revealing higher structural layer coefficients of 0.82 and 0.88 for HP mixtures under static and dynamic loading, respectively, as determined using data extracted from the NCAT study. These simulations, however, were based on fatigue models developed at a single temperature, which limited the ability to account for the effect of modulus. Moreover, due to the absence of available rutting models, the analyses primarily focused on cracking.
Method IV was also applied to a dataset from the New Hampshire (NH) DOT’s Auburn-Candia resurfacing study. In 2011, FHWA awarded the NH DOT a $2 million grant for implementing new technologies as part of the resurfacing of NH Route 101 between Auburn and Candia. This project included the evaluation of HP and neat asphalt mixtures. A structural layer coefficient of 0.57 was determined for HP mixtures under static loading, leading to a 33 percent reduction in asphalt layer thickness compared to a mix produced with 20 percent reclaimed material and a neat asphalt binder.
The third database in this study comes from the research conducted by UNR for FDOT. In this study, eight HP mixtures and eight corresponding control PMA mixtures were designed and evaluated in the laboratory for their engineering properties and performance characteristics. These 16 mixtures were then subjected to a full mechanistic analysis using the 3D-Move model. The initial fatigue-based structural layer coefficient for the HP mixtures was estimated to range from 0.33 to 1.32, compared to the typical 0.44 used by FDOT for PMA mixtures. Through advanced statistical analyses, an initial fatigue-based structural coefficient of 0.54 was determined for HP mixtures, which was further validated for other distress modes, such as rutting and reflective cracking.
Additional studies, not detailed in this article, involving an HP rehabilitation section built in 2010 and both a control and HP section built in 2015 at the NCAT test track, have recommended structural layer coefficients for HP mixtures as high as 0.75 and 0.92, based on in-situ strain measurements and laboratory fatigue performance data.
A shift towards thinner pavements?
The structural layer coefficients from these studies suggest that HP mixtures could enable the design of thinner pavement layers compared to conventional PMA mixtures. For example, a 5.00-inch-thick PMA layer might be reduced to 4.25 inches, 3.00 inches, or even 2.50 inches when adopting a structural layer coefficient of 0.54, 0.75, or 0.92 for HP mixtures, respectively. This raises critical questions about the validity and application of these findings and whether there should be considerations for a minimum thickness requirement.
One key consideration for HP mixtures is whether we should adopt thinner layers to achieve similar performance levels as conventional mixtures or focus on extending the pavement’s performance life by enhancing the mixture properties with additional polymer modification without reducing the layer thickness. The latter approach could unlock the potential for achieving perpetual pavements, where the pavement structure is designed to last indefinitely with minimal maintenance.
The debate continues, and the verdict is still undecided. Choosing between these two approaches – thinner layers for cost savings or maintaining thickness for extended performance – requires a thorough evaluation of project-specific factors such as traffic loads, environmental conditions and long-term maintenance strategies. Ultimately, the decision will hinge on whether the priority is achieving immediate cost savings or investing in more durable and longer-lasting pavement structures.
Closing remarks and ongoing efforts
The topic of high polymer modification is vast, and a single article cannot cover all its aspects. As part of a cooperative agreement between FHWA and UNR, supported by institutions such as AI, NCAT, VTRC, ATSE (Asphalt Testing Solutions & Engineering), and NAPA through the AIEI program (Asphalt | Innovate | Enlighten | Implement), the research team is gathering key information on the use of HP binders and mixtures. This effort includes visits with personnel from state DOTs such as New Jersey, Ohio, Oklahoma, Utah and Virginia, which are considered national leaders in using HP binders and mixtures. While the primary intent of these interviews is to document lessons learned, challenges, and best practices, the overarching objective is to conduct a comprehensive gap analysis and help develop strategies for the appropriate implementation of HP binders, which will benefit agencies either using HP binders for the first time or seeking to enhance their current specifications.
A case study of Florida’s experience with HP binders and mixtures, conducted as part of the FHWA’s Every-Day Counts (EDC)-6: Targeted Overlay Pavement Solutions (TOPS) Program, provided valuable insights into research, construction considerations, and cost factors. Moreover, in 2021, my colleague Dave Johnson, Senior Regional Engineer, elaborated in his article “Thick and Rich in Utah” (“Asphalt” magazine, Fall 2021), on some of the innovative work done in Utah featuring the installation of a single thick lift/layer of low-air void HP materials known as HiMod. Furthermore, Tom Kuennen’s article, “States Balance RAP Content for Successful High Polymer Use” in “Asphalt Pro” magazine, explored the role of HP modification in allowing increased use of reclaimed materials in asphalt mixtures without compromising performance. However, a lot of advancements have happened since then.
As sustainability remains a key focus, the use of high-performing materials like HP mixtures warrants a different perspective. While these materials may not rank highly in cradle-to-gate analyses, which focus on environmental impact from raw material extraction to plant departure due to the use of polymers and high production temperatures, they excel when considering their long-term impact during the use phase in a complete life-cycle assessment.
I look forward to exploring these topics and many others in future articles, highlighting the promising use of HP binders and mixtures.
Habbouche is an Asphalt Institute Regional Engineer based in Arizona.
