Three asphalt myths to consider
Welcome to the second installment of Mix Busters – truths about asphalt myths. We are emulating the theme of the Discovery Channel program, “Myth Busters.” Between Buzz and Dave, they have over 70 years of experience designing, constructing, maintaining and experimenting with asphalt pavements.
For 15 years, Myth Busters would address various myths or urban legends to try to determine if they were true or not. Similarly, within the asphalt world there are many long and often strongly held beliefs that may need to be examined objectively to establish if they are true or not.
Moreover, we have a willingness to question and challenge various “truths” we “know” about the design, construction and maintenance of asphalt pavements. Drawing heavily on the eight completed cycles of the Pavement Test Track operated by the National Center for Asphalt Technology (NCAT) at Auburn University and untold numbers of field projects, we will explore topics we recognize as needing to be examined for their validity.
We have been busy scanning the asphalt world for new topics to examine, seeking the truth. After considering numerous options, we offer a fresh and objective look at the following subjects.
For this installment, we focus on three myths related to pavement thickness design and perpetual pavements:
1. Layer coefficients should not change when using high-polymer-modified (HP) binders.
2. To achieve a perpetual pavement, it must be very thick.
3. It’s OK to pave lean, stiff binder mixes (intermediate layers) as long as we cover them up with healthy surface mixes.
MYTH: “Layer coefficients should not change when using high-polymer-modified (HP) binders”
High-polymer-modified (HP) binders are often utilized to improve structural performance in cases where increasing pavement thickness is impractical. For states that use layer coefficients for structural pavement design, this works because HP mixes have higher layer coefficients. For example, research sponsored by the Florida Department of Transportation found that HP mixes result in layer coefficients that are 23 percent higher than conventional mixes.
In other research (NCAT Report 12-10), NCAT found that HP mixes result in layer coefficients that are at least 43 percent higher than conventional mixes. This outcome assumed the NCAT Pavement Test Track would suddenly fail with the very next truck pass. This assumption is conservative because the HP test section was exhibiting no distress at the time it was taken out of service after 20 million equivalent single axle loadings (ESALs). In fact, it’s likely the HP section was a perpetual pavement (at an asphalt thickness of less than six inches on six inches of dense crushed granite base and a high-quality subgrade). Research to precisely quantify the layer coefficient increase at the NCAT Pavement Test Track is ongoing.
The enhanced layer coefficients of HP pavements are well documented, offering cost-effective solutions for achieving superior performance. Their enhanced performance characteristics present an opportunity to achieve significantly greater performance with minimally greater initial unit costs. The ability to build a perpetual pavement at thicknesses once thought to be impossible could initiate a dramatic cost/benefit improvement to an already very competitive material. To achieve a higher level of performance, resisting the urge to thin sections is encouraged. Dr. Jhony Habbouche summarized the quandary well in his article found in the October 2024 issue of Asphalt magazine.
“One key consideration for HP (high polymer) 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,” wrote Habbouche.
MYTH: “To achieve a perpetual pavement, it must be very thick.”
A common belief is that achieving perpetual pavement performance requires considerable thickness. A perpetual asphalt pavement is defined as a flexible pavement system designed such that the tensile strain at the bottom of the asphalt layers remains below the material’s endurance limit. The endurance limit is the threshold level of strain below which asphalt materials can theoretically withstand an infinite number of loading cycles without experiencing fatigue cracking. While the minimum thickness to achieve such performance varies, common values are greater than 12 inches for a pavement to have the opportunity to be perpetual. Is this really true? Does an asphalt pavement indeed need to be that thick to be perpetual or can it be thinner – perhaps significantly thinner? To address this topic, let’s look at the evidence.
Our first piece of evidence comes from the Asphalt Pavement Alliance’s Perpetual Pavement Award winners. There have been over 150 awards since 2001 when the first winners were announced. Analysis of these award-winning pavements reveals structural designs with total asphalt thicknesses typically ranging from 10 to 14 inches depending on traffic volume, climate, and subgrade conditions. These designs used pre-Superpave mix methodologies (Marshall or Hveem), without modified binders, which likely contributed to the misconception of necessary thickness.
The second piece of evidence comes from NCAT. Namely, as was discussed before, when using modern, advanced asphalt binders that have been engineered with performance-enhancing modifications, the pavements using such binders have shown consistently that they perform better. In fact, the HiMod NCAT section that was exhibiting perpetual performance referenced above was only 5.75 inches thick!
Does that mean that surprisingly thin sections can achieve perpetual performance anywhere? Unfortunately, no. NCAT researchers found that 14 inches were needed for perpetual performance on sections that were built to represent typical conditions found in Oklahoma. Therefore, pavement designers need to engineer perpetual pavements for their conditions. Factors that influence thickness design include:
• Traffic load: High-volume truck routes typically require greater thickness.
• Subgrade quality: Weak subgrades might necessitate thicker asphalt layers or stabilized bases.
• Climate considerations: Hot or freeze-thaw conditions may influence material selection and thickness.
Regardless of where a pavement is being designed and constructed, a perpetual pavement can be engineered for that site’s unique conditions. Moreover, especially when advanced materials are incorporated into the design, thicknesses that were once deemed impossible, can be perpetual, sometimes with less than six inches of asphalt!
For more information on the NCAT work please see https://eng.auburn. edu/research/centers/ncat/files/ reports/2012/rep12-10.pdf.
For further details on the documented benefits of polymer-modified asphalt binders, refer to Asphalt Institute publications ER-215 and IS-215.
MYTH: “It’s OK to pave lean, stiff binder mixes (intermediate layers) as long as we cover them up with healthy surface mixes.”
It’s often said that the long-term performance of a pavement structure is a function of using the right mix and materials in each layer, providing enough asphalt thickness to prevent deep distresses, and preserving/maintaining the surface. These are all elements of perpetual pavements that also apply to good practice with pavements that are not perpetual. While it’s tempting to focus solely on the surface mix because it’s the most visible part of the pavement structure, the reality is that every layer in the pavement should satisfy specific performance requirements. In addition to basic quality requirements for all mixes (e.g., resistance to stripping), surface layers should be durable and resistant to both rutting and cracking, bottom layers should be resistant to cracking, and intermediate layers should reduce deflections through a combination of thickness and stiffness.
A simple way to increase stiffness is to increase the percentage of reclaimed asphalt pavement (RAP) in the mix. While intermediate layers can be a great place to use higher RAP content mix designs, it should not be done indiscriminately. Balanced mix design (BMD) testing is a great way to ensure that higher RAP content mixes will have a sufficient quantity and quality of effective total binder content to exhibit good resistance to cracking. This is especially important when milling/inlaying two layers of pavement within a milled depth that does not completely eliminate existing cracking. The existing cracking will eventually reflect up into the inlaid asphalt, and the rate of progression should be expected to increase if the inlaid mix is lean and/or contains a lot of aged binder. This concern is compounded by the modern prevalence of multigenerational RAP (i.e., RAP that has been recycled multiple times since it was originally placed as a virgin mix decades ago).
Additionally, adjacent layers with significant stiffness differences can exhibit high interlayer tensile stresses under traffic loading. Good tack products at appropriate rates are even more important to prevent premature failures when leaner/stiffer mixes are placed either over or under more flexible layers (e.g. when placed under a crack-resistant surface mix). The importance of placing mixes in every layer of the pavement structure that satisfy all relevant quality requirements for new construction as well as rehabilitation cannot be overstated. Agencies should consider relaxing design air void requirements in BMD-proven mixes as an acknowledgment that volumetrics inform practitioners on the quantity of effective binder content but say little or nothing about quality. Striving to use as much RAP as possible can enhance environmental and economic stewardship, but only in ways that ensure good long-term performance.
Where to next?
Buzz and Dave encourage readers to share ideas for future investigations. Email your suggestions to asphaltmixbuster@gmail.com. If your idea is used, full credit will be given!
Dr. Buzz Powell, P.E., is the Asphalt Pavement Alliance Technical Director. Dave Johnson P.E. is an Asphalt Institute Senior Regional Engineer.
