Reheating asphalt mix influences moisture levels in aggregate over time

In an asphalt laboratory, ensuring accurate test results on reheated mix comes down to a lesson we can learn from a batch of leftover French fries: moisture migration over time.

Consider that the humble French fry is beautifully engineered: crispy on the outside, soft on the inside, nicely salted, and just hot enough to make anyone forget about the will-start-eating-healthy resolutions they made. Now, think about what happens when you take the leftover fries, put them in a container, let them go cold and soggy, and reheat them the next morning. The magic is gone. Technically, they are the same potatoes, but because of time, moisture has migrated, fundamentally changing the product quality.

Asphalt mixes also go through metamorphosis. Fresh asphalt mix that comes out of the plant is hot, the binder viscosity is low, aggregates are nicely coated, and the mix is highly workable. But if the sample is stored in a cardboard box and cooled to ambient temperature, the properties of the mix change.

Leaving the sample inside the oven for a few extra hours could allow the aggregates to absorb more binder.

This happens because aggregates have natural pores. While the mix is hot and the binder viscosity is low, the binder slowly travels through those pores. Technically, this is known as asphalt binder absorption (Pba).

Because this absorption is a one-way street, reheating the cold mix will never force the binder back to the surface. This means Pba directly steals from effective asphalt content (Pbe), which is the actual binder left on the outside coating aggregates. Mathematically, when Pba goes up, Pbe goes down, triggering a domino effect across several volumetric properties.

The severity of this metamorphosis depends on the percent of asphalt binder absorption, which depends on the aggregate type used in the mix. A low-absorption aggregate like granite isn’t as “thirsty” as a high-absorption aggregate like the volcanic basalt used in Hawaii.

How to Use BMD with Airfield Mixes

Reheating a sample that uses low-absorptive aggregates might only cause a minor nudge in the test results. On the other hand, reheating a sample with high-absorptive aggregates can be the difference between the mix passing or failing. To make matters worse, the number of times a sample is allowed to cool to ambient temperature and reheated to test temperature only compounds the problem.

This means two different labs could test the exact same split-sample of a mix produced using highly absorptive aggregates and get wildly different results, simply because one lab let the sample cool down completely and reheated the sample prior to testing.

Because the reheated sample has absorbed some of the binder, lowering the effective asphalt content, the mix is harsher to compact, leaving extra pockets of empty space known as air voids. To make matters worse, the reheating cycle also affects the maximum specific gravity, also known as the Rice value.

This is exactly where reheating of mixes strikes a project where it hurts. Typically, cores drilled from the project are compared against the Rice value to compute the percent density, which feeds the pay factor calculation. When the Rice value is calculated using a reheated sample, the value is skewed compared to the Rice value calculated using a fresh sample. This can easily lead to erroneous density values, making a good project in the field look like it failed on paper.

Future of Petroleum Asphalt Binder Supply

So, the question is, how do we prevent our lab results from turning into yesterday’s soggy fries? Here are some rules to ensure what’s paved is actually what’s tested:

Test the mix fresh whenever possible: This is perhaps the best defense against binder absorption. Remove the cooling phase to obtain the volumetric test results. However, this is easier said than done. Acceptance testing labs have their own challenges to test samples before they cool down to ambient temperature.

Establish a correlation factor: If you know samples will be cooled and reheated for independent testing, establishing a correlation factor early in the project to account for the data shift is a good idea.

Watch the clock: When a sample goes into the oven for reheating, it is critical to ensure the sample doesn’t stay inside the oven longer than it needs to after reaching testing temperature. Leaving the sample inside the oven for a few extra hours could allow the aggregates to absorb more binder.

Avoid multiple reheatings: To avoid compounding the problem, it’s best to avoid putting a sample through the vicious cycle of reheating-cooling-reheating.

At the end of the day, it’s important to make sure the lab results reflect the true quality of the pavement on the road. After all, nobody wins when a perfectly good paving job gets penalized over how the sample was handled. Just like nobody wants to be served yesterday’s soggy fries.

Jayanth Kumar Rayapeddi Kumar is a quality control engineer at Jas. W. Glover Ltd. in Hawaii and a licensed professional engineer in the state of Hawaii. He has over 14 years of experience in the asphalt industry and is an active member of ASTM Committees D04 (Road and Paving Materials) and E36 (Accreditation and Certification).

How to use BMD with airfield asphalt technology

Rutting is an important performance consideration in the design and evaluation of airfield asphalt because even small amounts of deformation can affect drainage, smoothness and operational safety. While existing Federal Aviation Administration (FAA) specifications include rutting evaluations, a recent study completed by the Airport Asphalt Pavement Technology Program (AAPTP), a cooperative agreement effort between the FAA and the National Asphalt Pavement Association (NAPA), focused on developing performance-based rutting tests and criteria that reflect the combined effects of aircraft loading, temperature and field conditions. These enhanced tests support the balanced mix design (BMD) approach to optimize durability and performance in airfield asphalt pavements.

The findings of the AAPTP report support potential updates to FAA P‑401.

Building a Practical Rutting Test Framework

The AAPTP study evaluated both laboratory- and plant-produced asphalt mixes from active airfield projects across the United States. The team included the University of Nevada, Reno (UNR), Rutgers University, the Texas A&M Transportation Institute, RDM International and Duval Engineering. Mixes represented a wide range of binder grades, aggregate properties, climatic regions and aircraft loading conditions.

Dr. Elie Hajj of UNR said the goal was to not only identify reliable rutting tests, but to determine how those tests could be implemented consistently throughout the project lifecycle.

“The aim was to move representative rutting mechanical tests beyond mix design and into verification and production to support a comprehensive BMD framework,” Hajj said.

The outcome is a framework agencies and contractors can use to standardize key test variables—such as air voids, temperature and specimen preparation—to improve consistency.

Balance Performance: BMD is not about maximizing one property at the expense of another. AAPTP paired the rutting study with a cracking study to support balanced performance across rutting, cracking and durability. That balance ultimately defines long-term pavement success.

Rethinking Aggregate Performance for Long-Term Friction Safety After One Million Cycles

Addressing Air Voids, Temperatures, 4 Tests

The study recommends running all rutting tests at 7% air voids (±0.5%). Traditionally, air void targets have varied by test method. For example, the Asphalt Pavement Analyzer (APA) is often run at design air voids, while the Hamburg Wheel-Track Test (HWTT) uses 7% per AASHTO procedures.

Evaluation of field density data from multiple airfields during the study supported the selection of a 7% target air-void level, as it captured a larger portion of the observed in-place air voids measurements. Importantly, the 7% target also enabled laboratory specimen preparation at practical heights, reducing the likelihood of aggregate damage associated with over‑compaction in the lab while remaining representative of densities measured in pavement mats and at longitudinal joints.

Why 7% Air Voids?

• Consistent with observed in-place air voids.

• Enables fabrication of specimens without inducing aggregate crushing.

• Simplifies testing protocols across rutting test methods.

• Eliminates confusion arising from different air-voids targets among test methods.

Current specifications typically require testing at 64°C for APA and 50°C for HWTT. Rather than testing all mixes at a single temperature, the framework selects test temperatures based on local climate using LTPPBind environmental PG values (50% reliability at the pavement surface, 12.5 mm rut depth). This results in temperatures typically ranging from 40°C to 64°C.

The study identified four rutting performance tests as reliable tools within a BMD framework, with calibrated criteria ensuring consistent mix design outcomes:

• Asphalt Pavement Analyzer (APA)
• Hamburg Wheel-Track Test (HWTT)
• High-Temperature Indirect Tensile Test (HT-IDT)
• Ideal Rutting Test (IDEAL-RT)

Together, these four tests provide agencies and contractors with a flexible toolkit that can be tailored to available equipment, project scale and the performance risk level of the airfield pavement area being paved.

Because HWTT combines rutting and moisture damage, the study recommends evaluating rut depth at 5,000 passes (instead of 20,000) to better isolate rutting behavior and improve correlation with other rutting tests.

Precise Specimen Prep: Specimen preparation had a measurable impact on results. Cutting or trimming specimens introduced variability, especially in HWTT under wet conditions. Using directly molded specimens will improve consistency across labs.

From Lab to Reality

To validate the criteria, pavement cores were collected from airfield sections with documented performance histories. Tests showed that mixes from sections with lower performance fell short of recommended criteria, while mixes from well-performing sections met them. This demonstrates that the framework can effectively identify mixes likely to deliver rut resistant, high-performing airfield pavements.

A multi-lab study showed most tests fall within about a 20% coefficient of variation. Monotonic tests showed less variability than repeated-load tests, and the project incorporated this variability into the established test criteria.

The study establishes rutting criteria tied to both climate and aircraft loading. Temperatures are climate-driven, while performance thresholds reflect aircraft weight and operational conditions. Although criteria vary by loading scenario, the recommendation is to design for the worst-case condition, which is slow or stationary loading.

Researchers also used mechanistic modeling (3D‑Move) to simulate aircraft stresses under realistic conditions, strengthening the link between lab results and field performance.

For contractors and agencies, the takeaway is that BMD provides a more reliable way to design asphalt mixes that perform in the field.

Navigating the EPA’s TCE Phase‑Out for Asphalt Labs

A Path to Implementation

The study outlines a phased approach to implementation, allowing contractors, labs and agencies to build confidence before full adoption.

Rutting performance testing can be incorporated into both mix design and verification processes. When the APA or HWTT is used for mix design, these tests can be paired with HT-IDT or IDEAL-RT for verification and acceptance. As agencies and producers gain experience with these methods, they can gradually introduce more reliance on HT-IDT and IDEAL-RT, allowing for higher testing frequency during production.

“The challenge with traditional rutting tests is that specimens have to be fabricated, conditioned and run over a long period of time,” said Adam Hand, PhD, of UNR. “You don’t want to keep paving for one or two days before getting an answer, so the index tests are going to be a good tool for getting an answer quickly or at least an indication of the anticipated performance.”

Asphalt Testing Solutions & Engineering Offers Asphalt Performance Testing and Mix Designs

Implications for FAA Specs

The findings support potential updates to FAA P‑401, including:

• Multiple approved rutting test options;
• Standardized air voids (7%);
• Climate-based test temperatures;
• Enhanced specimen preparation;
• Integration into quality control and acceptance; and
• Criteria tied to aircraft loading.

“BMD is a huge deal to bring to airfields,” Hand said. “The FAA specifications have not included these performance tests in the past, and with BMD, we’ll be able to assure better performance with a more reliable system.”

It’s important to note that the FAA already has rutting requirements in place: The AAPTP project was about advancing the science. The approach builds confidence in testing before full specification adoption and advances the readiness of BMD to support FAA and AAPTP goals for safer, more reliable and cost-efficient airfield pavements.

For contractors and agencies, the takeaway is that BMD provides a more reliable way to design asphalt mixes that perform in the field. Standardized air voids, climate-based temperatures, validated tests and consistent specimen preparation, combined with performance-based criteria, offer a practical path forward.

“Even when mixtures meet volumetric requirements, changes in binder source or formulation can affect performance,” Hajj said. “Communication between contractors, suppliers and agencies is critical.

“It will be important to monitor performance and continue refining as we learn more,” he added. “But this study gives us a strong foundation for moving forward.”

As airfield demands increase, that foundation will only become more important. “The Balanced Mix Design: Rutting Performance Tests” report is available from the AAPTP webpage. Visit airportasphalt.com to download the full report and supporting materials.

Brett Williams is senior director, Engineering & Technical Services, for the National Asphalt Pavement Association. He is the NAPA liaison for the Committee for Engineering Application and Practice and the Pavement Economics Committee Mixture Quality and Performance Task Force.

Solterra Materials, Phoenix, announced in March company growth, expanded operations across Arizona, and a new website. Recent statistics from P&S Intelligence have shown that the overall market size of the asphalt industry will expand to roughly $52.9 million by 2032. Solterra Materials, a leading Arizona-based asphalt materials provider and a division of Sunland Asphalt, is already seeing the effects of this expected growth.

Since its founding in 2018, Solterra Materials has expanded from 29 to 42 employees. This represents a nearly 45% increase for the company that operates across the state of Arizona, serving private, commercial, highway and infrastructure projects, and supports a wide range of asphalt needs, from private developments and commercial projects to large-scale highway and infrastructure work, as well as specialized asphalt solutions.

Along with promoting its team internally in 2026 and launching its new website (www.solterramaterials.com), a key component of Solterra Materials’ 2026 growth strategy is its enhanced quality control (QC) division, which features a fully accredited lab where the team can provide mix designs and QC services internally and for customers, giving additional confidence, project reliability and support.

“Across Arizona and nationwide, we’re seeing continued momentum in infrastructure investment, private development and roadway improvements,” said General Manager Scott Bollinger. “That growth is translating into increased demand for reliable, high-quality asphalt production and technical support. By expanding our team, strengthening our QA/QC capabilities and fully staffing our plant maintenance operations, we’re positioning Solterra Materials to meet that demand.”

In addition to team growth, Solterra Materials has also strengthened their fully staffed, in-house maintenance team responsible for plant repairs, equipment optimization and infrastructure improvements. This investment has enhanced production efficiency, minimized downtime and allowed the company to maintain consistent, high-performance output.

The company has also expanded its industry footprint through participation in Building Women in Construction (BWIC), through the Arizona Chapter of the Associated General Contractors (AZAGC), demonstrating its commitment to workforce development and industry leadership.

Take a look over the next few pages at the products and services that will assist the entire industry this season as our industry grows. AsphaltPro Magazine is using the next few product gallery departments to highlight new technology launched at the recent CONEXPO-CON/AGG in case you missed anything you need to enhance your bottom line.

New EX 300 Plant Boosts Efficiency and RAP Capacity

New ALLU Bucket Reduces Hauling and Crushing Costs

Astec Digital Platform Connects Plant and Paving Data

CASE Adds AutoDig and Comfort Features to Loaders

New Cedarapids HSI Plant Targets High-Capacity Crushing

New TalonTrax Plant Targets Recycling Efficiency

EDGE Launches SCREENPRO S16 Screening Plant

EvoQuip Showcases Cobra 290R Impact Crusher

John Deere Introduces SmartDetect Assist for Safety

ProStack Names Sync Aggregate Utah Distributor

Vortex Burner Delivers Multi-Fuel, Low NOx Performance

Webster HDRA Burner Reduces NOx in Asphalt Dryers

Simplifying the search for Optimum Asphalt Content (OAC)

Author’s Note: This article presents a simple procedure that can be used to estimate optimum asphalt content. A more detailed version of this article, titled “Simple Analytical Procedure to Estimate Optimum Asphalt Content,” was originally published in ASTM International’s Journal of Testing and Evaluation, Vol 53, No. 2, 2025.

Asphalt producers allocate significant personnel and material resources to develop hundreds of hot-mix asphalt (HMA) mix designs annually. Traditional mix design methods such as the Marshall, Superpave and Hveem methods require multiple samples to establish the optimum asphalt content (OAC). For example, the Marshall method requires at least 17 samples to determine the optimum asphalt content. This includes two samples for measuring theoretical maximum specific gravity (Gmm) and three samples at five different asphalt content levels to understand how asphalt content affects air voids. The Superpave method requires at least 12 samples to estimate OAC.

Express Your RAP, RAS Content

Many state and federal highway agencies are strongly pursuing the implementation of balanced mix design (BMD), which requires fabrication and testing of substantially more samples to develop a compliant asphalt mix. Unlike traditional volumetric mix design methods, all of which have volumetric requirements such as limits on the voids in mineral aggregate (VMA), voids filled with asphalt (VFA), etc., at the design asphalt content for a preselected design air voids level, BMD introduces performance testing to evaluate cracking and rutting potential of mixes into the mix design process.

While some state agencies have opted to retain traditional volumetric requirements as a starting point for mix development, others have moved toward a more performance-focused approach that may reduce emphasis on volumetrics. Nonetheless, since developing a mix design using BMD or otherwise is a detailed, time-consuming process that demands significant effort, volumetrics most likely will continue to play an important role, either as a screening tool or a starting point for trial blends.

Current practice on mix design requires specific volumetric criteria to ensure performance and durability. Among these, one of the difficult challenges for mix designers is achieving the minimum required VMA. The Asphalt Institute’s MS-2 manual even points out that it’s not uncommon for designers to finish their laboratory mix design, only to find out that the VMA falls short of the requirement. To overcome this, designers tweak the aggregate proportions and asphalt content until the mix meets the VMA criteria.

But the challenge does not stop at the design stage. During production, VMA can collapse because of aggregate breakdown, usually by about 0.2% to 0.5%, according to the article “Fix Your Mix” published in Asphalt Institute’s Asphalt magazine in June 2016. To stay ahead of this problem, designers typically aim for a VMA slightly above the minimum, giving their mixes extra leeway for real-world conditions.

Current mix design methods are iterative, relying on trial and error to select the right aggregate structure and binder content to meet various volumetric criteria. It is like trying to navigate an unfamiliar city using a paper map by unfolding and refolding the map, squinting at tiny street names, taking wrong turns, and often backtracking to reach the destination. It takes patience and a lot of effort. Yet, just like switching from paper maps to a GPS makes navigation smoother and faster, a simple tool would be useful to take the hassle out of estimating OAC.

Balance Your Mix Design for Asphalt

Proposed Estimating Procedure

The proposed procedure is based on three well-established equations used in volumetric asphalt mix design and a newly developed equation based on these.

Using these three equations, an analytical expression shown in Equation 4 is derived to estimate the optimum asphalt content (P̂_b). A detailed explanation of this equation is provided in the referenced ASTM paper.

Pa—Air voids (%)

Pb—Percent Binder Content by total weight of mix (%)

Gmb—Bulk Specific Gravity of compacted specimen

Gmm—Theoretical Maximum Specific Gravity of loose mix specimen

Gsb—Bulk Specific Gravity of combined aggregate blend

Gse—Effective Specific Gravity of aggregate

Gb—Bulk Specific Gravity of asphalt binder

VMA—Voids in Mineral Aggregate (%)

It is useful to note that, for fixed air voids (Pa), the estimated bulk specific gravity of a compacted sample (Gmb) decreases as asphalt content (Pb) increases in Equation 1. This occurs because the theoretical maximum specific gravity of the mix (Gmm) decreases with increasing asphalt content. Similarly, for a fixed VMA, the estimated Gmb increases as Pb increases in Equation 2. The intersection of these two curves represents the point at which the mix simultaneously satisfies the target Pa and VMA criteria. The corresponding Gmb at this point is referred to as the critical bulk specific gravity (Gmb−crit) and the corresponding Pb is the estimated OAC of the mix.

Based on Equations 3 and 4, only six inputs are required to estimate OAC. These inputs are:

• Trial asphalt content, Pb (at which the loose mix for Gmm testing is prepared)
• Gmm at trial asphalt content
• Gb of the binder
• Gsb of the combined aggregate blend
• Target air voids, Pa
• Target VMA for the mix

Compaction’s Role

There was no mention of compaction effort throughout this discussion, which raises an important question: Can we accurately estimate optimum asphalt content without factoring the compaction level? The Asphalt magazine article “A Look at Superpave5” from May 2019 addresses the issue of the effect of compaction level on Superpave mix design methodology by stating “A common myth is that laboratory compactive effort (N_design) will change asphalt content, but data does not support that.”

The Federal Highway Administration reinforces this in its December 2010 Tech Brief titled “Superpave Mix Design and Gyratory Compaction Levels” stating “there is a misconception that gyratory compaction level controls the voids and asphalt binder content of the mix. In truth, it is the gradation and aggregate properties and their resistance to compaction that controls the air voids and resulting binder content.” The Tech Brief also notes that, because of the need to stay competitive, asphalt producers modify their design aggregate structure within the design limits to make it possible to obtain a new aggregate skeleton such that a minimal amount of asphalt binder is used to achieve target air voids. It further points out that the air voids of a mix are controlled by VMA and not by compaction level. Other researchers have arrived at similar conclusions. While the above is true for the mix design process, it is also true that for given aggregate properties and a fixed gradation, as compaction level increases, the binder content must decrease and vice-versa to achieve the target air voids.

How AMIBIND Helps Crews Lower Temps Without Changing Plants

The premise of the procedure presented in this article is that the aggregate structure and compaction level have indeed a substantial effect on the resulting mix volumetrics. However, Equation 4 provides an estimated OAC achieved with some unspecified compaction level only for a trial aggregate blend. If volumetrics cannot be satisfied with that OAC at the selected “design” compaction level, then the designer will need to modify the aggregate structure. This is no different than with any other design method, but it is done with minimal testing. The procedure is particularly useful when working with new aggregates and blends. It offers a simple, analytical, and efficient starting point by estimating the likely optimal Pb based on the target Pa and VMA.

Based on this information, the designer can fabricate one set of specimens to check whether the mix meets all the target requirements. If it does, the mix is compliant. If not, the designer will know early on that adjustments to the aggregate structure are needed, without trial-and-error using multiple test batches. The single point verification ensures less waste of time, effort and materials.

From Equations to Application

With the six input values, a mix designer can quickly use Equations 3 and 4 to generate the estimated OAC for the blend. Once OAC is known, the designer can prepare two samples to determine Gmm, and compact two gyratory or three Marshall specimens to determine Gmb and calculate Pa and VMA. If the calculated volumetrics meet the target criteria, the job mix formula can be considered established. The same blend can then be tested at other asphalt contents to complete the mix design.

Evaluation of the Regressed Air Voids Approach for Mix Design

Confidence in the Procedure

The proposed procedure was validated using a combination of laboratory-prepared mixes, previously formulated designs from multiple asphalt producers, and long-term pavement performance (LTPP) data. More than 65 mix designs in total were evaluated.

Estimated OACs from the procedure were compared to the values from traditional mix design methods. The results demonstrated excellent agreement, with a correlation coefficient of 0.99. Full details of the validation process are provided in the ASTM paper.

Benefits of the Proposed Procedure

Optimum Asphalt Content: Solved Analytically

For any given aggregate blend, the OAC can be estimated analytically using Equation 4, requiring just six inputs, all of which are already part of traditional mix design methods. This equation is built on well-established volumetric equations that have long been the foundation of Marshall and Superpave mix design approaches.

A Quicker Route to Job Mix Formula

Unlike traditional mix design methods, which can require up to 17 and 10 samples using the Marshall and Superpave methods, respectively to establish the OAC, the proposed procedure significantly reduces laboratory effort. Assuming no alternative blends are being evaluated, OAC can be determined with four to five samples, which saves time and materials to establish the job mix formula.

Build the Test Section for Validation of Plant Produced Mixture

An Easier Way to Manage VMA

It’s not unusual for mix designers to reach the end of the mix design process and discover that the VMA criterion hasn’t been met. Also, it’s industry knowledge and experience that VMA can collapse during production. The proposed procedure offers a practical solution to both issues. It gives the mix designer the flexibility to input a target VMA that accounts for expected collapse during production. For instance, if the design VMA requirement for a mix is 15.0, but experience shows VMA typically drops by about 0.5% during production, the designer might enter 15.5 to estimate OAC. This proactive adjustment helps ensure the final mix meets targets not just during design but also in production.

Streamlining Testing for BMD

In the context of BMD, any approach that can simplify volumetric verification can be a valuable tool in a designer’s toolbox. In this regard, the proposed procedure may offer a way to ease some of the added workload that comes with BMD. By reducing the number of specimens needed to validate a selected aggregate blend, this method provides an opportunity to streamline the early stages of mix development.

Four Tips for Lab Sampling

Final Thoughts

Designing asphalt mixes to meet volumetric criteria can be simplified. The procedure presented in this article and detailed in the ASTM paper referenced earlier provides a more efficient, analytical approach to estimating OAC using fewer samples and familiar equations. We believe it offers a practical option that may help save time and resources, while simplifying the search for optimum asphalt content.

Request access to the longer, more technical paper titled “Simple Analytical Procedure to Estimate Optimum Asphalt Content” in Vol 53, Issue 2 of ASTM International’s Journal of Testing and Evaluation here: https://store.astm.org/jte20240242.html

---

Adrian Ricardo Archilla is a professor in the Department of Civil, Environmental, and Construction Engineering at the University of Hawaiʻi at Manoa, with more than 25 years of experience in asphalt materials characterization and pavement engineering research. Reach him at archilla@hawaii.edu. 

BATT’s groundbreaking million-cycle test sheds new light on aggregate performance for friction safety

In early 2024, the asphalt specialists at the Blankenship Asphalt Tech and Training Laboratory (BATT Lab) in Richmond, Kentucky, set out to challenge a long-standing assumption: that only dolomitic limestone aggregates can provide durable surface friction under traffic when compared to Kentucky aggregates containing silica and other hard minerals. Under the direction of Phil Blankenship, PE, MSCE, and led by Lab Operations Manager Zack McKay, BATT conducted what we believe is the first known one-million-cycle aggregate polishing test using the three-wheel polishing device (TWPD).

Historically, Kentucky’s reliance on limestone has been based on legacy data from quarry ledges, correlated with skid numbers obtained through locked-wheel or sideway-force coefficient routine investigation machine (SCRIM) testing. An open question is whether alternative Kentucky aggregates can also provide adequate long-term friction performance.

Bluegrass Testing Laboratory Focuses on Asphalt Mix Designs, Binder and Aggregate Testing

Traditionally, answering this question required constructing a highway test section and waiting 5-10 years for natural polishing under approximately 8-10 million vehicle passes. To accelerate the evaluation, three-wheel polishing (TWP), combined with dynamic friction testing (DFT), offers a promising laboratory approach.

Research from the National Center for Asphalt Technology (NCAT) suggests 100,000 to 150,000 TWPD passes may approximate typical traffic exposure, since the test is always conducted wet (a worst-case condition) with a contact pressure of 87 psi per tire—equivalent to a semi-truck tire footprint—multiplied by three tires. Still, caution is essential when applying this benchmark. If the polishing requirement is underestimated, an aggregate may appear acceptable in the lab yet lose friction prematurely in service. Unlike pavement cracking, which can often be managed, inadequate surface friction is a direct safety hazard. Because little work has been done to calibrate this method, BATT undertook a study to refine the approach while assisting a local aggregate producer and quarry operator.

Can alternative aggregates also provide adequate long-term friction performance? The million cycle TWPD test recently performed at the Blankenship Asphalt Tech and Training Laboratory (BATT Lab) in Richmond, Kentucky, offers answers.

The Million-Cycle Approach to Aggregate Wear

BATT’s Million Cycle TWPD test ran 24/7 for three months, requiring 270 hours of machine time and duplicate slabs to reduce variability and improve confidence in the results. Many DFT measures were made throughout the 1 million cycles pausing at 25,000, 75,000, 150,000, 200,000 and then every 100,000 thereafter to understand the effect of the extended polishing.

What the test revealed was striking: Even some of the best dolomite limestone surface declined over time, while the silica-based materials continued to resist polishing under extended loading. These findings challenge long-standing assumptions about aggregate wear and open the door to new thinking that could improve asphalt mix designs for long-term friction performance and durability.

While the silica-based material did not start with the higher friction values of the dolomite limestone, it maintained the friction longer than the limestone. This doesn’t mean that dolomite limestone is not well suited for high-traffic roads. Rather, it means that we should be able to use the silica-based aggregate or any aggregate combination as defined in the DFT where the polishing slope is flatter (changing less with continuous polishing) and thus maintaining friction.

The purpose of this extended testing was to validate assumptions and assist research into accelerated lab testing to certify aggregate mixes based on lab properties instead of waiting years to see aggregate wear in roadways. The main goal is always to improve the safety of asphalt pavements.

Beyond Rutting: A New Role for Aggregate Testing

While traditional tests like the Hamburg wheel track test focus on rutting and moisture damage, they stop short of evaluating aggregate polishing, which is a key contributor to loss of friction and skid resistance over time. That’s what makes the million cycle TWPD test that BATT conducted so significant. It pushed well beyond conventional testing windows that simulate aggregates’ long-term surface wear under real-world traffic conditions.

NCAT’s Nathan Moore shared that there is very little information published on what is a suitable amount of polishing to accurately simulate real-world traffic. Polishing times that are too short can produce results that favor limestone in short-term performance due to not applying enough traffic to simulate field polishing.

But the extended test performed by BATT showed that materials like granite and crushed gravel retain surface texture longer, potentially offering safer, more durable pavement surfaces.

While BATT does not recommend that million-cycle polishing be used in everyday testing, the experiment proved useful in taking these mixes to the limit. One-million-cycle polishing can lead to other issues such as stripping of the mix, which can cause variability in friction results, changing the microtexture of the pavemet. However, the stripping and friction loss we observed are real-world effects.

Navigating the EPA’s TCE Phase‑Out for Asphalt Labs

Locally Sourced Aggregates and the Case for Re-Evaluation

Limestone’s dominance in asphalt mixes isn’t solely based on lab test performance. It’s widely available, especially across the Midwest, making it a locally sourced, cost-effective option. It’s also easier to crush and tends to meet moisture susceptibility, stability and gradation requirements with less processing. Combined with agency specs written around its characteristics, these advantages have made limestone the go-to aggregate, even when harder aggregates may outperform it over time.

Aggregate is one of the costliest materials to transport in highway construction and hauling it long distances adds both financial and environmental burdens. If a polish-resistant aggregate is available closer to the project site, it can reduce costs, lower carbon impact and simplify logistics.

This raises an important consideration: some local materials may not currently be “approved,” yet they could still provide excellent long-term skid resistance if properly evaluated. BATT’s million-cycle polishing testing raises important questions:

• Is it time to rethink aggregate acceptance criteria based solely on acid insolubility residue, and consider TWP and DFT as additional mix design tools?
• Should we place greater priority on polish-resistant local materials that may deliver longer-lasting friction, reduced maintenance costs, and improved roadway safety?

R18 Certifies Higher Calibration Costs

KYTC: Leading the Way in Friction Safety Research

BATT’s work aligns closely with an ongoing research initiative by the Kentucky Transportation Center (KTC) and funded by the Kentucky Transportation Cabinet (KYTC) to improve pavement friction and reduce roadway fatalities.

In 2021, KYTC/KTC launched a project focused on integrating friction testing into BMD. Track sections at NCAT (S7A and S7B) were designed to test medium- and high-friction mixes while validating both the TWP and the DFT. The project aims to correlate lab-measured friction values and polishing cycles to real-world traffic performance and build them into performance-based specifications alongside rutting and cracking resistance.

These tests strive to make friction a core design criterion, not just a performance afterthought—a goal that aligns directly with the implications of BATT’s million cycle TWPD that pushes the limits of the test.

Polishing the Asphalt Surface: Tennessee Researches Aggregate Frictional Properties

Limestone’s Earlier Polishing Calls for Smarter, Safer Designs

The KYTC study also highlights a key geographic concern: sedimentary limestone is widely used in the Midwestern states like Kentucky, Missouri, Indiana, Ohio, etc., where it polishes more quickly under traffic. In contrast, more eastern states often use granite or other hard materials that are more durable under traffic.

 

The combination of soft aggregates and high traffic volumes makes friction loss more likely in these regions. That’s why designing asphalt mixes that better resist polishing over time is essential. This also has implications for how reclaimed asphalt pavement (RAP) is classified and incorporated, since most modern mixes contain reclaimed materials. These findings highlight the increasing focus on the importance of BMD and other performance testing methods—especially when using RAP, additives and regionally available aggregates—to ensure long-term safety and sustainability.

The one-million-cycle polishing test performed by BATT is more than a research milestone. If these findings are validated in further studies, they may spark a shift in how friction, polishing and long-term durability are considered in asphalt mix design.

With BATT, KYTC, NCAT and others leading the way, the future of asphalt design is moving toward data-driven, safety-focused solutions, where smart testing and long-term thinking determine which materials make it from lab to pavement.

Phil Blankenship, PE, is the president of the Blankenship Asphalt Tech and Training Laboratory (BATT Lab) and Zack McKay is the lab operations manager.

Editor’s Note: Reporting around the industry in late summer 2025 may have added some confusion to the discussion of when trichloroethylene (TCE) was banned. As early as Dec. 21, 2023, AASHTO petitioned EPA to grant the industry a 20-year phase-out period to allow state DOTs time “to investigate alternative methods for asphalt testing with the goal of eliminating TCE from their testing protocols.” Read more about that here. By Dec. 17, 2024, our industry was included in the final ruling’s 10-year phase-out period. See paragraph two below for more detailed information.

The Environmental Protection Agency (EPA) proposed a rule in 2023 to prohibit the production and use of trichloroethylene (TCE) in many industrial applications, including asphalt testing. After a period of comment review and discussion, the EPA published its final ruling in December 2024 in favor of a TCE ban.

To alleviate issues with immediately banning TCE from asphalt labs, a phase-out period has been identified: five years for open systems (this would include centrifuges and reflux systems used in binder extraction from asphalt mixtures) and 10 years for closed systems (this would include auto-extractors). However, those organizations continuing to use TCE during this phase-out period will be required to monitor and document procedures on training employees and establishing a Workplace Chemical Protection Program (WCPP).

ASTM D8159 currently allows for the use of tetrachlorethylene (PCE) and methylene chloride (DCM) as suitable alternatives.

The transition away from TCE will affect asphalt laboratories in several ways, including binder solubility testing and solvent-based extraction and recovery. Fortunately, options are available to asphalt labs to make the necessary adjustments in light of these rulemaking directives and concerns. To that end, the following insights are offered from the Asphalt Institute as a means of assisting asphalt test labs during the transition away from the use of TCE.

S.T.A.T.E. Testing Advances Hot Mix Testing Technology

Asphalt Binder Solubility

TCE is still the primary solvent used for ensuring that an asphalt binder is relatively free of non-organic mineral matter. The two most common test methods for this type of determination (often referred to as the “Solubility Test”) are AASHTO T 44 (Solubility of Bituminous Materials) and ASTM D2042 (Standard Test Method for Solubility of Asphalt Materials in Trichloroethylene or Toluene). AASHTO T 44 lists TCE or trichloroethane (TCA) as the allowable solvents, while ASTM D2042 permits both TCE and toluene. Of the two test methods, AASHTO T 44 is the most widely used in the United States but is the most restrictive in that it only allows for the use of chlorinated solvents.

In 2016, AASHTO T111 (Standard Method of Test for Mineral Matter or Ash in Asphalt Materials), commonly referred to as the “Ash Test,” began to replace solubility testing in AASHTO specifications for anionic, cationic and polymer-modified asphalt emulsions. ASTM emulsified asphalt specifications followed suit in 2020, allowing for the use of either D2042 or D8078 (Standard Test Method for Ash Content of Asphalt Binder and Emulsified Asphalt Residues). A primary reason for this change was to eliminate testing difficulties related to polymer-modified residues. For example, latex-modified residues are known to “gel” on the crucible during solubility testing, plugging the filter pad and making the test procedure difficult or even impossible to perform.

Like solubility testing, the Ash Test is meant to ensure that paving binders and emulsified asphalt residues are relatively free of mineral matter. Whereas the Solubility Test involves dissolving organic matter with a solvent, the Ash Test uses a muffle furnace to incinerate all organic material, leaving behind a percentage of inorganic mineral matter. The Ash Test procedure accommodates more complex binders while removing the risks associated with the use of chlorinated solvents.

Polishing the Asphalt Surface: Tennessee Researches Aggregate Frictional Properties

Solvent Extraction and Recovery of Asphalt Binder

The impact of the TCE ban may be felt more acutely for labs who perform solvent-based extraction and recovery of binder from asphalt mixtures. One alternative is to replace the use of TCE with toluene as outlined in ASTM D2172 (Standard Test Methods for Quantitative Extraction of Asphalt Binder from Asphalt Mixtures) and ASTM D7906 (Standard Practice for Recovery of Asphalt Binder from Solution Using Toluene and the Rotary Evaporator).

Toluene does not carry the same long-term exposure risks as TCE and is thought to be easier to remove from the binder during the recovery process. However, the flash point of toluene is notably lower than TCE (around 40°F); thus, labs must ensure that storage and handling procedures are updated, and engineering controls are put in place to mitigate any fire risk. For many agencies and organizations, the increased flammability risk makes toluene a non-starter. Additionally, the low flash point means that toluene is incompatible with the closed-loop systems used in auto-extractors.

Where does this leave labs who depend on auto-extraction systems? ASTM D8159 (Standard Test Method for Automated Extraction of Asphalt Binder from Asphalt Mixtures) currently allows for the use of tetrachlorethylene (PCE) and methylene chloride (DCM) as suitable alternatives. Like TCE, PCM and DCM are chlorinated solvents that carry their own risk to human health; if labs make the switch to either of these solvents, care should be taken, including proper personal protective equipment (PPE) such as appropriately rated chemical-resistant gloves, exposure monitors and solvent respirators. Engineering controls are also vital, including walk-in fume hoods to house extraction systems or snorkel vents that limit exposure to fumes.

Bluegrass Testing Laboratory Focuses on Asphalt Mix Designs, Binder and Aggregate Testing

The Road Ahead

Implementation of changes will be context-specific, and while innovation may lead to any number of viable alternatives, it will likely take time for these to emerge in the market.  Undoubtedly, there will be more questions to arise and challenges that the asphalt industry will face over the course of the TCE transition period. Working together and sharing knowledge can help everyone who depends on these solvents for accurate binder characterization and quality control.

Jason Wielinski is an Asphalt Institute regional engineer based in Ohio. Wes Cooper is the laboratory manager and senior research scientist at the Asphalt Institute Laboratory in Lexington, Kentucky.

---

Read next: What Asphalt Labs Must Do to Comply With the TCE Ban