With agencies increasingly adopting BMD, is the era of traditional volumetrics coming to an end, or what role will it play in the future of pavements?

As the asphalt industry evolves, Balanced Mix Design (BMD) is gaining traction as a performance-driven approach that emphasizes long-term pavement durability over traditional volumetric specifications. While volumetrics have been the foundation of mix design for decades, BMD focuses on directly evaluating performance indicators such as cracking and rutting resistance. This shift raises important questions about the future of volumetrics: Will they become obsolete, or will they continue to serve as a useful baseline in the mix design process?

Madhav Verma, MSc. NCAT Ph.D. Student

Volumetrics has played a major role in asphalt mixture mix designs, especially with the introduction of Superpave design. Balanced Mix Design (BMD) is relatively new, but because it provides additional assurance to contractors and DOTs about performance, it's gaining widespread acceptance. In my view, even with BMD providing direct performance confidence, volumetrics will never be out of the picture. Volumetric is a crucial tool in a mix design. Its role in the complete pavement industry might change, but it will never come to an end. Current BMD specifications also include volumetrics in three of the four approaches.

Firstly, for quality control purposes during paving, volumetrics offers an added benefit, as the time required to verify is less than for cracking and rutting testing. Performance can be fully integrated into the mix design, but determining volumetrics during the design stage can help contractors with quality control. It is very helpful for DOTs, also for pay factor determination.

Secondly, because asphalt mixtures are flexible, air voids play a major role in our industry. Even for performance testing, the air void level affects performance. During construction, pavement layer density and air void level also matter. Volumetrics helps mix designers to understand air void levels effectively. Volumetrics can also be one of the strategies to improve the performance of mix design by changing gradations and, in turn, altering voids in mineral aggregates (VMA). Another important factor, effective binder content or film thickness, can also be very well controlled by having proper volumetrics checked.

Thirdly, current performance tests, such as IDEAL-CT, have limitations for some types of mixtures. In these cases, volumetrics can help ensure proper design. In today's world, where the pavement industry uses many different additives, some of which behave differently, these performance tests can fail to identify them. Understanding volumetrics can be very helpful when they affect binder viscosity and, in turn, effective binder content.

Therefore, in my view, BMD definitely has many advantages, and it will become the main part of mix design, but having volumetrics as a tool in your box in the future.

David Vivanco, Ph.D., P.E. Asphalt Branch Manager, Oklahoma Department of Transportation.

I think that the increasing adoption of BMD does not necessarily signal the end of traditional volumetric mix design, but rather a redefinition of its role within the asphalt design framework. Volumetrics will transition from being the primary indicator of mixture quality to serving as a design starting point and a quality-control evaluation tool.

Volumetric parameters such as air voids, VMA, and VFA are assumed to be indirectly related to durability, rutting, and cracking. While this assumption is reasonable when materials and traffic conditions are relatively uniform, it has become less reliable with the widespread use of modified binders, high RAP contents, and increasing traffic demands. As a result, mixtures that fully satisfy volumetric requirements may still exhibit poor field performance, and the other way around. BMD addresses this limitation by incorporating performance testing to evaluate rutting and cracking resistance. However, in my opinion, BMD will not completely eliminate the need for volumetrics, in the future, volumetrics will likely function as a first-level filter in mix design, establishing reasonable bounds on mixture structure before performance testing is conducted. BMD will then be used to verify that the selected volumetric design achieves balanced resistance to a variety of distress mechanisms. This combined approach allows agencies to maintain the operational benefits of volumetric control while gaining the performance confidence offered by BMD and performance testing.

From an agency perspective, fully abandoning volumetrics would be impractical at the current implementation stages, as performance tests have shown not to be ideal to replace routine production control. Instead, the future of pavement mix design will be a system in which volumetrics ensure uniformity and constructability, and BMD ensures long-term performance and reliability. The era of volumetrics is not ending, but it is shifting. As agencies continue to adopt BMD, volumetric mix design will remain a critical component, but its role will shift from being a standalone design philosophy to a supporting element within a performance-based framework.

Nathan Moore, P.E. Assistant Director for Test Track Research | NCAT

As a cell phone owner, I don’t really care what quality control tests the manufacturer had to run to ensure my phone works. I just need it to meet my performance expectations. This doesn’t mean phone manufacturers should stop running internal quality checks, but it is the product's performance that matters to the customer.

Volumetrics have stubbornly remained a foundational aspect of mix design and mix procurement for a very long time. Historically, this approach made sense when mixes were simpler; essentially confirming the quantities of the asphalt mix (e.g., aggregate, asphalt, and air) equated to confirming the quality of the mix. However, with the influx of new materials over the past few decades, like RAP/RAS, rubber, compaction aids, WMA, fibers, etc., the link between volumetrics and performance has been broken. Dozens of experiments on the NCAT Test Track and elsewhere have proved this. One of the key ideas behind BMD was to replace criteria focused on the ingredients of the mix with performance criteria. Materials and proportions may still be important and influence decisions, but the ultimate target is performance instead of a recipe.

There is another side to this coin, though. For decades, asphalt mix designers have used volumetrics to design, monitor, and control their mixes. Easy checks like pill heights, Gse, Gmm, and Gmb can help a mix designer troubleshoot and provide reference points to adjust mixes. These fingerprints of an asphalt mix may still be used by mix designers and QC technicians to assess adherence to a design or make adjustments. They may not be reported to the DOT or look like the existing rules of thumb, but they can help a producer make quick, informed decisions. Until better quality control tools replace volumetrics, they will have a place in a contractor’s lab, but only as they relate to production consistency or reference points for design changes.

Finally, many state DOTs have volumetrics ingrained into their procurement and payment systems. Traditional volumetrics are quicker to check than BMD tests, have less test uncertainty, and are easy to test, making them preferred pay factors for many state agencies. Despite their inadequacies, changing pay factors can be challenging, which is one reason why we see many states attempting to blend BMD and volumetrics. Until the risks of BMD testing are better understood and quantified or DOT’s elect to move forward despite uncertainty, we will, unfortunately, most likely operate in this blended BMD/volumetrics environment for a while. In a perfect world, we’d all like mixes to be designed and specified by performance alone, but some of the hiccups we’re seeing in BMD implementation, especially during production for mix acceptance, hinge on the fact that DOT’s haven’t reduced or quantified the uncertainty and risk surrounding BMD testing to the level that we have with volumetrics. You can’t package and return a road as easily as you can a phone if it doesn’t meet your expectations. Thus, many people would default back to a system with less uncertainty or simply add BMD to the existing mix design and procurement system, thinking it is a less risky approach. In reality, adding the uncertainty of BMD to the existing volumetric-based systems shifts risk to contractors, resulting in bid prices that cost the agencies more money.

Derek Nener-Plante, M.S, P.E. Principal, DNP Infrastructure LLC.

As the asphalt industry evolves, Balanced Mix Design (BMD) is gaining traction as a performance-driven approach that emphasizes long-term pavement durability over traditional volumetric specifications. While volumetrics have been the foundation of mix design for decades, BMD focuses on directly evaluating performance indicators such as cracking and rutting resistance. This shift raises important questions about the future of volumetrics: Will they become obsolete, or will they continue to serve as a useful baseline in the mix design process?

Some in the asphalt industry are looking for Balanced Mix Design (BMD) to replace volumetrics in how we assess the quality of our asphalt mixtures — I prefer to think of it as an evolution. When volumetric analysis became the norm, it did not replace tools like asphalt content and gradation. Those properties are still tools in our current volumetric system, but we now interpret them differently than we once did. I believe we will view volumetrics in a similar way in the future: as measures that help us fine-tune the mixture to achieve performance. Will every mixture target 4.0% air voids or have to meet the same voids in mineral aggregate (VMA) criteria? No, but I envision producers establishing an air-void target (or acceptable range) and understanding the volume of effective binder as important for designing and producing quality asphalt mixtures.

What I often find missing in this discussion is the role of volumetrics in production. We already have extensive evidence that controlling asphalt content and gradation in production does not guarantee volumetric consistency, and it’s unlikely to control consistency with BMD measures as well. Given the added time and effort required to conduct BMD tests in a production environment, I see the continued use of volumetric properties for quality control and acceptance as a practical measure of mixture consistency. Are they perfect? Of course not — but they tell us more than asphalt content and gradation alone can. Volumetric tests can be performed more frequently to fill in the gaps that the more time-consuming BMD tests cannot, helping ensure the mixture being produced is consistent.

So, will volumetrics become obsolete? Unlikely. What’s more realistic is a future where volumetrics provide specification guardrails, help producers adjust mix designs to meet BMD criteria, and continue to serve as measures of consistency in production for quality control and acceptance.

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