Are current Superpave binder grading systems adequate for climate change, or are we designing pavements for yesterday’s weather?

Superpave binder grades are selected using historical temperature data, yet climate patterns are shifting rapidly with more intense heat waves, rainfall variability, and freeze–thaw cycles. In your view, does the current PG system still capture the demands of future pavement performance? Or is it time to rethink how we grade binders for a changing climate?

Liz Valenca NCAT Master's Student

This is a very interesting topic. When selecting the PG grade of a binder, we typically rely on historical temperature data. Some may argue that this approach is no longer fully reliable, as average temperatures in the United States have risen by about 2 °C over the last three decades. However, in my view, a 2 °C increase is unlikely to significantly affect PG selection; it is relatively minor when considering binder grading. For that reason, I do not think using predictive temperature data instead of historical data is necessary at this point.

In my opinion, the bigger challenge related to global warming and binder selection is the increasing prevalence of heat waves. Today, we experience 2–3 times as many extreme heat days per year compared to the 1980s. Heat waves are also lasting longer: events that previously lasted 2–3 days now commonly extend 4–6 days or more, and maximum daily temperatures during these events have risen by 2–5 °F.

When considering these effects on binder performance, there are two events to consider: 1) rutting shortly after the pavement is laid during heat waves, and 2) accelerated aging of the binder over time due to higher temperatures. These events can work in opposite directions. To prevent early rutting, one might consider increasing the PG high-temperature grade to avoid softening under extreme heat. Conversely, to reduce long-term stiffness associated with aging, one might select a lower PG grade to increase initial binder softness. That is why I believe grade bumping or reduction is not the correct solution.

That said, the challenge may not lie with Superpave binder grading itself. While I may not have all the answers to this issue, I believe we need to carefully plan construction schedules to avoid paving during severe heat waves and focus on more resilient binder products that are less susceptible to aging, such as polymer-modified binders or other emerging technologies.

Benjamin F. Bowers, Ph.D, P.E. McCartney-Chase Highway Engineering Distinguished Associate Professor, Auburn University.

When we think about adapting to a changing climate, a benefit of asphalt pavement is that it inherently climate adaptable. When considering resilience as it relates to binder, I typically think about temperature. The PG system is fundamentally grounded in selecting a binder based on temperature. The question I believe we should be asking is whether our regions for binder selection, represented in LTPPBind or elsewhere, have accounted for the changes that are occurring. Work by Dr. Shane Underwood and published in Nature Climate Change found that the currently recommended temperatures do not always account for our “new” highs and lows. So, is the grading system adequate for climate change? I would say absolutely – I think it was (consciously or unconsciously) made for it. But we need to make sure we’re selecting the target based on the temperatures the region is seeing today and tomorrow as opposed to only looking to the past. Fun side note: Binder modification may be the answer to handling other climate-related disruptions! But more on that later. Reference for Underwood’s paper and further reading provided below.

Underwood, BS, Guido, Z, Gudipudi, P, and Feinberg, Y (2017). Increased costs to US pavement infrastructure from future temperature rise. Nature Climate Change, 7, 704-707 (2017).

Sias, JE, Dave, EV, Underwood, BS, Bowers, BF, Harvey, JT, Henning, TFP, Tighe, SL, Jacobs, JM, Pregnolato, M, Qiao, Y, Mecray, E, Golalipour, A, Chamorro, A, and Hendrick, P (2025) Climate Change Impacts on Roadways. Nature Reviews Earth & Environment, 6, 555-573 (2025).

Faustina Keuliyan-Rodriguez Pavement Research Scientist, Heritage Research Group.

Superpave provided a major leap forward by introducing a performance-based binder grading system tied to pavement temperature extremes. It created a universal language for binder selection and captured binder’s viscoelastic properties, introducing considerable improvements past empirical methods.

Binder properties remain critical to pavement performance, and evolving modifiers, alternative materials, and production methods have highlighted the need to reconsider how we characterize binder behavior. Changing climate patterns further stretch this need; not only rising temperatures but also more frequent and severe climatic events impose greater demands on pavement systems.

Our industry is proactive, and progress toward climate integration is already evident. NCHRP 20 50(21) and 21A are advancing efforts to incorporate climate data into pavement design. Tools such as PGAC³ (Performance-Graded Asphalt Cement under a Changing Climate) and CAAST (Climate Adaptation and Asphalt Selection Tool) integrate future temperature projections into binder selection and design. These frameworks are already tackling our evolving climatic demands.

Research suggests that Superpave grades may widen under future temperature conditions, which might require stricter performance specifications, or even changes in modifiers or material selection for pavement design. Whether through improved parameters to better capture binder performance or adapting to new climatic grades, Superpave will likely not look the same. One fundamental strength of Superpave is its universality, as it provides a common framework across the industry. We should leverage this as we adjust requirements for future demands, and collaboration across industry, agencies, academia, and policy makers will be key to accomplishing this. Fast adoption of climate-informed practices will lead to lessons learned that will further drive decision-making and fruitful iterations. Climate change will not pause; the sooner we implement these strategies, the better prepared we will be to ensure durable, resilient pavements

Richard Willis, Ph.D. Vice President, Engineering, Research, & Technology, NAPA.

We asked Dr. Willis this month’s question, and he believes that one of the most important considerations is the typical service life of surface mixtures. As he noted: “If we look at how long a typical surface mix is in service, it may vary from 12-20 years plus or minus. At that point, it is milled, and then a new mix is put down. Critical temp rise is 1.5°C. I don’t have the data, but my guess is that most pavements could handle 1.5°C in the next 15-20 years!”

From this perspective, the industry may not be facing as severe a challenge in the near term as sometimes suggested. Many pavements, as Dr. Willis observed, are likely capable of accommodating this degree of temperature rise during a standard service interval without experiencing notable declines in performance.

However, he also emphasized the importance of remaining forward thinking, stating: “I do think we need to try to be more forward thinking and predictive, but I don’t think we may not be in as bad a place as some think

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