Using Asphalt Modifiers to Enhance Pavement Performance
BY Tanya Nash
Editor’s Note: The team at Asphalt Testing Solutions & Engineering (ATS) prepared this guide to using asphalt modifiers in enhancing pavement performance based on Tanya Nash’s presentation of the same title at the National Pavement Expo (NPE) Jan. 23-25, 2024, in Tampa. The next NPE takes place Jan. 29-31, 2025, in Tampa.
Balanced mix design (BMD) is not just about performance; it’s also about being able to create an economical and sustainable pavement. Innovation is progressing faster than ever, trying to create this BMD. How do transportation personnel know what’s available on the market and better yet, how do they know it works? The modifier world is vast and can be confusing. Some examples of the categories and modifiers available are illustrated in Figure 1.
Modify means to make minor or partial changes to improve something or make it less extreme. What do you think of when you hear “modified asphalt?” Depending on your role in the industry, the answer may be different. The modification could pertain to the liquid asphalt binder or the asphalt mixture. No matter what your role, the goal of meeting the specifications is the same for everyone. (See the players in Figure 2).
Why modify asphalt? One reason is due to higher demand on our pavements. The heavier loads and increased volumes create distresses that unmodified pavements cannot withstand. The goal of modification is to increase the service life of the pavement by addressing common failures such as rutting, moisture damage, raveling, cracking and bleeding.
Basic volumetrics of a mixture will only tell you so much about the long-term performance of the pavement. Waiting 10 years to determine if the mixture performed is impractical. But how do we determine which designs perform before we place them in the field? The balanced mix design process utilizes performance testing to try and answer this question and help us properly evaluate the different modifiers’ impacts on performance. When selecting the performance tests to use for the evaluation it is important that the tests and evaluation maintain three key ideas:
- Strong relationship to field performance
- Practicality: cost, time, complexity
- Repeatable, reproduceable tests
Another reason to modify asphalt is to address specification requirements. Specifications range from a recipe to performance driven specification. This creates either the “must use” modification or the need to use it to meet performance requirements. The key to the performance-driven specifications is to let the performance drive the innovation. Restricting the modifiers to a list starts falling back into a recipe type specification.
Think about the sustainability factors. It appears almost anything can be recycled into asphalt; is this true? While it’s important to consider the environmental aspects we do not want to sacrifice the quality or longevity of the pavement’s life. Sacrificing the pavement performance contributes to the environmental impact at the end of its service life. This may be greater than the benefit of using the recycled modification in the beginning. It is important that the entire life cycle is considered when making the decisions to use modifiers.
Economics also play a large role in asphalt production and paving. The value of asphalt modification is becoming more apparent, and agencies are starting to pay more for the increase in service life.
Even with these reasons, there are pros and cons of modified asphalt, such as those indicated in Figure 3. Let’s break them down next.
The next NPE takes place Jan. 29-31, 2025, in Tampa. Visit https://nationalpavementexpo.com/ for a recap of this year’s event and to see what’s coming up!
There are three factors to consider regarding performance: constructability, aging resistance, and rutting and cracking. Regarding constructability, some modifiers make the mix too stiff or viscous to compact or properly work by hand. They can also cause the mix to cool too quickly. However, other modifiers help in this area working as compaction aids or viscosity reducers.
Modifiers help the mix resist aging from oxidation. They can work as a “face cream” for the pavement as a surface treatment that reflects UV light. Modifiers can help with rutting or cracking, but it takes balance. Some modifiers will swing the pendulum too far one way or the other.
Quality assurance (QA) is vital to modified asphalt. We learn what is in the binder by testing and performing QA. Performance testing has evolved to include many tests used to verify different performance properties of the liquid or the mixture to differentiate non-modified and modified asphalt. Many modifier suppliers have completed initial lab and field evaluations in conjunction with state departments of transportation (DOTs) willing to work with them to construct, monitor and evaluate these test sections.
Specification updates are crucial as technologies are developed. As mentioned before, some owners are writing recipe specifications that may require modification or some specifications have performance driven criteria using BMD.
On Sept. 27, 2019 (effective Oct. 28, 2019), based on a century old Federal requirement, the outdated requirements in 23 CFR 635.411(a)-(e) were rescinded to encourage innovation in the development of highway transportation technology and methods. State specifications have been rewritten to replace words such as “exclusively” and “shall be produced with” which restrict producers and suppliers to innovate with new products and possible performance enhancers and cost savings.
Approved product lists (APL) traditionally approved products with decades old processes, and some have not been adjusted as new tests and products have evolved over time. With the mindset of driving innovation, many states have developed an innovative products list (IPL) that identifies products under consideration by the department that may not fall into a current APL category.
Further complicating quality control (QC) testing is defining the terms of acceptance of a mixture under performance testing. As the tests have progressed and the industry is using more of them, most are still looking at different ways to accept and pay for mixtures using these results.
The key to the performance-driven specifications is to let the performance drive the innovation.
Using more recycled materials is environmentally friendly, but this means less new materials are being introduced into the mix. To address this issue, modifiers have been developed specifically to mobilize or rejuvenate the aged binder of RAP and RAS.
Cost is always at the top of everyone’s mind. Modifiers can help with cost in that they can aid in virgin binder replacement, find effective use for waste materials, increase the use of recycled materials and provide a life cycle extension.
Applying these ideas relates to BMD. Danny Gierhart, P.E., at Asphalt Institute, has stated BMD is “asphalt mix design using performance tests to help balance the mixture’s resistance to rutting and cracking.”
BMD offers paths to promote innovation while meeting performance expectations, offering an opportunity for modifiers to play a vital role in the future of asphalt mixes. Modifiers bring valuable and innovative solutions that can help overcome deficiencies in the binder and the mix. They can be cost-effective and sustainable. Knowledge is key and collaboration between agencies, contractors and suppliers drives innovation. It is important for specifications to promote innovation, not prevent it.
Roads are a capital investment, and we must take a proactive approach to protect this asset. Applying the correct modifier in the appropriate scenario can help enhance pavement performance as well as extend the longevity of our roads.
Tanya Nash is the director of engineer at Asphalt Testing Solutions & Engineering (ATS), an approved Florida DOT Construction Training Qualification Program (CTQP) instructor and 2024 president of the Association of Modified Asphalt Producers (AMAP).