Mix design engineers are constantly tasked with determining the right ratios of aggregate and binder to develop asphalt mixtures that will meet performance requirements. Traditionally, mix designs have employed volumetric analysis relying on the evaluation of compacted samples to determine properties such as air voids and voids in mineral aggregate, among others. For instance, the Marshall method includes tests for stability and flow, while Superpave includes a more robust method incorporating traffic and climate impacts to design asphalt mixtures.
However, several industry variables have caused mix designers to consider advanced methods to achieve mix performance outcomes. The heightened focus on performance testing has become more important for various reasons such as binder aging and quality concerns, low binder content, and the need to increase reclaimed asphalt pavement (RAP) usage. One potential solution has been to incorporate modifiers and additives that can enhance the functional properties of bitumen in a mix. This can include anything from the addition of styrene-butadiene-styrene (SBS) to create polymer modified bitumen (PMB) to adding emulsifiers or anti-strip agents. Notably, adding these materials can dramatically alter roadway performance with respect to performance indicators including fatigue, durability, cracking, and rutting.
Complicating matters more has been the industry’s trend toward more sustainable paving solutions. Engineers are now asked to use bitumen of variable quality, incorporate additives, and aim for net-zero carbon emission benchmarks with their mixes without sacrificing performance. This has led to increasing use of RAP and warm mix asphalt (WMA) technologies. Using higher amounts of RAP in a mix requires the inclusion of an effective asphalt rejuvenator, and, similarly, producing a long-lasting WMA roadway requires the use of warm mix additives.
These dynamics have led to the development of a new approach to mix design that accounts for performance requirements without solely relying on volumetric properties.
What is Balanced Mix Design?
Balanced mix design (BMD) is an asphalt mix development method that achieves requirements through performance testing. Instead of relying solely on binder properties, aggregate specifications, volumetric measures, BMD seeks balanced performance across indicators such as rutting, cracking, aging, and fatigue. This approach also allows engineers to consider factors such as traffic volume, location, and climate when creating a mix.
Additional benefits of adopting BMD include the ability to:
- Incorporate higher amounts of recycled materials (RAP) through the use of asphalt rejuvenators
- Promote the use of WMA to reduce carbon emissions during production and compaction
- Use a combination of locally and globally sourced materials to manage mix costs
- Extend the life of the roadway to lower overall costs and enhance sustainability
Departments of Transportation already rolling out balanced mix design are seeing marked improvement in roadway performance and durability. For example, the New Jersey Department of Transportation (NJDOT) tracked the improvement of roadway surface quality from 2006 through 2022 after pivoting from Superpave to BMD. While adjusting for volumetric properties in the past did not necessarily yield the desired improved performance, durability and cracking resistance has improved through BMD development, surveying indicated a steady increase in NJ roadways with a “Good” rating compared to Fair or Deficient conditions. (Source: NEAUPG 2021)
How to Implement Balanced Mix Design
Carrying out a balanced mix design approach requires a different mentality than its volumetric counterparts. The U.S. Department of Transportation Federal Highway Administration breaks BMD implementation down into eight tasks, or stages, with various supporting subtasks. These can provide a basic framework for making the transition to BMD a reality.
- Motivate Teams: Advocates for BMD should start by recounting the benefits of switching to the methodology over traditional approaches. There may be resistance to change without clear definitions for improvement.
- Plan Benchmarks: BMD implementation is typically adopted to solve performance problems where traditional mix designs have failed to meet specifications. Establishing benchmarks for performance success from the start can help streamline getting the process underway. Additionally, project sponsors should conduct research to ensure BMD is a viable solution and uncover similar progress made by other DOTs that can expedite planning. The goals, timeline, and project scope set during this stage should guide the rest of the initiative forward.
- Select Performance Tests: Switching to BMD from volumetric assessments opens the door to a variety of performance tests. Project leaders should identify those tests most critical to assessing the achievement of the performance goals set during the prior stage. Common tests used for balanced mix design performance testing include:
- Rutting: Hamburg Wheel Track Test and Rapid Shear Rutting Test (IDEAL-CT)
- Cracking: Indirect Tensile Cracking Test (CT Index) and Low Temperature Semi-Circular Bend Test (SCB)
- Stiffness: Cantabro Test
- Fatigue: Flexural Bending Beam Test
- Acquire Resources: One limitation of BMD implementation is the need to procure or obtain access to the testing equipment needed to prepare and evaluate mix samples. Dedicating space, time, and funding to create the right laboratory environment can go a long way in creating testing efficiency and accuracy.
- Establish Baseline Data: Teams can start by reviewing historical mix test data in comparison to newly defined goals to identify required changes before getting started. In addition, benchmarking studies and shadow projects can demonstrate how existing mix designs would perform in testing and ensure that tests correlate to field performance.
- Develop Specifications: Setting reasonable, achievable acceptance criteria when developing new BMD projects is also essential. Testing sample methods, lots, sizes, and frequencies should be incorporated into testing plans for consistency. Pilot projects can also help clarify requirements.
- Provide Training: With new standards in place, companies can finalize BMD-related processes and update their training and certification requirements for employees. This includes the development of training assets such as manuals and videos for affected technicians.
- Implement BMD: Balanced mix design implementation can begin on a per-project basis once these stages have been completed. Since BMD is a different approach than traditional methods, teams will want to notify any impacted industry teams of the upcoming changes, providing clear information tying goals and scope with performance specifications.
As the asphalt community, including contractors (hot mix producers), design engineers, DOTs, and agencies, turn to BMD, Sripath remains committed to helping the entire asphalt industry work to improve the overall performance of asphalt roads. Our additives, including ReLIXER® (an asphalt rejuvenator), PGXpand® (a bitumen-friendly polymeric additive), and PHALANX® (a warm mix additive) are designed to address these needs.
To learn more about balanced mix design and how Sripath’s product portfolio could benefit your mixes with improved performance and sustainability in mind, email info@sripath.com.
