With the current changes in bitumen quality stemming from global sourcing and decreased focus on bitumen production by suppliers, engineers are looking for novel approaches to achieve performance requirements in asphalt mixes without relying on producers to meet specifications. This has led to the development of a balanced mix design in asphalt, which helps engineers formulate mixes that account for performance standards while offsetting physical characteristics with one another.
While this approach is fairly new within the asphalt industry, balanced mix design can be effective in guiding formulation to improve the overall durability of asphalt mixes and the roadway surfaces they produce.
Here, we will explore the details of balanced mix design in asphalt including its benefits, compatibility, and typical testing techniques.
Balanced mix design (BMD) is the method of developing an asphalt mix in which roadway specifications are addressed through performance testing. Rather than relying solely on binder or aggregate specifications to meet performance standards, all aspects of the mix (binder, aggregate, modifiers, and additives) are formulated to balance factors such as fatigue, rutting, and cracking characteristics.
Balanced mix design can help with meeting performance requirements in a mix that considers elements beyond virgin binder and aggregate. Traditional approaches such as volumetric mix design, which uses volume to determine component amounts for a mix including binder, aggregate, and air voids. BMD evaluates performance such as cracking and rutting resistance based on total mixture performance without regard to volume.
This approach allows plants to incorporate additional components in a mix including reclaimed asphalt pavement (RAP) and additives that can impact performance outcomes. Samples are formulated and tested based on performance metrics to determine the right balance for an optimal paved roadway surface. BMD not only helps address performance more reliably than volumetric mix design, but it also allows for more cost effective and sustainable innovation such as the incorporation of RAP.
Blending charts are graphical guidelines that can help with balanced mix design (BMD) when using reclaimed asphalt pavement (RAP). These tools help determine performance grade and the maximum amount of RAP that can typically be included in a mix compared to the amount of virgin binder required to meet performance requirements. While these charts serve as a standard, final asphalt mixtures should be sampled and tested to ensure desired outcomes. Blending charts in BMD can help engineers optimize and expedite mix design but may not account for extraneous factors such as climate and constituent(s) quality.
Balanced mix design (BMD) may help engineers meet budget requirements alongside performance specifications by optimizing the amount of each material required in a mix to achieve desired results. This process can help reduce waste byproducts, maximize use of reclaimed asphalt pavement (which is typically more cost effective than virgin binder and aggregate), and prevent overdosing of additives or modifiers to accomplish performance goals.
While balanced mix design does not require the use of additives, many engineers find BMD useful in formulating mixes that include additives or modifiers. With the changes in bitumen quality seen across the global asphalt industry and added reliance on performance testing rather than volumetric mix design, additives are becoming increasingly important in creating a balanced mix that seeks to achieve specific functional characteristics. BMD also enables engineers to sample and test different additives, and amounts of additives, including recycling agents, softeners, and anti-strip additives that can impact performance, expenses, durability and sustainability.
Balanced mix design (BMD) can include polymer modified bitumen (PMB) as a component when developing asphalt mixes with specific performance characteristics in mind. PMBs can help with rutting resistance, fatigue, and thermal cracking. PMBs may be dosed into a mix depending on binder and aggregate content and quality, making BMD an efficient way to test for mix optimization.
Balanced mix design applies to any asphalt mix formulation. The core difference between hot mix asphalt (HMA) and warm mix asphalt (WMA) is production and compaction temperature. BMD can help engineers formulate a warm mix that includes the right dosing of warm mix additive, which typically does not impact performance outcomes for the roadway surface. Both HMA and WMA will have requirements when it comes to performance measures such as cracking and rutting, which can be addressed during BMD development and testing.
Balanced mix design (BMD) looks to optimize performance by balancing functional characteristics of an asphalt mix. This can include factors such as cracking, rutting, aging, and fatigue. A variety of tests are used to assess performance depending on the distress being analyzed. The Hamburg Wheel Track Test and Rapid Shear Rutting Test (IDEAL-CT) are both used to determine rutting resistance. Cracking tests include Indirect Tensile Cracking Test (CT Index) and Low Temperature Semi-Circular Bend Test (SCB) among others. Other tests include the Cantabro Test for mixture toughness and the Flexural Bending Beam Test for fatigue.
Typically, the performance specifications for a particular roadway are defined by the department of transportation or governing body commissioning the construction work. In many cases, specific issues are being addressed or have parameters associated with them such as meeting high-traffic performance needs or incorporating a minimum amount of reclaimed asphalt pavement (RAP) to meet sustainability goals. Engineers are then tasked with developing a balanced mix design that meets the established performance criteria.
In the United States, industry standards for balanced mix design in asphalt have been established by the American Association of State Highway and Transportation Officials. Their “Standard Practice for Balanced Mix Design in Asphalt” includes detailed information on different approaches to BMD as well as testing criteria and evaluation methods.
Balanced mix design (BMD) is a method that looks to offset different performance metrics to create a durable roadway surface. In addition to general performance guidelines for cracking, rutting, and fatigue, BMD should account for localized variables such as climate, traffic volume, aggregate size, percent of reclaimed asphalt pavement, and air voids, among others.
Reclaimed asphalt pavement (RAP) is comprised of aged binder that requires an effective rejuvenator in order to restore its functional properties in a balanced mix design (BMD). Engineers should consider the amount of required RAP, often stipulated by the department of transportation, as well as the amount of virgin binder, virgin aggregate, and recycling (or rejuvenating) agent needed to meet performance requirements. Typically, higher amounts of RAP reduce the need for virgin materials when paired with an effective rejuvenator.
Balanced mix design (BMD) can be used to help departments of transportation and other agencies meet their sustainability goals. With the ability to formulate mixes with high amounts of reclaimed asphalt pavement (RAP), engineers can reduce the overall carbon footprint of a project. In addition, BMD can enable engineers to optimize the amount of all materials in a mix, including binder, aggregate, and additives, to meet performance requirements while limiting emissions. Formulating an asphalt mix using BMD can also ensure long-term performance of a roadway surface, reducing the need for environmentally impactful repairs or replacements.
Recycled binder availability (RBA) is the total amount of effective functional binder available in reclaimed asphalt pavement (RAP). RBA can impact performance including cracking, rutting, and durability and should be accounted for in BMD.
While balanced mix design (BMD) is typically a performance outcomes-based approach to asphalt mix formulation, four variations of BMD exist with differing levels of innovation and risk tolerance.
- Volumetric Design with Performance Verification: Relies primarily on volumetric requirements and validates performance through subsequent testing.
- Volumetric Design with Performance Optimization: Requires volumetric benchmarking but allows for changes based on performance testing results.
- Performance-Modified Volumetric Design: Reduces reliance on volumetric requirements if performance benchmarks are achieved.
- Performance Design: Relies solely on performance outcomes from testing to allow for innovation and iteration in mix design.
The goal of balanced mix design in asphalt is to produce a mix that can meet specified performance requirements by balancing how the mix responds to varying degrees and sources of distress. This can result in longer lasting roadway surfaces that can withstand high traffic and environmental exposure and provide for a more sustainable asphalt pavement.
Performance in balanced mix design (BMD) in asphalt can refer to a number of factors indicating a roadway surface’s ability to withstand distress. These include rutting, cracking, shearing, raveling, warping, and moisture damage. BMD seeks to balance performance across all factors to produce an asphalt mix that is durable for the long term.
Balanced Mix Design and Sripath®
Balanced mix design is an approach that prioritizes performance outcomes while enabling engineers and plants to work with the materials they have on hand. In many instances, introducing an additive to a mix can help solve performance problems through changes to the functional characteristics of the binder and aggregate.
Sripath engineers solutions that help the bitumen and asphalt industry optimize mix design with an eye to performance. With a portfolio of rejuvenators, oils, and polymers, Sripath works with customers to solve complex problems with practical, cost-effective, and sustainable solutions.
