Balanced Mix Design: When the PG Looks Right but the Mix Won’t Pass
Two strategies to bring results into alignment
At Blankenship Asphalt Tech & Training (BATT), we spend quite a bit of time helping clients interpret Balanced Mix Design (BMD) results. On paper, BMD may appear straightforward. The goal is to balance rutting and cracking performance. In practice, many mixes that initially appear good still struggle to meet BMD design requirements.
Balanced Mix Design isn’t a single test. It’s a framework that uses separate performance tests to evaluate rutting and cracking resistance. Rutting is typically assessed through tests such as Hamburg or APA, which focus on how well the aggregate structure and binder perform under load and high temperatures. Cracking performance is evaluated through tests like IDEAL-CT, Overlay Tester, SCB I-FIT, or DCT that reflect how well the asphalt mix responds to thermal and/or fatigue stresses.
Because these tests measure different distress mechanisms, a mix can look correct by PG and even perform well in rutting tests yet still miss cracking criteria. That is, the mix can be structurally sound and still not meet specification thresholds for cracking because it’s too stiff. When that happens, the goal isn’t to start over. It’s to understand what the test results are telling you and make targeted changes that bring rutting and cracking performance into balance. In other words, what levers can you pull to fix your mix?
Two recommended strategies for bringing mixes into balance:
Assuming the mix is well designed, meaning it has good VMA, VFA, and DP, now we must look to other methods to improve it.
Strategy #1: Use Additives and Binder Enhancements Strategically
Aramid and structural fibers
Fibers don’t change the PG directly but can reinforce the asphalt matrix and help resist crack propagation. Aramid fiber will improve cracking resistance, rutting resistance, fatigue life, and pavement modulus without changing the mix design.
Warm-mix asphalt (WMA) additives
WMA technologies can slightly reduce binder stiffness at intermediate temperatures, improving IDEAL-CT results while maintaining high-temperature performance. This is often enough to move a mix from “almost there” to passing without changing the specified PG.
Crumb rubber–modified binders
Rubber modification can add elasticity to the binder system, allowing it to stretch and recover rather than crack. In some cases, rubber improves cracking performance while still supporting strong rut resistance due to its high-temperature stiffness contribution.
Recycling agents
When RAP or RAS content stiffens the mix, recycling agents can restore flexibility. Used carefully, they improve cracking resistance without significantly affecting rutting performance.
Strategy #2: Don’t Rely on Just One Binder Source
Another common reason mixes struggle to meet BMD specs is relying on a single binder option. Binders with the same PG can behave differently depending on crude source, refining process, and chemistry. Remember to check with your refiner to understand what options you may have.
If a mix is close but not passing:
Ask suppliers about alternate binder sources or blends
Evaluate different polymer systems
Consider how RAP/RAS binder contributes to the total blend
Look at performance results—not just the PG label
Suppliers may have multiple crude streams available, and a different option may provide better cracking performance while still meeting rutting requirements. Looking beyond a single binder source can reveal solutions that aren’t obvious when focusing on only one.
How do you decide what to try?
Let the BMD results guide you. If none of the above seem to be working, there’s still hope. Any time you adjust a mix to improve cracking, verify that rutting performance still meets criteria. BMD requires balance—fixing one side of the equation should not create a problem on the other. The challenge comes when real-world mixes are close to meeting spec but not quite there.
Careful adjustments to additives, binder chemistry, and supplier options can make the difference. With a clear understanding of what performance tests are telling you, BMD becomes less about chasing numbers and more about producing mixes that perform reliably in the field.
