Balanced Mix Design in Liberty, MO
Posted On: Apr 30, 2020
By: John Findlay, P.E. – Project Engineer, City of Liberty, MO
The City of Liberty recently moved toward a balanced mix design (BDM) specification for the City’s annual city-wide street rehabilitation project which is predominately a mill and overlay program. After recently reviewing the City’s asphalt paving specifications, we decided to update our current asphalt mix design standards. Our decision to change was largely driven by information we learned at some recent MAPA conferences, webinars, and Lunch-n-Learns about the new wave of performance based specifications that is the newest movement in producing innovative BMD asphalt mixes. We were intrigued by the paradigm change in approach to mix design.
Typically, current mix designs like ours follow a specific heavily scripted “recipe” or standardized approach. These specifications will typically have set requirements for volumetric property requirements, aggregate type/properties, aggregate blend grading, binder type, recycle content, additives, etc. While these recipes may work, specifications have become convoluted and confounded over time with included specifications items often competing against each other in achieving the ultimate mix performance goal.
I contacted Brandon Atchison at MAPA to get more information about BMD and performance based specifications. I was truly impressed with his immediate initiative in helping us partner with MAPA and local asphalt producers and contractors for guidance in modifying our asphalt specifications.
We learned from MAPA, contractors, producers, and mix design specialists that a better approach for mix design lies with an optimized mix design methodology. An optimized mix design is one where the appropriate binder content and other mix items (aggregate type, aggregate blend grading, recycle type/content, binder grade, etc.) are selected and optimized to provide needed performance for the specific application. In an optimized mix design approach the desired mix performance is defined and the mix design specification opened up to allow for innovation on part of the mix designer to achieve the needed performance. The freedom and ability to innovate is key to transition mix design and ultimately mix performance to the next level.
The intent of trying this new optimized mix design is to allow innovation and engineering within the mix design development which is typically embraced by innovative and proactive producers. These producers have invested in personnel, training, equipment, and processes and thus understand and produce the highest quality, most consistent asphalt mix possible. Achieving mix design innovation requires the number of “rules and restrictions” during mix design to be greatly limited. Any designed mix must meet the needed performance characteristics for the given mix application and not just meet “historical” established specification requirements.
The result was a realistic, practical, and balanced mix design spec that we were able to introduce into our project. Balanced mix design principles will be used by producers to design an economical, crack resistant, and rut resistant asphalt mix that meets performance test based flexibility index and rut resistance testing criteria. We hope we are unlocking the key foundation points of optimized mix design: (1) use what works, (2) eliminate what doesn’t, and (3) be simple, practical and correct. “Good doesn’t have to be complicated” in order to develop a quality mix design and lasting performance.
This article highlights what resources our association offers to owners, consultants, and the public. As the state asphalt pavement association, one of our purposes is to be the “go to” resource to those working in the asphalt pavement industry. We encourage you to reach out to us with any questions you may have about the asphalt pavement industry.