People have been optimizing the design of asphalt mix to create more stable and durable pavements for
years. Although Superpave is currently used by most state highway agencies (SHA), its original intention
of incorporating performance tests into mixture analysis portion has not yet been realized. Also, current
volumetric analysis alone cannot adequately assess mix variables such as recycled materials, warm-mix
additives, polymers, rejuvenators, and fibers, and the proportions of mix components are highly
sensitive to specific gravity tested in the lab. Additionally, more and more state DOTs have indicated
that cracking and raveling have become the primary controlling factor for the service lives of asphalt
pavements, indicating an insufficient asphalt binder content in the mixture from Superpave. With the
awareness of deficiencies of volumetric mix design system and increased concerns with durability and
cracking issues, the asphalt industry and SHAs have turned their attentions to a more performance
based mix design method-the Balanced Mix Design (BMD).
BMD was defined as “asphalt mix design using performance tests on appropriately conditioned
specimens that address multiple modes of distress taking into consideration mix aging, traffic, climate
and location within the pavement structure.” It focuses on designing asphalt by incorporating
mechanical tests for performance evaluation into the design procedures, rather than simply meeting the
specified volumetric requirements. This approach aims to provide good resistance to both rutting and
cracking by selecting a “balanced” mixture. Compared to pure volume analysis which is limited to
evaluating material quantities, BMD can remove the volumetric limitations and improve long-term
pavement performance by making the roads more durable, more sustainable, and even cost less.
How is the BMD practically done here at Western Technologies? For the design, mechanical and
empirical data are used which means laboratory data is obtained to measure stability (rutting) and
durability (cracking) which is the mechanical data and existing performance data is used which is the
empirical data. From a graphical standpoint, the intersection of mechanical data (rutting and cracking)
produces the optimum design asphalt content.
Sample steps for BMP (reference: Minnesota DOT)
1) Select the materials for use according to the current practice. Aggregates should meet the consensus properties and gradation required for the particular application, and the asphalt grade should be selected according to the governing DOT binder guidelines.
2) Combine materials, mix, and short-term oven age (STOA) for 2 hours for the rutting test and long-term oven age (LTOA) for 4 hours for the cracking test at the suggested compaction temperature.
3) Using a volumetric design, define the asphalt content (ACv) meeting the requirement of 4.0 percent air voids at Ndesign.
4) Prepare samples at ACv, ACv +0.5 percent, and ACv −0.5 percent.
5) After aging, compact samples to 7±0.5 percent air voids. This level of target air voids represents what might be expected in field compaction.
6) Conduct cracking and rutting performance tests.
7) Select the asphalt content defined as the balanced asphalt content (ACB) according to the test results and accounting for the allowable variance of asphalt content in construction. Adding construction tolerance ensures that the resulting field mixture does not fall below the minimum required by the cracking performance testing.
For more information, contact us via our website wt-us.com.
Illinois Flexibility Index Test (IFIT) for Durability/Cracking
Hamburg Wheel Tracking (HWT) for Stability/Rutting
References:
National Asphalt Pavement Association. (2022). https://www.asphaltpavement.org/expertise/engineering/resources/bmd-resource-guide
Buchanan, Shane. (December 2017). https://ws.engr.illinois.edu/sitemanager/getfile.asp?id=1348
Newcomb, David and Zhou, Fuije. (June 2018). Balanced Design of Asphalt Mixtures. Minnesota Department of Transportation Report No. MN/RC 2018-22
