Letter from the Director

Momentum is Building for Balanced Mix Design

Momentum is a word that has several meanings. In sports, it is the perceptible shift in a contest when one team or competitor begins to gain the advantage. In physics, it is the property of a moving object calculated by the product of its mass and velocity.

In the world of asphalt pavements, it is clear that the concept of balanced mix design (BMD) is gaining momentum. BMD is a new era of mix design that promises to result in longer lasting asphalt pavements. The impetus for change is rooted in the recognition that some asphalt pavements have not performed to expectations even though they met the project’s specifications.

Most state DOTs have tweaked their mix design specifications to increase asphalt contents by a few tenths of a percent. However, these small changes have been unable to overcome some fundamental weaknesses of the current mix design method which depends primarily on the volume of effective asphalt to ensure mixture durability. In reality, the volume of effective asphalt is not easy to measure and it tells us nothing about the quality of the binder. The Superpave mix design system also relies on specified minimum properties of mix components (e.g. aggregate angularity criteria and PG binder specifications), but this approach misses the interactions that occur between materials. This has become more critical as the use of recycled materials has increased and more innovative additives are utilized in mixtures.

The concept of BMD utilizes simple mixture performance tests to more directly assess a mixture’s resistance to the types of distress that could occur on the project. The word “balanced” in balanced mix design infers that the mixture will have good resistance to rutting and cracking. Testing of the mix makes more sense than testing the individual components because the interactions are manifest in the performance test results. We want the mix performance tests to be simple enough so that every asphalt lab can perform them, but just being simple and fast is not good enough. The test results have to be demonstrably related to the mixture’s resistance to the particular distress. Just because we can measure a mix characteristic does not mean that characteristic is a good indication of performance.

In the last year, NCAT proposed a framework for BMD that is now working its way through AASHTO’s Committee on Materials and Pavements. We are nearing the completion of an experiment to determine which cracking tests are best suited for indicating top-down cracking and thermal cracking. We are also finalizing the analysis of a round-robin study to gain more information about the between-lab variability of the most popular mix performance tests. Many other researchers are conducting studies that will help fill in the gaps.

Momentum for BMD is evident by the number of workshops, webinars, training classes, research projects, and pilot projects that now have a focus on BMD. Day to day conversations among asphalt technologists in labs and offices all across the country now center on how to implement BMD tests in mix design and quality assurance testing. If you are a stakeholder in the world of asphalt pavements, you should be involved in work to fill in the gaps of the BMD framework. As the physics of momentum teaches us, it’s much easier to be involved in change at the beginning before the velocity takes off, and you are more likely to influence the direction of change when the mass is relatively small. Keep up the good work for better pavements.

Randy C. West, Ph.D., P.E. | Director & Research Professor