The original vision of the Superpave mix design system was to include three levels. Level I, envisioned for low traffic pavements, would rely primarily on traditional volumetric property requirements for mix design. Level II would be used for the majority of projects that carry moderate traffic levels and would include volumetric property requirements plus a limited set of mixture performance tests. Level III would be for high traffic pavements and would also start with a volumetric-based mix design followed by an expanded set of advanced performance tests. However, the proposed performance tests for Level II and III were never implemented, primarily because they were not practical for routine use for the thousands of mix designs used each year in the United States.
In the early years of Superpave implementation, the primary focus of mix design was on rutting resistance. Mix designs for moderate and high traffic projects were designed for improved rutting resistance by specifying a higher grade of asphalt binder and higher quality aggregates. Many states also added a rutting test requirement (e.g. the Asphalt Pavement Analyzer) for mix designs for moderate and high traffic projects. Twenty years later, most highway agencies now report that rutting problems have been virtually eliminated. However, as projects built using the Superpave system have matured, highway agencies have found that cracking of one form or another is the primary form of distress. There are several possible contributing factors to increased cracking, including issues with the mix designs, increased use of many different recycled materials and byproducts, problems with the quality of construction, and failure to adequately address underlying pavement distresses during pavement rehabilitation. It is now well recognized that the current mix design system has some shortcomings. Many highway agencies and the asphalt paving industry are revisiting the possibility of using mixture performance tests in the mix design process to improve the life of asphalt pavements.
In September 2015, the FHWA Expert Task Group on Mixtures and Construction formed a Task Force on Balanced Mix Design (BMD) that defined BMD 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.” BMD infers that the mixture is designed to balance between rutting resistance and cracking resistance using appropriately selected mixture performance tests rather than relying solely on volumetric guidelines.
The National Center for Asphalt Technology (NCAT) at Auburn University completed NCHRP 20-07/Task 406 Development of a Framework for Balanced Mix Design in August 2018. The framework proposed in that project allows SHAs to select performance tests of their choice for evaluation of mixture rutting resistance, cracking resistance, and moisture damage resistance. The project final report included an analysis of knowledge gaps and recommended research needs that should be addressed in order to continue progress toward the implementation of BMD. The final report also included two proposed provisional standards for BMD that were balloted by AASHTO COMP Technical Subcommittee 2d in January 2019. After NCAT revised the proposed standards based on the feedback from the initial ballot, they were unanimously approved as provisional standards in a concurrent Tech Section and main committee ballot in late 2019.
Many state highway agencies (SHAs) and the asphalt pavement industry have expressed interest in using BMD to design asphalt paving mixtures to improve pavement performance and extend the life span of asphalt pavements and overlays. Using the BMD approach is also expected to open the door to utilizing innovative materials and technologies to design asphalt mixtures and provide agencies with a more reliable way of accepting mixtures for asphalt pavements. As of February 2020, ten SHAs have
implemented BMD to limited degrees. Those SHAs are California, Illinois, Iowa, Louisiana, Missouri, New Jersey, Oklahoma, Texas, Vermont, and Virginia. Other states that are aggressively moving toward implementation include Alabama, Georgia, Kentucky, New York, Oregon, and Wisconsin. Based on a review of literature, 20 SHAs and two transportation consortiums have sponsored research projects relevant to BMD and mixture performance testing. The National Asphalt Pavement Association (NAPA) and many state asphalt associations are also actively encouraging efforts to deploy BMD.
To continue to advance the BMD approach toward implementation, SHAs and asphalt contractors need information and guidance on how to proceed. The first big decision in the path of implementation is the selection of the “best” performance tests for each type of distress? Numerous tests have been developed over the past few decades. Some are better suited for routine use in mix design and quality assurance while others are primarily focused on characterizing the fundamental properties of a mix to predict pavement response and damage. For a performance test to be included in BMD, the test results must correlate to measured field performance. Other important considerations include the test turnaround time, complexity, variability, equipment cost and availability, and sensitivity of test results to mix parameters.
Implementation of BMD will be as big of a change as was the implementation of Superpave mix design a little over 20 years ago. Some SHAs and contractors are diligently working toward implementation and have established a technical committee to work together to chart their path. Table 1 shows the general progress on key steps that need to be addressed in the path toward implementation of BMD. Some SHAs are further along with these steps than others. What many of these groups need is a summary of available information on the subject and a resource to answer questions.
The objective of this project is to assist SHAs and contractors in the early stages of BMD implementation by providing up-to-date information on using simple but robust mixture performance tests as part of mix design and quality assurance.
Work is in progress.