A Roadmap for BMD Implementation

Balanced Mix Design (BMD) represents a new era in how asphalt mixtures are designed and accepted and is a major step toward the implementation of performance specifications. As such, it continues to be a primary focus among many highway agencies and industry stakeholders. A few states have already implemented specifications using BMD tests and criteria as part of their routine asphalt paving projects, and several others are quickly moving toward that goal. Most states, however, are in the process of selecting the tests, gathering test data and trying to figure out how to establish criteria and address other gaps and questions prior to moving forward. 

Implementing BMD will be a big change for highway agencies and the asphalt paving industry, and a number of important decisions will need to be made along the way, such as: 

• Which BMD tests should we use? 
• How do we establish criteria for the new tests?  
• Do we add the BMD tests and criteria to existing mix design criteria or replace legacy criteria with BMD criteria? 
• Should BMD be used for special applications, specific categories of projects, or all mix types on all projects? 
• How should we deal with the effect of mixture aging in a BMD system? 
• What approach should an agency use to validate a contractor’s BMD test results? 
• Should BMD tests be used in quality control and acceptance decisions, and if so, at what frequency and what actions should be taken when a production mix sample fails the criteria? 
• What timeline is appropriate for achieving BMD implementation milestones?  

NCAT researchers recently completed a guide for BMD implementation as part of NCHRP project 10107. The guide presents NCAT’s recommendations based on experiences from Superpave implementation, guidance in existing AASHTO standards and reports, the evolving body of research on performance testing, and ongoing BMD implementation efforts among state highway agencies. The guide includes eight chapters that coincide with the eight major steps to implementation, as shown in Figure 1.  

Chapter One discusses the motivations for implementing BMD. Many highway agencies are motivated to try BMD because they are not satisfied with the field performance of their asphalt pavements built with current specifications. Some agencies understand the significant weaknesses with the volumetric mix design systems and the lack of correlation between  the legacy criteria to field performance. Some also believe that we have to change to a system that better evaluates a mixture’s resistance to performance so that we will be able to utilize technologies that are longer-lasting and pavement materials that are more sustainable. The chapter references case study reports from seven “early adopter” BMD states that have either begun to implement BMD or have already gone “full Monty” into BMD.   

Chapter Two covers key elements in planning the overall implementation process. The guide recommends identifying leaders for change in the agency and the industry, as well as a BMD technical committee to map out goals, identify resources, and address knowledge gaps. Major tasks and subtasks are listed, and an example Gantt chart is provided to aid in planning.  

Chapter Three presents guidance on selecting BMD tests and validating their relationships to pavement distresses. A critical step is to identify the jurisdiction’s primary modes of distress so that appropriate tests can be selected. The chapter discusses the impact of reheating and mixture aging on test results and acknowledges that decisions need to be made on these procedures that balance timeliness and the ability to reasonably represent the field conditions when the distresses develop. One of the most important recommendations of the guide is to validate the relationships between the lab test results and field performance on actual projects in each state. Although this can be done in several ways, the best approach is to build test sections for a group of validation experiments that will become the basis of setting appropriate BMD criteria. NCAT is now developing more detailed guidance specifically for field validation experiments through funding from CAPRI.  

Chapter Four covers considerations for acquiring equipment, allocation of laboratory space, staffing needed to work on implementation efforts, and preliminary training. In addition to purchasing new equipment for contractor and agency labs, all organizations will need to consider what additional human resources are needed to gear up for BMD. Training new and experienced personnel on the new tests as well as developing consistent sample preparation procedures will also be necessary. Participating in a proficiency testing program is recommended as an excellent way to make sure the BMD tests are being conducted properly. 

Chapter Five provides recommendations for establishing baseline data. The first part of the chapter discusses developing and analyzing benchmarking databases of existing mixtures. It is vital that benchmarking databases include detailed information and results for lab-prepared mixtures and plant-produced mixtures representing a wide range of materials used in the state. The second part of the chapter discusses shadow projects which involve sampling and BMD testing of numerous plant-produced mixtures at the same frequency currently used for acceptance testing. The primary goal of shadow project testing is to gather production variability data for the new tests, with the BMD test results for informational purposes only. The “within-lot” standard deviation or coefficient of variation are the key statistics of interest, which contractors need to set production targets to meet the agency’s BMD criteria with considering production variablity.. 

Chapter Six presents considerations needed to develop BMD specifications and possible adjustments to a state’s Quality Assurance Program. This chapter assumes that BMD testing will eventually become part of routine testing for QC and acceptance of plant-produced mixtures. For that to happen, agencies may need to consider revisions to their existing sampling and testing plans, reevaluate their acceptance quality characteristics (i.e., pay factor items), along with their incentive and disincentive provisions. Much of the data gathered in Chapter 5 will be needed to make appropriate decisions for pilot projects and specifications. Pilot projects are recommended for the first few years of implementation to gain familiarity with the tests and specifications and allow for adjustments in the system. 

Chapter Seven discusses possible modifications for an agency’s asphalt technician training, qualification, and laboratory accreditation programs. Ongoing research indicates that most BMD tests are more sensitive to sample handling and specimen preparation than volumetric properties; thus, agency and contractor testing personnel will need training on the new methods to ensure accurate and meaningful results. New inspection checklists will also be needed for laboratory accreditations. 

And finally, Chapter Eight presents basic guidance on transitioning to the new methods, specifications, and QA program to full implementation. The key point in this chapter is that the early stages of implementation should include a process for feedback and possible adjustments to test methods, specification criteria, sample frequencies and handling protocols, training efforts, etc., through agency and industry collaboration. The BMD technical committee suggested in Chapter 2 should provide a forum for discussion of issues and collaborative solutions throughout the process of BMD implementation. 

For a copy of the draft final BMD implementation guide, click this link or visit the NCAT website, www.ncat.us. 

Contact Randy West for more information about this research.