Balanced Mix Design (BMD) is a mix design process that adds additional virgin bitumen (the glue) in hot mix asphalt (HMA) while meeting laboratory performance tests such as rutting and cracking. Overall, the use of BMD allows us to focus on designing asphalt for performance.
Quality control mix design labs typically design HMA mixtures targeting 4% air voids—small airspaces or pockets of air that occur between the coated aggregate particles in the final compacted mix. By adding slightly more virgin bitumen, the air voids can drop below 4%.
Once the mix is done, it is tested using rutting tests such as the Hamburg Wheel Tracking Test and cracking tests such as the Indirect Tensile Asphalt Cracking Test (IDEALCT). The Hamburg Wheel Tracking Test uses stainless steel wheels on a weighted cantilevered arm, and in conjunction with water is used to evaluate the resistance to rutting and moisture susceptibility of asphalt mixtures. The IDEAL-CT is an indirect tension test that can determine the tracking potential of asphalt mixtures with a fracture mechanics-based parameter.
Some states—such as Illinois and New Jersey—have already begun to require the use of BMD, while others—such as Pennsylvania, New York, and Virginia—are evaluating how the current materials (bitumen and aggregates) perform in mix designs following the BMD methodology and then benchmarking the results against current mix design criteria and field performance.
In the end, the goal is to make more durable hot mix asphalt roads without sacrificing rutting and cracking. Road owners are seeking more durable HMA roads and understand the cost of high quality, and it is up to the quality control technical teams to design and work closely with production and construction to deliver on meeting the BMD specifications.