(Buzz Powell, NCAT photo)
Industry and agencies share a unified desire for pavements that perform well under the loads of the traveling public and the environments in which they are placed. Asphalt pavement mixture design has evolved as demands on pavements increase and as engineering continues to advance in building high-performance and cost-efficient pavements. That's led to balanced mix design (BMD), which is proving successful in places like Alabama. But it might still be "new" to some road owners and mix producers.
What is BMD, how did it come to be, and what are its benefits?
The Path to BMD
The Strategic Highway Research Program (SHRP) introduced the Superior Performing Asphalt Pavement system (Superpave) in 1993 to implement performance testing and volumetric design methods to account for traffic and environment, as well as a new asphalt binder grading system. The intention was to replace Marshall stability and flow testing with a new test(s) that measured basic material properties that were more predictive of actual rutting and cracking performance.
While the Simple Performance Test (SPT) has undergone several iterations and a name change, it has not matured into a test that can be used during construction. In response to this mixture performance testing void, several different laboratory tests have been developed, many of them focused on the cracking performance of asphalt mixtures. Additionally, efforts have been made to formally shift the design focus toward balancing the cracking and rutting performance and away from some of the now well-established volumetric requirements.
When you dissect this design and testing trajectory, the factors critical to widescale adoption of the BMD process become apparent, empowering the industry to move toward mixtures that can outperform their predecessors.
A BMD Success
Many states have begun implementing BMD or are in the process of doing so. One such example provides perspective on how simple the approach can be while demonstrating meaningful advantages over the volumetric-based system.
The example BMD implementation dates back to summer 2019, when two Alabama counties (Geneva and Houston) sought advice from the National Center for Asphalt Technology (NCAT) to improve asphalt mixture quality for the purchase order materials the counties procure through annual contracts. Their initial inquiry stemmed from the idea of eliminating reclaimed asphalt pavement (RAP) from the mixtures to address performance concerns the counties had experienced with their pavements. But NCAT presented a different solution.
NCAT offered a simplified BMD approach that focused on rutting and cracking performance while allowing for increased RAP use. The approach required satisfactory performance testing during construction to ensure contracted materials would meet the counties' performance expectations.
Let's review some of the keys to implementation:
The new county specification was developed using mix performance thresholds tied to NCAT Pavement Test Track findings from the 2015-2021 cracking group experiment, which correlated field performance to laboratory performance testing. The specification threshold values for IDEAL-CT (50) and hot-IDT (17 psi) testing results in mixes that perform well in both rutting and cracking in the Alabama climate, using local aggregates and asphalt binders, as determined by testing conducted on numerous mixes from the region.
Determination of threshold values was an important step because BMD implementation should always be based on the proven performance of jurisdictional materials and mixes. Local asphalt producers embraced the specification change and quickly responded by evaluating their existing Superpave mixtures and determining the design changes necessary to meet the new requirements. Producers also explored opportunities to be innovative, improve operational efficiency and increase the sustainability of their asphalt mixtures.
Interestingly, changes made to the Superpave mix designs moved the blends toward the Marshall mixtures used in Alabama in the past, with one significant change: increasing asphalt binder content by 0.5 to 0.75 percent.
While additional asphalt binder can improve cracking test results and mixture durability, its additional cost must be offset by improved performance. In the Alabama case, the BMD mixtures allowed producers to also increase RAP content up to 35 percent, where previous Superpave mixtures only included 20 percent RAP. Increased RAP usage required producers to pay close attention to its quality.
This opportunity for innovation in meeting the performance testing requirements led to bids for BMD asphalt mixtures that were cheaper than the traditional specification for purchase order asphalt mixtures from the same producer.
The new BMD specification addressed all the asphalt mixture performance requirements in just over 350 words. The specification removed the mixture volumetric requirements, replacing them with the performance testing requirements and frequencies, which eliminated confounding constraints to produce the desired outcome.
The specification used the IDEAL-CT and hot-IDT tests, which can be conducted with equipment commonly found in asphalt laboratories in Alabama. Doing so leveraged existing staff competency without additional capital investment to quickly bring new testing protocols online.
Additionally, the performance tests are run on specimens that are easy to fabricate with a gyratory compactor and do not require cutting, reducing opportunities for error in specimen preparation. Testing results can be obtained within four hours of retrieving the sample. Rapid findings are critical for using the performance data to proactively adjust asphalt mixture production operations to maintain quality within the expected range. Determining test results does not require significant calculations and the values needed are clearly understood when the test is completed, allowing technical staff to quickly ascertain what actions are required for quality control and acceptance.
While the county was initially leaning toward eliminating RAP from the mixtures, the simplified BMD solution allowed for RAP utilization to increase (from 20 to 35 percent), while also increasing the performance and service life of these pavements. Responsible use of recycled materials complemented with high-performing, long-life pavements ensure the industry can continue to meet the needs of the traveling public in the most sustainable and environmentally-friendly manner possible. If improved performance and increased recycling were not enough, the cost savings of the new BMD specification alone made the changes a huge win for all parties and shows how much promise BMD implementation offers to the asphalt industry.
The simple BMD specification lifted the volumetric requirements and established criteria tied to local field mixture performance, freeing the producer to develop new and innovative approaches to meet performance expectations, realize economic advantages, remain environmentally conscious by using local and recycled materials, and leverage technology to enhance performance. While the new performance requirements led to a significant increase in the most expensive component of the mix — asphalt binder — producers found innovative ways to contain, and even reduce, overall costs.
Positive Outcomes in Alabama
In Geneva and Houston counties, the new BMD specification has been embraced by contractors that are finding methods to increase mixture performance, embrace innovation and reduce costs. The counties plan to continue using the specification, as they have seen quality improvement in the mixtures. Contractors have reported mixes are easier to place and compact.
The new specification captured the attention of the Alabama Department of Transportation (ALDOT) and counties throughout the state. ALDOT has a long-term plan for implementing BMD, which includes state-level shadow projects and trial projects. While researchers continue to learn more about BMD testing methods, sample conditioning options, and local threshold performance limits, simple BMD is hard at work in Alabama counties advancing safe, durable, and sustainable asphalt pavements.
Buzz Powell is from the National Center for Asphalt Technology. Brett Williams is from NAPA.
(Reprinted with permission from the National Asphalt Pavement Association)
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