Impact of Production Variability on Balanced Mix Designs in Virginia

About the report:

Balanced Mix Design (BMD) promotes durable pavements by ensuring resistance to multiple modes of distress through laboratory performance testing. BMD can also address material variability concerns present in volumetric design and encourage sustainability by allowing more recycled materials and innovative technologies. An example of addressing material variability concerns includes changes in aggregate specific gravity, which can change the volumetric properties through production.  If performance testing is assessed through production, performance is verified despite inherent aggregate variability. Benchmarking studies and pilot projects have been conducted to select performance tests and threshold limits and evaluate the feasibility of implementing BMD in production. However, more information is needed regarding how plant variability can influence performance test results, specifically for Cantabro Mass Loss test, Asphalt Pavement Analyzer Rut Depth test (APA), and Indirect Tensile Strength Cracking test (IDT-CT), to fully implement performance testing into production.

The objective of this study was to evaluate the influence of production variability on performance test results to determine if mixtures balanced during design could become unbalanced during production. Fourteen currently accepted mixture designs, 10 BMD mixtures and 4 Superpave mixtures (to serve as a comparison between current standards and BMD designs), were recreated in the laboratory. Additionally, the mixes were adjusted to produce coarse and fine gradations and high and low binder contents according to currently accepted tolerance limits to simulate production variability. Two interaction mixtures assessing changes in gradation and binder content were evaluated as well as two critically aged mixtures.  

Based on performance results, it was determined that mixtures originally balanced could become unbalanced due to production variability. Mixtures showed excellent rutting resistance but could be susceptible to durability and cracking issues as the gradation and binder content change. The Cantabro, APA, and IDT-CT were sensitive to changes in binder content. Highbinder content improved Cantabro and IDT-CT results but reduced the mixtures resistance to rutting, though this increase generally did not cause results to fail the threshold limit. Gradation was not found to be significant for IDT-CT, however, a coarse gradation negatively influenced Cantabro results, and a fine gradation hindered APA results. When evaluating the interaction between changes in gradation and binder content, performance test results were sometimes further negatively influenced. Complexities in results could be a performance test response to changes in volumetric parameters. 

Based on the findings, this study recommends further refinement of the BMD specifications to ensure mixtures stay balanced through production despite variability. VTRC should also continue to assess the influence of volumetric parameters on performance test results since the interaction mixtures showed greater influences than other variations or different trends all together. Benefits of this study include furthering implementation of performance-based design and acceptance and informing VDOT regarding potential challenges with plant variations and their impact on volumetrics and performance results.

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