Influence of Asphalt Mix Design Parameters on Performance Testing Results for Balanced Mixture Design

Abstract

There is growing interest in implementing Balanced Mixture Design (BMD) to promote longevity and durability of asphalt pavement structures. Previous design methods left asphalt pavements susceptible to cracking or rutting. BMD evaluates multiple distresses in the laboratory to ensure mixtures are resistant to these distresses. Extensive research has been conducted on the BMD design methodology, especially using the Superpave Gyratory Compactor (SGC) for specimen preparation. There is a knowledge gap regarding how production variability influences performance test results and BMD implementation, and how performance tests are influenced by different compaction methods within a BMD framework. The influence of production variability on performance tests must be understood to set tolerance limits for production to ensure that balanced designs stay balanced through production. In addition, there is growing interest in using new asphalt mixture performance tests within a BMD framework using Marshall hammer compaction rather than the currently required SGC. This would provide global transferability of such tests since Marshall hammer compaction is more widely used outside of the United States. This work aims to fill these knowledge gaps by evaluating the following objectives: (1) evaluate the influence of current production tolerance limits for gradation and binder content for the Cantabro Test, the Asphalt Pavement Analyzer (APA) Test, and the Indirect Tensile Cracking Test (IDT-CT) to determine if mixtures balanced in design can remain balanced during production through plant and material variability (Chapter 3), (2) determine if differences in laboratory compaction or specimen size produce equivalent results for the IDT-CT and the High Temperature Indirect Tensile Test (HT-IDT) (Chapter 4), and (3) evaluate SGC and Marshall hammer compaction (with uniform specimen geometry) based on the internal structure using x-ray computed tomography (CT) and performance testing using the IDT-CT and the HT-IDT (Chapter 5). Chapter 3 evaluated fourteen mixture designs that were altered to produce a coarse and fine gradation and high and low binder content based on current allowable tolerance limits for Virginia and tested for performance. It was determined that designs could become unbalanced during production. All the performance tests were sensitive to changes in asphalt content. The Cantabro was also sensitive to a coarse gradation, and the APA was sensitive to a fine gradation. IDT-CT did not show a sensitivity to changes in gradation. Mixtures that became unbalanced were a result of failing the Cantabro or IDT-CT tests. Chapter 4 and 5 evaluated 11 mixtures based on IDT-CT and HT-IDT for SGC 150 mm and 100 mm diameter specimens and Marshall (101 mm diameter) specimens according to two one-sided t-test. From the results, it was determined that Marshall and 100 mm SGC specimens were not equivalent to standard 150 mm SGC specimens. However, when comparing Marshall to Superpave 100 mm specimens (Chapter 5), IDT-CT was found to be equivalent between methods. The HT-IDT was not equivalent but resulted in a correlation between compaction methods such that the results could be estimated using the regression equation. X-ray computed tomography was evaluated for a 101 mm Marshall specimen, a 100 mm SGC specimen, and a field core from the same mixture to evaluate if similarities or differences in performance results could be explained by the internal structure. The CT imaging determined that both SGC and Marshall were influenced by restricted movement of the asphalt mixture contacting the mold which increases the porosity at the top and bottom of the sample. The Marshall specimen also showed considerable aggregate fracturing that was not nearly as extensive for the SGC and field core. These differences did not influence the IDT-CT test, but they and/or other mechanical properties may be influencing the HT-IDT test. Findings from these studies will aid in the implementation of BMD in production as well as for Marshall compaction. Results from the production tolerance study (Chapter 3) will aid agencies in understanding how changes in gradation and asphalt content affect performance testing and how to best implement these tests during production. Conclusions from the compaction studies (Chapter 4 and 5) will help agencies using Marshall hammer compaction to implement new performance tests allowing for global implementation of BMD.