Nebraska Balanced Mix Design – Phase I

Typically, asphalt mixtures are designed under the Superpave system, in which the proportioning of the aggregates and asphalt binder is based on the aggregate quality characteristics and mixture volumetric properties such as air voids, voids in the mineral aggregate (VMA), and voids filled with asphalt (VFA). The estimation of the volumetric properties significantly depends on the accuracy of the estimated specific gravity of the mixture components, however, there are some serious concerns regarding the accuracy and variability of aggregate bulk specific gravity testing and the selection of suitable amount of asphalt in the mixture design. Mixtures designed with high amount of asphalt can be susceptible to rutting, while those with low asphalt content are prone to cracking, raveling, or other durability related pavement distresses. It should be pointed out that with the incorporation of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS), the latter concerns are intensified. Moreover, the effects of binders in these recycled materials are not completely clear in volumetric properties. For instance, the interaction of the recycled binders with the virgin binders, that affects the field performance, is unclear. On the other hand, the effects of warm-mix asphalt (WMA) additives, polymers, recycling agents (i.e., either rejuvenators or softening agents), antioxidants, and fibers cannot be evaluated in the conventional volumetric mix design method. One of the largest shortcomings of current design procedures is the lack of long-term materials aging protocol. Therefore, performance tests should be included as part of the mixture design procedure to ensure the desirable field pavement performance. Many performance tests have been proposed for the evaluation of the rutting resistance (e.g., Hamburg Wheel Track Tester, Gyratory Stability, IDEAL Rutting Test, and High-Temp Indirect Tensile Test), cracking resistance (e.g., Semi-circular Bending Test and IDELA Cracking Test, and moisture susceptibility (e.g., Indirect Tensile Strength, Hamburg Wheel Track Tester) of the asphalt mixtures. In Balanced Mix Design (BMD), two or more mechanical tests are coupled to quantify the mixture resistance against different forms of distress. In Texas, where the BMD concept originated, Hamburg Wheel Tracker Test (HWTT) and overlay test (OT) are employed to estimate the resistance to rutting and cracking, respectively. In Louisiana, HWTT and semi-circular bend (SCB) test (J-integral) are applied to assess rutting and cracking potential of mixtures, respectively. The Illinois approach is similar but uses a simpler SCB test that yields a parameter called Flexibility Index to assess cracking resistance. The Nebraska DOT conducted a research project (SPR-1(18) M070) to examine the feasibility of the BMD approach and develop a potential BMD method for Nebraska pavements. The results of this research indicate that the Gyratory Stability and the SCB tests for the evaluation of rutting and fatigue cracking potential of mixtures are applicable especially regarding the inexpensive equipment used to conduct all the necessary tests. Despite these efforts, there are still some gaps in the knowledge needed for future development of a detailed and comprehensive standard practice. Some of the gaps include: 1) applicability of the tests to quality assurance and control (QA/QC) testing, 2) suitability of the tests to assess the effects of non-traditional materials such as recycling agents and antioxidants on the mixtures performance, 3) relationship of test results to pavement performance, and 4) conditioning protocols for aging and moisture.