Evaluation of Engineering Properties of Hot Mix Asphalt Concrete for the Mechanistic-Empirical Pavement Design

Hot Mix Asphalt (HMA) is a viscoelastic material and has been broadly used in pavement structures. It is important to understand the mechanism of complex behaviors of HMA mixtures in field for improving pavement mechanical performance. Aggregate gradation and asphalt binder are two key factors that influence the engineering properties of HMA. The asphalt binder plays a significant role in elastic properties of HMA and it is the essential component that determines HMA's viscous behavior. Many research works suggest that Styrene-Butadiene-Styrene (SBS) polymer is a promising modifier to improve the asphalt binder, and hence to benefit the HMA viscoelastic properties. The specific beneficial characteristics and appropriate polymer concentration need to be identified. In addition, aggregate gradation requirements have been defined in Superpave mix design criteria. However, a potentially sound coarse mixture with the gradation curve passing below the coarse size limit may be disqualified from being used. There is a need to evaluate the Superpave gradation requirements by studying mixtures purposely designed exceeding the control limits. Moreover, the mechanical parameters adopted by AASHTO to characterize HMA properties are shifting from indirect diametral tensile (IDT) test to dynamic modulus test (DMT), because the DMT has the ability to simulate real traffic conditions and to record more viscoelastic information of HMA. Thus, the DMT and the IDT test for implementing the AASHTO Mechanistic-Empirical Design Guide (M-E PDG) are needed to be discussed. The primary objective of this research study was to evaluate the fracture mechanics properties of HMA concrete and to study the correlation between the DMT and the IDT test for Superpave mixtures. An experimental program was performed on asphalt mixtures with various types of materials. The laboratory testing program was developed by applying a viscoelastic fracture mechanics-based framework that appeared to be capable of describing the whole mechanical properties of HMA according to past research studies. The goals for these experiments are to evaluate the effect of aggregate type, the effect of gradation adjustment to control mix designs, and the effect of SBS polymer on fracture mechanics properties of HMA mixtures. Two standard coarse mixes were selected as control levels for fracture mechanics tests: one granite mixture and one limestone mixture. Each control mix design was modified to two different gradation levels with the control asphalt binder (PG 67-22) and three SBS polymer content levels (3.0%, 4.5%, and 6.0%) with the original aggregate gradation. The experimental program for dynamic complex modulus test involved 20 Superpave asphalt concrete mixtures commonly used in Florida with a range of aggregates and mix designs. Data evaluation of the test results indicated the increase of nominal maximum size aggregate amount by 5% to 15% to the standard coarse mix designs had negligible effect on HMA fracture mechanics properties. The SBS polymer-modified asphalt binder improved the fracture mechanics behavior of asphalt mixtures comprehensively. The limestone materials hold advantages over granite materials in improving the performance of thermal cracking at low service temperatures and the rutting resistance at high service temperatures. The master curve construction and linear regression analysis indicated that the total resilient modulus increased with an increase in dynamic modulus at a specific loading frequency. The resilient modulus values were comparable with the dynamic modulus values at the loading frequency of 4 Hz. A correlation relationship was developed for predicting the dynamic modulus from existing resilient modulus values of the asphalt concrete mixture in implementing the mechanistic-empirical pavement design.