Abstract:
HMA mix design consists of the optimum combination of two basic ingredients: aggregate and asphalt binder. Several alternative methods for the mix design including Marshall, Hveem and Superpave methods are in practice. In order to meet the diverse and often conflicting performance demands e.g. resistance to fatigue, deformation, cracking and moisture damage; durability; skid resistance; workability and economy, the mix designer generally manipulates three variables of aggregate, asphalt binder and the ratio of asphalt binder to aggregate. By manipulating these variables, mix design seeks to achieve aforementioned performance standards. A successful mix design results in a recommended mixture of aggregate and asphalt binder which includes aggregate gradation and asphalt binder type, often referred to as the job mix formula (JMF). The purpose of this study was to characterize the performance of various Hot Mix Asphalt (HMA) mixtures procured from highway construction projects in Pakistan based on their laboratory testing. Laboratory characterization tests mainly included simple performance test (SPT) dynamic modulus |E*| of Marshall designed HMA mixtures. Seven plant produced mixtures were selected for the study. Specimens were prepared using the Superpave Gyratory Compactor. Furthermore two dynamic modulus prediction models, namely Witczak and Hirsch were evaluated for HMA mixtures in Pakistan. Analysis of the results indicated that both the Witczak and Hirsch models mostly under predict the dynamic modulus values of HMA for pavements in Pakistan with average under prediction errors of 48% and 72%, respectively. The study investigated the relationships between the dynamic modulus and mixture composition examined in the study and established that temperature and loading frequency had a significant effect on the dynamic modulus of HMA. However, the effect of other factors like NMAS was found to be insignificant. The study also established default dynamic modulus values catalog for typical asphalt concrete mixtures of Pakistan at various temperatures and frequencies by developing the master curves of E* thus facilitating the implementation of performance based mechanistic empirical structural design and analysis approach.
