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Influence of Balanced Mix Design on Asphalt Mixture Decision Making, Predicted Pavement Performance, Traffic Diversion, and User costs
Civil and Environmental Engineering
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Throughout the years, laboratory tests have been developed and used to improve the performance-related predictions of a volumetric-based asphalt mixture design. Volumetric property requirements were mainly set for improving field pavement performance. These requirements have been typically established based on specific material properties. However, with the increase use of innovative and recycling materials, these requirements shifted from guidelines to achieve a better asphalt mixture design into guidelines limiting innovation and potential improvements in asphalt mixture designs. Therefore, it is time to start looking towards the ultimate goal of Superpave mixture design by moving into a balanced mix design (BMD). Four major approaches are available for BMD (AASHTO PP 105). Each of the approaches provides different levels of freedom and flexibility compared to the traditional volumetric-based mixture design. In this research, BMD approaches are explored using typical northern Nevada materials. An array of performance tests for each of rutting and cracking are being conducted to evaluate the impact of the test selection on BMD. Moreover, the influence of asphalt mixture components (e.g., binder source, binder content, RAP percentage, RA addition, gradation) on BMD are investigated. The influence of BMD on predictive pavement performance and pavement design was studied, using FlexPAVETM. Based on predictive pavement performance, maintenance and rehabilitation treatments frequencies were explored. As these treatments requires works zones (WZs) establishment, therefore a case study was conducted on US 395 NB investigating the influence of WZs on traffic diversion and user costs. Traffic diversion impact on the signalized intersections was determined and emergency traffic signal plan was established. User costs increase, due to WZs establishment, were calculated for the default and emergency signal timing plan. Time delay and vehicle operating costs (VOCs) were considered in the user costs. VOCs calculator web application, developed by the University of Nevada, Reno, was used for fuel and non-fuel VOCs estimation.The study outcome showed that BMD is influenced by asphalt mixture properties. Binder content and sources were found to have significant influence on the BMD performance. In addition, it was experienced that higher RAP content did not automatically implied worst asphalt mixture performance. Moreover, recycling agent (RA) dosage addition was found to be binder source dependent. Subsequently, performance tests of choice were found to have influence on the convergence of the BMD. Statistical study was performed on performance tests index parameters resulting in a significant linear correlation between Illinois flexibility index test (FIT) and indirect tensile cracking test (ITCT) outcomes. This correlation was found to be binder source dependent. No correlations were found between FIT, ITCT, and cyclic fatigue outcomes, or between stress sweep rutting (SSR) test and Hamburg wheel tracking test (HWTT) outcomes. Costs analysis was conducted on asphalt mixture resulting in a poor correlation between mixtures costs and performance. In other words, mixtures with the highest costs were not found to have the best overall performances. Based on pavement performance prediction, it was found that BMD mixtures, with similar performance tests index parameters magnitude, showed unique performances throughout the design life. Different pavement structure was required for the BMD mixtures to result in a similar pavement predicted performance. Maintenance and rehabilitation treatments frequencies varied based on the BMD of choice. Therefore, BMD influenced WZ frequencies. Traffic diversion study highlighted the importance of developing an emergency traffic signal plan. This plan allowed more vehicles to divert from the highway to the signalized intersections network, reducing the travel delay time. Consequently, user costs was found to be lower when signalized intersections were operating using the emergency signal timing plan. Therefore, it was concluded that BMD influence is not limited to asphalt design. However, it reaches pavement design, pavement predicted performance, maintenance and rehabilitation treatments frequencies, and user costs. This highlights the importance of agencies and contractor to have an adequately designed asphalt mixtures and pavements.