Mechanical, Rheological, and Spectroscopic Characterization and Environmental Assessment of Rejuvenated Asphalt Mixtures for Sustainable Pavements

Abstract

The use of Reclaimed Asphalt Pavement (RAP) in asphalt mixtures offers great environmental and economic benefits. RAP reduces the amount of virgin materials in asphalt mixtures and conserves the natural resources. In addition, it reduces the greenhouse gas emissions (GHG). Despite these benefits, RAP can negatively impact the performance, durability, and longevity of asphalt pavements leading to premature failure. Meanwhile, the pavement industry is moving towards the implementation of Balanced Mix Design (BMD) that offers innovation and flexibility in developing asphalt mix design. The BMD aims to provide asphalt mixtures with sufficient resistance to permanent deformation and cracking. This approach has offered a unique opportunity to integrate sustainable solutions in the construction of flexible pavements by using high RAP content and rejuvenators in asphalt mixtures. This study aims to develop and implement advanced test methods and tools to enhance the design and performance evaluation of asphalt mixtures with RAP to construct more sustainable pavements. The first phase of this research investigated the use of several types of rejuvenators or rejuvenators to enable the design of high RAP asphalt mixtures. This study used the BMD approach to optimize the dosage of rejuvenators at different RAP contents to ensure comparable or superior performance to control mixtures (i.e., mixtures without RAP or rejuvenators). The study examined the performance of five different types of rejuvenators used in asphalt mixtures containing RAP from different sources at various RAP contents. For this study, a comprehensive laboratory testing program was conducted to evaluate rutting, low temperature, and intermediate-temperature cracking resistance performance. The results demonstrated that rejuvenators can be used effectively at a high RAP content to produce asphalt mixtures that meet the performance criteria. Tall oil and waste vegetable oil rejuvenators were found effective in improving the mechanical properties of asphalt mixtures containing high RAP content compared to other examined rejuvenators. The rejuvenators were able to counter the detrimental impact of using higher contents of RAP in asphalt mixtures which leads to stiffer and brittle mixtures susceptible to premature cracking. Rejuvenators softened asphalt mixtures with high RAP content and restored its flexibility which greatly improves the resistance to intermediate- and low-temperature cracking resistance of the mixtures. Furthermore, the first phase of this study provided an economic and environmental comprehensive analysis of the benefits associated with the use of high RAP contents and rejuvenators in asphalt mixtures. The study investigated the economic benefits through examining the cost savings associated with decreasing the amount of virgin materials as well as the environmental benefits associated with the reduction in energy consumption and carbon dioxide equivalent (Co2-eq). The result clearly showed a positive impact of using rejuvenators and RAP in asphalt mixtures which led to cost savings and reduction in greenhouse gas emissions (GHG). The second phase of this study focused on the characterization of extracted and recovered asphalt binders from selected asphalt mixtures from the first phase of this study. The author measured the rheological, chemical, and molecular characteristics of the extracted asphalt binders. This study established and implemented a simple extraction and recovery technique in the laboratory to extract and recover the asphalt binder from mixtures tested in the first phase. This method reduces the amount of hazardous solvents used and doesn’t alter the properties of asphalt extracted and recovered binders. The binder was tested using a Dynamic Shear Rheometer (DSR) to evaluate the effect of rejuvenators on the rheological properties of aged binders. Several binder performance parameters were investigated including but not limited to Rheological value (R-value), Superpave intermediate temperature (PGI), crossover temperature (Tδ=45°), and Glover-Rowe parameter (G-R). Additionally, advanced characterization tools such as Fourier Transform Infrared (FTIR) and Electrospray Ionization (ESI) were used to further examine the binders at the spectroscopic level. Also, this phase investigated possible correlations between various mixture performance indices evaluated in the first phase of this study, and the binder rheological parameters evaluated in the second phase. The results of the second phase confirmed the findings of the first phase and demonstrated the efficiency of the rejuvenators in reducing the stiffness of aged RAP binder which reduced the complex modulus of the extracted and recovered binders. Various rheological parameters including PGI, G-R, and Tδ=45°, showed improved performance when rejuvenators were used with high RAP content mixtures. The advanced binder characterization using FTIR and ESI demonstrated that increased oxidation aging with the increase of RAP content in asphalt mixtures where the carbonyl index (IC=O) and the molecular weight increased with RAP binder. The use of rejuvenators reduced the binder oxidation in mixtures with high RAP content, especially when using softer binders (e.g., PG 58-34). The results of the ESI analyses showed that molecular weight and number increased with aging. There were also excellent correlations among binder rheological parameters (i.e., Wc, G-R, and PGI) and Tδ=45° and R-value. In addition, good correlations were observed between rheological parameters (i.e., G-R, PGI, LAS damage parameter B) and work of fracture and fracture energy-related parameters of the mixtures. These results explained the favorable impact of rejuvenators at the binder level and the role of the binder rheological characteristics on the cracking performance at the mixture level. The third phase of this study developed a monotonic direct tension test and Multi-Stage Semi-Circular Dynamic (MSSD) tension tests to examine cracking resistance and fatigue life of asphalt mixtures. The results were compared to the conventional indirect tensile (IDT) strength test. The direct tension and MSSD testing protocols addressed the limitations of the current testing protocols. This study evaluated the performance of asphalt mixtures with different mix compositions under direct tension loading. The monotonic direct tension test evaluated the dissipated energy and tensile stiffness index, while the MSSD test evaluated several fatigue parameters including the crack progression rate, critical fracture energy, dissipated energy change, and the number of cycles until failure. This study evaluated how these protocols capture the changes in mix composition, including variations in binder content (i.e., 4.8, 5.8, and 6.8%), air voids (i.e., 4, 7, and 10%), and binder grades (i.e., PG 58-28, PG 64-28, and PG 76-22). The results of the monotonic direct tension test demonstrated that mixtures with higher binder content (i.e., 6.8%) had higher dissipated energy indicating better cracking resistance compared to mixtures with lower binder contents (i.e., 4.8% and 5.8%). Also, mixtures with lower air voids (i.e., 4%) and mixtures with stiffer binder grades (i.e., PG 76-22) had higher dissipated energy compared to those with lower air voids or used soft binder grades. The results for the MSSD test demonstrated that mixtures with low binder content had higher critical fracture energy indicating stiffer mixtures; the same mixtures had a higher crack progression rate indicating lower fatigue resistance. Moreover, Pearson and Spearman correlation results between the monotonic tests indices showed excellent correlations between the force related parameters (e.g., Fmax and IRT) in the direct tension test and the strength parameters (e.g., IDTStrength and IDTModulus) in the indirect tensile strength test.