The use of hot in-place recycling (HIR) for pavement surface rehabilitation has gained increasing attention in recent years with the aim of achieving sustainable pavement. For the purpose of cost saving and the consumption of 100% reclaimed asphalt pavement (RAP), an in-place HIR train has been developed to combine various units including preheating, milling, rejuvenating, and compacting. Compared with the conventional hot-mix asphalt (HMA) surface treatment, the HIR technique might result in different pavement performances as a result of the low mixing temperature and insufficient quality control. This study aimed to conduct a comparative analysis of pavement surface rehabilitation using HIR and HMA, including the performance evaluation, pavement life prediction, and life cycle cost analysis (LCCA). HIR and HMA mixes were collected from construction areas and the nearby asphalt plant, respectively. The loose mixes were reheated and compacted for performance testing, including dynamic modulus tests, Superpave IDT tests, and moisture susceptibility tests. The AASHTOWare Pavement ME Design software was adopted to predict the pavement life with two rehabilitation techniques, followed by the LCCA with regard to the prediction results. Test results showed that pavement surface rehabilitation with HIR achieves acceptable performance and economic benefits. However, the HIR mixes are brittle and more susceptible to cracking and, therefore, have shorter pavement service lives than HMA.

The surface energy of aggregate in asphalt pavements has a huge impact on its moisture susceptibility. The commercial antistripping agents widely used in asphalt may not be effective enough to enhance the moisture resistance if large quantities of acidic aggregates exist in pavements. This is also the reason why moisture damage is still an issue in many U.S. regions primarily supplied with “problematic” aggregates. In this regard, a promising solution is to use waste-plastic-coated aggregate, the surface of which is fundamentally modified and has a high affinity with asphalt because of the hydrophobic nature of plastic. However, it has not been widely promoted in the industry since there needs to be a practical and reliable method for uniform coating and avoiding the agglomeration of fine aggregate. To this end, an entirely new coating technology (thermoplastic polyethylene powder coating [TPPC]) was developed to coat aggregate using polyethylene (PE) powder. This technology utilizes a cheap but highly wettable fluidized bed to stably disperse the hydrophobic PE powder. Then, the dispersion liquid is sprayed onto the hot aggregate where the fluidized bed will evaporate fast while the PE particles stay, melt, and coat the aggregate surface. The microstructures of the coated aggregate were directly observed by scanning electron microscopy. Surface energy-based thermodynamic parameters were used to demonstrate the effect of TPPC on the moisture resistance of asphalt mixtures. This technology can be extended to other types of waste plastic if suitable fluidized beds are developed.

The current trend in the asphalt industry is to increase the utilization of recycled asphalt pavement (RAP). In general, there are two types of RAP binder in RAP aggregates, the mobilized RAP binder and the immobilized RAP binder. The former can be blended into the virgin binder to coat aggregates during asphalt blending, while the latter can be treated as black rocks because they have already become an outer crust of RAP aggregates. Studies have focused on the mobilized RAP binder quantification and the significance of mobilized RAP binder to mixture performances. However, it remains unknown whether the immobilized RAP binder affects mixture performances. This study developed a laboratory procedure for immobilized RAP aggregates preparation and investigated the contribution of immobilized RAP binder to asphalt-aggregate interaction as well as the performance of 100% recycled asphalt mixtures. The work of adhesive energy and the $K\text{-}B\text{-}{G}^{*}$ model were adopted to characterize the asphalt-aggregate interaction for asphalt mortar. Resilient modulus and dissipated creep strain energy were used to evaluate the stiffness and cracking resistance of 100% recycled asphalt mixtures. Results showed that the immobilized RAP binder can strengthen the asphalt-aggregate interaction and improve the cracking resistance of 100% recycled asphalt mixtures by increasing total asphalt content. However, if the total asphalt content remains the same, the immobilized RAP binder increases stiffness but decreases the cracking resistance of 100% recycled asphalt mixtures.

Correct compaction is vital for asphalt mixture service life. An adequately compacted mixture with inferior properties can achieve better performance than a mixture with excellent properties but poorly compacted. This study investigated resistance to damage caused by over-compaction by utilizing the locking point concept. Over-compaction might cause damage to the aggregate structure and decrease service life. The locking point is defined as the moment during mixture compaction at which an aggregate skeleton is developed and becomes stable. Beyond the locking point, more compaction energy does not significantly increase mixture density and can damage aggregate particles. A total of 15 mixtures was utilized and evaluated using the gyratory compactor. Among them, five dense-graded plant mixtures contained different aggregates and binders, and 10 laboratory mixtures (three types: the surface, the base, and stone mastic asphalt [SMA]) were designed with the most popular coarse aggregates in Tennessee: hard limestone, soft limestone, gravel, and granite. The results of this study show that the highest locking point was reached by the mixtures containing gravel. The SMA mixtures have, on average, lower locking points than the dense-graded mixtures. Most of the dense-graded mixtures made with crushed stones failed in the range of +20 to +30 gyrations, whereas the samples made with gravels failed in the range of +30 to +40 gyrations, indicating that gravel seems to be the most resistant to damage.

Aggregate gradation is crucial for the performance of asphalt mixtures. However, the adequate mix design must include the compromise between performance and workability. This study used two compaction methods: the impact (Marshall Hammer, Humboldt, Elgin, Illinois) and gyratory [Superpave Gyratory Compactor (SGC), Pine Instrument Company, Grove City, Pennsylania], to investigate the influence of aggregate gradation on the compaction of asphalt mixture utilizing the concept of the locking point. For the impact compaction, an accelerometer was placed on the falling mass of the Marshall Hammer to record the asphalt mix response, with which the impact locking point was defined. For the gyratory compaction, the densification curves were utilized to define the locking point. Twelve mixes, three for each mixture type (base, intermediate, and surface) were designed and prepared in the laboratory. The asphalt mixes were tested for their locking points in relation to their type and gradation. Results indicated that the locking point could be determined for the majority of the mixtures compacted by both the impact and gyration methods, except for the mixture composed of the bigger stone and coarser gradation. The locking point of the asphalt mixture was strongly dependent on its gradation. A coarser mix resulted in a higher locking point. Thus, the maximum aggregate size was not the only factor that determined the locking point but the whole gradation.