Development of Balanced Mix Design for Thin Asphalt Overlays for Collector and Local Roads

Thin overlays are favored by local agencies due to their ability to extend the pavement'slifespan and enhance ride quality. However, the low thickness of thin overlays presents some inherent challenges. The use of conventional mixes for constructing thin overlays has led to numerous premature failures, primarily due to the relationship between compaction, the Nominal Maximum Aggregate Size (NMAS), and lift thickness. The current study's objective was to utilize a balanced mix design to enhance the quality of mixes used by local agencies by developing two new dense-graded mixes and one Stone Matrix Asphalt (SMA) mix. Local mixes were collected and studied, working closely with industry experts. This research work aimed to identify the performance characteristics of commonly used mixes, optimize these mixes, and design new mixes that better suit their intended application, thereby prolonging the life of overlays. The findings indicated that while the current mix designs are fundamentally wellstructured, they are not appropriate for the given application due to the unsuitability of a 12.5 mm NMAS for mix designs below 38 mm, especially considering that most overlays are less than that. The results also showed that the current mixes are already optimized in terms of cracking and rutting resistance. Three new mixes with 9.5 mm NMAS aggregates and SBS modified binder were designed. These include two dense-graded mixes using PG 76-22 SBS and PG 70-28 SBS modified binders, and one SMA mix utilizing the PG 76-22 SBS modified binder. All theseii mixes demonstrated better cracking properties compared to commonly used mixtures. While their rutting properties were either comparable or occasionally inferior but meeting the rutting criteria. Based on these findings, it can be proposed that the use of a 9.5 mm NMAS mix improves compaction and compatibility with lift thickness. Additionally, these mixes reduce susceptibility to cracking and extend service life of the overlay. To get a superior overlay mix, SMA can be employed as it had 2.5 times better CT Index compared to the conventional 12.5 mm mix.