Developing Balanced Mix Design Gyrations (Ndesign) for North Dakota’s Hot-Mix Asphalt Pavements

There is a need to develop a new mix design criterion for North Dakota especially for medium- and low-volume traffic pavements, where the durability performance of the hot-mix asphalt (HMA) is of the most importance. High Ndesign numbers tend to lower the asphalt binder, thus lowering the durability of the asphalt mix. In contrast, if Ndesign is reduced, it tends to increase the asphalt binder, thus improving the durability of the asphalt mix. The main objective of this study was to develop balanced mix Ndesign values for North Dakota pavements based on the HMA performance with respect to resistance to rutting, low-temperature cracking, and fatigue cracking. Project mix samples were constructed and tested based on Ndesign values of 75, 65, 55, or 50 gyrations. The sample matrix included fine aggregate angularity (FAA) values of 45, 43, and 40, representing high-, medium-, and low-volume pavements, respectively. The matrix also included the asphalt binders PG 58S-28, PG 58H-34, and PG 58H-28. An analysis of variance (ANOVA) was done on the performance results and showed that the results corresponding to the various gyration levels were significantly different and can be compared. Test results revealed that a lower number of gyrations and a higher binder content resulted in higher resistance to fatigue and thermal cracking, while rut resistance stayed above acceptable levels. Based on the test results, the authors recommend an Ndesign of 65 gyrations for the high-volume pavements and 50 gyrations for intermediate- and low-volume pavements.