A Look at the Bailey Method and Locking Point Concept in Superpave Mixture Design.

This study analyzes the physical and performance characteristics of asphalt concrete mixtures with aggregate structures designed using the Bailey method of aggregate gradation evaluation. Three aggregate types—limestone, sandstone, and granite—were considered in this study. Three different aggregate structures of 12.5-mm nominal maximum particle size were designed for each aggregate type.
Mixtures were designed for high traffic levels. The binder type selected was performance grade 76-22M. The compaction characteristics of the mixtures were analyzed using data from the Superpave gyratory compactor. Locking points and compaction indices were defined and determined for all the mixtures in the study. Simulative (Hamburg wheel tracking test) and fundamental (semicircular fracture and IT strength) tests were conducted to determine laboratory performance properties and evaluate the mixtures under different loading and environmental conditions. The design number of gyrations (Ndes) recommended by Superpave was compared to the locking points obtained from this study. The data indicate that the current Ndes level recommended by Superpave is much higher than the locking points of the mixtures and may subject the mixtures to high compaction energy for an extended period of time.
Selected mixtures were designed using the locking point as the design number of gyrations instead of the recommended Superpave Ndes. The data presented in this paper suggests that mixes with dense aggregate structures can be designed using their locking point instead of the recommended Ndes. The designed mixtures maintained good resistance to permanent deformation and maintained an adequate level of durability.