ABSTRACT
ABSTRACT: The importance of aggregate and soil layers within the pavement structure has been appreciated from the time that the California Department of Transportation initiated the use of the California Bearing Ratio in the 1920s. In the late 1950s, the American Association of Highway and Transportation Officials (AASHTO) Road Test assigned structural layer coefficients to all pavement layers including the unbound aggregates base and subbase. Subsequent satellite studies to the AASHTO Road Test assigned structural layer coefficients to chemically stabilized materials (CSMs) used as pavement layers. This lecture discusses significant advancements made within approximately the last 15 years in modeling the performance of unbound aggregates base and subbase layers and CSM layers. The importance of correctly considering not only the hardening and softening effects of stress state on unbound layers but also cross anisotropy are discussed. The new AASHTO mechanistic-empirical pavement design guide (MEPDG) accounts for stress state but not for anisotropy. Prior to development of the MEPDG, the AASHTO structural layer coefficients of CSMs were linked to resilient modulus values of the respective CSM; however, a substantial disconnect exists between moduli versus layer coefficient predictions for unbound aggregate base layers and similar relationships between modulus values and layer coefficients for CSMs. This lecture describes why this disconnect exists and how this is related to a difference in the mode of damage for the respective layers. The lecture discusses the adequacy of the MEPDG approach for assessing the structural value of CSMs as well as granular layers. The lecture further compares the MEPDG-type approach to other widely used structural pavement design methods such as the Texas Flexible Pavement method and the 1998 AASHTO method.
