Re-appraisal of the Specification for Aggregate Base Course (ABC)

The current specification for acceptance of Aggregate Base Course (ABC) materials consists of a band-type gradation specification, which is essentially a "recipe" that dictates the mass percentages of the individual particle sizes constituting the ABC. These specifications were developed about a half century ago with few adjustments since then and are similar to the majority of the departments of transportation (DOTs) around the country. The "recipe" specification is based on the assumption that the product will achieve the desired engineering performance as long as it meets gradation specifications and is placed and compacted properly in the field. The biggest disadvantage of the "recipe" specification is that it cannot quantify the mechanical behavior of the aggregates under different traffic and weather conditions, which will determine the stress states and moisture variations. Recent developments in mechanistic-empirical (ME) pavement design (i.e., Pavement ME Design) utilize mechanical material properties and structure to predict pavement performance, which includes properties of the unbound aggregates. Therefore, understanding the mechanical properties of ABC is critical for prediction of pavement performance, and consequently design. Unfortunately, the current ABC specification is disconnected with the design process and required parameters (e.g., resilient modulus). A more comprehensive approach to test, evaluate, approve, and accept aggregate base course material is needed. Gradation of the ABC alone is insufficient to adequately capture the performance of the pavement during the service life. Incorporating easy to measure physical characteristics of the ABC (e.g., angularity, shape, and texture) and aggregate packing theory into the material specification will aid in linking easily measured ABC properties to observed mechanical behavior, which will allow the new specification to be directly related to the ME design parameters. Additionally, the re-appraisal of the ABC specification should include tests that are easily implementable such that they can be practically used routinely. To best meet the objectives of the proposed research, first, a literature review of current practices for ABC specification used throughout the US, identification of critical properties governing ABC mechanical behavior, and state-of-the art test methods to efficiently characterize the identified properties will be conducted. The literature review will be followed by a testing program that focuses on both physical material properties and mechanical behavior of the ABC material. The testing program will be complimented with modeling the ABC material using aggregate packing theory and the discrete element method. The material and behavioral testing and modeling results will be compiled to develop a relationship between the material properties and pavement performance, which in term will be used to propose a new specification for ABC material.