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 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 the 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 content. Recent developments in mechanistic-empirical (ME) pavement design (i.e., Pavement ME Design) utilize mechanical material properties to predict pavement performance, which includes properties of the unbound aggregates. Therefore, understanding the mechanical properties of ABC is critical for the prediction of pavement performance, and consequently pavement design. Unfortunately, the current ABC specification is disconnected with the design process and required parameters (e.g., resilient modulus).
A more comprehensive approach to evaluate ABC 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 evaluation may aid in linking easily measured ABC properties to ME design parameters. To assess the ABC specification, a thorough testing program was developed. The testing program was complimented with analytically evaluating the ABC material using aggregate packing theory. The material and behavioral testing and analysis results were used to develop a relationship between the material properties and resilient modulus, which is a key input to Pavement ME Design.
The findings of the project indicate that the resilient behavior of ABC materials was insensitive to the gradation when inside the current NCDOT gradation specification. Furthermore, analytical packing theories confirmed that the current gradation band produces material with adequate performance. The surface texture and sphericity are statistically significant morphological properties that affect the resilient modulus of ABC. A regression analysis was performed to establish a predictive resilient modulus model; the inputs to the model include surface texture, and sphericity. Additionally, compaction processes (e.g., impact compaction) can degrade ABC materials that are susceptible to crushing. This in turn affects the resilient modulus of the ABC by changing the fabric of the material. The compaction method used in the laboratory should match the compaction processes in the field for a more representative resilient modulus; for example, if vibratory methods are used in the field, they should also be considered for the laboratory specimen preparation. Finally, the majority of the MR results for a wide range of degree of saturation are within a relatively narrow band, indicating that the inherent variability of the material may have a higher effect than saturation level on the MR results for the sources studied.
