Exploratory Evaluation of Cracking Performance of a 4.75 mm NMAS Overlay Using Full-Scale Accelerated Loading

State transportation agencies have begun to develop and implement specifications for 4.75mm Nominal Maximum Aggregate Size (NMAS) Superpave mixes with some specifications based on recommendations from a study conducted by the National Center for Asphalt Technology. These mixes restore surface texture and ride quality but also have advantages such as optimal use of available aggregates, accommodation of Reclaimed Asphalt Pavement (RAP), and application as an impermeable, thin preservation treatment. A trial 4.75mm NMAS from Virginia DOT was placed as a thin treatment on existing accelerated pavement test sections. The objectives of this study were to conduct full-scale load testing to gain confidence in a new mix design and to explore the ability of this thin treatment to curtail top down cracking. Several sub-sections, which were reserved and left unloaded from a preceding study, received a 25mmthick mill-and-fill with the 4.75mmNMAS mix. The construction produced a test section where half of the loaded wheel path was paved with the 4.75mm NMAS mix allowing a direct comparison of cracking performance with and without the thin treatment. Full scale accelerated aging was utilized to compare the fatigue cracking performance for four combinations; with and without 4.75mm NMAS treatment each with and without aging. Crack maps illustrated that the unaged 4.75mm mixture’s cracking performance exceeded the life of the sections without the treatment. Estimates of the increase in life provided by this thin treatment exceed 8 years. Forensic coring has shown top-down cracking to be the predominant distress. When aged, however, the 4.75mm treatment provided no additional life, but performed as well as an aged section without the treatment.