Fracture characterization of gap-graded asphalt mixtures and thin bonded wearing courses

Thin bonded wearing courses (TBWC) provide an efficient treatment option for deteriorated rigid and flexible pavement systems. As a surfacing layer placed over existing, deteriorated pavement, the overlay system should be designed to resist various forms of cracking, including: thermal, block, reflective, and top-down. Recent developments in fracture testing and numerical simulation techniques have provided stronger links between material fracture properties and field cracking performances of asphalt pavements. However, little work has been directed towards applying these tools to thin bonded wearing courses and overlay systems. This paper describes fracture characterization of TBWC through testing of cored field samples and laboratory prepared specimens. The fracture characterization was performed using the ASTM D7313-07b test protocol, which is currently one of the most widely utilized test specifications for low temperature fracture energy measurement of asphalt concrete. Laboratory samples were prepared to evaluate the effects of the tack-coat application rate, compaction effort (air void level), and overlay thickness on the fracture properties of the gap-graded asphalt concrete mixes and TBWC. The fracture energy results for field cores are compared with laboratory compacted specimens of plant produced hot-mix asphalt mixture, which was sampled during construction. Moreover comparisons of fracture toughness are made between typical dense-graded asphalt mixtures, laboratory compacted gap-graded mixture, and TBWC samples. The results indicate a higher fracture resistance of the gap-graded TBWC when compared to the typical wearing course mixtures. This is a significant finding since the greater air void levels of gap-graded mixtures are typically associated with lower fracture toughness. More work is needed to further explore the fracture behavior in TBWC, especially to study the effects of crack propagation orientation. The fracture characterization results and the testing techniques presented herein provide a laboratory analysis tool for design, control, and characterization of TBWC.