Comparing Different Laboratory Tests to Understand Fracture Cracking Potential in Asphalt Binders

Abstract: Fatigue and fracture cracking in asphalt pavements is one of the critical distresses, primarily caused due to long term age hardening of asphalt binders. The long-term aging (LTA) hardens asphalt binder, reducing its ability to withstand repeated traffic loads. Aging behaviour of asphalt binder varies based on their chemical composition. Therefore, understanding the aging susceptibility and associated impact on cracking performance of asphalt binders is essential for constructing durable asphaltic pavements.
The common test methods to evaluate fatigue/fracture cracking potential of asphalt binders are Superpave fatigue parameter (G*·sinδ), Linear Amplitude Sweep (LAS), and Double-Edge Notched Tension (DENT) test. Many researchers have demonstrated potential of these tests, however, field application of these tests during acceptance of asphalt binders supply may be limited by budget and skill constraints, particularly in developing countries where traditional penetration or viscosity grading system tests remain prevalent. Therefore, it is critical to come up with simple test method which can be used in the field, quick to conduct, and does not require high end instruments. Therefore, in the present study, potential of using softening point (SP) and ductility test parameters were explored and correlated with G*·sinδ, LAS and DENT. Further, chemical characteristics of asphalt binder (SARA analysis) was used to evaluate Gaestel Index (Ic), which accounts for saturates, aromatics, resins, and asphaltenes, and can help to understand quality of asphalt binders and associated correlation/impact on LTA performance. The potential of each of the laboratory tests was correlated with Ic values to identify strength of the parameter.
This study evaluates twelve unmodified VG40-grade binders collected from diverse domestic and imported sources. Short-term aging (STA) and LTA were simulated using the Rolling Thin Film Oven (RTFO) and Pressure Aging Vessel (PAV), respectively. The LTA asphalt binders were assessed for G*·sinδ, LAS, DENT, SP, ductility, and for chemical characterization (Ic). The results showed that after LTA, the SP of bitumen increased, while ductility decreased, indicating adverse impact on fracture cracking performance of asphalt binders. Further, both SP and ductility showed good correction with Ic values, emphasizing potential of these tests.
The widely used G*·sinδ and LAS (fatigue life, Nf) did not show expected trend with Ic values of asphalt binder, further, the parameter did not correlate with SP and ductility, and DENT. A good correlation was found between crack tip opening displacement (CTOD) parameter, derived from DENT, with SP and ductility, and Ic value.
The finding of the present work emphasis that G*·sinδ and LAS are not able to provide confidence on fatigue cracking of asphalt binder, while CTOD parameter is an effective tool. Further, both SP and ductility can be valuable tool for screening fracture susceptibility and durability aspects of asphalt binders.