Cracking prediction of asphalt concrete using fracture and strength tests

Strength and fracture-based tests have been developed to assess the cracking potential of asphalt concrete (AC). The major difference between them is the presence of a notch in fracture tests. The objective of this study was to investigate the correlations between AC strengths and energy-based indices obtained from these tests, theoretically and experimentally. Theoretically, the role of a notch is presented and validated experimentally using 13 various AC mixtures. Strength tests, Indirect Tensile Test (IDT) and Indirect Tensile Cracking Test (IDEAL-CT), and a fracture test, Illinois Flexibility Index Test (I-FIT) were used. In addition, digital image correlation (DIC) was utilised to monitor crack development and path, compute crack propagation speed, and identify location and mode of energy dissipation. Brittle AC exhibits limited plasticity in both strength and fracture tests. Hence, a correlation is, theoretically, expected between strength and fracture test results. Conversely, ductile AC exhibits high plasticity in a strength test, but limited plasticity in a fracture test due to the notch presence, which amplifies the stress around the crack tip. Hence, theoretically, no correlation is expected between strength and fracture test results. Both tests reported lower index values for brittle AC mixtures, but no trend for ductile AC mixtures. Using DIC, the process zone (PZ) magnitudes of strength specimens, such as IDEAL-CT, were greater than the fracture-process zone (FPZ) for fracture specimens, such as I-FIT. Although I-FIT exhibits a single well-defined crack path, its Flexibility Index (FI) variability is a direct result of crack propagation speed, which is a function of the AC inhomogeneity. The FI captures AC material-inherent variability, which is useful for AC risk assessment to control AC pavement premature cracking.