Investigating the fracture behaviors of asphalt mixtures at the intermediate temperature considering the detection of crack initiation.

• Crack initiation was detected using a strain gauge. • Calculation methods of the modified J-integral and modified fracture energy were proposed. • Fracture energy was divided in three parts: plastic energy, elastic energy, and surface energy. Crack initiation detection is of crucial significance for comprehending the entire fracture process behaviors, encompassing crack initiation and propagation. In the present study, strain gauges were utilized to detect crack initiation at a temperature of 25 °C, and the data related to crack initiation were employed for the calculations of the modified J-integral and modified fracture energy. Fracture energy was classified into three components: elastic energy, plastic energy, and surface energy. The improved fracture energy is the difference between the traditional fracture energy and the plastic energy. Semi-circular bending (SCB) tests were carried out on two types of asphalt mixtures, namely AC10 and AC16, with two distinct notch lengths of 15 mm and 30 mm. Crack mouth opening displacement (CMOD) was also recorded during the SCB tests. In addition, load-line-displacement (LLD) was also recorded. The results demonstrated that the peak of strain occurred earlier than that of the load, suggesting that specimens could endure further loads after the onset of a macro-crack. With the augmentation of notch length, the elastic energy, plastic energy, and surface energy decreased significantly for both AC10 and AC16. For both AC10 and AC16, approximately 70 % of the total energy was surface energy, which was utilized for crack propagation, and the proportion of plastic energy was lower than that of elastic energy. Plastic energy is predominantly employed for the plastic deformation of the asphalt binder. When the plastic energy was excluded from the total fracture energy, the disparity in fracture energy between specimens with different notch lengths was narrowed, indicating that the plastic energy was regarded as a key factor leading to the reduction in fracture due to the increased notch depth. When calculating the J-integrals using the data corresponding to crack initiation instead of the peak load data, the J-integrals decreased considerably, signifying that the surface energy within the range from crack initiation to the peak load point results in a significant increase in the J-integral. In comparison to AC16, AC10 exhibited higher J-integral, elastic energy, plastic energy, and surface energy, suggesting that AC10 demonstrated superior fracture resistance to crack initiation and propagation, which was mainly attributed to the porosity difference between AC10 and AC16 specimens. The load-LLD curves and load-CMOD curves generally exhibited a similar trend in energy parameters, while the load-CMOD curves manifested flatter curves within the descending phase. [ABSTRACT FROM AUTHOR]