# Investigation of Crack Propagation and Healing of Asphalt Concrete Using Digital Image Correlation
Therefore, it is important to further investigate the healing of asphalt concrete with modern instruments. Furthermore, during the past decade, different approaches were introduced to induce the self-healing capacity of asphalt concrete. These methods include induction heating [16], microwave heating [17], and rejuvenation [18; 19]. Nowadays, strain gauges and linear variable differential transformers (LVDT) are the most common measurement devices to perform mechanical tests on asphalt mixtures. These measurement devices are simple to use, but they are only able to measure the average deformation in the central region of the specimen and are not able to accurately measure the localized or non-uniform strain distributions. Furthermore, with these sensors measurement locations must be decided prior to the experiments. To solve these issues, two-dimensional digital image correlation (2D-DIC) has been introduced to conduct mechanical testing on asphalt mixtures [20; 21; 22]. One of the applications of this method is to investigate the crack formation and propagation in asphalt pavements [23; 24]. For instance, the cracking mechanism of different asphalt mixtures during three common fracture tests: the indirect tensile (IDT) test, the semi-circular bending (SCB) test, and the three-point bending (3 PB) test have been investigated using the DIC technique [25]. DIC can be useful to validate the finite element models predicting the propagation of the crack in pavements [26]. In another study, DIC has been applied for validation of hot mix asphalt mixtures (HMA) fracture mechanics model, a viscoelastic fracture mechanics-based crack growth law, which identifies the fracture energy of the mixtures as a fundamental crack growth threshold [27]. DIC has also been used to investigate the crack opening and closing behavior of asphalt concrete specimens containing iron oxide nanoparticles in double torsion tests and the effect of exposition of the specimens to an alternating magnetic field [28]. In another study, DIC was used to detect damage occurrence and to select the right time to start an inductive heating process to accomplish healing in the asphalt concrete specimens [29]. DIC was also used to investigate the healing mechanism in other materials, like concrete [30]. The purpose of this research is to provide an insight into crack propagation and healing of asphalt on its surface by measuring the strain map on the surface of a cylindrical asphalt mixture during indirect tensile fatigue tests and to propose some healing indices based on the surface strain. Therefore, a healing test, consisting of three stages, was developed. In the first stage, a cyclic load is applied for microcrack formation and propagation until the module of elasticity of the specimen reaches a certain termination value. In the second stage, the loading is stopped and the temperature is increased to initiate the healing in the sample during the rest time. In the final stage, a fatigue test similar to the test of the first stage is conducted until the module of elasticity reaches the same termination value. Images are captured during the experiment and using the DIC technique crack propagation and healing phenomenon are investigated. The structure of this paper is as follows: In Section 2 a description of the material production and preparation, experimental setup, and different methods used in this research are given. In Section 3 the overview of the measurement results and detailed analysis of the images to investigate crack propagation and healing are summarized. The paper is finalized with some conclusions in Section 4.