# Investigation of Crack Propagation and Healing of Asphalt Concrete Using Digital Image Correlation
## 2 Materials and Methods
### Experimental Setup  
The experiments were designed based on the test described in EN 12697-24:2012 [36], Annex E (Indirect tensile fatigue test on cylindrical shape specimens) and were conducted according to Figure 2 with a Pavetest DTS-30 servo-hydraulic dynamic testing system UTM. After the samples were conditioned to the desired temperature (20 \({}^{\circ}\)C), fatigue tests with a 0.1 s haversine load with a maximum load of 600 kPa followed by a rest period of 0.1 s were carried out. A Poisson's ratio of 0.35 was assumed for this asphalt mixture to determine its resilient modulus (stiffness). To stop the tests at a certain point, termination conditions based on the resilient horizontal strain and the resilient modulus were defined. According to the standard [36], the tests should stop when the resilient horizontal strain and the resilient modulus reach 200% and 50% of their initial values, respectively. However, in these experiments, to avoid having any macrocracks without the possibility of healing, the termination value was defined as reaching 140% of the initial resilient horizontal strain or 71% of the initial resilient modulus. Using these values, at the end of this step, there were no visible cracks on the surface of the specimens. In the second step, the temperature of the climate chamber was raised to 60 \({}^{\circ}\)C. This temperature was chosen slightly higher than the softening point of the bitumen used in this study. In previous research [1], samples were placed inside a wooden frame during the healing stage at an elevated temperature. This can help the cracks to close when the specimen expands due to the temperature increase. However, in this research, the samples were left at the exact same position ensuring that the images captured during the healing process can be compared with the previous images. After keeping the temperature of the climate chamber at 60 \({}^{\circ}\)C for 18 h it was lowered to 20 \({}^{\circ}\)C again. The samples were kept at this temperature for three hours and then the final fatigue tests were continued until the resilient modulus again reaches the termination value defined in the first step. It is expected that this approach will lead to a lower healing degree compared to Reference [1]. Figure 1: Grading curve of the designed mixture. The camera used in this research was a GigE uEye UI223 xRE-M IDS, with a resolution of 1024 \(\times\) 768 and frame rate of 30 fps in full resolution. The camera was aligned with four LED spots to generate the proper lighting condition in the images (Figure 2). The load signal was measured by the Pavetest's compact control and data acquisition system (CDAS) and amplified using an AM 502 differential amplifier and sent to the national instrument NI USB-6343 Multifunction DAQ, as a trigger signal. Using this signal, camera and LED spots were triggered at the desired cycles of the experiment to capture the images. Moreover, the camera captures images with a 20 ms delay after receiving a pulse; therefore, to have a perfect synchronization between the camera and LED spots, the camera was triggered 20 ms before the LED spots. The camera was focused manually and the exposure time was set to 18 ms in order to have the optimum brightness in the images. The camera was calibrated using an AI-08-BMB_9*9 Dantec Dynamics calibration plate and the Istra 4D software. Using the Istra 4D software it was possible to determine the projection parameter and take any distortion of the imaging or tilting of the object into account. For each specimen, first, one image was captured before the experiment starts as a reference image. Then, depending on the total duration of the fatigue tests, approximately 100 images were acquired using a logarithmic relation between the number of images and number of cycles so more images are captured at the end of the experiment when the cracks propagate faster. Finally, 1080 images were captured during the 18 h rest time (one image per minute). Figure 2 shows the experimental set up of the measurements.