## 4 Results and Discussion
### Fracture Trajectory Results  
#### 4.2.1 Mode I Fracture  
Figure 4 shows the fracture trajectories of the tested SCB, ENDB, BD and SENB specimens under mode I. For the pure mode I case, it is expected to see a self-similar and straight fracture path. The overall mode I fracture trajectory for the broken asphaltic samples is straight, but some deviations and local zig-zag type path is observed. The amount of deviation depends on some parameters such as the size of the aggregate,  
Figure 3: Typical load-displacement curves obtained by low and medium temperature tests; (**a**) ENDB, (**b**) SCB, (**c**) BD and (**d**) SENB. the distribution of the coarse aggregate inside the asphalt mixture and also the testing temperature. Indeed, the propagating crack rounds the aggregates located in the ligament and in these regions an interface fracture between the boundary of the aggregates and the mastic part can control the mechanism of the asphalt fracture. However, if a coarse aggregate is located exactly in front of the crack tip or in the middle of the ligament and is oriented normal to the crack propagation path, the fracture can grow through such aggregates. This is probably because breaking the aggregate needs lesser energy than moving along the longer path of the aggregate's boundary (i.e., interface of aggregate and mastic) to round the aggregate. For those situations in which there is no aggregate in the route of fracture, the crack growth can occur with less energy and with the driving force through the mastic part. The path of fracture in such regions is straight with no significant kinking. Figure 5 shows different regions (i.e., through the aggregates, through the mastic and through the boundary of aggregates or interface of aggregates/mastic) for the growth of propagating crack inside the texture of an asphalt mixture under mode I loading. Figure 4: Mode I fracture trajectory observed for some of the tested configurations at low and medium temperatures. The test temperature also plays a significant role in the trajectory of propagating the crack in the asphalt mixtures. This is mainly due to the visco-elastic nature of bitumen used in the mixture of asphalt concrete materials. At low temperature conditions, the bitumen behaves as a brittle and linear elastic material. The higher stiffness of bitumen at low temperatures will result in a brittle-type fracture for the whole asphalt mixture with higher load bearing capacity as shown in Figure 3 and Table 5. In this temperature condition, the fracture propagation shows less deviation from the straight line. However, under higher temperatures (such as room or medium temperature conditions) the fracture trajectory grows only through the mastic part or passes along the boundary of aggregate/soft mastic part. Indeed, for even small size aggregates located in front of the propagating crack, fracture dose not grow through or inside the aggregate and due to lower stiffness of mastic, it tends to round the aggregate and grow from the interface of aggregate/mastic. Figure 6 shows typical test samples tested at low and intermediate temperatures. As a conclusion, the heterogeneity and visco-elastic behavior of the asphalt mixtures can affect the cracking behavior of the asphalt mixtures. Increasing the amount of non-homogeneity and the size of aggregates and increasing the temperature results in greater deviation of fracture trajectory from the straight line. Figure 7 compares the mode I fracture  
Figure 5: Path of the propagating crack through either the mastic, aggregate or aggregate/mastic interface. Figure 6: Effect of aggregate distribution and temperature on the fracture trajectory of tested asphalt mixtures, (**a**) \(-15\)\({}^{\circ}\)C, (**b**) \(+15\)\({}^{\circ}\)C, (**c**) large aggregate in front of the crack, (**d**) small aggregate in front of the crack. trajectory of the investigated asphalt samples with the cracking path of an isotropic and brittle model material (e.g., Perspex or Polymethylmethacrylate that is known as PMMA).