13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 4. Results and discussion Mode-I fracture toughness (KIc) was determined for each cracked SCB specimen from following equation [16]: a Rt Y P K I π 2 f Ic = (1) where Pf and YI are the critical peak (or fracture) load and the geometry factor for the SCB specimen, respectively. YI is function of the crack length to radius ratio (a/R) and the loading span to radius ratio (S/R). Some analytical and numerical solutions are available for obtaining YI [e.g. 16,17]. For example, for the tested mode I samples in this research (i.e. with a/R = 0.27 and S/R = 0.67), the corresponding geometry factor has been determined by Ayatollahi and Aliha [16] using finite element analysis as: YI = 3.73. By recording the final fracture loads from the experiments, the corresponding values of KIc for the tested asphalt materials were calculated from Eq. (1) for different compositions of the asphalt mixture. Details of the experimental results including critical fracture load and corresponding value of fracture toughness for each specimen have been presented in Table 4. The first column in this Table defines the mix type and for easy understating, the specimens are designated as x-y-z, in which x indicates the type of aggregate (i.e L for limestone and S for siliceous), y indicates the aggregate size number (i.e. 4, 5 and 6) and z shows the binder type (i.e. 60 for 60/70 and 85 for 85/100). According to the obtained results the average Averages value of KIc for the tested mixtures varies between 0.6 MPa.m0.5 and 1MPa.m0.5. Fig. 4 compares the influence of asphalt characteristic specifications on the value of mode I fracture toughness. As seen from this figure, generally by increasing the void percentage (i.e. decreasing the number of gyratory rotations in the SGC machine) the value of KIc is reduced and this reduction is more pronounced for mixtures containing siliceous aggregates. As shown in Fig. 4a, it can be concluded that depending on the void percentage the value of fracture toughness for mixtures made of limestone is about 25 to 60% greater than the corresponding values of KIc for siliceous mixtures. A noticeable increase in the value of fracture toughness is also seen when the stiffer binder (i.e. bitumen 60/70) is used (see Fig. 4b). This is mainly because of the higher stiffness of the asphalt mixture which increases the crack growth resistance. Moreover, based on Fig. 4 (c) it is obvious that KIc increases when the size of aggregates becomes greater in the asphalt mixtutre. This can be attributed to the lesser amount of binder and greater amount of aggregates in the texture of asphalt mixture which can increase the general stiffness and strength and the required fracturing load of the material. Consequently, According to the results of this research, the risk of low temperature brittle fracture in the asphalt mixtures can be generally increased for siliceous aggregates, smaller aggregate sizes, higher percentages of voids and the higher penetration grades of the binders.
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