Issue 43

M. Fakhri et alii, Frattura ed Integrità Strutturale, 43 (2018) 113-132; DOI: 10.3221/IGF-ESIS.43.09 117 After preparing the asphalt mixtures, several cylindrical specimens with diameter of 150 mm and height of 145 mm were manufactured using the Superpave Gyratory Compactor (SGC) machine with 1.16±0.02 degrees tilt angel based on AASHTO TP-71 and pressure of 600±18 kPa (AASHTO T-312). Among different affecting parameters (such as the aggregate gradation, bitumen, void and etc.), as demonstrated in the literature, the air void content has the highest effect on fracture resistance behavior of HMA material [1, 39] or other mechanical properties of asphalt concretes [58-60]. Therefore, in order to investigate the influence of such parameter on the fracture behavior of asphalt concrete at intermediate test temperatures, different air void percentages (i.e. 3, 5 and 7%) were considered in preparing the HMA mixtures. Accordingly, the mixtures were compacted with different compaction energies of 30, 70 and 90 gyratory rotations, respectively. The manufactured cylindrical specimens were then sliced by a rotary diamond saw blade to obtain circular discs with approximate thickness of 27 mm. Each disc was then further sliced into two semi-circular samples as shown schematically in Fig. 1. Finally, a notch with length of 25 mm was created in the middle edge of the specimen by a 0.3 mm thick rotary diamond saw blade. The crack length over the semi-disc ratio (a/R) was equal to 0.33. Based on Ayatollahi et al. [61], the contribution of mode I and mode II components in the edge cracked SCB specimen varies by changing the bottom loading support (i.e. S 1 and S 2 defined in Fig. 2). Indeed, when the left loading support roller (S 2 ) moves from symmetric condition (i.e. S 1 =S 2 ), towards the crack location different combination of modes I and II are introduced in the SCB specimen. A mode mixity parameter (M e ) defined as: 1 2 tan e I II K M K          , is often used for determining the fraction of mode I (opening) and mode II (in-plane sliding) by means of stress intensity factors (K I and K II ). Fig. 2 shows the schematic of loading setup (i.e. the bottom loading support locations relative to the crack line) for introducing the chosen mode mixtures. By considering the left loading support distance (S 1 ) equal to 50 mm for all mode mixities, the distance of right variable support (S 2 ) was changed from 50 mm (for pure mode I) to 9 mm (for pure mode II) [61]. A total number of 229 cracked SCB samples were prepared for fracture tests. Tab. 4 presents the specifications of test specimens and loading conditions. Figure 1 : Schematic procedure of preparing semi-circular bend test specimens. Figure 2 : Different mode mixities considered for testing the semi-circular bend specimens.