13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- intensity factors were calculated and compared. 2. Specimen geometry and material While HMA behavior is strongly affected by temperature, at subzero temperatures, its mechanical behavior is often considered to be brittle or quasi-brittle. Hence, linear elastic fracture mechanics (LEFM) concept is adoptable for investigating the fracture behavior of HMA mixtures at low temperatures. Generally, the cracked asphalt concrete pavement may experience three main modes of fracture, mode-I, mod-II, and mode-III. Among these three modes, mode-I and mode-II are predominant modes of crack propagation in asphalt concrete pavements containing reflective or top-down cracks. Several test specimens such as single edge notched beam (SENB), disk-shaped compact tension (DC-T), semi-circular bend (SCB) and so on, have been used in the past by many researchers to study the fracture behavior of asphalt concretes. In this research, an improved SCB specimen was employed to conduct the experiments. For this purpose, first several cylindrical samples (130 mm in height, 150 mm in diameter) were prepared using superpave gyratory compactor (SGC) in the laboratory. These samples were sliced into several disks of 32 mm thick by means of a water-cooled masonry sawing machine. Each disk was then halved to obtain semi-circular shaped specimens. At the next stage, an artificial edge crack (20 mm in length) was generated within the specimen utilizing a water-cooled cutting machine with a very thin blade. The width of generated crack was 0.3 mm. According to the finite element simulations performed by Ayatollahi [9], in a SCB specimen of radius 75 mm containing an edge crack of length 20 mm, pure mode-II loading is achieved when the edge crack distance is 16 mm from the middle of the specimen (see Fig. 1). S1 and S2 are the left and right hand side distances of the lower fixtures from the middle of the specimen, respectively. For pure mode-II loading, the values of S1 and S2 were found from the FE analysis to be 50mm and 20 mm respectively. Figure 1. Test specimen used for pure mode-II fracture tests The asphalt concrete used to prepare the cylindrical samples was similar to the one which is widely used in Iran pavement systems. Aggregate gradation of the HMA mixtures used in this study (as described in Table 1) is within the range of the recommendations by Iran Highway Asphalt Paving
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