Issue 43

M. Fakhri et alii, Frattura ed Integrità Strutturale, 43 (2018) 113-132; DOI: 10.3221/IGF-ESIS.43.09 123 Under pure mode II loading condition, the change in fracture energy of asphalt mixtures show less sensitivity to aggregate type, comparing to mode I loading condition, showing that when the crack is to move with an angle from load line, it simply chooses a path with less effort which might be mastic phase. For this reason, aggregate type does not affect the fracture energy of AC under all loading rates the same way, no matter what the testing temperature is. Figure 11 : The effect of aggregate type on fracture energy at 25°C under pure mode II loading condition. The effect of air void content on fracture energy under pure mode II loading condition Fig. 12 shows the influence of air void on fracture energy of AC under pure mode II loading and low temperature condition for different loading rates. During this research 5 mm/min proved to be a specific loading rate specially at 5°C. In this case, at three loading rates (i.e. 1, 10 and 50 mm/min) by increasing air void, fracture energy grows to reach a maximum value for asphalt mixtures with air void content of 5% and decreases afterward, whereas this trend is exactly reverse under loading rate of 5 mm/min. Another outstanding result to be reported is that fracture energy reaches its lowest values under low and high loading rates (i.e. 1 and 50 mm/min) for all specimens with different air void contents, while it reaches its highest values under intermediate loading rates (i.e. 5 and 10 mm/min) at 5°C under pure mode II loading condition. At intermediate temperature (25°C) fracture energy is under the influence of loading rate, independent of asphalt mix air void content as shown in Fig. 13. Therefore, under each loading rate, fracture energy of asphalt mixtures with different air void contents are nearly the same. Moreover, fracture energy of AC specimens with air void content of 7% is more than other specimens, since at intermediate temperatures cracks tend to move toward weaker part of asphalt concrete matrix (mastic phase), though more air void contents, results in longer routs for cracks to grow and more fracture energy is needed for specimen to crack. The effect of testing temperature on fracture energy under pure mode II loading condition Fig. 14 presents the results of fracture tests for one type of asphalt mixtures containing air void of 3%, lime aggregate and bitumen type of PG64-22 under different loading temperatures, to investigate the effect of loading rate on AC fracture energy at wider range of temperatures (between 5 and 25°C). The mean values of fracture energy is the largest at 5C in the mentioned range of temperature, under pure mode II loading condition (similar to mode I loading condition). The peaky trend of fracture energy at 5°C changes to incremental trend at 15°C with an acute slope and this trend remains incremental at 25°C with a gentle slope, this time. Just like mode I fracture, Asphalt concrete specimens show the highest fracture resistance under pure mode II loading condition at 5°C temperature under 5 mm/min loading rate, which can be the result of quasi-brittle behavior of asphalt mix at near zero temperature under normal loading rates.