# The Effect of Ageing on Chemical and Mechanical Properties of Asphalt Mortar
## 3 Results and Discussion
### Dynamic Shear Rheometer  
#### 3.2.1 Frequency Sweep Tests  
At least three replicate samples were tested in DSR at each condition. Based on the time-temperature superposition (TTS) principle, master curves of complex shear modulus and phase angle were generated at reference temperature of 20 \({}^{\circ}\)C. Figure 6 shows the evolution of the rheological characteristics of bitumen with increased ageing time. The master curves of mortar_AM and mortar_MA at various ageing times are shown in Figure 7.  
As shown in Figure 6, the observed differences were more pronounced at low frequencies, while they were diluted at higher frequencies. Specifically, the variation of complex modulus between fresh and aged materials was about one order of magnitude at low frequencies. Moreover, at the highest frequency, all samples tended to reach an asymptote at a value of \(10^{8}\) Pa. On the contrary, the phase angle differed substantially for the whole frequency range. The greater the value of complex shear modulus, the harder was the bitumen; the smaller the phase angle, the more elastic was the bitumen.  
In Figure 7, it can be seen that the complex shear modulus of mortar sample increased as expected with increasing ageing, while the opposite was observed for the phase angle. Specifically, the variation of complex modulus between fresh and aged materials was one order of magnitude at low frequencies. Moreover, at the highest frequency, all samples tended to reach an asymptote at a value of \(10^{9}\) Pa. The threshold value at high frequency was one order of magnitude larger than that of bitumen due to the addition of filler and sand. On the other hand, the phase angle of the mortar samples differed  
Figure 6: Master curves of PEN 70/100 at different ageing times.  
Figure 7: Master curve of mortar samples at different ageing times: (**a**) mortar_AM; (**b**) mortar_MA.  
substantially for the whole frequency range. These results show that the material became stiffer and more brittle with ageing. The comparison between mortar_AM and mortar_MA is shown in Figure 8.  
Figure 8 shows that compared to mortar_AM, mortar_MA had lower values of complex shear modulus and higher values of phase angle. In other words, mortar_MA was softer under the same ageing conditions, thus the effect of ageing was less. This finding was in agreement with the FTIR results presented in Figure 5. Figure 9 shows the differences in the complex shear modulus and phase angle at 20 \({}^{\circ}\)C and at 10 Hz frequency for the bitumen, mortar_MA, and mortar_AM samples. The reason for choosing shear moduli at 10 Hz and 20 \({}^{\circ}\)C was that 10 Hz is the commonly used frequency in a dynamic test and 20 \({}^{\circ}\)C is considered to be room temperature [36; 37; 38].  
It can be observed that the complex shear modulus of mortar was significantly higher than that of bitumen, with the variation being about 200 times higher, while the phase angle of mortar was slightly smaller than that of bitumen. This high difference in stiffness occurred due to the presence of filler and sand particles, while the phase angle of mortar was mainly determined by the bitumen component.