# The Effect of Ageing on Chemical and Mechanical Properties of Asphalt Mortar
## 3 Results and Discussion
### Fourier Transform Infrared Spectrometer
#### 3.1.2 Ageing Index  
In this study, the effects of ageing were analyzed considering specific bands of wavenumber as defined by [PERSON] et al. [28; 29; 30] and the corresponding area under those bands. Then, the carbonyl and the sulfoxide indices were determined by dividing the area under a specific location of the spectrum by the sum of specific areas, Equations (1) and (2).  
\[\text{Carbonyl index }=\frac{A_{1700}}{\sum A} \tag{1}\]  
\[\text{Sulfoxide index }=\frac{A_{1030}}{\sum A} \tag{2}\]  
where \(\sum A=A_{(2953,2862)}+A_{1700}+A_{1600}+A_{1460}+A_{1376}+A_{1030}+A_{864}+A_ {814}+A_{743}+A_{724}\).  
Using the obtained spectra, the ageing indices (carbonyl and sulfoxide index) of each sample were calculated. Figure 5 shows the carbonyl and sulfoxide indices (average value of three measurements) of bitumen, mortar_AM, and mortar_MA. The error bar in Figure 5 is the standard deviation of the three measurements. It can be observed that under the same ageing conditions, the value of the carbonyl index for mortar_AM was similar to that of bitumen. This can be attributed to the fact that carbonyls in the mortar only came from bitumen as illustrated in Figure 4. Previous studies have shown that carbonyls are formed later than sulfoxides because sulfur is more reactive than carbon in bitumen. Later, carbonyls increases due to the full consumption of sulfur [31]. Similar trends and values were found for the mortar in our study as well.  
Moreover, it can be observed that the carbonyl index of mortar_MA was significantly smaller than that of mortar_AM (Figure 5a), indicating that mortar_MA had a lower ageing rate. This finding confirms the theory that the sand and filler particles present in the mortar can slow down the diffusion rate of oxygen due to an increased length of diffusion paths and consequently slow down the ageing process as well [32]. In addition, the diffusivity of mortar will decrease as the bitumen volume fraction decreases [33]. This happens because mortar is the mixtures of bitumen, filler, and sand, and its diffusivity should always be between that of the bitumen and the filler/sand. Moreover, the diffusivity of bitumen is much higher than that of filler/sand [34; 35].  
The sulfoxide index of mortar_AM differed significantly with that of bitumen (Figure 4b). This was because sulfoxides in the mortar came not only from the bitumen component but also from the filler and sand particles as illustrated in Figure 3. The sulfoxides content in filler and sand was much higher than that in bitumen, and this led to obvious change in the sulfoxide index of the mortar. Moreover, as another result, the deviation of the mortar results was obviously higher than that of bitumen. The reason for this might be because the measuring area of the FTIR test was limited, meaning the  
Figure 5: Function group index of mortar at different ageing conditions: (a) carbonyl index; (b) sulfoxide index.  
infrared radiation hit only one or few of the components of mortar. Another reason might be that the mix of mortar samples was not homogenous.