## 3 Results and Discussion
### Semi-Quantitative Analysis and Standard Curve  
FTIR has mostly been used for semi-qualitative analysis to identify different kinds of chemical functional groups in bitumen [30; 32]. In the previous research [23], the study utilized FTIR to qualitatively assess the blending state of virgin and RAP binders in RAM with a high percentage of  
Figure 4: The infrared spectrum of RAP: (**a**) Whole; (**b**) Partial. Figure 3: Infrared spectra of the virgin bitumen, reclaimed asphalt pavement (RAP), and carboxyl-terminated butadiene acrylonitrile (CTBN): (**A**) is virgin bitumen, (**B**) is RAP, and (**C**) is CTBN. RAP by counting the number of samples in which blending occurred. It was found that the virgin and RAP binders in the RAM did not fully blend; furthermore, the study indicated that the combined use of FTIR with a tracer constitutes a useful method for investigating the blending states of virgin and RAP binders. However, quantitative analysis used to directly investigate the actual blending degree of virgin and RAP binders in RAM with a high percentage of RAP is still lacking. Considering that the absorption peak intensity is influenced by the thickness of the sample film covering the KBr table, the structural indices of the chemical functional groups were used to semi-quantitatively analyze the proportions of CTBN in the samples for the sake of avoiding any effects associated with the film thickness. Following [PERSON] et al. [26], the structural indices considering the area of chemical functional groups can be calculated by the following equation:  
\[I_{CTBN}=\frac{A_{2238\ cm^{-1}}}{A_{2960\ cm^{-1}}} \tag{1}\]  
where ICTBN is the structural index of CTBN, \(A_{2238\ cm^{-1}}\) is the absorbance area of nitrile stretching vibrations and \(A_{2960\cm^{-1}}\) is the absorbance area of the antisymmetric vibrations of -CH\({}_{3}\) groups. The software package Peakfit (v4.12, Systat Software Inc, San Jose, California, US) was utilized to fit the spectra of the FTIR samples to obtain the absorption peak areas of the chemical functional groups [33; 34; 35]. After applying a spectral baseline correction, the spectrum was smoothed by a Savitsk-Golay function and a second derivative was carried out. By fitting the spectrum with a Gaussian peak [36], the number and location of sub-peaks can be estimated roughly. After multiplex fitting, the residual reaches a minimum, and the absorption peak area of the sub-peaks can be obtained quantitatively, as shown in Figure 5.  
To explore the reliability of ICTBN as a semi-quantitative analysis index for investigating the blending degree of virgin and RAP binders, the infrared spectra of virgin bitumen with 1 wt. %, 5 wt. % and 10 wt. % CTBN were tested to obtain the values of ICTBN. Correlation analysis and a one-way analysis of variance (one-way ANOVA) were performed to investigate whether the CTBN content had a significant effect on ICTBN. Each sample with a different CTBN content was measured three times. The data were analyzed to calculate the mean values (MV), standard deviations (SD) and coefficients of variation (CV) for each sample; the results are listed in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**CTBN Content**} & \multicolumn{3}{c}{**ICTBN**} \\ \cline{2-4}  & **MV** & **SD** & **CV** \\ \hline
1 wt. \% & 0.404 & 0.028 & 0.069 \\
5 wt. \% & 1.634 & 0.101 & 0.062 \\
10 wt. \% & 4.638 & 0.262 & 0.056 \\ \hline \end{tabular}
\end{table} 
Table 5: CTBN indices of the virgin bitumen with different CTBN contents. Figure 5: Characteristic absorption peak area at: (**a**) 2238 cm\({}^{-1}\); (**b**) 2960 cm\({}^{-1}\). Table 5 shows that the MV of I\({}_{\text{CTBN}}\) increased with an increase in the CTBN content, and the SD and CV results indicate that the CTBN was evenly distributed throughout the virgin bitumen.