## 2 Materials and Methods
### Methods  
The investigation involved subjecting the blends of asphalt binders and the investigated additives to short-term and long-term laboratory ageing performed using the rolling thin-film oven (RTFOT, EN 12607-1) and the pressure ageing vessel (PAV, EN 14769), respectively. The experiment was designed with a three-level, full factorial approach to evaluate the combined effects of both additives on the properties of the two asphalt binders. The investigated dosing ranges of these additives included the following:  
* additive A: 0.00%, 0.15%, and 0.30%;
* additive B: 0.0%, 1.0%, and 2.0%. The implemented design for both asphalt binders enabled the evaluation of linear, quadratic and interaction terms (as shown in Equation (1)) related to the effects of the investigated additives. The experiments were conducted with 3 replicates. \[Y=\beta_{0}+\beta_{1}X_{1}+\beta_{2}X_{2}+\beta_{3}X_{1}^{2}+\beta_{4}X_{2}^{2 }+\beta_{5}X_{1}X_{2} \tag{1}\]  
The effects of the additives under investigation were evaluated by Fourier-transform infrared spectroscopy using the attenuated total reflectance (ATR-FTIR) method. The Thermo-Scientific Nicolet is 5 FTIR Spectrometer (Waltham, MA, USA) and the PIKE Technologies GladiATR (Madison, WI, USA) attenuated total reflectance accessory with a diamond window were used. The changes in the chemical structure of the asphalt binders were evaluated using structural and chemical indices characterizing the relative changes in the amounts of compounds associated with oxidative ageing (sulfoxide and carbonyl indices) and the presence of the elastomeric modifier (polybutadiene and polystyrene index). These indices were calculated as a function of the areas under the peaks in the spectrograms, specific to excitation in particular bonds present in the molecules. The method for calculating structural indices was based on comparing the absorption bands, which were of interest, to those remaining invariant in the experiment. For this invariant reference, different values can typically be chosen, including a reference absorption band [72], a group of absorption bands [73] or the sum of all evaluated peaks [57] in the spectrum. In this method, the areas of absorption bands (peaks) of interest are compared to those reference values, and the structural indices are computed. An analysis of different reference values (peaks and groups of peaks) was conducted to evaluate their variability in the experimental data (Table 3). It was found that the sum of all peaks had the smallest relative variability  
Figure 2: Photographs of the additives used in this study: (**a**) additive A and (**b**) additive B.  
with a coefficient of variance of 1.01%. Therefore, this value was used for calculating the structural indices as shown in Table 4.  
In addition to the FTIR testing of the asphalt binders, their high-temperature performance was evaluated before and after RTFOT ageing. The values of high-temperature stiffness (G*/sin(\(\delta\))), as defined in AASHTO M320 and AASHTO T315, measured at 64 \({}^{\circ}\)C and 70 \({}^{\circ}\)C in the case of the 50/70- and 45/80-55-based binders, respectively, are presented, and the non-recoverable creep compliance (AASHTO T 350) measured at 58 \({}^{\circ}\)C and 64 \({}^{\circ}\)C in case of 50/70 and 45/80-55 is shown. A simple handling procedure for the samples was employed in this study. The bitumen samples were heated at 140 \({}^{\circ}\)C for three hours in closed containers, mixed with the appropriate additives and homogenized for their uniform distribution. Homogenization was achieved through blending for 15 min using a low-shear mixer at ca. 2000 rpm and at a sustained temperature of 140 \({}^{\circ}\)C. The testing was conducted directly after the necessary processing (e.g., ageing). The material for the FTIR measurements was poured into 100 mm diameter and 75 mm high metal containers, and the samples were obtained after cooling by scraping the material from an approx. 5 mm depth under the asphalt surface with a metal spatula.