*2.3. Methodology*

For the investigation of the RA granulates' coating, a procedure was set (Figure 5). First, the area to be measured is selected. Second, the grid of the points to be measured is defined. Third, the measured spectra are preprocessed. Fourth, the integrated typical bands are visualized. The intensity visualization of the bands on the microscopic image is defined as chemical imaging. The processing of the spectra and evaluation of the derived data was done using the Bruker OPUS™ software (v. 7.5). Each sample was analyzed following the proposed steps.

Figure 6 shows the polished surface of the samples and the areas selected to be analyzed (Areas 1 to 5). This figure shows only the exposed surface of the granulate without the resin. For illustrative purposes, the resin was removed graphically in Figure 6 to provide a clear view of the samples and their bituminous coating. The areas were selected with the purpose of observing potential traces of partially or non-blended aged bitumen and to track the presence of other components such as resin or fine particles. The focus was mainly on the interface of mortar and resin (Areas 2 and 4), and also on areas starting from the resin towards the stone, creating a profile (Areas 3 and 5). Area 1 is a single case where the analysis was performed in a significantly larger area. On average, the spatial resolution applied for each point of the analysis was 30 × 30 μm2.

**Figure 5.** Procedure for determination of the chemical imaging.

**Figure 6.** Samples and selected areas for analysis: Area 1 in GRA sample, Areas 2 and 3 in GRAB sample, Area 4 in MRAB sample, and Area 5 in GRAR sample.

The solidity of the studied samples does not allow probe penetration into the sample, which has a direct effect on the intensity of the absorbance signal. The software provides the solution of atmospheric compensation, which allows the correction of the signal. This step uses "physical models to estimate the amount of atmospheric gases in the single-channel spectra and therefore compensates disturbing H2O and/or CO2" [35]. The compensated areas are from 3600 to 4000 cm<sup>−</sup><sup>1</sup> for H2O compensation and from 2300 to 2400 cm<sup>−</sup><sup>1</sup> for CO2 compensation [35]. This step only affects the aforementioned bands, and it is suitable for the current research since those areas are free of absorption bands related to the RA samples (see Table 3).

The spectra were normalized using the min-max normalization method [35]. This method was applied by selecting the C–H stretching band (2990–2820 cm<sup>−</sup>1) as the band to be maximized. The result is that the absorbance of the most intense peak (at 2920 cm<sup>−</sup>1) is scaled to 2. A normalization step is required in order to avoid intensity problems caused by varying contact between the sample and the crystal. The C–H stretching band is the most appropriate since it is not affected by bitumen ageing [16] and it is present for both bituminous and resin zones.

For the visualization of the chemical images, specific bands were selected to be integrated. The integration method was used to quantitatively measure the area under the spectrum graph, which represents the intensity of a certain functional group, using wavelength limits as a baseline. The characteristic bands used in this study for each component are presented in Table 3. The tracing of the carbonyl group was the main focus since the presence of carbonyls indicates ageing of bitumen [17]. The carbonate group was used as a marker of the mastic/mortar area and the carboxylic acids group was used for the rejuvenator. Finally, the resin can be distinguished by the ring vibration of the epoxides bond. Figure 7 demonstrates the integration method of the characteristic infrared bands, integrated in spectra derived from different materials by means of FTIR-ATR spectroscopy. The chemical images can then be formed on the sample microscopic image, providing the opportunity to trace spatial intensity of the integrated functional groups.


**Table 3.** Distinctive bands of the individual components.

**Figure 7.** Carbonyl of RA bitumen (red) and neat bitumen (light grey) (**a**); carboxylic acids of rejuvenated bitumen (blue) and non-rejuvenated bitumen (red) (**b**); epoxides of resin (dark grey) and of bitumen (blue) (**c**); carbonate of bitumen with limestone filler (magenta) and bitumen without filler (light grey) (**d**).

#### **3. Results and Discussion**

Five surface areas of RA granulates were analyzed using FTIR microscopy in ATR mode. Using the infrared spectra derived, chemical images were formed by integrating the typical bands for each area separately. In Section 3.1 the spatial distribution of typical bands is presented, and in Section 3.2, the numerical distribution of carbonyl levels is exhibited only for the bituminous area.

#### *3.1. Chemical Imaging*

The selected areas (Areas 1 to 5) cover three basic situations: (i) original ageing state of RA bitumen, (ii) interaction between neat bitumen and RA, and (iii) rejuvenation of RA. In order to have comparable chemical imaging, the color plot scaling is the same between chemical maps of the same

band, except for the carbonate group, since larger variations in absorbance are noticed here compared to other areas.

Figure 8 shows the results of the chemical imaging analysis of the GRA sample. The spatial resolution of the area studied is 9800 × 5400 μm2. In Figure 8a, the integration of carbonyls is demonstrated, while Figure 8b,c show the carbonate and the epoxides, respectively. Here, it is noticeable that the resin area is distinguishable by the chemical imaging of the epoxide (level above 60). The level of carbonyl varies here between 0 and 5.5. In Figure 8b, larger aggregate particles (white areas) and also mortar areas can be seen from the integration of the carbonate.

**Figure 8.** Chemical imaging of the GRA sample (Area 1): carbonyl group (**a**); carbonate group (**b**); epoxides (**c**); spectral visualization of the selected points marked by colored dots in Figure 8c (**d**).

The chemical imaging analysis of Area 2 of the GRAB sample is presented in Figure 9. The spatial resolution of Area 2 is 390 × 2450 μm2. Figure 9a shows that the carbonyl level is lower compared to sample GRA (Figure 8a) and that the main part of Area 2 is resin free, except from a minor area at the upper side (Figure 9c). According to Figure 7a, the absence of carbonyls is an indication of unaged bituminous areas. Based on that observation and by detecting the carbonyl level of Area 2, the presence of a blend between aged and neat bitumen can be assumed in this area.

**Figure 9.** Chemical imaging of the GRAB sample (Area 2): carbonyl group (**a**); carbonate group (**b**); epoxides (**c**); spectral visualization of the selected points marked by colored dots in Figure 9b (**d**).

The third area is illustrated in Figure 10. A profile area of the bituminous coating of sample GRAB is presented here with the aim to track the mobilization in full depth towards the stone. The spatial resolution of Area 3 is 2800 × 610 μm2. Similar levels of carbonyls can be observed between Areas 1 and 3 (Figures 8a and 10a). On the other hand, the levels in this area (Figure 10a) differ from the levels in Area 2 (Figure 9a). A possible hypothesis is that a lower mixing temperature and less mixing time can lead to ineffective mixing between the neat bitumen and the RA mortar. Findings in literature support this hypothesis [36]. Another observation that strengthens this hypothesis is the presence of lower levels of carbonyls and the absence of high carbonate groups in Area 2 compared to area 3 (Figure 9a, Figure 10a, Figure 9b, and Figure 10b, correspondingly) which is an indication of a low degree of blending. The chemical imaging of alkyl levels of Area 3 is demonstrated in Figure 10c, which shows that Area 3 is a bituminous zone.

**Figure 10.** Chemical imaging of the GRAB sample (Area 3): carbonyl group (**a**); carbonate group (**b**); alkyl group (**c**); spectral visualization of the selected points marked by colored dots in Figure 10b (**d**).

The findings of the last two samples, MRAB and GRAR, are presented in Figures 11 and 12, respectively. The spatial resolution of the studied areas are 1000 × 340 μm<sup>2</sup> and 1580 × 1360 μm2, accordingly. Concerning the carbonyl level of Area 4, a stable concentration in an order of magnitude of 2 (Figure 11a) can be observed. Since the fabrication conditions of that sample are different compared to GRAB sample, i.e., mixed at a higher temperature for a longer period of time, one can assume a higher degree of blending, as well as a more homogenized dispersion of the carbonyls within the analyzed area. Figure 12 provides information regarding the chemical imaging of the GRAR sample where a rejuvenation agen<sup>t</sup> was sprayed on the RA sample. The carbonyl levels of Area 5 range from 0 to 3 (Figure 12a), which are lower compared to the concentration of carbonyl of the GRA reference sample (Figure 8a). The presence of rejuvenator can be verified by the chemical imaging in Figure 12c, where the carboxylic acids show values between 6 and 16, and also the absence of the same band in non-rejuvenated samples such as the MRAB sample (Figure 11c), where the absorption is below 0.

**Figure 11.** Chemical imaging of the MRAB sample (Area 4): carbonyl group (**a**); carbonate group (**b**); carboxylic acids (**c**); spectral visualization of the selected points marked by colored dots in Figure 11b,c (**d**).

**Figure 12.** Chemical imaging of the GRAR sample (Area 5): carbonyl group (**a**); carbonate group (**b**); carboxylic acids (**c**); spectral visualization of the selected points marked by colored dots in Figure 12b,c (**d**).

#### *3.2. Carbonyl Distribution as Indicator of Mobilization*

The carbonyl level can be used as an indicator for ageing of the bituminous zones and therefore also as an indication of blending. The quantified carbonyl is not always a band that belongs in a bituminous zone. For that reason, by focusing on the carbonyl levels deduced only from bituminous areas, it is possible to better understand the extent of the ageing or the rejuvenation effect on those areas. Based on the integration of typical bands, a bituminous coating can be defined as the area where the following conditions are simultaneously met:


Based on those requirements, the distribution of the carbonyl for all the areas is plotted in Figure 13a and the percentiles are presented in Table 4. Because of the large difference between the maximum level of carbonyl (27.94) and the level at the 97.5 percentile (7.16), values above the 97.5% limit can be considered as extreme measurements. Based on that observation, the carbonyl level, which can be considered as valid within a bituminous zone, for this research is between 0 and 7.16.


**Table 4.** Carbonyl levels at different percentiles of the distribution.

In Figure 13b–f, the carbonyl distributions of the individual studied zones are shown. Considering the GRA sample as the reference sample (Figure 13b), a shift can be observed towards lower carbonyl levels for the other 4 areas, which is an indication of blending. When comparing Areas 2 and 3 of the same GRAB sample (see Figure 13c,d), the probability of a carbonyl level between 0 and 1 is higher for Area 2 (66% vs. 50%), but on the other hand, some dispersed higher carbonyl levels (between 3 and 8) are present in Area 3 but not in Area 2. Those results are in line with the chemical imaging and indicate the presence of an outer zone with low carbonyl, which can possibly be interpreted as an area of a lower degree of blending consisting mostly of neat bitumen. On the other hand, Area 3 shows similar probability, for carbonyl levels higher than 1, compared to Area 1 of the GRA sample but higher probability for the lowest carbonyl levels between 0 and 1 (Figure 13b,d, respectively). Here, we can assume that an area of partial blending is present, as well as an inactive RA bituminous zone. Therefore, the assumption of full blending is not valid for these samples.

The MRAB sample exhibits a high probability of carbonyl levels between 1 and 2. This is an indication of a higher degree of blending, since lower and higher carbonyl levels are limited. The GRAR sample shows similar probabilities as Area 3 of the GRAB sample.

**Figure 13.** Probability of carbonyl levels in the bituminous zone: among all samples (**a**); GRA (**b**); GRAB Area 2 (**c**); GRAB Area 3 (**d**); MRAB (**e**); GRAR (**f**).

#### *3.3. Bituminous Mortar Coating of the RA Granulates*

Previous studies have discussed the mobilization of pure bitumen or mastic (bitumen and filler). Figure 14 shows part of the coating of sample MRAB. This particular sample was obtained from an actual mixture, and, as demonstrated before, for this sample, a higher degree of blending has been achieved. Therefore, it can be considered as the most appropriate representation of the result of an actual mixing procedure and thus a realistic bituminous coating of RA granulates. As can be seen in Figure 14, the surrounding coating also consists of visible fine particles larger than 63 μm (filler threshold) and smaller than 500 μm. This bituminous zone should be considered as the bituminous mortar area. Moreover, this statement is in line with the definition given for asphalt mortar in [16]. For that reason, the mobilization of mortar should be considered as a more realistic phenomenon when adding RA in bituminous mixtures rather than the mobilization of bitumen or mastic.

**Figure 14.** Bituminous mortar as coating of a RA granulate.
