**1. Introduction**

When asphalt pavements reach their end-of-life point, their layers must be renewed. The material removed from these layers is known as Reclaimed Asphalt (RA) (EN 13108-8:2016, [1]). RA is in fact an asphalt mixture consisting of aggregates and bitumen. The removed material is typically crushed in agglomerations of different sizes. Asphalt recycling is not only economically viable, since a considerable amount of bitumen can be replaced by the RA bitumen in the new asphalt mixture, but also environmentally friendly [2]. Previous research has shown the potential benefits of using RA on the mechanical performance of bituminous mixtures [3] and also provided recommendations concerning the proper exploitation of the material [4].

When a new bituminous mixture with the incorporation of RA is designed, in most design procedures, the assumption of full blending between the aged RA bitumen and the new bitumen is still being used. The mobilization or activation of RA bitumen is, till today, a "black box" for the asphalt sector; the actual degree of blending between new and aged bitumen is under research. Many studies question this practice and have demonstrated cases of partial blending [5,6] or zones where the phenomenon of "black rock" is present, meaning that part of the bitumen in the RA is inactive [7].

This gap in the scientific knowledge is of grea<sup>t</sup> importance, since overestimating the degree of blending can lead to mixtures with less active bitumen, which could significantly influence the mechanical performance of bituminous mixtures [8].

Previous studies have approached this problem in different ways. The first approach is by investigating the diffusion between two bituminous layers and studying the response of the blend by means of Dynamic Shear Rheometer (DSR) [9,10]. Other studies have modelled the activated bitumen modulus response by using the stiffness modulus of the corresponding mixture [11]. Researchers have also performed staged extractions to RA granulates in order to assess the properties of the recovered bitumen layer by layer in terms of shear modulus and infrared spectra analysis [12–14]. The latter approach contains some risks for interpretation of the results, as it is still unclear whether the solvents influence the bitumen structure and what their impact is on the properties of the recovered binder [15].

The past years, increasing attempts have demonstrated the ability of non-destructive tests to evaluate this problem. Attenuated total reflectance (ATR) spectra obtained by Fourier transform infrared (FTIR) spectroscopy is a widely utilized non-destructive test to study bitumen and especially to trace ageing evolution [16–18], to evaluate rejuvenation [19,20] or to evaluate the modification of bitumen [21,22]. ATR-FTIR spectroscopy is suitable to evaluate homogeneous individual components but not a heterogeneous composite material (such as an asphalt mixture), since it can only be applied in a single spot averaging the measuring spectra. For that purpose, more and more researchers draw attention to FTIR microscopy techniques.

ATR-FTIR imaging has been used on asphalt concrete samples with RA in order to demonstrate a method to evaluate the different components highlighting that sample imperfections strongly effect the quality of the measurements [23]. Besides ATR-FTIR imaging, X-ray fluorescence and infrared microscopy have been used in an additional study. The analysis was applied on the bitumen level using markers to track the mobilization of artificially aged bitumen. The conclusion was that ATR imaging was the most successful technique [24]. Other studies have evaluated the blending of asphalt concrete enriched with titanium dioxide (TiO2) as a tracer using Computer Tomography (CT), in macro- and micro-scale [25], and using environmental scanning electron microscopy (ESEM) [7,25]. A scanning electron microscope/energy dispersive spectrometer (SEM/EDS) evaluated the degree of blending of mixtures, with various RA quantities, again using TiO2 as a tracer [26]. Research has also been conducted using atomic force microscopy (AFM) on bitumen levels [26,27]. Another attempt at investigating the blending between two different bituminous binders was by nano-CT scanning images and nano-indentation tests, which did not provide clear and direct conclusions [28].

In order to study the blending process between aged and new bituminous layers, previous research simulated the ageing state of the RA bitumen with artificially aged bitumen in the laboratory; it has been demonstrated that long-term laboratory ageing techniques do not always correspond to field ageing [29,30]. On the other hand, several studies have used actual RA but with the addition of tracers to mark the mobilization, such as the TiO2. The question that might arise here is to what extent markers can influence the mobilization. Furthermore, earlier studies considered the mobilization of only bitumen or focused only on the diffusion process between two bituminous binders. The aforementioned procedures highlight the need of evaluating field-aged asphalt materials and focus on the actual blending conditions as it will happen in the bituminous coating of an RA granulate. In reality, the stones in the asphalt mixture are bonded not by pure bitumen but by the bituminous mortar (Figure 1). For clarification purposes, the following definitions are given: asphalt mortar is a combination of bitumen and fine particles (filler and sand smaller than 0.5 mm) and asphalt mastic is a combination of bitumen and filler.

In this study, actual RA granulates are used without the addition of foreign tracers and provide the opportunity to study the spatial distribution of neat bitumen within the mortar scale by means of ATR-FTIR microscopic measurements. This research aims firstly to assess the application of FTIR microscopy directly on actual RA granulates, secondly to fingerprint the different zones of components, and thirdly to trace rejuvenating agents and mobilization of neat bitumen within the coating of the granulates.

**Figure 1.** Asphalt mixture bonded by asphalt mortar.

#### **2. Materials and Methods**

For the purpose of achieving the aims of this research, an FTIR microscope (LUMOS by Bruker, Billerica, MA, USA) in ATR-mode was used to evaluate the coating of various RA samples. Characteristic absorbance bands were selected, which were typical for the components that form the final sample, i.e., bitumen, fines, rejuvenator, and resin (see Section 2.3). After the definition of the spectra, which is a graphical visualization of the infrared light absorbance as a function of the wavelength in the mid-infrared region of 4000—600 cm<sup>−</sup>1, data concerning the integration of the characteristic areas were analyzed and formed into a chemical imaging of the area studied (see Section 2.3). The integrated areas were further analyzed to inspect the numerical distribution of carbonyl levels within the bituminous zone (Section 3.2).

#### *2.1. Sample Preparation*

Four types of RA granulate samples were studied in this study. Among the samples, one is an untreated RA granulate (sample name: GRA) and three are treated RA granulates (sample names: GRAB, MRAB, and GRAR). A summary of the samples and their treatment is presented in Table 1. The RA used in this study consists of granite and limestone aggregates (inspected visually), and the bitumen after extraction and recovery has a penetration of 24 1/10 mm. The treated RA granulates were fabricated following three different procedures. First, sample GRAB was prepared by mixing the RA granulates with neat bitumen. Second, sample MRAB is an RA granulate that was derived from an actual asphalt concrete mixture (type AC14, see Table 2). Third, the GRAR sample was sprayed with rejuvenator. The preparation of the treated samples is more elaborately described in the following paragraphs.




**Table 2.** Composition of the AC14 mixture.

The GRAB samples were fabricated following the procedure described in § 5.2 of EN 12697-11:2012 [31]. In this standard, a method is described to create a uniform bituminous coating on mineral aggregates. An adequate amount of RA granulates (600 g) was preheated at 110 ◦C. In order to avoid excessive ageing of the RA binder, the preheating time was limited to two hours. The preheated granulates were then manually mixed together with 3% of 35/50 penetration grade neat bitumen (by RA granulates mass). The mixing conditions were a maximum of 2 min at 150 ◦C. Afterwards, the covered stones were spread and separated on silicone paper (Figure 2b). Visually traced agglomerations were rejected.

**Figure 2.** Original, untreated Reclaimed Asphalt (RA) granulates (**a**); RA granulates mixed with neat bitumen (**b**); RA granulates sprayed with rejuvenator (**c**).

The MRAB samples were obtained from an actually mixed AC14 mix with 55.7% of RA in the mixture. The composition of the mixture was designed in such way that the only source of coarser aggregates (larger than 8 mm) was the RA granulates. The mix composition is presented in Table 2. The production of the mixture was done according to EN 12697-35:2016 [32]. In contrast to the GRAB samples, the mixing took place for 5 min at a temperature of 180 ◦C instead of maximum 2 min at 150 ◦C. After mixing, only the larger stones (>8 mm) were selected for MRAB in order to assure only covered GRA was sampled.

Finally, the GRAR samples were prepared by spraying a crude tall oil-based rejuvenator on the samples. Six (6) RA granulates were sprayed with 2% rejuvenator on the RA mass (Figure 2c). Both RA and rejuvenator were at room temperature. In order to accelerate the diffusion of rejuvenator [33], the sprayed stones were placed for 45 min in an oven at 110 ◦C. This temperature is according to the heating limits of RA proposed in § 6.3 of EN 12697-35:2016 [32]. The amount of rejuvenator added can be considered as rather high when compared to other studies [34]. The purpose here is to make sure there is a traceable amount of rejuvenator.

A necessary step for the utilization of the FTIR microscope is the existence of a flat and even sample surface. In order to fulfil this requirement, the RA granulates were embedded in epoxy resin. For the preparation of the stone-resin samples, the following steps were followed: First, the samples were embedded in a plastic mold with resin (Figure 3a). The height of the stone-resin sample is

25 mm and the diameter 30 mm. The samples were then cured at room temperature for one day. Afterwards, the cured samples were demolded and polished with a mechanical grinding/polishing machine (Struers TegraForce 5) (Figure 3b). The polishing was done in two steps. During the first step, a 220 grit size sandpaper removed the larger part. During this step, the plateau is rinsed by cool water, which keeps the temperature low during the polishing. The second step uses a high friction paper together with a diamond paste (9 μm), providing an even final surface.

**Figure 3.** Plastic mold for preparation of the stone-resin samples (**a**) and the polishing machine (**b**).

## *2.2. FTIR Instrumentation*

In this study, an FTIR Microscope (Lumos by Bruker, Billerica, MA, USA) was used. The infrared analyses were performed using a germanium ATR crystal with a high refractive index (*<sup>n</sup>*Ge = 4), which permits the analysis of dark samples. The instrument is able to generate a chemical map by integrating the infrared spectra over a specified area. The integration methods used for this study are presented in Section 2.3. The stone-resin samples were placed on a mounting holder called the Micro-Vice Holder (Figure 4). Each spectrum was compiled from 32 scans with a resolution of 4.0 cm<sup>−</sup><sup>1</sup> in a range of 4000–600 cm<sup>−</sup>1. To avoid damaging the crystal, the applied contact pressure was limited to "low pressure".

**Figure 4.** Stone-resin sample mounted in the holder during spectra collection.
