*2.2. Rheological Behavior*

A dynamic shear rheometer (DSR) was used to determine the complex shear modulus (G\*). All five bitumens were tested. AASHTO-T-315 [25] was adopted for the analysis. This test method covered the determination of the dynamic shear modulus and phase angle of asphalt binder when tested in dynamic (oscillatory) shear using parallel plate test geometry. It was applicable to asphalt binders that have dynamic shear modulus values in the range from 100 Pa to 10 MPa. This range of modulus was typically obtained between 6 ◦C and 88 ◦C at an angular frequency of 10 rad/s. This test method was intended for determining the linear viscoelastic properties of asphalt binders, as required

for specification testing, and was not intended as a comprehensive procedure for full characterization of the viscoelastic properties of asphalt binder.

The shear modulus responses (G\*) of the bitumen measured in the laboratory were first modelled with the 2S2P1D model, a generalization of the Huet–Sayegh model [26]. Referring to the 2S2P1D model, the values of the DBN (Di Benedetto-Neifar) bodies were fixed in the Linier Visco-Elastic (LVE) domain. An alternative general model "2S2P1D" (generalization of the Huet–Sayegh model), valid for both the bituminous binders and mixes and based on a simple combination of physical elements (spring, dashpot, and parabolic elements), was proposed.

The introduced 2S2P1D model had seven constants (Figure 2). At a given temperature T, the seven constants of the 2S2P1D model can be determined. G\* is calculated following Olard and Di Benedetto [26]:

$$E'\_{\text{(i\omega\tau)}} = E\_{00} + \left(\text{E}\_{0} - \text{E}\_{00}\right) / \left(1 + \delta \left(\text{i}\omega\tau\right)^{(-\text{k})} + \left(\text{i}\omega\tau\right)^{(-\text{h})} - \left(\text{i}\omega\beta\tau\right)^{(-1)}\right),\tag{1}$$

where:


**Figure 2.** 2S2P1D schematic model.

When ωτ→0, the *E*\*(i<sup>ω</sup>t)→*E*<sup>00</sup> + iω (*E*<sup>0</sup> − *E*00)βτ × β.

#### *2.3. Fourier Transform Infrared Spectroscopy–Attenuated Total Reflection (FTIR–ATR) Spectrometry*

The FTIR–ATR analysis was performed with a Bruker Tensor 27 and a diamond ATR crystal. The analysis was performed 16 times per specimen with a 4 cm−<sup>1</sup> resolution and a band range of 4000–600 cm<sup>−</sup>1. Around 0.5 g of bitumen was placed with a spatula (at ambient temperature) on the crystal. The crystal was cleaned with a bitumen remover after each test followed by ethanol to remove traces of the bitumen remover.

Oxidation of hydrocarbons was associated, notably, with the increase of carbonyl C=O (around 1700 cm−1) and sulfoxide S=O (around 1030 cm−1) bonds. The carbonyl index (%) and sulfoxide index (%) are defined in Table 1 for C=O and S=O, respectively, with a higher value indicating relatively more oxidation. The peaks around 1460 cm−<sup>1</sup> and 1376 cm−<sup>1</sup> were for aliphatic C-CH3 groups, which served as baselines for the analysis, as they change relatively little during aging [27].


**Table 1.** Fourier transform infrared (FTIR) bands with bitumen aging [27].

#### *2.4. Environmental Scanning Electron Microscopy (ESEM) Analysis*

Environmental scanning electron microscopy (ESEM) observations were conducted in accordance with the settings developed previously for bitumen by Mikhailenko et al. [28]. Bitumen specimens were observed at room temperature immediately after being removed from the cooler with a FEI Quanta 250 FEG ESEM.

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

#### *3.1. Bitumen Performance Grade (PG)*

Rheological characterizations of all extracted and recovered bitumen from FR, CR, FRM, CRM, and control mix were performed following AASHTO T 315. Figure 3 presents the rheological properties of each asphalt bitumen using a dynamic shear rheometer (DSR).

**Figure 3.** Performance grade of recovered bitumen by dynamic shear rheometer (DSR) at 1.6 Hz.

It was important to note that the recovered bitumen from the fine RAP was much stiffer than the recovered bitumen from the coarse RAP, even for the same RAP source that was simply split into two different groups. For example, at 76 ◦C and 1.6 Hz, the fine RAP bitumen was 4.6 times stiffer than the coarse RAP bitumen. Basically, the slope of each trend showed the sensitivity of that bitumen to temperature change. The control mix had the lowest sensitivity to temperature changes. Recovered bitumen from CR and CRM showed almost the same sensitivities, but recovered bitumen from FRM had less sensitivity than FR. Both CRM and FRM had the same virgin bitumen quantity (2.2%) and RAP bitumen content (2.3%). It was seen that FRM was stiffer than CRM at 76 ◦C (4.8 and 2 kPa, respectively), but the CRM bitumen was less sensitive than FRM to temperature change (0.25 and 0.57, respectively). In terms of the coarse and fine RAP before and after mixing with virgin bitumen, there was a huge difference from FR to FRM. This may be due to the amount of active bitumen in FR. As it was shown in a previous work [8], more RAP bitumen was transferred from coarse RAP than fine RAP particles because fine RAP particles were covered by a clump of mastic that did not tend to participate as active bitumen in the mixture. Despite the fact that fine RAP particles had higher aged bitumen content, participation by volume was less for the bitumen surrounding the coarse RAP particles in HMA.

One method that has been extensively used to evaluate the PG of virgin bitumen, in adding to HMA containing a high amount of RAP, is the blending chart [29]. In blending, it is assumed that there is a linear relationship between the amount of RAP bitumen and the rigidity (or DSR results) at a given temperature. Figure 4 shows a blending chart for the tested mix. To get results for all mixes at the same temperature, linear best fit curves were drawn from the results shown in Figure 3 (all with R2 > 0.95), and results were calculated for missing temperatures. At 70 ◦C, G\*/sin(d) value of control mix (0.4 KPa) was referred to as 0% coarse rap content, and G\*/sin(d) value of CR (6.038 KPa) was referred to as 100% coarse rap content. With these two values, the linear relationship between coarse RAP bitumen content and rigidity was plotted in Figure 4. The linear best fit curve for fine rap content was drawn at 70 ◦C and 1.6 Hz as well as in Figure 4.

**Figure 4.** Blending charts for the fine and coarse RAP mixes at 70 ◦C and 1.6 Hz.

As shown in Figure 4, if we compare mixes with 51% FR or CR RAP, we have G\*/sin (d) values of 14.4 and 3.2 KPa respectively. It showed that a mix with CR binder had a huge difference in shear modulus from the same mix with FR. It might be because the bitumen aging rate around finer particles is faster than the binder surrounding coarse particles, but more studies are needed to precisely verify this hypothesis. It may also be because fine particles have more surface area than coarse particles. Clearly, 51% CR had the same impact as 11% FR in mixes. Thus, it was important to clarify properly characterized RAP bitumen for a given particle size before mixing with virgin materials.
