*2.3. Designing and Testing of Bituminous Mastics and Mixes* 2.3.1. Preparation of Mastic Samples

The mastic samples were fabricated by adding a suitable amount of fillers to preheated bitumen in order to maintain a filler/bitumen (f/b) ratio of 0.5–1.4 with a rate of 0.3 rises by weight; each sample was then mixed for 1 h. A shear blender with blending conditions of 2800 rpm at 160 ◦C was applied to attain homogeneity, as detailed in earlier studies [20]. Afterward, 100 gm of each mastic sample was stored in an airtight container for physical and morphological characterization.

#### 2.3.2. Determination of the Physical Properties of Mastic

The conventional bitumen tests of penetration and softening point were performed to assess the physical properties of the prepared mastics. Those two tests may be considered a significant indicator of the stiffening effect of the fillers in mastic. Again, a conventional approach was taken from the Shell bitumen handbook to forecast the thermal susceptibility of the mastic samples in respect of the penetration index [21]. Such an index for mastics is based on their penetration value and softening point, as shown in Equation (1). A lower value of a binder's penetration index indicates greater temperature susceptibility, whereas a higher value indicates more resistance to low-temperature cracking [22].

$$\text{Percentage Index} \ (\text{PI}) = \frac{1952 - 500 \log(Pen\_{25}) - 20SP}{50 \log(Pen\_{25}) - SP - 120} \tag{1}$$

where *Pen*<sup>25</sup> = Penetration value of the mastic at 25 ◦C, and *SP* = Softening point (◦C) of the mastic.

#### 2.3.3. Morphological Analysis by Optical Microscopy Test

A Ziuss optical microscope was used to study the extent of dispersion of fillers in the binder. A droplet of warm mastic was crammed between two microscope slides that were then bound by adhesive tape at both ends and left to dry for a day in a dirt-free Petri dish (Figure 2a). Then, they were studied using an optical microscope (Figure 2b) with a 20× magnification power; the images were captured by Axiovision software. The photos of 16 numbered slides (2 replicates × 2 types of filler × 4 f/b ratios) were captured and considered for analysis.

**Figure 2.** (**a**) Microscope slide having bituminous mastic (**b**) Optical microscope.

#### 2.3.4. Marshall and Volumetric Properties

The Marshall mix design method was used to evaluate the optimum bitumen content (OBC) of all mixes according to MS-2 [23] guidelines. For each mix, about 1200 g of aggregate was sieved at the chosen gradation (Table 2) with five bitumen contents (4.5–6.5%) and four filler contents (2%,4%, 6%, and 8% by the weight of aggregate). For each mix, 15 numbered samples (three for each bitumen content) were fabricated, and their volumetric properties, Marshall stability, and flow were calculated (ASTM D6927 [24]). The OBC of each mixture was considered as a binder percentage in the compacted samples with 4% air voids [25]. The air voids in the compacted specimens were measured according to ASTM D3203-17 [26]. The increase in filler content in the mix was achieved by reducing the fine aggregate fraction to maintain the required grading. In this study, the mix having 2% HL as filler was considered the conventional/control mix.


**Table 2.** Characteristics of Fillers.
