## Materials and Methods
### Preparation of GNP-Modified Asphalt  
In Pakistan, the asphalt binder selection is currently based upon penetration grading, whereas pavement engineers around the globe are adopting performance grading (PG). The concept of performance grading is based around the idea that the properties of the asphalt binder should be in accordance with the requirements put forth by the environmental conditions of the area in which it is used. According to the temperature zoning of Pakistan, in 70% of areas the required performance grade of a binder is 70-10 [13,14]. Unfortunately, PG 70-10 binder is not produced in any of the oil refineries in Pakistan [13]. The PG of the binder used in this study was 58-22, which is much softer than the required PG 70-10. The high PG values are more of a concern, as the temperature hardly falls below 0 \({}^{\circ}\)C in most of the areas of Pakistan. The aim of the study was to achieve a PG of 70 using GNPs. For this research, two percentages of GNPs were used (2% and 4%) by weight (23% and 46% by volume) of the binder. This choice was made on the basis of the performance grade, as PG 64 was achieved when 2% of GNPs were added to the binder and PG 70 was achieved upon the addition of 4% of GNPs. For performance grading, a Dynamic Shear Rheometer was used. The frequency was set as 10 rad/s and 25 mm geometry was used. The initial temperature was set at 58 \({}^{\circ}\)C. The highest temperature, where the value of G*/sin8(kPa) was reduced to 1.0 kPa or less, was termed the high-temperature performance grade (PG) of the binder. The variation in the performance grade of the binder after its modification with GNPs is presented in Figure 2. The PG of the base binder was 58 \({}^{\circ}\)C and it failed at 62.7 \({}^{\circ}\)C. Upon the addition of 2% of GNPs, the PG increased by one level, i.e., 64 \({}^{\circ}\)C. The failure temperature also increased from 62.7 \({}^{\circ}\)C to 65.7 \({}^{\circ}\)C. A more pronounced change was observed when 4% of GNPs were added to the asphalt binder. PG 70 was achieved after modifying the binder with 4% of GNPs and the failure temperature was 71.5 \({}^{\circ}\)C. The trend shows that, with the addition of GNPs to the asphalt binder, its performance grade increases. Prior to the mixing process, the asphalt binder was heated in an oven for 30 min at 158 \(\pm\) 5 \({}^{\circ}\)C. A glass rod was used to mix the GNPs into the binder for 10 min at 158 \(\pm\) 5 \({}^{\circ}\)C [8]. In order to ensure homogeneous mixing of the GNPs in the asphalt binder, Scanning Electron Microscope (SEM) (Vega3, TESCAN, Czech Republic) images were taken. The Scanning Electron Microscope has been shown in  
Figure 1: The aggregate gradation curve. Figure 3. Figure 4 shows the SEM image of the GNPs, in which the plate-like/flaky morphology of the GNPs can be seen. Figure 5 is the SEM image of the GNPs dispersed in the asphalt binder. In the image, homogeneously dispersed GNPs are visible in the binder. Figure 4. A Scanning Electron Microscope (SEM) image of the Graphene Nano-Platelets (GNPs) (100 \(\upmu\)m). Figure 3. The Scanning Electron Microscope (SEM) in the Mechanical Engineering Department (University of Engineering and Technology, Taxila). Figure 2. The performance grades (PGs) of the asphalt binders. According to the General Specifications of the National Highway Authority (NHA) of Pakistan, two aggregate gradations named class \"A\" and \"B\" are used for the construction of asphalt pavement wearing courses. Class A is coarser, while class B is finer [15]. For this study, the asphalt mixtures were prepared using the NHA class B aggregate gradation for the wearing course (19.5 mm Nominal Maximum Aggregate Size). To determine the Optimum Binder Content (OBC) of the GNP-modified asphalt, the Marshall Method of Mix Design was adopted. The asphalt mixes were prepared using a base binder and GNP-modified binders.