## 1 Introduction  
The use of reclaimed asphalt pavement (RAP) is referred to as the recycling of aged asphalt pavement since RAP is a suitable substitute for natural aggregate and binder. The utilisation of RAP could reduce energy consumption, mineral usages, relevant pollution, and costs [1, 2, 3, 4, 5, 6, 7, 8]. In fact, the use of RAP could reduce gas emissions and production costs by as much as 35% and 70%, respectively [8, 9, 10]. However, the application of RAP remains limited in many countries due to the lack of awareness and non-standard recycling requirements [10, 11, 12]. Therefore, there is a need to establish a RAP assessment framework, especially when various RAP proportions are used in a hot mix asphalt (HMA) design. As such, the use of 30% RAP in HMA was reported to cause little or no complications at all [13, 14, 15, 16]. Nevertheless, the incorporation of a high RAP content in HMA faces many challenges. For example, an asphalt mix with higher RAP content could result in higher rutting resistance due to changes in the viscoelastic property, which in turn could have a negative impact on thermal cracking at low temperatures [17]. Furthermore, high RAP content could influence the volumetric properties of the asphalt mix, which could affect its mechanical performance [16; 18]. It was also observed that high RAP content could reduce the fatigue resistance [19; 20; 21] and the workability of the mix [22; 23]. Despite the challenges, several studies indicated that the use of a specific amount of RAP enhances the performance characteristics of asphalt mixes. In terms of the performance against moisture damage, [PERSON] et al. [24] showed that a 35% RAP mix performed better than a 50% RAP mix, though both mixes outperformed the control mix. In another study, [PERSON] et al. [25] observed that a 50% RAP mix recorded a tensile strength ratio (TSR) value of nearly 95%, which indicated the low susceptibility against moisture damage. [PERSON] et al. [26] also studied the performance of 50% RAP mix with and without rejuvenator. The study reported that a 50% RAP mix with a rejuvenator increased the cracking resistance without compromising moisture damage or permanent deformation resistance as compared to a 50% RAP mix without a rejuvenator. It was also observed that the use of a rejuvenator slightly improved the TSR with no detrimental impact. In addition, the compaction process is vital to achieving optimal rutting resistance through proper interlocking aggregates [27]. In particular, the laboratory study indicated that the aged binder in RAP affects the mechanical properties of the mixes due to its internal interaction [28]. For instance, the stiffness, viscosity, and softening point of the binder were reported to increase as the aged binder increased [28; 29; 30]. Based on these studies, the selection of the appropriate temperature, rejuvenator, binder and manufacturing process are all crucial factors to achieve the highest performance of the RAP mix [8; 31; 32]. Currently, rejuvenators have received increasing attention in asphalt technologies due to their ability to revive the properties of the aged binder [33; 34; 35], which potentially prevents thermal cracks and brittleness [36]. Moreover, rejuvenators could increase the durability of asphalt mixes [37]. Waste engine oil (WEO) is one of the common rejuvenators that restores and softens the aged binder [38; 39; 40; 41] as well as enhances the workability of RAP mixes [23; 39; 42]. Although WEO can reduce the optimum binder content (OBC) [43], WEO has a lower percentage of volatile components since it is processed under high temperatures (above 220 \({}^{\circ}\)C) [9], which partly contributes to binder hardening. The selection of a proper dosage of rejuvenator is a critical issue [8; 44; 45; 46] since the excessive incorporation of rejuvenator affects the stripping, adhesion, rutting, and heat cracking while inadequate dosages can stiff the mix [47; 48; 49].