## 1 Introduction  
The use of modifiers such as polymers and recycled crumb rubber has been gaining popularity in recent decades. It has been successfully adopted by transportation agencies in hot mix asphalt (HMA) pavements due to the environmental benefits and ability to improve the performance of asphalt mixtures related to rutting and moisture-induced damage [1; 2; 3]. Rutting or permanent deformation is one of the main causes of distress in HMA pavements and is due to the rise in traffic volume and heavier loads [4; 5; 6]. It is defined as the accumulation of unrecoverable strain due to applied wheel loads, resulting in a reduction in the performance and service life of HMA pavements [7; 8]. Due to rutting, many problems arise, such as water accumulation, which results in the entry of water into the pavement layers and weakening of the structure [9]. In addition, water accumulation results in hydroplaning, which is a safety hazard for vehicles [10]. However, to tackle this rutting problem in the pavement, several studies have been conducted, resulting in the development of rutting models and concepts and the invention of testing methods. Most studies have evaluated the rutting properties of HMA pavements using wheel tracking test devices [4; 5; 6; 11; 12; 13]. Previous studies conducted on HMA mixtures with the use of polymers and recycled crumb rubber have exhibited a better resistance to rutting [1; 2; 11; 14; 15; 16]. In addition, studies carried out to evaluate the rutting resistance of unmodified and polymer-modified mixtures have shown that polymer-modified mixtures exhibit a better resistance [13; 16]. Studies on crumb rubber-modified mixtures have shown better rutting resistance [15]. It is important to note that the majority of the reported work on the rutting of asphalt binder has utilized the Superpave rutting parameter (G*/sin8), which is unsuitable for characterizingmodified asphalt binder due to the unaccountability of the delayed recovery response associated with modifiers [17, 18]. The refinement of the testing protocol over the years included the development of the Multiple Stress Creep Recovery (MSCR) test, which predict the high-temperature performance of modified asphalt binder better [19, 20]. Results have shown that the MSCR test provides a much better correlation with mixture rutting than the existing Superpave binder criteria [19]. Therefore, considering the limitations of G*/sin$, one of the aims of this research work was to investigate the effect of modifiers on the rutting properties of asphalt binders using the MSCR test. Likewise, moisture-induced damage is also one of the concerns in HMA pavements and is defined as the breakdown of the adhesive bond between aggregate and asphalt due to the presence of water, which, in turn, results in a loss of cohesion, strength, and stiffness [13, 21, 22]. Different indirect approaches, such as the Tensile Strength Ratio (TSR) test, the boiling test, the stripping test, etc., have been used in the past to evaluate moisture-induced damage properties [13, 21]. Previous studies conducted on HMA mixtures have a exhibited better resistance to moisture-induced damage with the use of polymers and recycled crumb rubber [11, 14, 15, 16]. In addition, studies carried out to evaluate the moisture-induced damage resistance of unmodified and polymer-modified mixtures have shown that polymer-modified mixtures exhibit a better resistance to moisture-induced damage [11, 16]. Studies on crumb rubber-modified mixtures have shown a detrimental effect on moisture-induced damage properties due to the higher air void content [15], which enables the loss of moisture at the asphalt-aggregate interface, causing more moisture damage [23]. However, every approach fails to answer whether failure has taken place at the aggregate-asphalt binder interface or within the asphalt binder matrix phase itself. Such information is essential so that further action can be taken to address it appropriately.