## 1 Introduction  
Bitumen pavement is a typical form of road pavement structure. With the continuous development of research methods, people have developed a clearer understanding of the internal stress and strain distribution law of the pavement structure under the influence of factors including environment and traffic [1; 2; 3; 4; 5], which makes it possible to design materials according to the stress characteristics and functions of pavements at different levels [6; 7]. According to a large number of studies, including the analysis of pavement distress and mechanical analysis, it has been found that in the entire pavement structure, the rutting distress occurs mostly in the area 5-10 cm below the surface (the middle layer), especially under heavy-load and slow traffic [8; 9; 10; 11; 12]. A survey conducted by the States Highway Bureaus in the United States during the 1990s revealed that 29.5% of road damage was attributed to rutting [13]. Similarly, in Japan, over 80% of overlay engineering for expressways during the same period was due to rutting. In the I and II zones of Gansu Province's road network, [PERSON] [14] indicated that expressways with permanent deformation exceeding 10 mm comprised 43.4% and 44.1% of the total expressway mileage in those regions, respectively. On the Lianhuo Expressway, sections with permanent deformation greater than 10 mm represented 79.2% of its total length. Data from 2014 regarding three expressways in Liaoning Province showed that rutting damage constituted 82.6-88.6% of bitumen pavement damage [15]. Research by [PERSON] et al. [16] underscored that the middle layer contributes up to 60% of the rutting depth in bitumen pavement. Therefore, in recent years, the construction of high-grade highways has generally increased the emphasis on the middle surface layer [17; 18]. It has been unanimously recognized that the rutting resistance of bituminous mixtures can be improved by adding polymer modifiers, anti-rutting agents, and high-modulus agents [19; 20; 21]. One of the most common polymer modifiers that is used to improve the high-temperature characteristics of bitumen binder is styrene-butadiene-styrene (SBS) [22; 23; 24; 25]. [PERSON] et al. [26] studied the rutting performance of bituminous mixtures containing different polymer modifiers. Compared with the conventional mixtures, polymer-modified bituminous mixtures showed more resistance to permanent deformation. Among the five modified bituminous mixtures studied, SBS-modified bituminous mixture had the most rutting resistance. Since the 1960s, bituminous mixtures produced with Recycled Tyre Rubber Modified Bitumen have been used in different parts of the world as solutions for different quality problems and, despite some drawbacks, in the majority of cases they have demonstrated an enhanced rutting resistance of the bituminous mixture [27; 28; 29]. Adding anti-rutting agent (ARA) in bituminous mixtures is also an effective means to prevent pavement rutting. With an increase in ARA content, the high-temperature rutting resistance increases remarkably, and the moisture-induced-damage resistance improves slightly [30; 31]. However, an excessive amount of ARA may deteriorate the fatigue life of bituminous mixtures [32]. High-modulus bitumen concrete (HMAC) was first mentioned in France in the 1960s and then high-modulus agent (HM)-modified bituminous mixtures were used in extremely high traffic conditions [33; 34; 35]. [PERSON] et al. [36] reported that when HMAC is applied to the middle surface layer, it can bear the maximum shear stress generated by vehicle load better and thus reduce rutting. However, some studies have found that the materials with high rutting resistance impaired low-temperature and water stability performances [37; 38]. In the seasonally frozen region of Northeast China, low-temperature cracking is an important failure mode of bitumen pavement, and there are also high requirements for the low-temperature performance of bituminous mixtures in this region.