## 1 Introduction  
As an essential accessory part of a bridge project, the quality of expansion joints is directly related to the quality and safety of the bridge project. Among all kinds of expansion joints, seamless expansion joints are widely used both in China and other countries because of their advantages of good integrity, durability, convenient construction, easy maintenance, and comfortable driving [1; 2; 3]. Traditional asphalt plug joints (APJs) are prone to problems such as high-temperature flow and low-temperature cracking in areas with significant temperature changes, making it challenging to meet the requirements of long-term bridge service. Up to now, many scholars have improved the performance of asphalt binders via polymer modification, such as SBR and SBS [4; 5]. SBR modifiers improve the low-temperature flexibility of asphalt materials, improving the low-temperature crack resistance of asphalt mixtures [6; 7; 8; 9; 10]. SBS modifiers can improve the high- and low-temperature performance and water stability of asphalt binder, and improve the adhesion of asphalt [11; 12; 13; 14; 15]. Someresearchers have also studied the performance of SBS/SBR-composite-modified asphalt to obtain modified asphalt with excellent high- and low-temperature performance. [PERSON] found that SBS/SBR-composite-modified asphalt mixtures have excellent low-temperature crack resistance [16]. [PERSON] found that when the SBS dosage is 3%, and the SBR dosage is 2%, the resulting composite-modified asphalt high- and low-temperature performance, and aging resistance is excellent [17]. [PERSON] used SBS, SBR, sulfur, and furfural extraction oil, four materials for matrix asphalt composite modification [18]. The composite-modified asphalt mixture of high- and low-temperature performance and water stability relative to the matrix asphalt have a substantial increase. Moreover, some scholars have also prepared APJs by adding resins instead of polymer bitumen. Polyurethane materials are thermosetting materials with good high- and low-temperature performance and excellent deformation adaptability, capable of absorbing transverse and micro-rotational deformation [19; 20; 21; 22]. However, the water resistance of polyurethane materials needs to be further improved. Epoxy asphalt is mainly made of epoxy resin, matrix asphalt, and a curing agent from a complex chemical reaction [23; 24]. The asphalt particles are bound in a three-dimensional network structure, forming an irreversible thermosetting material with the advantages of high strength, fatigue resistance, high erosion resistance, high-temperature performance, and better low-temperature fracture resistance [25; 26; 27; 28; 29; 30]. Epoxy asphalt has been widely used in bridge paving materials, especially in steel bridge deck paving, which has shown excellent use. [PERSON] et al. [31] developed a lightweight epoxy asphalt mixture (LEAM) to reduce the deadweight of the bridge deck pavement, and the addition of 70% rounded lightweight aggregate achieved the optimal performance of the steel bridge deck pavement. [PERSON] et al. [32] modified epoxy asphalt using crumb rubber (CR), and epoxy asphalt containing 30% CR was found to meet the application requirements of bridge expansion joints. Although previous studies have shown that SBR and SBS modifiers can significantly improve the high- and low-temperature performance and water stability of asphalt materials, the application of their composite modification techniques has mostly focused on ordinary pavement-surfacing materials, and research on seamless expansion joint materials for bridges has been more limited. In addition, most of the existing studies on the performance optimization of epoxy asphalt-based materials focus on a single environmental condition, and a systematic evaluation of their comprehensive performance is lacking. The aim of this study is to optimize the ratio of SBR and SBS modifiers to enhance the performance of thermosetting epoxy asphalt in seamless bridge expansion joints.