# Fatigue Resistance Characterization of Warm Asphalt Rubber by Multiple Approaches
## 1 Introduction  
Fatigue cracking is one of the most common types of distress that occur in asphalt pavement. The adhesive and cohesive performance of asphalt mastic deteriorates after repeated traffic loading together with the ultraviolet-irradiation aging. Fatigue cracks develop gradually within asphalt layers throughout the service period. Fatigue cracks always bring roughness and pothe concerns, eventually leading to the disintegration of the whole pavement surface. Therefore, a comprehensive understanding of asphalt fatigue is essential for the construction and maintenance of better flexible pavements. Previous studies have proposed several evaluating methods to predict the fatigue life ofasphalt materials, such as the Superpave fatigue factor (G*sin\(\delta\)) test [1] and linear amplitude sweep (LAS) test [2] for asphalt binder, the shear fatigue test for asphalt mortar [3], and the indirect tensile fatigue test (ITFT) [4] and four point bending beam (4 PB) test [5] for asphalt mixture. Although the above methods are mostly based on relationships between initial stress/strain and loading cycles to failure with different stress or strain levels, their predictions have sometimes contradicted each other, especially when new materials or additives with more complicated properties are incorporated [6]. Asphalt rubber (AR) mixture is considered a sustainable paving material as it consumes abundant waste rubber tires. Due to the incorporation of crumb rubber modifier (CRM) into asphalt fractions, the components of modified binder become more complex due to the interaction among various components and swelled CRM's particle effect [7]. AR has attracted great interest for its superior mechanical properties and tire-road noise reduction function [8; 9]. However, AR pavement should be constructed at an elevated temperature (175 \({}^{\circ}\)C-200 \({}^{\circ}\)C) due to its high-viscous binder behavior. Recently, warm mix asphalt (WMA) technologies has been developed to alleviate the workability concern of asphalt mixtures [10; 11; 12]. According to different working mechanisms, WMA can be divided into three categories: (i) foaming processes (using water foaming or foaming additives to decrease the binder viscosity by enlarging binder the volume); (ii) the addition of organic additives (using wax as a flow improvored to reduce the binder viscosity at the mixing temperature); and (iii) the addition of chemical additives (using surfactant agents to reduce binder-aggregate friction for easier coating). It is reported that with the aid of WMA additives, AR mixtures can be paved at a similar temperature (160 \({}^{\circ}\)C) to that used for conventional asphalt mixes [13]. Compared to AR, warm asphalt rubber (WAR) has more complex components and its rheological and mechanical properties are also influenced by the properties of WMA additives. Many studies have been conducted for mechanism investigation and performance evaluation of WAR [7; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. For example, [PERSON] et al. found that organic and chemical additives promoted the dissolution of CRM in asphalt fractions [12]. [PERSON] et al. found that during the blending process at a high temperature, CRM absorbs not only asphalt light fractions but also part of the components from organic additives [7]. The workability enhancing effect of WMA additives on AR are significant [14]. The mechanical performance evaluation always involves both binder and mixture tests. [PERSON] et al. reported that although the mixture performance may be affected by aggregate properties, the mixture test results have relatively good consistency with binder rheological characteristics [15]. For high temperature performance, the results of binder tests (both G*/sin\(\delta\) by temperature sweep and Jnr3.2 by multiple stress creep and recovery) showed good consistence with those of mixture rutting tests.