# Shear Properties of Asphalt Mixtures under Triaxial Compression
## 1 Introduction  
By the end of 2018, 142,500 km of expressway had been built in China, most of which are composed of the semi-rigid base asphalt pavement structure. The use of semi-rigid base reduces the deflection of the pavement, as well as the bending stress at the bottom of the asphalt layer [1]. Hence, the overall deformation resistance of the pavement structure is greatly enhanced. However, the shear stress in the surface layer is increased inadvertently [2]. Besides this, the overloading phenomenon is widespread in China. Therefore, it is of great importance to enhance the strength, especially the shear strength of asphalt mixtures in the hot areas in China [3]. Otherwise, the rutting damage will appear in the asphalt pavement due to insufficient capacity of shear resistance [4; 5; 6]. Thus, the performance of asphalt pavement will be greatly reduced, and the service life and service level of pavement will be seriously affected.  
In previous studies, the triaxial test and shear formula have been used for the analysis of shear resistance for asphalt mixtures [7; 8; 9]. In recent years, the Strategic Highway Research Program (SHRP) has proposed three types of tests to evaluate the shear deformation of materials, including triaxial creep and repeated loading tests, simple shear creep and repeated loading tests, and uniaxial creep and repeated loading tests [10; 11]. However, these tests have not been widely used in practice in China because of their time consumption. In addition, [PERSON] et al. developed a method for determining the criterion of shear strength based on the large accelerated loading test [12]. Nevertheless, the large accelerated loading test is expensive and complicated to operate. Only a few agencies in China have this equipment, which makes it difficult to be widely used [12]. In order to simplify the test method, [PERSON] et al. proposed the partial triaxial compression test to evaluate the shear resistance of asphalt mixtures [13]. Although this test method inherits many advantages of triaxial test, its applicability needs further verification because it is a new test method. In order to ensure the asphalt surfaces have adequate shear strength, the Specifications for Design of Highway Asphalt Pavement (JTG D50-2017) in China inspects the integrated penetration strength of the whole asphalt concrete surface layers; the essence is to control the shear strength from the view of asphalt mixtures. Although the uniaxial penetration test was proposed in the specification to simulate the complex stress states of asphalt materials in the pavement structure [14; 15; 16], it is unreasonable to solve the friction angle and cohesion strength by linear Mohr-Coulomb theory according to the combined tests of uniaxial penetration and unconfined compression, the reason being that the nonlinear shear properties of asphalt mixture were not taken into account [17; 18; 19].  
As mentioned above, on one hand, the shear failure of asphalt pavement materials usually occurs under the effect of overload and heavy load during the hot summer season [20]. On the other hand, the triaxial test has many advantages such as to simulate the complex stress state of the pavement structure, and to solve the friction angle and cohesion strength for asphalt pavement materials and structure design [21; 22; 23]. However, the use of the linear Mohr-Coulomb theory affects the accuracy of materials and structure design for asphalt pavement. Therefore, the triaxial tests were carried out at various temperatures and confining pressure levels for the widely used continuous-graded AC (Asphalt Concrete)-13, open-graded OGFC (Open Graded Friction Course)-13, and gap-graded SMA (Stone Mastic Asphalt)-13 asphalt mixtures in China [24]. Based on the results, a nonlinear Mohr-Coulomb failure envelope was established, and the nonlinear evolutional laws of cohesion strength and friction angle were also proposed. All of these are important for the design of asphalt pavement materials and structures.