# Experimental Study on Torsional Shear Testing of
## 1 Introduction  
Under a moving wheel load on pavement, the tire-pavement contact stress is in complex stress conditions [1]. The pavement surface is subjected to three-dimensional stress conditions, including vertical normal stress, longitudinal shear stress, and horizontal shear stress. During braking and turning, the shear stress exerted by wheels is more significant [2]. Research has shown that it is sufficient to accurately characterize the mechanical behavior of an asphalt mixture under three-dimensional stress for pavement response prediction and pavement design. At present, the shear test of asphalt mixtures mainly includes direct/indirect shear test, uniaxial shear test, triaxial compression test, and torsional shear test. The direct shear test is the compressional shear test in which a horizontal load is applied to the specimen in an unconfined compression load. The indirect shear test uses the inclined shear fixture to generate a certain angle between the specimen and the vertical axial force direction and to conduct the shear test by axial loads. The direct/indirect shear test has been widely used in the study of the interlayer shear performance of pavement structure (including the base course). The instruments and equipment adopted include self-developed equipment, improved geotechnical direct shear instrument, and fixture design on MTS or UTM. Superpave Shear Tester (SST), one of the testing device results of the Strategic Highway Research Program (SHRP) study, has been further improved based on the principle of a direct shear test by adding vertical dynamic loads, pressure, and temperature control. This testing device is capable of using both static and dynamic loadings in confined and unconfined conditions. The uniaxial shear test uses the Material Testing machine (MTS) or the Universal Testing Machine (UTM) with a servocontrol system to conduct static shear strength test and repeat shear test on a cylinder or hollow cylinder. In the cylinder uniaxial shear test or uniaxial penetration test, a stainless-steel indenter of a certain size is inserted into a cylindrical specimen to obtain the cohesion and the angle of internal friction from the measured penetration strength [3]. In the hollow cylinder uniaxial shear test, a hollow cylindrical specimen with limited inner and outer sides is adopted, and an axial load is applied to the inner wall of the specimen to achieve shear failure [4]. The triaxial compression test is a classic test to investigate the shear performance of asphalt mixtures, which can control the temperatures, internal and external pressures, and loading speeds. By applying axial and radial compression loads, the Mohr circle is obtained, and the cohesion and the angle of internal friction are calculated according to the Mohr-Coulomb criterion to obtain the shear strength of the material. The torsional shear test applies torsion loads on the specimen; it was used in early research to study the plastic yield of metal materials. In 1986, [PERSON] [5] at the University of California, Berkeley, USA, designed and constructed a hollow cylinder test to first develop the dynamic shear performance of the asphalt concrete under torsion loads. There were two forms of torsion test of the asphalt mixture: laboratory test and in situ test. [PERSON] [6] applied torsion loads through laboratory test equipment to measure the shear characteristics of pavement specimens. [PERSON] [7] designed the CISST, which directly measured the in situ shear characteristics of asphalt concrete. [PERSON] [8] extended the idea of applying torsion on the pavement surface and developed InSiSST to obtain the pure shear stress. In 2009, ASTM International published the DSR test method (ASTM D7552) [9] for the dynamic shear rheological test of asphalt mixtures, which could conduct dynamic shear modulus test and torsion creep test of asphalt mixtures, among other tests.