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, Sousa [
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. Zahw [
6] applied torsion loads through laboratory test equipment to measure the shear characteristics of pavement specimens. Abd [
7] designed the CISST, which directly measured the in situ shear characteristics of asphalt concrete. Goodman [
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. Li Yuhua [
10] developed a torsional shear piece of equipment, which could conduct compression torsion failure test and creep test for asphalt or an asphalt mixture. Ragnia [
11] applied shear–torque tests to assess the fatigue behavior of double-layered asphalt specimens.
In addition, on the basis of a torsional shear test, a triaxial compression load control system was added to form a torsional shear triaxial test. The different internal and external pressures being applied to a hollow cylinder in the test used a four-way loading system simultaneously or individually, which was an ideal test to study the development of the influence of intrinsic anisotropy and stress path correlation of asphalt mixtures. Over years of experimental research at the University of Nottingham to simulate the stress condition of a pavement surface under a moving vehicle, the authors in [
12] have implemented a wide range of stress path tests using multiobjective apparatus for measuring the comprehensive mechanical properties of asphalt mixtures.
Overall, the characteristics of the proposed torsional shear stress of the torsion test compared with other tests are as follows: (1) The pure shear state is obtained in the torsional shear test in unconfined compression. (2) The normal stress of cross sections is uniform in the torsional shear test under confined compression. (3) The mechanical theory of torsional shear tests can be used in the in-situ test to evaluate the shear strength and the permanent deformation of the pavement.
5. Experimental Plans
The torsional shear test included two types to study the failure envelope modeled by the stress point on the failure plane. The type I test in unconfined compression was used to evaluate the influence of the temperature and the loading speed on the cohesion, which was one of the shear strength parameters. The type II test in confined compression was used to evaluate the effects of the magnitude of normal stress on the shear strength.
At high temperature, the static shear loading results in significant damage of asphalt mixtures. A temperature of 40
C for the experimental plan was set in this paper. To be aware of the various results of failure tests, the strain rate of the torsional shear test in this paper was 0.01 per second compared with a corresponding strain rate of 0.0085 per second, i.e., the loading speed was 50.8 mm/min for the triaxial compression test of Tan et al. [
19], as well as a corresponding strain rate 0.01 per second, i.e., the loading speed was 50 mm/min for the uniaxial compression test of the Chinese standard (T 0714) [
16]. A rapid loading pattern was selected away from the viscous response of creep. The measurement of the loading speed of the torsional shear testing device was 2.4 rad/min with an unloading period, which corresponded to a strain rate of 0.01 per second. The test consisted of a set of four specimens to collect three valid data points. The temperature of the specimens and jigs were controlled by a heated air system device for 4 h before testing, as shown in
Figure 7. Under constant temperature conditions, the experimental work was accomplished in 1 min to avoid the laboratory temperature effect on the response of the material.