Advanced shear tester for evaluation of asphalt concrete under constant normal stiffness conditions

This paper presents motivation and details on the development of an advanced shear tester (AST) device capable of investigating the response of 150-mm cylindrical specimens under constant normal stiffness (CNS) conditions. Based on the laboratory experience and field observations from the soil and rock engineering, CNS conditions are particularly desirable when the normal stress changes considerably during the shearing process. Such situations occur in asphalt pavement structures especially under certain loading configurations. The CNS conditions are complementary to the constant normal load conditions that are applied in the current shear-mode devices incorporated in testing of asphalt concrete (AC) and pavement interface properties. This paper demonstrates the complex state of stress in the upper AC layer of a typical pavement under moving truck wheel and shows the need for the CNS device. In addition to outlining unique AST design features, this paper presents also the verification effort conducted on the solid Ultra-High Molecular Weight Polyethylene specimens. The AST results matched very well the reference data that were obtained under the elastic regime in the stress-controlled mode from the unconfined uniaxial compression and indirect tension tests. Finally, this paper demonstrates the example results collected on the solid AC specimens prepared with two different nominal maximum aggregate sizes. The analysis confirmed the ability of the AST device to capture both practical and fundamental phenomena occurring during the shearing process in the AC. It is believed that AST-type devices that are rather simple in design yet allow to induce complex state of stress in the AC specimens should be more recognised in the future research efforts.