IMPROVEMENTS TO THE DUPLICATE SHEAR TESTER

Abstract

Rutting in the asphalt concrete layer is a major distress in flexible pavements caused by the permanent shear deformation in asphalt mixes. The shear deformation is induced by heavy truck axis during hot summer days. Rutting affects directly the safety and users comfort. Therefore, developing of an appropriate test device for measuring the fundamental shear properties of asphalt mixes is important in order to improve the rutting resistance of asphalt mixes and pavement performance. The desired device should be simple, inexpensive, and accurate to be able to be incorporated for a quality control or quality assurance process, as well as to be accepted as a simple performance shear test device. The Duplicate Shear Tester (DST) is a device developed recently by Khajeh-Hosseini (2015) to measure the average shear asphalt properties of two specimens at the same time. Even though the DST has satisfied the objectives of a new test device for measuring the fundamental shear properties of asphalt mixes at low cost relative to the SST device, it provides reliable measurements solely for the dynamic shear modulus and phase angle of asphalt mixes at load frequencies ranged from 0.5 to 10 Hz. Therefore, this study aimed mainly to improve the DST as well as to assess the variability of the results obtained with the improved device by conducting the FSCH and RSCH tests.

The modified DST includes an additional mini linear rail system that ensures the movement of the aluminum plate only in the vertical plane. It replicates the loading conditions and constraints of the Superpave Shear Tester (SST) to measure the dynamic shear modulus, phase angle and permanent shear deformation as an average for two asphalt concrete specimens at the same time. This device is used to perform the two common SST tests: the FSCH and RSCH in accordance to the standard test method AASHTO-T 320. The tests were performed by utilizing a universal testing machine equipment (UTM-25) developed by IPCTM Global.

To evaluate the effectiveness of the modified DST, two dense graded asphalt mixes were tested for FSCH and RSCH tests. For each mix, eight pairs of specimens were tested. The FSCH tests were conducted first at 30°C to measure the shear dynamic modulus and phase angle of the mixes at ten different load frequencies ranged from 10 Hz to 0.01 Hz. The RSCH tests were performed at 50°C after that on the same samples to measure the permanent shear deformation of the specimens by applying 5,000 shear load cycles of controlled stress. Since the FSCH tests are performed in the linear viscoelastic domain of asphalt mixes, the same pairs of two mixes were used for both tests. The obtained results have proved the ability of the modified DST device to provide reliable measurements for shear dynamic modulus, phase angle, and shear permanent deformation of the tested asphalt mixes with relatively low coefficient of variations: 7% to 23% for the dynamic shear modulus (FSCH), 3% to 11% for the phase angle (FSCH), and about 20% for the permanent shear deformation at 5,000 shear load cycles (RSCH).