Superpave Shear Tester

In 1987, SHRP began a 5 year, $50 million study to address and provide solutions to the performance problems of HMA pavements in the United States (FHWA-SA-95-003, 1995). As part of the study, the Superpave Shear Tester (SST) was developed to become the performance test used in the mix design process. The initial testing required a total of 6 different test (AASHTO M-003, Determining the Shear and Stiffness Behavior of Modified and Unmodified Hot Mix Asphalt in the Superpave Shear Test). The tests included:

1. Uniaxial
2. Hydrostatic
3. Frequency Sweep at Constant Height (FSCH)
4. Simple Shear at Constant Height (SSCH)
5. Repeated Shear at Constant Stress Ratio (RSCSR)
6. Repeated Shear at Constant Height (RSCH)

The first two tests, as well as the Simple Shear, were mainly used for modeling purposes within the Superpave modeling program. However, inaccuracies within the results of the computer analysis, as well test complexities, resulted in eliminating three of the tests. The test now only utilizes the SSCH, FSCH, and RSCH modes (AASHTO TP7-01).

The development and selection of the Superpave Shear Tester (SST) by the SHRP researchers was based on the device having the capability of measuring properties under states of stress that are encountered within the entire rutting zone of the pavement, particularly near the surface. Since there are an infinite number of states of stress that could exist within the pavement, it would be impossible to truly simulate all of them considering the non-linear and viscous behavior of HMA. Realizing this (Sousa et al., 1993) the SHRP researchers concentrated on the most important aspects and simulative conditions of the HMA behavior.

The following summary of factors which significantly affect the behavior of HMA was taken from the SHRP research product entitled, Accelerated Performance-Related Tests for Asphalt-Aggregate Mixes and Their Use in Mix Design and Analysis Systems, SHRP-A-417.

1. Specimen Geometry: a) A six inch by 2 inch specimen can easily be obtained from any pavement section by coring, or from any typical compaction method; b) the state of stress is relatively uniform for the loads applied; c) the magnitude of loads needed to be applied can easily be achieved by the use of normal hydraulic equipment.
2. Rotation of Principle Axis: The test set-up permits the controlled rotation of principal axes of strain and stress which represent the conditions that impact rutting.
3. Repetitively Applied Loads: Work by the SHRP researchers has indicated that to accurately capture the rutting phenomena, repetitive loads are required. This type of loading is needed given the viscous nature of the binder (load frequency dependent) and also granular nature of the aggregate (aggregates behave differently under static and dynamic loading).
4. Dilation: As discussed earlier, the dilation plays an important role in the rutting behavior of HMA. The SST constrains the dilation, and by doing so, confining stresses are developed. It is in part due to the development of these confining stresses that a mix derives most of its stability against rutting. The SST allows this by implying a constant height on the specimen while under going a shear stress. In the constant height regime, the development of axial stresses (confining stress in the SST) is fully dependent on the dilatency characteristics of the HMA. A vertical LVDT is positioned on the specimen to measure the dilation. This in turn props the axial actuator to either create a compressive or tensile force on the sample, depending on the volume change characteristic of the specimen. In this configuration, the HMA will either resist permanent deformation by relying on the high binder stiffness to minimize shear strains or the aggregate structure stability developed by the axial stresses from the dilation. In the constant height test, these two mechanisms are free to fully develop their relative contribution to the resistance of permanent deformation.

The SST system used in the testing was fabricated by Interlaken Technologies Corp. It consists of two orthogonal tables which are mounted on bearing. The tables are connected to two hydraulic actuators which are controlled using servo-values under a feedback closed-loop system (Figure 4, 5, and 6). To insure that the shear and axial forces are transmitted to the specimen, aluminum caps are glued to the parallel faces of the specimen. The gluing device used with this system was again fabricated by Interlaken Technologies Corp. Hydraulic clamps are then used to securely fasten the glued caps to SST test platens.

Repeated Shear at Constant Height (RSCH)

The RSCH test involves applying a repeated haversine shear stress of 10 psi a sample that has the dimensions of 150 mm in diameter and 50 mm in height. The applied load has a duration of 0.1 seconds, with an unload time of 0.6 seconds. An axial load is applied to the sample during the test to ensure a constant height is applied at all times. The test procedure followed for this test was AASHTO TP7-01, test procedure C. The HMA sample is tested at a test temperature that corresponds to local pavement temperatures. In New Jersey, this is approximately 52oC.

Frequency Sweep at Constant Height (FSCH)

The FSCH test involves applying a sinusoidal shear strain of 0.0001 mm/mm (0.01 %) at each of the following loading frequencies – 10, 5, 1, 0.5, 0.2, 0.1, 0.05, 0.02, and 0.01 Hz. Fifty cycles are tested for 10 and 5 Hz, twenty cycles are used for 2 and 1 Hz, seven cycles are used for 0.5, 0.2, and 0.1 Hz, and four cycles are used for 0.05, 0.02, and 0.01 Hz. This follows the test procedure used for the testing (AASHTO TP7-01, test procedure A). The tests are typically conducted at 20, 40, and 52oC, as recommended in the AASHTO procedures. Due to the nature of HMA, the test produces a value known as the dynamic shear modulus, G*. As the load is applied, there is a small delay in the actual movement of the HMA. This delay is called the phase angle, f, and is schematically shown in Figure 7.

Figure 7 – Development of the Phase Angle in Visco-Elastic Materials

After the sample has been tested over a range of temperatures, a master stiffness curve can be developed. The master stiffness curve of HMA allows for the comparison of visco-elastic materials when testing has been conducted using different loading frequencies and temperatures. The master curve can be constructed using the time-temperature superposition principle.

Simple Shear at Constant Height (SSCH)

The SSCH test is essentially a shear creep test. The specimen is loaded at a stress rate of 70 kPa/sec until a pre-determined creep load is obtained. The creep load is based on the temperature for which it is tested. The creep loads conform to those recommended in AASHTO TP7-01 test procedure B, and are; 345 ± 5 kPa for 4oC, 105 ± 5 kPa for 20oC, and 35 ± 5 kPa for 40oC. The creep load is applied for 10 seconds and then the load is reduced to zero at a rate of 25 kPa/sec. Once the stress reaches zero, the shear strain is measured for another 10 seconds. The test is complete after these final 10 seconds at zero stress. AASHTO TP7-01, test procedure B recommends the calculation of the maximum shear strain that occurs during the test and also the permanent shear strain at the end of the test.