Penetration grading and viscosity grading are somewhat limited in their ability to fully characterize bitumen binder for use in HMA pavement. Therefore, as part of the Superpave research effort new binder tests and specifications were developed to more accurately and fully characterize bitumen binders for use in HMA pavements. These tests and specifications are specifically designed to address HMA pavement performance parameters such as rutting, fatigue cracking and thermal cracking.
Superpave performance grading (PG) is based on the idea that an HMA bitumen binder’s properties should be related to the conditions under which it is used. For bitumen binders, this involves expected climatic conditions as well as aging considerations. Therefore, the PG system uses a common battery of tests but specifies that a particular bitumen binder must pass these tests at specific temperatures that are dependent upon the specific climatic conditions in the area of use.
Superpave performance grading uses the following bitumen binder tests:
The Rolling Thin-Film Oven (RTFO) procedure provides simulated short term aged bitumen binder for physical property testing. Bitumen binder is exposed to elevated temperatures to simulate manufacturing and placement aging. The RTFO also provides a quantitative measure of the volatiles lost during the aging process.
The basic RTFO procedure takes unaged bitumen binder samples in cylindrical glass bottles and places these bottles in a rotating carriage within an oven. The carriage rotates within the oven while the 325°F (163°C) temperature ages the samples for 85 minutes. Samples are then stored for use in physical properties tests or the PAV.
The standard Rolling Thin-Film Oven test is:
AASHTO T 240 and ASTM D 2872. Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test)
The Pressure Aging Vessel (PAV) provides simulated long term aged bitumen binder for physical property testing. Bitumen binder is exposed to heat and pressure to simulate in-service aging over a 7 to 10 year period.
The basic PAV procedure takes RTFO aged bitumen binder samples, places them in stainless steel pans and then ages them for 20 hours in a heated vessel pressurized to 305 psi (2.10 MPa or 20.7 atmospheres). Samples are then stored for use in physical property tests.
The standard Pressure Aging Vessel procedure is found in:
AASHTO R 28: Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV)
The Rotational Viscometer (RV) is used to determine the viscosity of bitumen binders in the high temperature range of manufacturing and construction. This measurement is used in the Superpave PG asphalt binder specification. The RV test can be conducted at various temperatures, but since manufacturing and construction temperatures are fairly similar regardless of the environment, the test for Superpave PG asphalt binder specification is always conducted at 275°F (135°C).
The RV test helps ensure that the bitumen binder is sufficiently fluid for pumping and mixing. The basic RV test measures the torque required to maintain a constant rotational speed (20 RPM) of a cylindrical spindle while submerged in a bitumen binder at a constant temperature. This torque is then converted to a viscosity and displayed automatically by the RV.
The standard Rotational Viscometer procedure is found in:
AASHTO T 316 and ASTM D 4402: Viscosity Determination of Asphalt Binder Using Rotational Viscometer.
The dynamic shear rheometer (DSR) is used to characterize the viscous and elastic behavior of bitumen binders at medium to high temperatures. This characterization is used in the Superpave PG bitumen binder specification. As with other Superpave binder tests, the actual temperatures anticipated in the area where the bitumen binder will be placed determine the test temperatures used.
The basic DSR test uses a thin bitumen binder sample sandwiched between two circular plates. The lower plate is fixed while the upper plate oscillates back and forth across the sample at 10 rad/sec (1.59 Hz) to create a shearing action. DSR tests are conducted on unaged, RTFO aged and PAV aged bitumen binder samples. The test is largely software controlled.
The standard dynamic shear rheometer test is:
AASHTO T 315: Determining the Rheological Properties of Asphalt Binder Using a Dynamic Shear Rheometer (DSR)
The Bending Beam Rheometer (BBR) test provides a measure of low temperature stiffness and relaxation properties of bitumen binders. These parameters give an indication of a bitumen binder’s ability to resist low temperature cracking. The BBR is used in combination with the DTT to determine a bitumen binder’s low temperature PG grade. As with other Superpave binder tests, the actual temperatures anticipated in the area where the bitumen binder will be placed determine the test temperatures used.
The basic BBR test uses a small bitumen beam that is simply supported and immersed in a cold liquid bath. A load is applied to the center of the beam and its deflection is measured against time. Stiffness is calculated based on measured deflection and standard beam properties and a measure of how the bitumen binder relaxes the load induced stresses is also measured. BBR tests are conducted on PAV aged asphalt binder samples. The test is largely software controlled.
The standard BBR test is:
AASHTO T 313: Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR)
The Superpave PG binder specification involving the BBR is:
AASHTO PP 42: Determination of Low-Temperature Performance Grade (PG) of Asphalt Binders