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Pavements

MATC

Mixture Tests

Asphalt Mixture Performance Tester (AMPT)

The AMPT is used to test key properties of asphalt mixtures as part of each MATC project. The AMPT simulates field conditions such as traffic loading, rate of loading, temperature, and confinement that the pavement experiences during its service life. The AMPT equipment is capable of conducting several performance tests, including dynamic modulus (|E*|), flow number (Fn), cyclic fatigue, and stress sweep rutting. The computer-controlled servo-hydraulic testing machine is used to test cylindrical asphalt mixture specimens without the use of notches. The AMPT machine has a small environmental chamber that allows testing at controlled test temperatures.

Photo of exterior of an Asphalt Mixture Performance TesterPhoto of mixture specimen in an AMPT chamber ready to be tested

DYNAMIC MODULUS FOR MEASURING ASPHALT STIFFNESS

The |E*| test on the AMPT is performed according to AASHTO T 378. In this test, a sinusoidal, axial compressive stress is applied to maintain a targeted on-specimen axial strain amplitude. This test is performed at different temperatures and frequencies to better understand how the mixture will behave under a range of climatic and traffic loads. The applied stresses and resulting axial strains are used to calculate the |E*| and phase angle. |E*| is the primary asphalt materials input for a flexible pavement design when using the AASHTOWare Pavement ME Design® approach.

Graph showing typical stress and strain response during a dynamic modulus test. The y axis is stress or strain. The x axis is time. The graph shows 5 cycles during a dynamic modulus test. The stress and strain both vary sinusoidally. The peak of the strain lags the peak of the stress. The strain also shows a small upward drift with each cycle.  Parameters resulted from this test.

CYCLIC FATIGUE

Another test that can be run in the AMPT is the cyclic fatigue test. The test is a controlled actuator displacement cyclic tension ("pull-pull") test, conducted in accordance with AASHTO TP 107 for large specimens. Failure in this test occurs when localized damage accumulation give way to a visible macrocrack (visible crack). A crack in the middle 70 mm of the specimen is an indication of a successful fatigue test.

Photo of failed sample due to fatigue test showing mid-specimen failure
Photo credit: David Heisler
Phase angle versus number of cycles in AMPT fatigue test

Small-Specimen Testing

Small specimens are used in the AMPT to conduct dynamic modulus (following AASHTO TP132) and cyclic fatigue (following AASHTO TP 133) tests. The 38-mm diameter by 110-mm height specimens may present savings in terms of time and materials for practitioners looking to use the AMPT as part of a PEMD. Materials are saved since four test specimens are now generated from one large gyratory specimen and time is saved during the fabrication, drying, and gluing processes. These specimens can also be taken directly from field cores.

Photo shows close up view of technician cutting smaller specimens for AMPT device out of a field core specimen
Photo credit: David Heisler
Close up view of small specimen (post coring) to be used for cyclic fatigue and E-star testing in the AMPT device
Photo credit: David Heisler

ILLINOIS FLEXIBILITY INDEX TESTING (I-FIT)

The I-FIT is a semi-circular bending fracture test to determine the cracking potential of asphalt mixtures. The goal of the I-FIT is to generate the Flexibility Index (FI) parameter to properly ascertain the cracking potential of asphalt mixtures.

The I-FIT uses a semi-circular bending geometry in a loading frame capable of measuring load and displacement over time at room temperature. The test is run at a displacement rate of 50 mm/min on a specimen with a notched sawed into it to force the failure location. The I-FIT determines the cracking resistance of asphalt mixes through a fracture mechanics based parameter calculated from the Work of Fracture and the post-peak slope. The larger the FI the better the cracking resistance.

Equipment and test are versatile to be used with any asphalt mixture that can be fabricated to the requirements, as included in AASHTO TP124. The test equipment is simple in operation however the notching during fabrication requires a stable set-up.

Photo of the Illinois fatigue cracking test (or i-FIT) indirect tension tester with an asphalt specimen with a notch cut into the specimen loaded in the chamber

INDIRECT TENSILE ASPHALT CRACKING TEST (IDEAL-CT)

The IDEAL-CT is an indirect tension test to determine the cracking potential of asphalt mixtures. The goal of the IDEAL-CT is to generate the CTindex parameter to properly ascertain the cracking potential of asphalt mixtures.

The IDEAL-CT uses a simple loading frame capable of measuring load and displacement over time. The test is then run at room temperature with a monotonic loading rate of 50 mm/min in terms of cross-head displacement. The IDEAL-CT determines cracking resistance of asphalt mixes through a fracture mechanics–based parameter: Cracking Tolerance Index (CTindex). The larger the CTindex, the better the cracking resistance.

Equipment and test are versatile to be used with any asphalt mixture that can be fabricated to the requirements, as included in ASTM D8225. The equipment is simple in operation and use for testing and can be easily performed in field laboratories.

Photo of the ITC (or IDEAL CT) indirect tension tester with an asphalt specimen loaded in the chamber

OVERLAY TESTER

The Overlay Test is direct tension test to determine the reflective cracking potential of asphalt mixtures. The goal of the overlay test is to generate the critical fracture energy and cycles to failure parameters to properly ascertain the cracking potential of asphalt mixtures.

The OT applies repeated direct tension loads to specimens fixing one side of the specimen on one block and allowing the other side to slide to apply tension, as described in Tex-248-F. The sliding block applies tension in a cyclic triangular waveform to a constant maximum displacement of 0.025 in. The test is run on specimens cut from a gyratory pill in a machine equipped with an environmental chamber. The OT determines the reflective cracking resistance of asphalt mixes through the critical fracture energy parameter and the number of cycles to failure (where a 93% reduction of the maximum load occurs). The larger the critical fracture energy and cycles to failure the better the cracking resistance.

Equipment and test are versatile to be used with any asphalt mixture that can be fabricated to the requirements. The test equipment is relatively simple to operate but the specimen preparation requires a more stable set-up.

Photo of the overlay test (or Texas Overlay Test) direct tension test with an asphalt specimen loaded in the chamber

PREDICTIVE TESTS FOR RUTTING

Flow Number Test

The permanent deformation properties of asphalt mixtures can be evaluated using the flow number (Fn) test which is also described in AASHTO T 378. This test consists of measuring the permanent axial strains on an asphalt mixture specimen subjected to a repeated haversine, axial compressive load pulse of 0.1 second duration every 1.0 second. The specimen may be tested in an unconfined or a confined state, which requires a membrane and an applied static confining pressure. During the test, the number of load cycles and cumulative axial strains are continuously recorded. The number of load cycles corresponding to the minimum rate of change of permanent axial strain is referred as Fn. This index provides information on the potential rutting performance of the mixture.

two samples after flow number test; Typical flow curve generated by an unconfined flow number test.

Stress Sweep Rutting

Another rutting test available in the AMPT is the stress sweep rutting (SSR) test which is also described in AASHTO TP 134. The SSR test follows similar preparation procedures to the flow number test, but is subjected to different deviatoric stress pulses. The test is run at a reference temperature of 20°C and a higher temperature based on the pavement location. The outputs of the test are then used to generate a rutting mastercurve which describes the distress potential at various temperature and traffic combinations.

Photo of the stress sweep rutting (or SSR) test specimen, enclosed in a neoprene membrane, with an asphalt specimen loaded in the chamber of an Asphalt Mixture Performance Tester
Photo credit: David Heisler

Aggregate Imaging Measurement System (AIMS)

The Aggregate Imaging Measurement System (AIMS) is a non-contact video-based technology used to measure the geometry, shape, angularity, and texture of fine and coarse aggregates. This device is equipped with a high resolution digital camera, a variable magnification microscope, two different light configurations, and software for image data analysis. The aggregate properties are thus determined from the analysis of the digital images captured by the camera rather than the traditional manual methods. A detailed description of the test method and classification of the aggregate shape properties captured through AIMS can be found in AASHTO TP 81 and in the AIMS operational manual.

Photo of Pine Instrument’s Aggregate Imaging Measurement System (AIMS) Model AFA2A equipment.

 AIMS Output - Example.

Photo source: FHWA unless otherwise noted
Updated: 03/30/2020
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000