Asphalt Research Correspondent

New from the ARC: 

Fatigue Testing Using the Asphalt Mixture Performance Tester 

The Asphalt Mixture Performance Tester (AMPT), a small servo-hydraulic testing device, was developed by the National Cooperative Highway Research Program (NCHRP) to measure dynamic modulus and permanent deformation properties of asphalt mixtures in support of AASHTO Mechanistic-Empirical Pavement Design Guide and future asphalt mixture design procedures.  Now, Asphalt Research Consortium engineers and technicians at Advanced Asphalt Technologies, LLC, of Sterling, Virginia, are expanding the capabilities of the equipment to measure the fatigue resistance of asphalt using continuum damage analysis.

Shortcomings…

Procedures developed in NCHRP Projects 9-19 and 9-29 for using the AMPT to measure dynamic modulus and permanent deformation properties of mixtures were standardized as AASHTO Provisional Standards.  A major shortcoming, however, is the lack of a test to evaluate fatigue characteristics of asphalt concrete.  The only standard fatigue test available is the flexural fatigue test, AASHTO T321, and it has several shortcomings. The most significant is that the stress state is not uniform but varies over the depth of the specimen. What’s more, equipment for fabricating beam specimens is not widely available. 

Interlaken Technology Corporation developed a fixture (left) for gluing tension specimens for the fatigue testing.  The specimen is the standard AMPT test specimen with tension grips attached (right). 

References:

AASHTO PP 60-09, Provisional Standard Practice for Preparation of Cylindrical Performance Test Specimens Using the Superpave Gyratory Compactor (SGC).

AASHTO PP 61-09, Provisional Standard Practice for Developing Dynamic Modulus Master Curves for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT).

AASHTO TP 79-09, Provisional Standard Method of Test for Determining the Dynamic Modulus and Flow Number of Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester.

…and an Alternative

Engineers and technicians at Advanced Asphalt Technologies, LLC recognized that with minor modification, the equipment used to fabricate AMPT test specimens and conduct AMPT testing could be adapted to continuum damage fatigue testing.  Thus, a single piece of equipment costing approximately $80,000 could be used to perform mixture stiffness, permanent deformation, and fatigue testing.

Continuum damage analysis offers a promising alternative to flexural fatigue testing for characterizing the fatigue properties of asphalt mixtures.  Continuum damage analysis was first applied to asphalt concrete during research funded in part by the FHWA project, Fundamental Properties of Asphalts and Modified Asphalts. 

In continuum damage fatigue testing, a cylindrical specimen similar to that used in the AMPT is tested in cyclic tension-compression loading while the change in stiffness with load cycles is monitored.  Using continuum damage analysis, a comprehensive fatigue relationship for an asphalt concrete mixture can be developed from a limited number of tests. 

Four Specimens, Eight Hours

In addition to developing the prototype equipment to conduct fatigue testing in the AMPT, Advanced Asphalt Technologies has developed a simplified procedure for continuum damage testing and analysis.  This procedure requires testing four specimens at high and low strain levels at two temperatures.  The total testing time is approximately 8 hours, compared to several days required for a complete characterization using flexural fatigue testing. 

Recently, the Federal Highway Administration (FHWA) initiated a pooled fund study to help highway agencies implement AMPT activities.  Under this study, the FHWA will procure the instrument for participating agencies, a training course will be developed to train engineers and technicians, and further testing will be performed to improve performance relationships and criteria for a range of materials, climatic conditions, and pavements.  Twenty highway agencies, the FHWA, and the Ontario, Canada Ministry of Transportation are participating in Pooled Fund Study TPF-5 (178).

For more information contact, Ramon Bonaquist, Chief Operating Officer, Advanced Asphalt Technologies, LLC. ARC

Fatigue test in progress in the AMPT.