SHRP Deformation Test—Asphalt Mix Tests and Analysis Systemsby Carl L. Monismith, Univ of California at Berkeley, Berkeley, United States,
John A. Deacon, Univ of California at Berkeley, Berkeley, United States,
Abstract: From analyses of existing information on the permanent-deformation characteristics of asphalt/binder-aggregate mixes, as well as the development of permanent deformation in pavement structures, it was concluded that any test to measure permanent-deformation characteristics for both mix design and deformation prediction purposes should take into account the shear stresses developed within the zone in a well-designed pavement where significant permanent deformation could take place. This zone, in the upper three to four inches of the asphalt/binder-bound course(s), extends at least to the edges of the tires. Accordingly, equipment to define the permanent-deformation characteristics of asphalt/binder-aggregate mixes was developed permitting the application of representative stress states including specific level of shear stresses which occur in the upper part of the pavement structure. This new equipment permits simultaneous application of shear and axial loads to cylindrical specimens 6 or 8 inches in diameter and 2.0 and 3.5 inches high depending on the diameter. Confining pressures of up to 100 psi can be applied over a temperature range of -10° to +70 °C. Both shear and axial loads can be applied sinusoidally, repetitively, or sustained (creep loading). For sinusoidal loading, frequencies in the range 20 to 0.01 Hz (approximately 3 decades) are feasible. Repeated loads using a haversine pattern can be applied with a range in times of loading and rest periods. Shear stresses are transmitted to the specimen through end caps which have been bonded to it using an epoxy resin. Using this equipment, a test methodology has been developed which permits the definition of the important mix characteristics necessary to define its propensity for permanent deformation including 1) dilation under shear loading, 2) increase in stiffness with increase in hydrostatic pressure at higher temperatures, 3) temperature and rate of loading dependence, 4) residual permanent-deformation development with unloading, and 5) difference in response in creep and repeated loading. A recommended analysis system using this equipment is described. The system consists of two levels and recognizes that a range in testing requirements is desirable. For routine applications, the testing is not extensive and a framework for mix evaluation is presented. When unconventional mixes are used and more complex design applications are encountered, the amount of testing is increased so that the amount of rutting can be predicted for the site-specific traffic and environmental conditions.
Subject Headings: Shear stress | Axial loads | Repeated loads | Deformation (mechanics) | Load tests | Thermal loads | Asphalt pavements | Stress analysis
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