A re-examination and simplification of the original rut index concept for predicting rut susceptibility in aggregate bases is presented to eliminate some of the disadvantages of the original approach.The rut index can be determined from the results of a single cyclicload triaxial test performed at a confining stress of 6 psi, ratherthan at two confining stresses as originally proposed. The principal stress ratio to be used in the test varies from 2 to 6, depending on the structural strength of the pavement section. The resilient and permanent deformation characteristics of river gravel, granitic gneiss, shale, limestone, and quartzite aggregates were determined using the cyclic load triaxial test. Variables investigated included density, gradation, moisture content, and aggregate shape and surface characteristics. The revised rut index concept was used to evaluate and compare the relative permanent deformation behavior of these various unbound aggregates. The cubic-shaped, smooth rounded river gravel was found to be more than two times as susceptible to rutting as the crushed aggregates tested. The crushed aggregates were angular, blade, and disc shaped and had relatively rough surfaces. These aggregates generally performed similarly with respect to permanent deformation, although the visual appearance of the two blade-shaped aggregates was not as nice as the others. The use of a simple, slow triaxial shear test as a practical alternative to the conventional dynamic test was studied for evaluating the resilient and permanent characteristics of unbound base materials. The slow triaxial shear test was found to be suitable for evaluating the resilient modulus, but appeared not to be appropriate for evaluating permanent deformation characteristics. This paper appears in transportation research record no. 1227, Rigid and flexible pavement design and analysis: unbound granular materials, tire pressures, backcalculation, and design methods.
Samenvatting