Effects of fine aggregate angularity on asphalt mixture performance

Superpave aggregate qualification includes the fine aggregate angularity (FAA). The numerical value of the FAA is the voids in the mineral aggregate of the loosely packed fine aggregate. Use of FAA has been predicted on the philosophy that higher and lower values of FAA represent fine aggregate with high and low frictional characteristics, respectively. The degree of friction depends on the aggregate particle shape and texture. Higher friction is associated with increased rutting resistance. Fine aggregate angularity levels used in the Superpave system are below 40, 40 to 45 and above 45. The higher values are specified for layers near the surface and for higher traffic levels. Past and current experience shows that there are fine aggregates in mixtures performing well that are below the specified levels. There are also aggregates above these levels in mixtures that are not performing as desired. A study has been conducted utilizing the PURWheel laboratory wheel track tester and Superpave shear test (SST) to develop performance-based data on mixtures with various fine aggregates. A single asphalt (PG 64-22) and 9.5-mm coarse aggregate were used in the study. The coarse aggregate was selected to emphasis the fine aggregate performance. In Phase I of this study, six fine aggregates were used with FAA ranging from 39 to 49. In addition two of these aggregate were blended in various proportions to produce blends with FAA values of 43,45 and 46. In Phase II of this study, two of the six fine aggregates in Phase I were selected. One was slag sand and the other was limestone sand with a special gradation. Nine mixtures were designed and tested to evaluate the effects of adding natural sand, adding mineral fillers, and changing the gradation of the original aggregate blend. Wheel track tests results indicated that the tests were successful in delineating potential performance of the mixtures tested. Test results showed that FAA alone may not be adequate to evaluate the contribution of fine aggregate to the mixture performance. Other factors including gradation, absorption, affinity for asphalt, etc. would also affect mixture performance