Evaluation of superpave fine aggregate specification

Abstract

Superpave[] was the final product of the Strategic Highway Research Program's (SHRP) asphalt pavement research effort during 1987-93. Some aspects of the Superpave aggregate specifications are not universally accepted. The validity of the fine aggregate angularity (FAA) requirement is questioned by both the owner agencies and the paving and aggregate industries. The FAA test is based on the assumption that more fractured faces will result in higher void content in the loosely compacted sample; however, this is not always true. The aggregate industry has found that cubical shaped particles, even with 100% fractured faces, may not meet the FAA requirement for high-volume traffic. State agencies are concerned that local materials previously considered acceptable and which have provided good field performance, now cannot meet the Superpave requirements. Twenty three-fine aggregates from different part of the USA were tested using major angularity tests: FAA test, direct shear test, compacted aggregate resistance (CAR) test, image analysis Hough transform, and visual inspection. The results from those tests were compared with the available performance history. The FAA test method does not consistently identify angular, cubical aggregates as high quality materials. There is a fair correlation between the CAR stability value and angle of internal friction (AIF) from direct shear test. No correlation was found between FAA and CAR stability or between FAA and AIF. A good correlation was found between FAA and K-index from Hough transform method. Some cubical crushed aggregates whose FAA values were less than 45 gave very high values of CAR stability, AIF and K-index. The statistical analysis of the SHRP LTPP database reveals that there is no evidence of any good linear relationship between FAA and rutting. Of the methods evaluated, image analysis using Hough transform appears most promising for measuring fine aggregate angularity. Until a replacement method for FAA can be identified, the author recommends that the FAA criteria be lowered from 45. The FAA vs. rutting data analysis should be continued with a larger amount of data as the LTPP SHRP database is expanded.