Fine Aggregate Angularity from Geotechnical Perspective

Particle angularity is an important aspect in the performance of asphalt concrete pavements. When rounded materials are used, the shear strength is reduced resulting in rutting and shoving of the asphalt concrete mixtures. Currently, the Superpave design method stipulates that the fine aggregate angularity (FAA) via AASHTO T304 be used as a test specification for fine aggregate. The objectives of this paper are to examine a number of methods for characterizing fine aggregate characteristics of aggregates commonly used in asphalt concrete pavements. Five different sands comprising of five different gradations utilizing a partial factorial experiment. The 19 different gradation and aggregate source combinations are tested using the Fine Aggregate Angularity Test Method C as outlined by AASHTO T304 (2003) and ASTM C1252 (2003), the direct shear test, compacted aggregate resistance (CAR) test, modified light Clegg Hammer, and an aggregate imaging system device. Statistical analysis of the test results are conducted to evaluate if the various tests can discriminate between the five different sources of materials and five different gradations. This paper will discuss particle angularity from a geotechnical point of view and it will hopefully answer the question: Do these tests really measure fine aggregate angularity and which test or tests should be used to measure the angularity of sands for use in asphalt concrete mixtures? The experimental testing results show that the CAR test and AIMS properties (i.e. gradient angularity, radius angularity, and 2D Form) are able to distinguish between different gradations and aggregate type (manufactured versus natural sands). The direct shear test and modified light Clegg Hammer test were able to distinguish between natural versus manufactured sands, however they were not able to distinguish among the variations in gradation. Therefore the CAR test and/or AIMS device are suitable candidate tests for determining the angularity of fine aggregates. These methods of characterizing fine aggregate, however, should be compared to the performance of asphalt concrete mixtures through laboratory and field performance.