IMPACT OF FINES ON ASPHALT MIX DESIGN

Pavement engineers have known for some time that the particle shape and surface texture of aggregates play a significant role in the constructability, drivability, strength, and durability of asphalt concrete pavements. Superpave recommends use of a variety of aggregate tests to ensure minimum desirable aggregate characteristics or "consensus properties" in order to assure an acceptable level of performance. For fine aggregates, the recommended method is AASHTO TP33 (ASTM C1252) - Test Method for Uncompacted Void Content of Fine Aggregate (as Influenced by Particle Shape, Surface Texture, and Grading). In this investigation, three methods for classifying aggregate particle shape and texture - AASHTO TP33 (ASTM C1252), ASTM D3398 (Index of Particle Shape and Texture), and the flow rate method - were evaluated. These methods were used to rank four natural river sands and a crushed granite from good to poor performance based on the criteria established in each method. Test results indicate that all methods easily distinguished the crushed aggregate from the natural river sands. The AASTHO TP33 (ASTM C1252) and the flow rate methods were found to be somewhat less sensitive to slight differences in particle shape and texture than ASTM D3398. The flow rate test was also found to be dependent on the gradation of the aggregate while the index values determined by ASTM D3398 and AASHTO TP33 (ASTM C1252) seemed to be less dependent on grading. All the test methods were found to be repeatable, each having low coefficients of variation for all the aggregates tested. One of the objectives of this study was to show that increased amount of mineral fillers can be accommodated in asphalt mixtures without adversely affecting its rutting (permanent deformation performance). Results of this study clearly indicate that within the range of mineral filler content and type used in this study, increasing the amount of mineral filler has a beneficial effect on the rutting performance. However, although the rutting performance is enhanced, it should be noted that at higher mineral filler content, the asphalt content is reduced, which may have a detrimental effect on other mixture properties, such as fatigue, thermal cracking, and raveling.