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24 TABLE 6 Precision of ASTM D4791 F&E tests from AMRL Proficiency Samples 117 and 118 (43) Particle Number Sample 1 Sample 2 Single Operator Precision Size, of Multilaboratory Multilaboratory Sample 1 Sample 2 mm Labs Precision Precision Avg. 1S% D2S% Avg. 1S% D2S% 1S% D2S% 1S% D2S% 19.0 to 128 14.4 51.0 144.3 13.1 53.4 151.0 26.6 75.3 29.1 82.2 12.5 12.5 to 123 17.6 43.9 124.1 17.3 42.1 119.2 22.7 64.2 23.1 65.2 9.5 9.5 to 122 24.9 45.7 129.3 23.3 46.4 131.2 18.3 51.8 19.6 55.4 4.75 near the existing specification level of 10% particles exceed- resistance in an HMA mixture (47). The amount of friction ing the 51 ratio of maximum-to-minimum particle dimen- depends on the aggregate particle shape and texture. Higher sion. The research conducted to date generally supports the internal friction is associated with increased rutting resis- following: tance. AASHTO T304 is commonly referred to as the FAA test. FAA levels used in the Superpave method are below 40, Percentage of F&E changes with handling of the stock- 40 to 45, and above 45. The higher values are specified for pile and mixing. layers near the pavement surface and for higher traffic lev- Aggregate breakdown during compaction increases for els. AASHTO T304 was to be used in conjunction with the higher percentages of F&E. restricted zone to limit the amount of rounded natural sand in VMA generally increases with increasing percent F&E. high traffic mixes. There does not be appear to be a relationship between The angularity and texture of the fine aggregate also affect the percentage of F&E exceeding the 31 ratio--in the the packing characteristics of the HMA and, therefore, the range of approximately 10% to 40%--and performance. VMA of the compacted HMA. More angular or poorly shaped ASTM D4791 is a highly variable test procedure. Alter- particles or particles having a high degree of texture may not native methods of determining the percentage of F&E pack as tightly as rounded or smooth particles and, therefore, should be developed. This variability may mask relation- may provide greater VMA in the compacted HMA. ships with performance. The FAA test is an indirect measure of particle shape, angu- larity, and texture. The FAA test is based on the National Aggregate Association Flow Test (Method A) that is used to 2.4 METHODS OF MEASURING FAA AND evaluate the effect of the fine aggregate on the finish ability THEIR RELATIONSHIP TO PERFORMANCE of Portland cement concrete. The FAA value is defined as the percent air voids in a loosely compacted sample of fine aggre- 2.4.1 Introduction gate. The FAA test assumes that more angular particles or particles with more surface texture will not pack together as It has long been recognized that the characteristics of the fine aggregate component of HMA can have a significant and tightly as rounded or smooth particles would. sometimes dominant influence on mixture rutting and fatigue In AASHTO T304, a 190-g sample of fine aggregate of a cracking resistance (2, 34, 44, and 45). Kandhal et al. (46) prescribed gradation is allowed to flow through the orifice of have classified the test methods to describe aggregate angu- a funnel and fill a 100-cm3 calibrated cylinder. Excess mate- larity into two broad categories: direct and indirect. Direct rial is struck off, and the cylinder with aggregate is weighed. methods are defined as those wherein particle shape or tex- The uncompacted void content of the sample is then com- ture are measured and described qualitatively or quantita- puted using the loosely compacted weight of the aggregate, tively through direct measurement of individual particles. In the bulk dry specific gravity of the aggregate, and the cali- indirect methods, particle shape and texture are determined brated volume of the receiving cylinder. based on measurements of bulk properties. There are three methods for running AASHTO T304: Methods A, B, and C. The mass of the sample for all three methods is fixed at 190 g. Method A specifies a known gra- 2.4.2 Uncompacted Voids Content dation ranging from material passing the 2.36-mm sieve to in Fine Aggregate material retained on the 0.150-mm sieve. Method B specifies that the test be run on three individual size fractions: 2.36 to The Superpave method specifies AASHTO T304 (ASTM 1.18 mm, 1.18 to 0.600 mm, and 0.600 to 0.300 mm. The C1252), "Uncompacted Void Content in Fine Aggregate, reported void content for Method B is the average of the Method A," to ensure that the blend of fine aggregates in an results from the three individual size fractions. In Method C, HMA mixture has sufficient internal friction to provide rut- the test is run on the as-received gradation (48).