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25 The Superpave researchers chose Method A to limit the ity or shape under the current Superpave aggregate proper- effect of gradation, particularly material passing the 0.075-mm ties (52). Work by Hudson (50) indicates that the current sieve on the test result. For example, if one were to test a AASHTO T304 apparatus may not be appropriate for testing washed manufactured sand with a -0.075 mm sieve content particles in this size range. A third concern is related to the of 8% and a crushed screening produced from the same variability of the test procedure and its dependence on the aggregate and crushed with the same crusher settings with a fine aggregate dry bulk specific gravity (52). 0.075 mm sieve content of 14% under Method C, the crushed Finally, there is concern that the FAA test may not be screening would produce a lower FAA value than would the related to the rutting propensity of the HMA mixture. These washed manufactured sand even though the two materials concerns led to numerous studies evaluating the FAA test as have identical particle shape and texture. well as alternative tests to relate FAA, shape, and texture to Several studies have been conducted to compare Methods the rutting performance of HMA mixtures. Commonly used A, B, and C (4953). The studies have indicated a strong alternative tests will be discussed prior to efforts to relate relationship between Methods B and C, with Method B pro- FAA to HMA performance. ducing uncompacted void contents almost 5 points higher (49, 5153). Hossain et al. (49) observed that the uncom- pacted void contents were generally higher for smaller sized 2.4.3 Alternative Methods of Measuring FAA particles. Hudson (50) stated that, based on visual observa- tion, particle shape appeared to be constant with size. Thus, 2.4.3.1 Direct Tests (Digital Imaging Methods) particle texture may have a greater effect for smaller parti- cles. Roque et al. (51) noted the strong effect of texture in In the past few years, digital image processing technique AASHTO T304 tests. Hudson (50) states: has been introduced into the HMA industry to analyze macro- and microstructures of HMA, aggregates, air voids, gradation, Test method C relates to the materials "as-is," or "in-situ." and so on. Several researchers have attempted to use image Little or no shape information can be determined from this analysis to measure the FAA. Particle shape from image method as the reduction in voids content that would be attrib- uted to improved particle shape cannot be separated due to analysis, automated image analysis, and morphology analysis the influence of the sample gradation. from profile images and from 3-D images are some of the image analysis methods being used actively in recent years. Researchers have also investigated the effects of alternate gradations. Hossain et al. (49) evaluated a gradation typical Particle Shape from Image Analysis: This automated tech- of dense-graded HMA that included material passing the nique was developed at the University of Arkansas for FHWA 4.75-mm sieve and retained on the 2.35-mm sieve. Alternate (55). The fine aggregate is spread on a glass plate, and a high- gradations are strongly correlated with the Method A grada- resolution video camera is used to capture the image of each tion (49, 51). The blended uncompacted voids contents were particle. Modern digital imaging hardware, image analysis on average 2.4% lower when the material retained on the techniques, and computerized analysis were used to quantify 2.36-mm sieve was included (49). Hudson (50) stated that the aggregate shape. EAAP (ellipse-based area of the object current AASHTO T304 equipment was not suitable for test- divided by the perimeter squared) Index and Roundness Index ing the material passing the 4.75-mm sieve and retained on were found to have the most potential for predicting rutting the 2.36-mm sieve because the outlet orifice and the receiv- performance. ing container were both too small. Virginia Test Method 5, which uses an enlarged version of the AASHTO T304 appa- Automated Image Analysis: The automated image analy- ratus, produced identical uncompacted void contents when the sis approach was developed by Massad et al. (56). Two Method A grading was tested in both devices (53). Based on procedures--surface erosion-dilation technique and fractal- the preceding research, altering the AASHTO T304 Method behavior technique--were used to quantify FAA. The surface A gradation or fixtures would appear to shift uniformly the erosion-dilation technique consists of subjecting the aggre- uncompacted void contents for all aggregates. gate surface to a smoothing effect that causes the angularity Several concerns have been expressed regarding the use of elements to disappear from the image. The aggregate angular- the FAA test as a screening tool for rutting resistance of fine ity is measured in terms of a surface parameter, which is aggregate. There is concern that some 100% crushed particles defined as the area lost during the erosion-dilation process as do not meet the minimum requirements (>45) for mixes used a percentage of the total area of the original image. in the upper 100 mm of the pavement structure with traffic lev- The fractal-behavior technique uses image-analysis tech- els in excess of 3 million ESALs during the design life (54). niques to capture the aggregate boundary. Fractal length of Typically, these particles are extremely cubical in nature. A the boundary is the slope of effective-width-to-number-of- second concern is that particles passing the 4.75-mm sieve but cycles relationship. The fractal length increases with aggre- retained on the 2.36-mm sieve are not evaluated for angular- gate angularity.