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19 and flat and/or elongated particles. Increasing values of flat imum dimension, the particle is considered flat and elon- and/or elongated particles at 21 or 31 ratios tend to increase gated. The percentage of flat or elongated particles may be the particle index value and uncompacted voids. This indi- reported by weight or by particle count. The overall percent cates that the tests are highly influenced by particle shape. of F&E is based on a weighted average determined from the The uncompacted voids test is also sensitive to the texture of sample gradation and the flat and elongated percentages for coarse aggregate particles. Both of these tests combine the each size fraction. effects of shape, angularity, and texture. Digital imaging meth- The Superpave method allows no more than 10% F&E ods are being developed that can separately quantify these exceeding the 51 ratio for the combined aggregate blend used parameters. These will be discussed later in the report. in asphalt mixtures for pavements with >1 million ESALs in the design life (15, 27). The Superpave specifications are based on the blend of coarse aggregate used in the HMA, not 2.3 FLAT AND ELONGATED PARTICLES on an individual stockpile. The Stone Matrix Asphalt Tech- nical Working Group guide specification allows 5% 51 and 2.3.1 Background 20% 31 F&E (15, 28). The asphalt industry believes excessive flat and elongated particles (F&E) to be undesirable. Perfectly cubical aggre- gates may also be undesirable. Prior to the implementation of 2.3.2 Relationship Between F&E the Superpave method, several design procedures had speci- and Performance fications limiting the percent of F&E allowed in the mix. AASHTO M283 allowed no more than 15% combined F&E A limited number of studies were completed to relate the as tested in accordance with ASTM D4791. Roberts et al. effect of F&E on performance prior to SHRP (2933). Huber (24) state: "Flat and elongated particles impede compaction et al. (34) evaluated the effect of F&E on the volumetric and thus may prevent the development of satisfactory strength properties of two Superpave 19.0 NMAS mixes. The coarse in HMA." The Aggregate Handbook (25) states: "Specifica- aggregate was a crushed No. 57 stone produced from a lime- tions requiring particle-by-particle measurements vary widely stone source. Coarse aggregate was produced using both a in terms of limiting values and allowable percentages of defec- vertical shaft impact crusher and a cone crusher. Vertical shaft tive particles. Research has largely been unsuccessful in estab- impact crushers tend to produce more cubical aggregate. Nei- lishing criteria related to performance." It is generally believed ther crusher produced F&E that exceeded the 51 ratio. The that high percentages of F&E were undesirable because they vertical shaft impact crusher produced 9.0% and the cone hinder compaction and, if broken under the roller, expose crusher produced 19.4% particles exceeding the 31 ratio. uncoated aggregate surfaces. HMA was produced with aggregate from both crushers to The fourth questionnaire used in the Delphi process by meet each of the two gradations for a total of four mixes. Two SHRP to identify the consensus aggregate properties ranked laboratories tested each mix at constant asphalt content in the thin, elongated pieces eighth in terms of importance (1). Superpave gyratory compactor. Based on the sample density ASTM D4791, "Standard Test Method for Flat Particles, results, the authors concluded that F&E exceeding the 31 Elongated Particles, or Flat and Elongated Particles in Coarse ratio do not negatively impact volumetric properties (34); Aggregate," was recommended as the test method for thin, however, this was a very limited study from which to make elongated particles by the expert panel. USACE (26) origi- general conclusions about the effect of F&E on mixture vol- nally developed the method. umetric properties. When the Superpave method was first implemented, ASTM Brown et al. (35) evaluated the effect of five levels of F&E specified that the test be performed on the +9.5-mm material on the volumetric properties, aggregate breakdown, and mois- by size fraction (14). The Superpave method specified that ture susceptibility of stone matrix asphalt (SMA). An Arkansas the test be run on the +4.75-mm material (1). ASTM later limestone source was crushed to provide two different levels revised the standard to include the +4.75-mm material (16). of F&E. The F&E varied from 67 to 38 for the 21 ratio, 25 to The test is run by first performing a gradation on a represen- 3 for the 31 ratio, and 1 to 0 for the 51 ratio. SMA was tative sample of the coarse aggregate. One hundred particles produced with both coarse aggregates and 75/25, 50/50, and are split out for testing for each size fraction that has at least 25/75% blends of the two aggregates. There was a slight 10% retained. The Superpave method specifies that the par- trend of increasing VMA with increasing percentages of F&E. ticle is considered flat and elongated if the particle's maxi- The VMA increased 1.2% from the cubical to the more F&E mum dimension is five or more times the particle's minimum coarse aggregate. Gradation testing indicated a statistically dimension. The maximum and minimum dimension of each significant increase in aggregate breakdown on the 4.75-mm particle is measured or, alternatively, a proportional caliper sieve for higher levels of F&E. Breakdown increased by may be used. If a proportional caliper is used, the largest approximately 4% between the two extremes in particle shape dimension of the particle is used to set the caliper. If the par- for samples compacted with 50 blows of each face with a ticle can pass through an opening that is one-fifth of the max- Marshall hammer (35).

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20 Vavrik et al. (36) evaluated the effect of F&E on the vol- by the previous studies. Vavrik et al. (36) evaluated aggre- umetric properties and aggregate breakdown for gyratory- gate breakdown during gyratory compaction and noted that compacted samples. Aggregate was obtained from a dolo- for the dolomite mixture, the percent passing the No. 4 sieve mite and a gravel source. The coarse aggregate particles for (4.7-mm) increased by 3% to 5% and that the percent passing each source were sorted into particles whose maximum-to- the No. 8 (2.36-mm) sieve increased by 1% to 4% for the non- minimum dimension was less than the 31 ratio, greater than cubical blends as compared with the cubical blends. Similarly, the 31 ratio but less than the 51 ratio, and greater than the the percent passing the No. 4 sieve (4.75-mm) increased by 2% 51 ratio. A modified Superpave mixture design was devel- to 4% for the gravel mixtures. No corresponding increase was oped using the cubical (less than 31 ratio) coarse aggregate observed for the percent passing the No. 8 sieve for the gravel from each source. The same manufactured sand, natural sand, mixtures. The authors concluded that increased F&E resulted and mineral filler were used for both designs. The dolomite in increased aggregate breakdown and that the changes in mixture used 55% coarse aggregate with the fine aggregate the volumetric properties (including VMA) resulted from the being an 80% to 20% blend of manufactured and natural changes in gradation caused by aggregate breakdown (36). sand. The gravel mixture used 52% coarse aggregate with a Buchanan (37) evaluated the effect of six levels of F&E 70% to 30% split of manufactured to natural sand for the fine from two aggregate sources on the volumetric properties, rut- fraction. The design asphalt contents were chosen using the ting performance, and fatigue performance of a 12.5-mm locking point concept. The locking point is defined as the first NMAS Superpave mix design. The six levels of F&E con- occurrence of three gyrations at the same height preceded by sisted of the as-received aggregate from a limestone and a two gyrations at the same height. The locking point is the granite source as well as each of those aggregates crushed at number of gyrations at which the first of the three consecu- two different rotor speeds in a scale vertical shaft impact tive gyrations of the same height occur. The average locking crusher. The blend percents of F&E, volumetric properties, point for the cubical dolomite mixture was 101 gyrations, and rut depths are shown in Table 4. and the average locking point for the cubical gravel mixture Some of the F&E results for the limestone aggregate was 90 gyrations. appear anomalous. Higher rotor tip speeds should produce Four samples were then compacted at the optimum asphalt more cubical particles because the aggregate is thrown with content determined for the cubical coarse aggregate of each more energy against cascading aggregate in the crusher. This of the aggregate sources at each of four blends of F&E. The is consistent with the data shown in Table 4 except for the samples were compacted to the 110 gyrations. The volumet- limestone sample at 65 m/s. ric results are summarized in Table 3. The data indicate a Based on Table 4, the volumetric properties for the lime- trend of increasing VMA with increasing F&E as indicated stone aggregate match the findings of Huber et al. (34) with TABLE 3 Volumetric data for various levels of F&E for Illinois study (36) Material Air Voids, % VMA, % Locking Point Dolomite Coarse Aggregate Cubical 3.75 14.7 101 50-50-01 4.24 15.1 97 30-50-20 4.48 15.4 102 70-0-30 4.16 15.1 101 Gravel Coarse Aggregate Cubical 3.55 14.6 93 50-50-01 4.37 15.3 113 30-50-20 4.61 15.6 120 70-0-30 4.62 15.6 119 1 50-50-0 are the percentages of cubical particles with shape ratios (maximum-to-minimum dimensions) >3:1 but less than 5:1 and particles with shape ratios >5:1. TABLE 4 F&E levels and resulting mixture properties (37) Aggregate Type F & E Ratios Optimum VMA, % APA Rut 2:1 3:1 5:1 AC% Depth (Dry), mm Limestone As-Received 69.2 29.5 3.8 4.2 13.7 5.9 Limestone @ 55 m/s1 58.6 21.8 0.2 4.5 13.9 6.6 Limestone @ 65 m/s 72.0 16.2 3.7 4.2 13.7 6.2 Granite As-Received 85.4 57.0 23.0 5.0 14.2 9.2 Granite @ 45 m/s 42.9 14.4 0.4 4.6 13.4 6.2 Granite @ 68 m/s 35.1 2.1 0.1 4.5 13.4 6.1 1 Indicates the rotor tip speed on the vertical shaft impact crusher.

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21 little change over a moderate range of 31 particles. The gran- tensile properties. Repeated shear at constant height testing ite aggregate indicates decreasing VMA with decreasing F&E indicated that the rutting susceptibility of the mixtures with as noted by Brown et al. (35) and Vavrik et al. (36). Rut test- three levels of F&E appeared to be similar at 58C (38). ing was performed in the Asphalt Pavement Analyzer (APA) Aho et al. (39) evaluated the relationship between the per- at 64C with a 100-lb vertical load and 100 psi hose pressure. centages of F&E and aggregate breakdown during construc- There was not a statistically significant difference between the tion. Samples of six surface mixtures, representing a range of APA rut depths for the limestone mixtures. The rut depths for F&E contents, were sampled at the plant; the mixtures were the granite mixtures produced in the vertical shaft impact sampled behind the paver prior to compaction and in-place crusher were significantly less than the rut depth for the as- after compaction. The samples taken after compaction were received mixture; however, the granite mixes produced using taken 2 ft (0.6 m) from the corresponding sample taken the aggregate from the vertical impact crusher have failing behind the paver (assumed to be longitudinally). Aggregate VMA values and therefore lower asphalt contents. Constant samples were collected from both the quarry and the asphalt strain fatigue tests were performed according to AASHTO plant stockpiles for F&E and LA abrasion testing. The per- T321 at two strain levels. There was not a statistically signifi- centages of particles exceeding the 31 ratio ranged from cant difference in the fatigue results for either aggregate at the 8.1% to 54.1% and the percentages of particles exceeding the three levels of F&E evaluated (37); therefore, there does not 51 ratio ranged from 0.4% to 21.1% based on the average appear to be an effect of F&E on a mixture fatigue resistance. of eight tests. In addition, for three of the five aggregates, Oduroh et al. (38) evaluated the effect of three levels of gyratory samples were prepared to simulate aggregate break- F&E on compacted sample density, shear stiffness, and ten- down during mix design. Three samples each were com- sile properties. The goal of the study was to provide data to pacted to 4% and 7% air voids. states that are considering the 31 ratio in lieu of the 51 ratio Statistical analyses were performed to compare the per- as the definition of F&E. The three levels of F&E investi- cent passing the No. 4 (4.75-mm) sieve from extracted sam- gated were 0%, 15%, and 40% particles exceeding the 31 ples. Comparisons were made between the plant sample and ratio for maximum-to-minimum particle dimension. A single the sample taken behind the paver prior to compaction and Kentucky limestone coarse aggregate and Ohio River natural between the sample taken behind the paver prior to com- sand were used to produce a 12.5-mm NMAS mixture. The paction and the sample taken after compaction. Statistical samples were mixed with an unmodified performance grade differences were observed before and after compaction for (PG) 64-22 at an optimum asphalt content determined using two mixtures produced with dolomite aggregates and between Ndesign = 96. the plant and paver sample for one of the mixtures. The LA Performance tests were performed with the Superpave abrasion values of the two dolomite sources were 25% and Shear Tester (SST) and Indirect Tensile Tester (IDT) on sam- 26%. Breakdown was not observed when comparing samples ples prepared at optimum asphalt content with 0%, 15%, and taken before and after compaction for the remaining three mix- 40% F&E. Testing included frequency sweep at constant tures even though one mixture had 54.1% and 21.1% F&E, height, simple shear at constant height, repeated shear at con- based on the 31 and 51 ratios, respectively (39). A relation- stant height, and IDT tests. Based on the results of the test- ship was observed between lift thickness and breakdown. ing, adding 15% or 40% particles exceeding the 31 ratio did The recovered samples from each sampling location were not affect the compacted mixture density, shear stiffness, or also tested for F&E. As shown in Figure 2, testing indicated Figure 2. Interaction among F&E, LA abrasion, and aggregate breakdown (39).