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16 fractured face counts ranged from 54% to 93.7%. It was not gate." The two tests are highly correlated, producing an R2 = reported whether the fractured face count represents one or 0.94 (2, 19). AASHTO TP56 is preferable to ASTM D3398 two fractured faces. A stepwise regression was performed to because of the significant time required to perform ASTM identify the factors most related to the in-place rut depth. The D3398. One drawback to both tests is that the effects of par- regression identified traffic (accumulated ESALs), between ticle shape, angularity, and texture cannot be separated. wheel path asphalt content, between wheel path voids filled, Kandhal et al. (20) indicated that the particle shape and and between wheel path density (R2 = 76.6) (18). The rut angularity index obtained from ASTM D3398 increase depths and accumulated ESALs provided in the paper were sharply as the percentage of two fractured faces from ASTM converted to a rutting rate as recommended by Cross and D5821 increases above 80%. However, the authors note (20): Brown (17). Regression analysis between the reported frac- tured face counts and rutting rate indicated no relationship. Theoretically, 100% crushed particles would be preferable to Overall, the rutting rates were higher than those reported by use when employing gravel coarse aggregates in an HMA Cross and Brown. mix. However, the benefits that might be achieved by requir- ing the 2-face crushed count to be 100% should be weighed against the additional cost involved in the crushing operation. 2.2.3 Precision of ASTM D5821 Ahlrich (19) investigated 11 aggregate blends meeting the ASTM D5821 for fractured face count of coarse aggregate Federal Aviation Administration's P401 gradation. The blends reports a multilaboratory standard deviation of 5.2% for well- were produced by combining different percentages of crushed trained observers (16). Thus, the acceptable range between limestone, crushed gravel, uncrushed gravel, and natural sand. two properly conducted tests by two well-trained observers The blends were combined to produce 0%, 30%, 50%, 70%, would be 14.7%. This precision is based on an Ontario Min- and 100% crushed coarse aggregate particle counts. Opti- istry of Transportation study that included 34 observers' mum asphalt content was determined for each blend using evaluations of two samples of partially crushed gravel. Hand the USACE Gyratory Testing Machine. The resulting mix- et al. (13) reported the precision statement shown in Table 1 tures were tested for rutting resistance using a confined based on 10 laboratories' tests of four aggregates used at repeated-load permanent deformation test. WesTrack. Coarse aggregate shape, angularity and texture were eval- uated using the USACE test for fractured face count (CRD- 2.2.4 Alternative Methods of Measuring Coarse C 171), ASTM D3398, and the uncompacted voids in coarse Aggregate Angularity aggregate test (AASHTO TP56). Testing indicated a strong correlation (R2 = 0.98) between the uncompacted voids con- Alternative methods to ASTM D5821 have been investi- tent of the as-received material and the fractured face count. gated that combine shape, angularity, and texture into one Table 2 shows the correlations between individual tests and measure (2, 1921). Two alternatives that have received atten- three parameters from the confined repeated-load permanent tion are ASTM D3398, "Index of Aggregate Particle Shape deformation test. The combined (coarse and fine aggregate) and Texture," and AASHTO TP56, "Uncompacted Voids in particle index value (PI Composite) from ASTM D3398 Coarse Aggregate." The literature indicates that angular, appears to provide the best overall correlation. The particle rough-textured aggregates have a particle index value greater index value on the coarse material also provides good corre- than 14, whereas rounded and/or smooth aggregates have a lations. The percent crushed face count (PCP Composite) as particle index value less than 12 (19). Ahlrich (19) developed measured by CRD-C 171 for the composite coarse and fine the uncompacted voids in coarse aggregate test based on aggregate as well as the uncompacted voids in coarse aggre- ASTM C1252, "Uncompacted Void Content in Fine Aggre- gate are also good predictors. TABLE 1 Precision statement for both one or more and two or more fractured faces (13) Property and Index Type Standard Deviation, % Acceptable Range of Two Results One or More Fractured Faces Single-Operator Precision 1.1 3.0 Multilaboratory Precision 1.8 5.1 Two or More Fractured Faces Single-Operator Precision 1.8 5.1 Multilaboratory Precision 2.9 8.2

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17 TABLE 2 Rankings for correlations between aggregate characterization tests and permanent deformation results (19) Rank Permanent Strain (R2) Creep Modulus (R2) Slope of Deformation Curve (R2) 1 PCP Composite (0.87) PI Composite (0.73) PI Composite (0.71) 2 PI Composite (0.78) PI Coarse (0.69) PCP Composite (0.65) 3 PI Coarse, (0.68) PCP Composite (0.63) PI Coarse (0.52) 4 UV Coarse (0.65) UV Coarse (0.56) UV Coarse (0.43) 5 PCP Coarse (0.60) PCP Coarse (0.46) ASTM C1252 Method A (0.41) Note: PCP = percent crushed particles (CRD-C 171) PI = Particle Index Value (ASTM D3398) UV = uncompacted voids in coarse aggregate (AASHTO TP 56) Composite = both coarse and fine aggregate As described previously for the aggregates studied as part lish a single gradation for all of the mixes. The gradation is a of NCHRP Project 4-19, Kandhal and Parker (2) identified 12.5-mm NMAS mixture that approximately follows the AASHTO TP56, "Uncompacted Void Content in Coarse Superpave maximum density line. Rut testing was performed Aggregate," as being the test most related to the rutting per- with the GLWT and a confined repeated-load permanent formance of a coarse-graded mix produced using nine differ- deformation test. No single aggregate test provided a strong ent coarse aggregate sources and a single natural sand source. relationship with the performance of all the mixes. The high- Hossain et al. (21) studied the results from ASTM D3398, est correlation coefficient (R = -0.70) was for percent flat or "Index of Aggregate Particle Shape and Texture," and the elongated particles by particle count at the 51 ratio. uncompacted voids in coarse aggregate to measure coarse Ongoing research by Rismantojo (23) as part of NCHRP aggregate angularity. The effects of gradation and the per- Project 4-19(2) evaluated five coarse aggregates using accel- centage of flat and elongated particles were considered on erated loading. The aggregates included a dolomite, lime- the test results. Two standard gradations were developed for stone, natural gravel, granite, and traprock. Coarse aggregate the uncompacted voids test. The gradations were based on the tests performed as part of the study include maximum density line for gradations with maximum particle sizes of 12.5 or 19.0 mm. The particle index value and uncom- Flat and Elongated Particles (ASTM D4791) at the 21, pacted voids tests were run on both the individual size frac- 31, and 51 ratio; tions and the proposed blended gradations. Testing indicated Uncompacted Voids in Coarse Aggregate (AASHTO a good relationship between the calculated index using the TP56) Method A (standard grading) and Method B (indi- results from the individual size fractions and the measured vidual size fractions); values from blended samples representing the two standard Micro-Deval (AASHTO TP58); gradations. The authors recommended the use of a standard Magnesium Sulfate Soundness (AASHTO T104); gradation for relative comparison of coarse aggregate sources LA abrasion (ASTM C96 Type C); (21). The standard gradations established by Hossain et al. Bulk Specific Gravity (ASTM C127); and (21) were adopted by AASHTO TP56. For comparing grada- Water Absorption (ASTM C127). tions, the authors recommended testing the individual size fraction and then calculating the result for a target gradation. A correlation matrix was developed among the aggregate Flat and elongated particles tend to increase the measured tests. Several strong relationships were indicated between the uncompacted voids content and aggregate particle index. various forms of ASTM D4791 used in the study. A fair rela- However, flat and elongated particles are not desirable in tionship (R = 0.786, p-value = 0.064) was indicated between HMA (21). Relationships between percent flat and elongated flat or elongated particles and uncompacted voids in coarse particles and both uncompacted voids and particle index aggregate Method A. Figure 1 presents the combined data were obtained for the limited materials used in the study. The from NCHRP Projects 4-19 and 4-19(2). The regression line relationships were non-linear and were different for gravel in the figure excludes the slag and sandstone sources tested and crushed stone (based on the differences in angularity and as part of NCHRP Project 4-19 as outliers. Regression analy- texture between the two groups) (21, 22). The relationships sis for the combined data (including the outliers) produces an were developed for both the 31 and 51 ratios for flat and R2 = 0.24. The relationship is not significant at the 5% level elongated particles. with a p-value = 0.06. This indicates that although particle Hossain et al. (22) evaluated the rutting performance of 11 shape has a strong influence on uncompacted voids results, mixes produced with blends from four crushed gravel sources, texture--such as that found on the slag and sandstone--can a limestone source, a granite source, and a natural sand. also have a strong effect. Rutting models developed by Alabama DOT 416 Mix 4 specifications were used to estab- Kandhal and Parker (2) indicated that high percentages of flat