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18 55 Blast Furnace Slag Uncompacted Voids, % 50 Sandstone 45 y = 0.2287x + 38.809 R2 = 0.70 40 5 10 15 20 25 30 35 40 45 50 Flat or Elongated Particles 2:1 NCHRP 4-19 NCHRP 4-19 (2) Figure 1. Relationship between flat or elongated particles and uncompacted voids (2, 22). or elongated particles at the 21 or 51 ratio were undesir- stone were tested to 20,000 passes. A strong relationship was able. Rismantojo (23) notes that Kandhal and Parker's (2) identified between the uncompacted voids from both Methods conclusion that high percentages of flat or elongated particles A and B and the total rut depth at 5,000 passes (R = -0.947 are undesirable from a rutting stand point is unrealistic based and R = -0.983, respectively). This relationship is strongly on the relationship with uncompacted voids in coarse aggre- influenced by the uncrushed gravel mixture. When the gravel gates. Higher percentages of flat or elongated particles tend mix is excluded and only the four mixes that were tested to to produce higher uncompacted voids and higher uncom- 20,000 passes are analyzed, the uncompacted voids in the coarse aggregate (Method A) performed on the plant stock- pacted voids tend to produce mixtures with less rutting. pile material produces the best correlation with R = -0.758 Rismantojo (23) performed correlations between coarse (23). The relationship is not significant at the 5% level. aggregate properties and both mix volumetric properties and rutting performance. Flat or elongated particles at the 21 ratio were positively correlated with optimum asphalt con- 2.2.5 Summary of Research Related to Coarse tent. This indicates that for the aggregates tested, higher Aggregate Angularity asphalt contents resulted for mixes with higher percentages of flat or elongated particles. Thus, mixes containing flat and Numerous research studies have indicated improved rut elongated particles may be more durable. Flat and elongated resistance with increased percentages of fractured faces in particles at the 31 ratio were negatively correlated with the coarse aggregate. However, the current test method, ASTM density at Ninitial. As the percentage of flat and elongated par- D5791, is subjective, requiring the technician to visually ticles at the 31 ratio increased, the density at Ninitial decreased. determine the presence and number of fractured faces. The Thus, flat and elongated particles also make it easier to meet Superpave fractured face count specifications are based on the Ninitial requirements. VMA was correlated positively with the consensus of an expert panel and not on laboratory test the uncompacted voids in coarse aggregate for both Methods results. Research completed since the implementation of the A and B. Overall, Method B, which is the average of the Superpave method has focused on alternative tests that are uncompacted voids for three individual size fractions, pro- more quantitative and objective. duced better correlations. Several studies have evaluated the relationship between both Full-scale rutting tests were performed at the Indiana DOT the particle index value (ASTM D3398) and the coarse aggre- APT Facility in West Lafayette, Indiana. Five mixes were gate uncompacted voids test (AASHTO TP56) and rutting per- tested in the APT facility. The rounded gravel mix produced formance. Trends indicate that higher particle index values or 29.5 mm of rutting after 5,000 passes, at which time testing uncompacted voids contents produce more rut-resistant pave- was terminated. The other four sections containing quarried ments. Relationships have been identified between both tests