Cover Image

Not for Sale

View/Hide Left Panel
Click for next page ( 75

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 74
74 CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS The results of this review have emphasized the difficult 6.1.2 Fine and Elongated Particles nature of conducting research to relate aggregate properties and HMA performance. It is difficult to isolate the effects of The research revealed the following about F&E: the aggregate properties from other interactions with grada- tion and mixture volumetric properties. It appears as if the Extreme levels (>10% 51 ratio) of F&E are most likely shortcomings of a single property related to rutting resistance undesirable in HMA. can be overcome by other supporting properties. Increased levels of F&E increase aggregate breakdown These interactions emphasize the need for laboratory per- during handling, mixing, and placement. formance tests for HMA mixtures. If performance tests are The current test for F&E, ASTM D4791, is extremely adopted that have criteria in which agencies are confident, the variable (multilaboratory coefficient of variation of 35.3% overall performance of the mixture could be assessed instead for the 31 ratio); however, precision improves as the of relying solely on component screening tests--for exam- ratio of maximum-to-minimum dimension decreases. ple, if the blend uncompacted voids in fine aggregate were Seven states currently specify the 31 ratio; and one 43% for a given mixture to be placed on a high-volume road, province, the 41 ratio. A specification of a maximum the rutting properties of this mixture could be tested (at the of 20% of particles exceeding the 31 ratio has been contractor's expense) to show whether the mix should pro- adopted by five states. Research has been unable to establish that between 20% vide acceptable performance. and 40% F&E exceeding the 31 ratio is detrimental to HMA performance. In fact, some level of F&E may be 6.1 CONSENSUS AGGREGATE PROPERTIES desirable to meet minimum VMA requirements. If ASTM D4791 continues to be used, specifications The consensus aggregate properties have been adopted should be developed for the 21 or 31 ratio to improve by the majority of the responding agencies. F&E, specified the precision of measurements. Up to 40% F&E exceed- by 79% of the responding agencies, has the lowest level of ing the 31 ratio does not appear to be detrimental to implementation. pavement performance. Therefore a specification level of up to 40% particles exceeding the 31 ratio may be appropriate. 6.1.1 Coarse Aggregate Angularity Imaging methods have been developed to accurately and precisely measure coarse aggregate shape, texture, The research revealed the following about coarse aggre- and angularity and fine aggregate shape. The results with gate angularity: these methods have not yet been correlated with the per- formance of HMA. Increased coarse aggregate fractured faces provide Research should be conducted to relate digital means for increased rutting resistance. Increased particle index measuring aggregate shape, texture, and angularity to value or uncompacted voids in coarse aggregate also pavement performance as replacements for both coarse provide increased rutting resistance. The latter combine aggregate angularity and F&E tests. the effect of shape, angularity, and texture. The current Superpave specification levels for coarse aggregate angularity have been adopted by 39% of the 6.1.3 Fine Aggregate Angularity agencies that specify ASTM D5821 or an equivalent. Five states have more stringent criteria; and four states, The research revealed the following about FAA: less stringent requirements. There is little research to support the need for two frac- Currently, the Superpave mix design system does not tured face counts in excess of 95%. address the shape, texture, and angularity of the material