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72 CHAPTER 5 FUTURE RESEARCH NEEDS 5.1 INTRODUCTION Second, there is significant controversy over the use of AASHTO T304, "Uncompacted Void Content in Fine Aggre- The Superpave mix design system was developed as part gate," to measure FAA. The literature indicates that AASHTO of the SHRP. Aggregate properties were included in the Super- T304 is a reasonable screening tool, but it does classify some pave mix design system; however, their inclusion was not cubical crushed fines as being inappropriate for high-traffic based on laboratory research conducted as part of that study. volumes. The Compacted Aggregate Resistance (CAR) Test The current study is to provide a critical review of the aggre- has been proposed as an alternative to AASHTO T304. The gate research conducted since the completion of the SHRP CAR test may be more sensitive than AASHTO T304 by and to propose the research needed to fill any gaps. providing a greater range of test values. Several of the cubi- First, there is a need to emphasize the collection and report- cal crushed fines for which there is concern based on the ing of aggregate property data for both in-service pavements AASHTO T304 results perform well in the CAR test. and for accelerated loading facilities. More effort needs to be placed on capturing aggregate property data in national studies related to HMA performance. NCHRP Project 10-56, "Accel- 5.2 THE RELATIONSHIP BETWEEN TEST METHODS TO CHARACTERIZE AGGREGATE erated Pavement Testing: Data Guidelines," was recently com- SHAPE, TEXTURE, AND ANGULARITY pleted. NCHRP Project 10-56 (published as NCHRP Report AND PERFORMANCE OF HMA 512: Accelerated Pavement Testing: Data Guidelines [180]) recommended that aggregate characteristics including "source, Past studies have had difficulty relating coarse aggregate gradation, particle shape, surface texture, mineralogy, specific shape, texture, and angularity to field performance. This may, gravity, porosity, toughness, hardness, etc.," be collected for in part, be due to the subjectivity, inaccuracy, and variability each aggregate. However, an examination of their recom- of currently used test methods to measure coarse aggregate mended protocols suggests that F&E (ASTM D4791) and shape and angularity. New test methods are being evaluated as coarse aggregate angularity (ASTM D5821) are missing for part of NCHRP Project 4-30A to characterize aggregate shape, coarse aggregates. It is recommended that the micro-deval texture, and angularity. Devices are to be recommended for test (AASHTO TP58) and the methylene blue test be added application in both central and field labs. A study should be and that making samples available for image analysis be added conducted in two phases to relate the results of these tests as well. with expected performance. Phase I should be used to relate Two areas were selected for immediate research: (1) the the aggregate properties to laboratory performance tests for development of performance relationships and criteria for the rutting and fatigue. If possible, workability and compactabil- new imaging methods to measure aggregate shape, angularity, ity should also be evaluated. Phase II should be used to assess and texture and (2) an investigation of alternatives to and spec- aggregates from projects with documented field performance. ification limits for the uncompacted voids in fine aggregate Results from accelerated loading facilities may be invaluable test. There is still room for additional research to refine speci- in this phase. fication limits for such tests as coarse aggregate angularity, F&E (both of which could one day be replaced by imaging), 5.2.1 Laboratory Evaluation LA abrasion, and micro-deval. Additional refinement of the methylene blue test could make it a likely and more discrimi- State agencies should be contacted to identify a range of nating candidate to replace the sand equivalent test. aggregates that have both good and bad performance. Initial NCHRP is currently sponsoring Project 4-30, "Test Meth- screening should be conducted with the recommended devices ods for Characterizing Aggregate Shape, Texture and Angu- from NCHRP Project 4-30A. Based on this screening, approx- larity." The objective of this research is to identify or develop imately 10 materials having a range of shape, texture, and angu- test methods for both central and field laboratories to mea- larity should be selected. Such combinations as cubical shape sure shape, angularity, and texture. Additional research will and high texture, cubical shape and low texture, poor shape and be required to relate these properties to field performance. high texture, and poor shape and low texture should be targeted.