Aggregate Properties and the Performance of Superpave-Designed Hot-Mix Asphalt (2005)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

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

73 Mix designs should be developed using these materials. Artifi- cially produced gradations should be avoided because there may be an interaction between aggregate shape, texture, and angularity and material packing. Coarse gradations should be used to better assess coarse aggregate properties, and fine gra- dations should be produced to better assess fine aggregate prop- erties. For a given aggregate-gradation combination, a mix design should be developed that meets all of the Superpave cri- teria and that produces a reasonable level of VMA. Aggregate shape, texture, and angularity would be expected to affect field compactability, rutting, and, possibly, the fatigue characteristics of HMA mixtures. NCAT has developed a prototype HMA workability device (181). A device such as this may be able to assess the effects of aggregate shape, tex- ture, and angularity on the workability of HMA. This vibra- tory compactor has been used to successfully demonstrate the increased compactability of warm asphalt mixes (182). Since the vibratory compactor is a constant stress device, it can be used to apply constant compaction energy to compare the compactability of various mixes designed using the same number of gyrations. The rutting potential of the mixes should be assessed using a simple performance test such as a version of the Flow Num- ber Test. In addition, rutting resistance may be assessed using the Asphalt Pavement Analyzer, which has been used more widely by agencies (to date). Fatigue can be evaluated using the beam fatigue test. Though there is a demonstrated shift factor with field performance, fatigue results may be used to rank mixes. 5.2.2 Field Evaluation If the Phase I laboratory evaluation indicates relationships between aggregate shape, texture, and angularity and field performance, testing should be conducted on aggregates used in actual field sections. Because of the difficulties in accu- rately determining traffic levels in field test sections, acceler- ated loading facilities should be utilized as much as possible. Sections should be selected to represent a range of aggregate types, climatic regions, and traffic loads. 5.3 THE RELATIONSHIP BETWEEN THE COMPACTED AGGREGATE RESISTANCE TEST AND RUTTING PERFORMANCE OF HMA The CAR test has been proposed as an alternate to AASHTO T304. Testing has been conducted by a task group within TRB’s Superpave Aggregate and Mixture Expert Task Group to further develop the procedure. However, little testing has been conducted to relate the refined test procedure to field per- formance. A research study should be conducted on a national scale to relate the CAR test to performance. Companion test- ing should be performed with AASHTO T304. 5.3.1 CAR Test Parameters Based on recently performed work, the following test param- eters are recommended for the CAR test: • Gradation, – The blended fine aggregate should be tested using the same gradation expected in the fine aggregate fraction of the HMA (material passing the 4.75-mm sieve), and – Since some agencies may wish to use the test to approve sources, as-received stockpile samples should also be tested. • Samples should be blended with 3% moisture. • Samples should be compacted with 50 Marshall blows on one face. 5.3.2 Laboratory Evaluation State agencies should be contacted to identify a range of aggregates having both good and bad performance. Efforts should be made to identify several of the cubical, crushed materials that have AASHTO T304 uncompacted void con- tents between 43 and 45. Natural sands with similar uncom- pacted voids content should also be identified. Additional test- ing, such as petrography and the tests from NCHRP Project 4-30A applicable to fine aggregates, should be performed to assess these “borderline” materials. Fine gradations or gradations through the restricted zone should be produced to better assess fine aggregate properties. Two coarse aggregate sources are recommended: partially crushed gravel and crushed limestone. For a given aggregate- gradation combination, a mix design should be developed that meets all of the Superpave criteria and that produces a rea- sonable level of VMA. Rutting potential of the mixes should be assessed using a simple performance test such as a version of the Flow Number Test. In addition, samples might be assessed using the Asphalt Pavement Analyzer, which has been used more widely by agencies to date. 5.3.3 Field Evaluation If the Phase I laboratory evaluation indicates relationships between aggregate shape, texture, and angularity and field per- formance, testing should be conducted on aggregates used in actual field sections. Because of the difficulties in accurately determining traffic levels in field test sections, accelerated loading facilities should be utilized as much as possible. Sec- tions should be selected to represent a range of aggregate types, climatic regions, and traffic loads.