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3This section of the report presents the project background, objectives, and descriptions of the tasks performed in the research project. Project Background The properties of coarse and fine aggregates used in hot-mix asphalt (HMA) and hydraulic cement concrete mixtures, and unbound base and subbase layers influence the performance of the pavement system in which they are used. Particle shape, texture, and angularity are among the aggregate properties that have significant effect on performance. These properties vary widely with the type and source of aggregates and processing techniques. However the current aggregate specifications do not address in a direct manner the measurements of these properties, thus leading to inconsistent interpretation and use of test results. Several methods for measuring aggregate shape, texture, and angularity characteristics were developed in recent years, and others are being developed as part of ongoing research efforts. However an evaluation of these methods with respect to their practicality, labor requirements, ease of use, cost, versatility, field applicability, use in multiple-ratio shape analysis, and other factors has not been performed. Without this informa- tion, a rational recommendation for incorporating such test methods in aggregate specifications cannot be made. Research was needed to evaluate potential test methods and identify or develop suitable test methods for measuring relevant properties in central and field laboratories, and to develop recommen- dations to help improve specifications for aggregates used in highway pavements. This need was addressed in this NCHRP project. Research Objective The objective of this research was to identify or develop, for use in central and field laboratories, suitable test methods for measuring shape, texture, and angularity characteristics of aggregates used in HMA and hydraulic cement concrete mixtures, and unbound base and subbase layers of highway pavements. The research focused on the characteristics of coarse aggregates, but also considered the characteristics of fine aggregates. Scope of Study This study included collecting and reviewing informa- tion relative to the effects of shape, texture, and angularity characteristics of aggregate on the performance of HMA and hydraulic cement concrete mixtures, and unbound materials used in highway pavements. This information was obtained from domestic and foreign literature, contacts with public and private agencies and industry organizations, and other sources. The collected information was used to (1) identify aggregate particle characteristics that are likely to influence performance and (2) to identify test procedures currently used in the United States and other countries for measuring these characteristics. The merits and deficiencies of the test methods were evaluated based on the published information with consideration to relevance, practicality, labor require- ments, cost, duration of test, repeatability, versatility, field applicability, use in multiple ratio shape analysis, and other pertinent factors. The evaluation of all available test methods was followed with the execution of an experimental plan to evaluate and validate the most promising test methods, and if necessary, to develop new methods for measuring shape, texture, and angularity characteristics of aggregates. The test methods recommended as a result of the experimental evaluations are presented in Appendix A. Note: Appendixes B through E are not published herein. These appendixes are acces- sible as Web-Only Document 80 at http://trb.org/news/ blurb_detail.asp?id=7276. C H A P T E R 1 Introduction and Research Approach
Research Approach A comprehensive literature review was conducted during this study. The literature search focused on aggregate particle characteristics that are likely to influence pavement perform- ance (summarized in Appendix B). The literature search also covered image analysis methods for characterizing aggregate shape properties (summarized in Appendix C), and test pro- cedures currently used in the United States and other countries and those proposed as part of recent research for measuring these characteristics (summarized in Appendix D). The research also included critical evaluation of all available test methods based on the published literature, identification of test methods for experimental evaluation, conduct of statistically designed experiments to evaluate test methods, rank and recommenda- tion of test methods for measuring aggregate characteristics, and development of a methodology for classification of aggre- gates based on shape. Evaluation of Test Methods Information gathered from the literature was used (1) to conduct a comparative analysis of the available test methods and (2) to select test methods that will be subjected to inten- sive experimental evaluation in this study. The advantages and disadvantages of all test methods were summarized. Three steps were used to select test methods (Figure 1). In the first step, methods were categorized into direct and indirect. Direct methods were defined as those wherein particle characteris- tics (shape, texture, and angularity) are measured, described qualitatively, and possibly quantified through direct measure- ment of individual particles. In indirect methods, particle characteristics are lumped together as geometric irregularities and determined based on measurements of bulk properties. In the second step, methods that share the same analysis concept were grouped together to ensure that the selected candidate methods represent different analysis concepts. In the third step, tests were selected from each group based on practicality, labor requirements, cost, repeatability, versatility, and field applicability for further evaluation. Some of the currently used test methods were included in the intensive evaluation for comparison. The following test methods were considered for further experimental evaluation: Uncompacted Void Content of Fine Aggregates (AASHTO T 304), Uncompacted Void Content of Coarse Aggregates (AASHTO TP56), Compacted Aggregate Resistance (CAR), Percentage of Fractured Particles in Coarse Aggregate (ASTM D 5821), Flat and Elongated Coarse Aggre- gates (ASTM D 4791), Multiple Ratio Shape Analysis (MRA), VDG-40 Videograder, Buffalo Wire Works PSSDA, Camsizer, WipShape, Aggregate Imaging System (AIMS), University of Illinois Aggregate Image Analyzer (UIAIA), and Laser-Based Scanning Analysis System (LASS). 4 Further Evaluation Step 3: Select methods based on practicality, cost, labor requirements, and field applicability. All Test Methods Direct Indirect D-2 D-3 I-1 I-2 I-3 I-2-a D-3-a D-2-a Step 1: Divide methods into direct and indirect. I-3-a I-1-a Step 2: Divide methods based on analysis concepts. D-1 Figure 1. Approach for preliminary evaluation, screening, and prioritization of test methods.
Experimental Design and Statistical Analysis Aggregates were selected to cover a wide spectrum of origin, rock type, and characteristics. Three sizes of 13 coarse aggregate types and three sizes of five fine aggregate types were used in this study. Experienced individuals from the industry and high- way agencies assisted in selecting and providing these aggregates (photographs of representative samples of these aggregates are provided in Appendix E). Statistically-designed experiments were conducted to eval- uate repeatability, reproducibility, and accuracy of the selected methods. Information about cost and operational character- istics was gathered from vendors, researchers, and operators who are familiar with these systems. The operational charac- teristics of ease of use, portability, ability of interpreting data, readiness for implementation in central laboratories as well as field laboratories, and applicability of test method to measure different aggregate types and sizes were considered. Ranking and Recommendation of Test Methods The Analytical Hierarchy Process (AHP) was implemented in a program and used to provide a ranking, or a priority list of all the methods included in the evaluation. Criteria for the preferred characteristics of test methods were developed and used in the rankings. The AHP program provides flexibil- ity in defining the objectives, ranking criteria, and relative importance or priorities of the different criteria elements. Test methods recommended based on the AHP rankings are listed in Table 1. A statistical-based methodology was developed to (1) sum- marize the analysis results, (2) facilitate the comparison between the characteristics of different aggregate sources, and (3) classify aggregates based on the distribution of their characteristics. The methodology was presented in the form of macro-driven Microsoft ExcelÂ®, a Visual Basic application. 5 Coarse Aggregates Fine Aggregates Recommended Test Method Shape, Angularity and Texture Shape Only Angularity Aggregate Imaging System (AIMS) Recommended Recommended University of Illinois Aggregate Image Analyzer (UIAIA) Recommended Multiple Ratio Shape Analysis Recommended Table 1. Recommended test methods for measuring aggregate characteristics.