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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2007. Test Methods for Characterizing Aggregate Shape, Texture, and Angularity. Washington, DC: The National Academies Press. doi: 10.17226/14017.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2007. Test Methods for Characterizing Aggregate Shape, Texture, and Angularity. Washington, DC: The National Academies Press. doi: 10.17226/14017.
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S U M M A R Y The literature review conducted in this project revealed that the characteristics of coarse and fine aggregates used in hot-mix asphalt and hydraulic cement concrete mixtures, and unbound base and subbase layers influence the performance of the pavement system in which they are used. Aggregate characteristics can be identified by three independent components: shape (or form), angularity, and texture. Methods currently used for measuring these char- acteristics have several limitations: they are laborious, subjective, lack direct relation with performance parameters, and have a limited ability to separate the influence of angularity from that of texture. A number of research studies have shown that aggregates, especially coarse aggregates that exhibit high texture, do not necessarily have high angularity. Consequently, it is important to develop methods that are capable of quantifying each of the aggregate char- acteristics rather than a manifestation of their interactions. This study evaluated available test methods for measuring aggregate characteristics. The evaluation was conducted based on accuracy, repeatability, reproducibility, cost, ease of use, ease of interpretation of the results, readiness of the test for implementation, portability, and applicability for the different aggregate sizes and types. Thirteen different coarse aggregate types and five different fine aggregate types were used in this evaluation. The evaluation of imaging-based test methods considered both the characteristics of the image acquisition procedure and the accuracy of the image analysis methods. Evaluation of the accuracy of the image analysis methods was conducted in two steps. In the first step, all the analysis methods were used to quantify the characteristics of particle projections that geologists have used for visual evaluation of particles. This step helped to identify analysis methods that are capable of distinguishing between particles of distinct characteristics. These methods were further evaluated in step 2 through the analysis of images of the aggregates used in this study. This step identified the analysis methods that are able to accurately rank aggregates based on their characteristics. The analysis results revealed that some of the avail- able analysis methods do not distinguish between angularity and shape and some analysis methods do not distinguish between texture and angularity. Accuracy of the test methods was assessed through statistical analysis of the correlations between the results from these methods with measurements of shape using a digital caliper and visual rankings of surface irregularity and texture by experienced individuals. Analyses of repeatability and reproducibility results were conducted following the guide- lines of the American Society of Testing and Materials (ASTM) standards E 177, C 802, and C 670. The Analytical Hierarchy Process (AHP)—a process of developing a numerical score to rank test methods based on how each of these methods meets certain criteria of desirable characteristics—was used to rank the test methods. The desirable characteristics of repeat- ability, reproducibility, accuracy, operational characteristics, and applicability for different sources of aggregates were considered in the evaluation. Test Methods for Characterizing Aggregate Shape, Texture, and Angularity 1

2The Aggregate Imaging System (AIMS) was recommended for measuring the characteris- tics of both coarse and fine aggregates. The system employs methods based on sound scien- tific concepts for the analysis of shape, angularity, and texture and provides the distribution of each of the characteristics in an aggregate sample. It has very good control of lighting and provides repeatable and reproducible results. The University of Illinois Aggregate Image Analyzer (UIAIA) can also be used for measuring the shape, angularity, and texture of coarse aggregates. For measuring the coarse aggregate shape only, the Multiple Ratio Shape analysis method (MRA) was the most appropriate and is much cheaper than all the other test methods. Similar to the imaging systems, the MRA provides the distribution of shape in an aggregate sample, but it cannot be used for measuring angularity or texture. All these test methods can be used for routine analysis of aggregate characteristics as they require minimal training and provide an easy to use summary of the results. Proposed procedures for conducting these tests are provided in Appendix A. The ability of X-ray computed tomography (CT)—a nondestructive technique to capture the three dimensions of materials—to provide detailed measurements of aggregate charac- teristics was assessed. The X-ray CT proved to be a powerful tool, but is premature for use in the routine measurements of aggregate characteristics. The image processing techniques used in separating the particles in X-ray CT require substantial manual manipulation of images, which could influence the measurements of angularity and texture. A methodology for classification of aggregates based on their characteristics was developed in this project. The methodology unifies the methods used to measure the characteristics of fine and coarse aggregates, and describes these characteristics by cumulative distribution func- tions rather than average values, thus better defining the effects of blending and crushing on aggregate characteristics. This methodology can be used to (1) explore the influence of different processes such as crushing and blending on aggregate shape, (2) conduct quality control by detecting changes in the distribution of any of the characteristics, (3) relate the dis- tribution of different characteristics to performance, and (4) develop specifications based on the distribution of aggregate characteristics rather than average indices.

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TRB's National Cooperative Highway Research Program (NCHRP) Report 555: Test Methods for Characterizing Aggregate Shape, Texture, and Angularity examines a methodology for classifying aggregates based on the distribution of shape, texture, and angularity characteristics. The report also explores a test method that it is appropriate for use in central and field laboratories, which measures shape, texture, and angularity characteristics of aggregates used in hot-mix asphalt, hydraulic cement concrete, and unbound base and subbase layers of pavements. Appendixes for the report were published as NCHRP Web-Only Document 80.

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