Test Methods for Characterizing Aggregate Shape, Texture, and Angularity (2007)

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45 Aggregate characteristics influence the structural and func- tional properties of pavement materials. Aggregate shape spec- ifications have generally developed based on the correlation between an indirect measure of aggregate shape and labora- tory measurements of the physical and mechanical proper- ties of pavement layers (26–31). These indirect aggregate tests have limited ability to identify and separate the fundamental characteristics (shape, angularity, and texture) (31). These limitations have led to discrepancies in terms of the extent of the influence of aggregate characteristics on performance. Consequently, specifications developed based on these tests may stipulate the need for superior aggregate characteristics, or otherwise allow for the use of marginal shape properties (26–31). Developing accurate methods for measuring and classifying aggregate characteristics is needed to specify the appropriate aggregate characteristics for each specific pave- ment application. Test Methods The test methods that were evaluated in this study exhibit significant differences in their operations, the characteristics being measured, and the analysis methods. Based on the eval- uation of test methods, the following conclusions were reached: • The Aggregate Imaging System (AIMS) is suited for use as a unified system for measuring the characteristics of both coarse and fine aggregates. It is capable of analyzing particles passing sieve 37.5 mm (11⁄2 in.) and retained on sieve 25.0-mm (1.0 in.) and as small as particles retained on sieve #100 (0.15 mm). By capturing images of aggregates at specified resolutions, AIMS minimizes the influence of particle size on shape results. The results are presented in terms of cumulative distribution function rather than an average value only. The system has been used in the field and in a number of research studies. The system is equipped with an automated control of the top lighting for texture analysis, that significantly improved repeatability and reproducibility. • The University of Illinois Aggregate Image Analyzer (UIAIA) has been shown to be accurate in measuring the character- istics of coarse aggregates; results are presented in terms of the distribution of aggregate characteristics in an aggregate sample. The system has already been used in a number of studies. Researchers at the University of Illinois are pursu- ing further improvement of the system to allow analysis of aggregates irrespective of their color. • The Multiple Ratio Shape Analysis (MRA) is desirable for measuring the shape of coarse aggregates. It is inexpensive and provides the distribution of shape in an aggregate sam- ple making it very desirable if the shape of coarse aggregate is the only property being sought. This test has already been used in a number of research studies. These test methods appear to be appropriate for central laboratories. During the course of this study, members of the research team visited field laboratories of aggregate produc- tion companies to investigate the feasibility of using these test methods in field laboratories. They concluded that the space and facilities available can easily accommodate these test methods. In fact, the technicians and managers in some of these field laboratories have indicated that they already operate systems that include electronics similar to those used in the imaging systems, suggesting that the recommended test methods can easily be used in central and field laboratories. Use of Aggregate Acceptance Tests in Specifications The literature search has shown that state highway agencies vary significantly in the extent of their using tests for measuring aggregate characteristics and incorporating criteria based on these tests in specifications. These variations are caused by the laborious and subjective nature of the aggregate tests C H A P T E R 3 Interpretation, Appraisal, and Applications

and the lack of clear evidence of strong correlations with the performance of pavement layers. The proposed test methods are rapid and accurate, making them well suited for use and specifications: • Test methods currently used in practice that summarize the results in terms of average indices are of limited value. Such average indices do not reflect the changes in aggregate characteristics during production due to natural variation in aggregate composition or the processes used to produce these aggregates. Also, an average value alone does not indicate the likely performance of the pavement layer. The aggre- gate shape classification presented in this report is based on the distributions of aggregate characteristics in an aggre- gate sample, and yields the percentages of aggregate particles that belong to certain shape categories. Standard statistics can be employed to quantify the changes in these percentages, and consequently, develop specifications for the optimum percentages and allowable variations in these percentages in pavement layers. • The recommended test methods include procedures for measuring texture and the loss of texture due to polishing. This feature can be used as an indicator of pavement friction characteristics. • The distribution of aggregate characteristics can be measured rapidly and accurately as part of the quality control and qual- ity assurance programs to detect changes in production and allow adjustment when needed. • The recommended tests and classification methodology can be used to develop specifications for the combinations of aggregate characteristics needed to achieve the specific levels of pavement performance. 46