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45 CHAPTER 3 Interpretation, Appraisal, and Applications Aggregate characteristics influence the structural and func- analysis, that significantly improved repeatability and tional properties of pavement materials. Aggregate shape spec- reproducibility. ifications have generally developed based on the correlation · The University of Illinois Aggregate Image Analyzer (UIAIA) between an indirect measure of aggregate shape and labora- has been shown to be accurate in measuring the character- tory measurements of the physical and mechanical proper- istics of coarse aggregates; results are presented in terms of ties of pavement layers (2631). These indirect aggregate tests the distribution of aggregate characteristics in an aggregate have limited ability to identify and separate the fundamental sample. The system has already been used in a number of characteristics (shape, angularity, and texture) (31). These studies. Researchers at the University of Illinois are pursu- limitations have led to discrepancies in terms of the extent of ing further improvement of the system to allow analysis of the influence of aggregate characteristics on performance. aggregates irrespective of their color. Consequently, specifications developed based on these tests · The Multiple Ratio Shape Analysis (MRA) is desirable for may stipulate the need for superior aggregate characteristics, measuring the shape of coarse aggregates. It is inexpensive or otherwise allow for the use of marginal shape properties and provides the distribution of shape in an aggregate sam- (2631). Developing accurate methods for measuring and ple making it very desirable if the shape of coarse aggregate classifying aggregate characteristics is needed to specify the is the only property being sought. This test has already been appropriate aggregate characteristics for each specific pave- used in a number of research studies. ment application. These test methods appear to be appropriate for central laboratories. During the course of this study, members of the Test Methods research team visited field laboratories of aggregate produc- The test methods that were evaluated in this study exhibit tion companies to investigate the feasibility of using these test significant differences in their operations, the characteristics methods in field laboratories. They concluded that the space and being measured, and the analysis methods. Based on the eval- facilities available can easily accommodate these test methods. uation of test methods, the following conclusions were reached: In fact, the technicians and managers in some of these field laboratories have indicated that they already operate systems · The Aggregate Imaging System (AIMS) is suited for use that include electronics similar to those used in the imaging as a unified system for measuring the characteristics of systems, suggesting that the recommended test methods can both coarse and fine aggregates. It is capable of analyzing easily be used in central and field laboratories. 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 Use of Aggregate Acceptance Tests sieve #100 (0.15 mm). By capturing images of aggregates in Specifications at specified resolutions, AIMS minimizes the influence of particle size on shape results. The results are presented in The literature search has shown that state highway agencies terms of cumulative distribution function rather than an vary significantly in the extent of their using tests for measuring average value only. The system has been used in the field aggregate characteristics and incorporating criteria based on and in a number of research studies. The system is equipped these tests in specifications. These variations are caused by with an automated control of the top lighting for texture the laborious and subjective nature of the aggregate tests
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46 and the lack of clear evidence of strong correlations with the can be employed to quantify the changes in these percentages, performance of pavement layers. The proposed test methods and consequently, develop specifications for the optimum are rapid and accurate, making them well suited for use and percentages and allowable variations in these percentages specifications: in pavement layers. · The recommended test methods include procedures for · Test methods currently used in practice that summarize measuring texture and the loss of texture due to polishing. the results in terms of average indices are of limited value. This feature can be used as an indicator of pavement friction Such average indices do not reflect the changes in aggregate characteristics. characteristics during production due to natural variation in · The distribution of aggregate characteristics can be measured aggregate composition or the processes used to produce these rapidly and accurately as part of the quality control and qual- aggregates. Also, an average value alone does not indicate ity assurance programs to detect changes in production and the likely performance of the pavement layer. The aggre- allow adjustment when needed. gate shape classification presented in this report is based on · The recommended tests and classification methodology the distributions of aggregate characteristics in an aggre- can be used to develop specifications for the combinations gate sample, and yields the percentages of aggregate particles of aggregate characteristics needed to achieve the specific that belong to certain shape categories. Standard statistics levels of pavement performance.