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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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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.
