Test Methods for Characterizing Aggregate Shape, Texture, and Angularity




SUMMARY

Test Methods for Characterizing Aggregate

Shape, Texture, and Angularity

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 characteristics

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 characteristics

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 available

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 guidelines

of the American Society of Testing and Materials (ASTM) standards E 177, C 802, and

C670. 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 repeatability,

reproducibility, accuracy, operational characteristics, and applicability for different sources of aggregates

were considered in the evaluation.


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