Imaging indices for quantification of shape, angularity, and surface texture of aggregates

Three image indices are presented that characterize aggregates in a hierarchical scheme involving overall shape (or form), roundness of corners, and particle surface texture. The corresponding dimensionless image indices are called aspect ratio, angularity, and roughness, respectively. Calculating these indices requires that image-analysis techniques be used to find the following data for a sample of two-dimensional particle images: shortest and longest dimensions, perimeters, convex perimeters, and perimeters of ellipses with areas equivalent to those of the associated particles. To demonstrate the usefulness of these three dimensionless indices, selected sieve-size fractions for 25 fine-aggregate sources were subjected to image-analysis testing and to uncompacted void testing (ASTM C1252). Both uncompacted void contents and image-analysis indices could generally differentiate between natural sands and crushed fine aggregates. However, the image-analysis indices provided additional useful information. For example, the image-analysis results identified that two sources of crushed fine aggregates had relatively low uncompacted void contents because they had relatively smooth surface textures or relatively low aspect ratios. In addition, the image-analysis results were able to distinguish morphological differences between aggregate sources that were measured to have similar uncompacted void contents.