Evaluation of grain shape characterization methods for unbound aggregates

Résumé

A laboratory study is performed on an unbound aggregate of gneiss commonly used as base course in road pavements. Aggregate samples with four different grain shapes from the same quarry have been tested. The objective of this study was to evaluate different methods of grain shape characterization to see which methods that gives the most reliable results for the same materials on the parameters describing shape, angularity and surface texture. One group of methods for grain shape characterization is simple petrologic methods in combination with physical measurements of the aggregate shape. These methods are based on visual classifications of rock type and measurement of the aggregate dimensions giving several shape characteristics as flatness and elongation ratio, angularity, degree of sphericity, flakiness index, shape index etc. The most common test in this group is the flakiness index test. Image analyses are more sophisticated in their characterization of the aggregate shape. These methods are also more powerful in their ability to characterize surface roughness. Two
dimensional analysis give a picture of each grain in only two dimensions, i.e. the projection of the grain on one plane in a random position. The most advanced methods are the three
dimensional image analysis.