Image analysis of shape and size of fine aggregates

The genesis of gravel deposits and rocks cause large differences in grain-shape and grain-size distributions of aggregate products and these differences are most pronounced in the fine material. Quantification of fine material properties is essential for the ability to control the physical properties of the products in which the material is used, such as in concrete and asphalt. This paper presents a method for the characterization of the fine fraction of natural and crushed aggregates, using image analysis. The aim of the characterization is to investigate the differences in particle shape and size distribution among a wide range of natural and crushed fine aggregate materials. In a following project these properties will be related to the rheological properties of fresh concrete, with an aim to improve concrete proportioning procedures. The study involved materials from 26 different gravel pits and rock quarries from different parts of Sweden. The samples were sieved and prepared for image analysis. Material <63 mu m was analysed using a scanning electron microscope (back scatter mode), Fig. 1 and fractions 63-125 mu m and 125-250 mu m were analysed in thin sections, using polarization microscopy with W-light, Fig. 2. The thin sections were prepared using epoxy mixed with fluorescence, which ensures a good contrast between the mineral particles and the epoxy matrix. The data from the image analysis was then analysed statistically. The results show that image analysis of aggregates gives multiple possibilities of characterization and classification depending on the intended use of the material. There is a major and distinct difference between the shape of crushed and natural aggregates. Measurements of the aspect ratio ("Fshape") of different fractions of the aggregate shows that the largest differences between materials occur in the fine fractions, i.e. material <250 mu m. From that follows that the particle shape of an aggregate material is not the same for all fractions. Fine particles are much more elongated and/or flaky than large particles. Development of a general method for converting size distributions into the traditional volume or weight distribution is recommended. (C) 1998 Elsevier Science B.V. All rights reserved.