38 AGGREGATES The grading curve – a plot of the mass of material passing each sieve size – also determines the potential workability of mixtures and the space to be filled by binder and can be adjusted to suit particular purposes The grading curve can be designed to reduce the volume of space to less than 10% of the total volume, but at this level the aggregate becomes almost completely unworkable Particle Shape Particle shape is important in controlling the ability of the aggregate to compact, with or without a binder, and affects the adhesion of the binder to the aggregate surface Shapes are described as rounded, irregular, angular, flaky, or elongate, and can be combinations of these (Figure 2) The first three are essentially equidimensional The shape is assessed by measuring the longest, shortest, and intermediate axial diameters of the fragments In the ideal equidimensional fragment, the three diameters are the same Particles with ratios of the shortest to the intermediate and the intermediate to the longest diameters of above about 0.6 are normally regarded as equidimensional For many purposes, it is important that the aggregate particles have equant shape: their maximum and minimum dimensions must be very similar Spherical and equant particles of a given uniform size placed together have the lowest space between the particles Highly angular particles and flaky particles with high aspect ratios of the same grading can have much more space between the particles The shape of the particles can significantly affect the properties and composition of a mixture The overall space is also determined by the grading curve Sometimes highly flaky particles such as slate can be used in a mixture if they are accompanied by suitably graded and highly spherical particles Flakiness Index (British Standard 812) The flakiness index is measured on particles larger than 6.5 mm and is the weight percentage of particles that have a least dimension of less than 0.6 times the mean dimension The sample must be greater than 200 pieces The test is carried out using a standard plate that has elongate holes of a given size; the proportion passing through the appropriate hole gives a measure of the flakiness index Elongation Index (BS 812) The elongation index is the percentage of particles by mass having a long dimension that is more than 1.8 times the mean dimension This measurement is made with a standard gauge in which pegs are placed an appropriate distance apart Petrography Figure Examples of particular particle shapes (A) Well rounded spherical metaquartzite (B) Elongate angular quartzite (C) Rounded flaky limestone The petrography of the aggregate is mainly assessed on the basis of hand picking particles from a bulk sample Thin-section analysis either of selected pieces or of a crush or sand mounted in a resin is also employed The petrographic analysis is essential to determine the rock types present and hence to identify potential difficulties in the use of the material It allows recognition of potentially deleterious components and estimation of physical parameters The experienced petrographer, for example, can estimate the parameters relevant to the use of a material for road surfacing Published standards provide procedures for petrographic description, including the standards published by the American Society for Testing and