Particle size distribution is an important factor when it comes to aggregates. It is used to determine the dimensions of the pore space in cultivated soils, and is also used to classify particles according to their size. The flake index is a test method based on the procedures of British Standard BS 812, which is preferred by some state transport departments over the ASTM flat and elongated method. This test uses a slotted thickness gauge and a separate length gauge to classify particles.
Gradation, or particle size distribution, is the dispersion of individual particle sizes throughout the sample. It is typically reported graphically by plotting each of the particle sizes on a semi-log graph and drawing a best-fit curve between the points. This procedure is described in ASTM C136 (AASHTO T-2). Once the proportions of the individual fractions are determined and represented graphically as a gradation curve, it can be used for more than just a grain size report.
Particle size determinations on large aggregate samples are necessary to ensure that aggregates perform as intended for their specified use. The values can qualitatively group the aggregate with classification terms such as by space, open grade, or uniform grade to describe the particle distribution. In the ASTM D4791 test method, a proportional gauge is used to test and grade a representative sample of approximately 100 individual aggregate particles of each size fraction. The ratio of the mass of an aggregate to the mass of a volume of water equal to the volume of the aggregate particles is also important when it comes to particle size distribution.
Scanning electron microscopy results confirm that the addition of fine aggregates to coarse-grained aggregates provides more bridging contacts (connected by precipitation of calcium carbonate) between the coarse-aggregate particles and, therefore, the maximum UCS achieved was not necessarily associated with the maximum calcium carbonate precipitation. A sieve analysis or gradation test determines the distribution of aggregate particles by size within a given sample. A known mass of material is placed on top of a group of nested screens (arranged in decreasing order of size from top to bottom) and mechanically agitated for a designated period of time. The fine and coarse-grained aggregates were mixed in various percentages to obtain five different grain size distributions.
The flat and elongated particle test measures the dimensional ratios of the individual coarse-grained aggregate particles. It is essential to understand why particle size distribution is important for aggregates in order to ensure that they perform as intended for their specified use. Knowing how to effectively determine particle size distribution can help you make sure that your aggregates are up to standard.
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