Aggregates are categorized by size and use. The Potters Bar Aggregates uses are as sub-base, base, and surface for roads and highways, foundation support, as ballast, river bank and beach stabilization, gravity retaining wall systems, reinforced portland cement concrete, asphaltic concrete, masonry units, grouts, structural back-fill, decorative walls, and many others. Sizing is determined by screens going from less than #4 to over 6 inches and much greater for erosion protection. Fine aggregates such as sand are used for concrete, fill, grouts, masonry and as filtration media. Usually the category includes aggregate sizes that pass through several screens and are determined by the percent passing each screen size. The other important characteristics are soundness, durability, and chemical inertness or non-reactive. You can check the American Society for Testing and Materials (ASTM) standards, the Federal Highway Administration, the Corps of Engineers, the state Department of Transportation, Bureau of Reclamation, flood control districts and other agencies for their grading and quality requirements for each type of project.