When it comes to concrete mixes, the size of the aggregate used is an important factor. The usual range used is between 9.5 mm and 37.5 mm in diameter, with the most common size being 20 mm. Coarse-grated aggregates are all aggregates larger than 4.75 mm in size per piece, including boulders, cobblestones and gravel. Fine or crushed gravel is usually 4 to 8 mm; medium or partially crushed gravel measures 8 to 16 mm; coarse-grained or uncrushed gravel measures 16 to 64 mm; pavers measure 64 to 256 mm; and rocks have anything above 256. Classification refers to determining the particle size distribution for the aggregate.
Leveling limits and maximum aggregate size are specified because these properties affect the amount of aggregate used, as well as cement and water requirements, workability, pumpability, and concrete durability. In general, if the water-cement ratio is chosen correctly, a wide rating range can be used without a major effect on strength. When specifying void graded aggregates, certain particle sizes are omitted from the aggregate of the size continuum. Gradual separation aggregates are used to obtain uniform textures in concrete with exposed aggregate. Strict control of mix ratios is necessary to avoid segregation. A larger size, 40 mm, is more common in bulk concrete.
Larger aggregate diameters reduce the amount of cement and water needed due to their lower voids. Recycled concrete as an aggregate will generally have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. Alternative sources of aggregates or additional aggregate mixtures can be considered to approach the elusive ideal gradation that provides the best workability, pumpability, shrinkage reduction and economy (Figure). In general, 40mm size aggregate is used for normal strengths and 20mm size is used for high-strength concrete. It's important to use aggregates that meet ASTM C 33, Standard Specifications for Concrete Aggregates. The absorption and surface moisture of aggregates are simple but critically important aspects of producing concrete that consistently achieves the specified or desired strength. Both gravel and crushed stone are generally acceptable for manufacturing quality concrete (Photo), although gravel is generally preferred for exposed aggregate.
This concrete aggregate is often used for roof decking systems and works with most concrete, wood and metal structural platforms. A mixture of all types of aggregates, is suitable as a base for concrete in projects that do not require precise proportions of aggregate to cement. If you are looking for medium strength concrete, get 40mm aggregates, but if you are looking for high strength, 20mm is best. Recycled concrete is a viable source of aggregate and has been successfully used in granular subbases, soil cement and new concrete. In general, you'll want smaller coarse-grated aggregates for stronger concrete, with 20mm aggregates meeting the threshold for strong concrete and 40mm aggregates for normal strength concrete. To obtain a good concrete mix, aggregates must be clean, hard and strong particles, free of absorbed chemicals or coatings of clay and other fine materials that could cause concrete deterioration. Crushed stone is preferred in pavement mixes, since the higher paste-aggregate bond produces greater flexural strengths (Photo courtesy of PCA).
Using larger coarse-grated aggregates generally reduces the cost of a concrete mix by reducing requirements for cement, the most expensive ingredient. Photo 7 - The size distribution of fine to coarse-grained aggregates plays an important role in the workability and performance of concrete.