The size of aggregate used in construction is an important factor in determining the strength and durability of concrete. The usual range used is between 9.5 mm and 37.5 mm in diameter, with the most common size being 20 mm. Coarse-grained 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. Coarse-grayed aggregates are particles larger than 9.5 mm. 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. Crushed stone is preferred in pavement mixes, since the higher paste-aggregate bond produces greater flexural strengths (Photo courtesy of PCA).
Aggregates are one of the most important components of concrete that give concrete body and also reduce shrinkage. If there is a deficiency in a locally available fine aggregate, concrete may benefit from the addition of air entrainment, additional cement, or a supplemental cementitious material (SCM) to address these deficiencies. The fundamental relationship between the water-cement ratio and strength begins with the correction of the contribution or absorption of moisture by the aggregates. The ratios between coarse-grained and fine aggregates will change depending on the unique characteristics of each aggregate, the method of placement and the desired finish. This concrete aggregate is often used for roof decking systems and works with most concrete, wood and metal structural platforms. 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).
The use of slag aggregates from iron and steel production in construction dates back to the Romans, who used crushed slag from crude iron production of that time to build their roads. In general, 40mm size aggregate is used for normal strengths and 20mm size is used for high-strength concrete. However, the use of slag is no longer limited to roads, but slag aggregates are widely used in all types of civil works. Using larger coarse-grated aggregates generally reduces the cost of a concrete mix by reducing requirements for cement, the most expensive ingredient. 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. Therefore, it's essential to understand how different sizes of aggregate affect concrete in order to create strong and durable structures.
Both gravel and crushed stone are generally acceptable for manufacturing quality concrete (Photo), although gravel is generally preferred for exposed aggregate. A track record of good performance of a local aggregate also provides an indication of the material's performance in service.