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In concrete the major constituent in terms of volume is the aggregate. In essence concrete is like an artificial rock, consisting of aggregate materials bound together by hydrated cement. Aggregates are classified as coarse or sand (fine) depending on whether they pass through a 4mm sieve. They occur as uncrushed material, (natural sand and gravel) or as crushed material.
Fine aggregate (sand) is material which is less than 4mm in size. It can be purchased at builders merchants. They generally sell two types, coarse (or concrete/sharp) sand and fine (builders/soft) sand. As the name implies the coarse (or concreting) sand contains less of the smaller particles than the soft sand. Coarse sand should only be used for concreting jobs and soft sand should only be used for render or mortar.
Coarse aggregate generally lies between a 4mm and a 20mm sieve and is typically nearly half the weight of the concrete. Builders merchants will generally sell it as 4/20mm coarse aggregate.
As its name implies, ‘All-in’ aggregate contains both coarse and fine aggregate (often described as ‘ballast’). However, for most concrete work it is preferable to use separate aggregates, mixed in the appropriate proportions.
Building or soft sand should not be used in concrete, as the grading is finer than concreting fine aggregate and leads to increased water demand. It is usually used for mortars and renders.
Acknowledgement: The Concrete Society