The size of coarse-grained aggregates is a key factor in determining the strength of concrete. Generally, smaller coarse-grained aggregates are used for stronger concrete, with 20mm aggregates meeting the threshold for strong concrete and 40mm aggregates for normal strength concrete. Fine aggregates are used to fill the voids of coarse-grained aggregates, so the smaller the coarse-grained aggregates, the finer the fine aggregates should be. This increases the workability of concrete. The optimum maximum aggregate size should be determined by both the desired properties of fresh and hardened concrete and physical limitations.
Physical limitations are usually the gaps between reinforcing steel bars or simply the depth of the slab for a concrete pavement or floor. The nominal aggregate size is usually taken as the next screen size below the maximum aggregate size. To prevent mixing and cross-contamination of materials of different sizes, barriers should be used in areas where yard space is limited and storage piles are difficult to keep separate.BS EN 13877-1 limits the maximum size of concrete aggregate to one third of the space between reinforcing bars. This is also consistent with U.
S. standards and general practice around the world, although some agencies may have slightly different restrictions. For concrete pavements, the maximum size of coarse-grained aggregate should be small enough to be suspended within the fresh concrete mix without breaking the surface or impeding compaction. In addition, larger aggregates have a number of benefits for mechanical performance of concrete. There is also evidence that pavements constructed with smaller coarse-sized aggregates generally achieve more uniform flexural strength than those constructed with very large coarse-grained aggregates. It may be necessary to select an overall size to accommodate this rate or accept that service life can be reduced if a lower rate is used.
Changes in gradation, maximum size, unit weight, and moisture content can alter the character and performance of a concrete mix. To sum up, the definition of maximum aggregate size in concrete is the smallest sieve that 100% of aggregates will pass through. This should be taken into consideration when determining optimum aggregate size for concrete.