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 a slab for a concrete pavement or floor. Larger aggregate sizes also increase aggregate interlocking and therefore load transfer across the slab. In areas where yard space is limited and storage piles are difficult to keep separate, barriers should be used to prevent mixing and cross-contamination of materials from different sizes. The digital image analysis method has the potential to estimate the characteristics, that is, size and shape of aggregates quickly and accurately. Using nominal maximum aggregate size avoids any problems with very small amounts of slightly oversized aggregates that clash the entire classification of coarse-grained aggregates in a higher band, which is not really appropriate.
BS EN 13877-1 limits the maximum aggregate size to one-third of the slab depth, which is consistent with U. S. standards and general practice around the world, although some agencies may have slightly different restrictions. Changes in gradation, maximum size, unit weight, and moisture content can alter the character and performance of a concrete mix. The definition of maximum aggregate size in concrete is the smallest sieve that 100% of the aggregates will pass through.
For this reason, it's important to choose the largest available at a reasonable cost. In applications where flexural strength of concrete is critical to design, a smaller aggregate size may be more appropriate despite mechanical performance benefits of larger aggregates. BS EN 13877-1 limits the maximum size of concrete aggregate to one third of space between reinforcing bars. For concrete pavements, maximum size of coarse-grained aggregate should be small enough to be suspended within fresh concrete mix without breaking surface or impeding compaction. It may be necessary to select overall size to accommodate this rate or accept that service life can be reduced if lower rate is used. In contracts, construction or purchase agreements, term “maximum aggregate size” refers to smallest sieve that 100% of your aggregate can pass through.
This is more onerous restriction than U. standards that specify maximum aggregate size of ¾ of minimum spacing.