In practice, there is no maximum size for the aggregate used in concrete slabs. However, for structural applications, the highest size typically used is 40mm, unless a mass concreting project such as a dam is being built. In this case, the maximum size of coarse-grained aggregate can reach up to 150 mm. When analyzing aggregates, they are often classified by fine and coarse-grained materials.
This provides an estimate of how the concrete mix will behave and helps identify any deficiencies in the aggregates. Alternative sources of aggregates or additional aggregate mixtures can be considered to approximate the ideal gradation that provides the best workability, pumpability, shrinkage reduction and economy. Removing sand from a mix transforms conventional concrete into a fineless mix, also known as permeable concrete. This type of concrete qualifies for LEED points, a green building rating system developed by the US Green Building Council (USGBC), as it allows runoff to seep directly into the subgrade and recharge the water table.
To address excessive particle size segregation, batching is usually preceded by reworking the stacks around the circumference to remix the sizes, rather than working directly on the stack. The maximum aggregate size depends on the laying method and slab thickness. The size distribution of fine to coarse-grained aggregates plays an important role in concrete workability and performance. Both gravel and crushed stone produce quality concrete; however, gravel will have lower water demand compared to crushed stone.
Gravel is preferred for concrete with exposed aggregate in walkways and decorative applications, while crushed stone is preferred in pavement mixes due to its higher paste-aggregate bond which produces higher flexural strengths. The moisture content of the aggregate also plays a role in the workability of concrete. If the aggregates are too dry, they will absorb (steal) the water from the mixture; if they are too humid, excess moisture should be subtracted from the expected amount of mixing water. In massive concrete or PCC works, larger aggregates are used due to their lower consumption of cement which reduces heat of hydration and thermal stresses and shrinkage cracks.
Up to 20 mm of aggregate is used in RCC structures due to its ease of pouring concrete due to increased workability. It is important to ensure that good quality aggregates are purchased as verified by regular aggregate test results in accordance with ASTM C 33, Standard Specifications for Concrete Aggregates. Good quality aggregate must be clean, hard, strong, have durable particles and be free of absorbed harmful chemicals, clay coatings or other contaminants that may affect cement hydration or reduce paste-aggregate bonding.
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