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 constructed. In this case, the maximum size of coarse-grained aggregate can reach up to 150 mm. When analyzing aggregates, they are often classified as 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. Permeable concrete is a fineless mix, also known as permeable concrete, which allows runoff to seep directly into the subgrade and recharges the water table. This type of concrete qualifies for LEED points, a green building rating system developed by the U.
S. Green Building Council (USGBC). 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 recommended maximum size of the aggregate to traverse the clearance between the reinforcing steel and the roof dimension should also be taken into account.
The maximum aggregate size depends on 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 since it 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. For reinforced concrete slabs such as bridge decks, roof slabs and columns, and prestressed concrete elements, 16 — 20 mm is usually considered as MSA (maximum aggregate size). Optimized grading based on aggregate availability and project requirements will result in an economical concrete with good workability and finishability. 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. For concrete dams: 75 mm near the face where steel reinforcement is provided and 150 — 200 mm MSA in the rest of the dam. When sieved through a size of 4.75 mm, retained aggregate is known as coarse-grained aggregate.Ensure your concrete producer purchases good quality aggregates as verified by regular aggregate test results in accordance with ASTM C 33 Standard Specifications for Concrete Aggregates. Up to 20 mm of aggregate is used in RCC structures due to increased workability when pouring concrete.
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