Concrete is made of many components but mostly made up of a material called coarse aggregates and they are one of the essential components of concrete and occupy large volumes in the mix.
Its size leads to lower water demand in the mixture and has a lower surface area as compared to the small coarse aggregate size and for normal strength concrete rounded shape aggregates provide economical mix design.
These aggregates are larger size filler materials in construction and these aggregates are the particles that retain on 4.75mm sieve. Broken bricks, broken stones, pebbles, clinkers, and cinders, etc are used as coarse aggregates in concrete.
Major sources of these aggregates are dolomite aggregate, crushed stone or gravel and natural disintegration of rock and surface area of coarse aggregates is less than fine aggregates and they are mostly used in concrete, railways track ballast, etc.
Maximum Aggregate Size:
To achieve certain workability and maximum size of aggregate the factor of maximum aggregate size controls water demand and reduces the surface area of coarse aggregate increases.
If the surface area is higher than water demand to coat the particles and generate workability is greater and for a given weight, higher the maximum size of aggregate and lower is the surface area of coarse aggregates.
To coat particles and maintain the cohesiveness of concrete mix smaller maximum size of coarse aggregate would require greater fine aggregate content. 40mm down aggregate would have lower water and cement ratio for the same workability thus higher strength when compared to 20mm down aggregate and advantage of the higher maximum size of these aggregates can be taken to lower cement consumption because of its lower water demand.
Between the reinforcement bars, the maximum size of aggregate is often restricted by the clear cover and minimum distance. To produce dense and homogenous concrete this allows aggregates to pass through the reinforcement in congested areas.
Grading Of Coarse Aggregates:
Determination of particle size distribution for aggregate is grading and it affects the amount of water requirement, cement, workability, durability and pumps ability of concrete. To get cohesive and dense concrete grading of these aggregates are important and by smaller coarse aggregate particles voids are filled which are left by larger coarse aggregate particles.
For higher workability and improving of compatibility of concrete proper grading of these aggregate can minimize the possibility of segregation.
Shape Of Coarse Aggregate:
The shape of these aggregate is round, angular or irregular and due to lower surface area rounded aggregate has the lowest water demand and also requires lower mortar paste.
To yield the most economical mixes of concrete grades up to M35 these properties make rounded aggregates and in favor of angular aggregate with more surface area the possibility of bond failure would tilt the balance for a concrete grade of M40.
The flexural strength of concrete reduces and water demand and the tendency of segregation increases in elongated coarse aggregate particles.
Strength Of Coarse Aggregates:
By crushing strength of rocks, aggregate crushing value, aggregate impact value and abrasion value the material strength of these aggregate is indicated.
For applying a correction factor for aggregates in dry condition and determining water demand for concrete in saturated surface dry condition aggregates absorption is used and by weight, the aggregate can absorb water up to 2% and it can be high as 5% in some cases.
Uses Of Coarse Aggregate:
There are the following most common uses
- Mix in concrete
- Preparation of moisture break under slab and vapor barrier
- Part of base preparation for roadways and driveways
- To facilitate drainage for perimeter drains and retaining walls
- Used to backfill retaining walls
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