Aggregates are inert granular components that give concrete its bulk. Their grain size is an important factor. According to their particle size, they are divided into fine and coarse aggregates.
Coarse aggregates: What are they?
Coarse aggregates are those that cannot pass through an IS Sieve of 4.75mm. They are gravel or crushed stone that occurs naturally and is used to make concrete. Their largest size is 63 mm and higher. Blasting the stone quarries is typically how coarse aggregates are created. However, they are occasionally also hand-crushed. Stones of varying sizes can be crushed together in the machinery. On the other hand, only stones of comparable sizes are utilised when crushing by hand. To create high-quality concrete, coarse aggregates are mixed in a specific ratio.
Natural river gravels and hard crushed rocks (granite and limestone) are the most widely used aggregates for structural concrete.
Foamed slag, shattered bricks, clinkers, etc., are also utilised as coarse aggregates in non-structural concrete.
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Coarse aggregate: Characteristics
Size
Depending on how the concrete will be used, the aggregate size should change.
| Use | Maximum Size of Coarse Aggregate |
| Non-reinforced work | 40-75 mm |
| RC Foundation work | 40 mm |
| RC works (beams, columns, slabs) | 20 mm |
| Shell roof and thin members | 10 mm |
The shape of the aggregate
The form of the aggregate plays a crucial role in determining how easily concrete may be worked. The most popular aggregate forms are angular, cubical, or spherical.
Water absorption
Water absorption will be impacted by the aggregate’s porosity. Generally speaking, it shouldn’t absorb more water than 5%. The more water absorbed, the worse it is for the concrete’s workability and durability.
Specific gravity
Materials with high specific gravity are strong and long-lasting. Soft and porous materials, however, have a low specific gravity. In general, coarse aggregates should have a specific gravity between 2.6 and 2.9.
Free of impurities
The aggregates’ binding strength is weakened by impurities. Before usage, impurities like dust, clay, and chemicals should be eliminated, as well as organic pollutants like leaves, twigs, and shells. Otherwise, they will reduce the bond between the aggregates and the cement or bitumen.
Durability
Extreme weather conditions must be tolerated by aggregates. They must thus be sturdy and impervious to physical and chemical forces like rain, storms, etc. Additionally, they are vulnerable to loads like cars. Their level of hardiness is a key factor in determining how long they can live.
Surface texture
The surface texture of the cement or bitumen affects how well the aggregate adheres to it. Aggregates that are rough and porous adhere to cement better. It offers concrete strong bonding strength.
Coarse aggregate: Uses
Provides volume to concrete: The concrete gains bulk from coarse aggregate, which also lowers the project’s overall cost.
Railway ballast: In railway ballast, coarse materials are employed to evenly disperse the load.
Base building: These are used as the foundation for the driveways, pavements, and roads. They assist a system for load transmission.
Purification of water: They serve as the filter in water filtration systems to remove impurities from water.
Drainage techniques: These are employed in the construction of septic leach fields, retaining walls, and perimeter drains.
Rainwater collection: In systems for collecting rainwater, they make up the top layer.
Road construction: These are extensively used while building roads. Additionally, they are utilised to build impromptu road surfaces, such as truck tyre knock areas.
Building resistance to moisture: These are used as vapour barriers in building slabs to keep moisture out.
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Coarse aggregate: Classification
One of the best techniques for distinguishing coarse particles is by form. It has an impact on the concrete’s workability, strength, durability, and adhesiveness. Here are the several categories for forms that the coarse aggregates fall under:
Rounded aggregates
Natural stones, boulders, and gravel that may be found on riverbeds are rounded aggregates. Natural weathering, erosion, and attrition are what give them their rounded and smooth form. Low water-to-cement ratios are ideal for these aggregates’ performance. Their design reduces the number of voids (32–33%) and improves workability. However, roundness leads to inadequate bond strength and poor interlocking behaviour.
Irregular aggregates
They have a little stronger connection than spherical aggregates. However, they are still inappropriate for high-strength concrete. They have a vacancy rate of 35–38%. In low or medium-strength road building projects, they are employed.
Angular aggregates
Aggregates that are angular have edges that are distinct and crisp. They are unworkable because they contain a maximum proportion of voids (38–45%). However, because of their outstanding interlocking and bonding properties, they are used in high-strength concrete. The low workability issue is remedied by utilising round or smaller aggregates for the voids. These were employed for the high-strength concreting of structures and bridges.
Flaky aggregates
If the thickness of an aggregate is less than 60% of its mean dimension, it is considered flaky. More cement slurry is needed for binding because of the uneven shape’s increased void production. They impair concrete’s ability to flow and have insufficient crushing strength. Their use in the construction sector has been constrained by these characteristics.
Elongated aggregates
If the thickness of an aggregate is less than 180% of its mean dimension, it is referred to as elongated. Elongated aggregates are not desirable in buildings, much like flaky aggregates. Their lower workability is caused by their increased void percentage.
Aggregates that are flaky and elongated
If an aggregate’s length is more than its breadth and its width is greater than its thickness, the aggregate is both flaky and elongated. Furthermore, mixing concrete with certain kinds of particles is not recommended. The production of such aggregates frequently arises from inadequate rock crushing.
FAQs
Why is coarse aggregate important in construction?
Coarse aggregate provides structural strength, durability and resistance to wear in construction materials, making it an essential component in the construction industry.
What are the different types of coarse aggregate?
The types of coarse aggregate include gravel, crushed stone, and slag, among others. They can be made from natural materials like rock or minerals, or they can be recycled materials like recycled concrete or asphalt.
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