Aggregate is an essential component of concrete used in construction work. It is a granular material, such as gravel, crushed stone, or sand, used in the mixture of concrete to provide bulk and strength to the finished product.
The size of the coarse aggregate used in the concrete mix is crucial, as it determines the overall strength and durability of the finished structure.
Factors affecting the nominal maximum size of coarse aggregate
The size of coarse aggregate is determined by several factors, some of which are as follows:
- Workability: If the aggregate is too large, it can lead to poor workability, making it difficult to place and compact concrete.
- Durability: The use of large aggregates can result in higher porosity, leading to lower durability and strength of the concrete.
- Reinforcement: The nominal maximum size of coarse aggregate should allow the concrete to surround all reinforcement thoroughly and fill the corners of the form. If the aggregate is too large, it can lead to inadequate compaction and the formation of voids around the reinforcement.
Suitable nominal maximum size of coarse aggregate
For most concrete construction work, a 20 mm nominal maximum size of coarse aggregate is suitable.
This size allows for adequate workability, durability, and strength of the concrete. However, in some cases, larger or smaller sizes may be used.
Using larger sizes of aggregate
In cases where there is no restriction to the flow of concrete into sections, a larger nominal maximum size of coarse aggregate can be permitted.
In particular, a 40 mm or larger size can be used. This is because larger aggregates can lead to several benefits, including
- Reduced cement content: By using larger aggregates, the cement content can be reduced, resulting in cost savings.
- Reduced thermal cracking: The use of larger aggregates can help reduce the risk of thermal cracking, which can occur when concrete is exposed to high temperatures.
- Reduced drying shrinkage: The use of larger aggregates can also help reduce the risk of drying shrinkage, which can occur when the concrete dries out and contracts.
Using a small size aggregate
In some cases, a smaller nominal maximum size of coarse aggregate may be required. This is particularly true for concrete elements with thin sections, closely spaced reinforcement, or small covers.
In such cases, consideration should be given to the use of a 10 mm nominal maximum size.
This is because smaller aggregates can lead to several benefits, including
- Improved workability: By using smaller aggregates, the workability of the concrete can be improved, making it easier to place and compact.
- Improved compaction: Smaller aggregates can also help improve compaction, ensuring that the concrete can surround all reinforcement thoroughly and fill the corners of the form.
- Better durability: By using smaller aggregates, the concrete can achieve better durability, as the smaller size ensures better packing and less porosity.
Using plums in plain concrete work: Guidelines and limitations
IS 456:2000 allow the use of plums in plain concrete work, as long as they are above 160 mm in size and not more than 20 percent of the concrete volume. The plums can be of any reasonable size.
Guidelines for using plums in concrete construction
1. Permission from the engineer-in-charge
The use of plums in concrete construction must be specifically permitted by the engineer in charge. This is to ensure that the concrete will meet the desired strength, durability, and quality.
2. Maximum limit of 20% by volume
The maximum limit of plums that can be used in plain concrete work is 20% by volume of concrete. This is to ensure that the concrete does not become too heavy or too difficult to place and compact.
3. Even distribution
The plums must be distributed evenly throughout the concrete mix to ensure that the concrete has consistent strength and durability.
4. Minimum distance from the surface
The plums must not be closer than 150 mm from the surface. This is to ensure that the plums do not cause surface defects or weaken the concrete at the surface.
Limitations of using plums in concrete construction
1. Not suitable for reinforced concrete
Plums are not suitable for reinforced concrete construction as they can cause honeycombing and voids around the reinforcement.
2. Increased risk of cracking
The irregular shape and size of plums can increase the risk of cracking in concrete, particularly if they are not distributed evenly.
3. Reduced workability
The use of plums in concrete can reduce the workability of the concrete, making it more difficult to place and compact.
Aggregate size limitations for heavily reinforced concrete members
- When making heavily reinforced concrete members, like ribs of main beams, it is important to limit the size of the aggregate.
- The nominal maximum size of the aggregate should be no more than 5mm less than the minimum clear distance between the main bars.
- Alternatively, it should be no more than 5mm less than the minimum cover to the reinforcement, whichever is smaller.
Importance of aggregate size limitations for heavily reinforced members
1. Prevents voids and honeycombing
Using larger size aggregate in heavily reinforced members can cause voids and honeycombing around the reinforcement due to difficulty in placing the concrete mix.
2. Improves workability
Smaller size aggregate can help improve the workability of the concrete mix in heavily reinforced members, making it easier to place and compact.
3. Enhances durability
Using the appropriate size of aggregate can enhance the durability of the concrete structure by minimizing the risk of cracks and increasing the resistance to environmental factors like freeze-thaw cycles and chemical attacks.
Factors to consider when selecting an aggregate size for heavily reinforced members
1. Minimum clear distance between the main bars
This is the minimum distance between the main reinforcing bars that must be maintained to ensure proper concrete placement and avoid voids and honeycombing.
2. Minimum cover to reinforcement
This is the minimum thickness of concrete cover required to protect the reinforcing steel from corrosion and ensure adequate structural performance.
Aggregate size for different concreting work and structural elements
| Element | Minimum Size of Aggregate (mm) | Maximum Size of Aggregate (mm) |
| Footing | 20 | 40 |
| Column | 20 | 40 |
| Beam | 20 | 40 |
| Slab | 20 | 40 |
| PCC | 10 | 20 |
| RCC | 20 | 40 |
| Heavily reinforced concrete members | 5 | 5 |
