What is grading of aggregates??
- Grading of aggregates is procedure to find out the particle size distribution of various aggregates.
- This method helps to determine the aggregates size and percentage used in construction works.
- Coarse aggregates are graded as, 80mm, 63mm, 40mm, 20mm, 10mm
- Fine aggregates are graded as. 10mm, 4.75mm, 2.36mm, 1.18mm. 600micron, 300micron, 150 micron
- Basically, grading of aggregate is to find out size of specific aggregates and its retention on specific sieve size.
Classification of aggregates based on grading:
Depending on the percentage of size of aggerates present, aggregates are also classified as,
- Uniformly Graded Aggregates
- Well graded Aggregates
- Gap Graded Aggregates
- Open graded
- Poorly graded
1. Uniformly Graded Aggregates:
- In this type of aggregates, aggregates has more prominent percentage of particular size of aggregates. Eg. Aggregates which has 16mm size of aggregates 80%, and remaining 20% is combination of rest sizes.
- In uniformly graded aggregates, there will be high chances to form a void, and to fill up this void cement paste requirement is very high. Due to this concrete can be more porous.
- And the changes of high permeability and low stability is there
2. Well graded Aggregates
- This consists almost all sizes of particles and due to this voids in concrete is almost filled by small size aggregates and this form a compacted mass.
- Due to less voids formations, this is less permeable and has high stability.
- This is the most used and preferred aggregates, as it suitable for every type of construction.
3. Gap Graded Aggregates:
- In this type of aggregates, a certain size of aggregate in missing (almost has zero percentage).
- This has moderate voids, and moderate permeability and low stability.
- This are generally used for the construction, where economical mix is required.
- This is also used when to improve aesthetic purpose.
4. Open graded:
- In this type, the smaller size aggregates are present.
- Due to this, air voids are more and permeability is also more.
5. Poorly graded
- In this type, the particular size of aggregate in much and other are very less.
- Due to this, the void formation is more, stability is less and permeability is more.
Grading of aggregates
- The grading of aggregates is mainly depend on particle size of aggregates.
- To find out the particle size distribution, sieve analysis is performed.
- Grading is very important factor , which influences the strength and durability of concrete. Also this is important factor for mix design of concrete.
- Before selection on sand/aggregates for construction, their sieve analysis has to be made, to find out the gradation of aggregates, and depend on result decision has to be made.
Following are the grading limits of aggregates,
Grading Limit for Coarse Aggregates: (Based on Clause 4.1 and 4.2 of IS: 383- 1970)
|IS Sieve||Percentage passing for single sized aggregates of nominal size(mm)|
|63 mm||40 mm||20 mm||16 mm||12.5 mm||10 mm|
|63 mm||85 – 100||100||–||–||–||–|
|40 mm||0 – 30||85 – 100||100||–||–||–|
|20 mm||0 – 5||0 – 20||85 – 100||100||–||–|
|16 mm||–||–||–||85 – 100||100||–|
|12.5 mm||–||–||–||–||85 – 100||100|
|10 mm||0 – 5||0 – 5||0 – 20||0 – 30||0 – 45||85 – 100|
|4.75 mm||–||–||0 – 5||0 – 5||0 – 10||0 – 20|
|2.36 mm||–||–||–||–||–||0 – 5|
Grading Limits for Fine Aggregates: (Based on Clause 4.3 of IS: 383 – 1970)
|IS Sieve Designation||Percentage Passing|
|Grading Zone I||Grading Zone II||Grading Zone III||Grading Zone IV|
|4.75 mm||90 – 100||90 – 100||90 – 100||95 – 100|
|2.36 mm||60 – 95||75 – 100||85 – 100||95 – 100|
|1.18 mm||30 – 70||55 – 90||75 – 100||90 – 100|
|600 microns||15 – 34||35 – 59||60 – 79||80 – 100|
|300 microns||5 – 20||8 – 30||12 – 40||15 – 50|
|150 microns||0 – 10||0 – 10||0 – 10||0 – 15|