Classification of Aggregates based on Grain Size, Shape, Specific Gravity, Geological Origin


Classification of Aggregates Based on Grain Size, Shape, Specific Gravity, Geological Origin

Topics covered in this article:-
  • Classification of aggregates based on: Geological Origin
  • Classification of aggregates based on: Grain Size
  • Classification of aggregates based on: Shape
  • Classification of aggregates based on: Unit Weight or Specific Gravity
  • Quality test of aggregates
  • Deleterious materials and impurity
  • Grading of aggregate

Aggregates are inert materials which are mixed with binding material such as cement or lime for manufacturing of mortar or concrete. Aggregates are used as filler in mortar and concrete and also to reduce their cost.

The Characteristic of an Ideal Aggregate :-
  • It should be hard, strong and durable.
  • It should be dense, clear and free from any coating.
  • It should not contain flaky (angular) and elongated pieces.
  • It should not contain any material liable to attack steel reinforcement in case of reinforced concrete.

Aggregates are classified by many ways as follows:

  1. Artificial (Synthetic) aggregates – The aggregates made by human. Also known as synthetic aggregates. 
  2. Natural aggregates – The aggregates made by nature.
Click here to know more about recycled (artificial) aggregates.

1.) Fine Aggregates –The aggregates ‘whose particles pass through 4.75 mm IS sieve are defined as fine aggregate. Most commonly used fine aggregates are sand (pit quarry sand, river sand and sea sand) and crushed stone in powdered form.
* The percentage of deleterious materials should not exceed 5%.

2.) Coarse Aggregates – The aggregates ‘whose particles do not pass through 4.75 mm IS sieve’ are defined as coarse aggregates. Most commonly used coarse aggregates are crushed stone, pebble, gravel, road metal, broken pieces of burnt bricks, etc.

Fine Aggregates

Coarse Aggregates

1.) Rounded Aggregates – The rounded aggregates have its shape because of attrition. They are generally available in the form of seashore gravel (beach pebbles). The percentage of voids in the rounded aggregates is lesser than other (32%–33%). Rounded aggregates give minimum ratio of surface area to given volume and therefore require minimum water for lubrication. Hence these aggregates give more workability for the given water cement ratio in less amount. The only disadvantages is that the interlocking between its particles is less and hence the development of bond is poor. This is why rounded aggregate is not suitable for high strength concrete and for pavements subjected to tension.
The smooth and rounded aggregates are excellent aggregates for concrete.

Example:- Desert, windblown and seashore sands, river or seashore gravels.
The smooth and rounded aggregates are excellent aggregates for concrete.

Example:- Desert, windblown and seashore sands, river or seashore gravels.
Rounded Aggregates

2.) Angular Aggregates –
Angular aggregates have higher specific surface area than smooth rounded aggregates. Angular aggregates exhibit a better interlocking effect in concrete that contributes in strength of concrete. Angular aggregates assure higher compressive strength than rounded aggregates in concrete because of the interlocking between the angular aggregates due to their higher specific surface area. The percentage of voids in the angular aggregates is minimum (38%–45%). It requires more water for lubrication and hence it gives least workability for the given water-cement ratio in great amount.
Example: Crushed rocks of all types; talus, screes.
Check here - Angularity number test of coarse aggregates
Angular Aggregates

3.) Irregular Aggregates Irregular or partly rounded aggregates are partly shaped by attrition. These are available in the form of pit sands and gravel. The percentage of voids in the irregular aggregates is almost 32% to 33%. These aggregates give lesser workability than rounded aggregates for the given water cement ratio.

Example: Pit sands, cuboid rocks and gravels; land and dug flints.
4.) Flaky Aggregates – The aggregate is said to be flaky when the least dimension of aggregate is less than 3/5th (or 60%) of its mean dimension. Mean dimension is the average size through which the particles pass and the sieve size on which these are retained. For example, mean size of the particles passing through 20 mm sieve and retained on 16 mm sieve is (20+16)/2=18 mm. If the least dimension is less than 3/5 x (18) = 10.8 mm, then the material is classified as flaky. The thickness of flaky aggregate is small when compared with width and length of that aggregate.  Flaky aggregate tends to be oriented in one plane which affects the durability.
Example: Laminated Rocks, both naturally occurring as well as crushed.

5.) Elongated Aggregate – The aggregate is said to be elongated when its length is greater than 9/5th (180%) of its mean dimension. The particles is said to be elongated when its greatest dimension (length) is greater than nine-fifth (180%) of its mean dimension. The mean dimension is mentioned above.
6.) Flaky & Elongated Aggregate – The aggregate is said to be flaky and elongated when it satisfies both the above conditions. Generally elongated or flaky particles in excess of 10 to 15% are not desirable.

7.) Cubical Aggregates – The most of angular aggregates whose particles have length, breadth, and thickness approximately equal are said to be cubical aggregates.

  1. Light Weight Aggregates (LWA) – Lightweight aggregate (sometimes is called as low density aggregate) is a generic name of a group of aggregates for structural concrete, mortar, or plaster that weighs less than usual rock aggregates including natural sand, gravel, and crushed stones. Its bulk density is lower than that of common construction aggregate. Most lightweight aggregate is produced from materials such as clay, shale or slate, blast furnace slag, natural pumice, vermiculite, and perlite can be used as substitutes, however. To produce lightweight aggregate, the raw material (excluding pumice) is expanded to about twice the original volume of the raw material. Lightweight aggregates have been produced with pelletisation and thermal treatment of SSA. Concrete made with light weight aggregate has density less than 1100 kg/m³.
  2. Heavy Weight Aggregate (HWA) – Heavy weight aggregates is an aggregate that possess relatively high density and it is fundamental in a situation that high density concrete is necessary. Concrete made with heavy weight aggregate has density more than 2,080 kg/m³ and can range up to 4,485 kg/m³. Heavy weight aggregate is most commonly used for radiation shielding, counterweights and other applications where a high mass-to-volume ratio is desired. ASTM C637 covers aggregates used for radiation shielding.
  3. Normal Weight Aggregate (NWA) – Normal weight aggregate is an aggregate having characteristics between those of a lightweight aggregate and a heavy weight aggregate. Concrete made with normal weight aggregate has density in the range of between 1280 kg/m³–1920 kg/m³. Normal weight aggregates are obtained by draining riverbeds or mining and crunching formational material.

It’s actually a thin film of water around the sand grains and interlocking of air in between the sand grains and film of water.

  1. It affects the concrete mixture, its proportion, its strength and many such other aspects.
  2. Cracks develop in structure.
  3. Reduce the load bearing capacity, it’s strength and it’s compatibility.

Soundness of aggregate :-
  1. Soundness is the ability of aggregate to resist change of volume due to change of physical condition.
  2. These physical condition include: freezing and thawing, temperature change, alternative change of drying and wetting in normal condition and alternative change of drying and wetting in salt water.

Deleterious materialS and impurity:-
  1. Substances, who effect harmful physically or chemically on the aggregates are considered to be deleterious.
  2. Commonly deleterious materials are organic matters, clay, shale, coal iron pyrites, etc. which are weak, soft and fine.
  3. They affect the properties of concrete in fresh as well as in hardened state and are undesirable.
  4. They may be classified as those interfering with the process of hydration i.e. organic matters, coatings such as clay, etc. affecting the development of bond between aggregate and cement paste.
  5. The surface coating impurities in aggregate can be removed by adequate washing.

Grading of aggregate:-
  1. The particle-size distribution of an aggregate as determined by a sieve is termed as Grading of aggregate.
  2. The proper grading of an aggregate must be done so that the workability, density & volume stability of concrete may not be adversely affected by it.
  3. In good concrete all aggregate particles must be covered by cement paste.

Advantage of Grading:-
  1. It increases the economy.
  2. It increases the strength of concrete.
  3. It increases the durability of concrete.
  4. It also lower the shrinkage of concrete.

Types of Gradations:-
Well graded, poorly graded & gap graded.


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