(3) Main types of aggregates classification | Based on size, shape and geological origin

What are aggregates?

Aggregates are the important constituents in the making of concrete. The concrete becomes an artificial stone when relatively inert fine and coarse materials get a binding with cement↗ paste. The aggregates give the body to the concrete, reduce shrinkage, and make it economical. Generally, aggregates occupy 70 - 80% of the volume of concrete. It is therefore significantly important to obtain the right type am quality of aggregates at the site. Unlike cement, aggregates are not branded in our country and hence the suppliers and manufacturers seldom bother to give the required quality or specifications.

Aggregates influence the properties of concrete/mortar such as water requirement, cohesiveness, and workability of the concrete in the plastic stage. While they influence strength, density, durability, permeability, surface finish, and color in the hardened stage.

Requirements good properties of  Aggregate in concrete

Good aggregate should meet following requirements if the concrete is to be workable, strong, durable and economical

• It should be free from coatings of clay or other materials.
• It should have a rough surface texture.
• It should be sufficiently hard.
• It should be chemically inert.
• It should be strong enough.
• It should not absorb water beyond permissible limits.
• It should possess chemical stability.
• It should exhibit abrasion resisting property.
• It should exhibit resistance to freezing and thawing.

Main types of Aggregates Classification 

The aggregates are generally classified in the following three ways are the following given below with explanation.

1. According to the geological origin
2. According to size
3. According to shape

1. Classification according to the geological origin

The aggregates are usually derived from natural sources and may have been naturally reduced red size by crushing. According to the source or nature of the formation, the aggregates are classified into two types are the following given below.

• Natural aggregates 
• Artificial or processed aggregates.

(a) Natural aggregates

These are naturally occurring aggregates obtained from natural deposits of sand and gravel or from quarries by cutting rocks. The suitability of the locally available aggregate depends upon the geological history of the region. 

The cheapest naturally occurring aggregates are natural sand and gravels, which have been reduced to their present size by natural weathering agencies like water, wind, snow, etc. These aggregates are obtained from the beds of deep rivers but mostly they are not clean enough or well-graded to suit the quality requirements. They are likely to have silt or clay coatings. These should be washed and sieve before use, especially in RCC works.

The other, most commonly used source of naturally occurring aggregate is the quarried bedrock material. Crushed aggregates are obtained by breaking rocks by blasting, crushing, and sieving (screening), etc. 

Uses - These are mostly used for preparing plain and reinforced cement concrete to be used for all engineering works.

(b) Artificial aggregates

These are artificially manufactured or processed aggregates. The most widely used artificial coarse aggregates are cleanly broken bricks and air-cooled fresh blast furnace slag. The artificial fine aggregates may be sawdust, fly ash, etc.

If the crushing strength of the brick is 30-35 N/mm2, then it is suitable for use in mass concrete works but is not suitable for RCC works. The blast furnace slag is a by-product obtained simultaneously with pig iron in the blast furnace, which when comes in contact with air gets cooled slowly.

Uses - Following are the uses of Artificial Aggregates are the given below.

• Broken bricks are used as coarse aggregates in lean concretes.
• Broken bricks are also used in mass concrete works.
• Fire-resistant property of the concrete can be enhanced by using blast furnace slag.

Also read :-  Define Grading of Aggregate | Types, effects and their grading limits

2. Classification according to size

The size of aggregates used in RCC works may range from few centimeters and down to a few microns. The largest maximum size of aggregate practicable to handle under a given set of conditions should be used. Using large-sized aggregate results in

• Reduction of the cement content
• Reduction in water requirement
• Reduction in drying shrinkage.

However, the maximum size of aggregate to be used in a particular condition is governed by the following factors 

• Thickness of section
• Clear cover
• The clear spacing of reinforcement
• Mixing, handling, and placing techniques.

Mostly, the maximum size of aggregate should be as large as pe specified but should not exceed one-fourth of the minimum thickness of the member.

According to size, the aggregates are classified into the following three categories are given below.

1. Fine Aggregates 
2. Coarse Aggregates 
3. All-in Aggregates.

1. Fine Aggregates

The aggregates, most of which pass through 4.75 mm IS sieve and retained on 75 microns (i.e., 0.075 mm) IS sieve are termed as fine aggregates. The lower size limit of sand is about 0.07 mm. Material having a particle size between 0.06 mm and 0.002 mm is Classified as silt and particle size less than 0.002 mm are termed as clay. The sort deposits when consists of sand, silt, and clay in equal proportions are known as loam. 

The fine aggregates are helpful in filling up the voids of coarse aggregates. The fine aggregates may be natural sand or crushed stone sand or crushed gravel sand. According to size, the fine aggregates may be described as coarse, medium, and fine sands. As per IS: 383 -1970, the fine aggregates are divided into four grading zones.

2. Coarse aggregate

The aggregates, are most of which pass through 75 mm IS sieve and retained on 4.75 mm IS sieve is known as mainly coarse aggregates. The maximum and minimum size of coarse aggregate is 75 mm and 4.75 mm respectively. The coarse aggregate may be crushed gravels or uncrushed gravel or partially crushed gravel.

3. All-in-Aggregates

Naturally available aggregates comprising of different sizes and proportions of fine and coarse aggregate are termed as all-in aggregates. In such cases, adjustments for various deficient gratings may be added. These type aggregates are not generally used for making high-quality concrete.

3.Classification according to shape

The shape of aggregates is an important characteristic because they influence the properties of fresh concrete-like workability, etc. The shape of the aggregate is very much influenced by the type of crusher and reduction ratio i.e., the size of material fed into crusher to the size of its finished product. According to the shape, the aggregates are classified into the following types given below.

1. Rounded Aggregates
2. Irregular Aggregates
3. Angular Aggregates
4. Flaky and Elongated Aggregates

1. Rounded aggregate

These are fully water-worn or completely shaped by attrition e.g., River or seashore gravels, desert, seashore, and windblown sands. Rounded aggregates have minimum voids ranging from 32 to 33%. These aggregates are smooth textured and have good workability but have poor bond due to less interlocking. Therefore, rounded aggregates are not suitable for high strength concrete and pavements.


2. Irregular or partly rounded aggregates

These types of aggregates are naturally irregular or partly rounded by attrition e.g., Pit sands and gravels, land or dug flints, cuboid rocks, etc. Irregular aggregates have a higher percentage of voids ranging from 35 to 38%. Such type of particles requires more cement paste for a given workability. It gives less workability as compared to rounded aggregate for a given water-cement ratio. The interlocking between particles, though better than rounded aggregates still cannot be used for high strength concrete.


3. Angular aggregates

The aggregates possessing well-defined edges formed at the intersection of roughly planar faces are termed as angular aggregates (e.g., crushed rocks of all types). These types of aggregates have a maximum percentage of voids ranging from 38 to 40%. Angular aggregates are superior to rounded aggregates from the following two-point of view are the following given below.

• These aggregates exhibit a better interlocking effect in concrete, hence can be used for roads and pavements.
• The total surface area of rough-textured angular aggregate is more than smooth rounded aggregate for a given volume. If the surface area is greater, then the angular aggregate may show higher bond strength than rounded aggregates. But such aggregates requires more water for given workability than rounded aggregates.

Angular aggregates are best suitable for high strength concrete.

4. Flaky and Elongated Aggregates

(a) Flaky aggregates

The aggregate is termed as flaky when its least dimension (thickness) is less than 3/5 times its mean dimension. The mean dimension of the aggregate is the average of the sieve sizes through which the particles pass and retained respectively.

(b) Elongated aggregates

The aggregates are said to be elongated when its greatest dimensions (length) is greater than 9/5 its mean dimension. then the aggregates are termed as elongated aggregates. 

The angularity of aggregates generally affects the workability of the mix which is dependent on the interlocking of particles. Both elongated and flaky particles have adverse effects on the durability of concrete because they tend to be oriented in one plane during compaction.

For the good quality of concrete, the flakiness index and elongation index in aggregates should not be more than 10 - 15%.

Also read :-  

1. What is Hot Weather Concreting and define effects, defects, Precautions
2. What is Cold weather concreting and effects, defects and precaution during concreting
3. Main Physical and Chemical Properties of (OPC) cement