Coarse Aggregate | Classification of Coarse Aggregate | 8 Requirements, Uses & Properties

Different materials are used in the construction industry. Among them, aggregate is one. Aggregate is the inert granular material that is used in the concrete as the filler material and provides volume to the concrete. There are two types of aggregates naming fine aggregate and coarse aggregate. We are discussing coarse aggregate in this article.

  1. Introduction  

Aggregates having a size of more than 4.75 mm are said to be coarse aggregate. 

In another word; aggregates retained on a 4.75 mm sieve are coarse aggregate.

Coarse aggregate provides the volume to the concrete. It is used as a filler material in concrete, roads, etc.

  2. Origin of Coarse Aggregate  

Coarse aggregate is mainly obtained from quarries following different processes like blasting, crushing, and sieving.

The properties of the aggregate are dependent upon the type of rock whether it’s sedimentary, Igneous, or metamorphic.

  3. Requirement of Coarse Aggregate  

The coarse aggregate which is utilized in the development must have the subsequent properties:

a. It must be long-lasting and durable.

b. It must be hard and powerful enough to resist loads.

c. It must be free from any type of dust or organic materials otherwise it’ll reduce the bonding capacity with binding materials like cement, bitumen, etc.

d. It mustn’t show a reaction with the cement after mixing.

e. It should not be soft and also should not be highly porous.

f. It mustn’t absorb water more than 5%.

g. It must be chemically inert.

h. It should have a cubical or spherical or angular shape as far as possible.

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  4. Uses of Coarse Aggregate  

The uses of coarse aggregate are:

1. It is used for providing volume to the concrete which reduces the cost of the project.

2. It helps to boost the crushing strength of concrete.

3. It is used in the railway ballast which helps in the uniform distribution of load.

4. It is used in the base course of the flexible pavement to support the granular load transfer mechanism.

5. It is also used in water purification plants as a part of a filter to separate water and unwanted particles.

6. It is also used in the upper layer of the rainwater harvesting system.

  5. Classification of Coarse Aggregates  

  5.1. Classification of Coarse Aggregates Based on Shape  

The shape is one of the foremost effective ways of differentiating aggregates.

The aggregate we choose depends on the workability and result we want from our concrete.

Aggregates bought in batches from a reputable supplier are often consistent in shape but we can mix aggregate of different shapes as per requirement and purpose.

The different shapes of aggregates are:

a. Rounded

Rounded aggregates are natural aggregates that are smoothed by weathering, erosion, and attrition.

Rocks, stones, and gravels found in riverbeds are usually rounded aggregates.

These aggregates result in the minimum amount of voids; so they give more workability.

They are not used in high-strength concrete because their round shape results in poor interlocking behavior and weak bond strength.

These aggregates require a low amount of water-cement ratio.

Use: Used mainly in road construction for filling voids between angular aggregates.

b. Irregular

Irregular aggregates are shaped by attrition but aren’t completely rounded. so, they are also known as partially rounded aggregates.

These aggregates have little high bond strength as compared to rounded but are not suitable to use in high strength concrete.

Use: In low strength or medium-strength concrete, road construction, etc.

c. Angular

Angular aggregates are used for higher strength concrete due to their good interlocking property and high bond strength.

The workability of angular coarse aggregate is low, but this problem is minimized by filling voids with rounded or smaller aggregates.

Use: In the preparation of high-strength concrete which is used in building construction, bridge construction, etc.

d. Flaky

An aggregate is said to be flaky if its least dimension is less than 3/5 (0.6) of the mean dimension.

The concrete should be flowable and pumpable but the use of flaky aggregates reduces the flowing capacity of concrete.

They lead to segregation in concrete and harshness of concrete.

Flaky aggregates also have very low crushing strength so should not be used in high-strength concrete and road construction.

e. Elongated

The aggregate is said to be elongated if its greater length is greater than 9/5th of its mean dimension.

Elongated aggregates also have very low crushing strength so should not be used in high-strength concrete and road construction.

  5.2. Classification of Coarse Aggregates Based on Natural or Artificial   Formation:  

Basically, coarse aggregates are often classified as either natural or artificial.

1. Natural Aggregates
These are the aggregates that are found from natural sources.

Natural aggregates are further divided into two categories as stated below:

(a) Gravel:

The main origin of gravel is river beds, stream deposits, etc.

These aggregates are formed by weathering of bedrock and subsequent transportation and deposition by water, ice, gravity, etc.

(b) Crushed Aggregates:

Crushed aggregates are obtained from the quarries. They are widely available in the market at cheap.

Crushed aggregates are small rock fragments that are subjected to mechanical processing such as crushing, washing, and sizing.

 2. Artificial Aggregates:
Artificial aggregates are used because they are environment-friendly materials. They’re manufactured from various pollutant by-products such as ash, power station solid waste, rice husk ash, furnace slag, granite powder, iron ore slag, over burnt brickbats, etc.

By using these industrial by-products, we can reduce environmental pollution and protect dwindling natural resources.

  5.3 Classification of Coarse Aggregates According to the Size  

  6. Properties of Coarse Aggregate  

Some properties of coarse aggregates are:

Coarse aggregates used in road construction should have the following properties:

a.Strength:

The aggregate to be used in road construction should be sufficiently strong to withstand crushing and several stresses due to traffic wheel load.

b.Hardness:

The aggregates used in the surface course are subjected to rubbing or abrasion. So, the aggregate should be hard enough to resist wear and tear due to the abrasive action of traffic.

c.Toughness:

Resistance to impact is termed toughness. Aggregates used in the pavement are also subjected to impact due to moving wheel loads and hammering action due to jumping of tires. Hence, toughness is another desirable property of coarse aggregate.

d.Soundness/Durability:

The property to withstand adverse action of weather or the ability to remain strong over a long period is called durability or soundness. Aggregates are subjected to the physical and chemical action of rain and groundwater. Hence, aggregate should be durable to withstand weathering action.

e.Shape of aggregate 

Aggregates may be of different shapes such as rounded, flaky, elongated, etc. Flaky and Elongated aggregates have lower strength. So, generally round and angular are preferred.

f.Adhesion with bitumen

Adhesion simply means a binding property with other materials. So, coarse aggregate should have proper or good binding properties with bitumen/asphalt/tar.

g. Free from foreign particles

Foreign materials like dust, chemicals, etc reduce the binding property of aggregate i.e. adhesion between coarse aggregate and bitumen is reduced. So, the material should be free from foreign materials.

  6.2. Properties of Aggregates Used in Concrete  

Aggregates used in the concrete should have the following properties:

a. Composition

Aggregate should be free from materials that react with alkalies in cement because it may lead to cracking, excessive expansion, and deterioration of concrete mix. So, several tests should be carried out to ensure the presence of such materials before use.

b. Size & Shape 

The size and shape of aggregates directly influence the quantity of cement required in concrete mix and hence ultimately the economy of concrete. The largest coarse aggregates should be given priority according to the feasibility of the structure. According to IS-456; the maximum size of aggregates that can be used in PCC and RCC are:

a.1/4 th of the minimum dimension of the concrete member.

b. 1/5 th of the minimum dimension of the reinforced concrete member.

c. Surface Texture 

Proper bonding of aggregate with concrete depends upon surface texture, surface roughness, and porosity. When the surface of aggregate is rough and porous, it provides maximum strength and good bonding to concrete. Cement paste in concrete set inside the pores of aggregate which provides very strong bonding.

d. Specific Gravity

The ratio of the weight of oven-dried aggregates maintained for 24 hours at a temperature of 100 to 110⁰C, to the weight of the equal volume of water displaced by saturated dry surface aggregate is known as the specific gravity of aggregates.

Lower specific gravity indicates porous and weak materials and high specific gravity indicates strong and durable materials. The value of specific gravity should lie between  2.6 to 2.9. Specific gravity also plays a vital role in the concrete mix design.

Table: Specific gravity of materials

e. Bulk Density

Bulk density is the weight of the coarse aggregate required to fill a container of unit volume. Its unit is kg/liter.

Bulk density of coarse aggregate depends upon:

a. The shape of aggregate

b. Grading of the aggregate

c. Degree of the compaction

Table: Value of bulk density

f. Voids

Tiny space between particles of aggregates is called void. Voids between aggregate particles should be less because excessive voids lead to porous concrete.

Table: Average Void Percentage

g. Impact Value  

Aggregate should be resistive towards shock and impact. Impact value gives the resistive value of aggregate towards crushing. A higher impact value indicates the higher toughness of coarse aggregate.

Table: Values for mechanical properties

h.  Abrasion Value

Coarse aggregate should be hard enough to resist abrasion. This decreases the chance of wear and tear in concrete.

i. Crushing Value

The crushing value gives the resistance of a coarse aggregate to crushing under the application of compressive load.

j. Deleterious Materials

Coarse aggregate should be free from deleterious materials such as clay, silt, dust, salt contamination, organic materials, etc. because these materials weaken the bond.

k. Surface Water 

Coarser aggregates carry less water. So coarser aggregates should be used rather than finer according to conditions.

Table: Surface water carried by aggregates

  7. Grading of Coarse Aggregate  

The process of determining the particle size distribution of coarse aggregate is referred to as the grading of coarse aggregate.

Sieve analysis is also commonly known as Grading.