Coarse aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. They are obtained by natural disintegration or by artificial crushing of rocks. Gravel, cobble and boulders come under this category. The maximum size of aggregate can be 80 mm. In general, 40mm size aggregate used for plain cement concrete (PCC) and 20mm size is used for reinforced cement concrete (RCC).

The size is governed by the thickness of section, spacing of reinforcement, clear cover, mixing, handling and placing methods. For economy the maximum size should be as large as possible but not more than one-fourth of the minimum thickness of the member. For reinforced sections the maximum size should be at least 5 mm less than the clear spacing between the reinforcement and also at least 5 mm less than the clear cover. Aggregate more than 20 mm size is seldom used for reinforced cement concrete structural members. The size range of various coarse aggregates is given below.

Table 1: Size range of coarse aggregate

Coarse aggregate	Size variation (mm)
Fine gravel	4 – 8
Medium gravel	8 – 16
Coarse gravel	16 – 64
Cobbles	64 – 256
Boulders	>256

Classification of Aggregates as per Shape

The shape is one of the most effective ways of differentiating aggregates. Aggregate is derived from naturally occurring rocks by blasting or crushing etc., so, it is difficult to attain required shape of aggregate. But, the shape of aggregate will affect the workability of concrete. So, we should take care about the shape of aggregate. Aggregates are classified according to shape into the following types.

1) Rounded Aggregate

The rounded aggregates are completely shaped by attrition (the resistance of a granular material to wear) and available in the form of seashore gravel. Rounded aggregates result in the minimum percentage of voids (32 – 33%) hence gives more workability. They require a lesser amount of water-cement ratio. They are not considered for high-strength concrete because of poor interlocking behavior and weak bond strength. It is used mainly in road construction for filling voids between angular aggregates.

Example: River or seashore gravels, desert seashore and windblown sands

Fig. 1 Rounded Aggregate

2) Irregular Aggregates/ Partially Rounded Aggregates

The irregular or partly rounded aggregates are partly shaped by attrition and these are available in the form of pit sands and gravel. Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete. It is used in low strength or medium-strength concrete, road construction etc.

Example: Pit sands and gravels, land or dug flints, cuboid rock

Fig. 2 Irregular Aggregate

3) Angular Aggregates

The angular aggregates consist of well defined edges formed at the intersection of roughly planar surfaces and these are obtained by crushing the rocks. Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability but this problem is minimized by filling voids with rounded or smaller aggregates. They give 10-20% more compressive strength due to development of stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.

Example: Crushed rocks of all types; talus; screes

Fig. 3 Angular Aggregate

4) Flaky Aggregates

When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate, or on the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate or an aggregate is said to be flaky if its least dimension is less than 3/5 (0.6) of the mean dimension. 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.

Example: Laminated rocks

Fig. 4 Flaky Aggregate

5) Elongated Aggregates

When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension or 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.

Fig. 5 Elongated Aggregate

6) Flaky and Elongated Aggregates

When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. Flaky, elongated, flaky and elongated aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks.

Fig. 6 Flaky and Elongated Aggregate

Classification of Coarse Aggregates Based on Natural or Artificial Formation

Basically, coarse aggregates are 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. 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 in case environment-friendly materials. They are 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 natural resources.