11 Important Points to Remember For Civil Engineer About Coarse Aggregates.

In this article, I have explained 11 Important points About Coarse Aggregate. It comprises of Definition of Aggregates, Sieve used for Aggregates, Fineness Modulus, Specific Aggregates, Types, and Field Test of Coarse Aggregates.

The following are the 11 Important Points to Remember for Civil Engineer about Aggregates.

Definition of Aggregates.

Coarse Aggregates are the particles which are retained on 4.75 mm sieve. Coarse Aggregates are larger in size as compared to fine aggregates. It obtained by the processing of igneous rocks such as pumice, scoria, and tuff. Some of the most common natural aggregates are silica sand, gravel, crushed lightweight stone, etc. It is used to produce concrete and any other stonework.

IS Sieve used for Grading of Aggregates

The I.S. sieve used for grading of Aggregates is different sizes such as 80 mm, 63 mm, 40 mm, 20 mm, 16 mm, 12.5 mm, 10 mm, 4.75 mm, 2.36 mm.

Size of Coarse aggregate used for Concreting

The Size of Coarse aggregates ranges from 4.5 mm to 80 mm. The sizes of aggregates depend on the grade of concrete is to be made. For normal house or Residential building, 10 to 20 mm aggregates sizes are used. The angular shape of the aggregate is best for concrete making. The other sizes of aggregates used for concrete making are 40mm, 20mm, 12.5 mm and 10mm, etc.

Fineness Modulus

Fineness modulus is obtained by adding the percentage of the weight of material retained on each of the prescribed I.S. sieves and divided by 100.

For Coarse Particles SizesFineness Modulus  
20mm6 to 6.9
40mm6.9 to 7.5
75mm7.5 to 8

Specific Gravity

The specific aggregates particles ranges from 2.55 to 2.65.

Qualities of Good aggregates that are suitable for construction Work

  • Aggregate should be free from silt, Lumps, Clay, and organic impurities like Vegetation, grass, etc. which affects the setting, strength, and durability of concrete.
  • It should not absorb water more the 10% of their weight after 24 hours immersion in water.
  • Flakey and Elongated aggregate is not used generally but it can be used in very less amount.
  • It is best for concrete making or the aggregates obtained from the river.
  • Angular aggregates are best for Concrete preparation than the rounded aggregates because of the following reasons
  • Angular aggregates possess a high surface area with rough surface higher bond strength with aggregate than the rounded aggregate.

Types of Coarse Aggregates

On the Basis of Shape

Angular Aggregate

Angular particles have sharp, angular and rough and have High Surface area. Due to the rough surface, It has a strong bond between the surface of angular Aggregate and cement paste. It requires more water due to rough surface. It has a high W/C ratio. Coarse Aggregate has low strength as compared to Rounded aggregates. It is good in workability.

Flaky Aggregate

Flaky aggregates are one of the types of aggregate which is flat and elongated in shape. Elongated aggregates are those aggregate whose length is 1.8 times its mean dimension.

Rounded Aggregates

These aggregates are generally found in rivers and seashore. It has a smooth texture. Less surface area. Due to the smooth surface, it gives poor concrete because due to the lack of bonding between the surface aggregate and cement paste. It requires less water due to the smooth surface. It has a low W/c ratio. Due to the low W/C ratio results in higher strength as compared to angular aggregates.

On the Basis of unit weight.

  • Light weight coarse Aggregates whose unit weight 12 KN/m3
  • Normal weight coarse Aggregate whose unit weight is 12 KN/m3
  • Heavy weight aggregate whose unit weight is 20 KN/m3

Characteristics of Aggregates

  • Bond strength.
  • Shape and texture.
  • Moisture content.
  • Bulk density.
  • Specific gravity.
  • Fine modulus.
  • Soundness.

Factors affecting Alkali Aggregates

  • High Alkali content cement.
  • Reactive type of aggregates.
  • Availability of moisture content.
  • Temperature condition.

Laboratory Test of Aggregate

  • Crushing strength.
  • Water Absorption.
  • Aggregate Impact value (AIV) test.
  •  Specific gravity.
  •  Sieve analysis.
  • Fineness modulus .

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