Aggregates: Properties of Good Quality of Coarse Aggregates

Aggregate is one of the most important ingredients of Concrete. Aggregate occupy 70-80 % of the total volume of the concrete. Coarse Aggregate is a chemically inert material. Let us go in deep to study the details of the aggregate properties. In this article, I have covered all the important properties which play a major role in gaining strength.

Properties of Good Quality of Aggregate

  • Size.
  • Shape.
  • Texture.
  • Strength.
  • Specific gravity.
  • Absorption and Moisture content.
  • The soundness of aggregates.
  • Cleanliness.
  • Grading.

Let us study all the above properties in deep.

Read also:Sand: Types of Sand, Properties of Good Quality of Sand

Size

Aggregates are classified into two categories

Coarse Aggregate

The material which is retained on 4.75 mm of I.S. Sieve is known as Coarse aggregate.

Fine Aggregate

The material Retained on the 75 microns I.S. Sieve but passing through 4.75 mm I.S. sieve.

The maximum size of the aggregates used in the concrete making is 80 mm. using the Maximum size of the aggregate lead in the reduction of cement content, reduction of drying shrinkage, and reduction in water requirement. For reinforced concrete work, aggregate having a maximum size of 20 mm is generally considered satisfactory.

Shape of Coarse Aggregate

According to the shape, Aggregate can be classified into three  types:

Rounded Aggregate

Rounded aggregate means the aggregate which is round in shape. This aggregate has a smooth surface. Round aggregate found in river or seashore gravel, desert, and windblown sand.

Flaky Aggregate

Flaky aggregate means the aggregate which is flat and thin with respect to their width and length. This aggregate has low workability due to its shape. This aggregate generally found in laminated rocks. The flaky aggregate has a small thickness relative to the width and length.

Angular Aggregate

Angular Aggregate has a rough texture and has a high specific area then rounded aggregate.

Difference between Rounded Aggregate and Angular Aggregates

S.NO.Rounded AggregatesAngular Aggregates
    1.It has smooth texture.It has rough texture.
    2.Less surface area.High Surface area.
    3.Due to smooth surface ,it give poor concrete because due to the lack of bonding between the surface aggregate and cement paste.Due to rough surface, It has strong bond between surface of angular aggregate and cement paste.
    4.It requires less water due to smooth surface.It requires more water due to rough surface.
    5.It has low W/c ratio.It has high W/C  ratio.
    6.Due to low W/C ratio results in higher strength as compared to angular aggregates.It has low strength as compared to Rounded aggregates.
    7.Better in workability.Good in workability.

Texture

Surface texture means the external appearance of the particle. In the context of aggregate, aggregate is either smooth or rough in texture. Smooth surface leads to weak bonding between the surface and cement paste.

Strength

Strength is a significant property to be looked into before selecting the aggregate for concreting. The strength of aggregate means how much aggregate resists the loads. Strength depends on the bond between the cement paste and aggregates surface. The test for the strength of the aggregate

  • Aggregate crushing test.
  • Impact value Test of Aggregate.
  • Aggregate Abrasion test.

Specific Gravity of Coarse Aggregate

The specific gravity of aggregate can be determined by the ratio of the weight of a given volume of aggregate to the weight of an equal volume of water. The Specific Gravity of aggregate particles ranges from 2.55 to 2.65.

Absorption and Moisture content:

If the coarse Aggregates are dry then it absorbs more water while mixing it at batching plant which affects workability. If the aggregates contain surface moisture, it contributes an extra amount of water to the concrete mix which increases the water-cement ratio. Both conditions affect the quality of concrete.  It is often that at the site work we use dry coarse aggregate and moist fine aggregate. Aggregate should not absorb water more than 10 percent of their weight after 24 hours of immersion in water.

Soundness of Coarse Aggregate

The soundness of aggregates means aggregate resists the changes that occurred in aggregates due to temperature variation, alternate wetting and drying, freezing, and thawing. The aggregate that subjects more change in volume is said to be unsound aggregates.

Cleanliness

The Coarse aggregates should be free from silt, lumps, clay, and organic impurities like vegetation, etc which affect the setting, strength, and durability of aggregates.

Grading of Coarse Aggregate

It plays an important role in concrete strength. Grading of aggregate means the distribution of the particle size of the aggregate. Good Grading of the aggregates has better workability and durability.

Coarse Aggregate Grading (As per IS: 383)

63mm40mm20mm16mm12.5mm10mm
100
85-100100
 0-3085-100100
0-50-2085-100100
85-100100
85-100100
0-50-50-200-300-4585-100
0-50-50-100-20
0-5

I hope you have got a lot of information regarding coarse aggregates

Read also:

The function of Different types of cement and their advantage and Disadvantage

Cement: Raw Material, History, Chemical Composition, Functions of Compound.

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