Aggregate

Aggregate ll Types and Property of aggregates.

Aggregate is an important constituent in concrete. It occupies about three-quarters of the volume of concrete, it contributes significantly to the structural performance of concrete, especially strength, durability, and volume stability.

Different types of aggregate

  1. Normal weight aggregates
  2. Light-weight aggregates
  3. Heavy-weight aggregates
  4. Fibrous aggregates
  5. Gaseous aggregates

In 80% of construction works, we used normal weight aggregates only. So, in this article, we will discuss the normal weight aggregate only.

1. Normal weight aggregate

Normal weight aggregates are those aggregates having a specific gravity between 2.5 and 3.0 and a bulk density in the range 14.5 to 17.5 kN/m3. It includes the most widely used of all aggregates, namely gravels, crushed rocks, and blast furnace slag.

Normal weight aggregate is classified as coarse aggregate and fine aggregate based upon their sizes.

i. Coarse aggregate

Coarse aggregate is material which passes through 80mm sieve and retained on a 4.75mm sieve. It may be uncrushed gravel if it results from the natural disintegration of rock or crushed stone or crushed gravel if it is produced by crushing hard stone or gravel respectively.

  • It acts as the main filler, and forms the main bulk of concrete, around the surfaces of which the binding materials adhere in the form of film.
  • Broken stone, broken bricks, and gravel are generally used as coarse aggregates.
  • The crushing strength and water tightness of concrete and its resistance to wear and tear depend upon the aggregates. Further, aggregates balance the shrinkage and volume changes of concrete.

Requirement /Characteristics of coarse aggregate

  1. A good coarse aggregate should be hard and tough, angular or cubical in shape, absolutely clean, sound, fireresisting and durable.
  2. Free from chemical or coating of clay.
  3. Free from organic matter.

Function/Use of coarse aggregate

  1. It imparts greater volumetric stability and durability to concrete (as compared to the cement paste alone).
  2. It makes a solid and hard mass of concrete with cement and sand.
  3. It increased the crushing strength of concrete.
  4. It is cheaper than cement and hence directly helps in achieving economy in concrete manufacture.

ii. Fine aggregate (Sand)

Fine aggregate(sand) is a material which passes through 4.75mm sieve and retained on 75-micron sieve. It may be either natural sand e.g. river sand, pit sand, and sea sand or artificial sand i.e. prepared by crushing stones and gravel to powder form. It consists of small angular or rounded grains of silica.

Characteristics of Fine aggregate

  1. Clean, sharp and angular
  2. Hard strong and durable.
  3. Chemically an inert material
  4. Highly silicious and free from impurities such as clay, loam, dust, coal particles and organic matter.

Function/ Use of Fine aggregates/Sand on construction

  1. Sands fills the voids present in the coarse aggregate.
  2. Sand is formed by the decomposition of sandstone under the effect of weathering agencies.
  3. Sand helps minimizes shrinking and cracking of concrete.
  4. Sand is used in plain cement concrete (PCC).
  5. It assists in the hardening of cement by allowing water to penetrate through its voids.
  6. It prepares concrete economically, of any required strength, by varying its proportions.

Why aggregates are used in huge amount in concrete?

It is normal to raise a question in mind of fresh engineers that Why aggregates are used in a huge amount in concrete or construction works? For that, we need to know more about the properties of aggregates.

Properties of aggregate

Because of the following properties of aggregates are used in a huge amount in concrete to manage cost as well as strength.

i) Size of aggregates

The size of aggregates is designated by the maximum size of the individual aggregate present in a substantial amount. The aggregate is said to be of 20mm size it passes through a 20mm sieve and retained on a 16mm sieve. Based on the size of particles, aggregates are classified as follows:

  1. Coarse aggregate: It passes through an 80mm sieve and retained on a 4.75mm sieve.
  2. Fine aggregate: It passes through a 4.75mm sieve and retained on a 75-micron sieve.
  3. Particles between 0.075mm and 0.002mm are classified as silt and particles of size smaller than 0.002mm as clay.
    1. Silt and clay are undesirable particles and should be removed from aggregates used for making concrete.

ii) Shape of aggregate

Aggregates are classified as rounded, irregular or partly rounded, angular and flaky. The shape of the particles affects the voids in compacted aggregate. Aggregate of rounded shape is very appropriate from workability consideration, but it results in poor bonding between the passes and the cement paste.

 shape of aggregates
Pic Source: Wikimedia

Flaky and elongated particles are not desirable for making concrete. A flaky particle has thickness less than 0.6 times of mean sieve size. Mean sieve size is defined as the arithmetic average of a sieve size through which it passes and over which it is retained. Elongated particles have their longest dimension 1.8 times the mean sieve size.

These particles have the highest surface area and hence reduce the workability of concrete. They tend to orient in a plane during compaction resembling with stratified rock and so affect adversely the durability of concrete.

iii) Surface Texture of aggregate

The surface texture describes the nature of the surface of the aggregate. The aggregates are classified as glossy, smooth, granular, rough, crystalline, Honey-combed and porous. The shape and surface texture of aggregates influence the strength of concrete considerably.

Aggregates of a rough surface increases the strength because of the higher surface area resulting in greater adhesive force between the particles and cement paste. However, it reduces workability on account of more water required to wet the surface of the aggregates.

iv) Compressive strength of aggregate

Generally, the strength of aggregate is greater than the strength of normal concrete. The strength of aggregate is required to be made in the following situations:

  1. For production of high strength and ultra high strength concrete.
  2. When contemplating to use aggregates manufactured from weathered rocks.
  3. Aggregate manufactured by industrial purpose.

Also Read: How to determine the crushing value of aggregate?

iv) Modulus of Elasticity of aggregate

Modulus of Elasticity of aggregate depends on its composition, texture, and structure. The modulus of elasticity of aggregate will influence the properties of concrete w.r.t. shrinkage and elastic behavior and to a very small extent creep of concrete.

Many studies have been conducted to investigate the influence of modulus of elasticity of aggregate on the properties of concrete. One of the studies indicated that the ‘E’ of aggregate has a decided effect on the elastic property of concrete and that the relation of ‘E’ of aggregate to that of the concrete is not a linear function, but maybe expressed as an equation of exponential type.

v) Specific gravity of aggregate

In concrete technology, the specific gravity of aggregates is made use of in design calculation of concrete mixes. With the specific gravity of each constituent known, its weight can be converted into solid volume and hence a theoretical yield of concrete per unit volume can be calculated.

Specific gravity of aggregate is also required in calculating the compaction factor in connection with the workability measurements. The average specific gravity of the rocks varies from 2.6 to 2.8.

vi) Bulk density of the aggregate

The bulk density or unit weight of aggregate gives valuable information regarding the shape and grading of the aggregate. For a given specific gravity the angular aggregates show a lower bulk density.

The bulk density of aggregate is measured by filling a container of known volume in the standard manner and weighing it. Bulk density shows how densely the aggregate is packed when filled in a standard manner. The bulk density depends upon the particle size distribution and shape of the particles.

vii) Water absorption and surface moisture

Aggregates contain permeable holes through which moisture can permeate. There are four conditions of moisture content:

  1. Bone dry: When an aggregate does not contain any moisture. The condition is attained by drying aggregate at 100 degrees.
  2. Air-dry: When a saturated and surface dry aggregate is allowed to dry in air, a part of moisture evaporates and part of it is retained in the pores. This is known as air-dry aggregates. C for 24 hours.
  3. Saturated and surface dry: When all the pores in the aggregate are completely filled with moisture and there is no free moisture on its surface, it is said to be saturated and surface dry condition. This condition of aggregate is considered for the water cement ratio in the concrete mix design.
  4. Moist: When an aggregate has free moisture on its surface in addition to all pores filled, they are said to be in moist condition.

viii) Soundness

Soundness is the resistance of aggregates to physical disintegration forces such as heating and cooling, wetting and drying, freezing and thawing due to climate changes. The aggregate is said to be unsound when its volume changes due to physical disintegration forces resulting in deterioration of the concrete.

ix) Durability

Durability is the resistance to aggregates to chemically disintegrating forces. The most common chemical reaction, causing disintegration, is between the active silica constituents of the aggregate and alkalies in the cement.

These aggregates contain veins or inclusions of reactive silica, which when used with cement having a high alkali content forms alkalisilica gel. The gel is of the unlimited swelling type which causes disintegration of the aggregate.

Also Read: What is impact value test of aggregate?

Quality of good Aggregate for construction work:

  1. Any constituent which affects the hardening of the cement and durability of the hardened concrete adversely.
  2. It should be free from organic matter which reduces the hydraulic activity of cement and affects its normal setting and hardening.
  3. It should be free from constituents which decompose or change significantly in volume on exposure to the atmosphere or react adversely with the hardened cement paste.

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