Classification of Construction Aggregate Based on Density

Fig.1.Construction Aggregate
The aggregates can be classified based on the density of aggregates as

  1. Normal Weight Aggregate
  2. Heavy Weight Aggregate
  3. Lightweight Aggregate

1. Normal Weight Aggregate

Normal weight aggregates are commonly used aggregates. These will include sands, gravels, crushed rocks such as granite, basalt, quartz, sandstone, limestone and brick ballasts, etc. The most common normal weight aggregates are:
  1. Sand
  2. Gravel
  3. Crushed Stone
Normal weight aggregate is the common type of aggregate with a weight of 2200 to 2400kg/m3.

The normal weight aggregates have specific weights between 2.5 and 2.5. These aggregates can contribute to concrete that has a unit weight of 23 kN/m3 to 26 kN/m3 and a crushing strength at 28 days between 15 MPa and 40 MPa.

2. Heavy Weight Aggregate

The heavy-weight aggregates have unit weights from 28 kN/m3 to 29 kN/m3. The specific gravity of these aggregates will range from 2.8 to 2.9.

Fig.2.Magnetite Heavy Weight Aggregates

The most commonly used heavy-weight aggregates are Magnetite ( FeO2O3 ) Barytes (BaSO4) and scrap iron. These are used to manufacture heavy-weight concrete. The heavy-weight aggregate is more effectively used as a radiation shield.

The concrete that is manufactured with heavyweight aggregate will have a crushing strength in the range of 20 to 21 MPa. The cement-aggregate ratio will vary from 1.5 to 1.9 with a water-cement ratio between 0.5 to 0.65. These aggregates will produce dense and crack-free concrete. It is used in areas where there is a need for a high mass-to-volume ratio

Types of Heavy Weight Aggregates

  1. Goethite 
  2. Limonite 
  3. Barite 
  4. Ilmenite
  5. Magnetite 
  6. Hematite 
  7. Ferrophosphorus 
  8. Steel

Uses of Heavy Weight Aggregate

The heavyweight aggregates acquire high radiation and mechanical resistance which is to be used  in
  1. Radiation Shielding Units - Nuclear
  2. Structures near to Explosive Units
  3. Used for ballasting for pipelines - Offshore construction

Limitations of Heavy Weight Aggregates

  • Heavy-weight aggregates are not suitably graded
  • Produce concrete of inadequate workability
  • Results in segregation

3. Lightweight  Aggregate

The lightweight aggregates will have a unit weight of up to 12 kN/m3. These are mainly used to manufacture structural concrete and masonry blocks for the reduction of the self-weight of the structure.

  • The lightweight aggregates are natural or synthetic in nature. This will have a weight of 110kg/m3. 
  • The microstructure of LWA is porous and cellular in nature. This results in lightweight.
  • The absorption amount is higher. The concrete mix design with LWA is based on a modified approach.
  • Slump Loss is an issue faced during testing. Avoiding saturation of aggregates before batching can prevent this issue.
  • The main features of lightweight aggregates are:
    1. Low Density
    2. High water absorption
    3. Concrete is formed with lower workability
    4. Lower Modulus of Elasticity
    5. Lower Tensile Strength
    6. Rough Surface Texture
    7. High Shrinkage and Creep

Types of Lightweight Aggregates

Classification of  Lightweight Concrete

As per BIS, the LWA can have the following classification. 
    • Naturally Occurring LWA
    • Industrial by-product LWA
    • Artificial or Manufactured LWA

Naturally Occurred LWA

The natural aggregates are available naturally. These aggregates are chemically inert and usually has a relatively high amount of silica about 75%. The common examples of this type are:
  • Pumice
  • Silica Sand
  • Volcanic Tuffs
  • Gravel and Crushed Stone
  • Volcanic Slag

Industrial by-product LWA

These are basically categorized under artificial aggregates. But due to their availability, they are considered industrial aggregates. Some of them are:
  • Foamed Blast Furnace Slag
  • Sintered -Pulverised Fuel Ash

Artificial or Manufactured LWA

These are mostly produced in the rotary kiln. The raw materials are clay, shale, slate, or pulverized fuel ash. These are subjected to either expansion or agglomeration. The common examples are:
  • Expanded Shale
  • Expanded Clay
  • Brick Rubber
  • Pelletized material
  • Vermiculate

Fig.3.Expanded Clay Lightweight Aggregate

Examples of Lightweight Aggregates are:

  1. Bloated Clay: These are of size 5 to 20mm. These take a spherical shape and are hard, light, and porous. The water absorption is about 8 to 20%.
  2. Sintered Fly Ash: These have a unit weight of 10 kN/m3. These have a porous structure
  3. Expanded Shale: These are produced by passing the crushed shale through a rotary kiln at 110 degree Celsius. This is also known as herculite or haydite. These types have higher resistance to heat. This is used for the refractory lining, and fireproofing of the structural steel. It has better sound absorption. 

Uses of Light Weight Aggregate

Lightweight aggregates have proven their efficiency in the construction of structures like:
  1. Bridges
  2. Precast Industry
  3. Tunnel Construction
  4. Building Construction
  5. Offshore Construction
These structures will have less dead weight and internal curing capacity as these particles have the water-holding capacity to perform efficient hydration products in cement concrete.

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