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Lightweight aggregate concrete is prepared by using lightweight aggregate or low density aggregate such as volcanic pumice, clay, slate, shale, scoria, tuff and pellite. Concrete is considered to be lightweight is the density is not more than 2200 kg/m3, when compared to normal concrete which is 2300-2400 kg/m3 and a proportion of the aggregate, should have a density of less than 2000 kg/m3.

Fig 1: Light Weight Concrete

In this article we discuss the properties, characteristics, uses and weight per cubic feet of lightweight aggregate concrete.

Properties of Lightweight Aggregate Concrete

The properties of lightweight aggregate concrete are discussed below –

1. Particle Shape and Texture of Aggregate

The shape of the lightweight aggregate used in concrete may be in cubical, rounded, angular, or irregular shape. Textures may range from the fine pore, relatively smooth skins to highly irregular surfaces with large exposed pores.

Particle shape and surface texture can directly influences the workability, coarse-to-fine aggregate ratio, cement content requirements, and water demand in concrete mixtures.

2. Compressive Strength

Compressive strength levels commonly required by the construction industry for design strengths of cast-in-place, precast, or prestressed concrete is around 3,000 to 5,000 psi which can be easily obtained by lightweight aggregate concrete.

3.Density

The fresh density of lightweight concretes is a function of mixture proportions, air contents, water demand, particle density, and moisture content of the lightweight aggregate.

ACI 213 definition of structural-grade lightweight concrete that has
an equilibrium dry density ranging between 90 to 115 lb/ft³.

4. Absorption

Studies have revealed that high-quality lightweight concretes absorbed very little water and thus maintained their low density. The permeability of lightweight concrete was extremely low and generally equal to or significantly lower than that reported for normal weight concrete.

5. Internal Curing

Lightweight aggregate batched at a high degree of saturation may be substituted for normal weight aggregates to provide ?internal curing? in concrete containing a high volume of cementitious materials.

The reason is better hydration of the cementitious fraction provided by moisture available from the slowly released reservoir of absorbed water within the pores of the lightweight aggregate.

6. Thermal Conductivity

The thermal conductivity of concrete depends mainly on its density and moisture content but is also influenced by the size and distribution of the pores, the chemical composition of the solid components, their internal structure of light weight concrete.

As the LWC is low in density and moisture conduct is more due to pores, the thermal conductivity of this concrete is less when compared to normal concrete.

7. Fire Resistance

When tested according to the procedures of ASTM E 119, structural lightweight concrete slabs, walls, and beams have demonstrated greater fire-endurance periods than equivalent-thickness members made with concretes containing ordinary aggregate.

Characteristics of Lightweight Aggregate Concrete

  1. There should be uniformity in properties and composition.
  2. The aggregate should have a low specific weight to ensure worthwhile savings in the structure, in accordance with the appropriate ASTM specifications.
  3. Notwithstanding the desirability of having surface characteristics to provide a good bond, the aggregate should have a minimum of large external voids but a large number of smaller well-dispersed voids throughout the particles.
  4. Individual pieces of aggregate should be strong enough to withstand handling and mixing.
  5. The particles should bond well with cement and not react chemically to cement.
  6. The aggregate should be suitably graded for the intended use, in accordance with the appropriate ASTM specification.

Uses of Lightweight Aggregate Concrete

  1. Screeds and thickening for general purposes especially when such screeds or thickening and weight to floors roofs and other structural members.
  2. Screeds and walls where timber has to be attached by nailing.
  3. Casting structural steel to protect its against fire and corrosion or as a covering for architectural purposes.
  4. Heat insulation on roofs.
  5. Insulating water pipes.
  6. Construction of partition walls and panel walls in frame structures.
  7. Fixing bricks to receive nails from joinery, principally in domestic or domestic type construction.
  8. General insulation of walls.
  9. Surface rendered for external walls of small houses.
  10. It is also being used for reinforced concrete.

Weight of Lightweight Aggregate Concrete

The weight of the lightweight aggregate concrete is around 115 pounds per cubic foot, whereas the weight of the normal weight concrete is 145 pounds per cubic foot.

The less weight of light weight concrete is due to the usage of fine and course lightweight aggregate. When the complete aggregate is replaced with lightweight aggregate, the weight decreases the concrete density of by approx. 10 kilogram per cubic meter.

For Example, 1 square foot of 1.5-inch thick normal concrete weighs about 18 pounds. The same segment created from lightweight concrete weighs approximately 14.5 pounds.

Table 1: Difference between Lightweight concrete and conventional concrete

PropertiesLightweight ConcreteConventional Concrete
Weight20- 115 pounds per cubic foot130- 150 pounds per cubic foot
Compressive Strength7000+ psi8000 psi
Modulus of Elasticity65,000 psi for the ultra light weights to 3 million psi for the medium lightweight concretes2 – 6 million psi for the conventional concretes
ShrinkagePerilite – 0.1 to 0.2%, Vermiculite- 0.1 to 0.7%
Slag – 0.04 to 0.06%Shale, Slate and Clay – 0.02 to 0.08%
0.04 to 0.08 %
Thermal ConductivityBTU per hour per square foot per degree F. per inch
Perilite – 0.1 to 0.2%, Vermiculite- 0.1 to 0.7%
Slag – 0.04 to 0.06%
Shale, Slate and Clay – 0.02 to 0.08%
BTU per hour per square foot per degree F. per inch
Sand and gravel thermal conductivity ranges from 8.0 to 12.0 %
Fire Resistance4-hour rating for 4.5″
slabs with lightweight aggregate.
3- hour rating for 6-inch slabs made of trap rock, crushed, limestone and gravel aggregate.

Fasi Ur Rahman

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