LIGHT WEIGHT CONCRETE

K.V.S REDDY CIVIL

Definition: Concrete having a 28-day compressive strength greater than 17 Mpa and an airdried unit weight not greater than 1850 kg/m³.Composition: Similar to normal concrete except that it is made with lightweight aggregates or combination of lightweight and normal-weight aggregates. All lightweight concretes use both lightweight coarse and lightweight fine aggregates. Standard lightweight concretes used natural sand instead of lightweight fine aggregates.

Lightweight Concrete

Introduction: Light weight concrete is a special concrete which weighs lighter than conventional

concrete. Density of this concrete is considerably low (300 kg/m3 to 1850 kg/m3) when

compared to normal concrete (2200kg/m3 to 2600kg/m3). Basically there is only one method for making concrete light i.e by INCLUSION of air

in concrete. This is achieved in actual practice by there different ways i)By replacing the usual aggregate by cellular porous or LWA ii)By introducing gas or bubbles in mortar-aerated concrete iii)By omitting sand fraction from the aggregate-no fines concrete

Light weight aggregate concrete - UK, France & USA Aerated concrete - Scandinavian countries No – fines concrete is less popular

LWC can also be classified on the purpose for which it is used such as:

1. Structural light weight concrete2. Non-load bearing concrete3. Insulating concrete

Types of Lightweight Concrete

Light weight aggregate concrete: Basically two types of light weight aggregates

Natural aggregates Artificial aggregates

Natural light weight aggregates are less preferred over artificial aggregates.

Important natural aggregates – Pumice & Scoria Artificial aggregates are usually produced by expanding the rocks

such as Shale, Slate, Perlite, Vermiculite, etc., Type of aggregates decides the density of concrete. Density of concrete as low as 300 kg/m3 can be achieved. Compressive strength varies from 0.3Mpa to 40Mpa.

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Foamed Slag – was the first LWA suitable for reinforced concrete.

(that was produced in large quantity in (UK) Sintered Pulverised – fuel ash aggregate – is being used in the UK

for a variety of structural purposes and is being marketed under the trade name Lytag.

Expanded Clays and Shales – capable of achieving sufficiently high strength for prestressed concrete

Pumice – is used for reinforced concrete roof slab, mainly for industrial roofs

NATURAL LIGHT WEIGHT AGGREGATE

• Brick bats• Cinder, Clinker and breeze• Foamed slag• Bloated clay• Pulverised fuel ash• Expanded perlite

ARTIFICIAL LIGHT WEIGHT AGGREGATE

• Very often light weight Concrete is made by the use of light weight aggregatesusually different LWC have different densities based on the density concrete differs In various properties like bulk density, dry density compressive strength ,shrinkage and thermal conductivity

• Most of the light weight aggrregates have rapid absorption quality. This is one of the difficulties in normal mix design procedure to light weight concrete

• But it is possible to water-proof the LWA by coating it with bitumen.

• The coating of aggregate by bitumen may reduce the bond strength between aggregate and paste,it result in degree in compressive strength of concrete

LIGHT WEIGHT AGGREGATE CONCRETE

o The structural LWC is going to be one of the important materials of construction.

A concrete which is light in weight and sufficiently strong to be used in conjunction with steel reinforcement will be a material which is more economical than the conventional concrete

Structural LWA concrete is a concrete having 28 day compressive strength more than 17Mpaand 28 day air dried unit weight not exceeding 1850kg/m3

For practical reasons it is common practice to use normal sand as fine aggregate and light weight coarse aggregate of maximum size 19mm such light weight concrete is termed as standard light weight concrete

Structural Light Weight Concrete

• In case of high slump and over vibration the mortar goes down and aggregate tends to float

LWC exhibits higher moisture movement than the normal weight concrete

The higher magnitude of drying shrinkage couple with lower tensile strength makes the LWA concrete to undergo shrinkage cracksBut the higher extensibility and lower modulus of elasticity help to reduce the tensile cracks

Since LWC contains large percent of air it is naturally a better material with respect to a)sound absorptionb)sound proofing c)thermal insulation

WORKABILITY

Mix design of LWC: Difficult to decide water–cement ratio, due to variable water absorption

by aggregates. The lack of accurate value of absorption, specific gravity and free

moisture content in aggregate make it difficult to apply water-cement ratio accurately for mix-proportion

Generally done by trial mixing. The density of concrete made with saturated aggregate is higher and

the durability of such concrete, especially its resistance to frost is lower. Concrete with saturated aggregates will have higher density, which is

bad in freezing & thawing action. In rare cases, aggregates are coated with bitumen to overcome the

water absorption problem.

• Mixing procedure for light weight concrete may vary with different types of aggregates

The general practice for structural light weight concrete is to mix the aggregate and about 2/3 of the mixing water for period up to one minute prior to the addition of cement and the balancing mixing water.

MIXING PROCEDURE:

Advantages: Reduces the dead load of the building. Easy to handle and hence reduces the cost of transportation and

handling. Improves the workability. Relatively low thermal conductivity Comparatively more durable. But less resistant to abrasion. Has applications in pre-stressed concrete, high rise buildings & shell

roofs. Good resistance to freezing & thawing action when compared to

conventional concrete. Helps in disposal of industrial wastes like fly ash, clinker, slag etc.

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