Autoclaved Aerated Concrete Blocks Thaju Zaman E K

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History• Developed in Sweden in the 1920s in

response to increasing demands on timber supplies.

• Industries like Durox, Siporex etc. were established in the early 1940s.

• Soon started spreading to other parts of the world though its main hub remained at Western Europe

• Huge acceptance in Asia especially in the middle-east countries because of the strong demand in housing and commercial space.

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Introduction• AAC block is made of Portland cement, fine

aggregates (fly ash or sand), water and an expansion agent.

• The autoclaving process results in the production of air voids in the material, making it less dense, easy to cut/ mould and better insulating.

• Autoclave is a strong, pressurized, steam-heated vessel

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A multi-storey building made with AAC blocks

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Reduced Dead LoadsUsing AAC Blocks reduces the load on the

foundation and other structural components in a structure due to its lower self-weight.

About 55% reduction in weight of walls can be obtained when compared to that of walls made with clay bricks.

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AAC blocks uses fly ash (70% of its weight), thus provides the most constructive solution to the nation’s fly-ash utilization problem.

Fly ash is an industrial waste product and use of fly ash in the AAC block production process takes care of the issues concerned with its disposal.

Environmental impact

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Autoclaved Aerated Concrete has excellent acoustic performance and can be used as an effective sound barrier, e.g. (AAC wall panel).

The small air pockets that are generated during AAC production prevents the sound from travelling from one side of a wall to the other

Excellent acoustics

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An important reason for the AAC good fire resistance is it's is relatively homogeneous structure.

Unlike the normal concrete where the presence of coarse aggregate leads to deferential rates of expansion, cracking and disintegration.

Fire resistance

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Autoclaved Aerated Concrete is very airy thus allowing more diffusion of water, reducing humidity of the building. 

Ventilation 

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The remarkably good insulation properties of Autoclaved Aerated Concrete mean a pleasant interior environment is achieved.

In most cases the need for supplementary insulation can be avoided.

It has very low specific conductivity. This makes heating or cooling (Air Conditioning) very efficient.

Energy saving

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The accurate manufacturing process ensures that Autoclaved Aerated Concrete panels and blocks are always produced to size as they leave the factory.

This results in less on-site trimming and reduced quantities of mortar and finishing materials used.

Accuracy

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The low weight of AAC blocks helps in the easy loading and unloading of these items when transportation to work sites are required.

Besides it also helps the mason as he can easily lift and place the blocks.

Rapid on - site assembly

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AAC does not deteriorate over time and they retain good finishes even after many years.

They have better resistance against alternate cycles of heating and cooling.

Long life

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Composition

1. Fly-ash - 59%2. Cement (usually OPC grade 53) -33%3. Lime - 8%4. Aluminum powder - 0.07%

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AAC blocks coming out of the Autoclave vessel

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Close up view of air bubble formation during the production of AAC blocks

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PROPERTIES

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Physical propertiesDensity• Ranges between 300- 1800 kg/m3

according to a source (RILEM)• AAC is thus a unique industrial product

that covers such a range in apparent density.

• AAC products with density up to 350 kg/m3 can be used as load bearing construction material

• AAC with lower density are generally used for thermal insulation purposes

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Porosity• Classified as capillary pores, gel pores,

macro-pores due to deliberately entrained air, and micro-pores due to inadequate compaction.

• Fly ash is used as for more uniform distribution of air-voids by providing uniform coating on each bubble and thereby prevents merging of bubbles.

• It is observed that the strength, permeability, diffusivity, shrinkage and creep properties of AAC blocks are considerably related to its porosity and pore size distribution. 22

Permeability• Permeability of aerated concrete is greatly

influenced by the type, size and distribution of the pores, and not the pore volume.

• Pores are generally classified into two types - open pores and closed pores.

• Permeability of aerated concrete is contributed by the open pores and not the closed pores.

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Compositional Properties• Scanning Electron Microscopy (SEM)

showed that the micro capillaries in AAC are plate shaped crystals of 11.3 Å tobermorite with a double-chain silicate structure.

• The growth rate and the degree of orientation of this structure cause differential pore distribution.

• Tobermorite is a calcium silicate hydrate mineral with chemical formula:

Ca5Si6O16(OH)2·4H2O or Ca5Si6(O,OH)18·5H2O24

Mechanical Properties

Compressive Strength• Can carry loads of up to 8 MPa

approximately 50% of the compressive strength of regular concrete.

• Influenced by factors such as density, age, curing method, component and mix proportion.

• The rate of strength development is initially very high and it gets decreased with age.

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Thermal conductivity• Thermal conductivity increases as the

moisture content of AAC increases. • Also increases with the increasing density.• The amount of pores and their distribution

are also critical for thermal insulation. • Finer the pores, better the insulation.

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Durability• Carbonation is one of the main factors of

the ageing deterioration of AAC• In cold countries frost deteriorations are

observed in outer walls made from AAC• Surface scaling caused by freezing and

thawing;• the other is the wide cracks formed due to

the inner part AAC temperature being reduced to 0℃

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Typical Properties of Autoclaved Aerated Concrete

Dry density(kg/m3)

Compressive

Strength(MPa)

Flexural Strength

(MPa)

Modulus of Elasticity

(GPa)

Thermal Conductivity

(3% moisture)(W/mK)

450 3.2 0.65 1.6 0.12

525 4.0 0.75 2.0 0.14

600 4.5 0.85 2.4 0.16

675 6.3 1.00 2.5 0.18

750 7.5 1.25 2.7 0.2030

COMPARISON BETWEEN AAC BLOCKS AND

CLAY BRICKS• Fly ash is an industrial waste product and

its use in AAC block production process takes care of the issues concerned with its disposal whereas brick industry consumes our natural top soil

• The AAC block is dimensionally more accurate as it is produced with wire cut technology in a certified factory.

• AAC blocks have air voids and hence have better fire resisting property compared to red clay bricks.

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• Using AAC Blocks reduces the load on the foundation and other structural components in a structure due to its lower self-weight.

• AAC Blocks do not have any “micro-pores” or continuous ‟capillaries‟ through which exterior surface water can be absorbed to interiors. It means longer life to the paints and interiors free from growth of any kind of fungus, providing healthier and long lasting interiors to the occupants

• AAC Blocks can be easily cut, drilled, nailed, milled and grooved to fit individual requirements. Available in custom sizes. 32

AAC Blocks in India• Currently there are around 35 largescale

AAC blocks manufacturing plants across India with a heavy concentration near Surat, Gujarat.

• Besides there are a number of small scale manufacturers concentrated near the coal industries.

• More and more AAC blocks plants are set up across the India, as awareness about AAC blocks is growing.

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Cost of AAC blocks

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DisadvantagesFamiliarisation of the Product• Few contractors are currently familiar with

the product, and masons must adjust to using thin-set mortar as opposed to traditional cement-based mortar, which requires less precision in its application.

Scarcity of Manufacturing Plants.• Projects far from manufacturing facilities

will suffer from higher initial costs

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Conclusion• Autoclaved Aerated Concrete block is

different from conventional concrete in the mix materials and properties.

• Though the compressive strength of Autoclaved Aerated concrete is less than that of concrete, AAC blocks possess many beneficial factors such as lower density, enhanced thermal and sound insulation and reduced dead loads.

• Hence it is an ideal alternative for traditional wall brick or stone materials owing to its environmental impact and eco- friendly features. 36

Thank You

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