Aerated Concrete: Methods, Uses and Properties | Concrete Technology

Aerated concrete is one of the light weight concrete. It is a mixture of water, cement and finely crushed sand. Aerated concrete is obtained by introducing gas bubbles into the plastic mix of cement and sand mortar. The product obtained is cellular in structure containing voids of size 0.1 to 1 mm similar to sponge rubber. The skin of cells or voids must be able to withstand mixing and compaction pressure. This resulting concrete is known as aerated or cellular concrete, but strictly speaking, the use of word concrete is not appropriate as no coarse aggregate is used in it.

Properties of Aerated Concrete:

Aerated Concrete has the following properties:

1. It can be sawn, cut, nailed. It can hold nails.

2. It is quite durable.

3. The rate of water penetration through aerated concrete is low.

4. It has better resistance to frost.

5. Its water absorption is high. Hence untreated aerated concrete should not be exposed to aggressive atmosphere.

Uses of Aerated Concrete:

The aerated concrete usually is used for the following purposes:

1. Due to its low thermal conductivity and weight, mostly it is used for heat insulation purposes.

2. As it offers better fire resistance than ordinary concrete, it is used for fire proofing.

3. Structurally aerated concrete is used mostly in the form of precast members or autoclaved blocks. It can also be used for floor construction in place of hollow tile floor.

4. Recently it has been used for light insulation.

Methods of Production of Aerated Concrete:

There are two basic methods of producing aerated concrete. An appropriate name is given to each product.

1. Gas Concrete:

It is obtained by a chemical reaction generating a gas in the fresh mortar. When this mortar sets, it contains a large number of gas bubbles. The consistency of the mortar should be such that the gas produced may expand it, but gas should not escape from it i.e. the consistency of mortar should be correct. The speed of gas evolution, consistency of mortar and its setting time should match with each other.

For generating gas, finally divided aluminum powder is most commonly used. The proportion of aluminum powder may be 0.2% of the mass of the cement. The reaction between this active powder and calcium hydroxide or alkalis, liberates hydrogen bubbles. Powdered zinc or aluminum alloy can also be used. Sometimes hydrogen peroxide is used to produce oxygen bubbles.

2. Foamed Concrete:

It is produced by adding a foaming agent in the mix which introduces and stabilizes the air bubbles during mixing at high speed. The foaming agent usually used is some form of hydrolyzed protein or resin soap. In some processes stable pre formed foam is added to the mortar during mixing in an ordinary mixer.

The aerated concrete may be made without sand, but such concrete can only be used for nonstructural purposes such as for heat insulation. The density of aerated concrete produced without sand varies from 200 to 300 kg/m³. When the aerated concrete is produced by a mixture of cement and very fine sand, the density of the usual mixes varies from 500 to 1100 kg/m³. In the case of other light weight concretes, the strength of aerated concrete varies with the density. The thermal conductivity of aerated concrete also varies with its density.

According to HOFF, the strength of cellular concrete can be expressed as a function of void contents taken as the sum of the induced voids and volume of evaporated water.

The strength of a concrete having density of 500 kg/m³ is found in the range of 3 to 4 MPa (30 to 40 kg/cm² and thermal conductivity of about 0.1 J/m²S^oC/m and for a concrete with a density of 1400 kg/nr, the corresponding values of strength and thermal conductivity would be approximately 12 to 14 MPa and 0.4 J/m²S°C/m.

By comparison the conductivity of ordinary concrete has been found about 10 times more than that of cellular concrete. Further it should be noted that thermal conductivity increases linearly with the moisture content. At 20% moisture content the conductivity is found almost double of that when the moisture content is zero.

The modulus of elasticity of aerated concrete usually varies between 1.7 and 3.5 GPa (0.25 to 0.5 x 10⁶ PSi). Its creep expressed on the basis of stress/strength ratio, (creep per unit of stress) is found to be the same as that of ordinary concrete. However on the basis of equal stress, the specific creep of aerated concrete is found higher in comparison with ordinary concrete.

The thermal movements, shrinkage and moisture movements of aerated concrete have been found higher in comparison with light weight aggregate concrete of the same strength. But these can be reduced by autoclaving i.e. high steam curing. Autoclaving also improves the strength of the aerated concrete.

The permeability of high pressure steam cured aerated concrete decreases with the increase in its mois­ture content, but even when the concrete is dry, the permeability at low pressure is negligible. The relation between the wet density and compressive strength of aerated concrete is shown in Fig. 22.3 Fig. 22.4 shows the relation of dry density and autoclaved high pressure steam cured concrete. Flowing aerated concrete can be obtained by using a super plasticizer.