Introduction to Lightweight Concrete
Lightweight concrete is a special type of concrete which weighs lighter than conventional or normal concrete. Density of light weight concrete is conventionally low. Generally 300 kg/m³ to 2200 kg/m³ and thus normal concrete has density between 2200 kg/m³ to 2600 kg/m³.
Principle of Lightweight Concrete
The basic principle behind the making of lightweight concrete is by inducing the air in concrete. To do this there are several methods that can be adopted. Such as :
- Conventional aggregates in the concrete can be replaced by cellular porous aggregates. (Lightweight aggregate concrete)
- The air or gas bubbles can be inserted in concrete (Aerated concrete).
- During the preparation of concrete, sand should not be used and it should be omitted. (No- fines concrete).
Types of Lightweight Concrete
1. Lightweight Aggregate Concrete :
In the manufacturing of the lightweight aggregate concrete care is take that the aggregates used are of light weight. Natural aggregates and Artificial aggregates are mostly used. But in most cases artificial aggregates are mostly preferred than natural aggregates, as artificial aggregates can be modified according to the need.
In any case if natural aggregates are used then most common natural lightweight aggregate is Pumice and Scoria.
Artificial aggregates are generally obtained by modifying and expanding the rocks such as shale, slate, perlite, vermiculite, etc.
| Natural Lightweight Aggregates | Artificial Lightweight Aggregates |
|---|---|
| Pumice | Artificial Cinders |
| Diatomite | Coke breeze |
| Scoria | Bloated clay |
| Volcanic cinders | Expanded shales and slate |
| Sawdust | Thermocole beads |
| Rice husk | Sintered flyash |
Types of aggregates which are used in making of concrete decides the density of concrete. Density of concrete as low as 300 kg/m³ can be achieved by using this type of concrete. Compressive strength of LWC varies from 0.3 Mpa to 40 Mpa.
Also Read : What is Prestressed Concrete ?
2. No-Fines Concrete :
In making of No-fines concrete, fine aggregates are not used. They are generally omitted. The materials used consist on only cement, coarse aggregates and water.
The concept of not using the fine aggregates at the preparation of conventional concrete is that the, if only coarse aggregates are used then there will be larger voids in the concrete which will make it lightweight.
It is always good to use the coarse aggregates of same size ( 10- 20 mm) rather than well- graded aggregates, due to which the density of the concrete will be less. Density as low as 640 kg/m³ can be achieved by using lighter coarse aggregates.
Cement ratio is also very important in this type of lightweight concrete. The strength criteria totally depends on the cement content in the concrete.
Drying shrinkage is comparatively less, but shrinkage takes place rapidly than conventional concrete. Thermal conductivity is also comparatively less.This type of concrete has excellent architectural appearance.
3. Aerated Concrete :
Aerated concrete is made by inserting air or gas into a slurry composed of portland cement or lime and finely crushed siliceous filler so that when the mix sets and hardens, a uniformly cellular structure is formed. Though it is called aerated concrete a common product of aerated concrete in India is Siporex. There are several ways in which aerated concrete can be manufactured.
- It can be manufactured by the formation of gas by chemical reaction within the mass during liquid or plastic state.
- By mixing preformed stable foam with the slurry.
- By using finely powdered metal (usually aluminium powder) with the slurry and made to react with the calcium hydroxide liberated during the hydration process to give out large quantity of hydrogen gas. This hydrogen gas when contained in the slurry mix, gives the cellular structure.
Design Mix of Lightweight Concrete :
- During the preparation of design mix of lightweight concrete it is very difficult to decide water- cement ratio of the concrete, due to variable water absorption by aggregates. It is usually done by trial mixing.
- Pre- saturation of aggregates is done to avoid excessive absorption of water by aggregates. Concrete in which saturated aggregates are present 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.
Advantages of Lightweight Concrete :
- It reduces the dead load of the building.
- Easy to handle and hence reduces the cost of transportation and handling.
- Improves the workability.
- Lightweight concrete has a lower thermal conductivity.
- At the time of construction of the building, the rooms in which air- conditioning is to be installed, the use of lightweight concrete at such place will result in good thermal comfort and less power consumption.
- Comparatively more strong and durable.
- Good resistance to freezing & thawing action when compared to conventional concrete.
- The use of light weight concrete gives an outlet for industrial wastes such as fly ash, clinkers, slag etc, which create problem for disposal.
- It offers great fire resistance.
- Light weight concrete gives overall economy.
Disadvantages of Lightweight Concrete :
- Lightweight concrete is generally very sensitive with water content in the mixture.
- Difficult to place and finish because of porosity and angularity of the aggregate, hence skilled labours are required.
- Cement mortar in some cases may separate the aggregates and make it float on the surface.
- Mixing time is comparatively longer than conventional concrete, so as to assure proper mixing of concrete.
- Lightweight Concrete are porous and shows poor resistance.
Applications of Lightweight Concrete :
- Since the strength of LWC is low, it is used in the construction of roof slabs, small houses with load bearing walls etc.
- It is also used in the construction of stairs, windows, garden walls, etc.
- In large building construction this is used in the construction of partition walls.
- These are moulded in the form of slabs and used as thermal insulators inside the building.
Also Read : Different Types of Grades of Concrete