Concrete

 Concrete is a construction material composed of cement, fine aggregates (sand) and coarse aggregates (Crushed or natural stones) mixed with water which hardens with time. Portland cement is the commonly used type of cement for the production of concrete.

The proportion of Concrete:

The proportion of concrete depends upon

1.      cement,

2.      sand,

3.      aggregates and

4.      Water.

A mixture of Portland cement and water is called a paste. So, concrete can be called a mixture of paste, sand and aggregates. Sometimes rocks are used instead of aggregates.

The cement paste coat the surface of the fine and coarse aggregates when mixed thoroughly and binds them. Soon after mixing the components, hydration reaction starts which provides strength and a rock-solid concrete is obtained.

Materials are mixed in specific proportions to obtain the required strength. The strength of the mix is specified as M5, M10, M15, M20, M25, M30 etc, where M signifies Mix and 5, 10, 15 etc. as their strength in kN/m². 

1. Plain or Ordinary Concrete.

It is one of the most commonly used types of concrete. In this type of concrete, the essential constituents are cement, sand and coarse aggregates designed and mixed with a specified quantity of water.

The ratio of essential constituents may be varied within wide limits.  A very commonly used mix design, commonly known as Nominal Mix Design is 1:2:4.

Plain concrete is mostly used in the construction of pavements and in buildings, where very high tensile strength is not required. It is also used in the construction of Dams.

Among the most important properties of ordinary concrete, the following maybe mentioned.

• Density: 2200 – 2500 Kg/meter.cube.
• Compressive Strength: 200 – 500 Kg/centimeter.square.
• Tensile Strength: 50 – 100 Kg/centimeter.square.
• Durability: Very Satisfactory.
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2. Lightweight Concrete:

Any types of concrete having a density of less than 1920 Kg/m³ is classed as lightweight concrete.

Various types of aggregates that are used in the manufacturing of lightweight concrete include natural materials like pumice and scoria, artificial materials like expanded shales and clays and processed materials like perlite and vermiculite.

The single important property of lightweight concrete is it's very low thermal conductivity.

For example, Thermal conductivity – the k value, for plain concrete may be as high as 10-12. But the thermal conductivity of Lightweight concrete is about 0.3.

Lightweight Concretes are used, depending upon their composition, for thermal insulation, for protecting steel structures, they are also used in long span bridge decks, and even as building blocks.

Aerated Concrete is a variety of extremely lightweight concrete ( density 480-800 Kg/m³ ). This is obtained by using cement, sand, and powdered fuel ash as constituents.

 

3. High-density Concrete:

This type of concrete is also called heavyweight concrete.  In this concrete type, the density varies between 3000-4000 Kg/m³.

These types of concrete are prepared by using high density crushed rocks as coarse aggregates. Among such materials, Barytes is the most commonly used material, which has a specific gravity of 4.5.

They are mostly used in atomic power plants and other similar structures because it provides good protection from all type of radiations.

4. Reinforced Concrete:

It is also called RCC (Reinforced Cement Concrete). In this concrete type, steel in various forms is used as reinforcement to give very high tensile strength.

In fact, it is because of the combined action of plain concrete (having high compressive strength) and steel (having high tensile strength).

The steel reinforcement is cast in the form of rods, bars, meshes, and all conceivable shapes.

Every care is taken to ensure the maximum bond between the reinforcement and the concrete during the setting and hardening process.

Thus, the resulting material (RCC) is capable of bearing all types of stress in any type of construction. RCC is the most important concrete type.

5. Precast Concrete:

This term refers to numerous types of concrete shapes that are cast into moulds either in a factory or at the site.

However, they are not used in construction until they completely set and hardened in a controlled condition.

Some of the examples of Precast Concrete are; precast poles, fence posts, concrete lintels, staircase units, concrete blocks, and cast stones, etc.

These structural and decorative members are prepared in a well-equipped place where all arrangements are made for;

1. Perfect proportioning of the ingredients of concrete.

2. Thorough mixing of the cement, aggregates and water to obtain the mix of the desired design and consistency.

3. Careful handling during transport and placement in the perfect design moulds.

4. Perfect curing, under the controlled conditions of temperature and humidity. Even steam curing is used to obtain precast products having high strength in much less time.

5. The latest trend in the construction industry is to shift more and more to prefabricated concrete units in building construction.

 

6. Prestressed Concrete:

It is a special type of reinforced concrete in which the reinforcement bars are tensioned before being embedded in the concrete.

Such tensioned wires are held firm at each end while the concrete mix is placed. The result is that when the concrete sets and hardens, the whole concrete members, so the cast is put into compression.

This sort of arrangement makes the lower section of the reinforced concrete also stronger against tension, which is the principal cause of the development of tension cracks in un-tensioned reinforced concrete.

Since pre-stressing involves the use of jacks and tensioning equipment, pre-stressed concrete is also cast in the factories.

Some of its advantages are the following.

1. The potential compressive strength of concrete gets considerably increased.

2. The risk of the development of tension cracks in the lower sections of beams is considerably reduced.

3. The resistance to shear is greatly reduced. This eliminates the necessity of stirrups to a great extent.

4. Lighter members can be used than the un-tensioned (normal) reinforced-concrete.

5. The prestressed concrete is greatly favoured in the construction of;

• Bridges.
• Long span Roofs.
• Most structures with the heavy dead load.