Concrete as Building

MaterialLECTURE # 5

Concrete Concrete is a mixture of binding material, coarse aggregate, fine aggregate and

water in suitable proportion. Concrete is a versatile construction material: it is plastic and malleable when newly mixed, yet strong and durable when hardened.

Usually, cement or lime is used as binding material.

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and Portland cement, are an essential ingredient in concrete.

Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories--fine and coarse.

For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.

Fine aggregate are basically sands won from the land or the marine environment. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 9.5mm sieve.

Coarse aggregates are particles greater than 4.75mm, but generally range between 9.5mm to 37.5mm in diameter. They can either be from Primary, Secondary or Recycled sources.

Types of Concrete

Concrete can be classified in following categories;

1. Concrete can be classified according to binding material used

2. Concrete can be classified according to its design

3. Concrete can be classified according to purpose of use.

Classification based on Binding Material

1. Cement Concrete: The concrete consist of cement sand and coarse

aggregate mixed in a suitable proportion in additive to water is called cement

concrete.

Cement is used as a binding material, sand as fine aggregate and shingle gravel,

broken brick and crushed stone are used as coarse aggregate.

This type of concrete is used where strength and durability are of prime importance

2. Lime Concrete: The concrete consisting of lime, coarse aggregate and fine

aggregate mixed in suitable proportion in additive to water is called lime concrete.

In this type of concrete lime is generally used as a binding material; sand, cinders and

surkhi are used as fine aggregate and broken bricks and broken stones etc. as coarse

aggregate. This concrete has less strength but cheaper than cement concrete.

It is generally used for the sake of economy in foundation works, under floors, over roofs

and where cement is not easily or cheaply available in required quantity.

Classification of Concrete According to Design

According to design of concrete it is classified in to three categories

1. Reinforced Cement Concrete: The cement concrete in which

reinforcement is embedded in form of round bars 6mm to 32mm Dia or mesh of steel

rods is known as RCC. This type of concrete is equally strong in taking tensile stresses,

Compressive and shear stresses.

RCC is commonly used for construction of slab, lintel, beams, columns, footings etc.

2. Plain Concrete: The concrete in which no reinforcement is provided is called

plain cement concrete or mass cement concrete. This type of concrete is strong

in taking compressive stresses but is weak in taking tensile or shear stresses.

Plain cement is commonly used for foundation work or floorings of building.

3. Pre-Stressed cement concrete: The type of concrete in which high

compressive stresses are artificially induced before their actual use is called pre

stresses.

In this type of cement concrete steel bars are induced to pre tensioning before

placing the concrete and the reinforcement is released after final setting of concrete.

This type of concrete can take up high compressive and tensile stresses without the

development of cracks.

It is used where high stresses develop.

Classification According to purpose:

According to purpose concrete is classified into six categories

1. Vacuum Concrete: The cement concrete from which excessive water and

entrained air is sucked out by means of suction pump is called vacuum concrete.

The removal of excess water after placing it help in increasing the strength of

concrete by 15 to 20 %.

This concrete is used for all RCC work for better results.

2. Air entrained concrete: The cement concrete prepared by mixing

aluminum in it is called air entrained concrete, cellular/aerated concrete. This

concrete is light in weight and spongy in structure. This concrete is used in roofs for

heat and sound insulation purpose.

3. Light Weight Concrete: The concrete prepared by using coke breeze,

cinder or slag as coarse aggregate is called light weight concrete. This concrete is

light in weight and possesses heat insulating properties. It is used in making precast

structural units for partition and wall lining.

4. Sawdust Concrete: The concrete prepared by mixing Portland cement

with sawdust in specified proportions in addition to water is called sawdust

concrete, This concrete expands when it becomes wet and contracts when dry.

This concrete is used as a heat or sound insulating material

5. High early strength concrete: The concrete in which high early strength

cement is used is called high early strength concrete. This concrete sets and

hardens quickly as compared to ordinary cement concrete. This type of concret

is used in construction work especially in cold weather.

6. White and Colored Concrete: The concrete in which white or colored

cement is used along with colored aggregate is call color concrete or color

crete. This type is used for decorative purpose of floorings , skirting, walls, etc.

RCC

RCC is concrete that contains steel bars, called reinforcement bars, or rebars. This

combination works very well, as concrete is very strong in compression, easy to

produce at site, and inexpensive, and steel is very very strong in tension.

To make reinforced concrete, one first makes a mould, called formwork, that will

contain the liquid concrete and give it the form and shape we need. Then one

looks at the structural engineer's drawings and places in the steel reinforcement

bars, and ties them in place using wire. The tied steel is called a reinforcement

cage.

Once the steel is in place, one can start to prepare the concrete, by mixing

cement, sand, stone chips in a range of sizes, and water in a cement mixer, and

pouring in the liquid concrete into the formwork tilll exactly the right level is

reached.

The concrete will become hard in a matter of hours, but takes a month to reach its

full strength.

Proportions of Concrete

Description of Work Recommended proportion

Retaining structures, piles, precast

work

1:1.5:3

RCC beams slabs, columns, road

pavements

1:2:4

Foundations for building, Mass

RCC works

1:3:6

For mass concrete works 1:4:8

Curing of Concrete

Curing is the process in which the concrete is protected from loss of moisture and

kept within a reasonable temperature range.

The result of this process is increased strength and decreased permeability.

Curing is also a key player in mitigating cracks in the concrete, which severely

impacts durability