CONCRETE

 What is Concrete?

In the field of civil engineering and construction industry concrete plays a vital role in building strong and durable structures.

Concrete is a composite material. It consists of a binding material which when mixed in water, binds the inert particles of well graded fine and coarse aggregates.

Depending upon the cementing material it is classified as:

1) Cement Concrete 

2) Lime Concrete 

Other types are ;

1) Reinforced Cement Concrete

2) Prestressed Cement Concrete

3) Aerated Concrete 

4) Pretty-packed concrete 

5) Cellular concrete or foam concrete 

Selection of Materials 

1) Cement: Normally Ordinary Portland Cement is used. The storage of cement should be carefully attended.

2) Fine Aggregates: Particles of fine aggregates pass through 4.75 mm mesh and are retained on 0.15 mm mesh. Commonly used fine aggregates are :

a) Sand 

b) crushed stone

c) ash, cinder or surkhi.

Kinds of Sand :

1) Pit sand or quarry sand 

2) River sand 

3) Sea sand 

3) Coarse aggregates : The particles whose size is bigger than 4.5 mm but smaller than 7.5mm are called coarse aggregates. Commonly used coarse aggregates are :

1)Stone Ballast 

2) Gravel or Shingle

3) Brick Ballast 

4) Breeze and Clinker 

Note: The aggregate after immersion in cold water should not absorb more than 25 percent of its weight of water.

Note: The aggregates having size from 7.5mm to 15 mm are call CYCLOPEAN AGGREGATES.

4) Water: Only good clean water should be used for making concrete.It should not contain salts, silt and organic matter. Briefly, water that is good for drinking should be used for concrete making.

Batching of Ingredients 

There are two methods of Batching 

1) By weight 

2) By Volume 

Note: Volume of one bag of cement weighing 50kg is 34.5 liters.

Mixing of Concrete 

There are two methods of mixing concrete 

1) Hand Mixing 

2) Machine Mixing ( Concrete Mixers)

Placing of Concrete 

After mixing of concrete it should be used within 30 minutes. Following are methods of placing concrete 

1) Mortar pans

2) Wheelbarrows 

3) Chutes 

4) Pumps 

5) Barrow Lifts 

WATER-CEMENT RATIO

It is the ratio of water and cement ( by wight or by volume) used in the preparation of concrete.

SLUMP TEST OF CONCRETE

How to conduct slump test of concrete in the field of civil engineering ?

A cone 30cm high, 20cm dia at the base, 10cm diameter at the top and handless, is used to conduct this test.

The concrete is filled in layers of 7.5cm at a time and each layer is rammed 25 times with a metallic tamping rod 16mm in diameter and 60cm long. After filling the cone is lifted up. It is measured from the top of the cone to the top of concrete after the cone is removed.

The usual values of slump are 

Mass concert and road work - 2.5 to 5 cm

Ordinary beams and slabs - 5 to 10 cm

Columns, thin vertical sections and retaining walls- 7.5 to 12.5 cm