Friday, March 24, 2023

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Points to be kept in mind:

Density of Cement = 1440 kg/m^3

Density of Aggregates = 1450-1550 kg/m^3

Sand Density = 1450-1550 kg/m^3

Weight of 1 bag of Cement = 50 kg

Volume of 1 bag of cement = 34.7 liters = 0.0347 cubic meters = 1.226 cubic feet.

No of bags in 1 cubic meter = 28.8 bags

Specific gravity of cement = 3.15

Grade of cement = 33, 43 and 53 ( where 33, 43 and 53 is the compressive strength of cement in N/mm^2)

Grade M-20 Concrete - Ratio = 1 : 1.5 : 3 - Sum=5.5

Here M refers to Mix and 20 is its compressive strength.

Let us consider Volume of Concrete = 1 m^3

Now for Dry Concrete Volume will be = 1.54 m^3

Now Calculate Cement, Sand and Aggregate

1) Cement

Cement = 1.54 X 1/5.5 = 0.28 m^3

Density of Cement = 1440 kg/m^3

Weight of Cement = 1440 X 0.28 = 403.2

No of bags = 403.2/50 = 8

2) Sand

Sand = 1.54 X 1.5/5.5 = 0.42 m^3

Density of sand = 1450 kg/m^

Weight of sand = 1450 X 0.42 = 609 kg

3) Aggregate

Aggregate = 1.54 X 3/5.5 = 0.84 cubic meters

Density of aggregates = 1500 kg/m^3

Weight of aggregates = 1500 X 0.84 = 1260 kg.

QUALITIES OF GOOD CONCRETE

Good concrete must be durable, dense, watertight, workable and be able to resist wear and tear.

(1) STRENGTH

Concrete should be able to withstand the stress which it is subjected to.

(2) DURABILITY

Concrete should be able to withstand the weathering action such as of wind, rain, frost and varying temperature. When used in the construction of drains and sewers or structures in touch with sea water it should be able to resist the action of chemical salts.

(3) DENSITY

The concrete should be able to well compacted so that there are no voids or hollows left. It should weigh 3000 kg/cu metre.

(4) WATER TIGHTNESS

In case of constructing dams and elevated tanks and water reservoirs, the concrete should be water tight otherwise it will damage the structure and corrode the steel.

(5) WORKABILITY

It should be easily workable.

(6) RESISTANCE TO WEAR AND TEAR

Concrete should be able to withstand abrasive forces in case of floor and road construction.

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Thursday, March 23, 2023

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