Pre Stressed Concrete

What is Prestressed Concrete?

Prestressed Concrete is an architectural and structural material possessing great strength. The unique characteristics of prestressed concrete allow predetermined, engineering stresses to be placed in members to counteract stresses that occur when the unit is subjected to service loads. This is accomplished by combining the the best properties of two quality materials: high strength concrete for compression and high tensile strength steel strands for tension.

Actually, prestressing is quite simple. High tensile strands are stretched between abutments at each end of long casting beds. Concrete is then poured into the forms encasing the strands. As the concrete sets, it bonds to the tensioned steel. When the concrete reaches a specific strength, the strands are released from the abutments. This compresses the concrete, arches the member, and creates a built in resistance to service loads.

Prestressed Concrete Beam
Prestressed or pretensioned before it leaves the plant, a slight arch or camber is noticeable. Energy is stored in the unit by the action of the highly tensioned steel which places a high compression in the lower portion of the member. An upward force is thereby created which in effect relieves the beam of having to carry its own weight.

The upward force along the length of the beam counteracts the service loads applied to the member.

Ordinary Concrete Beam
Even without a load, the ordinary concrete beam must carry its own considerable weight - this leaves only a portion of its strength available to resist added loads.Under service loads, the bottom of the beam will develop hairline cracks.

TYPES OF CEMENT

TYPES OF CEMENT AND THEIR USES

TYPES OF CEMENT, ITS COMPOSITION AND USES

The following are the types of cement that are in practice:

1. Rapid Hardening Cement

2. Quick setting cement

3. Low Heat Cement

4. Sulphates resisting cement

5. Blast Furnace Slag Cement

6. High Alumina Cement

7. White Cement

8. Coloured cement

9. Pozzolanic Cement

10. Air Entraining Cement

11. Hydrographic cement

Table below shows different types of cement, their composition and uses:

Types of Cement

Composition

Purpose

Rapid Hardening Cement

Increased Lime content

Attains high strength in early days it is used in concrete where form work are removed at an early stage.

Quick setting cement

Small percentage of aluminium sulphate as an accelerator and reducing percentage of Gypsum with fine grinding

Used in works is to be completed in very short period and concreting in static and running water

Low Heat Cement

Manufactured by reducing tri-calcium aluminate

It is used in massive concrete construction like gravity dams

Sulphates resisting Cement

It is prepared by maintaining the percentage of tricalcium aluminate below 6% which increases power against sulphates

It is used in construction exposed to severe sulphate action by water and soil in places like canals linings, culverts, retaining walls, siphons etc.,

Blast Furnace Slag Cement

It is obtained by grinding the clinkers with about 60% slag and resembles more or less in properties of Portland cement

It can used for works economic considerations is predominant.

High Alumina Cement

It is obtained by melting mixture of bauxite and lime and grinding with the clinker it is rapid hardening cement with initial and final setting time of about 3.5 and 5 hours respectively

It is used in works where concrete is subjected to high temperatures, frost, and acidic action.

White Cement

It is prepared from raw materials free from Iron oxide.

It is more costly and is used for architectural purposes such as pre-cast curtain wall and facing panels, terrazzo surface etc.,

Coloured cement

It is produced by mixing mineral pigments with ordinary cement.

They are widely used for decorative works in floors

Pozzolanic Cement

It is prepared by grindin pozzolanic clinker with Portland cement

It is used in marine structures, sewage works, sewage works and for laying concrete under water such as bridges, piers, dams etc.,

Air Entraining Cement

It is produced by adding indigenous air entraining agents such as resins, glues, sodium salts of Sulphates etc during the grinding of clinker.

This type of cement is specially suited to improve the workability with smaller water cement ratio and to improve frost resistance of concrete.

Hydrographic cement

It is prepared by mixing water repelling chemicals

This cement has high workability and strength

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HOW TO CALCULATE THE CEMENT & AGGREGATE QTY FOR CONCRETE

take the ratio 1:1.5:3
cement =1
sand =1.5
metal =3
So what is the ratio we want to mixing 1+1.5+3=5.5
Volum of wet cement concret is 1.54 to 1.57
unit weight of cement bag is 1440 kg/cum
one bag of cement = 50/1440 = 0.034722 cum

SOLUTION : CEMENT = 1.54/5.5 =0.28
=0.28/0.0347 = 8.069 ( 8 bags)
one bag of cement =50 kg X 8.069 = 403.45 kgs.

SAND solution =0.28 X 1.5 = 0.42 Cum
1.5 is the mixing proportion
METAL SOLUTION = 0.28 x 3 = 0.84 cUM.
3 is the mixing proportion
1 Cum cement concrete is 1:1.5:3 ratio

cement = 403.45 kgs.
sand = 0.42 Cum.
metal = 0.84 Cum.

HOW TO CALCULATE CEMENT & SAND QTY FOR PLASTERING

10 feet= 3.05m

area of plaster = 3.05 x 3.05 = 9.3 m2
(convert thickness of plaster into m divide by 1000)
volume of plaster = 9.3 x (10/1000) = 0.09 m3
quantity of cement = 0.09/1+5 =0.09/6 = 0.02 m3
as cement comes in bags of 50 kg therefore volume of one
bag cement = o.0347 m3 (exact)
(or it may be taken 0.034 or 0.035 m3)

no. of cement bags = 0.02/0.0347 = 0.58 bags (almost 1 bag
of cement required for plaster )

quantity of sand = 0.02 x 5 = 0.10 m3

this is exact calculation
you can add 30% extra for wastage etc.

HOW TO CALCULATE CEMENT, SAND & BRICKS FOR BRICK MASONRY

Consider Brick Masonry for 1 cum with 1:5 ratio mix

1). Brick calculation
take brick size =200mmx100mmx100mm
convert in meter =0.2mx.1mx.1m
total volume of brick= .2x.1x.1=0.002 cum

calculate the numbers of brick for 1cum = 1cum/0.002cum= 500 numbers of bricks.

2). cement calculation
the ratio of cement mortar =1:5
sum =1+5=6
total dry mortar requried for 1 cum brick masonary=.30cum
so,
cement = (.30x1)/6= 0.05 cum
convert in KG=.05x1440=72KG
convert in cement bag= 72/50=1.44bag.

sand calculation
sand =(.30x5)/6=.25cum
convert in kg =.25x1440=360Kg.
to convert in cum
sand = .3*quantity of sand(5)/total of ratio
= .255 for one cum