Plain Cement Concrete(Grades of concrete)

Plain Cement Concrete ( PCC) | Preparation of Plain Cement Concrete | Uses of Plain Cement Concrete

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The cement concrete in which no reinforcement is provided is called  Plain Cement Concrete(PCC). It is simply a mixture of cement, sand and aggregate containing the suitable proportion of water.

It is sometimes called mass cement concrete or cement concrete.

It is considerably stronger in compression but weak in tension and shear.

 

Ingredients of  Plain cement concrete

 

1) Cement   

~ Portland Pozzolana Cement(PPC) is commonly used cement in  Plain Cement Concrete.

~ It should have a suitable degree of fineness.

 

2) Sand(Fine aggregate)

~ The sand used should be hard, sharp and angular.

~ The size of sand should be smaller than 5 mm (i.e. sand passing through 5 mm square sieve are used).

~ Sand should be free from dust, organic particles, etc.

~ Sea sand is not suitable in concrete because of very weak binding capacity.

 

3) Coarse aggregate

~ It is simply called as aggregate.

~ Aggregates used should be free from dust, foreign materials and also hard to resist enough stresses.

~ Generally, stone ballasts passed through the square sieve of 20 mm and retained in the square sieve of 5 mm are used as coarse aggregates.

~ Aggregates should be well graded so that the voids do not exceed 42%.

 

4) Water

~ Water to be used should be free from oils, acids, alkalis, salts and vegetable growths.

~ Water having a pH value less than 6 is not suitable for use in plain cement concrete.

 

Uses of Plain Cement Concrete 

Some of the uses of plain cement concrete are:

a. It is commonly used in the construction of the column foundation, massive gravity dams, flooring, etc.

b. It is used in rigid pavement construction (reinforcement-less rigid pavement).

c. In small scale canal construction; PCC is used.

d. It is also used in some stone masonry works.

 

Properties of Plain Cement Concrete(PCC)

Some of the properties of plain cement concrete are:

 

a. Strength

The PCC should have high compressive strength. The tensile strength should be 8-12% of compressive strength and shear strength should be 8-10% of compressive strength.

Compressive strength of the PCC depends upon the following:

i. Cement Content

ii. Water Cement Ratio

iii. Method of mixing, placing, compacting and curing.

iv. Quality of materials used

v. Age of the concrete.

 

b. Durability 

PCC should be able to resist climatical as well as chemical actions to be durable.

 

c. Workability 

PCC should be highly workable. It should be easy to mix, manage and transport. It should be free from bleeding and segregation.

 

d. Fire Resistance

PCC should be highly resistive towards the fire to prevent problems like firing, spalling of concrete, etc.

 

Preparation of Plain Cement Concrete (PCC)

Plain Cement Concrete preparation includes the following processes:

1) Selection and preparation of ingredients.

2) Mixing of ingredients

3) Transportation and placing

4) Compaction of Concrete

5) Finishing of surface

6) Curing of concrete

 

1) Selection and preparation of ingredients:

Various ingredients such as cement, sand, aggregate and water should be carefully selected according to the purpose and strength of concrete desired. These ingredients are then stored separately and properly.

 

2) Mixing of ingredients:

Ingredients of PCC are mixed properly with their relative proportion. Mixing may be of two types:

i) Hand mixing

ii) Machine mixing

i) Hand mixing:

It is the manual process of mixing of ingredients by labourers. This method of mixing is less suitable because more cement is required than in machine mixing.

 

ii) Machine mixing:

Machine mixing is the process of mixing the ingredients of concrete by the use of a machine and such concrete is called the machine mixed concrete.

A machine used for the mixing of the concrete is called a mixer. A machine used for the mixing can be operated manually or electrically. Machine mixed concrete is suitable if the large quantity of concrete to be prepared.

 

3) Transportation and placing: 

The prepared concrete is transported manually in an iron pan or mechanically through wheelbarrow or by buckets or by the pump. The media used in transportation depends upon the types and nature of the work.

The placing of concrete means the process of depositing to concrete in its required place and position as fast as possible.

 

4) Compaction of Concrete:  

To remove air bubbles or air voids and to make concrete more dense and compacted; the freshly placed concrete should be compacted with rollers, rod, and vibrators. This process is called compaction of concrete.

There may be needed needle vibrator and surface vibrator. Needle vibrator is generally used to compact the concrete in the small, narrow and deep area like a beam, column etc and Some time used in flat area but the surface vibrator is used in compaction of a flat area like road construction.

 

5) Finishing of surface:  

Finishing of a surface is also one of the important processes. Finishing means the removing of irregularities, voids etc present on the surface. After concreting the rough concrete surface; it is levelled and smoothen during the finishing process.

 

6) Curing of concrete

~ Wet curing by jute bags and clothes is done after 2 /3 hours of concrete laying.

~ After 24 hours, ponding or spraying curing is done for a minimum of 14 days.

~ In rainy areas, concrete should be covered to protect from water for 24 hours.

 

Dos & Don’ts of PCC Work

 

Dos

a. When PCC work is to be carried out underground; loose materials from the side of the pits or trenches should be removed.

b. De-watering should be done before PCC work when the water table is high.

c. Water should be added to the concrete according to the water-cement ratio.

d. Shuttering used in the PCC work should have the same size and thickness.

 

Don’ts 

a. Mixing of concrete on bare land should not be done.

b. PCC work without formwork should not be allowed.

c. Pouring of concrete from more than 1.5 m height should not be done.

 

 

For curing of concrete: Click here

 

 

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