All in the family of concrete

Overview:

This article explains the various types of materials required in mortar , concrete and reinforced cement concrete.

Before we go forward let us understand the basics:

Hydration of cement :

Cement reacts in the presence of Water and  a process known as hydration takes place . Heat is evolved during this process and hence it is known as exothermic .

Cement mortar:

When sand is added to the cement & water , it makes a substance which is known as cement mortar.

Cement mortar is used in joining , pointing and plastering .It can be made in various proportions of cement : sand such as :
1:1, 1:2 , 1:3 ,1:4 ,1:5 ,1:6.

The ratios 1:1, 1:2 are used for punning , the ratios 1:3 ,1:4 are used for plastering whereas the ratios 1:5 ,1:6 are used for joining of brick masonry and stone masonry.

SAND JUST RECEIVED AT SITE. TRUCK DIMENSIONS : 15 ' X 8 ' X 3' 6". VOLUME OF SAND :             420 CUBIC FOOT                   OR  4.2 BRASS
SAND JUST RECEIVED AT SITE. TRUCK DIMENSIONS : 15 ‘ X 8 ‘ X 3’ 6″.
VOLUME OF SAND : 420 CUBIC FOOT OR 4.2 BRASS

Cement concrete:

When aggregate is added in this mortar , it makes a substance known as cement concrete.

In order to make a dense concrete , it is necessary to properly select the particle sizes of fine aggregate ( sand ) and coarse aggregates and this process is known as blending .

Concrete is shown on drawing as              CC ( cement concrete ) or PCC ( Plain cement concrete ) and is used in :

Levelling course below footings
Bed concrete below flooring
Benching below underground pipes
Plinth protection
Coping on the top of parapet wall

Cement concrete is quite strong in compression but is rather weak in tension. Steel bars are excellent in tensile strength.
So , Steel bars are added in cement concrete to make it strong in tension .

2012-07-28-324

Reinforced Cement concrete:

The mechanism of Cement concrete and steel bars together is called reinforced cement concrete.
This has the compressive strength of concrete and also is blessed with the tensile strength due to presence of steel bars in the tension zone.
The working of steel bars in the concrete works on the basis of bond developed between the surface of concrete and steel bars .

IMG_2093

HYSD bars :
This bond is further improved by the  undulating or rough surface of the steel bars which is possible in tor-steel or ribbed bars.

Now in this context , let us make a list of what all is needed to make mortar , concrete and RCC .

A CEMENT BAG WEIGHS 50 KG AND HAS A VOLUME OF 35 LITRES.
A CEMENT BAG WEIGHS 50 KG AND HAS A VOLUME OF 35 LITRES.

Cement:

Can be Ordinary Portland Cement  or Pozzolona Portland Cement :
OPC is available in grades 43 & 53.
Whereas PPC is available in grade 33 only.

Meaning of grade of cement :
When it is said that the cement is 43 grade , it means that if a cube is made in a cement mortar using this cement and tested , the average crushing strength on 28th day shall not be less than                 430 Kg/Sq.cm.

The standard cube size is
7.07 cm x 7.07 cm x 7.07 cm.

Picture1

Sand:
It can be River sand or Crushed sand.
Both of them can be used in concrete in varying proportions.

The particle size has to be between 
75 microns to 4.75 mm .

One problem with river sand is its silt content and it should be controlled within 3% . In case , the silt content is more than 3% then the sand must be washed prior to it’s use.

If the particle sizes larger than 4.75 mm are present in larger proportions , then it is 
advisable to screen the sand prior to use in mortar.

Water:
For optimum results , the water should be free from chemicals and impurities and hence it is better to use the potable water for concrete.

Picture19

Aggregate:

The aggregate is designated by the mean size of aggregate ( MSA ) and the suggested size for RCC is 10 to 20 mm.

Admixtures:

These are of three groups:
Plasticizers , Retarders &
Water proofing chemicals.

Plasticizers help in improving workability of concrete without addition of extra water during mixing.
Retarders decelerate the setting time of concrete and thus transportation of ready mix concrete becomes feasible over long distances.
Waterproofing chemicals are used to make the concrete impermeable to passage of water.
These are used during the concreting of the terrace slab or sunk portions below toilets or balconies or wet areas of a building.

Steel bars:

Can be either Mild steel bars or HYSD bars.
Mild steel bars are available in grade Fe250.
High yield strength deformed bars
( HYSD ) are available in grades of Fe 415 , Fe 500 and Fe 550.

Meaning of grade :

Grade Fe415 means it has an ultimate tensile strength of 4150 Kg/Sq.cm.

Binding wire:
Used for binding the steel bars together . Main bars to distribution ,
Main bars to stirrups ,
Lapping of bars.

Mild steel wire 
GI wire
Suggested diameter is 1.6 mm.
Cover blocks:
Mortar blocks
PVC blocks
15 mm
25 mm
40 mm
50 mm

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