Q.
What is concrete?
Concrete is a mixture of cement, sand, stone aggregates and
water. |
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Q.
What is RCC?
If a concrete mix is placed in and around a cage of steel rods,
it is called Reinforced Cement Concrete (RCC). |
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Q.
What is mortar?
Mortar is a mix of cement, sand and water, to be used for brick
works/block works and plaster. |
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Q.
How much water should be added in a concrete mix of one bag of
cement for normal construction work?
Normally the amount of water that is required per bag of cement
is 25 -28 liters only. |
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Q.
How does color affect the quality of cement & its concrete?
Quality of cement has nothing to do with its color. |
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Q.
What precautions should one take for water to be used in
concrete construction?
- It is good to use potable quality of water.
It should be free from impurities and harmful ingredients.
- Seawater isn’t recommended.
- The water fit for mixing is fit for curing too.
- Use of minimum quantity of mixing water, consistent with
the degree of workability required to enable easy placing
and compaction of concrete, is advisable.
- Ensure that water is measured and added.
- Low water to cement ratio is essential for good
performance of the structure in the long run.
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Q.
What is the minimum recommended concrete mix proportion for RCC
works?
1:1.5:3, where 1 part of cement is to be mixed with 1.5 parts of
sand and 3 parts of coarse aggregates. Water requirement for the
mix would be less than 25 liters per bag of cement. |
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| Q.
What are the common mistakes, which affect the quality of
concrete? The ways in which concrete maybe spoilt are
many, most common of them being:
- Use of too much or too little water for mixing, or water
carelessly added during mixing.
- Incomplete mixing of aggregate with cement
- Improper grading of aggregates resulting in segregation
or bleeding of concrete.
- Inadequate compaction of concrete
Using concrete which has already begun to set.
- Placing of concrete on a dry foundation without properly
wetting it with water.
- Use of dirty aggregate or water containing earthy
matter, clay or lime.
- Too much troweling of the concrete surface.
- Leaving the finished concrete surface exposed to sun and
wind during the first ten days after placing without
protecting it and keeping it damp by proper methods of
curing.
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Q.
What are the factors responsible for governing the compressive
strength of concrete?
The compressive strength is governed by the following factors : |
(i) |
w/c ratio |
(ii) |
characteristics of cement |
(iii) |
characteristics of aggregates |
(iv) |
time of mixing |
(v) |
degree of compaction |
(vi) |
temperature and period of curing |
(vii) |
age of concrete |
(viii) |
air entertainment |
(ix) |
conditions of testing |
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Back to Questions |
Q.
Within how much time the freshly prepared mortar / concrete
should be consumed?
The mortar / concrete should be consumed as early as possible
after addition of water to it. The hydration of cement starts
the moment water is added to it. As the hydration progresses the
cement paste starts stiffening and loses its plasticity. The
concrete should not be disturbed after this. Normally, this is
about 45 – 50 minutes. |
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Q.
What does MPa stand for & what is its significance?
MPa is abbreviated form of mega Pascal, which is a unit of
pressure. 1 MPa is equivalent to a pressure of 10Kg /cm2. The
strength of concrete & cement is expressed in terms of pressure
a standard cube can withstand. The Ordinary Portland Cement,
commonly called OPC is available in three grades namely 33, 43 &
53 grades. Thus, for a 43 grade cement a standard cement & sand
mortar cube would give a minimum strength of 43 MPa or 430 Kg
/cm2 when tested under standard curing conditions for 28 days. |
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Q.
What are construction joints and where they should be provided
in concrete?
Construction joints are the joints provided between successive
pours of concrete that have been carried out after a time lag.
As far as possible the construction joints should be avoided and
every care should be taken to keep their numbers minimal. Since,
presence of these joints creates a plane of weakness within the
concrete body, these joints should be preplanned and their
location should be such that they are at places where they are
subjected to minimum bending moment and minimum shear force. |
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Q.
Why is compaction essential?
Green concrete has all the three phases – solids, water & air.
In order to make the concrete impervious & attain its maximum
strength it is required to remove the entrapped air from the
concrete mass when it is still in plastic state. If the air is
not removed completely, the concrete loses strength
considerably. It has been observed that 5% voids reduce the
strength by about 30% and 10% voids reduce the strength by over
50%.
Compaction eliminates air bubbles and brings enough fine
material both to the surface and against the forms to produce
the desired finish. One can use such hand tools as steel rods,
paddling sticks, or tampers, but mechanical vibrators are best.
Any compacting device must reach the bottom of the form and be
small enough to pass between reinforcing bars. Since the
strength of the concrete member depends on proper reinforcement
location, be careful not to displace the reinforcing steel. |
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