CONCRETE

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CONCRETE

Q. What is concrete?
Concrete is a mixture of cement, sand, stone aggregates and water.
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). 
Q. What is mortar?
Mortar is a mix of cement, sand and water, to be used for brick works/block works and plaster.
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.
Q. How does color affect the quality of cement & its concrete?
Quality of cement has nothing to do with its color.
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.
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.
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. 
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
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.
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 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.
CURING
Q. What is ‘curing’ and why is it so important?
The term ‘curing’ is used to include maintenance of a favorable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external and protection against extremes of temperature. 
Q. What is the correct method of curing?
If a concrete mix is placed in and around a cage of steel rods, it is called Reinforced Cement Concrete (RCC). 
Walls: Water should be sprinkled from the top such that it covers the whole area of the wall and it should be remain wet. 
Slab: Ponding should be done on the slab by constructing bunds of mortar of approximately 1mX1m and water should be stored. 
Beams and columns   :The beams and columns can be maintained wet by tying gunny bags around the periphery and by maintaining it wet always.
Q. What methods are commonly employed to ensure sufficient moisture for curing?
Ponding, continuous sprinkling, covering with wet cloth, cotton mats or similar materials, covering with specially prepared paper, polyethylene, sealing coat applied as a liquid commonly known as ‘curing compound’ which hardens to form a thin protective membrane, are some of the methods by which concrete is cured.
Q. When should curing be started and when is it complete? 
Curing should be started just after the surfaces begin to dry. Normally 7 to 14 days curing is considered adequate.
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