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?
1. It is good to use potable quality of water.
2. It should be free from impurities and harmful ingredients.
3. Seawater isn't recommended.
4. The water fit for mixing is fit for curing too.
5. Use of minimum quantity of mixing water, consistent with the degree of workability required to enable easy placing and compaction of concrete, is advisable.
6. Ensure that water is measured and added.
7. 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:
1. Use of too much or too little water for mixing, or water carelessly added during mixing.
2. Incomplete mixing of aggregate with cement
3. Improper grading of aggregates resulting in segregation or bleeding of concrete.
4. Inadequate compaction of concrete
5. Using concrete which has already begun to set.
6. Placing of concrete on a dry foundation without properly wetting it with water.
7. Use of dirty aggregate or water containing earthy matter, clay or lime.
8. Too much troweling of the concrete surface.
9. 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 :
1. w/c ratio
2.characteristics of cement
3.characteristics of aggregates
4.time of mixing
5.degree of compaction
6.temperature and period of curing
7.age of concrete
9.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. 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.
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.
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.