In this article Learn : cement concrete,cement concrete ingredients, selection of material,cement, fine aggregate,coarse aggregate,water,Batching of ingredients by weight and by volume mixing, placing of concrete
Table of Contents
It is a composite material where in a paste of cement, made with water, on solidification binds firmly together the various particles of inert materials like sand and stone ballast. The inert material called “aggregate” is well graded in size from fine sand to pebbles or stone ballast or brick ballast.
Cement concrete is an extremely versatile material of construction which is used for a variety of works ranging from small cottages to massive dams and bridges. Cement concrete is quite strong in compression but not so in tension. To make good this deficiency of concrete steel bars (reinforcement) are embedded in concrete. The concrete then is known as reinforced concrete. The four materials go to make cement concrete : (i) Cement ; (ii) Sand (i.e. fine aggregate); (iii) Stone/Brick ballast (i.e., coarse aggregate) and (iv) Water.
To get quality concrete due attention should be paid in choosing the constituents, in mixing them in correct proportions, in mixing the concrete in correct manner and finally in using it properly followed by proper curing.
1. Selection of materials.
Concrete forming materials shall be carefully selected so as to get quality concrete :
Normally ordinary Portland cement satisfying the requirements of the Indian Standards Institution is used. However, for special conditions the type of cement suiting the requirements is used.
Cement, being hygroscopic, attracts moisture quickly and sets, So storage of cement should be carefully attended to and no set or even partially set cement should be used.
(ii) Fine aggregates.
Sand and crushed stone are the commony used fine aggregates in cement concrete.
(iii) Coarse aggregate.
Stone ballast, gravel, shingle and brick ballast are the usual coarse aggregates used in making cement concrete. Size of aggregate shall depend upon the type of work and the reinforcement. The size of aggregate should be less than the distance between two consecutive steel bars in RCC.
Strength of concrete shall depend to a great extent on the voids in it. As such to have lesser voids so as to get stronger concrete the aggregates should be well graded i.e., they should have particles of various sizes so that the voids of bigger particles are filled up by the particles of smaller sizes.
Graded aggregates give solid and dense concrete (free from holes or voids) which is stronger, more durable and water tight.
Only good clean water should be used for making concrete. It should be free from silt, salts or any organic matter. Generally speaking, water that is good for drinking is good enough for concrete work.
2. Batching of ingredients.
There are two methods of batching the ingredients of concrete : (i) by weight, and (ii) by volume.
Except for very large projects the latter would do. The basis of batching by volume is generally one part of cement to n parts of sand and 2n parts of ballast. The ballast is usually twice the sand whereas the ratio of cement to sand depends upon the desired strength of concrete. The ratio being less when the strength needed is great and vice versa. .
Volume of one bag of cement weighing 50 kgs is 34.5 litres. When it is taken out of bags it becomes loose showing a considerable increase in volume. As such batching cement concrete by taking into account the volume of loose cement is likely to result in less cement being mixed in the concrete. So in batching the ingredients by volume materials corresponding to a whole number of cement bags should only be taken. However, if fraction of a bag has to be used then it should be done so by weighting.
A convenient method of measurement is to use an open measuring gauge box with a capacity of 34.5 litres. Batches of fine and coarse aggregates required could then be measured in multiples of these boxes in accordance with the required proportions of the ingredients. In measuring ‘sand’ due allowance for bulking, if any, should be made . Not doing so would result in under sanded concrete.
Mixing should preferably be done by machines called concrete mixers. If these machines are not readily available or are too expensive for the job on hand then perfectly good results could be obtained by hand mixing if the following procedure is adopted.
For hand mixing of cement concrete a closely jointed platform of timber or bricks large enough to turn over the mix without spilling should be prepared. One bag of cement and the corresponding quantity of sand required is spread over this platform. Cement and sand should then be turned over thoroughly with shovels at least three times to produce a mix of uniform colour Spread this mixture evenly over the correctly measured quantity of coarse aggregate in a layer. Now turn this mixture thoroughly with shovels so that the stone pieces are evenly distributed throughout the mixture of cement and sand. Then water is slowly poured on to the heap from a water can fitted with a hose and the turning of mixture continued till a workable, smooth and uniform mixture is obtained. Only the right amount of water should be used in mixing concrete .
4. Placing of concrete.
After mixing the concrete with water it should be used up within 30 minutes i.e., before the initial setting of cement starts. Only so much of concrete should be mixed with water that can be easily used up in 30 minutes. In no case should cement concrete be remixed with water and used after 30 minutes.
After the concrete has been placed in the proper place it should then be rammed with wooden tampers or iron rods. It is done so as to make cement mortar penetrate all corners particularly around the reinfocement bars in case of RCC work. In case of large and important project this work is done by mechanical vibrators.
A few hours after the concrete has been laid, the exposed surfaces of the concreted portion should be cured (kept wet) for at least ten days.