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Author Topic: notes on concrete  (Read 1133 times)

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September 22, 2011, 12:42:33 PM
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Consolidation Of Concrete

The term consolidation of concrete is used to mean the compaction between aggregate and aggregate; between aggregate and reinforcement; and between aggregate and forms.
The main aim of consolidation of concrete is to eliminate air bubbles and thus to give maximum density to the concrete. An intimate contact between concrete and reinforcement is ensured by proper consolidation.
The difference between voids and pores may be noted. The voids are the gaps between two individual particles. The pores represent the opening within the individual particles. The process of consolidation of concrete can be carried out either with hand or with the help of vibrators.
Hand consolidation: For unimportant works, the consolidation of concrete is carried out by hand methods which include ramming, tamping, spading and slicing with suitable tools.
The hand methods require use of a fairly wet concrete. It should however be remembered that wherever feasible, the hand compaction should be preferred because the use of vibrator my lead to the segregation of the aggregates. As a matter of fact, the concrete mixes which can be hand-compacted should not be compacted by the use of vibrators.
Vibrators: These are the mechanical devices which are used to compact concrete in the formwork. The advantages of vibrators over hand methods are as follows:

(1) It is possible by means of vibrators to make a harsh and stiff concrete mix, with a slump of about 40 mm or less, workable.

(2) The quality of concrete can be improved by use of vibrators as less water will be required or in other way, economy can be achieved by adopting a leaner mix when vibrators are used.

(3) The use of vibrators results in the reduction of consolidation time. Hence the vibrators are used where the rapid progress of work is of great importance.



The poker vibrates while it is being inserted. The internal vibrators should be inserted and withdrawn slowly and they should be operated continuously while they are being withdrawn. Otherwise holes will be formed inside the concrete. The vibrator can be placed vertically or at a slight inclination not exceeding 100 to the vertical with the view to avoid flow of concrete due to vibration int

These vibrators require more power because of loss of some power in vibrating the rigid shutters. They are also heavy and hence they cannot be clamped at as many points as possible for uniform compaction of concrete. The compaction by these vibrators is found to be effective only upto a distance of about 450 mm from the face of the formwork.

(4) Vibrating tables: These are in the form of a rigidly built steel platform mounted on flexible springs and they are found to be very electromagnetic action or electric motors. They are found to be very effective in compacting stiff and harsh concrete mixes and hence they are invariably used in the preparation of precast structural products in factories and test specimens in laboratories.
The tables are vibrated either mechanically or by placing the springs under the supports of tables. The frequency of vibration varies from 3000 to 7200 vibration per minute. The two parameters of vibrations are frequency and time and they are related as follows:
« Last Edit: January 01, 1970, 04:00:00 AM by Guest »


 

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