Vibrators are commonly used as a way of compacting concrete on site. In case of vibration, compacting gets effected by reduction in internal friction between various particles of concrete due to oscillation of particles which results in a dense and compact mass of concrete.

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2. Concrete Poker Risk Assessment Checklist
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Vibrations can either be

• Produced in the interior of the concrete using Internal vibrator, or
• Produced in concrete by vibrating formwork using Shutter form vibrators, or
• Produced on the surface of concrete using Surface vibrators, or
• Produced in the entire concrete mass using Vibrating table.

Therefore vibrators are commonly of 4 different types, such as:

1. Internal or Immersion or Poker or Needle Type Vibrator
2. Shutter form or External Type Vibrator
3. Surface or Screed or Pan Type Vibrators
4. Vibrating Tables

1. Internal Vibrator

They are also called immersion, poker or needle vibrators. They essentially consist of a power unit and a long flexible tube at the end of which a vibrating head is attached. Power is provided by electric motor, compressed air or petrol engine. The long tube houses a flexible shaft which rotates an eccentric weight inside the vibrating head. The frequency of the vibrator is about 700 cycles per minute.

The vibrating head is inserted in the concrete. They are most effective as the vibrating head comes into intimate contact with concrete.

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2. Form Vibrators

They are also called external vibrators. They are clamped to the formwork horizontally and vertically at suitable spacing not exceeding 90 cm in either direction. As the work progress they are shifted. They vibrate the concrete from the vibration of the forms and thus much energy is wasted.

They are used only if the use of internal vibrators is not practicable as in the case of thin and congested sections, arches and tunnel lining, etc.

3. Surface Vibrators

They are also called screed or pan vibrators. They are clamped to the screed. They vibrate the concrete from the surface at the time when screeding (striking off) of the concrete is carried out. They are effective only if depth of concrete is upto 20 cm. if the depth is more they can be used in combination with internal vibrators. They are used for long horizontal surfaces such as pavements and slabs.

4. Vibrating Tables

These are rigidly built steel platforms mounted on steel springs and driven by electric motors. The concrete is placed in moulds mounted on the platforms and securely clamped to the vibrating table, so that mould and concrete vibrate in union.

Vibrating tables are used for compacting stiff and harsh mixes used in precast structural members in factories and laboratory specimens.

What is a Poker Vibrator?

A poker vibrator consist of an electric weight encased in a housing and may be driven by an electric or pneumatic motor or a petrol or diesel engine. Poker diameters range from 25 to 75 mm.

How To Use Poker Vibrator?

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A poker vibrator should be used as follows

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• Insert the vibrator quickly. When compaction is complete, withdraw it slowly to avoid leaving a hole.
• Compaction is complete when the surface of the concrete gets a water sheen and air bubbles no longer break through it.
• Concrete must be placed in layers and the tip of the poker must penetrate the layer below to knit the concrete.
• The poker must be inserted at regular intervals in both directions. Points of insertion should be no more than 10 times poker diameter.
• Where void formers occur, concrete should be placed and compacted from one side until it has flowed under the obstruction.
• The poker must not touch the formwork.
• Slump of concrete for compacting by means of a poker vibrator is normally between 50 to 125 mm.