Needle Vibrator

                                           Needle type concrete vibrator

These concrete vibrators are also referred to as poker or immersion vibrator. These are extensively used in building industry. A needle vibrator consists of a cylindrical casing made of steel and is vibrated by means of a flexible drive with an outer tube run from an operating machine which can be run by electricity, petrol, diesel or compresses air. The frequency of vibrations of an immersion vibrator is very important as higher frequency gives better compaction. It varies from 3500 to 12000 cycles per minute. Frequencies below 3500 cycles per minute are inefficient for concrete compaction. The diameter of the vibrating part varies from 20 to 75 mm and its length varies from 25 to 90 cm.

ART OF USING NEEDLE VIBRATOR

The following points are important for efficient use of needle vibrators.

1.      When inserting the vibrator into freshly laid concrete, we should hold it upright and drop it slowly to an angle. This method will vibrate a larger area.

2.      Pull the vibrator out gradually so that the hole is filled up. It is also necessary o restrict the spacing of the introduction of the vibrator to less than 45 cm (18 inch) apart.

3.      We should not vibrate the formwork with a needle vibrator as it will damage the formwork. We should place it only upto 75 mm (3 inch) away from the formwork.

4.      When compacting concrete on top of another layer already placed, we must push the vibrator to a small distance (say about 75 mm) into the previously placed fresh lower layer to join the two concretes together.

5.      The depth to which the vibrator can be immersed is limited to the position of the flexible drive. The vibrator head can be let down into the concrete to the point of the flexible drive.

6.      Do not leave the vibrator running when it is not in concrete. Otherwise it can get heated up and the bearings can break down.

7.      Put the whole permissible length of the vibrator in the concrete when compacting fresh concrete. This step is necessary to prevent it from heating up. Immersion in the concrete cools it down.

8.      Avoid sharp bend of the flexible drive, especially at the vibrator head. Otherwise great strain is placed on the outer tube and internal flexible drive.

The amount of concrete that can be vibrated by one vibrator per hour varies from 4 to 30 m3 depending on the vibrator and the method of placing concrete.

Comparison Between Ordinary & Performance Graded Binders

The primary differences between ordinary graded binder and performance graded binder is given in the following table